Thermal to electrical transducers
The conversion of electrical energy to thermal energy proceeds with almost 100% efficiency, but no conversions from thermal energy to any other form ever approach much more than 50% efficiency. The reasons for this are summed up in the laws of thermodynamics, and are founded on the principle that we do not know how much heat an object contains, and we cannot remove all of it. Any change from thermal energy to another form must involve heat taken in by a converter at a high temperature and a lesser amount of heat given out at a lower temperature. The efficiency cannot then be greater than the fraction given by:
with the temperatures measured in the Kelvin scale whose zero point is equivalent to -273.16oC. This equation implies that 100% conversion is possible only if the converter exhausts its heat at 0K, which is not a practical proposition. In addition, this equation assumes that every other part of the conversion process operates at 100% efficiency.
The conversion from thermal to electrical energy is, on the large scale, carried out by way of steam generation, with the steam operating turbines that are coupled to alternators. The source of thermal energy can be nuclear. The use of the more direct gas turbine and alternator method is expensive and is used only for topping up the supply from conventional coal and nuclear power stations. One of the benefits of the inefficiency of the whole process can be the availability of large quantities of water at a useful domestic temperature of 40-60oC, and in some countries electricity generating stations also sell their waste heat in a type of scheme called CHP (combined heat and power). In the UK, the emphasis has always been on large-scale generation and grid distribution, and CHP has never been a practical proposition in such circumstances. The pioneer dream of a small nuclear station supplying electricity and heating for a self-contained community has never been realized, but it will have to become an option as the effects of burning half of the contents of the planet become more noticeable.
By comparison with the 40% efficiency that can be obtained from a well- designed and very large coal-fired power station, the efficiency of the generation of electricity from any other thermal transducers is extremely low. The most practical system in the past has used thermocouples stacked into large blocks that are heated at one end and cooled at the other, with the individual thermocouples connected in series so as to obtain a useful voltage. Several commercial units have been available in the past, and at one time the gas-fired radio receiver was, if not common, more than a museum piece. The Milnes converter was one of the most successful of these early units, and was manufactured in a factory in Tayport, Fife in the late forties, along with loudspeakers to the Milnes design. The efficiency of these thermocouple units, however, rarely exceeded 5%.
Other forms of thermal to electrical converters have achieved even lower efficiencies, and even with intensive development work, the efficiency figure of 10% is more of a dream than a reality. One method that was pursued for con- version of solar energy used the principle of thermal emission from a cathode heated by focused sunlight, with a large number of units connected in series in order to attain a reasonable voltage. This, however, proved to be as unreliable as any other solar source, and the ultimate comment on solar heating is provided by the advertisements in Australian papers offering to replace solar heating units by oil-fired boilers. If solar heating is uneconomic in Australia, its chances in the rest of the world are rather poor.