Thermal efficiency
) is a dimensionless performance measure of a device that uses thermal energy, such as an internal combustion engine, steam turbine, steam engine, boiler, furnace, refrigerator, ACs etc.
Because the input heat normally has a real financial cost, a memorable, generic definition of thermal efficiency is[1]
Efficiency must be less than 100% because there are inefficiencies such as friction and heat loss that convert the energy into alternative forms.
For example, a typical gasoline automobile engine operates at around 25% efficiency, and a large coal-fuelled electrical generating plant peaks at about 46%.
In a combined cycle plant, thermal efficiencies approach 60%.
Since a large fraction of the fuels produced worldwide go to powering heat engines, perhaps up to half of the useful energy produced worldwide is wasted in engine inefficiency, although modern cogeneration, combined cycle and energy recycling schemes are beginning to use this heat for other purposes.
The second law of thermodynamics puts a fundamental limit on the thermal efficiency of all heat engines.
Even an ideal, frictionless engine can't convert anywhere near 100% of its input heat into work.
The limiting factors are the temperature at which the heat enters the engine,
, and the temperature of the environment into which the engine exhausts its waste heat,
No device converting heat into mechanical energy, regardless of its construction, can exceed this efficiency.
For example, if an automobile engine burns gasoline at a temperature of
The efficiency of ordinary heat engines also generally increases with operating temperature, and advanced structural materials that allow engines to operate at higher temperatures is an active area of research.
Due to the other causes detailed below, practical engines have efficiencies far below the Carnot limit.
Carnot's theorem applies to thermodynamic cycles, where thermal energy is converted to mechanical work.
One of the factors determining efficiency is how heat is added to the working fluid in the cycle, and how it is removed.
An important parameter in the efficiency of combustion engines is the specific heat ratio of the air-fuel mixture, γ.
This varies somewhat with the fuel, but is generally close to the air value of 1.4.
The above efficiency formulas are based on simple idealized mathematical models of engines, with no friction and working fluids that obey simplified thermodynamic models.
Real engines have many departures from ideal behavior that waste energy, reducing actual efficiencies below the theoretical values given above.
Examples are: These factors may be accounted when analyzing thermodynamic cycles, however discussion of how to do so is outside the scope of this article.
An electric resistance heater has a thermal efficiency close to 100%.
[8] When comparing heating units, such as a highly efficient electric resistance heater to an 80% efficient natural gas-fuelled furnace, an economic analysis is needed to determine the most cost-effective choice.
Not stating whether an efficiency is HHV or LHV renders such numbers very misleading.
The limiting value of the Carnot 'efficiency' for these processes, with the equality theoretically achievable only with an ideal 'reversible' cycle, is: The same device used between the same temperatures is more efficient when considered as a heat pump than when considered as a refrigerator since This is because when heating, the work used to run the device is converted to heat and adds to the desired effect, whereas if the desired effect is cooling the heat resulting from the input work is just an unwanted by-product.
In the United States, in everyday usage the SEER is the more common measure of energy efficiency for cooling devices, as well as for heat pumps when in their heating mode.
However, for a more complete picture of heat exchanger efficiency, exergetic considerations must be taken into account.
