U.S. patent application number 17/111193 was filed with the patent office on 2021-07-22 for systems and methods for collecting a species.
This patent application is currently assigned to Massachusetts Institute of Technology. The applicant listed for this patent is Massachusetts Institute of Technology. Invention is credited to Maher Damak, Seyed Reza Mahmoudi, Kripa K. Varanasi.
Application Number | 20210220838 17/111193 |
Document ID | / |
Family ID | 1000005504831 |
Filed Date | 2021-07-22 |
United States Patent
Application |
20210220838 |
Kind Code |
A1 |
Damak; Maher ; et
al. |
July 22, 2021 |
SYSTEMS AND METHODS FOR COLLECTING A SPECIES
Abstract
Systems and methods related to the collection of a species from
a gas stream are generally provided. The systems and methods
described herein may allow for collection of a species such as a
fluid (e.g., water) with a relatively high collection efficiency.
Such systems and methods may be useful in various applications
including, for example, fog collection. In some embodiments, the
systems and methods enhance water collection from airborne fog to
produce usable water. Advantageously, the methods described herein
may, in some cases, incorporate ions into the gas stream such that
the species present in the gas stream follows electric field lines
and/or are attracted to a grounded (or charged) collector.
Advantageously, the systems and methods described herein may
suppress the adverse effects of natural conditions such as the
velocity and direction of the wind.
Inventors: |
Damak; Maher; (Cambridge,
MA) ; Mahmoudi; Seyed Reza; (Waltham, MA) ;
Varanasi; Kripa K.; (Lexington, MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Massachusetts Institute of Technology |
Cambridge |
MA |
US |
|
|
Assignee: |
Massachusetts Institute of
Technology
Cambridge
MA
|
Family ID: |
1000005504831 |
Appl. No.: |
17/111193 |
Filed: |
December 3, 2020 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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15763229 |
Mar 26, 2018 |
10882054 |
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PCT/US2016/054230 |
Sep 28, 2016 |
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17111193 |
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62233499 |
Sep 28, 2015 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C02F 1/78 20130101; C02F
1/4608 20130101; B03C 3/08 20130101; E03B 3/28 20130101; C02F
2303/04 20130101; B03C 3/47 20130101; B03C 3/09 20130101; Y02A
20/00 20180101; C02F 2201/782 20130101; B03C 3/013 20130101; B03C
3/41 20130101; B03C 3/12 20130101 |
International
Class: |
B03C 3/47 20060101
B03C003/47; B03C 3/09 20060101 B03C003/09; B03C 3/12 20060101
B03C003/12; B03C 3/41 20060101 B03C003/41; E03B 3/28 20060101
E03B003/28; B03C 3/08 20060101 B03C003/08; B03C 3/013 20060101
B03C003/013 |
Claims
1. A method of collecting a species present in a gas stream,
comprising: establishing a plurality of charged species in the gas
stream, electrically biased against a collector; and collecting the
charged species at the collector at a collection efficiency of
greater than or equal to 10%.
2. A method of collecting a species present in a gas stream,
comprising: arranging at least a first and second electrode so as
to apply an electric field to at least a portion of the gas stream
thereby urging the species toward the second electrode and
isolating at least a portion of the species from the gas stream,
wherein a minimum distance between the first electrode and the
second electrode is between 2 cm and 50 cm.
3-4. (canceled)
5. A collection system, comprising: a first electrode and a second
electrode configured to be positioned proximate the first
electrode; a power source in electrical communication with at least
the first electrode; and a collector, wherein the system is
configured to collect a species present within a gas stream with an
energy efficiency of greater than or equal to 1 liter per kWh of
energy applied in creating the field.
6-33. (canceled)
Description
RELATED APPLICATIONS
[0001] This application claims priority under 35 U.S.C. .sctn.
119(e) to U.S. Provisional Patent Application Ser. No. 62/233,499,
filed Sep. 28, 2015 and entitled "Enhanced Fog Collection with
Corona Discharge," which is incorporated herein by reference in its
entirety for all purposes.
FIELD
[0002] The present invention relates generally to systems and
methods for collection of a species from a gas stream such as the
collection of a liquid from a gas stream under the influence of an
electric field.
BACKGROUND
[0003] Although access to clean water is considered as a human
right, there are still over 1.1 billion people who lack access to
safe drinking water worldwide according to the World Water Council.
and this number is expected to increase, as water resources are
more and more polluted and scarce due to global warming. Water
scarcity can cause serious economical and social issues in the
regions where it occurs. One promising solution to provide clean
water to such regions is fog harvesting. Fog is a cloud that
touches the ground, composed of tiny droplets of diameters ranging
from 1 to 40 .mu.m with a typical diameter of 10 .mu.m. Fog
harvesting is particularly appropriate in remote, drought-prone
areas where rainwater harvesting is impossible and where water
transportation is prohibitively expensive. It can also be useful in
regions where water is currently available, but where non-renewable
groundwater is heavily used. Collecting water from fog can then
mitigate the depletion of groundwater reserves. If dense fog occurs
on a regular basis in such areas, then fog collection may be an
economically viable solution to meet the needs in water of local
populations. Areas prone to large fog formation are usually close
to oceans where fog clouds form over the water and are then
transported by the wind, but there are also some inland areas where
climatic conditions make it possible for a dense fog to form.
[0004] Fog collectors have been successfully implemented in 17
countries, generally to provide water to poor communities, even if
it has also been implemented in some developed countries such as
Spain. The technology used is simple and sustainable and the water
provided could be used in various applications: In addition to
drinking water for humans and animals, the collected water may be
used for cleaning, crop irrigation and afforestation.
[0005] Although fog-harvesting systems have been designed for
centuries, and even with the improvements of the last decades,
their efficiency remains dramatically low, generally around 2% for
the systems used in practice. Accordingly, improved compositions
and methods are needed.
SUMMARY
[0006] Methods and articles for the collection of species from a
gas stream as well as related components and methods associated
therewith are provided. The subject matter of the present invention
involves, in some cases, interrelated products, alternative
solutions to a particular problem, and/or a plurality of different
uses of one or more systems and/or articles.
[0007] In one aspect, methods for collection a species present in a
gas stream are provided. In some embodiments, the method comprises
establishing a plurality of charged species in the gas stream,
electrically biased against a collector and collecting the charged
species at the collector at a collection efficiency of greater than
or equal to 10%.
[0008] In some embodiments, the method comprises arranging at least
a first and second electrode so as to apply an electric field to at
least a portion of the gas stream thereby urging the species toward
the second electrode and isolating at least a portion of the
species from the gas stream, wherein a minimum distance between the
first electrode and the second electrode is between 2 cm and 50
cm.
[0009] In some embodiments, the method comprises arranging, within
the gas stream, a first electrode and a second electrode proximate
the first electrode, applying a potential to the first electrode
such that at least a portion of the fluid present in the gas stream
deposits on the second electrode and collecting the fluid. In some
embodiments, a minimum distance between the first electrode and the
second electrode is between 2 cm and 50 cm.
[0010] In some embodiments, the method comprises arranging, within
the gas stream, a first electrode and a second electrode proximate
the first electrode, applying a potential to the first electrode
such that at least a portion the fluid present in the gas stream
deposits on the second electrode and collecting the fluid at an
energy efficiency of greater than or equal to 1 liter per kWh of
energy applied in creating the field.
[0011] In another aspect, systems are provided. In some
embodiments, the system comprises a first electrode and a second
electrode configured to be positioned proximate the first
electrode, a power source in electrical communication with at least
the first electrode, and a collector. In some embodiments, the
system is configured to collect a species present within a gas
stream with an energy efficiency of greater than or equal to 1
liter per kWh of energy applied in creating the field. In certain
embodiments, the system is configured to collect a species present
within a gas stream with a collection efficiency of greater than or
equal to 10%.
[0012] In certain embodiments, the species comprises water.
[0013] In certain embodiments, the second electrode comprises a
mesh. In certain embodiments, the second electrode comprises
parallel wires. In certain embodiments, the first electrode
comprises needles. In certain embodiments, an average radius of
curvature of the needles is greater than or equal to 10
microns.
[0014] In certain embodiments, the second electrode is positioned
downstream of the first electrode. In certain embodiments, the
second electrode is positioned upstream of the first electrode. In
certain embodiments, the first electrode is held at a negative
potential. In certain embodiments, the first electrode is held at a
positive potential. In certain embodiments, the second electrode is
grounded. In certain embodiments, the second electrode is held at a
negative potential. In certain embodiments, the second electrode is
held at a positive potential. In certain embodiments, water is
collected on a surface of the second electrode facing the first
electrode. In certain embodiments, water is collected on a surface
of the second electrode not facing the first electrode.
[0015] In certain embodiments, a difference in potential between
the first electrode and the second electrode is greater than or
equal to 2 kV and less than or equal to 100 kV. In certain
embodiments, applying the potential to the first electrode
comprises ionizing at least a portion of the air stream. In certain
embodiments, the method comprises exposing the air steam to ozone.
In certain embodiments, the method comprises applying the potential
to the first electrode such that ozone is generated. In certain
embodiments, a corona discharge is generated by the first
electrode. In certain embodiments, the corona discharge purifies at
least a portion of the fluid collected.
[0016] In certain embodiments, the method or system comprises a
third electrode. In certain embodiments, the third electrode is
positioned downstream of the second electrode. In certain
embodiments, the third electrode is capable of charging the
species.
[0017] In certain embodiments, the electrode or component used to
charge species does not contribute to the electric field used to
collect the species. In certain embodiments, the electrode or
component used to charge species does contribute to the electric
field used to collect the species. In certain embodiments, the
combination of a perforated plate and a grounded electrode is used
to charge the species.
[0018] In certain embodiments, a Taylor cone of an ionic liquid is
used to generate space charge. In certain embodiments,
electrospraying a volatile liquid is used to generate space
charge.
[0019] Other advantages and novel features of the present invention
will become apparent from the following detailed description of
various non-limiting embodiments of the invention when considered
in conjunction with the accompanying figures. In cases where the
present specification and a document incorporated by reference
include conflicting and/or inconsistent disclosure, the present
specification shall control. If two or more documents incorporated
by reference include conflicting and/or inconsistent disclosure
with respect to each other, then the document having the later
effective date shall control.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] Non-limiting embodiments of the present invention will be
described by way of example with reference to the accompanying
figures, which are schematic and are not intended to be drawn to
scale. In the figures, each identical or nearly identical component
illustrated is typically represented by a single numeral. For
purposes of clarity, not every component is labeled in every
figure, nor is every component of each embodiment of the invention
shown where illustration is not necessary to allow those of
ordinary skill in the art to understand the invention. In the
figures:
[0021] FIG. 1A shows, in accordance with some embodiments, a
schematic illustration of a method of applying an electric field to
a gas stream comprising a species:
[0022] FIG. 1B shows, according to certain embodiments, a schematic
illustration of the trajectories of charged species under the
influence of an applied electric field;
[0023] FIG. 2A shows an exemplary schematic illustration of a
method where species are charged by a charge generator positioned
upstream of a collector,
[0024] FIG. 2B shows, according to certain embodiments, a schematic
illustration of a method where species are charged by a charge
generator positioned downstream of a collector;
[0025] FIG. 2C shows a schematic illustration of a method where
species are charged by a charge generator positioned neither
upstream nor downstream of a collector, according to certain
embodiments;
[0026] FIG. 3 shows a schematic illustration of a method where an
emitter that does not contribute to the applied electric field is
used to charge species, according to certain embodiments;
[0027] FIG. 4A shows a schematic illustration of a mesh, according
to one set of embodiments:
[0028] FIG. 4B shows a schematic illustration of a mesh, in
accordance with some embodiments:
[0029] FIG. 5A shows, according to certain embodiments, a schematic
of streamlines and particle trajectories and a photograph of
particle trajectories in the absence of an applied electric
field:
[0030] FIG. 5B shows, in accordance with certain embodiments, a
schematic of streamlines and particle trajectories and a photograph
of particle trajectories in the presence of an applied electric
field:
[0031] FIG. 6 shows a schematic depiction of an emitter electrode
and a collector, according to some embodiments;
[0032] FIG. 7 shows a schematic depiction of an emitter electrode
and a collector, according to certain embodiments:
[0033] FIG. 8 shows, in accordance with some embodiments, a
schematic depiction of an emitter electrode and a collector;
[0034] FIG. 9 shows a plot of the rate of the collection of fog in
L/(daym.sup.2) as a function of the applied voltage in kV according
to certain embodiments:
[0035] FIG. 10 shows the deposition efficiency as a function of Kc
for selected meshes, in accordance with some embodiments;
[0036] FIG. 11A shows, according to some embodiments, photographs
of a mesh at different times with and without an applied electric
field;
[0037] FIG. 11B shows, in accordance with some embodiments, a
photograph of the back surface of a mesh showing droplets collected
by turn-around:
[0038] FIG. 11C shows, according to certain embodiments, a
schematic illustration of the collection enhancement by doubling
the effective collection area;
[0039] FIG. 12A shows, in accordance with some embodiments, a
schematic illustration of a simplified experimental setup and
droplet trajectories;
[0040] FIG. 12B shows an exemplary schematic of the acceleration of
species in an electric field:
[0041] FIG. 12C shows, according to certain embodiments, a plot of
the added velocity as a function of V.sup.2;
[0042] FIG. 12D shows a schematic of the collection of charged
species, according to certain embodiment %:
[0043] FIG. 12E shows a plot of the nondimensional collection area
as a function of V.sup.2 for different wind speeds, according to
certain embodiments;
[0044] FIG. 13 shows the dependence of the nondimensional
collection area on the electrical number, according to one set of
embodiments;
[0045] FIG. 14A shows a schematic illustration of droplet
trajectories in a two-wire system, in accordance with some
embodiments;
[0046] FIG. 14B shows a photograph of species trajectories, in
accordance with certain embodiments:
[0047] FIG. 14C shows, according to certain embodiments, a plot
of
A in A 0 / D ##EQU00001##
as a function of Ke for different wire distances;
[0048] FIG. 15A shows, in accordance with certain embodiments,
photographs of meshes at different time intervals after fog
exposure;
[0049] FIG. 15B shows a plot of the mass of the collected water as
a function of Ke for different meshes, according to some
embodiments;
[0050] FIG. 15C shows a plot of the deposition efficiency as a
function of Ke for different meshes, according to certain
embodiments;
[0051] FIG. 16A shows a plot of the nondimensional collection area
as a function of the inverse of wind speed for five different
voltages; and
[0052] FIG. 16B shows a plot of the nondimensional collection area
for two wires as a function of Ke for three different wire
distances.
DETAILED DESCRIPTION
[0053] Systems and methods related to the collection of a species
from a gas stream are generally provided. The systems and methods
described herein may allow for collection of a species such as a
fluid (e.g., water) with a relatively high collection efficiency.
Such systems and methods may be useful in various applications
including, for example, fog collection. In some embodiments, the
systems and methods enhance water collection from airborne fog to
produce usable water. Traditional fog collection systems may be
limited by the distortion of hydrodynamic streamlines upstream of
the collector, preventing some of the species present in the gas
stream from reaching the collector, reducing the collection rate
and/or collection efficiency of the collection system.
Advantageously, the methods described herein may, in some cases,
incorporate ions into the gas stream such that the species present
in the gas stream follows electric field lines and/or are attracted
to a grounded (or charged) collector. For example, a corona
discharge may be used to charge the species such that the electric
field overcomes the forces generated by hydrodynamic streamlines,
resulting in an increased collection efficiency as compared to
traditional collection systems, while, in some cases, using
relatively low power consumption. In some embodiments, the ionic
discharge is established between an emitter (e.g., an electrode)
and a grounded collector (e.g., a mesh). Without wishing to be
bound by theory, the additional electric body force generally
changes the trajectory of the charged species such that it is
directed towards the mesh and the streamline distortion becomes
less important. Advantageously, the systems and methods described
herein may suppress the adverse effects of natural conditions such
as the velocity and direction of the wind. In some embodiments, two
or more surfaces (e.g., sides) of the collector may be used to
capture the species from the gas stream. In certain embodiments,
the systems and methods described herein may disinfect the
collected species. For example, in some cases, ozone is generated
during generation of the electric field such that the ozone
disinfects the collection species during collection. The system may
be scaled by, for example, increasing the number of emitters and/or
collectors.
[0054] In an exemplary embodiment, a plurality of charged species
may be established in a gas stream, the plurality of charged
species being electrically biased against a collector, such that
the charged species are collected at the collector. In another
exemplary embodiment, at least one electrode may be arranged to
apply an electric field to at least a portion of a gas stream, such
that a plurality of species are urged towards a collector (e.g., a
second electrode) and/or isolated from the stream.
[0055] As illustrated in FIG. 1A, in some embodiments, a gas stream
110 may comprise a plurality of species (e.g., fluid droplets) 120.
In some embodiments, the plurality of species may be charged. In
some such embodiments, plurality of species 120 may be urged
towards a collector 130. For example, in some cases, the plurality
of species may be charged such that they are electrically biased
against the collector. In some embodiments, the plurality of
species may be collected at collector 130.
[0056] In some embodiments, the plurality of species may not be
charged and flow according to hydrodynamic streamlines of the gas
stream (e.g., streamlines 115 in FIG. 1A). In certain embodiments,
however, the plurality of species may be charged such that the
plurality of species do not flow according to the hydrodynamic
streamlines of the gas stream. For example, in some embodiments, an
electric field may be applied to at least a portion of the gas
stream. In some such embodiments, the plurality of species may be
urged towards the collector (e.g., an electrode) and/or isolated
from the gas stream.
[0057] In some embodiments, the plurality of species may be
collected on the collector with a collection efficiency of greater
than or equal to 10%. Those skilled in the art, based upon the
teachings of the specification and descriptions of the various
techniques and arrangements described herein to affect efficiency,
would be able to, without undue experimentation, build a system
that has a collection efficiency of greater than or equal to 10%,
using any of the arrangements (e.g., meshes, electrodes, droplet
generations, etc.) described herein. That is to say, a significant
aspect of the invention is the development of a series of
parameters that lead to a collection efficiency of greater than or
equal to 10%, and the development of various factors that affect
efficiency such that one or more can be used in combination with
one or more of the other techniques described herein for generating
charged species (e.g., charged droplets) and collecting the
species, to achieve the collection efficiencies described
herein.
[0058] The collection efficiency of the system, as used herein, is
defined as the ratio of the rate at which the species is collected
by the collector to the rate at which the species flows through the
collector, expressed as a percentage. In some embodiments, the
efficiency may be greater than or equal to 5%, greater than or
equal to 10%, greater than or equal to 15%, greater than or equal
to 25%, greater than or equal to 50%, greater than or equal to 75%,
greater than or equal to 90%, or greater than or equal to 100%. In
certain embodiments, the efficiency may be less than or equal to
150, less than or equal to 90%, less than or equal to 75%, less
than or equal to 15%, or less than or equal to 10%. Combinations of
the above-referenced ranges are also possible (e.g., greater than
or equal to 10% and less than or equal to 150%). Other ranges are
also possible.
[0059] Certain methods for collection of a species may be able to
collect a relatively large amount of the species while consuming a
relatively low amount of power. Such systems may be considered to
be energy efficient. As used herein, the energy efficiency refers
to the ratio of the species collected in liters to the amount of
energy used to collect the species. In some embodiments, the energy
efficiency may be less than or equal to 20000 L/kWh, less than or
equal to 17550 L/kWh, less than or equal to 15000 L/kWh, less than
or equal to 12500 L/kWh, less than or equal to 10000 L/kWh, less
than or equal to 7500 L/kWh, 5000 L/kWh, less than or equal to 4750
L/kWh, less than or equal to 4500 L/kWh, less than or equal to 4250
L/kWh, less than or equal to 4000 L/kWh, less than or equal to 3750
L/kWh, less than or equal to 3500 L/kWh, less than or equal to 3250
L/kWh, less than or equal to 3000 L/kWh, less than or equal to 2750
L/kWh, less than or equal to 2500 L/kWh, less than or equal to 2250
L/kWh, less than or equal to 2000 L/kWh, less than or equal to 1750
L/kWh, less than or equal to 1500 UL/kWh, less than or equal to
1250 L/kWh, less than or equal to 1000 L/kWh, less than or equal to
750 L/kWh, less than or equal to 500 L/kWh, less than or equal to
250 L/kWh, less than or equal to 100 L/kWh, less than or equal to
75 L/kWh, less than or equal to 50 L/kWh, or less than or equal to
20 L/kWh. In certain embodiments, the energy efficiency may be
greater than or equal to 1 L/kWh, greater than or equal to 20
L/kWh, greater than or equal to 50 L/kWh, greater than or equal to
75 L/kWh, greater than or equal to 100 L/kWh, greater than or equal
to 250 L/kWh, greater than or equal to 500 L/kWh, greater than or
equal to 750 L/kWh, greater than or equal to 1000 L/kWh, greater
than or equal to 1250 L/kWh, greater than or equal to 1500 L/kWh,
greater than or equal to 1750 L/kWh, greater than or equal to 2000
L/kWh, greater than or equal to 2250 L/kWh, greater than or equal
to 2500 L/kWh, greater than or equal to 2750 L/kWh, greater than or
equal to 3000 L/kWh, greater than or equal to 3250 L/kWh, greater
than or equal to 3500 L/kWh, greater than or equal to 3750 L/kWh,
greater than or equal to 400 L/kWh, greater than or equal to 4250
L/kWh, greater than or equal to 4500 L/kWh, greater than or equal
to 4750 L/kWh, greater than or equal to 5000 L/kWh, greater than or
equal to 7500 L/kWh, greater than or equal to 10000 L/kWh, greater
than or equal to 12500 L/kWh, greater than or equal to 15000 L/kWh,
or greater than or equal to 17500 kWh. Combinations of the
above-referenced ranges are also possible (e.g., less than or equal
to 5000 L/kWh and greater than or equal to 1 L/kWh, less than or
equal to 5000 L/kWh and greater than or equal to 20 L/kWh, or less
than or equal to 20000 L/kWh and greater than or equal to 1 L/kWh).
Other ranges are also possible.
[0060] In some embodiments, the species may be collected at a
relatively high rate. According to some embodiments, collection may
occur at a rate of greater than or equal to 1 L/(daym.sup.2),
greater than or equal to 2.5 L/(daym.sup.2), greater than or equal
to 5 L/(daym.sup.2), greater than or equal to 10 L/(daym.sup.2),
greater than or equal to 25 L/(daym.sup.2), greater than or equal
to 50 L/(daym.sup.2), greater than or equal to 100 L/(daym.sup.2),
greater than or equal to 250 L/(daym.sup.2), or greater than or
equal to 500 L/(daym.sup.2). According to some embodiments,
collection may occur at a rate of less than or equal to 1000
L/(daym.sup.2), less than or equal to 500 L/(daym.sup.2), less than
or equal to 250 L/(daym.sup.2), less than or equal to 100
L/(daym.sup.2), less than or equal to 50 L/(daym.sup.2), less than
or equal to 25 L/(daym.sup.2), less than or equal to 10 L/(daym),
less than or equal to 5 L/(daym), or less than or equal to 2.5
L/(daym.sup.2). Combinations of the above-referenced ranges are
also possible (e.g., greater than or equal to 1 L/(daym.sup.2) and
less than or equal to 1000 L/(daym.sup.2)). Other ranges are also
possible.
[0061] The rate of collection of the species may be measured by any
suitable technique known to one of skill in the art. One technique
for measuring the rate of collection of the species is to expose
the collection apparatus to a uniform gas stream comprising the
species and to weigh and record the mass of the collection
apparatus as it is collecting the species. If the density of the
species is known (or experimentally determined), the collection
rate can be calculated by dividing the rate at which mass
accumulates on the collection apparatus by the density of the
species.
[0062] As described above, a significant aspect of the invention is
the development of various factors that affect efficiency such that
one or more can be used in combination with one or more of the
other techniques described herein for generating charged droplets
and collecting them to achieve this efficiency. Exemplary methods
and collection systems for generating and collecting charged
droplets are described in further detail below.
[0063] In some embodiments, the method may comprise removing the
species from the gas stream. FIG. 1B depicts one method for
removing the species from the gas stream. For example, as
illustrated in FIG. 1B, in some embodiments, gas stream 110
comprises plurality of species 120. Electric field 100 may be
applied to at least a portion of gas stream 110 such that the
plurality of species 120 are urged towards collector 130 (e.g., an
electrode). In some embodiments, in the absence of the electric
field, plurality of species 120 may flow following streamlines 115
(e.g., to position 120A). In certain embodiments, in the presence
of the electric field, flow of plurality of species 120 may be
affected by electric field 100 (e.g., to position 120B), as
compared to the flow in the absence of the electric field. In some
embodiments, electric field 100 urges plurality of species 120
towards and/or collect on collector 130 (e.g., collected species
125 on collector 130), whereas, in the absence of electric field
100, at least a portion of plurality of species 120 would not be
urged towards and/or collect on collector 130. In certain
embodiments, the plurality of species may be charged. In some
cases, ions may be added to the gas stream (e.g., via corona
discharge) such that the plurality of species are urged towards the
collector. In some cases, the plurality of species present in the
gas stream are moved (e.g., flowed) under the influence of forces
generated by both the applied electric field and the gas stream.
Those of ordinary skill in the art would understand, based upon the
teachings of this specification, that while FIG. 1B depicts a
uniform electric field, other electric fields could also be used to
direct the flow of the species (including, but not limited to,
electric fields where the magnitude and/or the direction of the
field vary spatially and/or temporally).
[0064] In FIG. 1B, species 120 has been urged towards collector 130
(e.g., an electrode) by the electric field such that it has
contacted the collector. In a gas stream comprising multiple
particles of the species to be at least partially isolated, at
least a portion of the particles may be urged by the electric field
to contact the collector. The collector may comprise any suitable
configuration. In some embodiments, the collector is a porous
substrate (e.g., a plate comprising a plurality of
perforations/openings, a mesh, a non-woven fiber web). In some
embodiments, the collector comprises a mesh (e.g., a conductive
mesh). In certain embodiments, the collector comprises a plurality
of wires (e.g., a plurality of substantially parallel wires). In
some cases, the collector may comprise a plate comprising a
plurality of openings that pass through its thickness. Other
collectors are also possible.
[0065] As described above, in certain embodiments an electrode may
be used to remove at least a portion of a charged species from a
gas stream. Some embodiments may comprise both a means for
generating space charge and an electrode to collect the charged
species. The means for generating space charge and the collector
may be positioned in any suitable manner with respect to each other
and with respect to the gas stream. For example, in some
embodiments, as shown in FIG. 2A, charge generator 225 (e.g., a
first electrode) used to charge the gas stream is positioned
upstream of collector 230 (e.g., a second electrode) used to
collect the plurality of species present within the gas stream. In
some embodiments, the first electrode is positioned proximate the
second electrode. In certain embodiments, as shown in FIG. 2B,
generator 225 may be positioned downstream of collector 230. In
some embodiments, as shown in FIG. 2C, generator 225 may be
positioned neither upstream nor downstream of collector 230. Other
arrangements of the charge generator with respect to the collector
are also possible. Additionally, while FIGS. 2A-C each show the
generation of negatively charged species by the charge generator,
embodiments in which the charge generator generates positively
charged species should also be understood to be encompassed by the
invention.
[0066] In embodiments comprising at least a charge generator (i.e.
a charge generator) and a collection electrode (i.e. a collector),
the minimum distance between the charge generator and the
collection electrode may be any suitable value. In some
embodiments, the charge generator and the collection electrode may
be relatively close together. For example, the minimum distance
between charge generator and the collection electrode may be less
than or equal to 50 cm, less than or equal to 40 cm, less than or
equal to 25 cm, less than or equal to 20 cm, less than or equal to
15 cm, less than or equal to 10 cm, less than or equal to 5 cm, or
less than or equal to 4 cm. In accordance with some embodiments,
the minimum distance between the charge generator and the
collection electrode may be greater than or equal to 2 cm, greater
than or equal to 4 cm, greater than or equal to 5 cm, greater than
or equal to 10 cm, greater than or equal to 15 cm, greater than or
equal to 20 cm, greater than or equal to 25 cm, or greater than or
equal to 40 cm. Combinations of the above-referenced ranges are
also possible (e.g., less than or equal to 50 cm and greater than
or equal to 2 cm, or less than or equal to 15 cm and greater than
or equal to 4 cm). Other ranges are also possible.
[0067] In some embodiments, a plurality of charged species may be
collected by more than one collector. For example, there may be
two, three, or more collectors. Without wishing to be bound by
theory, it is believed that the presence of additional collectors
may improve the collection efficiency and/or energy efficiency. In
some embodiments, the collectors may be positioned successively
downstream of each other.
[0068] In certain embodiments, collectors (e.g., electrodes)
positioned downstream may have relatively larger absolute values of
potential than electrodes positioned upstream (e.g., the first
electrode may have a potential of +5 V and the second electrode may
have a potential of +10 V, or the first electrode may have a
potential of -5 V and the second electrode may have a potential of
-10 V, etc.). This may allow the charged species to be subject to
increasing levels of force towards a collector as they flow through
the system. For instance, it may be possible for a second collector
to collect a portion of the charged species that are not collected
by the first electrode, and/or for a third collector to collect a
portion of the charged species that are not collected by the first
and second electrode, etc.
[0069] According to some embodiments, the species may be charged by
more than one charge generator. The species may be charged by two,
three, or more charge generators (e.g., electrodes). Each charge
generator may be independently positioned either upstream or
downstream of any fraction of the collectors. In some embodiments,
a first charge generator may be positioned upstream of all
collectors and a second charge generator may be positioned
downstream of at least a first collector. In this configuration,
the first charge generator may charge at least a portion of the
species before the species flows through any collectors and the
second charge generator may charge at least a portion of the
species not captured by at least the first collector. Without
wishing to be bound by theory, mechanisms for the collection of
species with this design may allow for species that are not charged
by the first charge generator to be charged by the second charge
generator and subsequently collected.
[0070] In certain embodiments, the charge generator comprises at
least one electrode. The electrode may be held at a potential such
that corona discharge occurs and space charge is generated. The
corona discharge may cause ionization of at least a portion of the
air stream. The space charge present due to corona discharge may
cause the species to become charged. In some such embodiments, the
difference in potential between the first electrode (e.g., which
charges the species by generating corona discharge) and the second
electrode (e.g., which collects the species) may result in the
formation of an electric field which directs the charged species
towards the second electrode.
[0071] In an exemplary embodiment, the charge generator comprises
two or more electrodes, three or more electrodes, or four or more
electrodes. For example, in some embodiments, the charge generator
may comprise a third electrode positioned downstream of the second
electrode. In some cases, the third electrode is capable of
charging the species.
[0072] In some embodiments, the charge generator does not
contribute to the electric field used to collect the charged
species. For instance, the species may be charged by a device that
is not an electrode, or it may be charged by a combination of
electrodes that together do not generate an appreciable electric
field outside the charging region. One non-limiting example of such
an article is the pair of an emitter and a grounded electrode,
which together compose the charge generator. After passing through
the emitter and grounded electrode, the species may be charged and
can then be attracted towards an electrode of opposite charge
(i.e., the collector). As depicted schematically in FIG. 3, gas
stream 300 comprises a plurality of species 320. In some
embodiments, emitter 340 and grounded electrode 350 are used to
generate charged species (but overall do not generate an electric
field outside of the grounded electrode). In some embodiments, the
grounded electrode may be a perforated plate (e.g., as depicted in
FIG. 3), although other geometries are also envisioned. Collector
330 can be held, in some cases, at the opposite charge to the
charge generated on the species so that it can attract and collect
the species (e.g., as shown in FIG. 3, the species can be
negatively charged by the emitter and collector 330 can be held at
positive charge; however, positively charged species attracted to
negatively charged electrodes are also contemplated).
[0073] In certain embodiments, the charge generator may comprise
charged fluids. For example, a potential may be applied to an ionic
liquid such that a Taylor cone is generated. The gas stream
comprising the species may flow through the Taylor cone such that
at least a portion of the charge therein is transferred to the
species. In some embodiments, ions may be ejected from a surface of
the Taylor cone (e.g., a tip) and form a space charge around the
surface of the Taylor cone. In certain embodiments, the gas stream
comprising the species may flow through the generated space charge
such that at least a portion of the charge therein is transferred
to the species.
[0074] In some embodiments, the charge generator may comprise
electrospray ionization. A volatile liquid may be electrosprayed.
In some embodiments, droplets having a net electric charge are
sprayed from the charge generator. In some such embodiments, at
least a portion of the sprayed droplets evaporate, leaving free
ions, such that a space charge is created. The gas stream
comprising the species may flow through the generated space charge
such that at least a portion of the charge therein is transferred
to the species.
[0075] As described above, the inventive systems and methods
described herein may provide for the collection of charged species
at relatively high collection efficiencies and/or high energy
efficiencies. The species may be collected in any suitable manner.
As used herein, collection refers to the accumulation of the
species at a defined location such that it can later be removed. In
some embodiments, collection of the species may comprise the use of
a collector. In some embodiments, the species may be at least
partially collected on an upstream surface of a collector. In some
embodiments, the species may be at least partially collected on a
downstream surface of a collector. In certain embodiments, the
species may be at least partially collected on a surface of a
collector facing a charge generating mechanism. In certain
embodiments, the species may be at least partially collected on a
surface of a collector not facing a charge generating mechanism.
The collector may be any suitable material that is at a potential
such that it will interact with the species such that the species
is attracted to the electrode. Features of the collector, such as
its design, will be discussed more fully below.
[0076] According to certain embodiments, collecting the species may
further comprise directing the species from a portion of the
collector at which it is collected to a different portion of the
collector. This may be accomplished, for example, with the aid of
any suitable device, non-limiting examples of which include
gutters, drains, storage containers, etc. In some embodiments,
collecting the species may further comprise removing the species
from the collector. For instance, a gutter on a collector may cause
the collected species to flow to a storage container, a drain pipe,
or the like.
[0077] As described above, in certain embodiments collecting the
species comprises using an electric field between a first electrode
and a second electrode to attract the species to the second
electrode. In certain embodiments, the first electrode may be
positively charged and the second electrode may be negatively
charged or may be grounded. In some embodiments, the first
electrode may be negatively charged and the second electrode may be
positively charged or may be grounded. In certain embodiments, it
may be advantageous for the first electrode to be negatively
charged. Without wishing to be bound by theory, a negatively
charged first electrode may be capable of generating corona
discharge that comprises ozone, thus exposing the air stream to
ozone. It may be beneficial to both charge a species and generate
ozone, because ozone may be capable of killing at least some
microorganisms. Accordingly, a first electrode that generates ozone
may be capable of at least partially disinfecting and/or purifying
the species.
[0078] In some embodiments, one or more electrodes is held at a
potential that has a relatively low absolute value. Any suitable
means may be utilized to hold one or more electrodes at a target
value, such as a power source. In some embodiments, a power source
may be placed in electrical communication with one or more
electrodes (e.g., at least the first electrode).
[0079] The power source may be a generator, a battery, etc. In
certain embodiments, one or more of the electrodes is held at a
potential with an absolute value of less than or equal to 10 kV,
less than or equal to 75 kV, less than or equal to 50 kV, less than
or equal to 30 V, less than or equal to 25 kV, less than or equal
to 20 kV, less than or equal to 15 kV, less than or equal to 10 kV
or less than or equal to 5 kV. In certain embodiments, one or more
of the electrodes is held at a potential with an absolute value of
greater than or equal to 2 kV, greater than or equal to 5 kV
greater than or equal to 10 kV, greater than or equal to 15 kV,
greater than or equal to 20 V greater than or equal to 25 kV
greater than or equal to 30 kV greater than or equal to 50 kV, or
greater than or equal to 75 kV. Combinations of the
above-referenced ranges are also possible (e.g., greater than or
equal to 2 kV and less than or equal to 100 kV, or greater than or
equal to 5 kV and less than or equal to 30 kV). Other ranges are
also possible. It should also be understood that the values above
refer to absolute values (e.g., an electrode held at a potential
with an absolute value of 2 kV may be held at a potential of +2 kV
or may be held at a potential of -2 kV).
[0080] In accordance with certain embodiments, the absolute value
of the difference in potential between an electrode that serves as
an emitter and an electrode that serves as a collector is
relatively low. In some embodiments, the absolute value of the
difference in potential between an electrode that serves as an
emitter and an electrode that serves as a collector is less than or
equal to 100 kV, less than or equal to less than or equal to 75 kV,
less than or equal to 50 kV, less than or equal to 30 kV, less than
or equal to 25 kV, less than or equal to 20 kV less than or equal
to 15 kV, less than or equal to 10 kV or less than or equal to 5
kV. In certain embodiments, the absolute value of the difference in
potential between an electrode that serves as an emitter and an
electrode that serves as a collector is greater than or equal to 2
kV greater than or equal to 5 kV greater than or equal to 10 kV,
greater than or equal to 15 kV greater than or equal to 20 kV,
greater than or equal to 25 kV, greater than or equal to 30 kV
greater than or equal to 50 kV, or greater than or equal to 75 kV.
Combinations of the above-referenced ranges are also possible
(e.g., greater than or equal to 2 kV and less than or equal to 100
kV, or greater than or equal to 5 kV and less than or equal to 30
kV). Other ranges are also possible. It should also be understood
that the values above refer to absolute values (e.g., an absolute
value of the difference in potential of 2 kV may be a difference in
potential of +2 kV or may be a difference in potential of -2
kV).
[0081] In some embodiments, the plurality of charged species is
electrically biased against the collector. The phrase electrically
biased against generally refers to having an electrical potential
such that there is a force of attraction (e.g., to the collector).
For instance, the charged species may be electrically biased
against the collector when it is at a potential such that it is
attracted to the collector. The absolute value of the electrical
bias generally refers to the absolute value of the difference in
potential between the two articles that are biased against each
other. In some embodiments, absolute value of the electrical bias
between the plurality of charged species and the collector is
greater than or equal to 2 kV, greater than or equal to 5 kV,
greater than or equal to 10 kV, greater than or equal to 15 kV,
greater than or equal to 20 kV, greater than or equal to 25 kV,
greater than or equal to 30 kV, greater than or equal to 50 kV or
greater than or equal to 75 kV. In some embodiments, the absolute
value of the electrical bias between the plurality of charged
species and the collector is less than or equal to 100 kV, less
than or equal to less than or equal to 75 kV, less than or equal to
50 kV, less than or equal to 30 kV less than or equal to 25 kV,
less than or equal to 20 kV less than or equal to 15 kV, less than
or equal to 10 kV, or less than or equal to 5 kV. Combinations of
the above-referenced ranges are also possible (e.g., greater than
or equal to 2 kV and less than or equal to 100 kV, or greater than
or equal to 5 kV and less than or equal to 30 kV). Other ranges are
also possible. It should also be understood that the values above
refer to absolute values (e.g., an absolute value of the electrical
bias of 2 kV may be an electrical bias of +2 kV or may be an
electrical bias of -2 kV).
[0082] In certain embodiments, the apparatus for collecting the
species may be operated while consuming a relatively low amount of
power (per surface area of the collector). In some embodiments, the
apparatus may be operated at a power of less than or equal to 500
W/m.sup.2, less than or equal to 4000 W/m.sup.2, 3000 W/m.sup.2,
2000 W/m.sup.2, 1000 W/m.sup.2, less than or equal to 750
W/m.sup.2, less than or equal to 500 W/m.sup.2, less than or equal
to 200 W/m.sup.2, less than or equal to 150 W/m.sup.2, less than or
equal to 100 W/m.sup.2, less than or equal to 75 W/m.sup.2, less
than or equal to 50 W/m.sup.2, less than or equal to 25 W/m.sup.2,
less than or equal to 20 W/m.sup.2, less than or equal to 15
W/m.sup.2, less than or equal to 10 W/m.sup.2, less than or equal
to 5 W/m.sup.2, less than or equal to 2.5 W/m.sup.2, less than or
equal to 1 W/m.sup.2, less than or equal to 0.5 W/m.sup.2, or less
than or equal to 0.25 W/m.sup.2. In some embodiments, the apparatus
may be operated at a power of greater than or equal to 0.1
W/m.sup.2, greater than or equal to 0.25 W/m.sup.2, greater than or
equal to 0.5 W/m.sup.2, greater than or equal to 1 W/m.sup.2,
greater than or equal to 2.5 W/m.sup.2, greater than or equal to 5
W/m.sup.2, greater than or equal to 10 W/m.sup.2, greater than or
equal to 15 W/m.sup.2, greater than or equal to 20 W/m.sup.2,
greater than or equal to 25 W/m.sup.2, greater than or equal to 50
W/m.sup.2, greater than or equal to 75 W/m.sup.2, greater than or
equal to 100 W/m.sup.2, greater than or equal to 150 W/m.sup.2,
greater than or equal to 200 W/m.sup.2, greater than or equal to
500 W/m.sup.2, greater than or equal to 750 W/m.sup.2, greater than
or equal to 1000 W/m.sup.2, greater than or equal to 2000
W/m.sup.2, greater than or equal to 3000 W/m.sup.2, or greater than
or equal to 4000 W/m.sup.2. Combinations of the above-referenced
ranges are also possible (e.g., less than or equal to 200 W/m.sup.2
and greater than or equal to 0.1 W/m.sup.2, less than or equal to 1
W/m.sup.2 and greater than or equal to 50 W/m.sup.2, or less than
or equal to 5000 W/m.sup.2 and greater than or equal to 1
W/m.sup.2). Other ranges are also possible. In an exemplary
embodiment, the apparatus may be operated at a power of less than
or equal to 200 W/m.sup.2 and greater than or equal to 0.1
W/m.sup.2.
[0083] In some embodiments, a first electrode (e.g., a charge
generator that is capable of at least partially charging the
species present within the gas stream) may comprise needles and/or
materials which comprise a surface with a relatively high radius of
curvature. Without wishing to be bound by theory, a relatively high
radius of curvature may be useful for generating a corona discharge
because it may result in a large potential gradient. In certain
embodiments, a first electrode may comprise a surface with a radius
of curvature of greater than or equal to 2 microns, greater than or
equal to 5 microns, greater than or equal to 10 microns, greater
than or equal to 15 microns, greater than or equal to 20 microns,
greater than or equal to 50 microns, greater than or equal to 100
microns, greater than or equal to 250 microns, greater than or
equal to 500 microns, greater than or equal to 1 mm, or greater
than or equal to 2.5 mm. In certain embodiments, a first electrode
may comprise a surface with a radius of curvature of less than or
equal to 5 mm, less than or equal to 2.5 mm, less than or equal to
1 mm, less than or equal to 500 microns, less than or equal to 250
microns, less than or equal to 100 microns, less than or equal to
50 microns, less than or equal to 20 microns, less than or equal to
15 microns, or less than or equal to 10 microns, less than or equal
to 5 microns. Combinations of the above-referenced ranges are also
possible (e.g., greater than or equal to 2 microns and less than or
equal to 250 microns, or greater than or equal to 100 microns and
less than or equal to 5 mm). Other ranges are also possible.
[0084] In certain embodiments, the system may comprise two or more
charge generators. For example, in some embodiments, a first
electrode comprises a plurality of needles (e.g., charge
generators). For instance, the first electrode may comprise an
array of needles. The ratio of the spacing between the needles (or
other charge generators) to the distance from the needles to a
second electrode may be any suitable value. In some embodiments,
ratio of the spacing between the needles to the distance from the
needles to a second electrode may be greater than or equal to 0.25,
greater than or equal to 0.5, greater than or equal to 1, greater
than or equal to 2.5, greater than or equal to 5, or greater than
or equal to 10. In some embodiments, the ratio of the spacing
between the needles to the distance from the needles to a second
electrode may be less than or equal to 15, less than or equal to
10, less than or equal to 5, less than or equal to 2.5, less than
or equal to 1, or less than or equal to 0.5. Combinations of the
above-referenced ranges are also possible (e.g., greater than or
equal to 0.25 and less than or equal to 15, or greater than or
equal to 0.5 and less than or equal to 5). Other ranges are also
possible. In some embodiments, the spacing between charge
generators is selected such that breakdown and/or arcing do not
occur.
[0085] In some embodiments, one or more electrodes may comprise a
mesh. For example, in some embodiments, the charge generator
comprises a mesh. In some In some embodiments, one or more second
electrodes (i.e., collectors) may comprise a mesh. Suitable
features of meshes will be described more fully below, and it
should be understood that these features may be present in either,
both, or none of a first and second electrode.
[0086] As shown in FIG. 4A, mesh 460 may comprise plurality of
wires 470 and openings 480. It should be noted that while mesh 460
is depicted as a square mesh, other mesh lattices am also
contemplated (e.g., triangular, rectangular, hexagonal,
non-periodic, etc.). In some embodiments, the mesh may only
comprise substantially parallel wires (e.g., mesh 462 as shown in
FIG. 4B comprising wires 470).
[0087] The openings in a mesh may have any suitable average minimum
cross-sectional dimension. In some embodiments, the openings may
have an average minimum cross-sectional dimension of greater than
or equal to 10 microns, greater than or equal to 20 microns,
greater than or equal to 50 microns, greater than or equal to 100
microns, greater than or equal to 200 microns, greater than or
equal to 500 microns, greater than or equal to 1 mm, greater than
or equal to 2 mm, greater than or equal to 5 mm, greater than or
equal to 10 mm, greater than or equal to 25 mm, greater than or
equal to 50 mm, or greater than or equal to 75 mm. In some
embodiments, the openings may have an average minimum
cross-sectional dimension of less than or equal to 100 mm, less
than or equal to 75 mm, less than or equal to 50 mm, less than or
equal to 25 mm, less than or equal to 10 mm, less than or equal to
5 mm, less than or equal to 2 mm, less than or equal to 1 mm, less
than or equal to 500 microns, less than or equal to 200 microns,
less than or equal to 100 microns, less than or equal to 50
microns, or less than or equal to 20 microns. Combinations of the
above-referenced ranges are also possible (e.g., greater than or
equal to 10 microns and less than or equal to 1 mm, greater than or
equal to 100 microns and less than or equal to 5 mm, greater than
or equal to 500 microns and less than or equal to 2 mm, greater
than or equal to 1 mm and less than or equal to 25 mm, or greater
than or equal to 2 mm and less than or equal to 100 mm). Other
ranges are also possible.
[0088] In some embodiments, the average minimum cross-sectional
dimension of the openings may be designed such that the average
minimum cross-sectional dimension is defined by:
D = D + 2 R c 2 R c < Ke / 5 , ##EQU00002##
where D* is the average minimum cross-sectional dimension of the
openings. R.sub.c is the average cross-sectional dimension of the
wires, K.sub.e is the ratio of electric and viscous forces defined
by:
Ke = 2 R p 0 V 2 .eta. g DR c ( U 0 + 2 R p .eta. g 0 V 2 d 2 ) ,
##EQU00003##
wherein: R.sub.p is the radius of the fog particles, .sub.0 is the
permittivity of free space. V is the voltage difference between the
emitter and the electrode, .eta..sub.g is the viscosity of the air.
D is the distance between the emitter and the collector, and
U.sub.0 is the wind speed.
[0089] The average cross-sectional dimension of the wires may be
any suitable value. In some embodiments, the average
cross-sectional dimension of the wires is greater than or equal to
0.5 mm, greater than or equal to 10 microns, greater than or equal
to 20 microns, greater than or equal to 50 microns, greater than or
equal to 100 microns, greater than or equal to 200 microns, greater
than or equal to 500 microns, greater than or equal to 1 mm,
greater than or equal to 2 mm, greater than or equal to 5 mm,
greater than or equal to 10 mm, greater than or equal to 25 mm,
greater than or equal to 50 mm, or greater than or equal to 75 mm.
In some embodiments, the average cross-sectional dimension of the
wires is less than or equal to 100 mm, less than or equal to less
than or equal to 75 mm, less than or equal to 50 mm, less than or
equal to 25 mm, less than or equal to 10 mm, less than or equal to
5 mm, less than or equal to 2 mm, less than or equal to 1 mm, less
than or equal to 500 microns, less than or equal to 200 microns,
less than or equal to 100 microns, less than or equal to 50
microns, or less than or equal to 20 microns. Combinations of the
above-referenced ranges are also possible (e.g., greater than or
equal to 10 microns and less than or equal to 1 mm, greater than or
equal to 100 microns and less than or equal to 5 mm, greater than
or equal to 500 microns and less than or equal to 2 mm, greater
than or equal to 1 mm and less than or equal to 25 mm, or greater
than or equal to 0.5 mm and less than or equal to 100 mm). Other
ranges are also possible.
[0090] In some embodiments, the system may be designed such that
the collection efficiency is defined by:
.eta. = SC 1 + [ 1.22 ( 1.3 SC + ( SC 1 - SC ) 2 ) ] Ke 10 ,
##EQU00004##
wherein SC is the fraction of the area occupied by the wires of the
collector divided by the total area of the collector.
[0091] In certain embodiments, openings occupy a relatively high
area fraction of the porous substrate (e.g., mesh). As used herein,
the area fraction is determined by measuring the total surface area
occupied by the openings divided by the largest surface area of the
porous substrate including openings, expressed as a percentage. In
accordance with some embodiments the openings occupy an area
fraction of the mesh of greater than or equal to 20%, greater than
or equal to 30%, greater than or equal to 40%, greater than or
equal to 50%, greater than or equal to 60%, greater than or equal
to 70%, greater than or equal to 80%, greater than or equal to 90%,
greater than or equal to 95%, greater than or equal to 98%, greater
than or equal to 99%, or greater than or equal to 99.5%. In
accordance with some embodiments, the openings occupy an area
fraction of the mesh of less than or equal to 99.9%, less than or
equal to 99.5%, less than or equal to 99%, less than or equal to
98%, less than or equal to 95%, less than or equal to 90%, less
than 80%, less than 70%, less than 60%, less than 50%, less than
40%, or less than 30%. Combinations of the above-referenced ranges
are also possible (e.g., greater than or equal to 20% and less than
or equal to 50%, greater than or equal to 40% and less than or
equal to 70%, greater than or equal to 60% and less than or equal
to 90%, or greater than or equal to 80% and less than or equal to
99.9%). Other ranges are also possible.
[0092] In certain embodiments, one or more of the parameters
described above (e.g., electrode voltage, opening diameter, etc.)
in relation to a first (i.e., emitter) and second (i.e.,
collection) electrode may be varied dynamically during collection
of the species. For example, one or more of the parameters
described above may be varied in response to changing environmental
conditions such that collection efficiency and/or energy efficiency
is maximized. In some embodiments, sensors may be incorporated into
the collection device that are capable of sensing the concentration
of species in the gas stream, the humidity, the wind direction, the
wind magnitude, etc. Then, one or more electrodes and/or wires may
be moved, or the potential applied to one or more electrodes may be
modified.
[0093] The species to be collected may comprise any suitable
compounds. In certain embodiments, the species to be collected may
comprise a liquid. In some embodiments, the species to be collected
may comprise water (e.g., in liquid, solid, and/or gaseous form).
The water may be substantially pure water, and/or it may comprise
one or more dissolved species present within the water. According
to some embodiments, the species to be collected may comprise fog.
In certain embodiments, the species to be collected may comprise an
organic compound.
[0094] In certain embodiments, the species may comprise liquid
droplets. The liquid droplets may have any suitable average
diameter. In some embodiments, the species may comprise liquid
droplets with an average diameter of greater than or equal to 100
nm, greater than or equal to 250 nm, greater than or equal to 500
nm, greater than or equal to 1 micron, greater than or equal to 2
microns, greater than or equal to 5 microns, greater than or equal
to 10 microns, greater than or equal to 15 microns, greater than or
equal to 20 microns, greater than or equal to 25 microns, greater
than or equal to 30 microns, greater than or equal to 35 microns,
greater than or equal to 40 microns, greater than or equal to 50
microns, greater than or equal to 100 microns, greater than or
equal to 250 microns, or greater than or equal to 500 microns. In
certain embodiments, the species may comprise liquid droplets with
an average diameter of less than or equal to 1 mm, less than or
equal to 500 microns, less than or equal to 250 microns, less than
or equal to 100 microns, less than or equal to 50 microns, less
than or equal to 40 microns, less than or equal to 35 microns, less
than or equal to 30 microns, less than or equal to 25 microns, less
than or equal to 20 microns, less than or equal to 15 microns, less
than or equal to 10 microns, less than or equal to 5 microns, less
than or equal to 2 microns, less than or equal to 1 micron, less
than or equal to 500 nm, or less than or equal to 250 nm.
Combinations of the above-referenced ranges are also possible
(e.g., greater than or equal to 100 nm and less than or equal to 1
micron, greater than or equal to 250 nm and less than or equal to
10 microns, greater than or equal to 1 micron and less than or
equal to 40 microns, greater than or equal to 1 micron and less
than or equal to 100 microns, greater than or equal to 10 microns
and less than or equal to 500 microns, or greater than or equal to
250 microns and less than or equal to 1 mm). Other ranges are also
possible.
[0095] The gas stream may comprise any suitable gaseous species. In
certain embodiments, the gaseous species may comprise air. Other
gaseous species are also possible. Those skilled in the art would
be capable of selecting suitable gas streams and gaseous species
which comprise a species to be collected based upon the teachings
of this specification.
[0096] In an exemplary embodiment, the system comprises a first
electrode and a second electrode configured to be positioned
proximate the first electrode, a power source in electrical
communication with at least the first electrode, and a collector.
In some embodiments, the system is configured to collect a fluid
with an energy efficiency of greater than or equal to 1 liter per
kWh of energy applied in creating the field and/or a collection
efficiency of greater than or equal to 10%.
[0097] In another exemplary embodiment, a species present in a gas
stream may be collected by establishing a plurality of charged
species in the gas stream, electrically biased against a collector
and collecting the charged species. In some embodiments, the
charged species is collected at the collector at a collection
efficiency of greater than or equal to 10%.
[0098] In yet another exemplary embodiment, at least a first and
second electrode may be arranged so as to apply an electric field
to at least a portion of a gas stream thereby urging a species
contained with the gas stream toward the second electrode. In some
embodiments, at least a portion of the species may be isolated from
the gas stream. In certain embodiments, a distance between the
first electrode and the second electrode is between 2 cm and 50
cm.
[0099] In another exemplary embodiment, a species present in a gas
stream may be collected by arranging, within the gas stream, a
first electrode and a second electrode proximate the first
electrode, applying a potential to the first electrode such that at
least a portion of the fluid present in the gas stream deposits on
the second electrode, and collecting the fluid. In some
embodiments, the distance between the first electrode and the
second electrode is between 2 cm and 50 cm. In certain embodiments,
the species is collected at an energy efficiency of greater than or
equal to 1 liter per kWh of energy applied in crating the
field.
[0100] U.S. Provisional Patent Application Ser. No. 62/233,499,
filed Sep. 28, 2015 and entitled "Enhanced Fog Collection with
Corona Discharge," is incorporated herein by reference in its
entirety for all purposes.
Examples
[0101] The most common design for fog collectors is a large woven
mesh that stands perpendicular to the fog-laden wind, held by a
frame. The wind blows fog into the mesh wires. Upon impact on the
wires, droplets stick to the mesh, coalesce with other incoming
droplets to become bigger, and when they reach a certain size, they
are shed by gravity into a gutter and are eventually carried to a
collection tank. Meshes are used instead of plates because a large
impermeable obstacle would lead to a deviation of the incoming
streamlines and consequently to the drag of the fog droplets away
from the plate, whereas a mesh causes a much smaller alteration of
the flow by letting air pass through its openings. Meshes used in
practice are made of polyethylene or polypropylene. Their
collection rates largely vary from location to other, but the
typical values in actual systems range from 1 to 10 L/m.sup.2/day.
The collection efficiency of a collector can be measured by
calculating the ratio between the rate of water collection in the
gutter and the rate of water flowing in an unperturbed stream tube
with the same area as the mesh. Many reasons may explain why the
efficiency cannot be 100%. First, for example, only the part of the
droplets that impacts the mesh wires can be captured. The rest of
the flow goes through the opening of the mesh and is never
captured. Then, among the droplets that impact the wires, some can
bounce on the wire and go away or be captured then re-entrained by
the wind before they shed due to gravity. Another problem is that
collected droplets that did not shed yet can partially clog the
mesh openings and form a large impermeable area that deviates
incoming streamlines. The meshes can be characterized by the wire
opening and the shade coefficient, which is the projected area
(surface area) of the wires divided by the total area of the mesh.
By tuning these two parameters, a balance can be found between
minimizing streamlines deviation and maximizing the fraction of
droplets that impact wires, to have a maximum efficiency.
[0102] The most common design for fog collectors is a large woven
mesh that stands perpendicular to the fog-laden wind, held by a
frame. The wind blows the fog into the mesh wires. Fog is a cloud
touching the ground, composed of tiny droplets of diameters ranging
from 1 to 40 .mu.m with a typical diameter of 10 .mu.m. Upon impact
on the wires, droplets stick to the mesh, coalesce with other
incoming droplets to grow bigger, and when they reach a critical
size, they are shed by gravity into a gutter and are eventually
carried to a collection tank. Meshes are used instead of plates
because a large impermeable obstacle may lead to a deviation of the
incoming streamlines and consequently to the drag of the fog
droplets away from the plate, whereas a mesh causes a much smaller
alteration of the flow by letting air pass through its openings.
Nevertheless, these meshes typically have very low efficiencies,
around 1-2%.
[0103] Several mechanisms limit the efficiency of such collectors.
The deviation of the streamlines around the collector is one of
them and constitutes the "aerodynamic efficiency" of the system.
The collector, of size L may deviate the flow in a region of size L
around it, thereby diminishing the number of fog particles directed
towards it. The actual number of particles directed towards the
mesh wires divided by the total number of particles that were
directed towards the collector far from it is the aerodynamic
efficiency, and this efficiency depends on the shading coefficient
(SC) of the mesh, which is the fraction of the projected area
(surface area) occupied by the mesh wires. A high SC may lead to a
plate-like situation where the streamlines are greatly deviated,
while a small SC may lead to a small efficiency because most of the
droplets may pass through the mesh openings. It has been shown that
a SC around 55% leads to a maximum aerodynamic efficiency.
[0104] Another limitation to efficiency is the shedding rate. If
captured droplets on the mesh cannot shed easily by gravity and
remain on the mesh they can decrease the efficiency by two
mechanisms. The mesh openings can be clogged by water making the
mesh locally act as a plate. Re-entrainment of the droplets due to
wind drag may also happen before these droplets are collected.
Improving the shedding rate has recently received significant
attention, and researchers have studied fog-harvesting animals and
plants for inspiration and developed coatings to improve the
shedding rates. Significant improvements have been reported.
However, the overall efficiency of these meshes remained low,
around 10%, suggesting that the main limitation is not the shedding
rate.
[0105] A significant bottleneck in the fog collection process is
the deviation of the droplets around the individual wires of the
mesh, captured by the deposition efficiency, which is the fraction
of droplets directed towards the wire that are collected. The flow
past a cylinder has been extensively studied. Far from the cylinder
the air streamlines are parallel and the fog particles'
trajectories follow them closely. Close to the cylinder, in a
region of characteristic size the radius of the cylinder R.sub.e,
the streamlines start deviating and going around the cylinder, as
schematically shown in FIG. 5A. In this region, fog particles are
subject to two forces: their inertia and drag forces exerted by the
air, and their fate may depend on the ratio of these forces. This
ratio is the Stokes number that can also be seen as the ratio of
the particle's timescale to the flow's timescale.
S t = .tau. patipcle .tau. flow = Inertia Drag = 2 R p 2 .rho. w U
9 .eta. b R c . ##EQU00005##
For low Stokes numbers, the particles trajectories may follow the
streamlines very closely, and few droplets will be collected. An
example is presented in the photograph of FIG. 5A that shows the
fog trajectories around a cylinder for St=0.05, with flow
separation. For high Stokes, the drag forces may not affect the
trajectories, and the droplets facing the cylinder may continue
their parallel horizontal movement and collide with the cylinder.
An empirical formula has been established for the deposition
efficiency:
.eta. d = St St + .pi. 2 . ##EQU00006##
In practice, however, very fine meshes, which are difficult to
make, are needed to have large Stokes numbers, and low deposition
efficiencies may significantly limit the fog collection rate.
[0106] In some cases, electrostatics have been proposed for the
purpose of fog abatement in roads and airports where it may be
dangerous because it reduces the visibility and enhances the risk
for accidents. These designs generally aim to clear relatively
large areas from fog and thus significantly distort the fog
trajectory over large distances (up to 500 m in some of the
designs).
[0107] Other prior methods use a high voltage needle and a grounded
plate and, since water molecules are polar, they align with the
electric field and move towards the regions of highest field. i.e.
the needle where the droplets coalesce and grow. As they become
larger, they also acquire charge from the needle by conduction. At
a certain point, the electrostatic repulsion overcomes the adhesion
force and the droplet leaves the needle and is transported towards
the collector under the influence of the electric field. However,
one exemplary disadvantage of this method is that charging the
droplets requires contact with the needle, which can be rapidly
covered with water. This may limit the performance of the system
and the collection rate, and/or may require a large number of
needles, which is energy consuming.
[0108] By contrast, in embodiments described herein, electric
fields cause relatively slight deflections in fog trajectories,
allowing droplets to hit the collection meshes rather of passing
between the wires, resulting in, for example, increased collection
efficiency (e.g., a collection efficiency of greater than or equal
to 10%). In addition, the methods and systems described herein
intend not to remove and get rid of the fog but to collect it and
disinfect it in situ to, for example, produce water that is usable
for drinking and irrigation.
[0109] A novel method leading to enhanced fog collection with
minimal power consumption is described herein. In one example, the
system consists of two electrodes: The first one, the emitter
electrode, is a needle or an array of needles, a needle being a
conductive object with a small radius of curvature, maintained at a
high voltage (order of magnitude: 5-25 kV). The second electrode is
the collector, and it is placed close to the first one and consists
in a grounded conductive mesh or plate. The collector is
perpendicular to the wind transporting fog droplets and the high
voltage needles are placed either just before or just after the
collector in the direction of the wind. The high voltage at the
first electrode causes corona discharge to occur at its vicinity. A
cloud of space charges is generated and accelerated towards the
droplets. They attach to water droplets and the droplets become
charged with the same sign as the emitter electrode. These charged
droplets are then attracted to the grounded collector under the
influence of the electric field between the two electrodes. This
additional force drives the fog droplets to the collector. When a
droplet arrives at the mesh/plate it is captured and as soon as a
few drops coalesce and become large enough to be shed by gravity,
they drop and are collected in a gutter that transports them to a
storage tank.
[0110] Far from the collector, the fog droplets follow the air
streamlines. As the wind approaches the collector, in the absence
of electric field, the streamlines deflect to avoid the obstacle.
If the collector is an impermeable plate, they deflect greatly and
entrain a large quantity of droplets. If the collector is a mesh,
streamlines can pass through the openings, and they slightly
deflect around the mesh wires. But, as the water drops have more
inertia than the air, they are deflected more slowly than the air
and they can hit the mesh fibers. When the droplets are charged due
to corona discharge, the collector attracts them and their
trajectories are modified and oriented towards the surface of the
collector. Many droplets that would have been deflected and passed
through the mesh openings can now be collected since the attractive
electric fore can overcome the drag force exerted by the air. FIG.
5A and FIG. 5B show the mechanism by which electrostatic forces may
overcome hydrodynamic forces and attract droplets towards the
collector. The trajectories of the fog droplets are modified and
are no longer the hydrodynamic streamlines.
[0111] One approach is schematically shown in FIG. 5B. Particles
are initially electrically neutral, and dielectrophoresis forces
are not sufficient to affect their trajectories. Therefore, a
corona discharge is used to inject a net charge into the droplets
and then the droplets are directed towards the collector using an
electric field. The application of a high voltage at the emitter
electrode produces an electric field, whose lines go from the
emitter to the grounded collector. This electric field does not
affect the air streamlines, but acts only on the charged fog
droplets. The case with a high voltage is shown in FIG. 5B, where
the electric forces are much larger than the air drag forces, which
makes the fog droplets follow the field lines and end up on both
sides of the collection wire. As can be seen in the photograph in
FIG. 5B, showing particles' trajectories in the presence of a
strong electric field, that particles are collected all over the
wire and that some particles that were not initially directed
towards the wire are still captured. Intuitively, this suggests
that the deposition efficiency, which was previously defined as the
fraction of droplets directed towards the wire that are collected,
can become higher than one.
[0112] Another phenomenon that often occurs with this system, when
a mesh is used, is termed the "turn-around". When the droplets are
charged with corona discharge, they are attracted to the collector
surface. However, not all of them are captured. Due to the inertia
of the wind, the electrostatic force may not be sufficient to
modify the trajectory of some droplets and make them go to the mesh
wires. These droplets pass through the openings and start to move
away from the mesh. However, these droplets are still charged and
they are still attracted to the grounded mesh. Due to this
attraction, part of the droplets may turn and go back to be
captured by the back surface of the mesh. This turn-around
phenomenon contributes significantly to enhancing the collection
efficiency since it allows doubling the used surface by using the
back surface of the mesh as well, and it offers a second chance to
catch the droplets that were let go when they arrived at the mesh.
This leads to deposition efficiencies that are often higher than
100%: Droplets can be collected from an area larger than the
projected area (surface area) of the collector, which represents
the 100% efficiency case in traditional collectors (this is never
reached: actual efficiencies are much lower in traditional
collectors).
[0113] As showed herein, experimentally and theoretically, the
deposition efficiency of such a system varies linearly with one
non-dimensional number Ke, the electrical number.
Ke = 2 R p 0 V 2 .eta. g dR c ( U 0 + 2 R p .eta. g 0 V 2 d 2 )
##EQU00007##
R.sub.p is the radius of the fog particles .sub.0 is the
permittivity of free space V is the voltage difference between the
emitter and the electrode .eta..sub.g is the viscosity of the air d
is the distance between the emitter and the collector R.sub.c is
the radius of the mesh wires U.sub.0 is the wind speed
[0114] This dependence allows predicting the voltage that should be
applied to have a certain efficiency and can be adjusted in
practice to compensate the effects of changes in wind speed for
example.
[0115] In addition, by analyzing the phase diagram of this system,
the optimal region of operation was identified, which can be
described by 2 parameters:
[0116] First, the relative added velocity U*, which represents the
ratio of the added velocity of the particles due to the
acceleration by the electric field and the wind velocity.
U * = 2 R p 0 V 2 U 0 .eta. g d 2 ##EQU00008##
Optimal operation is for U*<1. For U*>1 the system reaches
voltage saturation. This limitation in the reachable collection
rates essentially comes from the fact that, as the voltage is
increased, there is a higher electric force attracting the
particles, but the particles are also moving faster, which leaves
less time for the electric force to attract them.
[0117] Another important parameter is the spacing D between the
mesh wires, which is captured by the non-dimensional number D*.
D * = D + 2 R c 2 R c ##EQU00009##
When the distance between the wires is too small, all the droplets
between them are collected and increasing the voltage would be
ineffective, since there is no more fog to collect.
[0118] When using corona discharge to collect fog, one should aim
for efficiencies just below D*, while making sure that U*<1.
Discharge Electrode Before a Collecting Mesh
[0119] In this embodiment, a high voltage needle is placed before a
mesh (FIG. 6). Fog-laden wind arrives at the vicinity of the needle
where corona discharge is occurring. Charged ions attach to fog
droplets and the droplets are directed to the collector. Most
collected droplets are captured on the front side of the mesh.
However if droplets pass through the mesh, they are still subject
to the electric field caused by their image charge and to the
turn-around effect. Therefore part of these drops can still be
collected on the backside of the mesh.
Discharge Electrode after a Collecting Mesh
[0120] In this embodiment (FIG. 7), the high voltage needle is
placed after the mesh. Fog droplets arrive at the mesh without
having been subject to any charging. They are collected at the same
rate as they would have been without electric field. The part that
is not collected and that goes through the openings of the mesh
then encounters the discharges electrode. Droplets are charged and
there is an electric field that is caused by the potential
difference between the needle and the collector and that is much
higher than the image charge field and that drives the droplets to
the collector. When the electric field is sufficiently high, this
force overcomes the inertia of the droplets. Droplets change
direction and go back to the mesh where they are collected on the
backside. The turn-around phenomenon is more important here than in
the previous embodiment, and, since significant collection occurs
on both sides of the mesh, the effective collection surface is
doubled compared to a traditional system.
Vertical Parallel Wires as a Collector
[0121] In this embodiment (FIG. 8) the horizontal wires of the
collection mesh are eliminated. The previous analysis remains valid
with only vertical wires with the same spacing, and vertical wires
should have the same efficiencies as meshes. This embodiment
reduces by half the cost of materials needed. Moreover, it promotes
faster shedding of droplets. When droplets are large enough to
start shedding by gravity, horizontal wires may act as pinning
sites and hold the droplets at the joints between horizontal and
vertical wires, effectively slowing down the shedding. Faster
shedding with vertical wires implies that the wires may quickly be
dry again and ready to collect new droplets. This may increase the
collection rate.
Collection Rate with and without Field
[0122] Collection rates were experimentally measured with and
without corona discharge. The first embodiment, in which the
discharge electrode is placed before a collecting mesh, has been
used, and the mass of water collected after a certain amount of
time has been measured. The collecting surfaces were placed at a
distance of 3.5 cm from the discharge electrode. Two sets of
experiments were performed, one with moving air (speed 0.8 m/s) and
the other with stagnant fog. The results are shown in FIG. 9. In
the case where the fog is moving, shown in FIG. 9 (log-scale), the
collection rate of fog on a mesh (0.002'' openings, 41% shade
coefficient, stainless steel) was enhanced by two orders of
magnitude, going from 1 liter per day per m.sup.2 of mesh, similar
to what is reported in current applications, to 107 L/day/m.sup.2
for a discharge voltage of 11.2 kV.
[0123] FIG. 10 shows the deposition efficiency data as a function
of the electrical number Ke (roughly proportional to V.sup.2) for
different meshes and different voltages. As can be seen in this
figure, the efficiency increases linearly with Ke. This figure also
shows that the deposition efficiency may be well above 100%,
reaching values above 200% in some cases.
Visualization of the "Turn-Around Phenomenon" and Effect on the
Rate of the Collection
[0124] The enhancement in collection with corona discharge can also
be visually observed directly. Snapshots of a collecting mesh at
different times after exposure to fog, with and without electric
field, are shown in FIGS. 11A-C. It can be observed that, as soon
as the electric field is turned on, the number of collected
droplets on the mesh increases dramatically. There are much more
droplets on the mesh after one second of exposure with corona
discharge than after 20 seconds without discharge. In FIB. 11B, the
turn-around can be observed in some spots where there are droplets
that are collected on the backside of the mesh. This enhances the
collection rate since the area available for collection is doubled
with the turn-around, as schematically shown in FIG. 7C.
Collection on a Single Cylindrical Wire
[0125] The typical setup that is used in this study is shown in
FIG. 12A. A sharp electrode with a high curvature is placed at a
distance d from the collector, a horizontal cylindrical wire of
radius R.sub.c, d is much larger than R.sub.c, which is much larger
than the radius of the fog particles R.sub.c. The collector is
electrically grounded (V=0), while a high voltage V is applied to
the discharge electrode. When V is above a critical value, corona
discharge occurs and the air is ionized. Corona discharge occurs
when the electric field around the electrode is high enough to form
a plasma region: Electrons in the air are accelerated and have
enough energy to ionize the air atoms when they collide with them.
A chain reaction starts with every collision crating additional
electrons and ions. After a collision, electrons and ions are
pulled in opposite directions by the electric field, preventing
recombination. At a certain distance from the electrode, the
electric field can no more give enough energy to the electrons to
sustain the reaction. In this region, escaped ions travel freely in
the air towards the opposite electrode and potentially attach to
the fog particles they collide with. The particles then acquire a
net charge q of the same sign as V.
[0126] As d is much larger than R.sub.c, apart from small regions
around the emitter and the collector, the electric field lines are
essentially parallel and horizontal in the central region. It is
assumed in all the following that the electric field is not
disrupted by the presence of the fog, and inter-particle
interactions are neglected. The magnitude of the electric field can
then be estimated as
V d . ##EQU00010##
[0127] In the central region, fog particles undergo an acceleration
phase, whose mechanisms are presented in FIG. 12B. The particles
enter the region with the same velocity U.sub.0 as the wind
carrying them. Therefore, no drag force is exerted on them.
However, since they acquired a net charge from the corona discharge
and are in an electric field, an electric force acts on them. The
particles are accelerated and their velocity becomes higher than
U.sub.0, giving rise to a drag force. When the drag force becomes
equal to the electric force, the terminal velocity U.sub.f is
reached and the particle is not accelerated anymore.
[0128] The force balance in this phase can be written as
m d u .fwdarw. dt = 67 .pi..eta. g R p ( w .fwdarw. - u .fwdarw. )
+ q E .fwdarw. ##EQU00011##
Where {right arrow over (u)} is the particle's velocity,
.eta..sub.g is the air viscosity and {right arrow over (w)} is the
air velocity.
[0129] To detennine whether the particle will reach its terminal
velocity during the acceleration phase, the particle acceleration
time scale is computed, which is given by
.tau. particle = m 6 .pi..eta. g R p = 2 9 R p 2 .rho. w .eta. g
##EQU00012##
[0130] In this case, .tau..sub.particle is one to two orders of
magnitude smaller than the particle travel time from the emitter to
the collector. Thus, the particles should reach their terminal
velocity, which is given by a balance between drag and electric
forces.
6 .pi..eta. g R p ( U f - U 0 ) = qE ##EQU00013## U f = U 0 + qE 6
.pi..eta. g R p ##EQU00013.2##
[0131] Without wishing to be bound by theory, in the case of a weak
electric force, the terminal velocity will be close to the initial
velocity, whereas in the case of a high electric force, the
terminal velocity will be independent of the initial velocity and
directly proportional to the electric force.
[0132] FIG. 12C shows experimental measures of the added velocity
U.sub.f-U.sub.0 as a function of V.sup.2. The shaded region of the
graph corresponds to the case V<V.sub.corona where no charge
injection occurs. The only electric force occurring there is
dielectrophoresis, whose magnitude can be approximated by
2 .pi. R p 3 o V 2 d 3 . ##EQU00014##
In this force is around 6 orders of magnitude smaller than the
typical drag forces. As such, there is no added velocity before the
onset of corona discharge. In the second region of the graph,
charging occurs and the added velocity is generally proportional to
V.sup.2. The explanation of this proportionality may be that the
electric field is proportional to V, and the charge on the
particles is also proportional to V, which makes the electric force
qE grow as V.sup.2.
[0133] To estimate the particle charge and the electric force, the
continuity equation for electric fields is used. Water is a
conductor compared to air, the surrounding medium. The charge is
localized at the surface of the fog particles and the surface
charge per unit area can be estimated as .sigma.= .sub.0E.
[0134] Droplets can gain charge when the ions attach to them but
they cannot lose charge to the air (insulator), so the final charge
of a droplet can be determined by the maximum value of the electric
field it encounters in its trajectory. The surface charge can then
be estimated as
.sigma. = 0 V d . ##EQU00015##
More precise calculations give
.sigma. = 3 0 V d . ##EQU00016##
[0135] Thus
q = 12 .pi. R p 2 c 0 V d ##EQU00017##
the electric force in the central region is
F elec = 12 .pi. R p 2 0 V 2 d 2 ##EQU00018##
and the added velocity is
U f - U 0 = 2 P p .eta. g 0 V 2 d 2 ##EQU00019##
[0136] The linear interpolation in FIG. 12C has a slope of 0.006.
Using the above formula, U.sub.f-U.sub.0=0.008 V.sup.2, which
matches well the experimental values, considering the uncertainties
in measuring the parameters.
[0137] Here.
U * = 2 R p 0 V 2 U 0 .eta. g d 2 , ##EQU00020##
a nondimensional number expressing the ratio between the added
velocity due to the electric field and the wind velocity.
[0138] After the acceleration phase, the fog droplets eventually
approach the collector wire region, whose size is of the order of
R.sub.c. In this region, as shown in FIG. 12D, both the streamlines
and the electric field lines start deviating from the parallel
horizontal configuration. While streamlines go around the cylinder,
field lines bend towards this conductive collector and end up
perpendicularly to it. The electric field magnitude scales here
as
E ~ V R c . ##EQU00021##
In this region, the particles are still subject to drag and
electric forces. As the distance to the cylinder increases, the
electric field is weaker, while particles require a higher force to
be collected since they are farther. Therefore, there may be a
distance above which particles will not be collected, for a certain
intensity of the applied voltage. The area of collection A is then
defined as the projected area (surface area) of the flow of
incoming particles that are collected on the cylinder. It is
assumed that the cylinder has an infinite depth and in the
calculations are performed in two dimensions. Thus, the area A will
have the dimension of a length. Given the definition of the
deposition efficiency, it is exactly
.eta. d = A A 0 , ##EQU00022##
where A.sub.0 is the projected area (surface area) of the cylinder,
equal to 2R.sub.c.
[0139] The area of collection for different voltages and wind
speeds were experimentally measured, by imaging the region around
the cylinder. FIG. 12E presents the nondimensional area A/A.sub.0
as a function of V.sup.2, for different values of U.sub.0. It is
observed that, for relatively low voltages or high wind speeds
(region 11 of the plot in FIG. 12E). A/A.sub.0 increases with
V.sup.2, which can be explained again by the fact that the driving
electric force scales as V.sup.2. It is also noticed that A/A.sub.0
decreases with the wind speed, and actually scales as 1/U.sub.0,
again for relatively low voltages or high wind speeds. This can be
justified by the fact that, when U.sub.0 increases, the electric
force has less time to attract the droplets as they pass by the
collecting cylinder. Finally, it is seen that, for low wind speeds
and high voltages (region IV in FIG. 12E), A/A.sub.0 starts to
plateau, which means that, after a certain point, increasing the
voltage does not help anymore collecting more droplets. Region III
of the graph is the transition from the linear behavior to the
plateau.
[0140] FIG. 16A shows the nondimensional collection area as a
function of the inverse of wind speed for five different voltages.
FIG. 16B shows the nondimensional collection area for two wires as
a function of Ke for three different wire distances (closed shapes
represent A.sub.in and open shapes represent A.sub.out). A.sub.out
generally has a linear behavior with a slope close to that of a
single wire.
[0141] To rationalize these observations, the equation of movement,
around the cylinder is written once more.
m d u -> dt = 6 .pi..eta. g R p C c ( w -> - u -> ) + q E
-> ##EQU00023##
The equation is non-dimensionalized by dividing all distances by
R.sub.c, and velocities by U.sub.f
r _ = r R c , u _ = u -> U f , t _ == tU f R c ##EQU00024## d u
_ d t _ = 1 St ( w _ - u _ ) + Ke St E -> E -> ##EQU00024.2##
St = 2 R p 2 .rho. w U f 9 .eta. g R c ; Ke = qE 6 .pi..eta. g R p
U f ##EQU00024.3##
[0142] Ke is the ratio of electric and viscous forces, and it is
called the electrical number herein. Apart from these
nondimensional numbers, all the terms of the equation are of order
1. Around the cylinder E scales as V/R.sub.c and the expressions of
q and U.sub.f that were obtained previously can be used.
St = 2 R p 2 .rho. w ( U 0 + 2 R p .eta. g 0 V 2 d 2 ) 9 .eta. g R
c , Ke = 2 R p 0 V 2 .eta. g dR c ( U 0 + 2 R p .eta. g 0 V 2 d 2 )
##EQU00025##
[0143] Moreover, in the low Stokes limit, corresponding to
relatively large wires, the equation is further simplified into
0 = ( w _ - u _ ) + Ke E -> E -> ##EQU00026##
[0144] The electrical number should then govern the physics of the
problem, in particular the nondimensional collection area:
A A 0 = f ( K e ) ##EQU00027##
[0145] In FIG. 13. A/A.sub.0 is plotted as a function of Ke. and it
is seen that the previous data of different voltages and wind
speeds collapses into one linear master curve. This linear behavior
is expected, as Ke represents the relative amplitude of the driving
force causing the collection. The proportionality constant, is
determined experimentally (0.26 here) and allows predicting the
efficiency of such a system for any values of the problem
parameters.
[0146] This dependence on Ke explains the behaviors observed
earlier. For low U* (low voltages or high wind speeds), the term
containing V.sup.2 in the denominator is negligible and Ke scales
as V.sup.2. That is why the non-dimensional area increases linearly
with V.sup.2 in this regime (region II of FIG. 12E). However, for
high U*, U.sub.0 becomes negligible in the denominator and Ke tends
towards a constant
( d R c ) , ##EQU00028##
which is the plateau region. This can be called the voltage
saturation. This limitation in the reachable collection rates
essentially comes from the fact that, as the voltage is increased,
there is a higher electric force attracting the particles, but the
particles are also moving faster, which leaves less time for the
electric force to attract them. Eventually, at high voltages, these
two effects balance each other, and the collection cannot be
enhanced anymore. The vertical dotted lines in FIG. 12E represent
U*=0.5 for the two lowest wind speeds. This value gives an
approximation of when the initial velocity starts to be overcome by
the added electrical velocity, or, equivalently, when the
transition towards the voltage saturation starts.
[0147] It is expected that a similar behavior to hold for different
geometries, such as meshes that are ultimately the geometries of
interest here, with only the proportionality constant changing with
the geometry.
Collection on Two Parallel Cylindrical Wires
[0148] To extend the model developed as described above for single
cylindrical wires, fog collection on a system of two parallel
cylindrical wires is investigated. It is hypothesized that, when
the wires are far from each other, they behave as two single wires.
However, when they are close enough, they may start competing over
the same droplets between them, thereby limiting the collection, as
shown schematically in FIG. 14A. The distance D between the wires
is thus incorporated into the model through the nondimensional
number
D * = D + 2 R c 2 R c . ##EQU00029##
Two areas of collection. A.sub.in and A.sub.out, are also defined,
accounting for the projected area (surface area) of the flow of the
collected incoming particles, respectively in the inner and outer
pans of the system. FIG. 14B is a photograph showing the two
cylinders and the droplets trajectories, in a case where D is small
enough so that most of the droplets between the wires are
collected. When this happens (small D), a simple geometrical
analysis shows that A.sub.in reaches a saturation value that is
equal to
R c + D 2 , ##EQU00030##
or in nondimensional terms
A in A o = D * , ##EQU00031##
where A here is equal to R.sub.c since only one half of the
cylinder is being considered. This limitation is termed the spacing
saturation.
[0149] It was hypothesized that
A in A o and A out A o ##EQU00032##
a grow with the single wire law until the spacing saturation is
reached, at which point A.sub.in will plateau, while A.sub.out will
still follow the single wire law. Knowing that the saturation
happens when
A in A o = D * , ##EQU00033##
the single wire law
( A in A o = c . Ke , ##EQU00034##
where c is the proportionality constant) can be used to predict the
electrical number (or equivalently the voltage) at which spacing
saturation starts
Ke sat - D * c , ##EQU00035##
or, for a certain electrical number, to predict the critical
distance D.sub.sat* at which spacing saturation happens.
[0150] A.sub.in and A.sub.out were experimentally measured for
different Ke and D*. The results for A.sub.in are reported in FIG.
14C. It is observed that
.LAMBDA. i n .LAMBDA. o / D * ##EQU00036##
increases with Ke and plateaus at 1 for low values of D*. Before
the plateau,
A in A o ##EQU00037##
increases linearly with Ke, with a slope equal to what was measured
in the single wire case. The shaded zone of the graph,
representing
A i n A o > D * , ##EQU00038##
is inaccessible because of the competition over a finite number of
droplets that leads to saturation. The vertical dotted lines
represent the expected saturation Ke for the cases of D*=1.7 and
D*=4.2 from the previous model and it can be seen that plateaus are
observed beyond these lines for the corresponding curves.
[0151] For D*=11, spacing saturation is not observed here and is
expected to occur at a much higher Ke. However, such a high Ke
cannot be reached because of the voltage saturation, which may be a
limiting factor here, in some cases.
Collection on Meshes
[0152] Having rationalized the behavior of two parallel wires, the
macroscopic collection on the ultimate system of interest: meshes,
was studied. Similar experiments were conducted by placing a 5
cm.times.5 cm mesh perpendicularly to the fog-laden wind direction
and a petri dish under the mesh, and measuring the mass of the
collected water after a certain amount of time.
[0153] The difference in collection when corona discharge is
applied is immediately visible to the naked eye. As shown in FIG.
15A, the mesh is covered with water within a few seconds when a
high voltage is applied, while it is little wet after minutes of
exposure to fog when there is no electric field.
[0154] Five different meshes were used. All had the same wire
radius, but the spacing D between the wires was increased from Mesh
1 to Mesh 5. The shading coefficient SC, or the projected relative
surface of the wires, was thus decreased.
[0155] FIG. 15B shows the mass of collected water after five
minutes of exposure, for different electrical numbers Ke. The mass
m increases linearly with Ke for the five meshes, although the
proportionality constant varies from mesh to mesh. It is noticed
that the mesh with the highest collection is not the one with the
highest wire density; Mesh 2 has fewer wires but still collects
more water. Finally, meshes 1 and 2 start to plateau at high Ke,
which is due to spacing saturation. The vertical dashed lines in
FIG. 15B show the predicted spacing saturation limits from the two
parallel wires model for the three first meshes, the last two being
out of the scope of the graph. That model is not exactly valid here
since the geometry is different, with parallel and perpendicular
wires, but it can provide a good approximation of the onset of
spacing saturation.
[0156] The efficiency .eta. of the collection process as the ratio
of the collected mass and the total mass of water directed towards
the mesh is computed. As stated before,
.eta.=.eta..sub.a.eta..sub.d.eta..sub.other.
.eta. d = A A 0 ##EQU00039##
is the efficiency that was focused on in the previous analysis. It
was assumed that it is possible to neglect .eta..sub.other, or that
its effect will be similar on all the meshes. The aerodynamic
efficiency has been previously studied and it is given by
.eta. a = SC 1 + [ 1.22 ( 1.3 SC + ( SC 1 - SC ) 2 ) ] ,
##EQU00040##
where SC is the shading coefficient.
[0157] The deposition efficiency
.eta. d = 1 .eta. a m collected m total ##EQU00041##
can then be calculated. Its values are plotted in FIG. 15C as a
function of Kc, and it can be seen that the data collapses around a
linear curve, which shows that the models for single and double
wires holds for meshes.
[0158] The proportionality constant is different from the single
wire case because of the different geometry, but also because of
the effects of the other inefficiencies that were neglected her,
since what is really being plotted is .eta..sub.d.eta..sub.other.
These other inefficiencies may also explain the relatively higher
dispersion of the results around the linear curve, as compared to
the single wire case.
[0159] .eta..sub.d.eta..sub.other reaches values over 2, which
means that .eta..sub.d is much higher than one. Therefore, these
high values of .eta..sub.d can compensate other inefficiencies, and
lead to very high overall efficiencies for the collection
system.
[0160] When designing such an active collection mesh, the geometry
and operating voltage should be chosen so as to remain below the
voltage saturation and the spacing saturation, to maximize the
collection without spending any unnecessary energy.
CONCLUSION
[0161] It has been demonstrated that it is possible to dramatically
enhance the efficiency of fog collection on meshes by breaking the
traditional aerodynamic limitation, using corona discharge.
Particles are accelerated and directed towards the collector, as
electric forces overcome the hydrodynamic ones. It has been shown
that the problem is governed by four non-dimensional numbers St,
Ke, U* and D*. The deposition efficiency .eta..sub.d depends on the
first two numbers and, in particular, it has been shown that, in
the case of low St, .eta..sub.d is proportional to Ke. U* and D*
predict two important limitations that are the voltage and spacing
saturations. They should therefore serve as design parameters for
active collection meshes, to operate just below the saturations,
and fully optimize the water produced per energy spent. This method
may be combined with others, such as surface treatment of the
collector, to slightly enhance the efficiency. This experiment has
been performed exclusively in the low Stokes regime, and, while the
high Stokes regime may be the object of scientifically interesting
future work, the low Stokes regime is much more interesting in
practice, as minimizing the inertia force is advantageous here, and
manufacturing meshes with large wires is easier than making finer
ones. These results can be used to design efficient fog harvesters
in drought-prone areas and collect water for drinking, irrigation
and afforestation. Fog removal systems to increase visibility on
roads and airports may also be designed.
Experimental Set-Up and Procedure
[0162] Samples were placed 4 cm away from the outlet,
perpendicularly to the axis, of two concentric cylinders (6.3 cm, 5
cm inner diameters) from which a uniform stream of fog was coming.
Fog consisted in a cloud of air-suspended water droplets of radius
3.5 .mu.m, generated using an ultrasonic humidifier (Air-O-Swiss
AOS 7146) delivering a volume rate of up to 0.1 L/hour. Fog was
generated directly into the smaller cylinder through an orifice. At
the inlet of the larger cylinder, a speed-tunable fan (Thermaltake
Mobile Fan II External USB Cooling Fan) was placed to create the
airflow that would convect the fog towards the collection area. A
honeycomb flow straightener (Saxon Computers 120 mm Honeycomb
Airflow Straightener) was placed after the fan to ensure that the
wind velocity is uniform through the area of the cylinder, thus
reproducing real-fog conditions. The outlet velocity was measured
with an anemometer (Testo 405 Hot Wire Thermo-Anemometer) and was
spatially uniform within a 15% interval. Corona discharge was
produced by placing a sharp metallic needle inside the cylinders,
its tip coinciding with the outlet of the smaller cylinder. The
needle was connected to a high-voltage generator (Spellman SL600)
delivering voltages from 0 to -25 kV. Corona discharge was observed
to start at a voltage around -7.6 kV. In all experiments, the
collector was connected to the ground, setting its voltage at 0V.
All experiments were performed in ambient temperature and humidity
conditions.
Wires and Meshes
[0163] In single and two-wire experiments, cylindrical needles,
made of stainless steel, of length 4 cm, and of diameter 1.88 mm
were used as collectors.
[0164] 5 cm square meshes were used for collection tests. They were
purchased from McMaster-Carr (Corrosion-Resistant Type 304
Stainless Steel Wire Cloth), and their individual characteristics
are summarized in Table 1.
TABLE-US-00001 TABLE 1 Characteristics of meshes (collectors) Wire
diameter Opening size Mesh (in) (in) D* Open area Mesh 1 0.063
0.062 1.98 25% Mesh 2 0.063 0.104 2.65 39% Mesh 3 0.063 0.187 3.97
56% Mesh 4 0.063 0.270 5.29 66% Mesh 5 0.063 0.437 7.94 76%
[0165] While several embodiments of the present invention have been
described and illustrated herein, those of ordinary skill in the
art will readily envision a variety of other mean % and/or
structures for performing the functions and/or obtaining the
results and/or one or more of the advantages described herein, and
each of such variations and/or modifications is deemed to be within
the scope of the present invention. More generally, those skilled
in the art will readily appreciate that all parameters, dimensions,
materials, and configurations described herein are meant to be
exemplary and that the actual parameters, dimensions, materials,
and/or configurations will depend upon the specific application or
applications for which the teachings of the present invention
is/are used. Those skilled in the art will recognize, or be able to
ascertain using no more than routine experimentation, many
equivalents to the specific embodiments of the invention described
herein. It is, therefore, to be understood that the foregoing
embodiments are presented by way of example only and that, within
the scope of the appended claims and equivalents thereto, the
invention may be practiced otherwise than as specifically described
and claimed. The present invention is directed to each individual
feature, system, article, material, kit, and/or method described
herein. In addition, any combination of two or more such features,
systems, articles, materials, kits, and/or methods, if such
features, systems, articles, materials, kits, and/or methods are
not mutually inconsistent, is included within the scope of the
present invention.
[0166] All definitions, as defined and used herein, should be
understood to control over dictionary definitions, definitions in
documents incorporated by reference, and/or ordinary meanings of
the defined terms.
[0167] The indefinite articles "a" and "an" as used herein in the
specification and in the claims, unless clearly indicated to the
contrary, should be understood to mean "at least one." The phrase
"and/or," as used herein in the specification and in the claims,
should be understood to mean "either or both" of the elements so
conjoined, i.e., elements that are conjunctively present in some
cases and disjunctively present in other cases. Multiple elements
listed with "and/or" should be construed in the same fashion. i.e.,
"one or more" of the elements so conjoined. Other elements may
optionally be present other than the elements specifically
identified by the "and/or" clause, whether related or unrelated to
those elements specifically identified. Thus, as a non-limiting
example, a reference to "A and/or B", when used in conjunction with
open-ended language such as "comprising" can refer, in one
embodiment, to A only (optionally including elements other than B);
in another embodiment, to B only (optionally including elements
other than A); in yet another embodiment, to both A and B
(optionally including other elements); etc.
[0168] As used herein in the specification and in the claims, "or"
should be understood to have the same meaning as "and/or" as
defined above. For example, when separating items in a list. "or"
or "and/or" shall be interpreted as being inclusive. i.e., the
inclusion of at least one, but also including more than one, of a
number or list of elements, and, optionally, additional unlisted
items. Only terms clearly indicated to the contrary, such as "only
one of" or "exactly one of," or, when used in the claims,
"consisting of" will refer to the inclusion of exactly one element
of a number or list of elements. In general, the term "or" as used
herein shall only be interpreted as indicating exclusive
alternatives (i.e. "one or the other but not both") when preceded
by terms of exclusivity, such as "either." "one of" "only one of,"
or "exactly one of." "Consisting essentially of," when used in the
claims, shall have its ordinary meaning as used in the field of
patent law.
[0169] As used herein in the specification and in the claims, the
phrase "at least one." in reference to a list of one or more
elements, should be understood to mean at least one element
selected from any one or more of the elements in the list of
elements, but not necessarily including at least one of each and
every element specifically listed within the list of elements and
not excluding any combinations of elements in the list of elements.
This definition also allows that elements may optionally be present
other than the elements specifically identified within the list of
elements to which the phrase "at least one" refers, whether related
or unrelated to those elements specifically identified. Thus, as a
non-limiting example "at least one of A and B" (or, equivalently,
"at least one of A or B," or, equivalently "at least one of A
and/or B") can refer, in one embodiment, to at least one,
optionally including more than one, A, with no B present (and
optionally including elements other than B); in another embodiment,
to at least one, optionally including more than one, B, with no A
present (and optionally including elements other than A); in yet
another embodiment, to at least one, optionally including more than
one, A. and at least one, optionally including more than one. B
(and optionally including other elements); etc.
[0170] It should also be understood that, unless clearly indicated
to the contrary, in any methods claimed herein that include more
than one step or act, the order of the steps or acts of the method
is not necessarily limited to the order in which the steps or acts
of the method are recited.
[0171] In the claims, as well as in the specification above, all
transitional phrases such as "comprising." "including," "carrying."
"having." "containing," "involving," "holding," "composed of," and
the like are to be understood to be open-ended. i.e., to mean
including but not limited to. Only the transitional phrases
"consisting of" and "consisting essentially of" shall be closed or
semi-closed transitional phrases, respectively, as set forth in the
United States Patent Office Manual of Patent Examining Procedures.
Section 2111.03.
[0172] Any terms as used herein related to shape, orientation,
alignment, and/or geometric relationship of or between, for
example, one or more articles, structures, forces, fields, flows,
directions/trajectories, and/or subcomponents thereof and/or
combinations thereof and/or any other tangible or intangible
elements not listed above amenable to characterization by such
terms, unless otherwise defined or indicated, shall be understood
to not require absolute conformance to a mathematical definition of
such term, but, rather, shall be understood to indicate conformance
to the mathematical definition of such term to the extent possible
for the subject matter so characterized as would be understood by
one skilled in the art most closely related to such subject matter.
Examples of such terms related to shape, orientation, and/or
geometric relationship include, but are not limited to terms
descriptive of: shape--such as, round, square, circular/circle,
rectangular/rectangle, triangular/triangle, cylindrical/cylinder,
elliptical/ellipse, (n)polygonal/(n)polygon, etc.; angular
orientation -such as perpendicular, orthogonal, parallel, vertical,
horizontal, collinear, etc.; contour and/or trajectory--such as,
plane/planar, coplanar, hemispherical, semi-hemispherical,
line/linear, hyperbolic, parabolic, flat, curved, straight,
arcuate, sinusoidal, tangent/tangential, etc.; direction--such as,
north, south, east, west, etc.; surface and/or bulk material
properties and/or spatial/temporal resolution and/or
distribution--such as, smooth, reflective, transparent, clear,
opaque, rigid, impermeable, uniform(ly), inert, non-wettable,
insoluble, steady, invariant, constant, homogeneous, etc.; as well
as many others that would be apparent to those skilled in the
relevant arts. As one example, a fabricated article that would
described herein as being "square" would not require such article
to have faces or sides that are perfectly planar or linear and that
intersect at angles of exactly 90 degrees (indeed, such an article
can only exist as a mathematical abstraction), but rather, the
shape of such article should be interpreted as approximating a
"square," as defined mathematically, to an extent typically
achievable and achieved for the recited fabrication technique as
would be understood by those skilled in the art or as specifically
described. As another example, two or more fabricated articles that
would described herein as being "aligned" would not require such
articles to have faces or sides that are perfectly aligned (indeed,
such an article can only exist as a mathematical abstraction), but
rather, the arrangement of such articles should be interpreted as
approximating "aligned," as defined mathematically, to an extent
typically achievable and achieved for the recited fabrication
technique as would be understood by those skilled in the art or as
specifically described.
* * * * *