Similar to vapor venting thermal storage, hydride venting thermal storage offers the potential for lower mass and volume systems, when compared with conventional PCM thermal storage systems. Thermal storage is provided by the release of the absorbed hydrogen when heat is applied. Micro-encapsulated metal hydrides are capable of storing huge amounts of heat with rapid reaction kinetics. Metal hydrides have a superior volumetric heat storage capacity when compared to phase change materials. A commonly used metal hydride, LaNi5, has a theoretical heat storage capacity of 1200kJ/liter, while paraffin wax PCM material only has a heat storage capacity of 160-200kJ/liter. A greater heat storage density results in a more compact system with a tighter temperature control.
ACT has already demonstrated a hydride thermal storage system for multiple cycles. In this system, the heat input released hydrogen from the metal hydride, which was then stored in a tank. A hydrogen compressor was then used to drive the hydrogen back into metal hydride to recharge the system. Unfortunately, the added mass of the tank and compressor (using current technology) made the system less competitive than a simple PCM heat sink.
However, theoretical calculations have shown that a vented hydride system has a very large reduction in volume, when compared with a conventional PCM system. As shown in Table 1, a micro-encapsulated, vented hydride system would have one-half of the mass of a PCM system, but only 7% of the volume.
Table 1. Comparison of Mass and Volume for PCM and Metal Hydride Venting Systems
|3 kW for 45 sec = 0.135 MJ||PCM||1.5 kg||2.0 liter|
|Metal Hydride||0.78 kg||0.13 liter|
|1 kW for 30 min = 1.8 MJ||PCM||20.5 kg||26.6 liter|
|Metal Hydride||10.3 kg||1.69 liter|