Thermal Energy Storage

This invention relates to the storage of thermal energy, and particularly to a thermal energy storage material, a method of preparing this material, and a system incorporating the material.

Salt hydrates are known for their ability to store thermal energy. These salt hydrates absorb thermal energy as they melt or decompose and release heat as they revert to the solid hydrated crystalline form. Na.sub.2 SO.sub.4.10H.sub.2 O is the most commonly reported hydrate since it is relatively inexpensive and has high heat storage capacity. However, Na.sub.2 SO.sub.4.10H.sub.2 O has a number of undesirable characteristics that present difficulties when used in thermal energy storage systems. Although, ideally, the decomposition of Na.sub.2 SO.sub.4.10H.sub.2 O can be reversed on cooling, complete reversibility is difficult to achieve because rapid sedimentation of the decomposition product (anhydrous Na.sub.2 SO.sub.4) occurs. U.S. Pat. No. 3,986,969 to M. Telkes disclosed the use of thickening agents to reduce this sedimentation and improve reversibility. D. R. Biswas, "Thermal Energy Storage Using Sodium Sulfate Decahydrate and Water," Solar Energy 19, 99 (1977), discloses the use of excess water. Excess water ensures complete reversibility but the energy capacity of the system is reduced and the heat delivery occurs at a steadily declining temperature. In static systems, supercooling is reported and heat acceptance and delivery rates vary with the state of charge.

An object of the present invention is to provide a thermal storage composition which delivers thermal energy at a substantially constant temperature as it crystallizes.

Another object is to provide a thermal storage composition for which there is no significant deterioration with repeated cycling.

Another object is to provide a thermal energy storage system that allows recovery of heat at a substantially constant temperature and substantially constant delivery rate and maximizes the heat recoverable from a given volume of storage material.

It has been found that these objects can be met by a material comprising Na.sub.2 SO.sub.4, Na.sub.2 HPO.sub.4 and water, preferably with Na.sub.2 SO.sub.4 and Na.sub.2 HPO.sub.4 in approximately equal molar proportions, and sufficient water to form the hydrates Na.sub.2 SO.sub.4.10H.sub.2 O and Na.sub.2 HPO.sub.4.12H.sub.2 O, respectively.

Although it is not understood with certainty why the present mixture of the hydrates of Na.sub.2 SO.sub.4 and Na.sub.2 HPO.sub.4 should perform differently from the salts individually, it is believed that the unique properties are due to the fact that no mixed crystals or eutectics are formed. It appears that the proximity of crystallization temperatures of these two hydrates is a factor. Analytical results are consistent with the simultaneous but independent growth of Na.sub.2 SO.sub.4.10H.sub.2 O and Na.sub.2 HPO.sub.4.12H.sub.2 O crystals. At complete solidification, the total of water used equals the sum of the stoichiometric requirements of the two hydrates. However, up to this point, the individual crystallizing salt hydrates contact a liquid phase that, in effect, is rich in water. This reduces the possibility of anhydrous salt formation, making the system fully reversible.

According to one aspect of the present invention, there is provided a thermal energy storage material comprising a mixture of Na.sub.2 SO.sub.4, Na.sub.2 HPO.sub.4 and water, with a Na.sub.2 SO.sub.4 :Na.sub.2 HPO.sub.4 mole ratio of from 1.1:0.9 to 1:2, and sufficient water to convert all the Na.sub.2 SO.sub.4 and Na.sub.2 HPO.sub.4 to Na.sub.2 SO.sub.4.10H.sub.2 O and Na.sub.2 HPO.sub.4.12H.sub.2 O, respectively.

The above storage material can best be produced by first dissolving all the selected quantity of Na.sub.2 SO.sub.4 in water and then adding and dissolving the selected quantity of Na.sub.2 HPO.sub.4. The balance of the water, if any, needed to form the hydrates of the salts, as described above, can then be added. Note that any water contained in the original salts used in the mixture preparation should be included in the calculation of the total water needed in the system to form the hydrates Na.sub.2 SO.sub.4.10H.sub.2 O and Na.sub.2 HPO.sub.4.12H.sub.2 O, respectively.

According to another aspect of the present invention, there is provided a thermal energy storage system comprising the above thermal energy storage material, an energy transfer fluid immiscible with the storage material, and means for passing said fluid through the energy storage material for exchange of thermal energy.

FIG. 1 illustrates one embodiment of a system in accordance with the present invention.

FIG. 2 illustrates graphically the temperature profile of two runs referred to in the Example, comparing the present invention with a system using Na.sub.2 SO.sub.4.10H.sub.2 O alone.

FIG. 3 illustrates the energy delivery rate for the runs shown in FIG. 2.

The thermal energy storage material of the present invention comprises a mixture of Na.sub.2 SO.sub.4, Na.sub.2 HPO.sub.4 and water. The best performance in terms of constancy of energy delivery temperature and rate, and reversibility, was found to be obtainable with equal molar quantities of Na.sub.2 SO.sub.4 and Na.sub.2 HPO.sub.4 and sufficient water to convert all the salt to the hydrate form Na.sub.2 SO.sub.4.10H.sub.2 O and Na.sub.2 HPO.sub.4.12H.sub.2 O. It was found that such a mixture crystallizes at about 28.degree. C.

It appears that a useful thermal energy storage material is obtainable with a Na.sub.2 SO.sub.4 :Na.sub.2 HPO.sub.4 mole ratio range of from 1.1:0.9 to 1:2. As the Na.sub.2 SO.sub.4 :Na.sub.2 HPO.sub.4 mole ratio is increased beyond 1.1:0.9, there is an increasing tendency for anhydrous salt formation and irreversibility. With the Na.sub.2 SO.sub.4 :Na.sub.2 HPO.sub.4 mole ratio reduced below 1:2, there is an increasing tendency for formation of the hydrate Na.sub.2 HPO.sub.4.7H.sub.2 O instead of the desired Na.sub.2 HPO.sub.4.12H.sub.2 O.

After the Na.sub.2 SO.sub.4 :Na.sub.2 HPO.sub.4 ratio has been selected, the proportion of water required to convert all salt to the hydrate form can be calculated. When the quantities of all the constituents have been determined, the mixture can be prepared. It was found that the desired material could be obtained if all the selected Na.sub.2 SO.sub.4 is first mixed with sufficient water to dissolve it readily, following which the selected quantity of Na.sub.2 HPO.sub.4 is added, followed by the balance of the water, if any. If however, the anhydrous salts are mixed together first and then mixed with water, incomplete dissolution of Na.sub.2 SO.sub.4 results.

FIG. 1 illustrates a system utilizing the present thermal energy storage material. A vessel 1 contains the thermal energy storage medium 2 and an immiscible thermal energy transfer fluid 3. A pump 4 circulates the energy transfer fluid for contact with the energy storage material 2 in the container 1 by means of the inlet pipe 5 and outlet pipe 6. Three-way valves 7 and 8 control the flow of fluid to the heat exchangers 9 and 10, respectively. In a solar heating system, for example, the heat exchanger 9 may form part of a solar collector while 10 provides heat for a building. A salt trap 11 containing water through which the energy transfer fluid bubbles collects any salt carried over from outlet 6 and so protects downstream parts of the system from salt deposition.

In operation, the energy storage material crystallizes to the hydrate form as heat is removed from the fluid at the heat exchanger 10, and decomposes as heat is supplied at 9.

It will be understood that the system can be arranged in various ways, or used in various applications benefiting from thermal energy storage.

Two thermal energy storage systems were compared, one using aqueous Na.sub.2 SO.sub.4, and the other using Na.sub.2 SO.sub.4 and Na.sub.2 HPO.sub.4 in accordance with the present invention. Both compositions were tested with the same apparatus using aliphatic hydrocarbon solvent, specifically Varsol .sup.R as the heat transfer fluid (HTF). The apparatus similar to that illustrated in FIG. 1. The storage container volume was 4 cu.ft. containing 250 lbs. of the energy storage material and the balance heat transfer fluid. For the Na.sub.2 SO.sub.4, in order to provide full reversibility, additional water in excess of the stoichiometric requirement for the hydrate was used. The amount of excess water approximated the amount recommended by D. R. Biswas, referred to heretofore, namely, 31.8% water by weight. Without such excess of water, anhydrous Na.sub.2 SO.sub.4 forms and reversibility is lost after a few cycles. The actual values of excess water are given in Table 1.

Tables 1 and 2 show samples of data obtained from numerous runs for the Na.sub.2 SO.sub.4 and Na.sub.2 SO.sub.4 --Na.sub.2 HPO.sub.4 systems, respectively. To provide a consistent basis of comparison, the storage capacity is listed in terms of latent heat obtained by subtracting the specific, or sensible, heat resulting from the temperature drop of the run. The data indicates that the Na.sub.2 SO.sub.4 --Na.sub.2 HPO.sub.4 mixture provided a significantly higher latent heat output per unit volume, indicating a greater storage capacity and consequently, allowing the use of a smaller storage container for a given energy storage capacity. Also, the lower "% standard deviation" indicates a more constant power output from the present material.

FIG. 2 provides a comparison of the temperature profile during a typical run for both a Na.sub.2 SO.sub.4 and a Na.sub.2 SO.sub.4 --Na.sub.2 HPO.sub.4 mixture of the present invention. FIG. 2 shows that the Na.sub.2 SO.sub.4 --Na.sub.2 HPO.sub.4 mixture provides a relatively constant delivery temperature, while the Na.sub.2 SO.sub.4 has a temperature drop that makes difficult full utilization of the latent energy available for the heating of buildings, without the use of a heat pump. FIG. 3 shows that the energy output for a given mass of heat transfer fluid remains relatively constant for the Na.sub.2 SO.sub.4 --Na.sub.2 HPO.sub.4 mixture as compared with Na.sub.2 SO.sub.4. Mixtures prepared from the formulation and by the method disclosed here were cycled a multitude of times through freezing and thawing experiments. The material was found to be completely reversible throughout these repeated cycles and no evidence of phase separation was found.

Further experiments were done to determine the applicability of the composition in a static system in closed containers. This simple containment has been used for proprietary thickened sodium sulfate mixes in solar space heating storage applications. In order to establish the practicality of the composition in this type of static containment, a number of glass vials, 60 ml volume, were filled with the mixture and closed with a cap. By immersion in a controlled temperature bath, the mixture was cycled between the fully charged liquid state and the fully discharged solid state. It was found that the system was completely reversible and no undissolved material accumulated. Crystallization did not occur until 22.degree. C. indicating supercooling of about 6.degree. C.