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Specific power
From Wikipedia, the free encyclopedia
(Redirected from Power density)
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In engineering, the terms specific power or power density can refer to power either per unit of mass, volume or area. The following sections give several examples of where these terms are used.
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- Heat engines can be characterized by their specific power, which is typically given in kilowatts per litre of engine displacement (in the U.S. also horsepower per cubic inch). The result offers an approximation of the peak-power output of an engine. This is not to be confused with fuel efficiency, since high-efficiency often requires a lean fuel-air ratio, and thus lower power density. A modern high-performance car engine makes in excess of 75 kW/L (1.65 hp/in³).
- With batteries, specific power usually refers to the power-to-weight ratio, measured in kilowatts per kilogram (generally, W/kg). Fuels are usually measured in terms of energy density, since the available power is usually not a limitation. A high-octane fuel actually burns more slowly, and due to engine dynamics this can actually allow an engine to produce more power. In high performance vehicles, the specific power determines the vehicle's acceleration.
- The specific power on a mass basis is also used to characterize solar panels for use in aerospace applications where this parameter is critical for an efficient launch.
- The intensity of electromagnetic waves can be expressed in W/m². An example of such a quantity is the solar constant.
- Wind turbines are often compared using a specific power measuring watts per square meter of turbine disk area, which is pi times the length of a blade squared. This measure is also commonly used for solar panels, at least for typical applications.