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What Is Primary Energy?
In introductory energy-system analysis, energy sources can be classified according to their origin and the conversions that occur before useful energy reaches consumers.
The EE 555 Renewable Energy Systems course material at LUMS introduces energy sources including petroleum, natural gas, coal, nuclear, hydro, biomass, solar, wind, geothermal and tidal resources. It then distinguishes between the energy resource itself and technologies used to convert that resource into useful forms such as electricity.
A useful way of thinking about primary energy is energy in the form in which it exists before subsequent human-controlled energy conversion.
Coal contains chemical energy.
Petroleum contains chemical energy.
Natural gas contains chemical energy.
Uranium contains nuclear energy.
Water stored at elevation possesses gravitational potential energy.
Moving air possesses kinetic energy.
Solar radiation carries radiant energy.
Electricity, by contrast, generally has to be produced by converting another form of energy.
This distinction becomes important the moment we begin comparing technologies.
A Coal Power Plant Illustrates the Problem
Suppose a thermal power station receives energy from coal.
The coal contains chemical energy.
Combustion converts that into thermal energy.
Heat produces steam.
Steam drives a turbine.
The turbine produces mechanical rotation.
A generator finally converts that mechanical energy into electricity.
The chain can be simplified as:
Chemical energy → thermal energy → mechanical energy → electrical energy
At every stage, we must remember a fundamental principle from thermodynamics.
Energy is conserved.
But useful output from a real conversion process is less than the required input for the intended purpose.
That is why:
Efficiency = Useful Output / Required Input
The EE 555 material introduces exactly this distinction through the first and second laws of thermodynamics: energy is conserved, while energy conversion involves losses in terms of useful output.
This has a major implication for national energy statistics.
The amount of primary energy entering an electricity system is not necessarily equal to the amount of electrical energy eventually delivered.
