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Co-Generation |
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Cogeneration, also referred to as combined heat and power (CHP), is the use of a heat engine or a power station to simultaneously generate both electricity and useful heat. Cogeneration is more energy efficient than the separate generation of electricity and thermal energy. Heat that is normally wasted in conventional power generation is recovered as useful energy for satisfying an existing thermal demand, the heating and cooling of the building and water supply, thus avoiding the losses that would otherwise be incurred from separate generation of power. Conventional electricity generation is inherently inefficient, converting only about a third of a fuel’s potential energy into usable energy. The significant increase in efficiency with CHP results in lower fuel consumption and reduced emissions compared with separate generation of heat and power. This reduced primary fuel consumption is key to the environmental benefits of cogeneration because burning the same fuel more efficiently means fewer emissions for the same level of output.
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Contents
1.Definition
2. Use/Application
a. Established Techniques
b. Emerging Trends
3. Use an Integrated Approach
4. Resources
5. Associated Strategies
6. Case Studies |
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Definition |
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Cogeneration, also referred to as combined heat and power (CHP), is the use of a heat engine or a power station to simultaneously generate both electricity and useful heat. Cogeneration is more energy efficient than the separate generation of electricity and thermal energy. Heat that is normally wasted in conventional power generation is recovered as useful energy for satisfying an existing thermal demand, the heating and cooling of the building and water supply, thus avoiding the losses that would otherwise be incurred from separate generation of power. Conventional electricity generation is inherently inefficient, converting only about a third of a fuel’s potential energy into usable energy. The significant increase in efficiency with CHP results in lower fuel consumption and reduced emissions compared with separate generation of heat and power. This reduced primary fuel consumption is key to the environmental benefits of cogeneration because burning the same fuel more efficiently means fewer emissions for the same level of output.
CHP systems consist of a number of individual components: the heat engine, generator, heat recovery, and electrical interconnection, which are all configured into an integrated whole. Heat engines for cogenerator systems include reciprocating engines, combustion or gas turbines, steam turbines, microturbines, and fuel cells. These heat engines are capable of burning a variety of fuels, including natural gas, coal, oil, and alternative fuels such as biomass, solar, and hydrogen to produce shaft power or mechanical energy. Although the mechanical energy created is most often used to drive a generator to produce electricity, it can also be used to drive rotating equipment such as compressors, pumps, and fans. Thermal energy from the system can be used in direct process applications or indirectly to
produce steam, hot water, hot air for drying, or chilled water for process cooling.
Cogeneration is not a new concept. It has successfully been used for more than a century, including industrial, district energy, institutional, government, commercial, and residential applications. CHP can be used in virtually any stationary application that uses waste heat from onsite power for cooling or heating. In fact, in the 1920s, more than half of the electricity used in the United States was produced by some form of cogeneration. |
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Use / Application |
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Fueled by environmental concerns, unease over energy security, and a host of other factors, interest in cogeneration technologies has been growing among energy customers, regulators, legislators, and developers. Installing a cogeneration system designed to meet the thermal and electrical base loads of a commercial or residential building can greatly increase the building’s operational efficiency and reduce energy costs and emission of greenhouse gases.
Cogeneration is most effective when the generating units are placed at or near the building for which they supply energy. Although central or onsite CHP systems have been used in commercial buildings, there are now a number of units available for the residential market.
Cogeneration systems have the potential for a wide range of applications and lower emissions than separate heat and power generation systems because of their higher efficiencies. The advantages of cogeneration include the following:
- Cogeneration is versatile and can be coupled with existing and planned technologies for many different applications in the industrial, commercial, and residential sectors.
- The simultaneous production of useful thermal and electrical energy leads to increased fuel efficiency and lower emissions compared with conventional systems.
- Cogeneration units can be strategically located at the point of energy use. Such onsite generation avoids grid transmission and distribution losses associated with decentralized power stations.
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Established Techniques |
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Commercial cogeneration unit
| Commercial Buildings
Cogeneration has been used for many years in commercial buildings for large-scale applications such as generating energy for industrial complexes, college campuses, hospitals, and commercial buildings in campus-like settings where there is considerable power and thermal demand. Currently, reciprocating engines are the most common and most efficient prime movers (engines) used in commercial cogeneration systems because of their cost, reliability, and availability. However, microturbines, fuel cells, and Stirling engines may be economically viable for cogeneration in the next few years as technology advances.
Cogeneration is gaining popularity for use in high-rise commercials office buildings as technology has improved over the years, allowing it to become more compact and modular and, therefore, cheaper to install. Depending on the size of the system, cogeneration can be a fully self-reliant energy source, generating enough electric power onsite for the entire building, while capturing the waste heat from the generating equipment, which is usually a gas reciprocating engine, combustion turbine, microturbine, or fuel cell. The recovered waste heat is “free energy” that can be used for space heating, domestic hot water production, and space cooling.
Residential Buildings
Because of the large number of successful residential installations in Europe and Japan, several manufacturers are now offering models in the United States. Once available only to large commercial buildings, cogeneration systems are now being produced on a scale that is safe, practical, and affordable to homeowners. A residential CHP system uses fuel such as natural gas to produce heat and electricity simultaneously. The electricity can be used for any household device such as lights and appliances, and the heat produced can be used for water heating and space heating. Microcogeneration units range in capacity from about 1 kW to 6 kW and are about the size of a major appliance. Installation may be performed initially by specialists and, after the technology matures, by an experienced plumber, electrician, or HVAC technician. Units come as grid-tied systems that connect to utility power as backup or as stand-alone systems for remote residences. One 6-kW unit can provide up to 10 GPM of hot water at 140º F to 150° F. This waste heat can be used to heat an entire home; water for domestic use, swimming pools, and spas; or even as an energy source for heat-driven (absorption) cooling systems.
Commercial cogeneration unit
| Current units on the market with small-capacity engines can simultaneously produce 1.2 kW of electric power and 11,000 Btus of heat in the form of hot water. These systems can be combined with a high-efficiency, natural gas–fueled warm air furnace or boiler for supplemental space heating. The small engines tend to burn very cleanly—exceeding all emissions requirements for carbon dioxide and nitrogen oxides. The primary challenge for getting the highest efficiency and best economic return on a cogeneration system is to fully use all of the thermal energy produced when generating electricity. As the technology develops, various operating regimes will be tested to optimize the energy available based on variables such as the loads in the home, the climate, and the season. CHP systems are extremely efficient, offering combined heat- and power-generating efficiency of about 90 percent, compared with about 30 to 40 percent for electricity from a central power station.
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Emerging Trends
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Decentralized Generation
Our current centralized electricity-generation system wastes over two-thirds of the energy contained in the fuel and continues to produce ever-increasing carbon and other harmful emissions because of a continued demand for energy worldwide. At least half of this wasted energy could be recaptured if we shift from centralized generation to distributed systems that cogenerate power and thermal energy onsite or nearby. Cogeneration offers significant, economy-wide energy-efficiency improvement and emissions reductions. Besides saving energy and reducing emissions, distributed generation also addresses emerging congestion problems within the electricity transmission and distribution grid.
Trigeneration
Also known as combined heating, cooling, and power generation (CHCP), trigeneration takes cogeneration one step further, using one energy source to simultaneously produce mechanical power, heating, and cooling. The additional cooling step produces chilled water for air conditioning to cool a building or for process use with the addition of absorption chillers. By combining a cogeneration system with an absorption refrigeration system, it is possible to achieve overall efficiencies (power and air-conditioning refrigeration) of up to 75 percent, increasing both annual capacity and efficiency of the CHCP system and greatly reducing overall emissions.
Combined Technologies
Cogeneration when combined with other technologies shows promise in effectiveness and efficiency. One such “combining of technologies” is with fuel cells. Fuel cells show electrical efficiencies of 40 to 49 percent in comparison with other competing technologies that are about 10 to 14 percent lower. Fuel cell systems used in cogeneration can achieve an electrical efficiency of close to 80 to 85 percent. Although fuel cell technology is still not the most cost-effective technology, combining the cogeneration aspect has drawn renewed interest in the marketplace.
- Nuvera, a fuel cell manufacturer, has developed a CHP fuel cell power system slated for Beta testing and precommercialization to begin by 2008. This new cogeneration system will generate approximately 5 kW of electricity and 7 kW of heat and will provide a consistent or “base load” output of thermal and electrical energy. The electric grid and conventional heating equipment are then used to support peak load demand. The CHP system is appropriately sized for small commercial customers who have a steady, consistent demand for thermal and electrical energy, such as hospitals, hotels, dormitories, restaurants, and swimming pools.
- FuelCell Energy Inc., will install a 1.2-MW CHP power plant that will be located at the site of Turlock’s Regional Water Quality Control Facility, in California’s Central Valley by the summer of 2008. Fuel cells will generate power to run the wastewater plant from methane gas generated from treatment of wastewater on site. This combined technology will reduce the site’s carbon footprint by 5,200 tons annually, compared with a typical power plant, and will make significant savings on the cost of fuel. The power plant is said to be 47 percent efficient and can achieve up to 80 percent efficiency by using the waste heat as a cogeneration system.
National Policy
During the past couple of years, cogeneration has become an important part of the national energy debate. The United States has taken the first steps toward setting in place policies to promote cogeneration by establishing a national target.
The Department of Energy and the Environmental Protection Agency have begun to review the means for achieving this target. According to the American Council for an Energy Efficient Economy (ACEEE), the primary barriers to greater cogeneration usage are not economic or technical but are regulatory and institutional. The ACEEE states that the private sector must work with government regulators and policymakers to ensure that competition and incentives for innovation are preserved, while creating a favorable regulatory environment for CHP for both environmental and “bottom-line” benefits. |
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Use an Integrated Approach |
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A new way of thinking must be adopted in order to meet the goal of reducing carbon emissions associated with buildings. Your solutions can begin by integrating four possible methods. None works alone, and they are not all relevant in considering every strategy. However, considering the following tactics is necessary:
- Reduce the overall energy use in your building
- An onsite cogeneration system will reduce the overall energy use of a commercial or residential building by decreased distribution losses and through the recapturing and reuse of formally lost heat.
- Specify energy-efficient equipment and technologies
- Combining cogeneration technology with energy-efficient equipment as part of the overall building design will further reduce energy usage and overall emissions.
- Use renewable strategies and purchase green power
- The use of CHP systems combined with renewables such as solar energy capture is one of many combined overall building strategies that will further reduce the energy load and carbon emissions of a building.
- Educate building owners, operators, and occupants
- Provide information on function and operations of installed technology.
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Last modified at 2/26/2009 10:23 PM by jamie nace
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