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Cogeneration or combined heat and power ( CHP) is the use of a heat engine Cogeneration and Cogeneration Schematic, www.clarke-energy.com, retrieved 26.11.11 or power station to simultaneously generate electricity and useful heat. Trigeneration or combined cooling, heat and power ( CCHP) refers to the simultaneous generation of electricity and useful heating and cooling from the combustion of a fuel or a solar heat collector. A plant producing electricity, heat and cold is called a trigeneration or polygeneration plant.
Cogeneration is a thermodynamically efficient use of fuel. In separate production of electricity, some energy must be discarded as waste heat, but in cogeneration this thermal energy is put to use. All thermal power plants emit heat during electricity generation, which can be released into the natural environment through , flue gas, or by other means. In contrast, CHP captures some or all of the by-product for heating, 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 ( CHPDH). Small CHP plants are an example of decentralized energy. By-product heat at moderate temperatures (100–180 °C, 212–356 °F) can also be used in for cooling.
The supply of high-temperature heat first drives a gas or steam turbine-powered generator and the resulting low-temperature waste heat is then used for water or space heating as described in cogeneration. Trigeneration differs from cogeneration in that the waste heat is used for both heating and cooling, typically in an absorption refrigerator. CCHP systems can attain higher overall efficiencies than cogeneration or traditional power plants. In the United States, the application of trigeneration in buildings is called building cooling, heating and power ( BCHP). Heating and cooling output may operate concurrently or alternately depending on need and system construction.
Cogeneration was practiced in some of the earliest installations of electrical generation. Before central stations distributed power, industries generating their own power used exhaust steam for process heating. Large office and apartment buildings, hotels and stores commonly generated their own power and used waste steam for building heat. Due to the high cost of early purchased power, these CHP operations continued for many years after utility electricity became available. ξ1 Cogeneration is still common in pulp and paper mills, refineries and chemical plants.
In the United States, Consolidated 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 (or approximately 2.5 GW). Other major cogeneration companies in the United States include Recycled Energy Development, and leading advocates include Tom Casten and Amory Lovins.
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. Steam at ordinary process heating conditions still has a considerable amount of enthalpy that could be used for power generation, so cogeneration has lost opportunity cost. Conversely, simply generating steam at process pressure instead of high enough pressure to generate power at the top end also has lost opportunity cost. (See: Steam turbine#Steam supply and exhaust conditions) The capital and operating cost of high pressure boilers, turbines and generators are substantial, and this equipment is normally operated continuously, which usually limits self generated power to large scale operations.
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 from heating systems in cold climates.
It is also possible to run a heat driven operation combined with a heat pump, where the excess electricity (as heat demand is the defining factor on utilization) is used to drive a heat pump. As heat demand increases, more electricity is generated to drive the heat pump, with the waste heat also heating the heating fluid.
Thermal efficiency in a trigeneration system is defined as:
Where:
Typical trigeneration models have losses as in any system. The energy distribution below is represented as a percent of total input energy:
Conventional central coal- or nuclear-powered power stations convert only about 33% of their input heat to electricity. The remaining 67% emerges from the turbines as low-grade waste heat with no significant local uses so it is usually rejected to the environment. These low conversion efficiencies strongly suggest that productive uses be found for this waste heat, and in some countries these plants do produce byproduct steam that can be sold to customers.
But if no practical uses can be found for the waste heat from a central power station, e.g., due to distance from potential customers, then moving generation to where the waste heat can find uses may be of great benefit. Even though the efficiency of a small distributed electrical generator may be lower than a large central power plant, the use of its waste heat for local heating and cooling can result in an overall use of the primary fuel supply as great as 80%. This provides substantial financial and environmental benefits.
Most industrial countries generate the majority of their electrical power needs in large centralized facilities with capacity for large electrical power output. These plants have excellent economies of scale, but usually transmit electricity long distances resulting in sizable losses, negatively affect the environment. Large power plants can use cogeneration or trigeneration systems only when sufficient need exists in immediate geographic vicinity for an industrial complex, additional power plant or a city. An example of cogeneration with trigeneration applications in a major city is the New York City steam system.
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, or industrial and municipal waste (see incineration).
Some cogeneration plants combine gas and solar photovoltaic generation to further improve technical and environmental performance.A.C. Oliveira, C. Afonso, J. Matos, S. Riffat, M. Nguyen and P. Doherty, " A Combined Heat and Power System for Buildings driven by Solar Energy and Gas", Applied Thermal Engineering, vol. 22, Iss. 6, pp. 587-593 (2002).
HRSGs used in the CHP industry are distinguished from conventional steam generators by the following main features:
Delta-ee consultants stated in 2013 that with 64% of global sales the fuel cell micro-combined heat and power passed the conventional systems in sales in 2012. The fuel cell industry review 2013 20.000 units where sold in Japan in 2012 overall within the Ene Farm project. With a Lifetime of around 60,000 hours. For PEM fuel cell units, which shut down at night, this equates to an estimated lifetime of between ten and fifteen years. Latest developments in the Ene-Farm scheme For a price of $22,600 before installation. Launch of new 'Ene-Farm' home fuel cell product more affordable and easier to install For 2013 a state subsidy for 50,000 units is in place.
The development of small scale CHP systems has provided the opportunity for in-house power backup of residential-scale photovoltaic (PV) arrays.J. M. Pearce, “Expanding Photovoltaic Penetration with Residential Distributed Generation from Hybrid Solar Photovoltaic Combined Heat and Power Systems”, Energy 34, pp. 1947-1954 (2009). [8] Open access The results of a 2011 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. 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. A. Nosrat and J. M. Pearce, “Dispatch Strategy and Model for Hybrid Photovoltaic and Combined Heating, Cooling, and Power Systems”, Applied Energy 88 (2011) 3270–3276. [10] Open access These trigeneration photovoltaic systems have the potential to save even more energy and further reduce emissions compared to conventional sources of power, heating and cooling.A.H. Nosrat, L.G. Swan, J.M. Pearce, "Improved Performance of Hybrid Photovoltaic-Trigeneration Systems Over Photovoltaic-Cogen Systems Including Effects of Battery Storage", Energy 49, pp. 366-374 (2013). DOI, open access.
MicroCHP installations use five different technologies: microturbines, internal combustion engines, , closed cycle and . One author indicated in 2008 that MicroCHP based on Stirling engines is the most cost effective of the so-called microgeneration technologies in abating carbon emissions; What is microgeneration? Jeremy Harrison, Claverton Energy Group Conference, Bath, Oct 24th 2008 A 2013 UK report from Ecuity Consulting stated that MCHP is the most cost-effective method of utilising gas to generate energy at the domestic level. The role of micro CHP in a smart energy world Micro CHP report powers heated discussion about UK energy future however, advances in reciprocation engine technology are adding efficiency to CHP plant, particularly in the biogas field. MiniCHP ranges and efficiencies Aug 15 2009 As both MiniCHP and CHP have been shown to reduce emissions Pehnt, M. (2008). Environmental impacts of distributed energy systems—The case of micro cogeneration. Environmental science & policy, 11(1), 25-37. they could play a large role in the field of CO2 reduction from buildings, where more than 14% of emissions can be saved using CHP in buildings.http://alfagy.com/what-is-chp/133-kaarsberg-t-rfiskum-jromm-a-rosenfeld-j-koomey-and-wpteagan-1998-qcombined-heat-and-power-chp-or-cogeneration-for-saving-energy-and-carbon-in-commercial-buildingsq.html "Combined Heat and Power (CHP or Cogeneration) for Saving Energy and Carbon in Commercial Buildings."
See also Cost of electricity by source
As a whole, the European Union generates 11% of its electricity using cogeneration, saving Europe an estimated 35 Mtoe per annum a day. 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.
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. 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.
The European public–private partnership Fuel Cells and Hydrogen Joint Undertaking Seventh Framework Programme project ene.field deploys in 2017 Fiona Riddoch- Session II.pdf 5th stakeholders general assembly of the FCH JU up 1,000 residential fuel cell Combined Heat and Power (micro-CHP) installations in 12 states. Per 2012 the first 2 installations have taken place. ene.field European-wide field trials for residential fuel cell micro-CHP ene.field Grant No 303462
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.
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.
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."
The United States Department of Energy has an aggressive goal of having CHP constitute
20% of generation capacity by the year 2030. Eight Clean Energy Application Centers 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.
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. 'Recycling' Energy Seen Saving Companies Money. By David Schaper. May 22, 2008. Morning Edition. National Public Radio.
Applications in power generation systems
Non-renewable
Any of the following conventional power plants may be converted to a CCHP system:
Renewable
See also
Further reading
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References
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