U.S. patent number 3,906,250 [Application Number 05/480,623] was granted by the patent office on 1975-09-16 for method and apparatus for generating power utilizing pressure-retarded-osmosis.
This patent grant is currently assigned to Ben-Gurion University of the Negev Research & Development Authority. Invention is credited to Sidney Loeb.
United States Patent |
3,906,250 |
Loeb |
September 16, 1975 |
**Please see images for:
( Certificate of Correction ) ** |
Method and apparatus for generating power utilizing
pressure-retarded-osmosis
Abstract
A method and apparatus are described for generating power by
utilizing prure-retarded-osmosis. A first liquid having a
relatively high osmotic pressure is introduced at a relatively high
hydraulic pressure into a first pathway in which it contacts one
face of a semi-permeable membrane, and a second liquid having a
lower osmotic pressure is introduced at a lower hydraulic pressure
into a second pathway in which it contacts the opposite face of the
membrane. At every point in the two pathways, the hydraulic
pressure difference between the liquids on the opposite faces of
the membrane is maintained at a value which is less than the
osmotic pressure difference between the liquids. Part of the second
liquid passes by pressure-retarded-osmosis through the
semi-permeable membrane, forming a pressurized mixed solution of
greater volume than that of the first liquid introduced into the
first pathway. The potential energy stored in the pressurized mixed
solution is then converted into useful energy, such as electrical
or mechanical power. According to a further feature included in
some of the described embodiments, after the potential energy
stored in the pressurized mixed solution is converted into useful
energy, the first and second liquids are recovered by separating
from the mixed solution a quantity of the second liquid equal to
the quantity which passed through the membrane, the original
temperatures of the so-recovered first and second liquids are
restored, the original hydraulic pressure difference is reapplied
between the recovered first and second liquids, and the recovered
first and second liquids are then recycled through the first and
second pathways.
Inventors: |
Loeb; Sidney (Beersheba,
IL) |
Assignee: |
Ben-Gurion University of the Negev
Research & Development Authority (Beersheba,
IL)
|
Family
ID: |
26320487 |
Appl.
No.: |
05/480,623 |
Filed: |
June 19, 1974 |
Foreign Application Priority Data
|
|
|
|
|
Jul 3, 1974 [IL] |
|
|
42658 |
May 10, 1974 [IL] |
|
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44799 |
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Current U.S.
Class: |
290/1R; 60/673;
310/300; 60/649; 210/652 |
Current CPC
Class: |
F03G
7/005 (20130101); B01D 61/002 (20130101) |
Current International
Class: |
F03G
7/00 (20060101); F03G 007/06 () |
Field of
Search: |
;290/1,2 ;310/2,651
;60/641-673 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Schaefer; Robert K.
Assistant Examiner: Redman; John W.
Attorney, Agent or Firm: Barish; Benjamin J.
Claims
What is claimed is:
1. A method of generating power comprising: applying a hydraulic
pressure to a first liquid of a first osmotic pressure and
introducing same into a first pathway which is at least partially
defined by one face of a semipermeable membrane; introducing a
second liquid having a lower hydraulic pressure and a lower osmotic
pressure into a second pathway which is at least partially defined
by the opposite face of the membrane; maintaining the hydraulic
pressure difference between liquids on the opposite faces of the
membrane at a pressure difference which is less than the osmotic
pressure difference between the liquids, at every point in the two
pathways, thus effecting by Pressure-Retarded-Osmosis a passage of
at least part of the second liquid through the semipermeable
membrane, forming a pressurized mixed solution of greater volume
than said first liquid introduced into said first pathway; and
converting the potential energy stored in the pressurized mixed
solution to useful energy.
2. The method according to claim 1, wherein said energy is
converted by passing the increased-volume pressurized first liquid
through a turbine generator to generate electrical power.
3. The method according to claim 1, wherein said first liquid is
passed through the first pathway in counter-flow with respect to
the second liquid in the second pathway.
4. The method according to claim 1, wherein the first and second
liquids are saline water solutions having different osmotic
pressures.
5. The method according to claim 4, wherein the first liquid is sea
water and the second liquid is water having a lower concentration
of salt than sea water.
6. The method according to claim 4, wherein the first liquid is a
naturally available salt-water body, and the second liquid is
derived from a river or ocean feeding the salt-water body.
7. The method according to claim 4, wherein the first liquid is a
body of water to which salt has been artifically added to produce
the higher osmotic pressure solution.
8. The method according to claim 1, wherein the first liquid is fed
from an evaporation pond, and wherein the evaporation pond is fed
from the outlet of the first pathway after the energy in the first
liquid has been converted to power.
9. The method according to claim 8, including the further step of
recovering the water evaporated from the pond.
10. The method according to claim 8, including the further step of
recovering the concentrated solution and/or solutes resulting from
the evaporation of water from the pond.
11. Apparatus for generating power from heat comprising: a
semipermeable membrane; means for applying a hydraulic pressure to
a first liquid of a first osmotic pressure and introducing same
into a first pathway which is at least partially defined by one
face of the semipermeable membrane; means for introducing a second
liquid having a lower hydraulic pressure and a lower osmotic
pressure into a second pathway which is at least partially defined
by the opposite face of the membrane; means for maintaining the
hydraulic pressure difference between liquids on the opposite faces
of the membrane at a pressure difference which is less than the
osmotic pressure difference between the liquids, at every point in
the two pathways, thus effecting by Pressure-Retarded-Osmosis a
passage of at least part of the second liquid through the
semipermeable membrane, forming a pressurized mixed solution of
greater volume than said first liquid introduced into said first
pathway; and means for converting the potential energy stored in
the pressurized mixed solution to useful energy.
12. Apparatus according to claim 11, wherein said converting means
comprises a turbine generator generating electrical power.
13. Apparatus according to claim 11, wherein said means for
applying a hydraulic pressure comprises a liquid pump.
14. Apparatus according to claim 13, further including means for
applying a portion of said energy to drive the pump.
15. Apparatus according to claim 11, further including an
evaporation pond, means feeding the first liquid from the outlet
end of the evaporation pond into the inlet end of the first
pathway, and means feeding the first liquid from the outlet end of
the first pathway after passing through said conversion means into
the inlet end of the evaporation pond.
16. A multi-stage system for generating power, each stage
comprising apparatus of claim 11, the hydraulic pressure of the
first liquid in the first stage being highest and progressively
decreasing through all the stages.
17. The method of claim 1 including the further steps of:
recovering the first and second liquids by separating from said
mixed solution a quantity of second liquid substantially equal to
the quantity which passed through the membrane and mixed with the
first liquid; restoring substantially the original temperatures to
the recovered first and second liquids; reapplying the above
mentioned hydraulic pressure difference between the recovered first
and second liquids; and recycling the recovered first and second
liquids through the first and second pathways respectively.
18. The method according to claim 17, wherein the recovering of
said first and second liquids is effected by thermal
separation.
19. The method according to claim 18, wherein said thermal
separation is effected by distillation.
20. The method according to claim 18, wherein said thermal
separation is effected by using as said first and second liquids
solutions of two liquid species whose miscibility is a function of
temperature.
21. The method according to claim 18, wherein the thermal
separation is effected by using as said first liquid a solution of
a solvent and a solute whose solubility is a function of
temperature.
22. The method according to claim 18, wherein said thermal
separation is effected by using solar energy as the energy
source.
23. A heat engine including the apparatus according to claim 11,
further including: separating means for recovering the first and
second liquids by separating from said mixed solution a quantity of
second liquid substantially equal to the quantity which passed
through the membrane and mixed with the first liquid; temperature
restoral means for restoring substantially the original
temperatures to the recovered first and second liquids; means for
reapplying the above mentioned hydraulic pressure difference
between the recovered first and second liquids; and means for
recycling the recovered first and second liquids through the first
and second pathways respectively.
24. The heat engine according to claim 23, wherein said separating
means and temperature restoral means are thermal means.
25. The heat engine according to claim 24, wherein said thermal
separating means comprises a distillation device.
26. The heat engine according to claim 25, wherein both said first
and second liquids include dichlorodifluoromethane as the solvent,
and the solute is a low molecular weight compound such as
ethanol.
27. The heat engine according to claim 24, wherein the two liquids
are solutions of two liquid species whose miscibility is a function
of temperature, and wherein said thermal separating means comprises
means for changing the temperature of the mixed solution, such that
the mixed solution will separate into the first and second liquids
thus substantially recovering them.
28. The heat engine according to claim 27, wherein one of the said
liquid species is methanol and the other is hexane.
29. The heat engine according to claim 27, wherein one of the said
liquid species is triethylamine, and the other is water.
30. The heat engine according to claim 24, wherein the first liquid
is a solution of a solvent and a solute whose solubility is a
function of temperature, and wherein said thermal separating means
including means utilizing said latter property to precipitate the
solute.
31. The heat engine according to claim 30, wherein said first
liquid is a solution of water and potassium nitrate, and said
second liquid is water.
Description
BACKGROUND OF THE INVENTION
The present invention relates to a method and apparatus for
generating electrical or mechanical power, and particularly to a
method and apparatus which generates the power by exploiting
certain naturally available sources which have not heretofore been
exploited at all, or solar energy which has heretofore been
exploited only by the use of very expensive and/or low-efficiency
devices.
With the widely-predicted energy crisis looming in the not too
distant future because of the dwindling supplies of fossil fuels,
it has become vital to develop new sources of energy not based on
fossil fuels. The present invention provides a new method and
apparatus for generating power by utilizing two liquids having
different osmotic pressures, and particularly by exploiting the
osmotic interaction between such two liquids.
The different osmotic pressure liquids that may be used for
generating the power can be obtained from a number of sources, both
naturally occurring and man-made, for example: (1) sea water
(liquids of higher osmotic pressure) fed by fresh water (liquid of
lower osmotic pressure) from a river; (2) brines or salt water
bodies (e.g., The Dead Sea) fed by sea water or fresh water; (3)
water "fueled" by artificially adding salt therein from naturally
occuring salt bodies (to produce the higher osmotic pressure
liquid) interacting with water not so "fueled"; and (4) evaporation
ponds (both naturally occurring and man-made) interacting with
lower-concentration water solutions.
Theoretically all of the foregoing sources could be used for
generating power in accordance with the invention, as will be more
fully described below. Osmotic power stations utilizing power
sources such as the Dead Sea would even appear to be competitive in
price with present power stations. When evaporation ponds are used,
they can be man-made at any desired location to convert the solar
energy at that location to mechanical or electrical power. They can
be used therefore instead of the known solar energy converters
(e.g., photovoltaic devices and solar heat collectors including
heat engines) and would even appear to be substantially less
expensive than such devices.
BRIEF SUMMARY OF THE INVENTION
The present invention employs Pressure-Retarded-Osmosis (PRO) for
the production of useful energy from the above naturally available
energy sources. The invention also enables the PRO process to be
employed where the above natural energy sources are not available.
A number of both types of systems are described below for purposes
of example.
The basic Pressure-Retarded-Osmosis (PRO) method and apparatus for
generating power is generally characterized by an arrangement for
performing the steps of: applying a hydraulic pressure to a first
liquid of a first osmotic pressure and introducing same into a
first pathway which is at least partially defined by one face of a
semipermeable membrane; introducing a second liquid having a lower
hydraulic pressure and a lower osmotic pressure into a second
pathway which is at least partially defined by the opposite face of
the membrane; maintaining the hydraulic pressure difference between
liquids on the opposite faces of the membrane at a pressure
difference which is less than the osmotic pressure difference
between the liquids, at every point in the two pathways, thus
effecting by Pressure-Retarded-Osmosis a passage of at least part
of the second liquid through the semipermeable membrane, forming a
pressurized mixed solution of greater volume than said first liquid
introduced into the first pathway; and converting the potential
energy stored in the pressurized mixed solution to useful energy,
such as electrical or mechanical energy.
The semipermeable membrane used is preferably one that passes only
solvent and no solute (perfectly semipermeable), but it may also be
an imperfect one, i.e., one that passes some solute.
The first part (Part 1) of the description below (particularly
FIGS. 1-6) concerns the basic technique as it may be applied with
respect to each of the four naturally-occurring and man-made water
sources mentioned above. The second part (Part 2) of the
description below (particularly FIGS. 7-14) concerns the manner of
enabling such PRO power plants to be applicable for the production
of power where the above natural energy sources are not
economically available.
One limitation in the systems described in Part 1 (FIGS. 1-6) is
that such systems can be used, as a practical matter, only in
certain areas of the World. the only osmotic power plant scheme
described in Part 1 below which can be considered to approach
general applicability for production of economic power is that of
FIG. 6, in which water evaporation ponds are used for
reconcentration of the mixed solution, i.e., brine diluted in the
Pressure-Retarded-Osmosis (PRO) unit. However, as will be discussed
in Part 2, the area required for evaporation or distillation would
be in the order of 1,000 square kilometers for a 1,000 megawatt
power plant utilizing evaporation ponds. This area is so large that
it would restrict the use of osmotic power plants to areas which in
effect already possess evaporation ponds, such as the Dead Sea and
Great Salt Lake.
Another limitation on the general use of the osmotic power plants
described in Part 1 (FIGS. 1-6) is that an expendable source of a
low-osmotic pressure aqueous solution, such as ocean or river
water, must be available in the region of the evaporation ponds.
Large quantities of this solution are expended. For example, in the
1,000 megawatt plant using evaporation ponds described below in
FIG. 6, a daily quantity of 4,300,000 cubic meters of water of low
osmotic pressure aqueous solution must be expended. Even if large
quantities of such solutions are available, such as in sea water,
charges for pumping, filtering, etc., may be exorbitant.
Part 2 of the description below (FIGS. 7-14) therefore describes
systems for making the basic PRO techniques of general
applicability, by eliminating both of the above limitations, namely
by enabling the solar energy collecting area to be greatly reduced,
and by obviating the need of expendable material altogether.
The latter aspects of the invention are accomplished by providing
arrangements which, after the potential energy stored in the
pressurized mixed solution is converted to useful energy such as
elecrical or mechanical energy, the following additional steps are
performed: recover the first and second liquids by separating from
the mixed solution a quantity of second liquid substantially equal
to the quantity which passed through the membrane and mixed with
the first liquid; restore substantially the original temperatures
to the recovered first and second liquids; reapply the above
mentioned hydraulic pressure difference between the recovered first
and second liquids; and recycle the recovered first and second
liquids through the first and second pathways respectively.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention is herein described, somewhat diagrammatically and by
way of example only, with reference to the accompanying drawings,
wherein:
FIG. 1 illustrates the basic osmosis process for the case of sea
water and fresh water on opposite sides of a semi-permeable
menbrane;
FIGS. 2a and 2b illustrate the basic pressure-retarded osmosis
process on which the present invention is founded;
FIG. 3 illustrates a continuous pressure-retarded osmosis process
for producing energy from sea water at the mouth of a river, and
FIG. 3a is a pressure-energy diagram relating thereto;
FIG. 4 illustrates a continuous pressure-retarded osmosis process
for producing energy from Dead Sea brine at the mouth of the River
Jordan, and FIG. 4a is a pressure-energy diagram relating
thereto;
FIG. 5 illustrates the pressure-retarded osmosis process for the
production of energy in a multi-stage system, and FIG. 5a is a
pressure-energy diagram relating thereto;
FIG. 6 illustrates the pressure-retarded osmosis process for the
production of energy from concentrated brine provided by
evaporation ponds.
FIG. 7 is a block diagram illustrating the generalized concept of a
PRO heat engine for generating power.
FIG. 8 is a block diagram of a PRO heat engine constructed in
accordance with the Carnot Cycle, utilizing solar energy as a heat
source;
FIG. 9 is a block diagram illustrating a portion of a PRO heat
engine constructed in accordance with the invention, FIG. 3 being
essentially the Pressure-Retarded-Osmosis (PRO) section described
in FIGS. 2-6 above;
FIG. 10 is a block diagram illustrating a PRO heat engine using
distillation in the thermal unmixing section;
FIG. 11 is a block diagram illustrating a PRO heat engine with a
distillation plant using dichlorodifluoromethane as the
solvent;
FIG. 12a illustrates a methanol-hexane binary liquid system whose
miscibility is a function of temperature, this system being useful
in effecting the thermal separation of a mixed solution containing
these two species into a diluted and a concentrated solution
respectively;
FIG. 12b illustrates a triethylamine-water binary liquid system
whose miscibility is also a function of temperature, which system
could therefore be useful in effecting the thermal separation;
FIG. 13 is a block diagram illustrating a PRO heat engine using a
FIG. 12a type binary liquid circulation system (methanol-hexane);
and
FIG. 14 is a block diagram of a PRO heat engine using a solution
containing a solute whose solubility is a function of
temperature.
PART 1 - BASIC PRO PROCESS (FIGS. 1-6)
Ordinary Osmosis (FIG. 1)
Referring to FIG. 1 illustrating the ordinary osmosic process,
there is shown a vessel 2 divided into two chambers 4, 6 by means
of a semipermeable membrane 8. Chamber 4 contains fresh water
having zero osmotic pressure and also zero atmospheric pressure.
Chamber 6 contains sea water having an osmotic pressure of 25
atmospheres and a hydraulic pressure of zero atmospheres.
Water permeates through the membrane 8 from the fresh water side 4
to the sea water side 6 because of the osmotic pressure differences
of .DELTA..pi. = 25 - 0 = 25 atmospheres. The flux, J, of the
permeant water, is given by:
J = A .DELTA. .pi. (Eq. 1)
where:
J is the flux of permeant, i.e., flow rate per unit area (a
convenient flux unit is cubic meters per day and square meter
m.sup.3 /m.sup.2 day);
.DELTA..pi. is the osmotic pressure difference (atmospheres) across
the membrane, and is 25 for the case of sea water-fresh water;
A is a constant which depends on membrane properties.
PRESSURE-RETARDED OSMOSIS (PRO) - FIGS. 2a and 2b
Now, as shown in FIG. 2a, let a hydraulic pressure, P, less than 25
atmospheres, be applied on the sea water side 6, i.e., let work be
done on the sea water. Fresh water will still permeate to the sea
water, i.e., against the hydraulic pressure gradient, but at a
reduced rate given by:
J' = A (.DELTA..pi. - P) (Eq. 2)
where (.DELTA..pi. - P) is the effective driving force, and
represents a more general statement of effective driving force than
does Equation 1.
If the process is permitted to continue, the volume under a
pressure, P, will increase on the sea water side 6 (FIG. 2b). The
total brine thus compressed has a potential for furnishing
mechanical energy by passage through a water turbine. the energy
furnished will exceed the energy (work) originally done on the sea
water by a fraction .DELTA.V/V where .DELTA.V is the volume of
permeant which has passed through the membrane and V is the
original volume of sea water. The excess mechanical energy
available is given by P.DELTA.V.
The general process of permeating against a hydraulic pressure
gradient is called "Pressure-Retarded-Osmosis" (PRO). In the above
example this is a transient process and would ultimately stop
because of dilution of sea water by the permeant. However, the
process could be carried out continuously, and several techniques
for continuous conversion are described below with respect to
different energy sources.
PRO PROCESS APPLIED TO SEA WATER AT MOUTH OF RIVER - FIGS. 3,
3a
FIG. 3 illustrates a continuous pressure-retarded process for the
production of energy from sea water at the mouth of a river, and
FIG. 3a is the related pressure-energy diagram. As will be shown,
large quantities of useful energy can be continuously extracted
from sea water close to the mouth of rivers.
In FIG. 3 the pressure-retarded osmosis (PRO) apparatus is
generally designated 10, and the semipermeable membranes are
generally designated 12. The membranes define a first pathway
(arrow 14) for the liquid of higher osmotic pressure, e.g., sea
water having an osmotic pressure of 25 atmospheres. The hydraulic
pressure of the sea water is raised by pump 16 before it is
introduced into pathway 14 of the PRO apparatus. The sea water
exits from the apparatus into a hydroturbine 17 which converts the
energy gained by it into electrical energy. The river water is
inletted at the opposite side of the PRO apparatus so as to flow
through path 18 in (preferably) counter-flow to path 14 of the sea
water, and in contact with the opposite face of the semipermeable
membranes 12. The river water is maintained at essentially zero
pressure.
Pressure-retarded osmosis is thus effected through the
semipermeable membranes 12 in the direction of arrow 19, the
permeant thereby increasing the volume of the sea water in pathway
14. The sea water in that path thus gains energy which is in excess
of that expended in pressurizing the sea water by pump 16, and this
energy is converted to electrical power by hydroturbine 17.
As one example, 1 cubic meter of sea water (as a basis) at zero
atmospheres hydraulic pressure and 25 atmospheres osmotic pressure
(Point A in FIGS. 3 and 3a) is compressed by pump 16 to 10
atmospheres hydraulic pressure (Point B), thus absorbing mechanical
energy equal to (1) (10) = 10 cubic meter - atmospheres (m.sup.3
atm) or 0.28 kilowatt hours (KWH) (Area ABEF in FIG. 3a). (For the
sake of simplicity in illustrating this process no allowance is
made in the discussion for inefficiencies in mechanical equipment,
frictional losses in fluid flow, etc.) The sea water is then passed
through pathway 14 of the PRO apparatus 10 at the hydraulic
pressure of 10 atmospheres in counterflow to river water at zero
hydraulic pressure flowing through pathway 18 on the other side of
the membranes 12. The sea water absorbs 0.6 m.sup.3 of permeant
through the membranes. Thus 1.6 m.sup.3 of diluted brine leave the
PRO apparatus at a hydraulic pressure of 10 atmospheres (Point
C).
As the sea water passes through hydroturbine generator 17, its
hydraulic pressure is released to zero (Point D) in the process of
delivering (1.6) (10) = 16 m.sup.3 atm or 0.45 KWH of energy (Area
CDEF in FIG. 3a). The net energy delivery for the 0.6 cubic meters
of permeant is (0.6) (10) = 6 m.sup.3 atm (Area ABCD in FIG. 3a).
On the basis of one cubic meter of permeant; 10 m.sup.3 atm or 0.28
KWH are deliviered, i.e., the energy/permeant ratio is 0.28
KWH/m.sup.3 permeant.
The possible power available is limited, in principle, only by the
amount of water available from the river source for permeation
through the membranes. For example, the Mississipi river delivers
56,000,000 cubic meters of water each day to the Gulf of Mexico. If
it is assumed that 3/4 of this is permeated through PRO stacks the
daily energy available would be 12,000,000 KWH, i.e., a power
output of about 500 megawatts.
The pump may be driven directly with the hydroturbine as shown by
the broken line mechanical connection 15 in FIG. 3, instead of
having a separate electric motor for the pump. Such direct coupling
might reduce capital costs and also reduce sources of energy loss
due to electric motor inefficiency.
The most attractive feature of this application of PRO (and some
subsequent ones to be described) is the fact that the "fuel" system
sea water - river, is provided without man-made apparatus, i.e.,
the solar energy is converted to a (slightly) high osmotic pressure
solution by natural means. Unfortunately the capital cost of PRO
plants for energy delivered by the use of the sea water-river water
combination would usually be excessive for two reasons, both
traceable to the relatively low osmotic pressure of sea water:
First, the energy/permeant ratio, kilowatt hours per cubic meter of
permeant, is too low, being only 0.28 KWH/m.sup.3. Second the
capital cost of the PRO apparatus would be too high. Present
equipment for reverse osmosis, another osmotic precess utilizing
appreciable hydraulic pressures, will cost in the order of 60
dollars per daily cubic meter of water permeated ($D/M.sup.3), but
at effective driving forces (.DELTA.II - P) in the order of 40
atmospheres. However in the sea water-river case the average
effective driving force would be in the order of 10 atmospheres
(see FIG. 3).
This low value of effective driving would given an undesirably low
value of the Flux, J, and thus increase the capital cost of the
membrane equipment. Therefore, it is assumed that the ratio of the
capital cost ($) to permeant flow rate, m.sup.3 /.sub.d would be
150 $D/m.sup.3 for the sea water-river water case. Based on the
above values the capital cost per kilowatt can be calculated:
##EQU1## where 24 is hours per day.
This cost is far too high when compared with present power plant
capital costs in the order of 200 $/KW.
From the above calculation on the use of PRO with sea water it is
clear that to make PRO economical as an energy conversion method,
the energy/permeant ratio must be increased and/or the capital
cost/permeant flow rate ratio must be decreased. Both of these
results can be accomplished by increasing the hydraulic pressure,
but the water-receiving solution must also have a correspondingly
higher osmotic pressure, i.e., greater than the hydraulic pressure,
as required in PRO. For the satisfaction of this requirement,
naturally available brines, in strongly saline bodies of water such
as the Dead Sea, are ideal if a lower osmotic pressure solution is
also available such as sea water, brackish springs, or a river
draining into the saline body.
PRO PROCESS APPLIED TO DEAD SEA BRINE AT MOUTH OF JORDAN -FIGS.
4,4a
FIGS. 4 and 4a show an economical scheme for using PRO together
with such an osmotic sink solution, as can be provided by the Dead
Sea and other naturally salty bodies of water. FIG. 4 illustrates
the flow diagram, and FIG. 4a illustrates the pressure-energy
diagram.
In FIG. 4, the pressure-retarded osmosis apparatus is generally
designated 20, and includes the semipermeable membranes 22 dividing
the apparatus into a first flow pathway 24 and a second
(preferably) counter-flow pathway 28, each pathway being partially
defined by the opposite faces of the semipermeable membranes 22.
The first pathway 24 is for the Dead Sea brine whose hydraulic
pressure is raised by pump 26 before being introduced into the
inlet of that pathway. The outlet of pathway 24 leads to a
hydroturbine 27 which converts the energy gained by the first
liquid to electrical power. The second pathway 28 is for the lower
osmotic pressure liquid supplied for example by the Jordan River.
The permeant from the latter water pathway passes through membranes
22 by pressure-retarded osmosis, as shown by arrow 29, thereby
increasing the volume of the liquid flowing through pathway 24,
producing therein a quantity of energy which is in excess of that
expended by pump 26.
As one example, 1 cubic meter of Dead Sea brine (.pi. = 940 atm) at
zero pressure gauge (Point A in FIGS. 4 and 4a) is compressed to a
hydraulic pressure of 200 atmospheres (Point B) after which it is
passed through the PRO apparatus 20 at this pressure in counterflow
to the Jordan River water at zero hydraulic pressure on the other
side of the membrane. (In the calculations on Dead Sea brine, it is
assumed that the osmotic pressure-concentration relations will be
the same as for magnesium chloride). Each cubic meter of Dead Sea
brine receives 0.8 cubic meters of permeant 29 at 200 atmospheres
pressure (Point C) after which the diluted solution (.pi. = 515
atm) passes through the hydroturbine generator 27 where its
hydraulic pressure is reduced to zero (Point D) in delivering a net
energy output for the 0.8 m.sup.3 of permeant, of 160 m.sup.3 atm
(Areas ABCD in FIG. 4a). On the basis of one cubic meter of
permeant 200m.sup.3 atm or 5.6KWH are delivered, i.e., the
energy/permeant ratio is now 5.6 KWH/m.sup.3 permeant.
As can be seen in FIG. 4 the effective driving force, (.DELTA..pi.
- P), at the Dead Sea Brine and Jordan River inlets of the PRO unit
are 738 and 114.5 atmospheres respectively. Because of these high
effective driving forces, which will give high flux values for
permeant, the capital cost of the PRO unit should now be much lower
than in the sea water case where effective driving forces were very
low. It is assumed that for Dead Sea Brine use the cost will be 60
$/m.sup.3 of permeant. This is in the order of magnitude of present
reverse osmosis equipment capital costs.
Thus capital cost per kilowatt hour now becomes: ##EQU2##
To this cost must be added the cost of the hydroturbine generator,
estimated at 75 $/KW. The total installed cost will then be 315
$/KW. This figure is low enough to merit comparison of PRO with
existing power plants. Assuming that the capital cost is paid for
at 8 percent per annum, the contribution of the capital cost to the
cost of the power is: ##EQU3##
From the foregoing analysis it is clear that, generally speaking,
the higher the hydraulic pressure the cheaper will be the capital
cost per kilowatt. However, as the hydraulic pressure is increased
there must be, as can be seen from considering FIG. 4a, a
simultaneous decrease in the permissible ratio of diluted Dead Sea
Brine to Dead Sea Brine entering the apparatus. High values of this
ratio are desirable to allow for hydraulic pressure losses due to
friction in the system and for the fact that the pump and
hydroturbine have efficiencies less than 100 percent.
(Incidentally, it can be demonstrated that for positive net energy
delivery from the system the ratio of diluted to entering brine
must exceed 1/(hydraulic efficiency) (pump efficiency)
(Hydroturbine efficiency) where hydraulic efficiency is the ratio
of the hydraulic pressure entering the hydroturbine to the
hydraulic pressure leaving the pump).
These two apparently mutually exclusive requirements, high
hydraulic pressure and high ratio of diluted to entering Dead Sea
Brine, can both be met by staging.
PRO 3-STAGE PROCESS - FIGS. 5, 5a
FIGS. 5 and 5a show a possible 3-stage unit. Each stage consists of
one of the pressure-retarded osmosis apparatuses described above
(these being designated 30, 40 and 50, respectively) and a
hydroturbine (37, 47, 57) at the output end of the higher osmotic
pressure liquid pathway. A pump 36 at the inlet end of the first
stage raises the hydraulic pressure of the high osmotic pressure
liquid (e.g., Dead Sea Brine) to a very high/hydraulic pressure,
for example 350 atmospheres. This pressure is reduced to about 200
atmospheres in hydroturbine 37 at the end of the first stage, and
is inletted at this pressure into the second stage 40. The
hydroturbine 47 at the outlet of the second stage drops by pressure
further, for example to about 108 atmospheres, before the liquid is
introduced into the third stage 50, dropping to 0 atmospheres at
the outlet of hydroturbine 57 of the third stage.
The low osmotic pressure liquid pathway is not illustrated in FIG.
5, but it will be appreciated that it would be as described in the
previous examples.
Thus, in each stage water from the low osmotic pressure liquid
permeates into the high osmotic pressure liquid, causing the latter
to gain energy; while the hydraulic pressure of the high osmotic
pressure liquid is very high at the first stage, and is
successively lowered while energy is delivered by the respective
hydroturbine. By this means, hydraulic pressures as high as 350
atmospheres may be utilized, and the final ratio of diluted to
entering Dead Sea Brine may be about 2.2, as shown.
In some parts of the World solid salt is stored in large quantities
on or below the surface. This salt could be used as "fuel" for
energy conversion in accordance with the invention by adding the
salt to the water to produce the higher osmotic pressure liquid. In
this case the costs would be greater than for the previous
processes, because of the cost of dissolving the salt, and also the
cost of disposing the diluted salt solution. Nevertheless, this use
of salt as "fuel" could very well still be commercially feasible in
some locations.
In the methods described above, naturally occurring bodies of water
or salt or used for producing the power in accordance with the
invention. The invention, however, may also be exploited by
artificial or man-made means, by the use of artificial solar or
evaporation ponds which collect the solar energy and produce the
high osmotic pressure solution.
PRO PROCESS APPLIED TO EVAPORATION PONDS - FIG. 6
FIG. 6 illustrates the flow diagram of a process in accordance with
the invention applied to brines of Dead Sea concentrations produced
by evaporation ponds. In this case, the pressure-retarded osmosis
apparatus 60 is fed by concentrated brine from the outlet end of an
evaporation pond 70, this brine being first pressurized, e.g., to
about 200 atmospheres by pump 66. The diluted brine exiting from
apparatus 60 is passed through hydroturbine 67, where its hydraulic
pressure drops to 0, and is then introduced into the inlet and of
the evaporation pond 70. The lower osmotic pressure solution may be
river water which is concentrated in the apparatus, after which it
may be discharged or salvaged for other use.
The new feature provided by the evaporation pond system of FIG. 6
amounts to passing from point D to point A in the diagram of FIG.
4a, and this will add somewhat to the costs of the process. The
unit cost of the evaporation pond is estimated at 100 dollars per
daily cubic of water evaporated. The cost of the evaporation ponds
per kilowatt hour will then be: ##EQU4##
The total capital cost per kilowatt hour will be: 315 + 428 = 743
$/KW
The above capital cost, although considerably more expensive than
the cost of presently built power plants, is much cheaper than the
estimated cost of solar energy power plants.
The efficiency of the process from the standpoint of solar energy
conversion, can be calculated from the scheme using evaporation
ponds. In this scheme 5.6 KWH are delivered per cubic meter of
water permeated. In gram calories this is (4.8)(10).sup.6 gm
Cal/m.sup.3. To evaporate a cubic meter of water requires
(580)(10).sup.6 gm Cal/m.sup.3 of solar energy. Therefore the
thermal efficiency of the process is (4.8/580)(100) = 0.83 percent.
The thermal efficiency could be improved by operating at a
hydraulic pressure much closer to the osmotic pressure, but this
would require a very low ratio of diluted Dead Sea Brine to Dead
Sea Brine entering the PRO apparatus.
In the above described processes, the low osmotic pressure solution
is concentrated. Thus, the process could also be used not only for
the production of power, but also for the recovery of concentrated
solution or solutes.
Also, while the energy gained by the higher osmotic pressure liquid
is converted to power by passing same through a hydroturbine
generator, it will be appreciated that this energy can be stored in
the liquid itself, e.g., by using it for pumping same to a higher
elevation until needed, or until subsequently used for generating
power.
PART 2 - PRO PROCESS OF MORE GENERAL APPLICABILITY (FIGS. 7-14)
PRO HEAT ENGINE, GENERALIZED CONCEPT (FIG. 7)
As indicated earlier, the systems shown in FIGS. 3-6 to illustrate
the basic PRO process would be of limited applicability because of
the need for large solar energy collecting surfaces and the need
for expendable material. Now described are systems which eliminate
both of the above limitations and therefore make the PRO process of
more general applicability.
The new system, as shown in its most generalized form in FIG. 7,
consists of two sections. First, there is the pressure-retarded
osmosi (PRO) section 102, which delivers work available from the
free energy decrease occurring during mixing of a diluted and a
concentrated solution. Second, there is the thermal unmixing
section 104, in which heat absorption and rejection supplies energy
for free energy recovery, i.e. unmixing and temperature restoral to
the two solutions after which they are returned via lines 106, 108
to the PRO section 102, thus completing the cycle. The thermal
unmixing process can involve intermediate gaseous or solid phases
in the course of restoring the original solutions or can be
conducted so that the original liquid solutions are directly
produced.
The above description fits in every respect the definition of a
heat engine, i.e., a man-made device which makes it possible for a
working substance to undergo a cyclic process in the conversion of
heat into work. Thus the invention is believed to represent the
first use of any osmotically-operated power process as the work
producing component of a heat engine. The invention is also
believed to represent the first use of pressure-retarded osmosis as
the work-producing component of a heat engine.
The heat source, diagrammatically indicated by arrow 110 in FIG. 7,
for effecting the thermal unmixing in section 104, could
theoretically be any energy source such as fossil fuels or nuclear
energy. However, because of the dwindling energy resources from
fossil fuel, only solar energy will hereinafter be mentioned.
With regard to solar energy collectors, several promising types
exist. These include solar ponds, i.e., ponds in which the solar
energy maintains a gradation in temperature by means of a gradation
in salt concentration, and solar heating devices such as those
incorporating selective coatings for trapping solar energy in the
form of heat. With both of these types the heat can be extracted by
the passage through the unit of a heat-transfer fluid. Solar
distillation plants are also energy collectors. In addition to this
service they also perform the unmixing functions.
The heat sink into which the heat from the thermal unmixing section
104 is rejected as indicated by arrow 112, is preferably of the
liquid type such as sea water or river water. However ambient air
can also serve as an appropriate heat sink. It is assumed herein
that any of these potential sinks can provide a heat rejecting
temperatures not exceeding 25.degree. C.
In FIG. 7, the net work output from the PRO section 102 is
indicated by arrow 114 and block 116, the mechanical and hydraulic
losses from the PRO section being passed to the heat sink as
indicated by arrow 118.
PRO HEAT ENGINE AS CARNOT ENGINE (FIG. 8)
FIG. 8, in which the parts corresponding to FIG. 7 are
correspondingly numbered, shows an idealized PRO heat engine
operating between temperature limits of 100.degree. and 25.degree.
centigrade. It is assumed that its heat source is a solar energy
collector 120 which loses (arrow 123) half the solar energy
incident upon it (arrow 121). This PRO engine is subject to the
maximum efficiency imposed by the law of Carnot: ##EQU5##
where 273.degree. is absolute temperature at 0.degree.
centigrade.
Based on this limitation and the above assumptions it is seen that,
of 100 units of energy incident on the solar energy collector, a
maximum of 10 units can be obtained as useful work. The useful work
obtained from an actual PRO engine must be measured against this
criterion, i.e., the useful work from the PRO engine will always be
less than, but should approach as closely as possible to, 10
percent of the energy incident on the solar energy collector.
Because the PRO heat engine obtains its work from the free energy
decrease during mixing of the solutions, the maximum work
obtainable, i.e., the Carnot work, is equal in magnitude to this
decrease in free energy.
Following are a number of important advantages of the PRO heat
engine:
1. By making pressure retarded osmosis the work-producing process
of a heat engine, the working liquids undergo a cyclic process and
are therefore not expended at all. The production of useful energy
by PRO becomes of general applicability, requiring only the
availability of solar or other energy source and no material
resource expenditure. This extends the usefulness of the technique
of the above-cited patent specification in which applicability of
the systems therein described as examples was limited to locations
having an expendable source of low osmotic pressure solution such
as river water, brackish water, or sea water.
2. The PRO heat engine can be efficient in the conversion of energy
to work. As will be shown, the thermal efficiency of the PRO engine
can be in the order of at least 60 percent of that
thermodynamically possible. This means that when operating between
the limitations of the previous section 100.degree. to 25.degree.
centrigrade, and with 50 percent losses in the solar energy,
collector, the PRO heat engine will convert (0.6)(10) = 6 percent
of the incident solar energy to useful work.
This thermal efficiency will enable the solar collector area to be
drastically reduced in comparison to what is required with the
evaporation ponds described as examples in the above-cited patent
application. For a 1,000 megawatt PRO heat engine this area need be
only in the order of 60-70 kilometers as compared with about 1,000
square kilometers for evaporation ponds.
3. The PRO heat engine possesses practical advantages over existing
heat engines. Most existing heat engines utilize the vapor power
cycle. In such a cycle, a working fluid under pressure is vaporized
by addition of heat from a high temperature heat source. It then
does work of expansion in a turbine or engine, after which it is
condensed by heat removal to a low temperature heat sink. The
liquid is compressed to the original pressure, thus completing the
cycle.
Because of the phase changes occurring in the vapor power cycle, it
must be conducted within limits which add to the cost of necessary
equipment. For example the expansion of saturated steam in a heat
engine is accompanied by partial condensation to liquid water.
However, this liquid content cannot exceed 10 or 12 percent because
of excessive wear on turbine blades or engine pistons. Thus the
expansion of the steam is limited. This limitation can be minimized
by super-heating the steam prior to expansion, but this adds to the
cost of the equipment, and for the low heat source temperatures
considered herein of 100.degree. C, no additional efficiency is
gained.
An additional limitation of the vapor power cycle using steam lies
in the fact that all the vapor must be finally condensed. This
means that at the heat rejection temperature of 25.degree. C, a
vacuum must be maintained in the condenser, including means for
continuous removal of air which might leak in through pump seals,
etc.
The PRO heat engine overcomes the above limitations of the vapor
power cycle because no vapor exists during the power production
part of the cycle (and indeed need not exist in any part of the
cycle, as will be seen).
4. Another very important advantage of the PRO heat engine is that
it provides a liquid under high hydraulic pressure as the energy
producing fluid. By this means the engine can utilize a high
pressure hydroturbine. The work is used here to mean any
liquid-driven turbine, in contrast to a steam turbine or other
turbine driven by gas or vapors other than a steam turbine.
Hydroturbines are more efficient than steam turbines or other vapor
turbines. Furthermore, a hydroturbine is inherently safer since at
high pressures much less energy is stored in a liquid than in a
gas.
PRO SECTION OF HEAT ENGINE (FIG. 9)
FIG. 9 illustrates the PRO section 102 (of FIG. 8) of the heat
engine; FIGS. 10-14 (described below) illustrate different
arrangements which may be used for the thermal unmixing section 104
(of FIG. 8).
The pressure-retarded osmosis (PRO) section, consisting of a pump
122, the membrane unit 124, and a hydroturbine 126, is shown in
FIG. 9 as it would operate under ideal conditions. A concentrated
solution, by which is meant one having a high osmotic pressure
(.pi..sub.high), and having a volume of V cubic meters (m.sup.3) is
pressurized by pump 122 to a hydraulic pressure P atmospheres (atm)
requiring a work input of PV cubic meter atmospheres (m.sup.3 atm),
after which it is pumped via line 127 into the high pressure side
of the membrane unit 124. Simultaneously a diluted solution, by
which is meant one having a low osmotic pressure, (.pi..sub.low),
and having a volume of .DELTA.V m.sup.3 is pumped (by a pump not
shown) via line 128 into the low hydraulic pressure side of the
membrane unit 124. The diluted solution permeates through the
membranes against the hydraulic pressure P because it is arranged
that everywhere in the unit P>.DELTA.P where .DELTA.P is the
osmotic pressure difference (atm) between the solutions on each
side of the membrane. This is the fundamental principle of
pressure-retarded osmosis, as described above.
A volume (V + .DELTA.V) m.sup.3 of mixed solution is sent to
hydroturbine 126 at the pressure P atm. Thus the hydroturbine
delivers P (V + .DELTA. V) m.sup.3 atm of work (via connection 129)
in the course of reducing the pressure of the mixed solution of
zero. The net output of work is equal to the difference between the
output from the hydroturbine and the input to the pump, i.e., the
net work is (P.DELTA.V)( m.sup.3) atm.
It is important to understand that net work is obtained only from
.DELTA.V, the volume of permeant liquid passing through the
membranes. In order to minimize the size of the membrane unit it
may be stated as a first guideline:
Guideline 1: the ratio should be maximized of net work delivered to
volume of liquid passed through the membranes.
This is accomplished by using a high hydraulic pressure. However P
must be less than .DELTA.P everywhere in the unit, as described
above, and the minimum .DELTA.P occurs between the diluted solution
and the mixed solution. Therefore it follows as a corollary to
Guideline 1 that the osmotic pressure difference between the mixed
solution and the diluted solution should be high.
It should be realized that Guideline 1 is also appropriate for the
thermal unmixing section (104, FIGS. 7 and 8). If the ratio is high
of net work delivered to volume of permeate passed through the
membranes, then less mixed solution must be separated by the
thermal unmixing techniques.
Thermal unmixing techniques may be divided into several categories
depending on the nature of the intermediate phases employed in the
unmixing. Thus it is possible to utilize vapor and solid
intermediate phase as well as to divide the mixed solution directly
into diluted and concentrated solutions.
The best thermal unmixing technique would seem to be that which
minimizes the thermal energy input requirement. A guideline for
carrying out this requirement can be cleaned from FIG. 8. Since
this is a Carnot cycle, the thermal efficiency is maximized.
Therefore the ratio of thermal energy input to net work is
minimized. Since the net work is equal to the free energy decrease,
it is possible to say that the ratio of thermal energy input to
free energy decrease is also minimized. This value is 50/10 = 5 for
the temperature chosen. In any actual plant, the ratio will be
higher but this value of five may be considered as a target at
which to aim.
Therefore it may be stated as a second guideline:
Guideline 2: Within thermodynamic limitations, the ratio of thermal
energy input to free energy decrease should be minimized in the PRO
heat engine.
THERMAL UNMIXING BY DISTILLATION (FIGS. 10 and 11)
The use of distillation as the thermal unmixing process is shown in
general terms in the heat engine of FIG. 10, parts corresponding to
those in FIG. 9, being correspondingly numbered. The distillation
plant 130, divides the unpressurized mixed solution from input 132
into a first output 134 of V m.sup.3 unpressurized concentrated
solution having a high value of osmotic pressure (.pi..sub.high),
and a second output stream 136 of a diluted solution in the form of
a vapor. The concentrated solution in stream 134 is joined to the
PRO section of FIG. 9 at the pump 122. Meanwhile, the diluted
solution vapor in stream 136 is condensed in condenser 138 and
cooled, thus rejecting part of the incoming thermal energy to the
heat sink as shown by arrow 140, after which the diluted solution
is joined via line 128 to the PRO section at the low pressure side
of the membrane unit. The remainder of the cycle is as described
above in the discussion on the PRO section, as a result of which
the unpressurized mixed solution of FIG. 9 is sent to the
distillation plant for thermal unmixing, and the cycle completed.
The heat for the distillation plant is supplied from a solar energy
collector 142.
Water is a poor choice as a solvent in a distillation plant in the
thermal unmixing section of a PRO heat engine. Specifically it
fails badly to meet Guidline 2 because of its very high volumetric
latent of vaporization of 580 cal/cm.sup.3. Even with concentrated
salt solutions for which the free energy of separation will be in
the order of 5 cal/cm.sup.3, the ratio of heat input to free energy
of separation will be in the order of 580/5 = 120. This factor
would increase the solar collector area requirement for a 1000
megawatt plant from the theoretical minimum value of about 4
km.sup.2 (assuming an average insolation flux of 520 calories per
day on each square centimeter) to 500 km.sup.2, if water were used
in a simple distillation process such as solar distillation. If it
is more realistically assumed that the solar collector efficiency
is about 50 percent, 1000 km.sup.2 of solar collector area would be
required. Other losses might increase the area requirement
further.
The second guideline may in principle be approached more closely in
distillation plant by employing the use of "heat multiplying"
distillation plants such as multiple effect or multistage flash
distillation plants. In these the arrangement is such that one
kilogram of steam (or its thermal equivalent) entering from the
heat source is capable of vaporizing, say 10 pounds of steam, thus
increasing the efficiency of the process 10-fold. However there is
a limitation on the heat multiplying capability. In each effect or
stage of the plant, the vapor condenses at the boiling point of
pure water characteristic of the pressure in the effect. However
the concentrated brine in the same effect boils at a higher
temperature because of its salt content. This phenomenon in known
as boiling point elevation (BPE). Its effect is cumulative from
effect-to-effect and the possible heat multiplying capability of
the plant is limited by the available source-to-sink temperatures
such that: ##EQU6##
Now we know that boiling point elevation increases directly with
osmotic pressure difference such that we may say: ##EQU7##
However it was stated as a corollary to the first design guideline
that the osmotic pressure difference between the mixed solution and
the diluted solution should be as high as possible. The complete
implementation of this guideline would reduce the heat multiplying
capability of the plant drastically. This capability is further
reduced by the fact that for solar energy heat sources discussed
herein, the difference between source and sink temperature is low.
Therefore a distillation plant to be described subsequently is not
of the heat multiplying type, and it is explained why such a plant
would not be feasible with solar collectors as the energy source.
However it is recognized that heat-multiplying distillation plants
might be efficacious under other conditions.
The disadvantage of water as the solvent phase in a PRO heat engine
may be overcome by using a solvent with a low value of volumetric
latent of vaporization. Virtually all liquids have a volumetric
latent heat of vaporization lower than water. Some of these such as
the halogenated organic compounds, and especially those containing
fluorine, are exceptional in this regard. For example
dichlorodifluoromethane C C1.sub.2 F.sub.2, a commerically
available refrigerant ("Freon-12") has a volumetric latent heat of
vaporization of only 44 cal/cm.sup.3. By the use of such materials
Guideline 2 is approached more closely than with water. From a
practical standpoint the use of Freon 12 means that the solar
energy collecting area can be an order of magnitude less in area
than if water is used, all other things being the same.
FIG. 11 (parts corresponding to FIG. 10 being correspondingly
numbered) illustrates a PRO heat engine with a distillation plant
using C Cl.sub.2 F.sub.2, dichlorodifluoromethane, as the solvent
in the dilute solution. For this calculation the following
conditions and/or assumptions were utilized:
1. A relatively non-volatile solute of molecular weight 46 was
used; e.g., Ethanol meets this requirement.
2. The osmotic pressure of the mixed solution was 275 atmospheres.
This value is sufficiently high that a high hydraulic pressure in
the order of 255 atm can be used, according to Guideline 1.
3. The efficiencies of the solar collector, the pressurizing pump,
and the hydroturbine were assumed to be 50 percent (thermal), 95
percent (mechanical), and 95 percent (mechanical),
respectively.
4. It was assumed that there was a 2 atmosphere pressure drop due
to hydraulic friction in any unit of the apparatus.
5. Raoult's Law was assumed to apply to all solutions.
6. The system was operated so that 1 volume of concentrated
solution was diluted with 0.4 volume of solvent, C Cl.sub.2
F.sub.2.
7. The density of all solutions was assumed to be one.
8. The temperature of the condensate leaving the condenser was
25.degree. C.
Based on these assumptions, the operating conditions of FIG. 11
were obtained. Of special interest is the requirement that the
concentrated solution have a temperature of 94.degree. C, as
compared with the condensate temperature of 25.degree. C. The
temperature of 94.degree. C is due chiefly to two requirements:
first, the mol fraction of CCl.sub.2 F.sub.2 must be low in the
concentrated solution in order to provide a high enough osmotic
pressure to the concentrated solution, second, the partial pressure
of CCl.sub.2 F.sub.2 in the concentrated solution must be at least
equal to the partial pressure of pure CCl.sub.2 F.sub.2 in the
condensate at 25.degree. C in order to condensate this solvent.
Even with the use of a single-effect distillation plant, the
temperature of 94.degree. C is approaching the assumed upper
temperature limit from a solar collector. For this reason it is
clear why heat multiplying plants may not be feasible with solar
collectors. The total temperature difference must be at least the
sum of the minimum permissible temperature differences (boiling
point elevation) in each stage, and this sum might exceed the
temperature difference possible with a solar energy collector.
Of most interest are the figures showing the distribution of
energy, based on 100 units of solar energy entering the solar
collector. It is seen that the net work is 4.5 units, i.e., the
overall efficiency is 4.5 percent. This means that a PRO heat
engine using CCl.sub.2 F.sub.2 in this manner would require 4/.045
= 90 km.sup.2 area for a 1000 megawatt plant, as compared with
about 1000 km.sup.2 for a plant using water as the solvent.
As stated above, Freon 12 is an exceptional material. It may be one
of the relatively few solvents with a latent heat of vaporization
sufficiently low to overcome the basic deficiency when distillation
is used as the thermal umixing technique, namely that the latent
heat of vaporization is usually too high in comparison to the free
energy of separation.
THERMAL UNMIXING BY SEPARATION INTO TWO LIQUID PHASES (FIGS. 12a,
12b, 13)
There may also be used a thermal unmixing system involving
separation into two liquid phases, a technique which should be
inherently more efficient than distillation since no latent heat of
vaporization is involved.
A number of binary liquid systems are distinguished by the fact
that their mutual solubility is a strong function of temperature.
FIG. 12a shows such a system, methanol-hexane. Above a temperature
of 42.6.degree. C, known as the upper consolute temperature, the
two species are miscible in all proportions. Below this
temperature, say at 25.degree. C, the liquids are only partially
miscible, and two liquid phases exist in equilibrium, a 5 percent
solution (by weight) of the methanol and a 95 percent solution of
methanol. These two solutions are called conjugate solutions.
This behavior immediately suggests means of thermal unmixing in a
PRO heat engine. Assume that a methanol-hexane solution is 22
percent methanol and at a temperature appreciably higher than
42.6.degree. C. The solution is cooled down to 25.degree. C at
which temperature the 5 and 95 percent methanol solutions form. The
amount of each solution will be determined by the lever arms y and
X such that: ##EQU8##
These two solutions can be reheated above 42.6.degree. to a
temperature region where they are again naturally and completely
miscible. At this temperature the total free energy of these two
separated solutions is higher than that of the solution obtained by
mixing them. (The free energy of a system decreases when it
undergoes a natural process). Therefore the decrease in free energy
upon mixing can be utilized to produce useful energy by means of
the PRO section. After passage through the hydroturbine, the mixed
solution is ready for cooling, and the cycle is completed.
The utility of this method is not limited to liquids having an
upper consolute temperature. Binary systems such as triethylamine
and H.sub.2 O exhibit a lower consolute temperature, as can be seen
in FIG. 12b. With such systems unmixing is accomplished by a
temperature rise, followed by cooling of the separated liquids to
bring them into the miscible range. Next, mixing occurs in the PRO
section to produce useful energy; the mixed solution is heated to
separate the concentrated and diluted solutions; and the cycle is
completed.
The advantage over distillation of either type of such binary
systems in the PRO heat engine is that the phase changes are
liquid-liquid and not liquid-vapor. Thus the thermal energy input
need not include values required by the latent heat of
vaporization, but can approach much more closely to the free energy
of separation in accordance with Guideline 2.
FIG. 13 (parts corresponding to those of FIG. 11 being
correspondingly numbered) shows a PRO heat engine including a
liquid separator, generally designated 156 which employs
methanol-hexane as the liquid-system whose miscibility is a
function of temperature. The methanol-hexane system was chosen to
illustrate this method because its upper consolute temperature of
42.6.degree. C is between the maximum temperature of 100.degree. C,
assumed to be the heat source temperature with a solar energy
collector 142 and 25.degree. C, assumed to be the lowest
temperature to which the system can be conveniently cooled.
The following other conditions and assumptions were utilized:
1. The concentrated solution, diluted solution, and mixed solutions
have the compositions of FIG. 12a. The weight ratio of concentrated
to diluted solution is also as shown in FIG. 12a, i.e., 100/40 =
2.5, and this is equivalent, for reasons of density difference, to
a volume ratio of 1/0.36 = 2.8.
2. Raoult's Law applies to all solutions in the PRO section.
3. Efficiencies of the solar collector 142, the pump 122, and the
hydroturbine 126 are as before 50, 95 and 95 percent
respectively.
Based on these conditions and assumptions, the conditions of FIG.
13 were obtained. The mixed solution temperature, after passing
through the hydroturbine 126, is 95.degree. C. It passes through a
heat exchanger 152 where it is partially cooled to 48.degree. C in
the course of preheating the separated solutions. The mixed
solution is then cooled to 25.degree. C by heat rejection to the
cooler (arrow 154). This causes the concentrated and diluted
solutions to form. These are divided in the separator 156, after
which they pass through the heat exchanger 152 where they are
heated to 72.degree. C. They then pass through the solar energy
collector 142 where they are heated to 100.degree. C. The
concentrated solution is compressed by pump 122 to 360 atm, which
warms it to 124.degree. C after which it passes into the membrane
unit 124 where it absorbs the diluted solution, so that the
temperature is reduced to 117.degree. C. In the membrane unit, the
pressure drops to 358 atm on the mixed solution, after which it is
depressurized through the hydroturbine 126, producing work, and
reducing the temperature of the solution to 95.degree. C. This
completes the cycle.
Also shown in FIG. 13 are the energy distributions. We consider the
solar energy incident on the collector as 100 percent. Half of this
is lost in the solar collector, and thus 50 percent goes forward to
the heat engine. If this PRO engine were ideal, i.e. employed the
Carnot cycle, then 10 percent of the solar energy incident on the
solar collector would be available for work as shown in FIG. 8.
However the cycle is not a Carnot cycle since temperature
absorption and rejection are not all accomplished at the maximum
and minimum temperature. respectively. Therefore it is assumed that
only 9 percent of the solar energy is available to supply the free
energy of separation. This is a higher value than was obtained with
distillation and follows from the fact that the separation into the
two solutions is accomplished without an intermediate vapor phase,
and thus no energy must be utilized to supply the latent heat of
vaporization.
Of this 9 percent, 6 percent will be available for useful work and
3 percent will be unavailable due to mechanical and hydraulic
losses in the circulating streams, and to the fact that some
fraction of the available free energy is lost in providing an
adequate driving force in the PRO section for permeant
transfer.
THERMAL UNMIXING BY USING SOLUTE WHOSE SOLUBILITY IS A FUNCTION OF
TEMPERATURE (FIG. 14)
It is well known that solubility of salts and other solutes is a
function of temperature. This behaviour in which can intermediate
solid phase is produced, can be used as a basis for operation of
the thermal unmixing section of a PRO heat engine. The technique
can be used for solutes whose solubility either decreases or
increases with temperature. This is illustrated in FIG. 14 for a
solute whose solubility increases with temperature. The solubility
characteristics of potassium nitrate are such that it could meet
the solution and precipitation requirements shown.
As shown in FIG. 14 (parts corresponding to those of FIG. 13 being
correspondingly numbered), the unpressurized mixed solution,
containing 60 parts salt and 80 parts water, is 95.degree. C after
passing through the hydroturbine 126. It passes through a heat
exchanger 152 where it is cooled to 48.degree. C in the course of
preheating the filtrate (to be discussed) and the unpressurized
concentrated solution. The mixed solution, which is now almost
saturated with regard to the salts, is then cooled to 25.degree. C
by means of the cooler 160. This causes salt to precipitate. The
slurry of salt and solution is sent to a filter 162 which separates
the slurry into a solid phase containing 30 parts of salt and a
filtrate containing 30 parts of salt and 80 parts of water. The
filtrate is warmed to 72.degree. C by passing through the heat
exchanger via line 164. The hot filtrate is divided into two parts.
One part, the diluted solution, containing 11 parts salt and 29
parts water, is sent via line 166 to the solar energy collector 142
for final heating. The remainder, 19 parts salt and 51 parts water,
is sent via line 168 to the dissolver 170 where it dissolves the 30
parts of salt from the filter.
The solution emerging from the dissolver is the unpressurized
concentrated solution, and it contains 49 parts salt and 51 parts
water. It is assumed that in the dissolving process the solution
temperature drops to 55.degree. C. (The actual temperature attained
will depend on the heat of solution, which can be positive or
negative). The concentrated solution is passed via line 172 through
the heat exchanger 152 where its temperature is raised to
72.degree. C, the same as that of the diluted solution. These
solutions then go through the same processes in the PRO section as
described for partially miscible liquids, the cycle being completed
with the unpressurized mixed solution emerging from the
hydroturbine 126.
This technique for thermal unmixing has the same basic limitation
as distillation, namely the changing of one of the components into
a phase other than a liquid phase. The energy required for this may
be high compared to the free energy of separation. However it
appears that by a judicious choice of solute this process can be
made efficient.
Many other variations and applications of the invention will be
apparent.
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