Heat engines & refrigerators worksheet with answers

A power plant delivers a net power output of 400 MW with a thermal efficiency of 55%. Find the rate Q̇_L at which heat is rejected to the cooling water.

A refrigerator removes heat from its cold space at a rate of 9.57 kW while its compressor uses 2.9 kW of power. Find its coefficient of performance COP_R.

A heat engine receives heat from a source at 800 °C and rejects heat to the surroundings at 10 °C. Its actual thermal efficiency is 44%. Find its second-law efficiency η_II, the ratio of the actual efficiency to the Carnot efficiency.

A heat engine works between a hot reservoir at 725 °C and a cold reservoir at 25 °C. Find the highest possible thermal efficiency η_max.

Two Carnot engines work in series. Engine A receives heat from a reservoir at 750 °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.

A Carnot heat engine receives 170 kW of heat from a source at 600 °C and rejects heat to the surroundings at 20 °C. All of its work drives a Carnot refrigerator that removes heat from a freezer at 0 °C and also rejects heat to the surroundings at 20 °C. Find the rate Q̇_L at which heat is removed from the freezer.