Skip to main content Skip to navigation links

NFCRC Tutorial: Energy Conversion

previous link: hydroelectric.html next link: fuelcell.html

Energy conversion involves transformation of energy (or matter in the case of nuclear energy) from a form that is available to us in nature into a useful form of energy. Radiant energy from the sun is converted in nature to the mechanical energy of the wind and waves and, through photosynthesis, into the stored energy in plants. Over the years, the stored energy in plants has been accumulated in deposits of coal, oil and gas. Thus, energy is made available to us by nature in the form of chemical bond energy of fossil fuels which may be harnessed relatively easily or as sun light which may be directly converted to heat of electricity. Another form of energy that is also available is nuclear energy which involves the conversion of matter into energy.

The three particularly useful forms of energy to humans are: mechanical energy to operate machines, thermal energy to process materials and to heat living spaces, and radiant energy to provide light. Thermal energy is derived directly by burning or combusting fossil fuels; mechanical energy is derived from heat energy in engines of various types; and radiant energy is provided by the luminosity of the flames of burning fuels.

Thus, in most current applications, the energy contained in fossil fuels and biomass as the chemical bond energy is harnessed by combustion. These combustion based sources provide as much as 90% of the current energy supply in the U.S. The combustion process, however, produces pollutants and conversion efficiency of the fuel bound energy to electricity with the intermediate step involving combustion is limited by the temperature at which the heat may be utilized. A more efficient and environmentally responsible way of converting this fuel bound energy consists of utilizing fuel cells which convert the fuel bound energy directly into electricity. Fuel cells, however require clean fuels such as natural gas. Dirty fuels such as coal and biomass may be converted to clean fuels by partial combustion or gasification followed by cleaning the gas thus derived, and then either fueling a fuel cell or a gas turbine with the clean gas.

Electrical energy is used to transfer energy over distances to be converted useful forms such as mechanical, heat, or radiant energy. Electrical energy is a very useful intermediate form of energy. The following table summarizes a comparison of the efficiency and Nox emissions of the various types of natural gas fired electricity generation methods for small (<20 MW). The part load performance of these cycles is presented in the figure following the table. Note that the increase in heat rate (fuel consumption per unit of power produced) remains relatively flat for the fuel cell based plants.

Thermal and Environmental Performance of
Small (<20 MW)Plants (Natural Gas Fired)

 

Reciprocating
Engine

Gas Turbine
(Simple Cycle)

Gas Turbine
Combine
d Cycle

Fuel Cell
(SOFC)

Fuel Cell
(SOFC)
with Gas
Turbine

Efficiency
(Fuel LHV to
Electricity)%

35-46

30-37

40-51

45

> 60

NOx ppmvd
(15% O2)

80

15

15

<2

<5

 

Part Load Performance chart: part load performance

 

previous link: hydroelectric.html next link: fuelcell.html