Overview and Benefits of Cogeneration
– Cogeneration is the simultaneous generation of electricity and useful heat.
– It is a more efficient use of fuel or heat, as wasted heat from electricity generation is put to productive use.
– Combined heat and power (CHP) plants recover otherwise wasted thermal energy for heating.
– Cogeneration can be used in decentralized energy systems, such as small CHP plants.
– By-product heat at moderate temperatures can be used in absorption refrigerators for cooling.
– CHP is most efficient when heat can be used on-site or very close to it.
– Efficiency is reduced when heat must be transported over longer distances.
– Electricity can be transmitted over longer distances with less energy loss compared to heat.
– Deployment of CHP depends on heat uses in the vicinity of the heat engine.
– Cogeneration reduces carbon footprint compared to on-site steam generation and importing electric power from the grid.
– Cogeneration is common in district heating systems, central heating systems of larger buildings, and industrial thermal production processes.
Types of Cogeneration Plants
– Steam turbines in cogeneration plants are designed for extraction or exhaust at lower pressures.
– Combined cycle plants can be used for cogeneration by extracting heat using a heating system.
– Topping cycle plants produce electricity from a steam turbine and condense steam for district heating or water desalination.
– Bottoming cycle plants produce high temperature heat for industrial processes and use waste heat recovery boilers.
– Large cogeneration systems provide heating water and power for industrial sites or towns.
– Gas turbine CHP plants use waste heat from gas turbines, typically fueled by natural gas.
– Gas engine CHP plants use reciprocating gas engines, usually fueled by natural gas.
– Biofuel engine CHP plants use adapted gas or diesel engines and reduce fossil fuel consumption.
– Wood gasifier CHP plants gasify wood biofuel in a high temperature environment to power gas engines.
– Combined cycle power plants can be adapted for CHP.
– Molten-carbonate fuel cells and solid oxide fuel cells have hot exhaust suitable for heating.
– Nuclear power plants can bleed partially expanded steam for heating, extracting heat for every MW of electricity lost.
– Some cogeneration plants combine gas and solar photovoltaic generation for improved performance.
MicroCHP and Trigeneration
– Micro combined heat and power (MicroCHP) is a distributed energy resource.
– MicroCHP installations are usually less than 5 kW and convert energy to electricity and heat.
– Fuel cell MicroCHP surpassed conventional systems in global sales in 2012.
– Different technologies used in MicroCHP installations include microturbines, internal combustion engines, Stirling engines, closed-cycle steam engines, and fuel cells.
– MicroCHP based on Stirling engines is considered the most cost-effective in reducing carbon emissions.
– Trigeneration plants produce electricity, heat, and cold.
– Cogeneration systems linked to absorption or adsorption chillers use waste heat for refrigeration.
Industrial Cogeneration and Heat Recovery
– Output capacity of industrial cogeneration units ranges from 5 MW to 25 MW.
– Industrial cogeneration is a viable off-grid option for remote applications.
– Heat Recovery Steam Generators (HRSG) use hot exhaust gases to heat up water and generate steam.
– HRSGs are designed based on specific features of gas turbines or reciprocating engines.
– Cogeneration using biomass includes using plant or animal matter for heat or electricity.
– Brazil is a world reference in biomass energy generation, particularly in the sugar and alcohol sector.
– Sugarcane bagasse is burned to produce steam for power generation in the sugar and alcohol sector.
– Cogeneration in sugarcane industries is growing in Brazil and helps reduce reliance on fossil fuels for power generation.
Efficiency, Costs, and Policy
– CHP has a Coefficient of Performance (COP) of 6 compared to a heat pump.
– Heat pump efficiency depends on the temperature difference between hot and cold ends.
– Conventional central coal or nuclear power stations convert about 33-45% of input heat to electricity.
– Brayton cycle power plants can operate at up to 60% efficiency.
– The fully installed cost per kW electrical for a gas-fired plant is around £400/kW (US$577).
– Cogeneration can provide financial benefits through the use of waste heat for local heating and cooling.
– The overall use of primary fuel supply can be as high as 80% with cogeneration.
– Cogeneration has been incorporated into the energy policy of the European Union through the CHP Directive.
– Denmark, the Netherlands, and Finland have the most intensive cogeneration economies in Europe.
– Germany aims to double its electricity cogeneration by 2020. Source: https://en.wikipedia.org/wiki/Cogeneration
Cogeneration or combined heat and power (CHP) is the use of a heat engine or power station to generate electricity and useful heat at the same time.
Cogeneration is a more efficient use of fuel or heat, because otherwise-wasted heat from electricity generation is put to some productive use. Combined heat and power (CHP) plants recover otherwise wasted thermal energy for heating. This is also called combined heat and power district heating. 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 absorption refrigerators for cooling.
The supply of high-temperature heat first drives a gas or steam turbine-powered generator. The resulting low-temperature waste heat is then used for water or space heating. At smaller scales (typically below 1 MW), a gas engine or diesel engine may be used. Cogeneration is also common with geothermal power plants as they often produce relatively low grade heat. Binary cycles may be necessary to reach acceptable thermal efficiency for electricity generation at all. Cogeneration is less commonly employed in nuclear power plants as NIMBY and safety considerations have often kept them further from population centers than comparable chemical power plants and district heating is less efficient in lower population density areas due to transmission losses.
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.
