Using Encapsulated Phase Change Salts for Concentrated Solar Power Plant
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Storing thermal energy as latent heat of fusion in phase change material (PCM), such as inorganic salt mixtures, can improve the energy density by as much as 50% while reducing the cost by over 40%. However, to discharge stored energy from PCMs, which has low thermal conductivity, requires a large heat transfer area which drives up the cost. Fortunately, salts encapsulated into small capsules can provide high specific surface area thus alleviating this problem. However, a technical barrier with encapsulating salts is that when it is produced, a void must be created inside the shell to allow for expansion of salt when it is heated above its melting point to 550 °C. Terrafore's method to economically create this void consists of using a sacrificial polymer which is coated as the middle layer between the salt prill and the shell material. The polymer is selected such that it decomposes much below the melting point of salt to gas leaving a void in the capsule. Salts with different melting points are encapsulated using the same recipe and contained in a packed bed consisting of salts with progressively higher melting points from bottom to top of the tank. This container serves as a cascaded energy storage medium to store heat transferred from the sensible heat energy collected in solar collectors. Mathematical models indicate that over 90% of salt in the capsules undergo phase change improving energy density by over 50% from a sensible-only thermal storage. Another advantage of this method is that it requires only a single tank as opposed to the two-tanks used in a sensible heat storage, thereby reducing the cost from a nominal $27 per kWht to $16 per kWht and coming close to the SunShot goal for thermal storage of $15 per kWht.