Metal hydride-based cooling system for fuel cell electric vehicles: Achieving a temperature lift of 40 K

The open metal hydride cooling system (MHCS) is a promising technology to recover compression energy required to fill the pressure tank of a fuel cell electric vehicle. By means of the thermochemical reaction of hydrogen with metal hydrides, the MHCS directly converts the available pressure difference into a heat pump effect to replace the conventional energy-consuming air-conditioning. Previously published studies already demonstrated a high specific power under vehicle-relevant operation. However, only a maximum temperature lift of 20 K was achieved so far, that is considered as too low for application as a vehicle air-conditioner. In order to achieve a high temperature lift, the underlaying thermochemical reaction behavior requires a high pressure ratio. To further reach a high specific power and efficiency at elevated temperature lifts, additionally the applied reactor has to facilitate a high heat transfer and low heat capacity. Thus, in this study experimental investigations using a high pressure ratio of 70 to 5 bar are conducted with a proven reactor-material setup. The comprehensive characterization on different discharge rates show a maximum specific cooling power of 386 W/kgMH at a temperature lift of 30 K and 229 W/kgMH at 40 K, where application-relevant temperatures of 5 °C for the cooling side and 45 °C for rejection side are reached. On the basis of these results a scaling of the lab-scale reactor is calculated revealing a total efficiency improvement on vehicle level in the order of 5 % using the open MHCS.