MAKING THE MOST OF Available Energy.(economy of cogeneration power plants)(Industry Overview)

When is cogeneration economical?

Most energy managers know that cogeneration is using a primary energy source such as a reciprocating engine or a gas turbine to produce at least two forms of useful energy. For example, heat can be extracted from several points on an engine and used for room heating and cooling, domestic water heating, water chilling (using an absorption chiller), and industrial processes.

Cogeneration can be as attractive for an apartment building, health club, or hospital as for an industrial facility. Although cogeneration plants range up to hundreds of megawatts in size, systems of less than 1 megawatt (MW) provide good opportunities for smaller facilities to reduce their energy costs.

These smaller systems will usually be based on spark-ignited, gas-fueled engines, diesels, or microturbines, which are most readily configured to produce hot water as opposed to steam.

Cogeneration is inherently more efficient than a conventional system, which will turn a generator shaft but wastes the rest of the energy contained in its fuel. Most of the energy contained in the gas or oil used to fuel a power plant does not end up at the generator bus bar. Most is rejected to the air as hot exhaust gas, or to the cooling water from condensing steam, hot lube oil, or engine jackets.

Most of the power generated in the U.S. today comes from large steam plants with efficiencies of about 35%. Gas turbines, diesels, and spark-ignited reciprocating engines normally range from 25-40% efficiency. This means that of the thousand (actually 1032) British thermal units (Btu) in every cubic foot of natural gas only 250-400 Btu make it to the generator bus bar. Of the 0.3 kilowatt-hours (kWh) that would be available from a hypothetical 100% efficient generator set, only 0.08-0.12 kWh are ever harnessed to power electrical loads.

Using these same engines in cogeneration makes efficiencies of 80-90% possible. Although only 250-400 Btu are available at the generator busbar, another 400-650 Btu in the form of heat can serve other useful purposes (see figure 1). The thermal output could be thought of as free energy, but just because something is free does not make it a good deal. In this case the heat and electric power must be put to work as well. Cogeneration offers tremendous economic and environmental benefits for the right facility, but not for every facility.