ACT has been developing thermal storage technologies based on metal hydrides and phase change materials (PCM).
PCMs are traditional thermal storage materials because of their relative maturity. Metal hydrides are relatively new for thermal storage applications. The biggest advantage of metal hydrides is the volumetric heat capacity. For example, Ca0.2Mm0.8Ni5, a common metal hydride, has a heat capacity of 819.3 MJ/m3, while Rubitherm RT2, a common PCM, has a heat capacity of 177.5 MJ/m3. Metal hydrides for a wide range of operating temperatures are readily available from commercial sources.
Both metal hydrides and PCMs, in the form of raw materials, have very low thermal conductivities (on the order of 0.1W/m-K). This is a detriment to designing a compact thermal storage system. Metal fins or graphite foams are often used with PCMs to enhance the heat transfer. For metal hydrides, ACT utilizes a “micro-encapsulation technique” to achieve a thermal conductivity of 5W/m-K or greater.
The figure below shows a prototype thermal storage test article that has a heat storage volume of 5cc. The test article consists of a heat pipe, a heat storage material container and a water cooling jacket. A cartridge heater is attached to the heat pipe evaporator to provide heat input. The test article is insulated from the ambient to minimize the heat loss during testing.
Two test articles of identical dimensions were fabricated and tested at the same conditions. One of the articles used a metal hydride and the other used a PCM in the heat storage container. Above is a photo of the test setup.
The figure to the left shows the temperature measurements for both devices (pink color for the PCM device and blue color for the metal hydride device). The heat input was cycled between 0 and 15W, with the 15W input maintained for 370 seconds during each cycle. The cooling water was turned off during heat storage and on during regeneration. The metal hydride device was able to absorb the heat input and maintain the heat source at a lower temperature than the PCM device. The metal hydride device also regenerated more quickly than the PCM device.
Based on the subscale prototype test results, ACT conducted a subsystem level design analysis of using both metal hydrides and PCMs for some high power applications. This analysis was based on actual system specifications. The metal hydride and PCM heat storage designs included all the necessary accessories and supporting structures.
The following conclusions are applicable to one of the systems analyzed:
- The metal hydride heat storage module volume is less than 1/3 of the PCM module.
- A smaller volume results in less resistance to the system coolant flow.
- The metal hydride module is approximately 32.5% lower mass than the PCM module.
ACT is continuing the technology development for multiple applications including military high power lasers and consumer products.