Similar to vapor venting thermal storage, hydride thermal storage offers the potential for lower mass and volume systems, when compared with conventional PCM thermal storage systems.
When a hydrogen atmosphere surrounds a metal, hydrogen molecules can split into atoms at the metal surface, allowing the hydrogen ions to permeate into the metal. Metal hydride systems are fabricated using materials that can store significant amounts of hydrogen. They have been used in many applications such as battery electrode material and as a hydrogen storage medium for automotive applications.
They are also suitable for thermal storage systems, since a metal hydride gives off heat when hydrogen is absorbed, and stores heat when hydrogen is desorbed. The hydriding/dehydriding reaction of a metal hydride can be written as:
where M is the metal (or metal alloy), x is a non-stoichiometric constant and ΔHf is the hydride formation heat. As illustrated in Figure 1, metal hydrides can absorb large amounts of hydrogen via bulk chemisorption and subsequent hydriding reactions.
Thermal storage is provided by the release of the absorbed hydrogen when heat is applied. Metal hydrides are capable of storing huge amounts of heat with rapid reaction kinetics. Theoretically, when one (1) mole of H2 gas is released/absorbed, the metal hydride can absorb/release approximately 25-35 kJ of heat. 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. The gravimetric heat storage capacity of metal hydrides is around 200 kJ/kg.
For the temperature range around 65-80°C, Lanthanum based metal hydrides (e.g., LaNi5) or Calcium-Mischmetal based metal hydrides (e.g., Ca0.7Mm0.4Ni5) could be used, which offers adequate dissociation pressures, fast reaction, low hysteresis, and easy activation. Hydrogen dissociation pressures for these two hydride systems are shown in Table 1. Besides the commercial metal hydrides, numerous research materials are available that provide better performance, and can be tailored for particular applications.
Table 1. Hydrogen Pressure as a Function of Temperature for Two Hydrides
|Ca0.7Mm0.3Ni5||2.5 psia||216 psia||282 psia|
|LaNi5||0.7 psia||124 psia||168 psia|
Conductivity and Multiple Hydriding/Dehydriding Cycles
Conventional hydride materials with a polycrystalline structure have relatively low hydrogen/heat storage capacity by weight and relatively slow absorption/desorption kinetics. Substantial improvements in the hydriding/dehydriding properties of metal hydrides could possibly be achieved by the formation of nanocrystalline structures using non-equilibrium processing techniques such as mechanical alloying or high-energy ball-milling. It has been shown that nanocrystalline LaNi5 type metal hydrides lead to advanced nanocrystalline intermetallics representing a new generation of metal hydride materials with the following characteristics: high storage capacity, stable temperature-pressure cycling capacity during the life-time of the system, low hysteresis, and good corrosion stability. A scanning electron micrograph of another commonly used high temperature metal hydride, MgH2, after ball milling is shown in 2. The surface and morphology of Mg particles after milling were rough and irregular. SEM observation showed small catalyst particle clusters covering larger Mg particles after milling. The surface area and effective grain boundaries of Mg hydrides significantly increased, resulting in enhanced sorption kinetics by providing a greatly increased number of reactive sites during high energy ball milling.
There are two factors that must be considered when using nanocrystalline hydrides. The first is low effective thermal conductivity. Typical hydride particles are generally small, to improve the reaction kinetics by minimizing the permeation length. Small particles, even if sintered, have a much lower thermal conductivity than the bulk material. Either high conductivity fins or heat pipes are used to improve the thermal conductivity, in a similar fashion to PCM heat sinks.
In most metal hydrides undergoing hydriding/dehydriding cycles, high volume strains lead to “decrepitation” (or pulverization) of metal hydrides into a powdered bed of micron-sized particles. The low thermal conductivity of such a powder bed (effective thermal conductivity = ~10-1 W/m-K) limits the heat transfer and, therefore, retards the apparent kinetics. ACT has used a metal hydride micro-encapsulation technique to coat the hydride powder particle with a thin (submicron size) copper layer using an electroless plating technique, as shown in Figure 3. This micro-encapsulation allows the decrepitated metal hydride particles to be contained inside a thin copper shell even after many hydriding/dehydriding cycles. Provided with a driving force, hydrogen readily diffuses through the thin copper layer. The micro-encapsulated technique can bring the hydride powder particle thermal conductivity up to ~3-5 W/m-K, which is a greater than 50 times improvement in effective thermal conductivity.
ACT has developed a multiple-cycle hydride system, and is looking into single use hydride venting systems:
- Single Use Hydride Venting Systems (One to a Few Cycles) offer a potential volume reduction of 90 percent or more, when compared with PCM systems.
- Hydride Thermal Storage for Multiple Cycles (Dampening)
. Chan-yeol Seo, Yeong Yoo and Zahir Dehouche, “Hydrogen sorption kinetics of nanocrystalline Mg-based composites using proton conductive ceramic catalysts”, Proceedings International Hydrogen Energy Congress and Exhibition IHEC 2005, Istanbul, Turkey, 13-15 July 2005