Heat engines & refrigerators worksheet with answers

A power plant delivers a net power output of 400 MW with a thermal efficiency of 35%. Find the rate of heat input Q̇_H from the fuel.

A refrigerator removes heat from its cold space at a rate of 2 kW while its compressor uses 0.5 kW of power. Find its coefficient of performance COP_R.

A reversible refrigerator keeps a cold room at -30 °C by removing 0.5 kW of heat, and rejects heat to the surroundings at 20 °C. Find the rate Q̇_H at which it rejects heat.

A heat engine receives heat from a source at 450 °C and rejects heat to the surroundings at 25 °C. Its actual thermal efficiency is 38%. Find its second-law efficiency η_II, the ratio of the actual efficiency to the Carnot efficiency.

A refrigerator keeps its cold space at -18 °C in a kitchen at 32 °C, removing heat from the cold space at 4 kW. Find the minimum power input Ẇ_min.

Two Carnot engines work in series. Engine A receives heat from a reservoir at 950 °C and rejects heat to an intermediate reservoir at temperature T_m. Engine B takes all that heat and rejects heat to a reservoir at 35 °C. Both engines produce the same work. Find T_m in °C.