CHP—Combined Heat and Power

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MI Summary

Combined Heat and Power (CHP)

Combined Heat and Power (CHP), also known as cogeneration, involves the use of a heat engine or a power station to generate both electricity and useful heat simultaneously. This enables CHP to use the heat that would be wasted in a conventional power plant, allowing for about 30% more efficiency. This means that less fuel needs to be consumed to produce the same amount of useful energy and consequently pollution is also reduced.

There are two different types of plants, the topping cycle plant produces electricity first, and then the exhausted steam is used for heating. Conversely, bottoming cycling plants produce high heats for an industrial process and then a waste heat recovery boiler feeds an electrical plant.

Text of Article

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.

Conventional power plants emit the heat created as a byproduct of electricity generation into the environment through cooling towers, as flue gas, or by other means. CHP or a bottoming cycle captures the byproduct 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.

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.[1][2]

Byproduct 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.

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 energy performs useful work

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 70%, compared with at most 40% for the 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 motor becomes a CHP plant in winter, when the reject heat is useful for warming the interior of the vehicle. This example scores 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[citation needed].

Types of plants

Topping cycle plants produce electricity first, then the exhausted steam is used for heating. Flames naturally produce heats suitable for a boiler. The hot water from condensed steam is well-suited for space and water heating.

Bottoming cycle plants produce high heats for an industrial process, 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 rarer.

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 beause small gas and diesel engines are less expensive than small gas- or oil-fired steam-electric plants.

Some cogeneration plants are fired by biomass [3], or industrial and municipal waste (see incineration).

MicroCHP

"Micro cogeneration" is a so called distributed energy resource (DER). the installation is usually in a house or small business[1]. 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.

Current (2007) MicroCHP installations use four different technologies: internal combustion engines, stirling engines, closed cycle steam engines and fuel cells.

In the Press

Summaries

Full article: Amory Lovins: How to Face Today's Greatest Energy Challenges (14-Aug-07)

No one has done more to change the world of energy, both its intellectual underpinnings and its real-world practice, than Lovins. Beginning with a seminal Foreign Affairs article in 1976 -- "Energy Strategy: The Road Not Taken?" which introduced the "soft path" to energy -- Lovins shifted the focus from bigger to smarter, from more to more-with-less. He's consulted with businesses, governments, and militaries on how to achieve organizational goals using less energy and less money. His books and articles are legion.

We see this now in the electricity business. A sixth of the world's electricity and a third of the world's new electricity comes from micropower* -- that is, combined heat and power (also called cogeneration) and distributed renewables.

Micropower provides anywhere from a sixth to over half of all electricity in most of the industrial countries. This is not a minor activity anymore; it's well over $100 billion a year in assets. And it's essentially all private risk capital.

So in 2005, micropower added 11 times as much capacity and four times as much output as nuclear worldwide, and not a single new nuclear project on the planet is funded by private risk capital. What does this tell you? I think it tells you that nuclear, and indeed other central power stations, have associated costs and financial risks that make them unattractive to private investors.

There are much smarter and much dumber approaches to biofuels, and biofuels do not need to have the problems you refer to.

Full article: It's not easy being Green (7-Aug-07)

This is an overview article that covers many topic: data centre consolidation, wind and wave power, energy losses in transmisson, nuclear energy, CHP etc.

  • Taking power from green energy suppliers seems reasonable—but what happens if all organisations and homes did this? There is nowhere near enough green power being produced.
  • The greenest means of generating centralised power may yet appear to be nuclear—provided you are willing to disassociate the long term green costs of decommissioning from those of the actual power.
  • Limits in the basic efficiencies of turning raw materials to electricity mean 60 per cent of possible energy is already lost before we get any electricity out of a power station that uses fossil fuels.
  • The National Grid leads to further losses of nearly 8% of the remaining power through resistive heat dissipation, and further inefficiencies at substations and distribution to local points of usage account for yet more waste.
  • Finally we get losses due to the inefficiencies of the electrical appliances themselves.
  • Around 70% of energy having been lost before we get to use it, and we then waste more ourselves.
  • IT is estimated to use 5% of all power and targets are set to bring this down to 4%.
  • Heat generated from the data centre could be used elsewhere in the building, for example to heat or pre-heat water.
  • If we could bring the power generation closer to the point of use, we could utilise the heat generated in the creation of electricity within the building's Heating, Ventilation and Air Conditioning requirements. Commonly known as combined heat and power (CHP), such units were considered during the 1990s but haven't made much impact as power utilities have chosen to go with the lower maintenance and running costs of centralised power plant.
  • The transmission of gas has also become more efficient over time, with low losses. Gas powered CHP units can have overall efficiencies greater than 75 per cent, so we are more than doubling the base efficiency compared to large centralised electricity generation with no heat recovery.
  • Another option is for businesses to run at a fixed load and sell excess electricity and heat to the National Grid and local community (a concept known as Community CHP, or CCHP). The solution therefore not only creates greener, more efficient electricity but removes or cuts heating costs and can provide revenues for organisations to boot.

Full article: Interview with Thomas Casten (13-Aug-07)

Factories and power plants are wasting energy. If it could be captured and put to good use, greenhouse gas emissions could be substantially reduced, at a profit. 38% of U.S. CO2 comes from the generation of electricity -- a bigger percentage than transportation or anything else -- and that number is growing.

While electricity travels relatively economically through the wires -- you lose 9 percent of it on average -- heat takes about seven times as much energy to travel. So if you've got a power plant located 50 miles from Seattle, there is no economic way to move the waste heat from that power plant to downtown Seattle. By moving the power local, he's maybe 85% efficient.

It is cheaper per kilowatt capacity to build a generating plant 100 miles out of Seattle than in downtown Seattle. Expensive real estate, tight spaces, difficult to be small scale. But that's absolutely the wrong question. You don't care. What you care about is how much capital it will cost to deliver a new kilowatt to you.

Let's say you're building a new apartment building, and it's going to be new electric load on the system; it's going to need 10 megawatts. The question is: how much capital are we going to spend to generate and deliver 10 megawatts to that apartment house? Well, it turns out that the wires, the distribution, the transmission, the substations, and the auxiliary equipment you need cost more than the central plant.

So local power costs less capital. It also uses half the fuel. It also puts out half or less of the pollution. It's also far less vulnerable to extreme weather and terrorists than central stations.

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