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| Anaerobic digestion · Biomass |
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| Cogeneration · Energy efficiency |
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Cogeneration (also combined heat and power, CHP) is the use of a heat engine[1] or a power station to simultaneously generate both electricity and useful heat.
All thermal power plants emit a certain amount of heat during electricity generation. This can be released into the natural environment through cooling towers, flue gas, or by other means. By contrast, CHP captures some or all of the by-product heat for heating purposes, either very close to the plant, or—especially in Scandinavia and eastern Europe—as hot water for district heating with temperatures ranging from approximately 80 to 130 °C. This is also called Combined Heat and Power District Heating or CHPDH. Small CHP plants are an example of decentralized energy.[2]
Cogeneration is common in industries that use both steam and electricity, such as pulp and paper mills, refineries and chemical plants. Steam turbines for cogeneration are designed for extraction of steam at lower pressures after it has passed through a number of turbine stages, or they may be designed for final exhaust at back pressure (non-condensing), or both. A typical power generation turbine in a paper mill may have extraction pressures of 160 psig (1.103 MPa) and 60 psig (0.41 MPa). A typical back pressure may be 60 psig (0.41 MPa). In practice these pressures are custom designed for each facility. The extracted or exhaust steam is used for process heating, such as drying paper, evaporation, heat for chemical reactions or distillation. See: Steam turbine#Steam supply and exhaust conditions
In the United States, Con Edison distributes 66 billion kilograms of 350 °F/180 °C steam each year through its seven cogeneration plants to 100,000 buildings in Manhattan—the biggest steam district in the United States. The peak delivery is 10 million pounds per hour (corresponding to approx. 2.5 GW)[3][4] (This steam distribution system is the reason for the steaming manholes often seen in "gritty" New York movies.)
Other major cogeneration companies in the United States include Recycled Energy Development[5] and leading advocates include Tom Casten and Amory Lovins.
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[6] or more generally polygeneration plant. Cogeneration is a thermodynamically efficient use of fuel. In separate production of electricity, some energy must be rejected as waste heat, but in cogeneration this thermal energy is put to good use.
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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 thermal energy into electricity. In most heat engines, a bit more than half is lost as excess heat (see: Second law of thermodynamics and Carnot's theorem). By capturing the excess heat, CHP uses heat that would be wasted in a conventional power plant, potentially reaching an efficiency of up to 80%,[7] for the best conventional plants. This means that less fuel needs to be consumed to produce the same amount of useful energy.
Some tri-cycle plants have used a combined cycle in which several thermodynamic cycles produced electricity, 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%.
The viability of CHP (sometimes termed utilisation factor), especially in smaller CHP installations, depends on a good baseload of operation, both in terms of an on-site (or near site) electrical demand and heat demand. In practice, 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, the latter being less advantageous in terms of its utilisation factor and thus its overall efficiency. The viability can be greatly increased where opportunities for Trigeneration exist. In such cases, the heat from the CHP plant is also used as a primary energy source to deliver cooling by means of an absorption chiller.
CHP is most efficient when 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. The 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 cities, 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.
CHP is one of the most cost-efficient methods of reducing carbon emissions of heating in cold climates.[8]
Cogeneration facilities typically operate at pressures significantly below that of 100% condensing power plants because of possible contamination of returned condensate from process steam and because much condensate is not recovered, meaning having to treat more make-up water. Boiler feed water must be completely oxygen free and de-mineralized, and the higher the pressure the more critical the level of purity of the feed water.
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 utilized for e.g. district heating or water desalination.
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:
Smaller cogeneration units may use a reciprocating engine or Stirling engine. The heat is removed from the exhaust and radiator. The systems are popular in small sizes 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,[12] or industrial and municipal waste (see incineration).
A heat recovery steam generator (HRSG) is a steam boiler that uses hot exhaust gases from the gas turbines or reciprocating engines in a CHP plant to heat up water and generate steam. The steam, in turn, drives a steam turbine and/or is used in industrial processes that require heat.
HRSGs used in the CHP industry are distinguished from conventional steam generators by the following main features:
Typically, for a gas-fired plant the fully installed cost per kW electrical is around £400/kW, which is comparable with large central power stations.[10]
See also Cost of electricity by source
Europe has actively incorporated cogeneration into its energy policy via the CHP Directive. In September 2008 at a hearing of the European Parliament’s Urban Lodgment Intergroup, Energy Commissioner Andris Piebalgs is quoted as saying, “security of supply really starts with energy efficiency.”[13] Energy efficiency and cogeneration are recognized in the opening paragraphs of the European Union’s Cogeneration Directive 2004/08/EC. This directive intends to support cogeneration and establish a method for calculating cogeneration abilities per country. The development of cogeneration has been very uneven over the years and has been dominated throughout the last decades by national circumstances.
As a whole, the European Union generates 11% of its electricity using cogeneration, saving Europe an estimated 35 Mtoe per annum a day.[14] However, there is large difference between Member States with variations of the energy savings between 2% and 60%. Europe has the three countries with the world’s most intensive cogeneration economies: Denmark, the Netherlands and Finland.[15]
Other European countries are also making great efforts to increase efficiency. Germany reported that at present, over 50% of the country’s total electricity demand could be provided through cogeneration. So far, Germany has set the target to double its electricity cogeneration from 12.5% of the country’s electricity to 25% of the country’s electricity by 2020 and has passed supporting legislation accordingly in “Federal Ministry of Economics and Technology, (BMWi), Germany, August 2007. The UK is also actively supporting combined heat and power. In light of UK’s goal to achieve a 60% reduction in carbon dioxide emissions by 2050, the government has set the target to source at least 15% of its government electricity use from CHP by 2010.[16] Other UK measures to encourage CHP growth are financial incentives, grant support, a greater regulatory framework, and government leadership and partnership.
According to the IEA 2008 modeling of cogeneration expansion for the G8 countries, the expansion of cogeneration in France, Germany, Italy and the UK alone would effectively double the existing primary fuel savings by 2030. This would increase Europe’s savings from today’s 155.69 Twh to 465 Twh in 2030. It would also result in a 16% to 29% increase in each country’s total cogenerated electricity by 2030.
Governments are being assisted in their CHP endeavors by organizations like COGEN Europe who serve as an information hub for the most recent updates within Europe’s energy policy. COGEN is Europe’s umbrella organization representing the interests of the cogeneration industry, users of the technology and promoting its benefits in the EU and the wider Europe. The association is supported by the key players in the industry including gas and electricity companies, ESCOs, equipment suppliers, consultancies, national promotion organisations, financial and other service companies.
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.[17] Recycling allowed Edison’s plant to achieve approximately 50 percent efficiency.
By the early 1900s, regulations emerged to promote rural electrification through the construction of centralized plants managed by regional utilities. These regulations not only promoted electrification throughout the countryside, but they also discouraged decentralized power generation, such as cogeneration. As Recycled Energy Development CEO Sean Casten testified to Congress, they even went so far as to make it illegal for non-utilities to sell power.[18]
By 1978, Congress recognized that efficiency at central power plants had stagnated and sought to encourage improved efficiency with the Public Utility Regulatory Policies Act (PURPA), which encouraged utilities to buy power from other energy producers.
The U.S. DOE has an aggressive goal of having CHP comprise of 20% of the US generation capacity by the year 2030. Eight Clean Energy Application Centers have been established across the nation whose mission is to develop the required technology application knowledge and educational infrastructure necessary to lead “clean energy” (Combined Heat and Power, Waste Heat Recovery and District Energy) technologies as viable energy options and reduce any perceived risks associated with their implementation. The focus of the Application Centers is to provide an outreach and technology deployment program for end users, policy makers, utilities, and industry stakeholders.
Cogeneration plants proliferated, soon producing about 8 percent of all energy in the United States.[19] 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 2008 Tom Casten, chairman of Recycled Energy Development, said that "We think we could make about 19 to 20 percent of U.S. electricity with heat that is currently thrown away by industry."[20]
Outside of the United States, energy recycling is 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, also obtain a much higher share of their energy from decentralized sources.[19][20]
"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. This recent development of small scale CHP systems has provided the opportunity for in-house power backup of residential-scale photovoltaic (PV) arrays.[21] The results of a recent study show that a PV+CHP hybrid system not only has the potential to radically reduce energy waste in the status quo electrical and heating systems, but it also enables the share of solar PV to be expanded by about a factor of five. [22] In some regions, in order to reduce waste from excess heat, an absorption chiller has been proposed to utilize the CHP-produced thermal energy for cooling of PV-CHP system. [23] These trigen+PV systems have the potential to save even more energy.
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. In this size range the viability or utilisation factor of the CHP plant is very important to consider since it will greatly affect the efficiency and cost effectiveness (payback) of the CHP plant. The utilisation factor is essentially the calculated hours of operation of the CHP plant expressed as a percentage of the total number of hours in a year. If less than 40% then the application of CHP is considered to be unviable. To be viable a good baseload for electrical demand and heat demand must exist. Such baseloads arise where building occupation or process activities are extended or continuous in operation. This typically includes for hospitals, prisons, manufacturing processes, swimming pools, airports, hotels, apartment blocks, etc.
Current (2007) Micro- and MiniCHP installations use five different technologies: microturbines, internal combustion engines, stirling engines, closed cycle steam engines and fuel cells. One author indicates that MicroCHP based on Stirling engines is the most cost effective of the so called microgeneration technologies in abating carbon emissions;[24] however, advances in reciprocation engine technology are adding efficiency to CHP plant, particularly in the biogas field.[25] MiniCHP has a large role to play in the field of CO2 reduction from buildings where more than 14% of emissions can be saved by 2010 using CHP in buildings according to the author.[26]
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