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Thermal energy storage using saturated salt solutions
Enrry.b Vol. 5. pp 1085-1090 Pergrmon Press Ltd.. 1980. Prmted
THERMAL
m Grear
Britam
ENERGY STORAGE USING SALT SOLUTIONS?
SATURATED
M. A. BELL and I. E. SMITH School of Mechanical Engineering, Cranfield Institute of Technology, Cranfield, Bedfordshire MK43 OAL, England (Received 14 February 1980)
Abstract-In order to reduce the volume required to store low grade thermal energy in water, various systems using phase change materials (PCMs)have been proposed. However, in order to overcome the poor heat transfer characteristics associated with the solidification of the PCM on heat transfer surfaces, large surface areas need to be provided. This is often achieved by encapsulation in either large or small containers, which has the effect of increasing the cost and reducing the effective energy density. Furthermore PCMs can only accept and release heat at one particular temperature. In an attempt to overcome these limitations thermal energy storage using saturated salt solutions has been examined. The energy density for a number of promising salts has been calculated and confirmed by experiment. Energy density increases of up to 4 times that of water are possible, depending on the salt used and the temperature swing permitted. The deposition of crystals from the solution on heat exchanger surfaces has been overcome by the use of a novel self-cleaning technique.
NOTATION Cs C” C, h, h, MH M” s T
Tsar W
X
specific heat of a saturated salt solution mixture, kJ/kg”K specific heat of hydrated salt crystals, kJ/mol”K specific heat of anhydrous salt, kJ/mol”K heat of solution, kJ/mol heat of solution at infinite dilution, kJ/mol molecular weight of a salt hydrate molecular weight of an anhydrous salt solubility, mol/kg water temperature, “K upper saturation temperature, “K water molecule, z Hz0 number of hydration water molecules
INTRODUCTION
Oil and coal have traditionally been regarded as sources of energy. It is only recently that proper significance has been given to the fact that they are stores of energy, and as such will eventually become exhaused. These energy stores must be conserved in order that time may be bought to enable the harnessing of renewable energy sources. Thermal energy storage can be employed both as a conservation measure and in the harnessing of renewable energy sources, in particular, solar energy. Approximately 20% of the total U.K. energy consumption is used to create comfortable artificial climates within buildings.’ Switching wholly to solar energy to achieve this end could thus effect a very large energy saving in the immediate future as well as providing the long term solution to environmental heating. Such a proposition, however, necessitates the use of an interseasonal thermal energy store, where solar energy collected during the summer months can be used for heating during winter. Preliminary estimates, assuming a well insulated building, indicate that such a store using water as the storage medium, would occupy approx. 15% of the building volume, increasing to 50% for more conventional buildings.* iThis research was sponsored jointly by the U.K. Science Research Council and the Commission European Communities. 1085
of the
1086
M. A. BELL and I. E. SMITH
The economic and architectural implications of this approach are such that any means of reducing the necessary volume for thermal storage would enhance its viability, even if it involves some additional cost.
THERMAL
STORAGE
USING
SATURATED
SALT
SOLUTIONS
A concept for a thermal store using saturated solutions was first suggested by Smith3 and depends on the fact that (a) many inorganic salts dissolve endothermically (i.e. they absorb heat when they go into solution) and (b) their solubility increases with temperature. The result is that, if a salt solution remains saturated over a temperature range, it can absorb a greater amount of heat per unit volume than either the salt or water by itself. The effective specific heat of a saturated salt solution can be calculated by combining the specific heats of the components in conjunction with the product of the heat of solution and the solubility gradient. This procedure leads to an expression for salts dissolving in water of the form
C = 4.1g5Cl + 0.018 X
s
S(T)1+ [S&T) - s(T)]c, + S(T)C, - /IS’(T). m kJ/kgK. 1 + 0.001S(&,,)M, (1)
The energy per unit weight over a given temperature swing can be obtained by integrating Eq. (1). The energy density is obtained by multiplying the result by a representative solution density, usually the density of the solution at the upper saturation temperature. When using Eqn (1) it should be noted that (a) when T > &r, h = 0 & S(T) = S(T,,r) and (b) for a non-hydrate, X = 0, CH = 0 and MH becomes MA. Examination of eqn (1) shows that, for an inorganic salt or a salt hydrate to exceed the volumetric storage capacity of water significantly, it must have the following properties: (a) a large negative heat of solution (h); (b) a large positive temperature coefficient of solubility [S’(T)] ; (c) a high density as a saturated solution; (d) a high specific heat as a solid (C, and C,). In addition, economic and practical considerations lead to the following desirable characteristics: (e) it should be cheap and plentiful; (f) it should be non-corrosive and non-toxic; (g) the crystals should be easily disloged from surfaces on which they form; (h) it should dissolve rapidly when in contact with unsaturated solution; (i) it should crystallize readily and not become supersaturated on cooling.
EVALUATION
OF
POTENTIAL
SALTS
The results of a survey of promising salts for thermal energy storage are shown in Fig. 1. A base temperature of 30°C has been selected as being the lowest at which thermal energy is useful, e.g. for space heating. The curves were calculated using step-wise integration of the above expression, together with values for the heat of solution, solubility, etc. extracted from the literature. Literature values for the heat of solution are usually given for large or infinite dilution, whereas saturated solutions are concentrated. Where information is available on the heat of formation at various concentrations, this can be used to deduce k; otherwise h, has to be used. For non-hydrates considerable error may occur if h, is used, but for hydrates (for which the heat of solution tends to the heat of fusion at the melting point) much less error is incurred, since for many hydrates the heat of fusion is not too different from the heat of solution. The saturated solution density used to calculate the energy density was calculated by summing the mass of water and anhydrous salt and dividing by the sum of their volumes
Thermal
energy
storage
/
I
30
10
using
saturated
50 TEMPERATURE
Fig. I. Calculated
energy
densities
salt solutions
60
10x7
70
IOC)
for salt solutions.
(see Fig. 1). The knee in the curves in Fig. 1 occurs at the upper saturation temperature. which may be prescribed by one of the following three criteria: (i) Sufficient solution must be available at the datum temperature to permit the solution to be pumped to a heat exchanger. This requirement limits the amount of salt which can be present at this temperature and hence also limits the upper saturation temperature, implying that for salt hydrates the upper saturation temperature will be below the melting temperature of the hydrate. (ii) For some salt hydrates, depending on the concentration, separation into a lower hydrate or anhydrous salt can occur as the temperature rises. Because reconversion to the hydrate does not rapidly occur when the temperature falls, this separation must be avoided. The limit can be determined from a phase diagram with &,r limited by the maximum allowable proportion of salt in order to avoid separation4 (iii) When neither of the two preceding conditions is limiting, q:,,,- may be limited arbitrarily by the maximum anticipated store temperature.
EXPERIMENTAL
DETERMINATIONS
OF
ENERGY
DENSITY
Experimental measurements5 on some of the more promising salts, were made using conventional calorimetric techniques. A sealed sample of salt solution was heated to a given temperature and then rapidly transferred to an adiabatic calorimeter. Small corrections were necessary to refer all of the measurements to a base temperature of 30°C. The experimental results for potassium nitrate, tri-sodium phosphate duodecahydrate and sodium acetate trihydrate are shown in Fig. 2. The solid lines show the calculated values. The results generally show excellent agreement. The calculated values underestimate the measured energy density, as is shown for CH,COONa.3H,O. This discrepancy is due mainly to a lack of knowledge regarding the way in which h varies with concentration
M. A. BELL and I. E. SMITH
1088
TEMPERATURE
(“C)
Fig. 2. Measured energy densities for some salt solutions.
and temperature. The experimental results for CH3COONa3H20 sity increase of 2.2 times that of water between the temperatures
HEAT
INPUT
TO
A SATURATED
SOLUTION
show an energy den30 and 70°C.
STORE
Heat could be supplied to a salt solution store simply by pumping the solution to, say, a solar collector, where it is heated, and then returned to the store where it can dissolve additional salt because of its increased temperature. In practice, this procedure is undesirable. Whenever a solution with a saturation temperature above ambient exists in pipes and pumps, crystals will form when the pump is off and the solution temperature falls (e.g. at night). These crystals cause blockage and fouling such that circulation is difficult or impossible to restart. A better approach would be to circulate the solution through an integral heat exchanger which derives its supply from a solar panel. Of prime importance to a salt solution store is the need for the crystals to dissolve rapidly in unsaturated solution. Failure to do so will result in a high temperature of the solution, a large mass of undissolved crystals, and a low energy storage density. Recent studies6 using KN03 have shown that an unsaturated solution can emerge very close to saturation after one pass at moderate velocities (0.3-1.0 m/s) through a crystal bed with initial depth of 100 mm. Dissolution rate constants were measured and showed a strong dependence on the degree of unsaturation of the incoming solution and a rather weaker dependence on fluid-dynamic parameters such as the crystal size and fluid velocity. HEAT
EXTRACTION
FROM
A SATURATED
SOLUTION
STORE
Heat must be extracted from a salt-solution store by means of a heat-exchanger transferring heat to a secondary fluid. However, as heat is withdrawn from the saturated solution, crystals form on the heat-exchanger surfaces. Unless these crystals can be dislodged, they will accumulate on the surfaces and reduce the rate of heat transfer. Attempts by Kauffman et ~1.’ to overcome this problem and prevent fouling by coating surfaces with non-stick materials (teflon, PTFE etc.) proved unsuccessful, and tests carried out by the authors have confirmed this negative result, However, a heatexchanger embodying a fluidized region of solid spheres inside the heat-exchanger tubes has demonstrated an ability to dislodge the crystals as they form. Because the crystals are small in comparison with the fluidized spheres, once detached they are entrained in the
Thermal energy storage using saturated salt solutions
1089
flow and expelled from the heat-exchanger. Development of this heat-exchanger is continuing for this application, as well as for other applications such as the extraction of heat from dirty liquids (e.g. sewage or industrial effluent).
THERMAL
ENERGY
STORE
CAPITAL
COSTS
The cost of a thermal store comprises the cost of the containing vessel, the storage medium and auxiliary equipment such as heat exchangers, pumps, etc. These costs have been estimated for four different types of store and are presented in Fig. 3. The quoted costs refer to the storage media in their installed state, i.e. for encapsulated materials the voidage is included and for saturated salts the water is included. The cost bands cover the range from current costs for bulk production to a somewhat lower cost.
3o
B
ENCAPSULATED
m
SATURATED
m
ENCAPSULATED
WAX SOLUTION
OF CH,COONa
No,SOI.IOH,O
1;11:1:11;1 WATER
25
?
0 x
3 < -
20
15
5 s 10
05
1
10 STORAGE
CAPACITY
( GJ /
Fig. 3. Thermal store capital costs.
An attempt to include the cost of heat-exchangers has been made where the size and cost of a heat-exchanger is reflected in the number of charge/discharge cycles per year (n). For large values of n, a proportionately larger heat-exchanger is required because of the higher rates of energy transfer. It may be seen that, for large capacity stores, water is competitive although the store volume is large. Encapsulated wax is expensive, owing primarily to its high material cost. Encapsulated Glauber’s salt may be competitive with water storage at the low capacity end and is similar to saturated solution storage at the high capacity end because the material costs are generally similar. However, no account has been taken of the fact that gelling agents and nucleating agents are required for Glauber’s salt, which tend to increase the cost and reduce the storage capacity. We conclude that saturated solution stores are less expensive than encapsulated organic materials but are always more expensive than an equivalent water store.
1090
M. A. BELL and I. E. SMITH CONCLUSIONS
An extensive evaluation of potentially useful salts for thermal energy storage shows that the hydrates of calcium nitrate, sodium sulphide, sodium thiosulphate, sodium acetate and tri-sodium phosphate could be used effectively in a saturated solution thermal energy store. Experimental measurements have shown that the energy density increases over that of water vary from 3.9 (3040°C for Ca(N0&.H20) to 1.5 (3&7O”C for Na3P04.12Hz0). Sodium acetate tri-hydrate, over the temperature range 3&7O”C, offers a volumetric reduction over water of -45%. Although a saturated solution store may be cheaper than a store using an organic PCM, its adoption, along with all other types of increased energy density stores, would only be warranted when the benefits of its reduced volume outweigh the increase in capital cost. REFERENCES 1. U.K. Department of Energy, Digest of U.K. Energy Statistics. HMSO, London (1977). 2. P. W. O’Callaghan, Energy for Industry, p. 383. Pergamon Press Oxford (1979). 3. I. E. Smith, Aspects of Energy Conversion (edited by I. M. Blair, B. D. Jones and A. J. Van Horn. Pergamon Press, Oxford (1976). 4. R. D. Biswas, Solar Energy 19, 99 (1977). 5. P. W. O’Callaghan et al., “Thermal Energy Storage Systems”, Progress Rep. No. 4, EEC Contract 398-78-EEUK (1979). 6. M. J. Nissen, “Resaturation of Salt Solutions for Application to Thermal Energy Storage”, M.Sc thesis, Cranfield Institute of Technology, Cranfield, Bedford MK43 OAL, England (1979). 7. K. W. Kauffman, H. G. Lorsch, and D. M. Kyllonen, “Thermal Energy Storage by means of Saturated Aqueous Solutions”, Final Rep., U.S. Department of Energy Contract No. EY-76-C-05-5158 (1977).
THERMAL
m Grear
Britam
ENERGY STORAGE USING SALT SOLUTIONS?
SATURATED
M. A. BELL and I. E. SMITH School of Mechanical Engineering, Cranfield Institute of Technology, Cranfield, Bedfordshire MK43 OAL, England (Received 14 February 1980)
Abstract-In order to reduce the volume required to store low grade thermal energy in water, various systems using phase change materials (PCMs)have been proposed. However, in order to overcome the poor heat transfer characteristics associated with the solidification of the PCM on heat transfer surfaces, large surface areas need to be provided. This is often achieved by encapsulation in either large or small containers, which has the effect of increasing the cost and reducing the effective energy density. Furthermore PCMs can only accept and release heat at one particular temperature. In an attempt to overcome these limitations thermal energy storage using saturated salt solutions has been examined. The energy density for a number of promising salts has been calculated and confirmed by experiment. Energy density increases of up to 4 times that of water are possible, depending on the salt used and the temperature swing permitted. The deposition of crystals from the solution on heat exchanger surfaces has been overcome by the use of a novel self-cleaning technique.
NOTATION Cs C” C, h, h, MH M” s T
Tsar W
X
specific heat of a saturated salt solution mixture, kJ/kg”K specific heat of hydrated salt crystals, kJ/mol”K specific heat of anhydrous salt, kJ/mol”K heat of solution, kJ/mol heat of solution at infinite dilution, kJ/mol molecular weight of a salt hydrate molecular weight of an anhydrous salt solubility, mol/kg water temperature, “K upper saturation temperature, “K water molecule, z Hz0 number of hydration water molecules
INTRODUCTION
Oil and coal have traditionally been regarded as sources of energy. It is only recently that proper significance has been given to the fact that they are stores of energy, and as such will eventually become exhaused. These energy stores must be conserved in order that time may be bought to enable the harnessing of renewable energy sources. Thermal energy storage can be employed both as a conservation measure and in the harnessing of renewable energy sources, in particular, solar energy. Approximately 20% of the total U.K. energy consumption is used to create comfortable artificial climates within buildings.’ Switching wholly to solar energy to achieve this end could thus effect a very large energy saving in the immediate future as well as providing the long term solution to environmental heating. Such a proposition, however, necessitates the use of an interseasonal thermal energy store, where solar energy collected during the summer months can be used for heating during winter. Preliminary estimates, assuming a well insulated building, indicate that such a store using water as the storage medium, would occupy approx. 15% of the building volume, increasing to 50% for more conventional buildings.* iThis research was sponsored jointly by the U.K. Science Research Council and the Commission European Communities. 1085
of the
1086
M. A. BELL and I. E. SMITH
The economic and architectural implications of this approach are such that any means of reducing the necessary volume for thermal storage would enhance its viability, even if it involves some additional cost.
THERMAL
STORAGE
USING
SATURATED
SALT
SOLUTIONS
A concept for a thermal store using saturated solutions was first suggested by Smith3 and depends on the fact that (a) many inorganic salts dissolve endothermically (i.e. they absorb heat when they go into solution) and (b) their solubility increases with temperature. The result is that, if a salt solution remains saturated over a temperature range, it can absorb a greater amount of heat per unit volume than either the salt or water by itself. The effective specific heat of a saturated salt solution can be calculated by combining the specific heats of the components in conjunction with the product of the heat of solution and the solubility gradient. This procedure leads to an expression for salts dissolving in water of the form
C = 4.1g5Cl + 0.018 X
s
S(T)1+ [S&T) - s(T)]c, + S(T)C, - /IS’(T). m kJ/kgK. 1 + 0.001S(&,,)M, (1)
The energy per unit weight over a given temperature swing can be obtained by integrating Eq. (1). The energy density is obtained by multiplying the result by a representative solution density, usually the density of the solution at the upper saturation temperature. When using Eqn (1) it should be noted that (a) when T > &r, h = 0 & S(T) = S(T,,r) and (b) for a non-hydrate, X = 0, CH = 0 and MH becomes MA. Examination of eqn (1) shows that, for an inorganic salt or a salt hydrate to exceed the volumetric storage capacity of water significantly, it must have the following properties: (a) a large negative heat of solution (h); (b) a large positive temperature coefficient of solubility [S’(T)] ; (c) a high density as a saturated solution; (d) a high specific heat as a solid (C, and C,). In addition, economic and practical considerations lead to the following desirable characteristics: (e) it should be cheap and plentiful; (f) it should be non-corrosive and non-toxic; (g) the crystals should be easily disloged from surfaces on which they form; (h) it should dissolve rapidly when in contact with unsaturated solution; (i) it should crystallize readily and not become supersaturated on cooling.
EVALUATION
OF
POTENTIAL
SALTS
The results of a survey of promising salts for thermal energy storage are shown in Fig. 1. A base temperature of 30°C has been selected as being the lowest at which thermal energy is useful, e.g. for space heating. The curves were calculated using step-wise integration of the above expression, together with values for the heat of solution, solubility, etc. extracted from the literature. Literature values for the heat of solution are usually given for large or infinite dilution, whereas saturated solutions are concentrated. Where information is available on the heat of formation at various concentrations, this can be used to deduce k; otherwise h, has to be used. For non-hydrates considerable error may occur if h, is used, but for hydrates (for which the heat of solution tends to the heat of fusion at the melting point) much less error is incurred, since for many hydrates the heat of fusion is not too different from the heat of solution. The saturated solution density used to calculate the energy density was calculated by summing the mass of water and anhydrous salt and dividing by the sum of their volumes
Thermal
energy
storage
/
I
30
10
using
saturated
50 TEMPERATURE
Fig. I. Calculated
energy
densities
salt solutions
60
10x7
70
IOC)
for salt solutions.
(see Fig. 1). The knee in the curves in Fig. 1 occurs at the upper saturation temperature. which may be prescribed by one of the following three criteria: (i) Sufficient solution must be available at the datum temperature to permit the solution to be pumped to a heat exchanger. This requirement limits the amount of salt which can be present at this temperature and hence also limits the upper saturation temperature, implying that for salt hydrates the upper saturation temperature will be below the melting temperature of the hydrate. (ii) For some salt hydrates, depending on the concentration, separation into a lower hydrate or anhydrous salt can occur as the temperature rises. Because reconversion to the hydrate does not rapidly occur when the temperature falls, this separation must be avoided. The limit can be determined from a phase diagram with &,r limited by the maximum allowable proportion of salt in order to avoid separation4 (iii) When neither of the two preceding conditions is limiting, q:,,,- may be limited arbitrarily by the maximum anticipated store temperature.
EXPERIMENTAL
DETERMINATIONS
OF
ENERGY
DENSITY
Experimental measurements5 on some of the more promising salts, were made using conventional calorimetric techniques. A sealed sample of salt solution was heated to a given temperature and then rapidly transferred to an adiabatic calorimeter. Small corrections were necessary to refer all of the measurements to a base temperature of 30°C. The experimental results for potassium nitrate, tri-sodium phosphate duodecahydrate and sodium acetate trihydrate are shown in Fig. 2. The solid lines show the calculated values. The results generally show excellent agreement. The calculated values underestimate the measured energy density, as is shown for CH,COONa.3H,O. This discrepancy is due mainly to a lack of knowledge regarding the way in which h varies with concentration
M. A. BELL and I. E. SMITH
1088
TEMPERATURE
(“C)
Fig. 2. Measured energy densities for some salt solutions.
and temperature. The experimental results for CH3COONa3H20 sity increase of 2.2 times that of water between the temperatures
HEAT
INPUT
TO
A SATURATED
SOLUTION
show an energy den30 and 70°C.
STORE
Heat could be supplied to a salt solution store simply by pumping the solution to, say, a solar collector, where it is heated, and then returned to the store where it can dissolve additional salt because of its increased temperature. In practice, this procedure is undesirable. Whenever a solution with a saturation temperature above ambient exists in pipes and pumps, crystals will form when the pump is off and the solution temperature falls (e.g. at night). These crystals cause blockage and fouling such that circulation is difficult or impossible to restart. A better approach would be to circulate the solution through an integral heat exchanger which derives its supply from a solar panel. Of prime importance to a salt solution store is the need for the crystals to dissolve rapidly in unsaturated solution. Failure to do so will result in a high temperature of the solution, a large mass of undissolved crystals, and a low energy storage density. Recent studies6 using KN03 have shown that an unsaturated solution can emerge very close to saturation after one pass at moderate velocities (0.3-1.0 m/s) through a crystal bed with initial depth of 100 mm. Dissolution rate constants were measured and showed a strong dependence on the degree of unsaturation of the incoming solution and a rather weaker dependence on fluid-dynamic parameters such as the crystal size and fluid velocity. HEAT
EXTRACTION
FROM
A SATURATED
SOLUTION
STORE
Heat must be extracted from a salt-solution store by means of a heat-exchanger transferring heat to a secondary fluid. However, as heat is withdrawn from the saturated solution, crystals form on the heat-exchanger surfaces. Unless these crystals can be dislodged, they will accumulate on the surfaces and reduce the rate of heat transfer. Attempts by Kauffman et ~1.’ to overcome this problem and prevent fouling by coating surfaces with non-stick materials (teflon, PTFE etc.) proved unsuccessful, and tests carried out by the authors have confirmed this negative result, However, a heatexchanger embodying a fluidized region of solid spheres inside the heat-exchanger tubes has demonstrated an ability to dislodge the crystals as they form. Because the crystals are small in comparison with the fluidized spheres, once detached they are entrained in the
Thermal energy storage using saturated salt solutions
1089
flow and expelled from the heat-exchanger. Development of this heat-exchanger is continuing for this application, as well as for other applications such as the extraction of heat from dirty liquids (e.g. sewage or industrial effluent).
THERMAL
ENERGY
STORE
CAPITAL
COSTS
The cost of a thermal store comprises the cost of the containing vessel, the storage medium and auxiliary equipment such as heat exchangers, pumps, etc. These costs have been estimated for four different types of store and are presented in Fig. 3. The quoted costs refer to the storage media in their installed state, i.e. for encapsulated materials the voidage is included and for saturated salts the water is included. The cost bands cover the range from current costs for bulk production to a somewhat lower cost.
3o
B
ENCAPSULATED
m
SATURATED
m
ENCAPSULATED
WAX SOLUTION
OF CH,COONa
No,SOI.IOH,O
1;11:1:11;1 WATER
25
?
0 x
3 < -
20
15
5 s 10
05
1
10 STORAGE
CAPACITY
( GJ /
Fig. 3. Thermal store capital costs.
An attempt to include the cost of heat-exchangers has been made where the size and cost of a heat-exchanger is reflected in the number of charge/discharge cycles per year (n). For large values of n, a proportionately larger heat-exchanger is required because of the higher rates of energy transfer. It may be seen that, for large capacity stores, water is competitive although the store volume is large. Encapsulated wax is expensive, owing primarily to its high material cost. Encapsulated Glauber’s salt may be competitive with water storage at the low capacity end and is similar to saturated solution storage at the high capacity end because the material costs are generally similar. However, no account has been taken of the fact that gelling agents and nucleating agents are required for Glauber’s salt, which tend to increase the cost and reduce the storage capacity. We conclude that saturated solution stores are less expensive than encapsulated organic materials but are always more expensive than an equivalent water store.
1090
M. A. BELL and I. E. SMITH CONCLUSIONS
An extensive evaluation of potentially useful salts for thermal energy storage shows that the hydrates of calcium nitrate, sodium sulphide, sodium thiosulphate, sodium acetate and tri-sodium phosphate could be used effectively in a saturated solution thermal energy store. Experimental measurements have shown that the energy density increases over that of water vary from 3.9 (3040°C for Ca(N0&.H20) to 1.5 (3&7O”C for Na3P04.12Hz0). Sodium acetate tri-hydrate, over the temperature range 3&7O”C, offers a volumetric reduction over water of -45%. Although a saturated solution store may be cheaper than a store using an organic PCM, its adoption, along with all other types of increased energy density stores, would only be warranted when the benefits of its reduced volume outweigh the increase in capital cost. REFERENCES 1. U.K. Department of Energy, Digest of U.K. Energy Statistics. HMSO, London (1977). 2. P. W. O’Callaghan, Energy for Industry, p. 383. Pergamon Press Oxford (1979). 3. I. E. Smith, Aspects of Energy Conversion (edited by I. M. Blair, B. D. Jones and A. J. Van Horn. Pergamon Press, Oxford (1976). 4. R. D. Biswas, Solar Energy 19, 99 (1977). 5. P. W. O’Callaghan et al., “Thermal Energy Storage Systems”, Progress Rep. No. 4, EEC Contract 398-78-EEUK (1979). 6. M. J. Nissen, “Resaturation of Salt Solutions for Application to Thermal Energy Storage”, M.Sc thesis, Cranfield Institute of Technology, Cranfield, Bedford MK43 OAL, England (1979). 7. K. W. Kauffman, H. G. Lorsch, and D. M. Kyllonen, “Thermal Energy Storage by means of Saturated Aqueous Solutions”, Final Rep., U.S. Department of Energy Contract No. EY-76-C-05-5158 (1977).