Hydrogen as a battery for a rooftop household solar power generation unit

Decentralization of electrical power generation using rooftop solar units is projected to develop to not only mitigate power losses along transmission and distribution lines, but to control greenhouse gases emissions. Due to intermittency of solar energy, traditional batteries are used to store energy. However, batteries have several drawbacks such as limited lifespan, low storage capacity, uncontrolled discharge when not connected to a load and limited number of charge/discharge cycles. In this paper, the feasibility of using hydrogen as a battery is analyzed where hydrogen is produced by the extra diurnal generated electricity by a rooftop household solar power generation unit and utilized in a fuel cell system to generate the required electrical power at night. In the proposed design, two rooftop concentrated photovoltaic thermal (CPVT) systems coupled with an organic Rankine cycle (ORC) are used to generate electricity during 9.5 h per day and the extra power is utilized in an electrolyzer to produce hydrogen. Various working fluids (Isobutane, R134a, R245fa and R123) are used in the ORC system to analyze the maximum feasible power generation by this section. Under the operating conditions, the generated power by ORC as well as its efficiency are evaluated for various working fluids and the most efficient working fluid is selected. The required power for the compressor in the hydrogen storage process is calculated and the number of electrolyzer cells required for the hydrogen production system is determined. The results indicate that the hybrid CPVT-ORC system produces 2.378 kW of electricity at 160 suns. Supplying 65% of the produced electricity to an electrolyzer, 0.2606 kg of hydrogen is produced and stored for nightly use in a fuel cell system. This amount of hydrogen can generate the required electrical power at night while the efficiency of electrolyzer is more than 70%