You are tasked with designing a system to supply energy to a new block of 30 eco flats, providing heat (150 kWh per flat) and electricity (at a constant rate of 5 KW per flat) for one day (24 hours). Electricity will be supplied by proton exchange membrane fuel cells (PEMFCs), fed by hydrogen, and operating under standard conditions (P = 1atm, T = 25C), with a fuel utilisation of 80% and oxygen utilisation of 100%. For the given property, it would be appropriate to install commercial stacks of 15 individual cells, arranged in series. Hydrogen gas necessary for the 1-day operation is stored in 100L tanks, at 700 atm and room temperature, T = 25C. Heat will be delivered by an air-source cascade heat pump system, operating under the vapour-compression refrigeration cycle, between temperatures of 10C (assume air temperature is stable) and 100C, and using water as the working fluid. The cascade system has two cycles, and an intermediate temperature of 46C. In the heat pump subsystem:

Sketch of the vapour-compression refrigeration cycle (T-S diagram)