12-4-applications-of-thermodynamics-heat-engines-heat-pumps-and-refrigerators_summary

This section is about understanding heat engines, heat pumps, and refrigerators in terms of the laws of thermodynamics, explaining thermal efficiency, and solving problems related to it. Heat engines, such as car engines, generators, and steam turbines, use thermodynamic properties to convert heat into work. They work by heating a gas in a cylinder, increasing its pressure and temperature, causing it to do work as it expands. The gas then cools, decreasing its pressure, and is returned to its original state, thus completing a cycle. The efficiency of a heat engine is determined by the amount of heat transferred out of the hot reservoir, the amount of heat transferred into the cold reservoir, and the work done by the engine. The thermal efficiency is defined as the ratio of the gross work output to the heat energy input. According to the second law of thermodynamics, a heat engine cannot have perfect conversion of heat into work, as there is a minimum amount of heat that cannot be used for work due to the increase in entropy (disorder) of the system. Heat pumps, air conditioners, and refrigerators utilize heat transfer of energy from low to high temperatures. They require work input, which produces a transfer of energy by heat. Heat pumps are used to transfer energy to a warm environment, such as a home in the winter, while air conditioners and refrigerators are used to cool indoor spaces by transferring heat outdoors. The quality of these devices is judged by how much energy is transferred by heat into the warm space compared with the amount of work input required. Refrigerators and air conditioners do not create cold; they merely transfer heat from the inside to the outside. While heat pumps and refrigerators are heat engines operated backward, the quality that is considered the energy benefit in a heat pump is waste heat in a refrigerator. Thermal efficiency is a measure of how efficiently a machine converts energy into useful work, demonstrating how much output energy is produced for a given input energy. The efficiency of a heat engine is the output of net work divided by heat-transferred energy into the engine. Efficiency is always less than 100 percent due to the laws of thermodynamics, and the energy lost is usually transferred as heat to the environment. Solving thermal efficiency problems requires calculating the net work and heat input to the engine, and then using the formula for efficiency to find the efficiency percentage.