Combined Heat and Power (Cogeneration)

Conventional electric power generation is only about 40 percent efficient—it leaves large amounts of waste heat. Cogeneration (also called combined heat and power, or CHP) is the use of this waste heat for heating buildings, providing hot water, and applying to industrial uses. The CHP cycle is about 80 percent efficient, obtaining twice as much energy from the same amount of fuel as conventional electrical generation. In the short to midterm, cogeneration promises to save significant amounts of fossil fuels. It is a transition technology, making better use of fossil fuel sources that provide very intense heat, but eventually it will become unsustainable as fossil fuel stocks (oil, gas, and coal) decline.

The concept of cogeneration is more familiar than most people think, for every automobile is also a cogeneration plant. An automobile's engine generates motive power for the wheels, but in the process it generates a lot of waste heat. Some of that heat runs the [Page 96]car's heater, warming the occupants of the car in cold weather without using extra energy. There are now schemes planned to generate the electricity needed for the car's accessories from waste engine heat. This would be an example of trigeneration: first is the motive power for the wheels, second is the heat for the passengers, and third is the electricity generated. The car still needs a radiator, however, because the automobile's occupants do not always want extra heat, and the engine usually generates more heat than they can use in any case.

The first commercial electrical generation plant, built by Thomas Edison in New York City, was more efficient than most modern power plants. Edison realized that the waste heat from his steam boilers could be sold profitably to nearby buildings to provide heat and hot water after the high-pressure steam had lost its power but was still very hot. This is the normal application of CHP.

During the course of the 20th century, however, this mode of electrical generation fell out of favor. Rising electrical demand drove utilities to build ever-larger power plants that could not be located in cities because many of them were fueled by coal. Later, regulation of utility rates gave power companies little incentive to operate more efficiently. Usually they were (and are) allowed to charge consumers for the cost of generation and transmission plus a reasonable profit, regardless of what those costs were. As a result, cogeneration stagnated, and utilities found ways to dissipate “waste” heat from electrical generation into rivers, lakes, and oceans. Cooling towers were developed that took in prodigious amounts of water, evaporating about half of it and returning the rest, hot and polluted, to the river or lake from which it came. Urban process heat was generated cheaply and conveniently from fossil fuels in furnaces without the bother of tie-ins to outside heat sources. Labor and maintenance costs were lower per kilowatt-hour produced because a large plant requires only a little more care than a small one.

However, the era of practicality of such inefficient power generation is coming to an end with the approaching decline in the availability of fossil fuels.

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