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A refrigerator-heat-pump desalination scheme for fresh-water and salt recovery
Ennerg~ Vol. 9. No. 7. P,T 583-588, Printed in the U.S.A.
1984 0
03&5442/84 S3.00 + .oo 1984 Pergamon Press Ltd.
A REFRIGERATOR-HEAT-PUMP DESALINATION SCHEME FOR FRESH-WATER AND SALT RECOVERY
ENEL-DSR-CRIS,
M. &AL1 Via Omato 90/14, 20162 Milano, Italy
(Received 8 September 1983) Abstract-This study concerns a refrigerator-heat-pump desalination scheme (RHPDS), which allows energy-efficient recovery of fresh water and salt from the sea. In this scheme, a salt-water chamber is continuously refilled with sea water via atmospheric pressure. Sea water is evaporated into a vacuum chamber and the water vapor is condensed on top of a fresh-water chamber. A refrigerator-heat-pump circuit maintains the two water chambers at suitably different operating temperatures and allows efficient recovery of the latent heat of condensation. The scheme is analyzed with special consideration to potential exploitation of renewable energy sources such as solar and wind energy.
INTRODUCTION
Fresh water and salt are essential for the life of man and for innumerable living beings. They have consequently been sought after since time immemorial and attempts at recovering them have been made even in the most hostile environments. Starting with the industrial revolution and up to the present time, plants of ever-increasing complexity have been designed for the recovery of either fresh water or of salt or of both from sea water, brackish water, and hypersaline water bodies. *AThese developments notwithstanding, lack or scarcity of fresh water and salt remain serious problems for many regions of the earth, as was recently emphasized by UNESCO, which declared the decade 1980-90 officially as dedicated to fostering research to ensure a sufficient supply of fresh water for all countries in need. In this paper, we introduce a refrigerator-heat-pump desalination scheme (RHPDS) for energy-efficient recovery of fresh water and salt from the sea. Since the operation of the RHPDS is based on the control of vapor pressures of sea water and fresh water, the scheme itself will be described after we have dealt with the vapor-pressure difference between sea and fresh water. The special features of the RHPDS are analyzed in some detail, and proper consideration is given to the important potential exploitation of any available energy source such as waste heat from thermoelectric power plants and solar energy or wind energy. VAPOR-PRESSURE
DIFFERENCE
BETWEEN
SEA
AND
FRESH
WATER
In the temperature range 273-373 K, the vapor pressure of sea water is N 1.84% less than that of fresh water.’ The difference between the two pressures increases with temperature, as is shown in Fig. 1. If, as shown in Fig. 2, two vessels containing deaerated fresh and sea water are kept at the same temperature T, 273 < T < 373 (K), and have their tops connected, fresh water will distill into the sea-water vessel, unless the fresh-water level is so much lower than the sea-water level for thermodynamic equilibrium to be maintained by gravity. For example, if T = 298 K, the equilibrium level difference should be w 240 m, as can be verified by applying the barometric law to the water-vapor pressure. Clearly, the vapor pressure of sea water must be increased by some means if we want water to be distilled from a sea-water vessel onto a fresh-water vessel. Inspection of Fig. 1 suggests that distillation of sea water can be obtained if the distilled-fresh-water temperature is kept lower than the sea-water temperature by a suitable amount. This simple observation represents the physical foundation for the operation of the RHPDS. 583
M.
584
REALI
800
700
500
500
2 LOO
s CL'
300
200
100
363
373
Fig. 1. Temperature dependences of the fresh-water vapor pressure p”, and of the vapor-pressure difference between fresh and sea water Ap,, in the temperature range 273-373 K.
Uniform
Temperature
I
,yy-Y
‘.
.. ; ‘.
4---r -_ -- -
__.
_
_
Sea
z--- -
-
Water
-FE?Sh
water
Fig. 2. Two vessels containing deaerated fresh and sea water and kept at the same temperature T (273 < T < 373 K) are connected at their tops. Water is distilled from the fresh-water vessel to the sea-water vessel, unless the fresh-water level is so much below the sea-water level that gravity can balance the vapor-pressure difference between fresh and sea water (at 298 K, the equilibrium level difference would be _ 240m).
Important physical properties of water to be taken into account for a proper design are the latent heat of vaporization I and the specific heat cp. Numerical values of 1 in the temperature range 273-373 K are presented in Fig. 3 (the data are from Ref. 5). In the same temperature range, cp shows only slight variations, which have a negligible practical influence on the operation of the RHPDS. The values? of cp are obtained from 10, and are, respectively, 4.127, 4.193, 4.182, q.=(273+Jx lO)(K)withJ=0,1,2,..., 4.179, 4.179, 4.181, 4.185, 4.190, 4.197, 4.205, and 4.216 (kJ/kg-K).
A refrigerator-heat-pump
desalination
scheme for fresh-water and salt recovery
585
1
2500
2400
-
2300
-
w
2200. 213
293
373
353
333
313
Temperature PKI
Fig. 3. Temperature
dependence
of the latent heat of vaporization 273-373 K.
of water (I)
in the range
Table 1. Selected values of boiling-point elevations of sea-salt solutions and of the vapor pressure of saturated brine for 273-373 K. vapor pressure of sat&d brine.
Boiling point elevation of sea-salt solutions, 'K
3.45
Weight % sea salt 4.0 I 6.0 8.0
10.0 I-12.0
0
0.253
0.295
0.459
0.637
0.835
1.053
10
0.276
0.323
0.503
0.701
0.920
1.164
, 0.300 ~0.312
0.351
0.547
0.765
1.006
1.275
0.365
0.570
0.797
1.049
1.330
0.184
0.324
0.380
0.593
0.830
1.093
1.386
0.198
0.350
0.409
0.640
0.896
1.181
1.499
0.213
0.376
0.440
0.689
0.965
1.272
1.614
0.228
0.403
0.472
0.739
1.035
1.364
1.731
70
0.244
0.431
0.505
0.790
1.107
1.459
1.851
80
0.261
0.460
0.538
0.843
1.180
1.556
1.973
90
0.278
0.489
0.573
0.897
1.256
1.655
2.098
100
0.295
0.953
1.334
1.756
2.225
20
25 30 40
44
50 60
117
The thermodynamic properties when the goal is salt recovery from to a 12.0% by weight of salinity) temperature range 273-373 K, are THE
I
0.520
0.609
of concentrated sea-salt solutions are of special interest the sea. Selected values of boiling-point elevations (up and of the vapor pressure of saturated brine in the given in Table 1.6*7
REFRIGERATOR-HEAT-PUMP
DESALINATION
SCHEME
(RHPDS)
The RHPDS for fresh water and salt recovery from the sea is shown diagrammatically in Fig. 4. The functioning of this desalination plant is as follows. Sea water is continuously allowed to flow into the sea-water chamber (2) and this flow is driven by atmospheric pressure. The fresh water in the fresh-water chamber (4) and the sea water in the sea-water chamber are kept at suitably different temperatures T,and T,,respectively, with T,c T,, by means of the RHP (7) which is assumed to be of the vapor-compression type.? Sea water evaporates into the vacuum chamber (3) and the water vapor diffuses to the fresh-water chamber and condenses on top. Atmospheric gases (in particular, NZ, 02, and tDifferent
RHP cycles, in particular the absorption
type, might also be of interest.
586
M.
REALI
.~ F Sea
Fresh
Water
I
Sea Water Level
---
--------
1
----_-
-
Fig. 4. Simplified schematic diagram of the RHPDS. The numbers have the following meanings: (l), sea-water inlet; (2), sea-water chamber; (3), water vapor, atmospheric gases, and vacuum chamber; (4), distilled-fresh-water chamber; (5) fresh-water outlet; (6) brine outlet; (7) vaporcompression refrigerator-heat-pump (RHP). A venting system (not shown) maintains adequate low pressures in the vacuum chamber.
CO&, which are dissolved in sea water and are freed in the vaporization process, are continuously withdrawn through a venting system (not shown in Fig. 4), in order to maintain adequate vacuum conditions in the vacuum chamber. Fresh water is recovered or pumped away from the bottom of the fresh-water chamber through outlet (5). Non-volatile salts dissolved in sea water are driven by gravity toward the bottom of the sea-water chamber, while sea water evaporates and the resulting brine is pumped away through outlet (6). However, if hyperconcentrated brine is produced, it is carried away by some special mechanical device (see below). The RHP absorbs the heat of condensation Q, of water vapor at temperature T, and delivers the heat Q, required to raise the temperature of the sea water from Tf to T, and to vaporize it at temperature T,. Specific designs of the RHPDS should comply with local requirements for fresh-water and/or salt production. If only a fresh-water supply is needed, the sea-water chamber should be flushed at a rate sufficiently high for the brine produced to have a salinity value just slightly higher than that of sea water. If, on the other hand, production of both fresh water and salt or only of salt are required, the RHPDS should be designed for continuous operation even in the presence of a concentrated brine. Potential clogging problems due to evaporation-induced precipitation (initially gypsum) should be kept under control if the sea-water chamber and the outlet channel at its bottom are of sufficiently large sizes. For salt production, it might prove more efficient to have two or perhaps three RHPDS operating in series. The brine from the first RHPDS would refill both the evaporator and the condenser chamber of the second RHPDS, and the more concentrated brine produced in the latter would similarly refill both chambers of a third RHPDS. Also the two water chambers of the first RHPDS could be refilled with sea water if production of fresh water is not needed. Potential technical advantages of the series arrangement are evaluated for an overall economic analysis. The degree of removal of water from the brine in the evaporator chamber of an RHPDS designed for salt production (the last one, in a two or three stage plant) should be almost complete. Salt will precipitate at the bottom of the evaporator chamber and will be carried away by mechanical means (e.g., a belt conveyer of suitable design). Scale problems will be minimized by using scraping devices to clean the heat-transfer surfaces continuously. However, it may prove to be advantageous, at selected sites, to produce concentrated brine of N 20% salinity and drain it off to a final evaporation plant (e.g., a plant exploiting solar and wind energy, or an evaporation plant exploiting industrial or geothermal steam). In Japan, sea water is concentrated about sixfold by using an electrodialysis process before final evaporation.8*9
A refrigerator-heat-pump
desalination
scheme for fresh-water and salt
recovery
587
We now discuss the thermodynamic energy efficiency of the RHPDS. Basic principles are used in the design. The discussion is limited to the RHPDS designed for fresh-water production only. Extension to other schemes is straightforward. Since only a small temperature difference (T, - T,) is required between sea and fresh water (see Fig. 1 and Table l), the RHPDS is expected to have a high energy efficiency. To clarify this point, we choose the water temperatures T, and T/ to be 298 and 293 K, respectively. Ideal performance coefficients of refrigerators and heat pumps operating between a heat source at temperature TJand a heat sink at temperature T, are, respectively,
c, = T-K - T,)= Q,AQs- Q,), chp =
r,l(T, - T/>= Qsl(Qs- Q,h
(2)
with T, = 298 K and Tf = 293 K, we find $,, = 59.6 and Q, N Qr+ (l/60) Q,. The heat of condensation of 1 kg of water vapor at 293 K is H,,,, (293 K) = I(293 K) x 1 kg = 2453.8 kJ. The heat required to raise the temperature of 1 kg of water in sea water from 293 to 298 K and then to vaporize it at 298 K is H, (293-+298 K) 1: cp x 1 kg x 5 K + 1 (298 K) x 1 kg z (4.182 x 5 + 2440) kJ N 2461 W. If we equate Q, to H,w (293 K), we see that an ideal performance RHPDS would work satisfactorily in the chosen temperature range, since it could produce somewhat more heat (Q, 1: 2453[ 1 + (l/60)] kJ = 2494 kJ} than is necessary to vaporize 1 kg of sea water at 298 K. Clearly, actual performances in a real RHPDS will be smaller than the ideal values; however, since the theoretical minimum work for desalting sea water is - 2.93 kJ/kg, our calculations suggest that the RHPDS has high energy efficiency. In any distillation method, heat transfer is a critical, rate-limiting factor. Rate regulation is possible in the RHPDS by adjusting the value of the operating temperature difference T, - T, and by increasing or decreasing the heat-exchange surfaces. An efficient design for the RHPDS water-distillation process may well be achieved in view of the balanced heat-transfer operations required for condensation of water vapor and vaporization of sea water. Unwanted heat losses may, however, occur and, to keep these under control, the walls of the vacuum and water chambers may have to be thermally insulated. Because the operating temperatures of the RHPDS are close to ambient temperatures, scale problems, which are generally critical in desalination by distillation, should not appear. Also, a venting system for the vacuum chamber might not be necessary, since atmospheric gases, which would be freed by the vaporization of sea water at T,, would dissolve again in the distilled fresh water at the lower temperature Tp Clearly, the water levels in the fresh- and sea-water chambers would have minimum values if the RHPDS had no venting system. Sea water may have to be deaerated, before it enters the salt-water chamber, for effective control of heat-exchanger corrosion. The expression water-vaporchamber, rather than vacuum chamber, would be appropriate. Some energy saving in the RHPDS is also obtainable by using free work of atmospheric air for replenishing the sea water. The RHPDS is conceptually simple and also shows design versatility, which allows the exploitation of renewable energy sources, such as solar and wind energy, as well as the use of industrial heat sources that may be available at the desalination site. The unit may be designed to exploit a combination of different energy sources. The variability and intermittency of renewable energy sources are not critical design factors. Solar radiation could be used, either for direct heating of sea water or to generate the power required by the RHPDS by means of photovoltaic cells. Wind energy could be tapped to obtain power to run the RHPDS; it could also be used either to cool fresh water or to heat sea water, this heating being achieved, e.g., by having one or more paddle-wheels mechanically coupled to a wind turbine and rotating inside the sea-water chamber. Among industrial heat sources, waste heat from thermoelectric power plants is of considerable potential interest. It is available in large amounts and is supplied with a useful difference (5 10 K) in temperature between the heated water discharged and atmospheric temperature. The refrigerator-heat-pump concept may be usefully applied in reverse to
588
M.
salinity-gradient, vapor-pressure in a forthcoming study. Acknowledgemenrs-The interest in this work.
REALI
power conversion
author thanks F. Oppenheimer
schemes.’ This idea will be developed
for useful comments and A. K. Oppenheim for his
REFERENCES 1. R. P. Multhauf, Neptune’s Gift-A Hisrory of Common Salt. The Johns Hopkins University Press, Baltimore, MD (1978). 2. “Saline water conversion research-Desalination Report No. 74”, July 1981. California Water Resources Center, University of California, Berkeley, 47th St. & Hoffman Blvd., Richmond, CA 94804. 3. K. S. Spiegler, Salt- Water Purification, 2nd Edn. Plenum Press, New York (1977). 4. M. A. S. Malik, G. N. Tiwari, A. Kumar, and M. S. Sodha, Solar Distillation. Pergamon Press, Oxford (1982). 5. N. B. Vargaftik, Tables on the Thermophysical Properties of Liquids and Gases, 2nd Edn. Hemisphere Publishing Corporation, Washington, D.C. (1975). 6. L. A. Bromley; D. Sir@, P. Ra;, S. Sridhar, and S. M. Read, AZChE J. 20, 326 (1974). 7. M. Olsson. G. L. Wick. and J. D. Isaacs. Science 206. 452 (1979). 8. H. P. Gregor and C. D: Gregor, Scientl& Am. 239, 88 (1978). ’ 9. I. Nakamura, Chem. Econ. Engng Reu. 13(6), 26 (1981).
1984 0
03&5442/84 S3.00 + .oo 1984 Pergamon Press Ltd.
A REFRIGERATOR-HEAT-PUMP DESALINATION SCHEME FOR FRESH-WATER AND SALT RECOVERY
ENEL-DSR-CRIS,
M. &AL1 Via Omato 90/14, 20162 Milano, Italy
(Received 8 September 1983) Abstract-This study concerns a refrigerator-heat-pump desalination scheme (RHPDS), which allows energy-efficient recovery of fresh water and salt from the sea. In this scheme, a salt-water chamber is continuously refilled with sea water via atmospheric pressure. Sea water is evaporated into a vacuum chamber and the water vapor is condensed on top of a fresh-water chamber. A refrigerator-heat-pump circuit maintains the two water chambers at suitably different operating temperatures and allows efficient recovery of the latent heat of condensation. The scheme is analyzed with special consideration to potential exploitation of renewable energy sources such as solar and wind energy.
INTRODUCTION
Fresh water and salt are essential for the life of man and for innumerable living beings. They have consequently been sought after since time immemorial and attempts at recovering them have been made even in the most hostile environments. Starting with the industrial revolution and up to the present time, plants of ever-increasing complexity have been designed for the recovery of either fresh water or of salt or of both from sea water, brackish water, and hypersaline water bodies. *AThese developments notwithstanding, lack or scarcity of fresh water and salt remain serious problems for many regions of the earth, as was recently emphasized by UNESCO, which declared the decade 1980-90 officially as dedicated to fostering research to ensure a sufficient supply of fresh water for all countries in need. In this paper, we introduce a refrigerator-heat-pump desalination scheme (RHPDS) for energy-efficient recovery of fresh water and salt from the sea. Since the operation of the RHPDS is based on the control of vapor pressures of sea water and fresh water, the scheme itself will be described after we have dealt with the vapor-pressure difference between sea and fresh water. The special features of the RHPDS are analyzed in some detail, and proper consideration is given to the important potential exploitation of any available energy source such as waste heat from thermoelectric power plants and solar energy or wind energy. VAPOR-PRESSURE
DIFFERENCE
BETWEEN
SEA
AND
FRESH
WATER
In the temperature range 273-373 K, the vapor pressure of sea water is N 1.84% less than that of fresh water.’ The difference between the two pressures increases with temperature, as is shown in Fig. 1. If, as shown in Fig. 2, two vessels containing deaerated fresh and sea water are kept at the same temperature T, 273 < T < 373 (K), and have their tops connected, fresh water will distill into the sea-water vessel, unless the fresh-water level is so much lower than the sea-water level for thermodynamic equilibrium to be maintained by gravity. For example, if T = 298 K, the equilibrium level difference should be w 240 m, as can be verified by applying the barometric law to the water-vapor pressure. Clearly, the vapor pressure of sea water must be increased by some means if we want water to be distilled from a sea-water vessel onto a fresh-water vessel. Inspection of Fig. 1 suggests that distillation of sea water can be obtained if the distilled-fresh-water temperature is kept lower than the sea-water temperature by a suitable amount. This simple observation represents the physical foundation for the operation of the RHPDS. 583
M.
584
REALI
800
700
500
500
2 LOO
s CL'
300
200
100
363
373
Fig. 1. Temperature dependences of the fresh-water vapor pressure p”, and of the vapor-pressure difference between fresh and sea water Ap,, in the temperature range 273-373 K.
Uniform
Temperature
I
,yy-Y
‘.
.. ; ‘.
4---r -_ -- -
__.
_
_
Sea
z--- -
-
Water
-FE?Sh
water
Fig. 2. Two vessels containing deaerated fresh and sea water and kept at the same temperature T (273 < T < 373 K) are connected at their tops. Water is distilled from the fresh-water vessel to the sea-water vessel, unless the fresh-water level is so much below the sea-water level that gravity can balance the vapor-pressure difference between fresh and sea water (at 298 K, the equilibrium level difference would be _ 240m).
Important physical properties of water to be taken into account for a proper design are the latent heat of vaporization I and the specific heat cp. Numerical values of 1 in the temperature range 273-373 K are presented in Fig. 3 (the data are from Ref. 5). In the same temperature range, cp shows only slight variations, which have a negligible practical influence on the operation of the RHPDS. The values? of cp are obtained from 10, and are, respectively, 4.127, 4.193, 4.182, q.=(273+Jx lO)(K)withJ=0,1,2,..., 4.179, 4.179, 4.181, 4.185, 4.190, 4.197, 4.205, and 4.216 (kJ/kg-K).
A refrigerator-heat-pump
desalination
scheme for fresh-water and salt recovery
585
1
2500
2400
-
2300
-
w
2200. 213
293
373
353
333
313
Temperature PKI
Fig. 3. Temperature
dependence
of the latent heat of vaporization 273-373 K.
of water (I)
in the range
Table 1. Selected values of boiling-point elevations of sea-salt solutions and of the vapor pressure of saturated brine for 273-373 K. vapor pressure of sat&d brine.
Boiling point elevation of sea-salt solutions, 'K
3.45
Weight % sea salt 4.0 I 6.0 8.0
10.0 I-12.0
0
0.253
0.295
0.459
0.637
0.835
1.053
10
0.276
0.323
0.503
0.701
0.920
1.164
, 0.300 ~0.312
0.351
0.547
0.765
1.006
1.275
0.365
0.570
0.797
1.049
1.330
0.184
0.324
0.380
0.593
0.830
1.093
1.386
0.198
0.350
0.409
0.640
0.896
1.181
1.499
0.213
0.376
0.440
0.689
0.965
1.272
1.614
0.228
0.403
0.472
0.739
1.035
1.364
1.731
70
0.244
0.431
0.505
0.790
1.107
1.459
1.851
80
0.261
0.460
0.538
0.843
1.180
1.556
1.973
90
0.278
0.489
0.573
0.897
1.256
1.655
2.098
100
0.295
0.953
1.334
1.756
2.225
20
25 30 40
44
50 60
117
The thermodynamic properties when the goal is salt recovery from to a 12.0% by weight of salinity) temperature range 273-373 K, are THE
I
0.520
0.609
of concentrated sea-salt solutions are of special interest the sea. Selected values of boiling-point elevations (up and of the vapor pressure of saturated brine in the given in Table 1.6*7
REFRIGERATOR-HEAT-PUMP
DESALINATION
SCHEME
(RHPDS)
The RHPDS for fresh water and salt recovery from the sea is shown diagrammatically in Fig. 4. The functioning of this desalination plant is as follows. Sea water is continuously allowed to flow into the sea-water chamber (2) and this flow is driven by atmospheric pressure. The fresh water in the fresh-water chamber (4) and the sea water in the sea-water chamber are kept at suitably different temperatures T,and T,,respectively, with T,c T,, by means of the RHP (7) which is assumed to be of the vapor-compression type.? Sea water evaporates into the vacuum chamber (3) and the water vapor diffuses to the fresh-water chamber and condenses on top. Atmospheric gases (in particular, NZ, 02, and tDifferent
RHP cycles, in particular the absorption
type, might also be of interest.
586
M.
REALI
.~ F Sea
Fresh
Water
I
Sea Water Level
---
--------
1
----_-
-
Fig. 4. Simplified schematic diagram of the RHPDS. The numbers have the following meanings: (l), sea-water inlet; (2), sea-water chamber; (3), water vapor, atmospheric gases, and vacuum chamber; (4), distilled-fresh-water chamber; (5) fresh-water outlet; (6) brine outlet; (7) vaporcompression refrigerator-heat-pump (RHP). A venting system (not shown) maintains adequate low pressures in the vacuum chamber.
CO&, which are dissolved in sea water and are freed in the vaporization process, are continuously withdrawn through a venting system (not shown in Fig. 4), in order to maintain adequate vacuum conditions in the vacuum chamber. Fresh water is recovered or pumped away from the bottom of the fresh-water chamber through outlet (5). Non-volatile salts dissolved in sea water are driven by gravity toward the bottom of the sea-water chamber, while sea water evaporates and the resulting brine is pumped away through outlet (6). However, if hyperconcentrated brine is produced, it is carried away by some special mechanical device (see below). The RHP absorbs the heat of condensation Q, of water vapor at temperature T, and delivers the heat Q, required to raise the temperature of the sea water from Tf to T, and to vaporize it at temperature T,. Specific designs of the RHPDS should comply with local requirements for fresh-water and/or salt production. If only a fresh-water supply is needed, the sea-water chamber should be flushed at a rate sufficiently high for the brine produced to have a salinity value just slightly higher than that of sea water. If, on the other hand, production of both fresh water and salt or only of salt are required, the RHPDS should be designed for continuous operation even in the presence of a concentrated brine. Potential clogging problems due to evaporation-induced precipitation (initially gypsum) should be kept under control if the sea-water chamber and the outlet channel at its bottom are of sufficiently large sizes. For salt production, it might prove more efficient to have two or perhaps three RHPDS operating in series. The brine from the first RHPDS would refill both the evaporator and the condenser chamber of the second RHPDS, and the more concentrated brine produced in the latter would similarly refill both chambers of a third RHPDS. Also the two water chambers of the first RHPDS could be refilled with sea water if production of fresh water is not needed. Potential technical advantages of the series arrangement are evaluated for an overall economic analysis. The degree of removal of water from the brine in the evaporator chamber of an RHPDS designed for salt production (the last one, in a two or three stage plant) should be almost complete. Salt will precipitate at the bottom of the evaporator chamber and will be carried away by mechanical means (e.g., a belt conveyer of suitable design). Scale problems will be minimized by using scraping devices to clean the heat-transfer surfaces continuously. However, it may prove to be advantageous, at selected sites, to produce concentrated brine of N 20% salinity and drain it off to a final evaporation plant (e.g., a plant exploiting solar and wind energy, or an evaporation plant exploiting industrial or geothermal steam). In Japan, sea water is concentrated about sixfold by using an electrodialysis process before final evaporation.8*9
A refrigerator-heat-pump
desalination
scheme for fresh-water and salt
recovery
587
We now discuss the thermodynamic energy efficiency of the RHPDS. Basic principles are used in the design. The discussion is limited to the RHPDS designed for fresh-water production only. Extension to other schemes is straightforward. Since only a small temperature difference (T, - T,) is required between sea and fresh water (see Fig. 1 and Table l), the RHPDS is expected to have a high energy efficiency. To clarify this point, we choose the water temperatures T, and T/ to be 298 and 293 K, respectively. Ideal performance coefficients of refrigerators and heat pumps operating between a heat source at temperature TJand a heat sink at temperature T, are, respectively,
c, = T-K - T,)= Q,AQs- Q,), chp =
r,l(T, - T/>= Qsl(Qs- Q,h
(2)
with T, = 298 K and Tf = 293 K, we find $,, = 59.6 and Q, N Qr+ (l/60) Q,. The heat of condensation of 1 kg of water vapor at 293 K is H,,,, (293 K) = I(293 K) x 1 kg = 2453.8 kJ. The heat required to raise the temperature of 1 kg of water in sea water from 293 to 298 K and then to vaporize it at 298 K is H, (293-+298 K) 1: cp x 1 kg x 5 K + 1 (298 K) x 1 kg z (4.182 x 5 + 2440) kJ N 2461 W. If we equate Q, to H,w (293 K), we see that an ideal performance RHPDS would work satisfactorily in the chosen temperature range, since it could produce somewhat more heat (Q, 1: 2453[ 1 + (l/60)] kJ = 2494 kJ} than is necessary to vaporize 1 kg of sea water at 298 K. Clearly, actual performances in a real RHPDS will be smaller than the ideal values; however, since the theoretical minimum work for desalting sea water is - 2.93 kJ/kg, our calculations suggest that the RHPDS has high energy efficiency. In any distillation method, heat transfer is a critical, rate-limiting factor. Rate regulation is possible in the RHPDS by adjusting the value of the operating temperature difference T, - T, and by increasing or decreasing the heat-exchange surfaces. An efficient design for the RHPDS water-distillation process may well be achieved in view of the balanced heat-transfer operations required for condensation of water vapor and vaporization of sea water. Unwanted heat losses may, however, occur and, to keep these under control, the walls of the vacuum and water chambers may have to be thermally insulated. Because the operating temperatures of the RHPDS are close to ambient temperatures, scale problems, which are generally critical in desalination by distillation, should not appear. Also, a venting system for the vacuum chamber might not be necessary, since atmospheric gases, which would be freed by the vaporization of sea water at T,, would dissolve again in the distilled fresh water at the lower temperature Tp Clearly, the water levels in the fresh- and sea-water chambers would have minimum values if the RHPDS had no venting system. Sea water may have to be deaerated, before it enters the salt-water chamber, for effective control of heat-exchanger corrosion. The expression water-vaporchamber, rather than vacuum chamber, would be appropriate. Some energy saving in the RHPDS is also obtainable by using free work of atmospheric air for replenishing the sea water. The RHPDS is conceptually simple and also shows design versatility, which allows the exploitation of renewable energy sources, such as solar and wind energy, as well as the use of industrial heat sources that may be available at the desalination site. The unit may be designed to exploit a combination of different energy sources. The variability and intermittency of renewable energy sources are not critical design factors. Solar radiation could be used, either for direct heating of sea water or to generate the power required by the RHPDS by means of photovoltaic cells. Wind energy could be tapped to obtain power to run the RHPDS; it could also be used either to cool fresh water or to heat sea water, this heating being achieved, e.g., by having one or more paddle-wheels mechanically coupled to a wind turbine and rotating inside the sea-water chamber. Among industrial heat sources, waste heat from thermoelectric power plants is of considerable potential interest. It is available in large amounts and is supplied with a useful difference (5 10 K) in temperature between the heated water discharged and atmospheric temperature. The refrigerator-heat-pump concept may be usefully applied in reverse to
588
M.
salinity-gradient, vapor-pressure in a forthcoming study. Acknowledgemenrs-The interest in this work.
REALI
power conversion
author thanks F. Oppenheimer
schemes.’ This idea will be developed
for useful comments and A. K. Oppenheim for his
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