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Cogeneration (also combined heat and power, CHP) is the use of a heat engine or a power station to simultaneously generate both electricity and useful heat. Distributed generation, including combined heat and power (CHP), can be distinguished from central energy resources in several respects. These distributed energy resources are small, modular, and come in a range of capacities from kilowatts to megawatts. They comprise a portfolio of technologies that can be located onsite or nearby the location where the energy is used. They provide the consumer with a greater choice, local control, and more efficient waste utilization to boost efficiency and lower emissions.
Conventional power plants emit the heat created as a by-product of electricity generation into the environment through cooling towers, flue gas, or by other means. CHP or a bottoming cycle captures the by-product heat for domestic or industrial heating purposes, either very close to the plant, or especially in Scandinavia and eastern Europe for distribution through pipes to heat local housing. This is also called decentralized energy.
By-product heat at moderate temperatures (212-356°F/100-180°C) can also be used in absorption chillers for cooling. A plant producing electricity, heat and cold is sometimes called trigeneration or more generally: polygeneration plant.
Perhaps the first modern use of energy recycling was done by Thomas Edison. His 1882 Pearl Street Station, the world’s first commercial power plant, was a combined heat and power plant, producing both electricity and thermal energy while using waste heat to warm neighboring buildings.Recycling allowed Edison’s plant to achieve approximately 50 percent efficiency.
Overview
Thermal power plants (including those that use fissile elements or burn coal, petroleum, or natural gas), and heat engines in general, do not convert all of their available energy into electricity. In most heat engines, a bit more than half is wasted as excess heat (see: Second law of thermodynamics). By capturing the excess heat, CHP uses heat that would be wasted in a conventional power plant, potentially reaching an efficiency of up to 89%, compared with 55% for the best conventional plants. This means that less fuel needs to be consumed to produce the same amount of useful energy. Also, less pollution is produced for a given economic benefit.
Some tri-cycle plants have utilized a combined cycle in which several thermodynamic cycles produced electricity, and then a heating system was used as a condenser of the power plant's bottoming cycle. For example, the RU-25 MHD generator in Moscow heated a boiler for a conventional steam powerplant, whose condensate was then used for space heat. A more modern system might use a gas turbine powered by natural gas, whose exhaust powers a steam plant, whose condensate provides heat. Tri-cycle plants can have thermal efficiencies above 80%.
An exact match between the heat and electricity needs rarely exists. A CHP plant can either meet the need for heat (heat driven operation) or be run as a power plant with some use of its waste heat.
CHP is most efficient when the heat can be used on site or very close to it. Overall efficiency is reduced when the heat must be transported over longer distances. This requires heavily insulated pipes, which are expensive and inefficient; whereas electricity can be transmitted along a comparatively simple wire, and over much longer distances for the same energy loss.
A car engine becomes a CHP plant in winter, when the reject heat is useful for warming the interior of the vehicle. This example illustrates the point that deployment of CHP depends on heat uses in the vicinity of the heat engine.
Cogeneration plants are commonly found in district heating systems of big towns, 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 (TEOR) plants often produce a substantial amount of excess electricity. After generating electricity, these plants pump leftover steam into heavy 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 is transmitted to Los Angeles
System Concepts
• The portfolio of distributed generation technologies includes, for example, photovoltaic systems, fuel cells, natural gas engines, industrial turbines, microturbines, energy-storage devices, wind turbines, and concentrating solar power collectors. These technologies can meet a variety of consumer energy needs including continuous power, backup power, remote power, and peak shaving. They can be installed directly on the consumer’s premise or located nearby in district energy systems, power parks, and mini-grids.
• CHP technologies have the potential to take all of the distributed generation technologies one step further in pollution prevention by utilizing the waste heat from the generation of electricity for the making of steam, heating of water, or for the production of cooling energy. The average power plant in the United States converts approximately one-third of the input energy into output electricity and then discards the remaining two-thirds of the energy as waste heat. Integrated DG systems with CHP similarly produce electricity at 30% to 45% efficiency, but then capture much of the waste heat to make steam, heat, or cool water –or meet other thermal needs and increase the overall efficiency of the system to greater than 70%.
Types of plants
Topping cycle plants primarily produce electricity from a steam turbine. The exhausted steam is then condensed, and the low temperature heat released from this condensation is utilised for e.g. district heating.
Bottoming cycle plants produce high temperature heat for industrial processes, then a waste heat recovery boiler feeds an electrical plant. Bottoming cycle plants are only used when the industrial process requires very high temperatures, such as furnaces for glass and metal manufacturing, so they are less common.
Large cogeneration systems provide heating water and power for an industrial site or an entire town. Common CHP plant types are:
• Gas turbine CHP plants using the waste heat in the flue gas of gas turbines
• Combined cycle power plants adapted for CHP
• Steam turbine CHP plants that use the heating system as the steam condenser for the steam turbine.
• Molten-carbonate fuel cells have a hot exhaust, very suitable for heating.
Smaller cogeneration units may use a reciprocating engine or Stirling engine. The heat is removed from the exhaust and the radiator. These systems are popular in small sizes because small gas and diesel engines are less expensive than small gas- or oil-fired steam-electric plants:
• Advanced industrial turbines and microturbines – combustion turbines are a class of electric-generation devices that produce high temperature, high-pressure gas to induce shaft rotation by impingement of the gas on a series of specially designed blades. Simple cycle efficiencies range from 21% to 40%. Turbines produce high-quality heat and can be used for CHP production. Microturbines are small combustion turbines with outputs of 25-1,000 kW. Microturbines evolved from automotive and truck turbochargers.
• Fuel cells – power is produced in fuel cells electrochemically by passing a hydrogen-rich fuel over an anode and air over a cathode and separating the two by an electrolyte in producing electricity. The only byproducts are heat, water, and carbon dioxide. • Natural Gas Engines – the reciprocating engine is widespread and well-known technology. Spark ignition gas-fired units (the focus here) typically use natural gas or propane. Capacities are typically in the 0.5- to 5-megawatt range
MicroCHP
"Micro cogeneration" is a so called distributed energy resource (DER). The installation is usually less than 5 kWe in a house or small business. Instead of burning fuel to merely heat space or water, some of the energy is converted to electricity in addition to heat. This electricity can be used within the home or business or, if permitted by the grid management, sold back into the electric power grid.
MiniCHP
"Mini cogeneration" is a so called distributed energy resource (DER). the installation is usually more than 5 kWe and less than 500 kWe in a building or medium sized business Current (2007) Micro- andMiniCHP installations use five different technologies: microturbines, internal combustion engines, stirling engines, closed cycle steam engines and fuel cells
Percentage of energy produced by cogeneration
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, also obtain a much higher share of their energy from decentralized sources.
Cogeneration plants proliferated, soon producing about 8 percent of all energy in the U.S. However, the bill left implementation and enforcement up to individual states, resulting in little or nothing being done in many parts of the country. In the United States, Con Edison produces 30 billion pounds of steam each year through its seven cogeneration plants (which boil water to 1,000°F/538°C before pumping it to 100,000 buildings in Manhattan, the biggest commercial steam system in the world.
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Cogeneration 
