ENERGY CONVERSION PROCESSES

24.13 ENERGY CONVERSION PROCESSES

As shown in Figure 24.4 , energy occurs in several forms and must be converted to other forms. The efficiencies of conversion vary over a wide range. Conversion of electrical energy to radiant energy by incandescent light bulbs is very inefficient—less than 5% of the energy is converted to visible light and the remainder is wasted as heat. At the other end of the scale, a large electrical generator is around 80% efficient in producing electrical energy from mechanical energy. The once much-publicized Wankel rotary engine converts chemical to mechanical energy with an efficiency of about 18%, compared with 25% for a gasoline-powered piston engine and about 37% for a diesel engine. A modern coal-fired steam-generating power plant converts chemical energy to electrical energy with an overall efficiency of about 40%.

Figure 24.4 Kinds of energy and examples of conversion between them, with conversion efficiency percentages.

One of the most significant energy conversion processes is that of thermal energy to mechanical energy in a heat engine such as a steam turbine. The Carnot equation,

T Percent efficiency = 1 - T 2 x 100 (24.13.1)

states that the percent efficiency is given by a fraction involving the inlet temperature (for example, of steam), T 1 , and the outlet temperature, T 2 . These temperatures are states that the percent efficiency is given by a fraction involving the inlet temperature (for example, of steam), T 1 , and the outlet temperature, T 2 . These temperatures are

Some of the greatest efficiency advances in the conversion of chemical to mechanical or electrical energy have been made by increasing the peak inlet temper- ature in heat engines. The use of superheated steam has raised T 1 in a steam power plant from around 550 K in 1900 to about 850 K at present. Improved materials and engineering design, therefore, have resulted in large energy savings.

The efficiency of nuclear power plants is limited by the maximum temperatures attainable. Reactor cores would be damaged by the high temperatures used in fossil- fuel-fired boilers and have a maximum temperature of approximately 620 K. Because of this limitation, the overall efficiency of conversion of nuclear energy to electricity is about 30%.

Most of the 60% of energy from fossil-fuel-fired power plants and 70% of energy from nuclear power plants that is not converted to electricity is dissipated as heat, either into the atmosphere or into bodies of water and streams. The latter is thermal pollution, which may either harm aquatic life or, in some cases, actually increase bio- activity in the water to the benefit of some species. This waste heat is potentially very useful in applications like home heating, water desalination, and aquaculture (growth of plants in water).

Some devices for the conversion of energy are shown in Figure 24.5 . Substantial advances have been made in energy conversion technology over many decades and more can be projected for the future. The use of higher temperatures and larger generating units have increased the overall efficiency of fossil-fueled electrical power generation from less than 4% in 1900 to more than 40%. An approximately four fold increase in the energy-use efficiency of rail transport occurred during the 1940s and 1950s with the replacement of steam locomotives with diesel locomotives. During the coming decades, increased efficiency can be anticipated from such techniques as combined power cycles in connection with generation of electricity. Magnetohydrodynamics ( Figure 24.7 ) may be developed as a very efficient energy source used in combination with conventional steam generation. Entirely new devices such as thermonuclear reactors for the direct conversion of nuclear fusion energy to electricity will possibly be developed.