![]() ![]() In real thermodynamic systems or in real heat engines, a part of the overall cycle inefficiency is due to the losses by the individual components. Therefore, heat engines must have lower efficiencies than limits on their efficiency due to the inherent irreversibility of the heat engine cycle they use. But all real thermodynamic processes are somehow irreversible. For example, when the hot reservoir have T hot of 400☌ (673K) and T cold of about 20☌ (293K), the maximum (ideal) efficiency will be: = 1 – T cold/T hot = 1 – 293/673 = 56%. According to the Carnot principle, no engine can be more efficient than a reversible engine ( a Carnot heat engine) operating between the same high temperature and low temperature reservoirs. This upper limit is called the Carnot efficiency. There is an overall theoretical upper limit to the efficiency of conversion of heat to work in any heat engine. This inefficiency can be attributed to three causes. Be careful when you compare it with efficiencies of wind or hydro power (wind turbines are not heat engines), there is no energy conversion between the thermal and mechanical energy.Īs was discussed, an efficiency can range between 0 and 1. The thermal efficiencies are usually below 50% and often far below. In short, it is very difficult to convert thermal energy to mechanical energy. ![]() ![]() In general, the efficiency of even the best heat engines is quite low. Note that, η th could be 100% only if the waste heat Q C will be zero. To give the efficiency as a percent, we multiply the previous formula by 100. Therefore we can rewrite the formula for thermal efficiency as: Since energy is conserved according to the first law of thermodynamicsand energy cannot be be converted to work completely, the heat input, Q H, must equal the work done, W, plus the heat that must be dissipated as waste heat Q C into the environment. Since it is dimensionless number, we must always express W, Q H, and Q C in the same units. For a refrigeration or heat pumps, thermal efficiency indicates the extent to which the energy added by work is converted to net heat output. It is a dimensionless performance measure of a heat engine that uses thermal energy, such as a steam turbine, an internal combustion engine, or a refrigerator. The thermal efficiency, η th, represents the fraction of heat, Q H, that is converted to work. As a result of this statement, we define the thermal efficiency, η th, of any heat engine as the ratio of the work it does, W, to the heat input at the high temperature, Q H. ![]()
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