Cogeneration refers to facilities that produce electricity and heat simultaneously, usually by recycling heat that would otherwise be expelled into the environment.
Excess heat from a cogeneration plant can also be captured and used to cool, by passing the heat through absorption chillers. Facilities that produce electricity, heat, and air conditioning are sometimes called trigeneration or polygeneration plants.
What is the World’s Largest Cogeneration System?
Con Edison delivers 30 billion pounds of steam per year at temperatures of 350 °F (180 °C) from its seven cogeneration plants to 100,000 buildings in Manhattan — the largest steam-heated district in the world. Peak delivery is 10 million pounds of steam per hour. (Hence the steaming manholes often seen in movies set in New York.)
How do Cogeneration Plants Recycle Heat?
Power plants that produce electricity from heat – for example, by burning coal, petroleum, natural gas, or by nuclear fission, are unable to convert all their thermal energy into electricity. In most of traditional power plants, a bit more than half the heat is not used.  By capturing this heat, cogeneration plants can theoretically achieve up to 89% efficiency, compared with 55% for the best conventional plants. This means that less fuel needs to be consumed to produce the same amount of energy.
Is Cogeneration an Economically Viable Option?
The viability of cogeneration depends on ongoing demand for electricity and heat. In practice, a plant’s capacity to produce heat and electricity rarely matches the ongoing demand exactly. The viability increases greatly where opportunities for trigeneration exist. In a modern trigeneration system, natural gas might power an electrical generator, and the exhaust from the generator might power a steam power plant. The excess heat from the steam plant could then be captured for space heating. Such tri-cycle plants can have thermal efficiencies above 80%. The excess heat from the cogeneration plant could also be captured to deliver cooling by means of an absorption chiller.
Cogeneration is most efficient when the heat can be used on site or nearby. Overall efficiency is greatly reduced when the heat must be transported over longer distances, requiring heavily insulated pipes, which are expensive and inefficient.
An automobile becomes a cogeneration plant in winter, when excess heat from the engine is used to warm the interior. This illustrates how efficient cogeneration depends on using heat in the vicinity of the heat engine.
What Kinds of Organizations Use Cogeneration?
Cogeneration plants are commonly found in city heating systems, hospitals, prisons, oil refineries, paper mills, wastewater treatment plants, thermal-enhanced oil recovery wells, and industrial plants with large heating needs.
Thermally enhanced oil recovery plants generate electricity, then pump leftover steam into oil wells so that the oil will flow more easily, increasing production. TEOR cogeneration plants in Kern County, California produce so much electricity that it cannot all be used locally, and the excess is transmitted to Los Angeles, 95 miles away.
Is Cogeneration a Good Option for Heating in Cold Climates?
Cogeneration is one of the most cost-efficient methods for reducing carbon emissions while producing heat in cold climates.
What Are the Different Types of Cogeneration Plants?
Topping cycle plants produce electricity primarily using steam turbines. The exhausted steam is condensed, and the low-temperature heat released from this condensation is used for municipal heating, water desalination, etc.
Bottoming cycle plants produce high-temperature heat for industrial processes. Waste heat is recovered in a boiler that drives an electrical plant. Bottoming cycle plants are only used when the industrial process requires very high temperatures, such as in furnaces for glass and metal manufacturing; so they are less common.
What Technologies Are Used in Cogeneration Plants?
  • Gas turbine cogeneration plants capture waste heat from gas turbines. These plants are generally manufactured as fully packaged units that can be installed with simple connections to the site’s gas, electrical, and heating systems.
  • Biofuel engine cogeneration plants are similar in design to gas turbine cogeneration plants, except that they use a biofuel-powered gas or diesel engine. Using biofuel reduces hydrocarbon fuel consumption, and therefore carbon emissions. A variant is wood gasifier cogeneration, where wood pellets or chips are gasified in a zero-oxygen, high-temperature environment, and the resulting gas is used to power the gas engine.
  • Steam turbine cogeneration plants use excess heat to condense steam for a steam turbine.
  • Molten-carbonate fuel cells have a hot exhaust that is very suitable for heating.
  • Smaller cogeneration units can capture heat from the exhaust and radiator of a reciprocating engine or Stirling engine. These systems are popular because small gas and diesel engines are less expensive than small gas- or oil-fired steam-electric plants.
  • Some cogeneration plants are fired by biomass, or industrial or municipal waste.
  • Heat Recovery Steam Generators (HRSG). The hot exhaust gases from turbines or reciprocating engines are captured to heat water and generate steam. The steam drives an electrical generator, or is used in industrial processes that require heat.
How Recently Did Cogeneration Come to the U.S.?
In 1882, Thomas Edison’s Pearl Street Station, the world’s first commercial power plant, produced both electricity and thermal energy, while using waste heat to warm neighboring buildings. Recycling allowed Edison’s plant to achieve approximately 50 percent efficiency.
By the early 1900s, as the use of electricity began to spread, monopolistic regulations discouraged small, local power production, including cogeneration.
By 1978, Congress recognized that the efficiency of large, central power plants had stagnated. It  sought to improve efficiency by passing the Public Utility Regulatory Policies Act (PURPA), which encouraged utilities to buy power from other energy producers.
The U.S. Department of Energy has set an aggressive goal that cogeneration should comprise 20% of US generation capacity by 2030. The DOE established eight Clean Energy Application Centers to develop technologies for producing “clean energy” through combined heat and power production, waste heat recovery, and “district energy” systems that provide heating and cooling to public spaces such as commercial buildings, condominiums, hotels, sports facilities, universities, and government complexes.
As a direct result of the DOE’s initiatives, cogeneration plants began to proliferate, soon producing about 8 percent of all energy in the U.S. However, the bill left implementation and enforcement up to the states, resulting in little or no activity in many parts of the country.
Some sources estimate that cogeneration could produce 19 to 20 percent of U.S. electricity.
Is Cogeneration Used in Other Countries?
Outside the U.S., energy recycling is even more common. Denmark is probably the most active energy recycler, obtaining about 55% of its energy from cogeneration and waste heat recovery. Other large countries, including Germany, Russia, and India, obtain a much larager share of their total energy from decentralized sources.
Is Small-Scale Cogeneration Practical?
In “micro cogeneration,” instead of burning fuel solely to heat water or home or business space, some of the heat is converted to electricity. A micro cogeneration installation usually produces less than 5 kWe for a house or small business. This electricity can be used in the home or business, and in some locations it can sold to the electric power grid.
A “mini cogeneration” installation can produce 5 kWe to 500 kWe in a building or medium-sized business. Whether the installation will pay off depends largely on the “utilization factor,” – the estimated yearly hours of operation of the cogeneration plant, as a percentage of the total hours in a year. With a utilization factor of under 40%, cogeneration is considered non-viable. Thus, mini cogeneration is most practical when there is a simultaneous demand for electricity and heat. Such a situation typically arises where building occupation or activities are extended or continuous – for example, in hospitals, prisons, manufacturing, swimming pools, airports, hotels, apartment complexes, etc.
Micro and mini cogeneration applications currently use five technologies: microturbines, internal combustion engines, Stirling engines, closed-cycle steam engines, and fuel cells.