Encapsulated Phase Change Materials (EPCM) Thermal Energy Storage (TES)

Terrafore reduced the cost of storing thermal energy by developing a cascaded Encapsulated Phase Change material (EPCM) Thermal Energy Storage (TES), towards meeting the Department of Energy’s SunShot cost goal of $15 per kWh for thermal storage.

With funding from the Department of Energy, Terrafore successfully solved the energy storage problem using phase change salts which increased the energy density by 50% thus reducing the amount of salt required to store energy. To achieve this, Terrafore, working with Southwest Research Institute, broke through the technology barrier to successfully create an open space in the capsules so that solid salt can expand when it melts. Making capsules with phase change materials melting at high temperatures has never been done before. By using a single tank packed with three different types of salt- capsules, they reduced the container size by 50%.

(The heat collected in a solar receiver by a heat transfer fluid is sensible heat. For example, in a solar tower receiver, a molten salt heat transfer fluid collects heat by heating the fluid from 300 C to 560 C) Using mathematical models, Terrafore showed that to efficiently transfer the sensible heat collected in a solar receiver by heating a heat transfer fluid and to efficiently use the phase change latent heat of fusion in salts, the capsules size should be about 15 mm; it also needs selecting at least three salts melting at specified temperatures. For example, in a solar tower receiver heat is collected by heating a molten salt heat transfer fluid from 300ᵒC to 560ᵒC. For this system, Terrafore’s models showed that the melting point of the salt at the bottom third of the tank should be about 15C higher than the cold heat transfer fluid or about 315ᵒC, the melting point of salt at the top third of the tank should be about 15C lower than the high operating temperature or about 545ᵒC, and the remainder third in the middle of the tank can be between 380ᵒC to 450ᵒC. However, a cost optimal solution for amounts of salt in each layer is determined by the cost of salts, their latent heat of fusion, whether they are readily available in large quantities and other properties. Terrafore’s mathematical model can be used to select the optimum phase change salt mixtures.

Terrafore compared the cost of the conventional two-tank molten salt storage system with their single tank EPCM-TES, and showed that the costs of TES can be reduced by over 40% in the near term and potentially achieve the SunShot goal of $15 per kWh.

Even though Terrafore proved the concept of making capsules and they survived up to 5000 thermal cycles in an accelerated test rig built by Terrafore, the process of making the capsules using commercial coating equipment has yet to be optimized and needs to scale up. In each thermal cycle, the salt in the capsule was cycled between melting and freezing in a hot and cold molten salt mixture. Terrafore is making this patented technology available for licensing to CSP developers and other interested parties for taking this next step.

Terrafore displayed this innovative TES technology at the 2014 ARPA-E Transformative Energy Solutions Summit Showcase, in Washington DC on February 24-26, 2014.

For further information, please contact:

Anoop Mathur at Anoop.mathur@terraforetechnologies.com , phone number 951-313-6333

Technical publication pdf document