U.S. patent application number 15/341607 was filed with the patent office on 2017-03-16 for indirect evaporative cooler using membrane-contained, liquid desiccant for dehumidification.
The applicant listed for this patent is Alliance for Sustainable Energy, LLC. Invention is credited to Eric Joseph KOZUBAL.
Application Number | 20170074530 15/341607 |
Document ID | / |
Family ID | 40901366 |
Filed Date | 2017-03-16 |
United States Patent
Application |
20170074530 |
Kind Code |
A1 |
KOZUBAL; Eric Joseph |
March 16, 2017 |
Indirect Evaporative Cooler Using Membrane-Contained, Liquid
Desiccant for Dehumidification
Abstract
An indirect evaporative cooler for cooling inlet supply air from
a first temperature to a second, lower temperature using a stream
of liquid coolant and a stream of exhaust or purge air. The cooler
includes a first flow channel for inlet supply air and a second
flow channel adjacent the first for exhaust air. The first and
second flow channels are defined in part by sheets of a membrane
permeable to water vapor such that mass is transferred as a vapor
through the membrane from the inlet supply air to a contained
liquid desiccant for dehumidification and also to the exhaust air
as heat is transferred from the inlet supply air to the liquid
coolant. A separation wall divides the liquid desiccant and the
coolant but allows heat to be transferred from the supply air to
the coolant which releases water vapor to the counter or cross
flowing exhaust air.
Inventors: |
KOZUBAL; Eric Joseph;
(Superior, CO) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Alliance for Sustainable Energy, LLC |
Golden |
CO |
US |
|
|
Family ID: |
40901366 |
Appl. No.: |
15/341607 |
Filed: |
November 2, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14286577 |
May 23, 2014 |
9518784 |
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15341607 |
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12864071 |
Sep 2, 2010 |
8769971 |
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PCT/US2008/052016 |
Jan 25, 2008 |
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14286577 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F24F 1/0007 20130101;
F24F 3/147 20130101; F28D 15/02 20130101; F24F 5/0035 20130101;
F24F 2003/1458 20130101; F24F 13/30 20130101; Y02B 30/545 20130101;
F24F 2003/1435 20130101; F24F 3/1411 20130101; F28D 21/0015
20130101; F24F 3/1417 20130101; Y02B 30/54 20130101 |
International
Class: |
F24F 3/14 20060101
F24F003/14; F24F 5/00 20060101 F24F005/00; F24F 13/30 20060101
F24F013/30; F24F 3/147 20060101 F24F003/147 |
Goverment Interests
CONTRACTUAL ORIGIN
[0002] The United States Government has rights in this invention
under Contract No. DE-AC36-08G028308 between the United States
Department of Energy and Alliance for Sustainable Energy, LLC, the
Manager and Operator of the National Renewable Energy Laboratory.
Claims
1-26. (canceled)
27. A method of conditioning a supply air stream, comprising:
dehumidifying the supply air stream to provide a dehumidified air
stream, wherein the dehumidifying includes directing the supply air
stream through a channel defined in part by a surface of a vapor
permeable membrane containing liquid desiccant; and cooling the
dehumidified air stream from a first temperature to a lower second
temperature using indirect evaporative cooling.
28. The method of claim 27, wherein the liquid desiccant comprises
a weak desiccant.
29. The method of claim 27, wherein the liquid desiccant comprises
a regenerable desiccant.
30. The method of claim 27, wherein the liquid desiccant comprises
a glycol.
31. The method of claim 27, wherein the liquid desiccant comprises
a salt concentrate or ionic salt solution.
32. The method of claim 31, wherein the liquid desiccant comprises
lithium chloride.
33. The method of claim 31, wherein the liquid desiccant comprises
calcium chloride.
34. The method of claim 27, further comprising transferring heat
from the liquid desiccant to a coolant flowing in a layer of
wicking material in heat transfer contact with the liquid
desiccant.
35. The method of claim 34, wherein the liquid desiccant comprises
a weak desiccant.
36. The method of claim 34, wherein the liquid desiccant comprises
a regenerable desiccant.
37. The method of claim 34, wherein the liquid desiccant comprises
a glycol.
38. The method of claim 34, wherein the liquid desiccant comprises
a salt concentrate or ionic salt solution.
39. The method of claim 27, further comprising humidifying the
cooled air stream by providing a flow of water adjacent to the
cooled air stream, whereby a temperature of the cooled air stream
is reduced to a post-humidification temperature.
40. The method of claim 39, wherein the humidifying includes
containing the flow of water with a vapor permeable membrane or
within a layer of wicking material.
41. The method of claim 39, wherein the liquid desiccant comprises
a weak desiccant.
42. The method of claim 39, wherein the liquid desiccant comprises
a regenerable desiccant.
43. The method of claim 39, wherein the liquid desiccant comprises
a glycol.
44. The method of claim 39, wherein the liquid desiccant comprises
a salt concentrate or ionic salt solution.
45. The method of claim 44, wherein the liquid desiccant comprises
lithium chloride.
46. The method of claim 44, wherein the liquid desiccant comprises
calcium chloride.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a Continuation of U.S. application Ser.
No. 14/286,577, filed May 23, 2014, which is a Continuation of U.S.
patent application Ser. No. 12/864,071, filed Sep. 2, 2010, which
is a National Stage Entry of International Application No.
PCT/US08/52016, filed Jan. 25, 2008, the contents of which are each
incorporated by reference in their entirety.
BACKGROUND
[0003] Air conditioning is used worldwide to provide comfortable
and healthy indoor environments that are properly ventilated and
cooled and that have adequate humidity control. While being useful
for conditioning supply air, conventional air conditioning systems
are costly to operate as they use large amounts of energy (e.g.,
electricity). With the growing demand for energy, the cost of air
conditioning is expected to increase, and there is a growing demand
for more efficient air conditioning methods and technologies.
Additionally, there are increasing demands for cooling technologies
that do not use chemicals and materials, such as many conventional
refrigerants, that may damage the environment if released or
leaked. Maintenance is also a concern with many air conditioning
technologies, and, as a result, any new technology that is
perceived as having increased maintenance requirements, especially
for residential use, will be resisted by the marketplace.
[0004] Evaporative coolers are used in some cases to address air
conditioning demands or needs, but due to a number of limitations,
conventional evaporative coolers have not been widely adopted for
use in commercial or residential buildings. Evaporative coolers,
which are often called swamp coolers, are devices that use simple
evaporation of water in air to provide cooling in contrast to
conventional air conditioners that use refrigeration or absorption
devices using the vapor-compression or absorption refrigeration
cycles. The use of evaporative cooling has typically been limited
to climates where the air is hot and humidity is low such as in the
western United States. In such dry climates, the installation and
operating costs of a conventional evaporative cooler can be lower
than refrigerative air conditioning. Residential and industrial
evaporative coolers typically use direct evaporative cooling with
warm dry air being mixed with water to change the water to vapor
and using the latent heat of evaporation to create cool moist air
(e.g., cool air with a relative humidity of 50 to 70 percent). For
example, the evaporative cooler may be provided in an enclosed
metal or plastic box with vented sides containing a fan or blower,
an electric motor to operate the fan, and a water pump to wet
evaporative cooling pads. To provide cooling, the fan draws ambient
air through vents on the unit's sides and through the dampened
pads. Heat in the air evaporates water from the pads, which are
continually moistened to continue the cooling process. The cooled,
moist air is then delivered to the building via a vent in the roof
or a wall.
[0005] While having an operation cost of about one fourth of
refrigerated air conditioning, evaporative coolers have not been
widely used to address needs for higher efficiency and lower cost
conditioning technologies. One problem with many sump coolers is
that in certain conditions these evaporative coolers cannot operate
to provide adequately cooled air. For example, air may only be
cooled to about 75.degree. F. when the input air is 90.degree. F.
and 50 percent relative humidity, and such cooling may not be
adequate to cool a particular space. The problem may get worse as
temperatures increase such as to temperatures well over 100.degree.
F. as found in many locations the southwest portion of the United
States and elsewhere. As a result, the air conditioning system may
need to include refrigerated air conditioning to cool the outlet
air from the evaporative cooler, which results in a system that is
more expensive to purchase, operate, and maintain.
[0006] Additionally, conventional evaporative coolers provide no
dehumidification of the air and, in fact, often output air at 80 to
90 percent relative humidity, which may only be acceptable in very
dry environments as very humid air reduces the rate of evaporation
for occupants of the building (e.g., reduces comfort levels) and
can cause condensation resulting in corrosion or other problems.
Dehumidification is provided as a second or later stage in some
evaporative coolers such as by wicking a liquid desiccant along a
wall of the air flow channel or chamber, but such systems have not
been widely adopted due to increased operating and maintenance
costs and concerns of having the desiccant expelled with the
conditioned air. In general, maintenance is a concern with
evaporative coolers as the evaporation process can result in
mineral deposits on the cooling pads and other surfaces of the
cooler that need to be cleaned or replaced to maintain the
efficiency of the system, and the water supply line needs to be
protected against freezing during the off season such as by
draining the system. Due to these and other concerns, evaporative
cooling is unlikely to be widely used to provide an energy
efficient, air conditioning alternative for commercial and
residential applications until significant improvements are made to
reduce maintenance concerns while improving achievable cooling
(e.g., providing adequately cooled output air for direct use in a
building).
[0007] The foregoing examples of the related art and limitations
related therewith are intended to be illustrative and not
exclusive. Other limitations of the related art will become
apparent to those of skill in the art upon a reading of the
specification and a study of the drawings.
SUMMARY
[0008] The following embodiments and aspects thereof are described
and illustrated in conjunction with systems, tools and methods that
are meant to be exemplary and illustrative, not limiting in scope.
In various embodiments, one or more of the above-described problems
have been reduced or eliminated, while other embodiments are
directed to other improvements.
[0009] This is achieved, in part, by providing a mass/heat transfer
assembly for use in indirect evaporative cooler or heat exchangers.
The assembly is formed of alternating stacks each including a first
(or upper) layer or sheet of membrane material, a separation wall,
and a second (or lower) layer or sheet of membrane material. The
membrane or membrane material for each layer is permeable to water
molecules in the vapor state while the separation wall is
impermeable to water but allows heat transfer (e.g., is a thin
layer and/or is made of materials that conduct heat). In a first
one of adjacent pairs of stacks, coolant such as water flows
between the first membrane layer and the separation wall and liquid
desiccant flows between the separation wall and the second membrane
layer while in the second or next one of the adjacent pairs of
stacks the flow order is reversed. This ordering is repeated
throughout the mass/heat transfer assembly to form alternating
supply and exhaust air flow channels or chambers. Supply air (or
air to be conditioned) is directed through a channel between a
first pair of stacks while a portion of the pre-cooled exhaust air
(e.g., a fraction of the supply air that is cooled by flowing
through the stacks) is directed through a chamber between a second
or next pair of stacks (e.g., typically in a counterflow
arrangement relative to the flow of the incoming supply air).
Liquid desiccant is provided proximate to the supply inlet airflow
while coolant such as water is provided proximate to the exhaust
airflow (i.e., a fraction of supply outlet airflow directed to be
exhausted) with the air only being separated from these flowing
liquids by the water permeable membrane. The supply air inlet
airflow, supply outlet airflow, exhaust airflow, liquid desiccant
flow, and coolant flow are plumbed such as via one or more manifold
assemblies to the mass/heat transfer assembly, which can be
provided in a housing as a single unit (e.g., an indirect
evaporative cooler).
[0010] In a typical embodiment, dehumidification and evaporative
cooling are accomplished by separation of the air to be processed
and the liquid and/or gas substances (e.g., liquid desiccant,
water, desiccated air, and the like) by a membrane. The membrane is
formed of one or more substance or materials to be permeable to
water molecules in the vapor state. The permeation of the water
molecules through the membrane is a driving force behind (or
enables) dehumidification (or dehumidification in some
implementations) and evaporative cooling of one or more process air
streams. As described above, multiple air streams can be arranged
to flow through chambers in the mass/heat transfer assembly such
that a secondary (purge) air stream, such as the exhaust airflow of
pre-cooled supply air, is humidified and absorbs enthalpy from a
primary (process) air stream, such as the supply inlet airflow that
can then be directed to a building as supply outlet airflow (e.g.,
make up air for a residential or commercial building or the like).
The process air stream is sensibly cooled and is, in some
embodiments, simultaneously dehumidified by providing a liquid
desiccant flow contained by membranes defining the sidewalls of the
supply inlet airflow channel or chamber.
[0011] The membrane is also used in some embodiments to define
sidewalls of the exhaust (e.g., counter) airflow channel or chamber
such that the membrane controls or separates coolant liquid from
the exhaust air stream. Wicking materials/surfaces or other devices
may be used to contain or control water flow (e.g., direct-contact
wicking surfaces could be used in combination with the use of the
liquid desiccant containment by a membrane), but membrane liquid
control facilitates fabrication of the stacks or manifold structure
useful for heat and mass exchanger/assembly configurations
described herein that provide cooling, dehumidification, and/or
humidification. In such configurations, the air streams can be
arranged in counter-flow, counter-flow with pre-cooled exhaust air,
cross-flow, parallel flow, and impinging flow to perform desired
simultaneous heat and mass transfer in the evaporative cooling
units.
[0012] By way of example, but not limitation, an embodiment
includes an indirect evaporative cooler for cooling a stream of
inlet supply air from a first temperature to a second, lower
temperature using a stream of liquid coolant and a stream of
exhaust or purge air. The cooler includes a first flow channel
through which the stream of inlet supply air flows and a second
flow channel adjacent the first flow channel through which the
stream of exhaust air, at a lower temperature than the inlet or
first temperature of the supply air, flows. The second flow channel
is formed or defined in part by a sheet of a membrane or membrane
material that is permeable to water vapor but that otherwise
contains the liquid coolant. In this manner, the coolant flows on a
side of the membrane (and not in direct contact with) the air in
the second flow channel but mass is transferred as a vapor through
the membrane to the exhaust air when or in response to heat being
transferred from the inlet supply air to the liquid coolant. In
some cases or configurations, as will become clear, the supply air
stream (or inlet supply air) is cooled and dehumidified in this
first stage. A second stage may be provided to sensibly cool the
air stream to a very cool temperature, which could be below the
dewpoint of the original supply inlet air as it was dehumidified
initially or in the first state to allow this.
[0013] A separation wall that is spaced apart from the sheet of
membrane is used to define a flow channel for the liquid coolant,
with the wall being formed from a material (such as plastic) that
is impermeable to the liquid coolant but that conducts or allows
the heat to be transferred from the inlet air supply to the
coolant. A second sheet of membrane may be spaced apart from the
opposite side of this separation wall to define a flow channel for
a liquid desiccant, and during operation, water vapor is
transferred from the stream of inlet supply air through the
membrane to the liquid desiccant, which results in the inlet supply
air being concurrently cooled and dehumidified. The membrane is
effective for resisting or even fully blocking flow of the liquid
coolant and the liquid desiccant while allowing flow of water
vapor, and in some embodiments, the coolant is water and the
desiccant is a halide salt solution (e.g., a weak desiccant such as
CaCl or the like). The exhaust air in some cases is a redirected
portion of the stream of inlet supply air after it has been cooled
to the second, lower temperature (e.g., as it is exiting the first
flow channel), and the exhaust air may flow in a direction through
the second flow channel that is cross, counter, or a combination of
these relative to the supply air flowing in the first flow
channel.
[0014] In another exemplary embodiment, a method is provided for
conditioning a process or return air for a residential or
commercial building. The method includes first directing the
process air through a first flow channel and second directing a
stream or volume of liquid desiccant adjacent one or more walls
defining the first flow channel, the liquid desiccant is separated
from the process air by a membrane (e.g., the membrane provides the
walls) that contains the liquid desiccant and also allows water
vapor from the process air to flow into and be absorbed by the
liquid desiccant, which dehumidifies the process air. The method
further includes concurrent with the first and second directing,
third directing a stream of purge air through a second flow channel
proximate to the first flow channel (e.g., parallel and adjacent).
The purge air is at a temperature lower than all or at least a
substantial portion of the process air in the first flow channel,
and in some cases, the purge air is a fraction of the dehumidified
process air exiting the first flow channel that is directed in a
counter flow direction relative to the process air through the
second flow channel. The method also includes fourth directing a
stream of liquid coolant adjacent a wall of the second flow
channel. The liquid coolant is also separated from the air by a
membrane that is permeable to vapor from the coolant such that mass
is transferred from the coolant to the purge air. The method
provides for concurrent (or single stage) dehumidification and
cooling of the process air.
[0015] According to another aspect, a mass and heat transfer
assembly is provided for use in an indirect evaporative cooler or
exchanger device. The assembly includes a first stack including an
upper membrane, a lower membrane, and a separation wall between the
upper and lower membranes. The upper and lower membranes are
permeable to water in vapor form and the separation wall is
substantially impermeable to liquid and vapor. Second and third
stacks are provided that also each includes an upper membrane, a
lower membrane, and a separation wall positioned therebetween. In
the assembly, the first stack and second stacks are spaced apart
(such as less than about 0.25 to 0.5 inches apart) to define a flow
channel for receiving a first stream of air (e.g., air to be
conditioned) and the second and third stacks are spaced apart to
define a flow channel for a second stream of air (e.g., purge or
exhaust air directed in cross or counter flow relative to the first
stream of air). In some configurations and/or operating modes, the
device does only evaporative cooling and no dehumidification. Such
that the membranes are only used on the purge side and the other
side of the wall is left bare for the supply air to exchange
heat.
[0016] The first, second, and third stacks may be considered a set
of stacks, and the assembly includes a plurality of such sets of
stacks to define a plurality of air flow channels spaced apart by
the stacks or layers of membranes and separation walls. A divider
or separator may be provided in the flow channels to maintain
spacing of the membranes while allowing flow of the air streams in
the channels. The assembly may further include in the first stack a
liquid coolant flowing between the upper membrane and the
separation wall and a liquid desiccant flowing between the
separation wall and the lower membrane. In the second stack, a
liquid desiccant flows between the upper membrane and the
separation wall while a liquid coolant flows between the separation
wall and the lower membrane. In the third stack, liquid desiccant
flows between the upper membrane and the separation wall while
liquid coolant flows between the separation wall and the lower
membrane. The liquid coolant may be water and during operation
water vapor may be transferred from the coolant through the
membrane to the second stream of air. The liquid desiccant may be a
salt solution (such as weak desiccant such as CaCl or the like) and
during operation or use of the assembly water vapor may be
transferred from the first stream of air through the membrane to
the liquid desiccant, whereby the first stream of air is
simultaneously dehumidified and cooled to a lower temperature.
[0017] In addition to the exemplary aspects and embodiments
described above, further aspects and embodiments will become
apparent by reference to the drawings and by study of the following
descriptions.
BRIEF DESCRIPTION OF THE DETAILED DRAWINGS
[0018] Exemplary embodiments are illustrated in referenced figures
of the drawings. It is intended that the embodiments and figures
disclosed herein are to be considered illustrative rather than
limiting.
[0019] FIG. 1 illustrates in schematic form an evaporative cooler
or heat exchanger including an exemplary representative of a
permeable membrane stack or assembly for use in providing indirect
evaporative cooling concurrently with dehumidification in an
integral unit or single stage;
[0020] FIG. 2 illustrates another an exemplary representation of an
evaporative cooler showing an assembly of membrane/wall/membrane
stacks used in combination to direct the supply and exhaust
airflows relative to membrane-contained liquid desiccant and
coolant (e.g., cooling water) to achieve cooling and
dehumidification;
[0021] FIG. 3 illustrates an evaporative cooler similar to that
shown in FIG. 2 but being configured with integral counterflow
passages for exhaust/cooled air;
[0022] FIG. 4 is a top view of an exemplary heat exchanger
illustrating air flows through a plurality of channels or chambers
provided by membrane-based assemblies such as those shown in FIGS.
1-3 or other embodiments shown or described herein;
[0023] FIGS. 5A and 5B illustrates an exemplary modeling of an
evaporative cooler or counterflow heat/mass exchanger such as one
with the stack assembly shown in FIG. 2 and flow arrangement shown
in FIG. 4;
[0024] FIG. 6 is a graph of air flow and surface temperatures along
the length of the exchanger modeled as shown in FIGS. 5A and
5B;
[0025] FIG. 7 is a graph of humidity ratios of the air along the
length of the exchanger modeled as shown in FIGS. 5A and 5B;
[0026] FIG. 8 is a graph showing concentration of liquid desiccant
flowing through the modeled heat exchanger of FIGS. 5A and 5B;
[0027] FIG. 9 is a psychrometric chart showing the cooling and
dehumidifying process modeled as shown in FIGS. 5A and 5B;
[0028] FIG. 10 is a top view of another exemplary heat exchanger
illustrating air flows through a plurality of channels or chambers
provided by membrane-based assemblies such as those shown in FIGS.
1-3 or other embodiments shown or described herein;
[0029] FIG. 11 is a top view of another exemplary heat exchanger
similar to those shown in FIGS. 4 and 10 showing a differing unit
arrangement with differing exhaust airflows;
[0030] FIG. 12 is a psychrometric chart showing the cooling and
dehumidifying process modeled similar to the modeling shown in
FIGS. 5A and 5B for the configuration of a heat exchanger shown in
FIG. 10;
[0031] FIG. 13 illustrates a HVAC system using an indirect
evaporative cooler to provide conditioned air to a building;
and
[0032] FIG. 14 is a psychrometric chart providing results of one
test of a prototype fabricated similar to the embodiment of FIG. 4
with the stack assembly of FIG. 2.
DESCRIPTION
[0033] The following provides a description of exemplary indirect
evaporative coolers with dehumidification and mass/heat transfer
assemblies for such coolers that provide inlet air stream chambers
with sidewalls defined by permeable membrane sheets containing
liquid desiccant. The assemblies also include outlet or exhaust air
stream chambers (such as in counterflow to the inlet air streams)
with sidewalls defined by permeable membrane sheets containing
coolant such as water. In embodiments described below, the membrane
is "permeable" in the sense that moisture in the form of a vapor
(e.g., water in the vapor state) generally can permeate readily
through the membrane such as from an inlet supply air and from
liquid coolant via evaporation. However, the membrane generally
contains or blocks moisture in the form of a liquid from flowing
through as it is instead directed to flow within the channel or
chamber. In some cases, water in the liquid state is contained by
the membrane at pressures less than about 20 psi and more typically
less than about 5 psi. The coolant and the liquid desiccant in some
embodiments is maintained at pressures below about 2 psi, and the
permeable membrane contains moisture such as water in the liquid
state while water vapor permeates the membrane.
[0034] As will become clear from the following description, use of
the assemblies such as for evaporative coolers or mass/heat
exchangers provides a number of benefits. The inlet or process air
stream can be cooled and dehumidified simultaneously or in a single
chamber/stage, and this combined action reduces system size and
cost as well as the number of required components and equipment
(e.g., do not require a multi-stage unit or device to cool and then
to dehumidify and/or further cool with refrigerant or the like).
The combination of liquid desiccant dehumidification with indirect
evaporative cooling provides very high energy transfer rates due to
evaporation and absorption. The design creates a liquid desiccant
system that does not require separate equipment for liquid
desiccant cooling (e.g., a separate cooling tower or chiller). The
stacked arrangements or multi-layered mass/heat transfer assemblies
(or manifolded flow chambers/channels) enable ultra-low flow liquid
desiccant designs. This is due in part to the enhanced geometry of
the assembly and its ability to decrease the liquid desiccant's
temperature to a lower temperature than achievable with traditional
cooling tower technologies. Hence, in the cooler, there are higher
concentration gradients of liquid desiccant (e.g., more than 20
percentage points of lithium chloride (LiCl) and similar gradients
for other desiccants), which provides the following advantages: (a)
a higher thermal coefficient of performance (COP) to regenerate the
desiccant (i.e., to remove water from the desiccant) for reuse in
the cooler; (b) less desiccant storage requirements due to better
utilization; and (c) ability to use desiccants that are less
expensive than LiCl such as calcium chloride (CaCl), which may not
be used in conventional systems because their absorption properties
are not as favorable as LiCl but lower temperature operation
provided by the cooler embodiments described herein makes the
properties of this and other "weaker" desiccants more acceptable or
favorable.
[0035] The use of membranes as chamber sidewalls facilitates
fabrication of counter-flow and counter-flow with pre-cooled
exhaust air embodiments. Liquid desiccant containment with water
molecule-permeable membranes eliminates liquid desiccant "carry
over" in which small droplets of desiccant are passed into the air
stream as is a concern with direct contact arrangements. The
embodiments described herein also provide considerable reduction or
even elimination of deposited solids during the process of water
evaporation or adsorption (and liquid flow rates can be maintained
at levels that are high enough to further control potential
deposits) whereas fouling leads to increased maintenance and
operating costs with prior evaporative coolers.
[0036] FIG. 1 illustrates in a schematic an evaporative cooler (or
mass/heat exchanger) 100 that is useful for providing concurrent or
simultaneous dehumidifying and cooling of a process or inlet air
stream 120 (e.g., outdoor or process air to be cooled and
conditioned prior to being fed into a building ventilation system).
The cooler 100 is shown in simplified form with a housing shown in
dashed lines, without inlet and outlet ducts, plumbing, and/or
manifolds. Also, the cooler 100 is shown with a single mass/heat
transfer stack 110 whereas in a typical cooler 100 there would be
numerous stacks 110 provided by repeating the configuration shown
(e.g. by alternating the liquid passed through the chamber defined
by the membrane and wall) to provide an assembly with a plurality
of air and liquid flow channels or chambers to provide the desired
mass and heat transfer functions described for the stack 110.
[0037] As shown, an inlet air stream 120 is directed in a chamber
or channel defined in part by a sheet or layer of a membrane 112.
Liquid desiccant 124 flows in an adjacent chamber or channel on the
other side of the membrane 112. The liquid desiccant 124 is
contained by the membrane 112, which is permeable to water
molecules in a liquid or vapor state but generally not to the
components of the liquid desiccant 124. The chamber for the
desiccant flow 124 is also defined by a sheet or layer of material
that is impermeable to fluid flow (i.e., a separation wall) 114 so
as to contain the liquid desiccant 124 in the chamber or flow path.
The chamber for stream 120 is also defined by an opposing membrane
(not shown) that is used to contain another flow of liquid
desiccant. In this manner, heat is passed or removed from the inlet
air stream 120 and transferred to the liquid desiccant flow 124
(and the desiccant behind the opposite sidewall/membrane (not
shown)). Concurrently, the inlet air stream 120 is dehumidified as
water 130 is removed by passing through the permeable membrane 112
into liquid desiccant 124.
[0038] The liquid (or gas) desiccant 124 may take many forms to act
to dehumidify and cool the air stream 120 as it passes over the
membrane 112. Desiccant 124 is generally any hygroscopic liquid
used to remove or absorb water and water vapor from an air stream
such as stream 120. Preferably, the desiccant 124 chosen would be a
regenerable desiccant (e.g., a desiccant that can have the absorbed
water separated and/or removed) such as a glycol (diethylene,
triethylene, tetraethylene, or the like), a salt concentrate or
ionic salt solution such as LiCl, CaCl, or the like, or other
desiccants. The membrane 112 may be formed of any material that
functions to contain liquid desiccant 124 and, typically, coolant
126 (e.g., water or the like) while also being permeable to
molecules of water in liquid or vapor state. For example, polymer
membranes may be used that have pores that are about the size or
just bigger than a water molecule and, in some cases, that are also
adapted to provide water molecules with high mobility through the
membrane 112. In one particular embodiment, the membrane 112 is
formed from a membrane material as described in detail U.S. Pat.
No. 6,413,298 to Wnek, which is incorporated in its entirety herein
by reference. The membrane material may also be obtained from a
number distributors or manufacturers such as, but not limited to,
Dias-Analytic Corporation, Odessa, Fla., U.S.A. The membranes 112,
118 and separation wall 114 preferably also are formed from
materials that are resistive to the corrosive effects of the
desiccant, and in this regard, may be fabricated from a polymer or
plastic with the wall in some cases being formed of a corrosion
resistant metal or alloy, which provides a higher thermal
conductivity compared with a plastic.
[0039] The embodiment 100 shown is configured for counter-flow of
the pre-cooled exhaust air stream 128 (relative to the inlet air
stream 120). Other embodiments may use cross (at about a 90 degree
flow path) or quasi-counter flow (e.g., not directly counter or
opposite in direction but transverse such as a greater than 90
degree angle flow path relative to air stream 120). The exhaust air
stream 128 flows in a channel or chamber defined by a sheet or
layer of membrane (e.g., second or lower membrane) 118 and an upper
membrane of another stack (not shown). The separation wall 114 and
membrane 118 define a flow chamber or channel for coolant flow 126,
which is typically a flow of water or the like. Heat is transferred
from the liquid desiccant 124 to the coolant 126 through the
separation wall, and the coolant 126 is cooled as heat and mass
(e.g., water or other moisture 132) is transferred to the exhaust
stream 128 via membrane 118. Heat transfer is not shown but
generally is flowing through the membrane 112 to the liquid
desiccant 124, through the separation wall 114 from the liquid
desiccant 124 to the coolant 126, and through the membrane 118 from
the coolant 126 to the exhaust air stream 128. The membranes 112,
118 are relatively thin with a thickness, t.sub.mem, that typically
is less than 0.25 inches and more typically less than about 0.1
inches such as 100 to 130 microns or the like. The membrane 112,
118 may have a tendency to expand outward if unrestrained, and, in
some embodiments, such as that shown in FIG. 3, a divider or "flow
field" support is provided in the inlet air stream 120 and exhaust
air stream 128 (i.e., in the airflow chambers) to maintain the
separation of the adjacent membranes (e.g., a plastic or metallic
mesh with holes or openings for air flow and a zig-zag, S or
W-shaped, or other cross section (or side view) that provides many
relatively small contact points with the membranes 112, 118). The
separation wall 114 also typically is relatively thin to facilitate
heat transfer between the desiccant 124 and coolant 126 such as
with a thickness, t.sub.wall, of less than 0.125 inches or the
like. The flow chambers for the air, desiccant, and coolant are
also generally relatively thin with some applications using
chambers less than 1 inch thick (or in depth) while others use
chambers less than about 0.5 inches such as about 0.25 inches or
less.
[0040] FIG. 2 illustrates an indirect evaporative cooler 210
utilizing the membrane/separation wall/membrane stack or assembly
configuration to provide a mass/heat transfer exchanger device in
which dehumidification and cooling occur within a single stage and,
therefore, an integral or unitary device. In some embodiments (not
shown), there is no desiccant side membrane or desiccant flow.
Thus, these embodiments are useful for providing an indirect
evaporative cooler in which the membrane contains liquid coolant
but not liquid desiccant and the membrane typically would not be
provided on the supply air side (or in these channels) to provide
better heat transfer surfaces with the separation wall. As shown in
FIG. 2, the cooler 210 includes a mass/heat transfer assembly
formed from stacks or devices 212, 230, 240 and such an assembly of
stack would typically be repeated to provide a plurality of inlet
and exhaust air, coolant, and desiccant flow channels or chambers
in the cooler 210. As shown, each set of stacks (or layered
assemblies or devices) 212, 230, 240 is formed similarly to include
a membrane, a separation wall, and a membrane, with the membrane
being permeable to water on the molecular level to allow mass and
heat transfer and the wall being impermeable (or nearly so) to only
allow heat transfer and not mass transfer.
[0041] Specifically, the stack 212 includes an upper membrane layer
214, a separation wall 216, and a lower membrane layer 218.
Dividers or spacers (not shown) would typically be provided to
space these layers apart to define flow channels for coolant 215
and for liquid desiccant 217. For example, the separators may be
configured to also provide a connection to a supply line for
coolant and for regenerated desiccant, provide a manifold(s) to
direct flow through the various stacks 212, 230, 240, and provide a
connection to a return line for the coolant and diluted desiccant.
The stacks 230 and 240 likewise include an upper membrane layer
232, 242, a separation wall 234, 244, and a lower membrane layer
238, 248. The stack 240 has coolant (such as water) 243 directed in
the chamber between the upper membrane 242 and wall 244 and
desiccant 246 flowing between the wall 244 and lower membrane layer
248 similar to stack 212. In contrast, the stack 230 has liquid
desiccant 233 directed to flow in the chamber defined by the upper
membrane layer 232 and wall 234 and has coolant 236 directed to
flow in the chamber or channel defined by the wall 234 and lower
membrane layer 238.
[0042] The cooler 210 includes ducting and the like (not shown) to
direct supply inlet air 250 through the channel or flow path
between the stack 212 and the stack 230. The arrangement of the
stacks 212, 230, 240 and contained fluids results in the supply
inlet air 250 being passed over the surfaces of the membranes 218,
232 that are containing liquid desiccant 217, 233. As a result,
supply outlet air 254 is output that is dehumidified as moisture in
the air 250 is absorbed by the desiccant 217, 233 via permeable
membrane 218, 232, and the air 254 is also cooled by the
interaction with desiccant 217, 233. The cooling effect in the
cooler 210 is in part effected by a fraction of supply outlet air
254 being redirected in the cooler 210 by ducting/manifolds (not
shown) to flow as pre-cooled exhaust air 255 through the channel or
flow path between stacks 230, 240 to be output as warmer and
moister air 258. Heat passes from desiccant 233 through wall 236 to
coolant 236 (with similar heat transfer occurring in stacks 212,
240), and the coolant 236 is able to transfer heat and mass (e.g.,
water molecules) via membrane 238 to the incoming exhaust air 255.
As discussed above, the stack pattern or set provided by 212, 230,
240 would typically be repeated within the cooler 210 to create a
mass/heat transfer assembly with numerous, parallel flow channels
for air, coolant, and desiccant.
[0043] The cooler 210 is shown as a counter flow exchanger, but
other flow patterns may be used to practice the desiccant-based
dehumidification and cooling described herein. For example, cross
flow patterns may readily be established as well as quasi (or not
fully opposite) counter flow patterns. These patterns may be
achieved by altering the manifolding and/or ducting/plumbing of the
cooler as well as the dividers provided between the stacks.
Additionally, the counter flow passages may be provided integral to
the stack assembly rather than externally as is the case in the
cooler 210. For example, the cooler 310 has a similar stack
arrangement as shown in the cooler 210 of FIG. 2 except that it
includes a counterflow baffle or dividing wall 360 on the end of
the flow channels for inlet air 250 and exhaust air 258. The
counterflow divider 360 allows a majority of the cooled air to exit
the stacks as supply outlet air 354 (e.g., more than about 50
percent and more typically 60 to 90 percent or more of the air flow
250). A smaller portion (e.g., a volume equal to the make up
outdoor air or the like) is directed by divider 360 to flow between
stacks 230, 240 as pre-cooled exhaust air 355. FIG. 3 also
illustrates the use of a divider or flow field baffle 370 that
functions to maintain a separation of membranes in the stacks 212,
230, 240 separate (or at about their original thickness rather than
puffed out or expanded as may occur with some permeable membranes).
The dividers 370 may take many forms such as a mesh with a wavy
pattern (e.g., an S or W-shaped side or cross sectional view), with
the mesh selected to provide as little resistance to air flow as
practical while still providing adequate strength. Also, it is
desirable to limit the number of contact points or areas with the
membranes as these can block moisture transfer from the air 250 and
to the air 355.
[0044] FIG. 4 illustrates an indirect evaporative cooler 400 of one
embodiment. A housing 410 is provided for supporting a mass/heat
transfer assembly such as one formed with the stack sets shown in
FIGS. 1-3. As shown, the housing 410 includes a first end 412 with
an inlet 414 for supply inlet airflow 415 and an outlet 416 for
exhaust airflow 417. The cooler 4100 further includes a second end
418 opposite the first end 412 that provides an outlet or vent for
directing supply outlet airflow 420 to an end-use device or system
(e.g., an inlet or supply for return air to a building). The second
end 418 is also configured to redirect a portion 426 of the cooled
(and, in some operating modes, dehumidified) air 426 for use in
counter flow cooling of the supply inlet airflow 415. A prototype
of the cooler 400 was fabricated with a stack assembly as shown in
FIG. 2 with 32 desiccant channels. The prototype was tested with 10
liters per minute (LPM) flow (or about 0.3 LPM per desiccant
channel). Coolant was provided as water at a water flow rate of
about 1.25 to 2.00 times the evaporation rate. The evaporation rate
for this prototype was about 1.33 gallons/ton-hr or about 5
liters/ton-hr, which provides a water or coolant flow rate of about
6-10 liters/ton-hr of cooling. Of course, these are exemplary and
not limiting flow rates, and it is expected that the flow rates of
liquid desiccant and coolant will depend on numerous factors and
will be matched to a particular channel design and cooling need as
well as other considerations.
[0045] An indirect evaporative cooler such as the cooler 400 using
stack sets as shown in FIG. 2 may be modeled to determine the
effectiveness of the use of a permeable membrane to contain coolant
and liquid desiccant. FIGS. 5A and 5B provide a diagram 500 of such
modeling showing use of stacks 212, 230, and 240 as discussed with
reference to FIG. 2 to cool inlet or process air and to also
dehumidify this air in the same stage or process. The inputs to the
model 500 are shown, and results for a typical inlet air condition
are provided, with results and modeling being performed in this
case with Engineering Equation Solver (EES). The numeric values
shown in boxes or with squares around them are input values (or
assumed typical operating conditions), and the values outside or
without boxes are outputs or results of the modeling. The modeling
results shown in the diagram 500 are believed to be
self-explanatory to those skilled in the heating, ventilation, and
air conditioning (HVAC) arts and do not require detailed
explanation to understand the achieved effectiveness of the
embodiments using membrane containment in indirect evaporative
coolers; however, the following provides a graphical description of
some of the results in the model 500.
[0046] FIG. 6 illustrates a graph or diagram 610 showing the
temperatures of the air flows in the channels between the stacks
(e.g., in an evaporative cooler using such mass/heat transfer
assembly described herein). The graph 610 also shows surface
temperatures along the length of the counterflow mass/heat
exchanger (e.g., exchanger 400 with stack arrangements as shown in
FIG. 2). Specifically, the graph 610 shows the temperature of
supply air with line 612, the temperature of exhaust/purge air with
line 614, the temperature of the desiccant side membrane surface
(e.g., at the interface of the membrane and the supply air) with
line 616, the dewpoint temperature of the desiccant side membrane
surface (e.g., at the interface of the membrane and the supply air)
with line 620, and the temperature of the water side membrane
surface (e.g., at the interface of the membrane and the
exhaust/purge air) with line 618.
[0047] FIG. 7 is a graph or diagram 710 showing the humidity ratios
of the air along the length of the counterflow heat/mass exchanger.
Specifically, the graph 710 shows the bulk humidity ratio of the
supply air with line 712, the bulk humidity ratio of exhaust/purge
air with line 714, the humidity ratio of the air in close proximity
to the desiccant side membrane surface (e.g., at the interface of
the membrane and the supply air) with line 716, and the humidity
ratio of the air in close proximity to the water side membrane
surface (e.g., at the interface of the membrane and the
exhaust/purge air) with line 718. FIG. 8 illustrates a graph 810
showing with line 815 the concentration of desiccant (in this
particular modeling the desiccant is LiCl) as it flows concurrent
with the supply air flow down the length of the counterflow
mass/heat exchanger. As shown with line 815, the desiccant is
getting weaker as it flows through the channel between the membrane
and the separation wall as it absorbs water molecules from the air,
e.g., the concentration of the desiccant is dropping from about 44
percent down to about 24 percent in this particular modeling
example (which results from the membrane being characterized as
permeable (at a particular input rate or setting) to water
molecules in the flowing air at these operating conditions).
[0048] FIG. 9 shows the process of model 500 of FIGS. 5A and 5B in
a psychrometric chart 910. The supply air shown with line 912 can
be seen to be gradually losing humidity (in kilograms water
vapor/kilograms dry air or kg.sub.v/kg.sub.da). The supply air 912
has its temperature initially rise slightly due to the large heat
flow of vapor sorption into the desiccant. As the supply air 912
continues down the length of the exchanger (or flow channel or
chamber between membrane layers or walls of adjacent stacks
containing liquid desiccant), the temperature then drops to a
cooler/drier condition that at the inlet. At the exit of the
exchanger, the supply air 912 is split into two streams. The
majority of the air is supplied to the cooled space, and the
minority of the air (such as less than about 50 percent and more
typically less than about 30 percent of the volume) gets funneled
into the exhaust/purge side (or exhaust/counterflow channels
between the membrane walls containing coolant) of the heat/mass
exchanger or cooler, which is shown with the line 916. The exhaust
air 916 has a low dewpoint, and, thus, it can pick up a large
amount of heat evaporatively. The pre-cooled exhaust or purge air
916 picks up water vapor (and associated heat of vaporization) from
the wet side channel. The air 916 exits out of the unit with a much
higher enthalpy than either the supply inlet or exit shown with
line 912. The diagram 910 also shows the humidity ratio and
temperature of the supply air in close proximity to the desiccant
side membrane surface (ds) with line 918.
[0049] The following table shows results in tabulated form for
modeling of FIGS. 5A and 5B for inlet and outlet air flows. As
shown, a wide range of temperatures and humidity levels can be
chosen and input into the model 500. In the configuration whose
results are shown in the table, the equivalent wet bulb
effectiveness with the desiccant flow turned off (e.g., in some
operating modes it may not be required or useful to utilize the
desiccant to dehumidify the air) would be 113 percent, which means
the cooler is able to cool the supply air below the inlet wet bulb
temperature.
TABLE-US-00001 TABLE Inlet and outlet conditions from model runs
(.degree. F. and kg/kg) Run # T.sub.supply, in T.sub.supply, out
T.sub.exhaust, out .omega..sub.supply, in .omega..sub.supply, out
.omega..sub.exhaust, out 1 27.7 21.11 31.55 0.0133 0.00892 0.0289 2
50.0 33.7 50.7 0.0319 0.0179 0.0834 3 50.0 20.7 41.0 0.0077 0.00406
0.0494 4 30.0 13.1 27.2 0.00262 0.00158 0.0226 5 30.0 18.9 42.55
0.0269 0.0137 0.0547 6 15.0 16.9 25.4 0.0105 0.00418 0.0207 7 15.0
11.9 20.0 0.00528 0.00203 0.0147 where LiCl Inlet Concentration =
44%; flow ratio (flow exhaust/(flow exhaust + flow supply) = 0.3;
supply outlet face velocity = 175 SCFM; and ambient pressure =
101.3 kPa.
[0050] The cooler 210 of FIG. 2 may be thought of as a
desiccant-enhanced, indirect evaporative cooler that utilizes a
membranes or layers of membrane material that is permeable to water
molecules to provide desired liquid containment. A standard
psychrometric chart (such as one at 14.7 psi ambient pressure and
other typical parameters) may be used to view lines of equal
sensible heat ratios (SHRs) originating at a typical room setpoint.
For vapor compression dehumidification, a SHR of less than about
0.7 is difficult to attain without reheat (e.g., given reasonable
evaporator temperatures). Also, it is psychrometrically impossible
to attain a SHR of less than about 0.6 without reheat, and
attempting such a SHR often leads to frozen evaporator coils that
require defrost cycles. The desiccant-enhanced, indirect
evaporative cooler, such as shown in FIG. 2 at 200, addresses this
problem with a unique, new process (as has been described above and
is presented in more detail below).
[0051] It may be useful at this point to review the process with
reference to FIGS. 2 and 3. FIGS. 2 and 3 show diagrams describing
the inner flow channels of the unit or assembly for use in an
evaporative cooler 210, 320. The mixed return/outdoor air is shown
by the arrow 250 (e.g., return air from a conditioned space along
with outdoor make up air such as 400 cfm/ton supply and 175 cfm/ton
outdoor air or the like). The air 250 is dehumidified by the
desiccant 217, 233 through the membrane 218, 232. This lowers both
the dew point and temperature of this air stream until it is output
at 254 or 354. At the exit of the supply air passage (between the
liquid desiccant-containing membranes), a portion of the air is
fractioned off as shown with arrows 255 and 355 and sent through an
adjacent passage (between the coolant-containing membranes 238,
242) which picks up moisture from the water layer 236, 243 through
the membrane 238, 242. The heat of evaporation is a source of
cooling that acts to remove the sensible heat and heat of
absorption from the supply air stream 250. This air is then
exhausted (purged) out at 254, 354.
[0052] Heat exchanger configuration shown at 400 in FIG. 4 has been
built in the laboratory by the inventors and was modeled as shown
in FIGS. 5A and 5B. Other options for flow/housing designs are
shown in configuration with the cooler 1000 of FIG. 10 and the
cooler 1100 of FIG. 11. The cooler 1000 is shown to have a housing
1010 with a first portion or end 1012 and a second portion or end
1020. The first portion 1012 is configured with inlet or vents for
receiving supply inlet airflow 1013 as well as input exhaust
airflow 1014, and the first portion 1012 also includes vents or
outlets for outputting exhaust airflow 1015 from the unit 1000. The
second portion 1020 is configured (e.g., with manifolds and other
components to direct air flow) with outlets for supply outlet
airflow 1022 with a portion 1025 being redirected back into the
housing 1010 as shown at arrows 1027 to provide counterflow for a
fraction of the channel provided for supply inlet airflow 1013
(with exhaust airflow 1014 provided as a cross flow in the other or
initial portion of the channel) and then this air is exhausted from
the housing portion 1020 at 1028. The input exhaust airflow 1014
may be return air to be exhausted or outdoor air (e.g., from the
building space). This approach 1000 improves the efficiency by
utilizing a smaller purge airflow 1025, 1027, and it is typically
preferred to limiting purge air flow to increase or maintain
desirable efficiency.
[0053] Referring again to FIG. 4, operation of the cooler 400 is
expected to have the cooling process shown in the psychrometric
chart 910 of FIG. 9. As shown, line 912 represents the supply air
flow while line 916 represents the purge air flow stream. The
desiccant side air boundary layer is represented with line 918. The
chart shows graphically how the dehumidification driver for the
cooler 400 is advantageously utilized to provide a more effective
cooler. The cooler 400 may use even a weak desiccant such as CaCl
solution to provide significant dehumidification, and this is due
in part to the cold temperatures that are achieved with the
configuration of the cooler 400 that allow weak desiccants to
attain high dehumidification potential.
[0054] The configuration shown with cooler 1000 of FIG. 10 was
modeled to determine the desirability of its performance, and the
results are provided in psychrometric chart 1200 of FIG. 12. In the
chart 1200, line 1210 represents supply air, line 1212 represents
ambient exhaust air, line 1214 represents desiccant side surface
temperatures, line 1220 represents the supply air post cooling,
line 1224 represents the purge air post cooling, and line 1230 is
the sensible heat ratio line (SHR) in which the load on the
building follows. So, for example, a building will have 0.67 units
of sensible heat and 0.33 units of latent heat added to the space
to arrive at the return air condition, which is the middle diamond
at 80.degree. F. and about 70 grains/lb, and that point may be
considered the return air condition. The first point of line 1210
is the "mixed air" condition, which is a 30/70 mixture of outdoor
air and return air. The two-stage approach to cooling provided by
cooler 1000 allows the process to be split into two distinct
sections of dehumidification plus a post cooling stages (e.g.,
sensible cooling only stage in which, for example, there is no
desiccant layer and dehumidification and only evaporative cooling
is provided). The cooler 1000 is, of course, only one example of
numerous configurations that may be implemented to provide two or
more stage cooling using the membrane containment features
described herein, and it shows the possibility of attaining nearly
any SHR desired (e.g., in this case, a SHR of about 0.67). In the
modeling to provide the chart 1200, a 1 cubic foot core (or
mass/heat transfer assembly) was used with 176 SCFM, and a flow
ratio of about 0.3 (e.g., 30 percent purge and 70 percent supply
air). Also, the return air was at 80.degree. F. and 40 percent
relative humidity, ambient air was at 86.degree. F. and 60 percent
relative humidity, and the liquid desiccant fed into the assembly
was 44 percent LiCl (but other desiccants such as solutions of salt
(such as, but not limited to, halide salts) and water that are
about 20 to 40 percent salt by weight may be used). The assembly
was able to provide 0.5 tons of building cooling with just this 1
cubic foot at about 7 Btu/lb. As can be appreciated from this
example and modeling, the use of membranes to contain desiccant and
coolant (e.g., to contain liquids) enable indirect evaporative
coolers to be produced that are much more compact than prior
designs, that are easier to maintain (e.g., have less or no fouling
issues), and that are more efficient in producing cooling (e.g.,
with simultaneous dehumidification and cooling to provide an
evaporative cooler that can condition as well as cool process
air).
[0055] FIG. 11 illustrates an evaporative cooler 1100 providing
another counterflow arrangement in which the counterflow cooling
air (or pre-cooled supply air) is directly opposite in direction
but only for a selected length (such as half to 80 or 90 percent or
more of the length) of the stacks or flow chambers (e.g., when full
counterflow is not required or desired). As shown, the cooler 1100
includes a housing 1110 containing a plurality of stacks or sets of
stacks configured as a mass/heat transfer assembly (as discussed
above) with alternating flow channels for supply inlet airflow 1112
and for counterflow air (e.g., redirected supply outlet airflow
1114). The housing 1110 includes venting and/or manifolding for
directing the supply inlet airflow 1112 (e.g., outdoor make up air
and return air) into channels between desiccant containing
membranes and to output the cooled and, often, dehumidified supply
outlet airflow 1114. The cooler 1100 further includes ducting,
manifolding, and the like for redirecting a fraction of the supply
outlet airflow back into the housing 1110 to provide cooling
counterflow air as shown at 1116 (e.g., into flow channels between
coolant containing membranes). The counterflow air 1116 typically
does not travel along the entire length of the housing 1110 but is,
instead, discharged out a side vent at some point along a channel
length (e.g., at a distance about 60 to 80 percent of the length).
Such a configuration is useful to tune a cooler 1100 for particular
operating environments (e.g., to provide a desired amount of
cooling to the supply outlet airflow based on outside air
temperatures and humidities and other operating parameters).
[0056] The stack and membrane technology described herein are
readily applicable to a number of indirect evaporative cooler
designs (with and without use of liquid desiccant for
dehumidification) and applications. However, it may be useful to
discuss the use of the technology within an air conditioning or
HVAC system with the belief that those skilled in the art will
readily understand that the technology is useful in many other such
systems. FIG. 13 illustrates a simplified air conditioning system
1300 in which the membrane technology may be provided to provide
desiccant dehumidification and evaporative cooling to condition air
within a building 1310 (e.g., a residential or commercial building
or other structure requiring conditioned and cooled air). As shown,
the system 1300 includes a cooler 1320 with a housing 1322 that is
used to house a membrane stack assembly, such as described above
with reference to FIGS. 1-12. A fan or blower 13224 is provided to
draw in outside or make up air 1325 and move return air 1326 from
the building 1310. The fan 1324 pushes these two air streams as
inlet supply air through the stacks as described above (e.g.,
adjacent liquid desiccant contained in membrane in embodiments
providing dehumidification or adjacent separation walls in
embodiments with just evaporative cooling). The cooled (and,
typically, conditioned air is output at 1330 as supply to the
building 1310 and a portion is returned 1332 as purge or pre-cooled
exhaust air that passes on the coolant or evaporative cooling side
of the stacks in housing 1322 and then out as exhaust 1328. Coolant
is provided in the form of a water supply and drain 1334 to the
housing (and through the stack assembly), and liquid desiccant is
provided at 1338 as supply and drain. The desiccant 1338 is
regenerated with a regenerator system 1340 including, in this
example, a desiccant boiler 1342.
[0057] The desiccant enhanced indirect evaporative cooler (DE-IDEC)
1320 is the portion of the system 1300 that takes strong desiccant
and water to provide cooling to building 1310. The system 1300
provides both sensible and latent cooling to building 1310 on
demand and in proportion to the demand, e.g., the system 1300 can
provide cooling in the form of 100 percent sensible, 100 percent
latent, or any combination thereof. The DE-IDEC 1320 uses some
portion of outdoor air 1325 with equal exhaust air 1328 to reject
the heat load outside of the building 1310. The DE-IDEC 1320 itself
can sit inside or outside of the building envelope because it has
no wet surfaces and the liquid streams 1334, 1338 are closed loop.
This makes system 1300 acceptable for indoor use and for placement
of cooler 1320 inside the building 1310. The water source (or
coolant source, not shown) for water or coolant 1334 is not
required to be potable, and the system 1300 is compact enough to be
acceptable by building managers. The electricity usage is much less
than that of typical vapor compression systems or units (e.g., less
than 0.2 kW/ton peak compared with 1.2 kW/ton typical for
conventional compression units).
[0058] The regenerator 1340 is another of the significant
components to the operation of the system 1300. This unit 1340
takes the weakened desiccant from the DE-IDEC 1320 and applies heat
with boiler 1342 (see list of heat sources below) to drive off the
moisture contained in the desiccant 1338. The result is a desiccant
1338 that has higher salt concentration and can be re-used by the
DE-IDEC 1320 (e.g., in the membrane contained/defined flow channels
adjacent to supply inlet air 1325, 1326). A list of heat sources
suitable for desiccant regeneration may include: (a) gas or other
fossil fuel; (b) solar heat; (c) waste heat from any waste heat
stream such as combine heat and power plant; and (d) waste heat
from a condenser unit originating from a vapor compression
cycle.
[0059] The inventors performed a test of a prototype fabricated
similar to the cooler shown in FIG. 4 with a stack assembly such as
shown in FIG. 2. FIG. 14 provides results of the testing for this
proof of concept prototype that was constructed and tested at
104.degree. F. and 93 grains/lb inlet air. The prototype was tested
with and without desiccant flow, but with membranes provided to
define liquid desiccant flow channels. Without the desiccant flow,
the indirect evaporative cooler had a wet-bulb effectiveness of
73%. When desiccant was turned on (with 41% LiCl solution as the
desiccant), the effectiveness was 63% and had 12 grains/lb of
dehumidification. This resulted in a sensible heat ratio of 0.73.
The prototype did not reach model expectations as explained above,
and this was likely due to prototype defects creating non-uniform
air, water, and desiccant flow distribution.
[0060] While a number of exemplary aspects and embodiments have
been discussed above, those of skill in the art will recognize
certain modifications, permutations, additions, and
sub-combinations thereof. It is therefore intended that the
following appended claims and claims hereafter introduced are
interpreted to include modifications, permutations, additions, and
sub-combinations to the exemplary aspects and embodiments discussed
above as are within their true spirit and scope. The above
description concentrated or stressed designs of heat/mass transfer
assemblies for use in providing unique indirect evaporative
coolers. Those skilled in the art will recognize that the coolers
described can readily be included in more complete HVAC systems for
residential and commercial use. Such HVAC systems would include
plumbing and components to circulate liquid desiccant to and from
the cooler at desirable and controllable flow rates. These systems
would also include a regenerator for the desiccant (e.g., one that
heats the liquid desiccant to remove absorbed water such as heat
provided by solar panels, electrical heaters, or the like). The
regenerator also include a sump and lines for recovering potable
water from the desiccant and storage would be provided for the
desiccant prior to it being pumped or fed to the cooler. Portions
of the system that come into contact with the desiccant typically
would be fabricated of corrosion resistant materials such as
certain metals or, more typically, plastics. The HVAC system would
also include ducting and other components such as fans or blowers
for moving the return air from the building through the cooler and
back to the cooled spaces, for moving make up air through the
cooler and into the cooled spaces, and for discharging any purge or
exhaust air. A coolant supply system with piping and pumps/valving
(as necessary) would also be provided to provide coolant such as
potable water to the cooler stacks (e.g., channels between
membranes and separation walls).
[0061] The embodiments shown typically discussed ongoing use of the
liquid desiccant to dehumidify the supply or process air. However,
in many operating conditions, the cooler may be operated without
desiccant flow, and these operating conditions may be considered
"free evaporative cooling" conditions (or zones on a psychrometric
chart). "Free cooling" is exemplified by cooling efficiency so high
that the cost of energy to run the system is of no consequence. For
example, cooling without drying/dehumidifying may be performed by
coolers described herein when humidity ratio is below about 80 (and
the dry bulb temperatures are above 60.degree. F.) while cooling
and drying may be required above this humidity ratio at which point
the cooler can be operated with flowing liquid desiccant. Such
"free" cooling is practical relatively large numbers of days in
less humid areas of the world (such as the southwest portion of the
United States).
[0062] Embodiments of an indirect evaporative cooler according to
the above description and attached figures can be provided as a
single unit that provides an integral heat and mass transfer device
utilizing a number of separation walls. The transfer device or
assembly uses membrane containment and air flows do not come in
direct contact with desiccant or water (coolant). The coolers use
evaporative cooling (e.g., of water from the air flows across the
membranes) to drive heat and mass exchange, with heat being
transferred through the separation walls between liquid desiccant
and coolant. The heat exchange is between two counter and/or cross
flowing air streams. The mass exchange, such as during
dehumidification, is generally the transfer of water vapor from the
inlet supply air or process air through a water molecule-permeable
membrane to a liquid state (e.g., to absorption by the liquid
desiccant). The evaporative section of the coolers drives heat
through the separation wall and expels that heat by evaporation
from the coolant/water to an air stream (e.g., again water vapor is
transferred through the permeable membrane but to a vapor state in
the exhaust or counter/cross flow airstream).
* * * * *