Wind turbines are often compared using a specific power measuring watts per square meter of turbine disk area, which is, where r is the length of a blade. This measure is also commonly used for solar panels, at least for typical applications.
Radiance is surface power density per unit of solid angle in a specific direction. Spectral radiance is radiance per unit of frequency at a specific frequency.
Surface power density is an important factor in comparison of industrial energy sources. Measured in W/m2 it describes the amount of power obtained per unit of area used by the specific energy source, including all supporting infrastructure, manufacturing, mining of fuel and decommissioning. Fossil fuels and nuclear power are characterized by high power density which means large power can be drawn from power plants occupying relatively small area. Renewable energy sources have power density at least three orders of magnitude smaller and for the same energy output they need to occupy accordingly larger area. The following table shows median surface power density of renewable and non-renewable energy sources.
Energy source
Median PD
Fossil gas
482.10
Nuclear power
240.81
Oil
194.61
Coal
135.10
Solar power
6.63
Geothermal
2.24
Wind power
1.84
Hydropower
0.14
Biomass
0.08
Background
As an electromagnetic wave travels through space, energy is transferred from the source to other objects. The rate of this energy transfer depends on the strength of the EM field components. Simply put, the rate of energy transfer per unit area is the product of the electric field strength times the magnetic field strength. where The above equation yields units of W/m2. In the USA the units of mW/cm2, are more often used when making surveys. One mW/cm2 is the same power density as 10 W/m2. The following equation can be used to obtain these units directly: The simplified relationships stated above apply at distances of about two or more wavelengths from the radiating source. This distance can be a far distance at low frequencies, and is called the far field. Here the ratio between E and H becomes a fixed constant and is called the characteristic impedance of free space. Under these conditions we can determine the power density by measuring only the E field component and calculating the power density from it. This fixed relationship is useful for measuring radio frequency or microwave fields. Since power is the rate of energy transfer, and the squares of E and H are proportional to power, E2 and H2 are proportional to the energy transfer rate and the energy absorption of a given material.
Far field
The region extending farther than about 2 wavelengths away from the source is called the far field. As the source emits electromagnetic radiation of a given wavelength, the far-field electric component of the waveE, the far-field magnetic componentH, and power density are related by the equations: E = H × 377 and Pd = E × H.
