Using the potential energy from high pressure tanks in fuel cell vehicles for powering an innovative air-conditioning process based on metal hydrides

Kurzfassung

Introduction By means of the reversible reaction of metal hydrides (MH) with hydrogen the so far unused potential energy of the compressed gas in a hydrogen pressure tanks can be utilised for air conditioning of the cabin and therefore improve the overall system efficiency. At present, the main challenge of this technology is the low power to mass ratio and low achieved temperature lift of the reactor component. That is why in this work a new reactor design is proposed and experimentally characterised, which shows mayor improvements in the key performance indicators. Hydrogen powered vehicles are a promising technology especially for heavy duty vehicles with high range requirements and short refueling times to increase the share of renewable energy in the transport sector. State of the art on-board storage is realised with high pressure tanks (350 bar or 700 bar). For the compression work roughly 15 % of the energy content is needed at the refueling station [1] and during unloading not recovered in any way. The presented technology developed at the German Aero Space Center (DLR) can recover parts of this energy while unloading the tank during the driving cycle. The exothermic and endothermic reaction of metal hydrides can be used to generate refrigeration power for instance to use it in the cabin´s air-conditioning and therefore reduce the electric energy demand for the conventional air conditioning system. The application of this technology finally leads to an increased driving range and reduced fuel costs. Objectives Materials and Methods The working principle of the metal hydride cooling system is based on the following: Two reactors are arranged in the hydrogen supply path between a high-pressure tank and a hydrogen consumer (e.g. fuel cell). They are operating in anticyclic mode. One reactor exothermically absorbs hydrogen on medium-pressure level of (~70 bar) releasing heat to the ambient. The second reactor endothermically desorbs hydrogen at low pressure (~5 bar) level producing cold. As soon as the specific hydrogen capacity of the metal hydride in the reactors is reached, the first reactor switches from absorption to desorption and the second from desorption to absorption, confer Fig. 1. In this way a quasi-continuous hydrogen flow from a medium to a low-pressure side is realised, where no hydrogen is consumed. While at the same time heat is pumped quasi-continuously from a cold temperature level to the ambient level. A functional demonstration of this system in an application relevant configuration was previously published by the DLR in [2]. However, to reach the requirements of a mobile system in the automotive environment a significant higher power output with a lower system weight has to be realised. On the same time the efficiency should be high to use as much as possible of the available potential energy. Another requirement is the high temperature lift, which is necessary for mobile air-conditioning systems. The previous system showed a maximum specific power of ~275 W/kgMH and a cooling efficiency of ~60 % at 10 K temperature lift as its key performance indicators. For a higher temperature lift of 20 K however the specific power was significantly reduced to 60-140 W/kgMH and the efficiency to 30-40 %. The focus of this work is the new design of a reactor, which addresses these requirements. The new design has been deduced using a lumped model including simplified heat and mass transfer equations. It is based on the principle geometry of a pressure cylinder (15 mm inner diameter) with an integrated heat exchanger capability. Additive manufacturing is used to realize the integration of heat transfer fluid channels. With this manufacturing technology a thin walled lightweight design made of aluminium with high strength and heat conductivity could be realized. To improve the outer heat transfer micro fluid channels with a hydraulic diameter <1 mm are used in the outer shell of the pressure cylinder as such reactors are mainly heat transport limited in their thermal power capability. Furthermore, the metal hydride is not utilized as loose powder, but in form of compressed metal hydride-graphite composites with 15 wt.-% graphite to realize a high heat conductivity inside [3]. The built-up reactor incorporates 200 g of a room temperature type metal hydride based on TiMn1.5. Results The realized reactor shows a ratio of its own heat capacity to the MH’s heat capacity of 1,5. This valued is half the one, which showed the previous published reactor. On the same time the heat transport resistance is just increased by ~7 %. As a result, less thermal losses due to the empty reactor’s heat capacity is expected, which leads to a higher specific power at high temperature lifts (>10 K). After this design is presented comprehensively in the conference contribution, subsequently experimental results of its characterisation in the laboratory environment will be presented. This includes the key performance indicators as they are the MH mass specific thermal power, the cooling efficiency and temperature lift. The measurements are conducted at reference boundary conditions (30 °C / 20 °C and 35 bar / 5 bar) for comparison with previous reactor concepts as well on a broad variety of different boundary conditions to describe its operation characteristic on different temperatures (0-20 °C cold side, 30-55 °C hot side), pressure (35-70 bar medium-pressure side) and mass flow conditions. The experimental data should also show the expected trade-off between power and efficiency when varying the hydrogen mass flow. Conclusions and Acknowledgments The presented new reactor concept based on an integral design realised with additive manufacturing in combination with the utilisation of metal hydride-graphite composites is supposed to meet the requirements of a specific power of at least 250 W/kgMH and a cooling efficiency of at least 50 % at a temperature lift of 30 K. With this achievement the application relevance of this technology is significantly increased. References [1] M. Klell, H. Eichlseder, A. Trattner, Wasserstoff in der Fahrzeugtechnik, 4th ed., Springer Vieweg, 2018 [2] C. Weckerle, M. Nasir, R. Hegner, I. Bürger, M. Linder, Applied Energy 259 (2020) 114187. [3] C. Pohlmann, L. Röntzsch, F. Heubner, T. Weißgärber, B. Kieback, Journal of Power Sources 231 (2013) 97-105.