1. The figure below shows operating data for a compressed air energy storage system using off-peak electricity to power a compressor that fills a cavern with pressurized air. The cavern has a volume of lOs m3 and initially holds air at 290K and 1 bar, which corresponds to ambient conditions. After filling, the air in the cavern is at 790K and 21 bar. Determine (a) the initial and final mass of air in the cavern and (b) the work required by the compressor (in Gf).
2. A high-efficiency passenger car has a frontal area of 2.4 m2, a drag coefficient of 0.25, a rolling resistance value of 0.01, maximum tractive force at low speed of 2800 N, maximum power at high speed of 32 kW, and a curb weight of 1200 kg. Calculate: (a) power requirement at a cruising speed of 100 km/h, (b) maximum speed, and (c) maximum gradability.
3. A fuel cell stack consists of 75 fuel cells arranged in series, each one 12 cm2 in size. The stack operates at 80°C and the amount of current generated is 4.257 A/cm2. The fuel cell operates at a system pressure of 1 MPa using a supply of pure hydrogen and oxygen in ambient air. The partial pressure of the water in the outflow is 0.5 MPa. The reference voltage for a fuel cell at 80°C is Ev= 1.18 V. Assume any losses to be negligible and that the voltage at which it operates is the open circuit voltage; in reality, the voltage would be lower due to the current flow. What is the output of the fuel cell stack when operating at full capacity, in kW?
