Off-grid test results of a solar-powered hydrogen refuelling station for fuel cell powered Unmanned Aerial Vehicles

• A renewable-based off-grid solution to refuel fuel cell powered UAVs is presented.
• An alkaline electrolyser's dynamic response under off-grid operation is characterized.
• Attention is paid to the overall efficiency of the refuelling system operating in off-grid mode.
• For aerospace applications, performance & reliability of the electrolyser require optimization.

Fuel cell (FC) propulsion for small (MTOW < 25 kg) Unmanned Aerial Vehicles (UAVs) provides a route for lower capital cost, environmentally friendlier and low noise operation. Most FC-based UAVs tested to date rely on compressed gas cylinders delivered to the point of use and used to refill the UAV hydrogen tanks on-site or chemical hydride systems to produce hydrogen on-board. An attractive alternative option is to produce hydrogen on-site from an off-grid renewable source according to the UAV fuel demand. A prototype off-grid solar-based hydrogen refuelling station for UAVs was developed for that purpose by Boeing Research & Technology Europe. A test program was carried out to evaluate the dynamic response of the hydrogen UAV refuelling system operating in an off-grid manner (disconnected from the AC grid). The system comprises a concentrated photovoltaic (CPV) array, an alkaline electrolyser, a low pressure hydrogen buffer tank and the required power electronics. The electrolyser was connected to the CPV source in an off-grid manner. The results from the off-grid tests are presented in this paper.From the results obtained, the operation of the CPV array and the CPV inverter connected to the electrolyser were both satisfactory, and no significant operational issues were observed. The overall energy efficiency of the CPV-inverter-electrolyser system was below 40%, mainly due to the excessively long start-up periods during which the electrolyser is not producing hydrogen, the slow dynamic response due to regular pressurization and depressurization cycles of the electrolyser and the excessive power consumption of the electrolyser auxiliaries. It is expected that if some optimization is carried out, mainly related to the control system of the electrolyser, the overall energy efficiency should increase and the dynamic response of the electrolyser can be improved.