Systems And Methods For Thermal Energy Storage
In one embodiment, a thermal energy storage system includes a first cascade containing a first phase change material and a second cascade containing a second phase change material, wherein the melting point of the first phase change material is different than the melting point of the second phase material.
This application claims priority to co-pending U.S. Provisional Application Ser. No. 61/553,598 filed Oct. 31, 2011, which is hereby incorporated by reference herein in its entirety.
BACKGROUNDThermal energy storage will be essential for solar thermal power plants to feed into the power grid in the future. At present, synthetic oils or molten salts are used for thermal energy storage. Both of these materials store energy as sensible heat, which requires a large amount of the material. Recently, phase change materials (PCMs) that store heat as latent heat have been suggested for use in thermal energy storage. PCMs can store a great deal more heat per unit mass, and therefore require much less material. However, PCMs store latent heat at only one unique temperature for that material, while heat is often needed across a range of temperatures. Although cascaded PCM storage has been suggested, such systems still do not match the needed storage/discharge characteristics. Furthermore, PCM heat conductivities are very small, which creates an obstacle that must be overcome to make full use of them in thermal energy storage applications.
In view of the foregoing discussion, it can be appreciated that it would be desirable to have an improved system and method for thermal energy storage.
The present disclosure may be better understood with reference to the following figures. Matching reference numerals designate corresponding parts throughout the figures, which are not necessarily drawn to scale.
As described above, it would be desirable to have an improved system and method for thermal energy storage, which could be used in solar thermal power plants. More particularly, it would be desirable to have such a system and method that utilizes the desirable properties of phase change materials (PCMs). Disclosed herein are examples of such systems and methods.
In the following disclosure, various embodiments are described. It is to be understood that those embodiments are example implementations of the disclosed inventions and that alternative embodiments are possible. All such embodiments are intended to fall within the scope of this disclosure.
The capsule wall 26 defines an interior space 28 that is filled with a thermal energy storage material 30. In some embodiments, the thermal energy storage material 30 comprises a single PCM. In other embodiments, the thermal energy storage material 30 can comprise a mixture of two or more different PCMs that have different phase change temperatures. The PCMs can, for example, comprise one or more salts, such as sodium nitrate. Preferably, the mixture of PCMs is non-eutectic so that the mixture will not behave as a single material. In some embodiments, the phase change temperatures of the PCMs in the mixture fall within a relatively narrow range. For example, the capsule 24 could contain three different PCMs, a first PCM having a phase change temperature of approximately 300° C., a second PCM having a phase change temperature of approximately 305° C., and a third PCM having a phase change temperature of approximately 310° C. In such a case, the PCMs would sequentially change from solid to liquid as they are heated and their respective phase change temperatures are crossed, and would likewise sequentially change from liquid to solid as they are cooled and the phase change temperatures are again crossed. Capsule shapes other than spherical, such as egg shapes, ellipsoids, rectangular, conical, or even odd shapes are also possible.
When the thermal energy storage system 10 includes cascades containing PCM capsules, such as capsules 24 or 32, the heat transfer liquid can flow through the cascades and between the capsules, making direct contact with those capsules. With reference to
In some embodiments, each cascade contains capsules that each contain a same single PCM. For example, the first cascade 12 could comprise capsules containing a single PCM that changes phase at a temperature of approximately 300° C., the second cascade 14 could comprise capsules containing a single PCM that changes phase at a temperature of approximately 350° C., and the third cascade 16 could comprise capsules containing a single PCM that changes phase at a temperature of approximately 400° C.
In other embodiments, each cascade contains capsules containing different single PCMs, wherein each PCM changes phase within a relatively narrow range of temperatures. For example, the first cascade 12 could contain three different types of capsules, a first type of capsule that contains a single PCM that changes phase at approximately 300° C., a second type of capsule that contains a single PCM that changes phase at approximately 305° C., and third type of capsule that contains a single PCM that changes phase at approximately 310° C., such that a range of approximately 300° C. to 310° C. is covered. Each of the second and the third cascades 14 and 16 could likewise contain multiple types of capsules that together cover a different range of temperature.
In still other embodiments, each cascade can contain capsules that each contain multiple PCMs, wherein each PCM changes phase within a relatively narrow range of temperatures. For example, the first cascade 12 could contain capsules that contain multiple PCMs that each changes phase within the range of approximately 300° C. to 310° C., the second cascade 14 could contain capsules that contain multiple PCMs that each changes phase with the range of approximately 350° C. to approximately 360° C., and the third cascade 16 could contain capsules that contain multiple PCMs that each changes phase with the range of approximately 400° C. to 410° C.
In yet other embodiments, combinations of the above-described embodiments could also be used.
The thermal energy storage material within the cascades 52, 54, and 56 comprises PCMs. In some embodiments, the PCMs are in bulk form, meaning that they are not encapsulated in capsules or other discrete elements. A single different PCM can be contained within each cascade 52, 54, and 56. For example, the first cascade 52 can contain a first PCM that changes phases a relatively high temperature (e.g., 370° C. to 410° C.), the second cascade 54 can contain a second PCM that changes phases a medium temperature (e.g., 340° C. to 370° C.), and the third cascade 56 can contain a third PCM that changes phases a relatively low temperature (e.g., 300° C. to 340° C.). In general, other temperatures and temperature ranges can be used. In other embodiments, each cascade can contain a non-eutectic mixture of multiple PCMs, each of which changes phase within a relatively narrow range of temperatures. For example, the first cascade 52 could contain a mixture of multiple PCMs that each changes phase within the range of approximately 300° C. to 310° C., the second cascade 54 could contain a mixture of multiple PCMs that each changes phase with the range of approximately 350° C. to 360° C., and the third cascade 56 could contain a mixture of multiple PCMs that each changes phase with the range of approximately 400° C. to 410° C.
In the above-described systems, each cascade comprised one or more PCMs. It is noted, however, that one or more of the cascades could include sensible heat storage material. For instance, if the system included three cascades, the highest and lowest temperature cascades could comprise one or more PCMs, and the intermediate temperature cascade could comprise sensible heat storage material.
Claims
1. A thermal energy storage system comprising:
- first cascade containing a first phase change material; and
- a second cascade containing a second phase change material;
- wherein the melting point of the first phase change material is different than the melting point of the second phase material.
2. The system of claim 1, wherein the first and second cascades comprise first and second storage tanks and further comprising pipes that enable a heat transfer liquid to flow from one tank to the other tank.
3. The system of claim 1, wherein the first and second cascades comprise first and second compartments of a single storage tank.
4. The system of claim 1, wherein at least one of the phase change materials is contained within thermal energy storage capsules.
5. The system of claim 4, wherein the thermal energy storage capsules comprise rigid capsule walls.
6. The system of claim 5, wherein the capsule walls are spherical.
7. The system of claim 4, wherein the thermal energy storage capsules contain a single phase change material.
8. The system of claim 4, wherein the thermal energy storage capsules contain a non-eutectic mixture of multiple phase change materials.
9. The system of claim 4, wherein the thermal energy storage capsules contain phase change material particles suspended within a liquid.
10. The system of claim 4, wherein at least one of the cascades contains multiple types of thermal energy storage capsules, each type of capsule containing a different phase change material.
11. The system of claim 1, wherein at least one of the cascades comprises a single phase change material in bulk.
12. The system of claim 1, wherein at least one of the cascades comprises a non-eutectic mixture of multiple phase change materials in bulk.
13. The system of claim 1, wherein at least one of the cascades comprises a piece of foam that contains a phase change material.
14. The system of claim 13, wherein the piece of foam contains a single phase change material.
15. The system of claim 13, wherein the piece of foam contains a non-eutectic mixture of multiple phase change materials.
16. A thermal energy storage capsule comprising:
- a capsule wall that defines an interior space; and
- a phase change material contained within the interior space.
17. The capsule of claim 16, wherein the capsule wall is spherical.
18. The capsule of claim 17, wherein the capsule has an outer diameter of approximately 1 to 2 inches.
19. The capsule of claim 16, wherein the capsule contains a single phase change material.
20. The capsule of claim 16, wherein the capsule contains a non-eutectic mixture of multiple phase change materials.
21. The capsule of claim 16, wherein the capsule contains phase change material particles suspended within a liquid.
Type: Application
Filed: Oct 31, 2012
Publication Date: May 2, 2013
Inventors: Dharendra Yogi Goswami (Tampa, FL), Elias K. Stefanakos (Tampa, FL), Nitin Goel (Ghaziabad)
Application Number: 13/665,389