The second law of thermodynamics is considered to be the most fundamental law of science. It explains not only the working of engines, refrigerators and other equipments used in our daily life, but also highly advanced theories like big bang, expansion of universe, heat death etc.
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Another important application of second law of thermodynamics is found in refrigerators and heat pumps, which are based on the principle of reversed Carnot cycle or reversed Carnot theorem. The second law of thermodynamics says that heat can flow from high temperature reservoir to low temperature reservoir only, however, if you want to move the heat from low temperature to high temperature reservoir, then the external work has be supplied.
The original Carnot cycle corresponds to the heat engines in which work is produced. In this cycle heat is absorbed from the high temperature reservoir and released to the low temperature reservoir and the work is produced. In the reversed Carnot cycle, the work is supplied to the cycle, and the heat is transferred from low temperature reservoir to the high temperature reservoir. This is what exactly the second law of thermodynamics says.
In our refrigerator, the freezer is maintained at low temperature by removing the heat from it and throwing it to the atmosphere which is at higher temperature. This does not happen automatically, we have to supply electricity which is used to run the compressor of the refrigerator.
The same principle is applied to the air-conditioner and heat pump also. The air-conditioner helps to keep the room cool when outside temperature is very high by removing heat from the room maintained at low temperature. The heat pump is used to keep the room hot when outside temperature is very low by absorbing the heat from atmosphere and supplying to the room.
In the heat engine Carnot cycle, the large is the temperature difference between hot and low temperature reservoir, more is the work produced in the cycle. In the reversed heat engine Carnot cycle, larger is the difference between low and high temperature reservoir more is the work required in the cycle.
The engines used in the automotives and the compressor used in the refrigerators, air-conditioners and heat pump seem to form the major applications of the second law of thermodynamics in our day-to-day lives. However, the fact is that these are just a few of innumerable other applications of the second law of thermodynamics. Recently an interesting program was relayed on BBC explaining the scope of the second law of thermodynamics. The recorded version of this program is available here.