In 1824, a French engineer and physicist, Nicolas Léonard Sadi Carnot advanced the study of the second law by forming a principle (also called Carnot’s rule) that specifies limits on the maximum efficiency any heat engine can obtain. There is no energy conversion between thermal and mechanical energy. Be careful when comparing it with efficiencies of wind or hydropower (wind turbines are not heat engines). 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 is 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:
#Thermo engineering calculator plus#
Since energy is conserved according to the first law of thermodynamicsand energy cannot 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 a dimensionless number, we must always express W, Q H, and Q C in the same units. For 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, converted to work. The thermal efficiency of the simple Rankine cycle and in terms of specific enthalpies would be: The thermal efficiency of the Brayton cycle in terms of the compressor pressure ratio (PR = p 2/p 1), which is the parameter commonly used: The air-standard Otto cycle thermal efficiency is a function of compression ratio and κ = c p /c v. 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.