A student designs a portable refrigeration chest powered by a battery-driven heat pump. The chest contains food packages that must be kept at 275 K while the external environment is at 300 K. The walls of the chest are good thermal insulators except for a small removable ventilation plug.
During normal operation the heat pump removes 500 J of energy from the interior each minute and expels 650 J to the surroundings; the electrical input to the pump is 150 J per minute. The student wishes to investigate how different boundary choices affect entropy bookkeeping.
(a) With the ventilation plug installed, treat the refrigerated space alone (everything inside the inner wall) as the system. Identify the system type and determine qualitatively whether its entropy is increasing, decreasing, or constant during steady-state operation. Explain.
(b) Now remove the plug so that a small mass flow of cold air leaves the chest each minute and is replaced by warm ambient air that is cooled by the heat pump. Under these conditions, re-classify the refrigerated space as a system and predict the direction of its entropy change. Justify carefully, emphasizing the role of mass transfer.
(c) Next, expand the boundary to include the refrigerated space, the heat pump, and the immediate layer of ambient air that receives the expelled 650 J each minute. Classify the resulting system and discuss the sign of its entropy change during steady operation.
(d) Propose one practical modification, other than improving insulation, that would reduce the total entropy generation associated with operating the chest. Justify your proposal by explaining how it alters energy or matter transfers relative to the chosen system in part (c).