Metal hydrides are a special type of metallic alloys. They can absorb and desorb hydrogen reversibly, while releasing and absorbing thermal energy, respectively. Since the entropy of hydride is lower in comparison to metal and gaseous hydrogen phase at ambient and elevated temperatures, hydride formation is exothermic, while the reverse reaction (hydrogen release) is endothermic. The reaction equilibrium can be described by the Van’t Hoff expression as shown in Figure 1. When pressurised, most metals bind strongly with hydrogen, resulting in stable metal hydrides that can be used to store hydrogen conveniently on board vehicles. Examples of metal hydrides are LaNi5H6, MgH2, and NaAlH4. Metal hydrides can be liquids or powders that are usually stored in tanks at approximately 1 MPa (10 bar). As the pressure is reduced or the temperature is increased (between 120 °C and 200 °C), hydrogen is released. The metal hydride can be recharged without the use of a high pressure compressed gas or cryogenic liquid. When designing efficient metal hydride systems, the critical material properties to manipulate are thermal conductivity, heat of reaction and activation energy.

Metal hydride storage has a low risk of accidental leaks since the hydrogen is stored within the metal hydride crystal and requires energy to be released. In addition, metal hydride storage has an energy density (kWh/m3) that is about three times higher than compressed storage and cryogenic storage. According to the historical Hydride Information Center database of DOE, 2722 metal hydrides are reported by 2014.

Metal hydrides are used for their hydrogen storage and compressors capabilities. Metal hydrides are also used for heat storage, heat pumps and isotope separation. The uses include sensors, activators, purification, thermal storage and refrigeration. Metal hydrides are often used in fuel cell applications that use hydrogen as a fuel. Nickel hydrides are often found in various types of batteries, particularly NiMH batteries. Nickel metal hydride batteries rely on hydrides of rare earth intermetallic compounds, such as lanthanum or neodymium bonded with cobalt or manganese. Lithium hydrides and sodium borohydride both serve as reducing agents in chemistry applications. Most hydrides behave as reducing agents in chemical reactions.

The dual metal hydride system can operate flexibly in three distinct thermodynamic cycles: cooling, heat pumping, and converting low-grade heat to high-grade heat (heat upgrade cycle), which is unique to this metal hydride heat pump system because these cycles occur in the single system. This implies the system can be greatly modified by adjusting the metal hydride pair with different thermodynamic properties (e.g., enthalpy and entropy) and thus leveraging the metal/hydrogen interaction characteristics, which makes it possible to respond to various input waste-heat temperature sources. It does not require any harmful chemicals. Figure shows a temperature and pressure changes of typical cooling/heat pump cycle of a dual metal hydride pair contained in two reactors.

Conventional heat transfer modes, named heat conduction, heat convection, and heat radiation, always transport heat upon temperature gradient as a driving force, i.e., heat flow from high temperature to low temperature. Meanwhile, thermal wave/resonance mode of heat transfer comes from cross-coupling among different transport processes in a medium and can transport heat isothermally or even from low temperature to high temperature. This mode of heat transfer is characterized with its wave-type distributions of temperature or various spatial/temporal derivatives depending on tunable cross coupling. Thermal waves/resonance shows strong predominance over all the other three modes of heat transfer. By using the thermal wave effect, heat or cooling is not dissipated into ambient environment but accumulated inside adiabatic channel, which makes it possible to overcome the heat transfer problem of the metal hydride system.