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 = 25°C), 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 = 25°C.
Heat will be delivered by an air-source cascade heat pump system, operating under the vapour-compression refrigeration cycle, between temperatures of 10°C (assume air temperature is stable) and 100°C, and using water as the working fluid. The cascade system has two cycles, and an intermediate temperature of 46°C.
You are requested to prepare a report for the client, containing the following sections and information:
Section 1. Calculations and estimations of the energy parameters, justifying assumptions made.
In the fuel cells subsystem:
(i) Minimum number of hydrogen tanks required for storage. [20 marks]
(ii) Overall subsystem efficiency. [10 marks]
In the heat pump subsystem:
(i) Sketch of the vapour-compression refrigeration cycle (T-S diagram) [5 marks
(ii) Work done at the compressor. [15 marks]
(iii) Overall subsystem efficiency. [10 marks]