Combined Hydrogen and Heat Storage System
An Austrian research center demonstrated a technology where the dissipation heat generated by a fuel cell is partly absorbed by a metal hydride hydrogen storage material. This allows the fuel cell to be cooled while liberating hydrogen, reducing the need for thermal management hardware. A possible terrestrial application are self-sustained habitats.
Batteries do not scale well if large amounts of energy (MWhrs) have to be stored at tens of kW performance. In this area and above, a closed-loop reversible fuel cell system (RFCS) can outperform batteries. A photovoltaic RFCS may be used as a base-loadable renewable energy source.
A major obstacle to fuel cell technology implementation is the hydrogen storage problem. Reversible metal hydrides are an effective method of storing hydrogen in the solid state; the gravimetric and volumetric storage densities achievable exceed those of a commercial pressurized steel vessel. There is a suitable metal hydride material for virtually any temperature and pressure window. Usually the operating temperature of the fuel cell defines this window. In order to be reversible, the desorption has to be endothermic, the absorption an exothermic process. However, most technological applications of interest (e.g. automotive) require the storage of several kilograms of hydrogen, resulting in tens of kilograms of high storage capacity metal hydride (~40+ kJ per mol H2). At this scale, the fueling process results in a substantial amount of heat, and thus heat management becomes a key feature. Radiators add substantial bulk mass, reducing overall system efficiency and rendering many otherwise practical metal hydride hydrogen storage applications unattractive (e.g. automotive).
This detrimental property of hydrogen storage in metal hydrides is turned into an advantage by the technology offered. A high-temperature fuel cell generates about 60 kJ per mol H2 during steady state operation. The best state-of-the-art metal hydride compatible with the temperature window of a high temperature polymer electrolyte membrane fuel cell (HT-PEMFC <200�C) is Ti-catalyzed NaAlH4, which absorbs about 40 kJ per mole desorbed hydrogen. Hence about two thirds of the HT-PEMFC dissipation heat may be absorbed by the metal hydride and utilized for the liberation of hydrogen. The metal hydride has the dual purpose of buffering heat and hydrogen over time, thus the need for thermal hardware can be substantially reduced.
Key to this technology is a heat exchanger for the coupling of heat and hydrogen mass flows. The sophisticated design needed could not be realized by conventional manufacturing techniques and was prepared by selective laser melting. The viability of the technology has been successfully demonstrated in a laboratory environment.
The initial target application of this technology is a RFCS as secondary energy supply for a 39 kW telecommunications satellite during eclipse. However, this technology may also have application in ships, submarines, offshore platforms or remote housing, which have a demand for high electrical energy storage at high electrical performance.
Innovations & Advantages
A RFCS is rather complex and comprises five subsystems: the fuel cell, the storage containers, electrolyzer, heat management and system control. A harmonious integration of these subsystems is imperative for system efficiency; otherwise gains at the individual subsystem level may be reduced or even nullified. The present technology is an outstanding example of such a harmonious integration. Synergies are created between the storage container and heat management subsystem, and the fuel cell is being cooled by the hydrogen source.
The innovative coupling of heat and hydrogen storage is a fine example of the optimizations needed for advancing the RFCS technological readiness to a mature level. (The RFCS technology in general is still experimental / TRL-4.) The full benefit from this technology is obtained if electrical energy has to be provided and substantial amounts of heat are generated, especially in confined environments like satellites or submarines.
Fuel cell powered submarines with low storage capacity interstitial metal hydrides (~25 kJ per mol H2, ~1.5 wt.% H2) are already a mature technology. The innovative heat exchanger technology of the offer allows the employment of medium temperature Na-Al-H metal hydrides with about double the hydrogen storage capacity (~3 wt.%) at about the same heat evolution. The price of sodium and aluminum is substantially lower compared to the elements contained in classical metal hydrides such as HydralloyTM.
This advantage in cost and performance may help to make the RFCS technology attractive for civilian applications such as cargo ships and offshore energy platforms and remote housing and research stations.
Current and Potential Domains of Application