Cogeneration

Cogeneration is the simultaneous generation of electrical and thermal energy where both forms of energy are put to productive use. The addition of cogeneration capability to generating facilities and industries that produce large amounts of heat energy helps ensure that waste heat (usually in the form of steam or hot water) is used efficiently for heating, industrial use, agriculture or conversion into electricity.

Nuclear energy plants, for example, produce enormous quantities of heated water as a byproduct of creating steam to drive generator turbines, and prior to efficient cogeneration this heat was expelled from the plant into local reservoirs and habitats. Often, this hot water is cycled through housing or industrial facilities in the surrounding area as a source of heat, or applied to agricultural purposes. Some cogeneration systems produce cold water, not hot water, and the cold water is used to supplement or replace cooling systems in areas where cooling is desirable.

When a facility cogenerates a given quantity or type of usable energy, the producer of that energy may become entitled to special benefits under the Public Utility Regulatory Policies Act (PURPA) of 1978. These benefits include the right to produce energy from cogeneration whether this energy is needed or not (a right not guaranteed in other circumstances), and possibly the right to sell cogenerated energy at favorable avoided-cost prices to utilities. To further stimulate cogeneration, utilities may even be forced to purchase that energy at this favorable price under the terms of PURPA.

There are two main types of cogeneration concepts: "Topping Cycle" plants and "Bottoming Cycle" plants.

Topping Cycle
A topping cycle cogeneration plant generates electricity as its main energy product. Facilities that operate a topping cycle produce the electricity for their own use and may sell any excess power to a utility. That is, if they are a qualified facility (QF) as recognized by the Federal Energy Regulatory Commission (FERC). Microturbine cogeneration is considered a topping cycle.

Bottoming Cycle
A bottoming cycle plant generates heat first in a furnace or a boiler. These facilities are typically heavy industries such as glass or metals manufacturing where very high temperature furnaces are used. The excess heat is used to generate steam that is then used to operate a steam turbine that generates electricity. The plant may sell any excess power to a utility, provided they are a QF. Bottoming cycle plants are much less common than topping cycle plants.