Micro combined heat and power or
microCHP is an extension of the now well established idea of
cogenerationCogeneration is the use of a heat engine or a power station to simultaneously generate both electricity and useful heat...
to the single/multi family home or small office building.
Overview
In the majority of energy applications, energy is required in multiple forms. These energy forms typically include some combination of:
heating, ventilation, and air conditioningHVAC is an acronym that stands for the closely related functions of "Heating, Ventilating, and Air Conditioning"-the technology of indoor environmental comfort...
,
mechanical energyIn physics, mechanical energy describes the sum of potential energy and kinetic energy present in the components of a mechanical system.-Simplifying assumptions:...
and
electric powerElectric power is defined as the rate at which electrical energy is transferred by an electric circuit. The SI unit of power is the watt.When electric current flows in a circuit, it can transfer energy to do mechanical or thermodynamic work...
. Often, these additional forms of energy are produced by a
heat engineA heat engine is a physical or theoretical device that converts thermal energy to mechanical output. The mechanical output is called work, and the thermal energy input is called heat. Heat engines typically run on a specific thermodynamic cycle...
, running on a source of high-temperature heat. A heat engine can never have perfect efficiency, according to the
second law of thermodynamicsThe second law of thermodynamics is an expression of the universal principle of entropy, stating that the entropy of an isolated system which is not in equilibrium will tend to increase over time, approaching a maximum value at equilibrium, and that the entropy change dS of a system undergoing any...
, therefore a heat engine will always produce a surplus of low-temperature heat. This is commonly referred to as "waste heat" or "secondary heat", or "low-grade heat". This heat is useful for the majority of heating applications, however, it is sometimes not practical to transport heat energy over long distances, unlike
electricityElectricity is a general term that encompasses a variety of phenomena resulting from the presence and flow of electric charge...
or
fuelFuel is any material that is burned or altered to obtain energy and to heat or to move object. Fuel releases its energy either through a chemical reaction means, such as combustion, or nuclear means, such as nuclear fission or nuclear fusion...
energy. By transporting fuel, however, the "waste heat" is essentially being transported along with the fuel, before the waste heat is actually produced.
To make efficient use of energy, the "waste heat" must be used purposefully. Since it is practical to transport electricity, but not always practical to transport waste heat, an energy efficient system must generate electricity near locations where the waste heat can be put to good use. This is known as a
combined heat and powerCogeneration is the use of a heat engine or a power station to simultaneously generate both electricity and useful heat...
(
CHPCHP can mean:* California Highway Patrol* Christian Heritage Party* Cumhuriyet Halk Partisi -Other:* Consolidated Health Plans* Children's Hospital of Pittsburgh* Christian Heritage Party* Christian Historical Party...
) system, or "
cogenerationCogeneration is the use of a heat engine or a power station to simultaneously generate both electricity and useful heat...
".
In a central power plant, the supply of "waste heat" may exceed the local heat demand. In such cases, if it is not desirable to reduce the power production, the excess waste heat must disposed in e.g.
cooling towerCooling towers are heat removal devices used to transfer process waste heat to the atmosphere. Cooling towers may either use the evaporation of water to remove process heat and cool the working fluid to near the wet-bulb air temperature or rely solely on air to cool the working fluid to near the ...
s or sea cooling without being used. A way to avoid excess waste heat is to reduce the fuel input to the CHP plant, reducing both the heat and power output to balance the heat demand. In doing this, the power production is limited by the heat demand.
CHP systems are able to increase the total energy utilization of primary energy sources, such as fuel and concentrated solar thermal energy. Thus CHP has been steadily gaining popularity in all sectors of the energy economy, due to the increased costs of fuels, particularly oil-based fuels, and due to environmental concerns, particularly
climate changeClimate change is a change in the statistical distribution of weather over periods of time that range from decades to millions of years. It can be a change in the average weather or a change in the distribution of weather events around an average...
.
In a traditional power plant delivering electricity to consumers, about 30% of the heat content of the primary heat energy source, such as
biomassBiomass, a renewable energy source, is biological material derived from living, or recently living organisms, such as wood, waste, and alcohol fuels. Biomass is commonly plant matter grown to generate electricity or produce heat. For example, forest residues , yard clippings and wood chips may be...
,
coalCoal is a readily combustible black or brownish-black sedimentary rock normally occurring in rock strata in layers or veins called coal beds. The harder forms, such as anthracite coal, can be regarded as metamorphic rock because of later exposure to elevated temperature and pressure...
, solar thermal,
natural gasNatural gas is a gas consisting primarily of methane. It is found associated with fossil fuels, in coal beds, as methane clathrates, and is created by methanogenic organisms in marshes, bogs, and landfills...
,
petroleumPetroleum or crude oil is a naturally occurring, flammable liquid found in rock formations in the Earth consisting of a complex mixture of hydrocarbons of various molecular weights, plus other organic compounds.The term "petroleum" was first used in the treatise De Natura Fossilium, published in...
or
uraniumNuclear power is power produced from controlled nuclear reactions. Commercial plants in use to date use nuclear fission reactions....
, reaches the consumer, although the efficiency can be 20% for very old plants and 45% for newer gas plants. In contrast, a CHP system converts 15%-42% of the primary heat to electricity, and most of the remaining heat is captured for
hot waterWater heating is a thermodynamic process using an energy source to heat water above its initial temperature. Typical domestic uses of hot water are for cooking, cleaning, bathing, and space heating...
or
space heatingSpace heating is the heating of a space, usually enclosed, such as a house or room. A space heater keeps the air and surroundings at a comfortable temperature for people or animals, or even plants in a greenhouse. Space heating generally warms a small space, and is usually held in contrast with...
. In total, as much as 90% of the heat from the primary energy source goes to useful purposes when heat production does not exceed the demand.
CHP systems have benefited the industrial sector since the energy crisis of the 1970s. For three decades, these larger CHP systems were more economically justifiable than micro-CHP, due to the economy of scale. After the year 2000, micro-CHP has become cost effective in many markets around the world, due to rising energy costs. The development of micro-CHP systems has also been facilitated by recent technological developments of small heat engines. This includes improved performance and/or cost-effectiveness of
Stirling engineA Stirling engine is a heat engine that operates by cyclic compression and expansion of air or other gas, the working fluid, at different temperature levels such that there is a net conversion of heat energy to mechanical work....
s, steam engines, gas turbines,
diesel engineA diesel engine is an internal combustion engine that uses the heat of compression to initiate ignition to burn the fuel, which is injected into the combustion chamber during the final stage of compression...
s and
Otto engineThe Otto engine was a large stationary single-cylinder internal combustion four-stroke engine designed by Nicolaus Otto. It was a low-RPM machine, and only fired occasionally since it was a hit-and-miss engine.-Open-frame mechanical design:...
s.
Micro-CHP systems
Micro-CHP systems’ chief difference from their larger-scale kin is in the operating parameter-driven operation. In many cases industrial CHP systems primarily
generate electricityElectricity generation is the process of creating electricity from other forms of energy.The fundamental principles of electricity generation were discovered during the 1820s and early 1830s by the British scientist Michael Faraday...
and heat is a useful by-product. Contrarily, micro-CHP systems, which operate in homes or small commercial buildings, are driven by heat-demand, delivering
electricityElectricity is a general term that encompasses a variety of phenomena resulting from the presence and flow of electric charge...
as the byproduct. Because of this operating model and because of the fluctuating electrical demand of the structures they would tend to operate-in, homes and small commercial buildings, micro-CHP systems will often generate more electricity than is instantly being demanded.
To date, micro-CHP systems achieve much of their savings, and thus attractiveness to consumers, through a "generate-and-resell" or
net meteringNet metering is an electricity policy for consumers who own renewable energy facilities, such as wind, solar power or home fuel cells. "Net", in this context, is used in the sense of meaning "what remains after deductions" — in this case, the deduction of any energy outflows from metered energy...
model wherein home-generated power exceeding the instantaneous in-home needs is sold back to the electrical utility. This system is efficient because the energy used is distributed and used instantaneously over the electrical grid. The main losses are in the transmission from the source to the consumer which will typically be less than losses incurred by storing energy locally or generating power at less than the peak efficiency of the micro-CHP system. So, from a purely technical standpoint net-metering is very efficient.
Another positive to net-metering is the fact that it is fairly easy to configure. The user's electrical meter is simply able to record electrical power exiting as well as entering the home or business. As such, it records the net amount of power entering the home. For a grid with relatively few micro-CHP users, no design changes to the electrical grid need be made. Additionally, in the
United StatesThe United States of America is a federal constitutional republic comprising fifty states and a federal district...
, federal and now many state regulations require utility operators to compensate anyone adding power to the grid. From the standpoint of grid operator, these points present operational and technical as well as administrative burdens. As a consequence, most grid operators compensate non-
utilityIn economics, utility is a measure of the relative satisfaction from, or desirability of, consumption of various goods and services. Given this measure, one may speak meaningfully of increasing or decreasing utility, and thereby explain economic behavior in terms of attempts to increase one's utility...
power-contributors at less-than or equal-to the rate they charge their customers. While this compensation scheme may seem almost fair at first glance, it only represents the consumer’s cost-savings of not purchasing utility power versus the true cost of generation and operation to the micro-CHP operator. Thus from the standpoint of micro-CHP operators, net-metering is not ideal.
While net-metering is a very efficient mechanism for using excess energy generated by a micro-CHP system, it is not without its detractors. Of the detractors' main points, the first to consider is that while the main generating source on the electrical grid is a large commercial generator, net-metering generators "spill" power to the grid in a haphazard and unpredictable fashion. However, the effect is negligible if there are only a small percentage of customers generating electricity and each of them generates a relatively small amount of electricity. When turning on an oven or space heater, about the same amount of electricity is drawn from the grid as a home generator puts out. If the percentage of homes with generating systems becomes large, then the effect on the grid may become significant. Coordination among the generating systems in homes and the rest of the grid may be necessary for reliable operation and to prevent damage to the grid.
In an evaluation from 2008 by Claverton Energy Group, Stirling engined micro CHP was deemed the most cost effective of the various microgeneration technologies in abating carbon in the UK.
System Types and Technologies
Micro-CHP systems are currently based on several different technologies:
- Internal combustion engine
The internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine the expansion of the high temperature and pressure gases, which are produced by the combustion, directly applies force to a movable...
s
- Stirling engine
A Stirling engine is a heat engine that operates by cyclic compression and expansion of air or other gas, the working fluid, at different temperature levels such that there is a net conversion of heat energy to mechanical work....
s
- Steam engine
A steam engine is a heat engine that performs mechanical work using steam as its working fluid.The idea of using boiling water to produce mechanical motion has a long history, going back about 2000 years...
s
- Microturbines
- Fuel cell
A fuel cell is an electrochemical cell that produces electricity from a replenishable fuel tank. The electricity is generated through the reaction, triggered in the presence of an electrolyte, between the fuel and an oxidant . The reactants flow into the cell, and the reaction products flow out of...
s
Fuels and Engine Types
The majority of cogeneration systems use natural gas for fuel, because natural gas burns easily and cleanly (though it does emit CO
2), it can be inexpensive, it is available in most areas and is easily transported through pipelines, which already exist for many homes. Natural gas is suitable for internal combustion engines, such as
Otto engineThe Otto engine was a large stationary single-cylinder internal combustion four-stroke engine designed by Nicolaus Otto. It was a low-RPM machine, and only fired occasionally since it was a hit-and-miss engine.-Open-frame mechanical design:...
and
gas turbineA gas turbine, also called a combustion turbine, is a rotary engine that extracts energy from a flow of combustion gas. It has an upstream compressor coupled to a downstream turbine, and a combustion chamber in-between. Energy is added to the gas stream in the combustor, where air is mixed with...
systems, because it burns without producing ash, soot or tar. Gas turbines are used in many small systems due to their high efficiency, small size, clean combustion, durability and low maintenance requirements. Gas turbines designed with
foil bearingFoil bearings are a type of air bearing. A shaft is supported by a compliant, spring loaded foil journal lining. Once the shaft is spinning fast enough, the working fluid , pushes the foil away from the shaft so that there is no more contact...
s and air-cooling, operate without lubricating oil or coolants. The waste heat of gas turbines is mostly in the exhaust, whereas the waste heat of
reciprocating internal combustion engineA reciprocating engine, also often known as a piston engine, is a heat engine that uses one or more reciprocating pistons to convert pressure into a rotating motion. This article describes the common features of all types...
s, is split between the exhaust and cooling system.
The future of combined heat and power, particularly for homes and small businesses, will continue to be affected by the price of fuel, including natural gas. As fuel prices continue to climb, this will make the economics more favorable for energy conservation measures, and more
efficient energy useEfficient energy use, sometimes simply called energy efficiency, is using less energy to provide the same level of energy service.For example, insulating a home allows a building to use less heating and cooling energy to achieve the same temperature...
, including CHP and micro-CHP.
Fuels
There are many types of fuels and sources of heat that may be considered for micro-CHP. The properties of these sources vary in terms of system cost, heat cost, environmental effects, convenience, ease of transportation and storage, system maintenance, and system life.
Some of the heat sources and fuels that are being considered for use with micro-CHP include:
biomassBiomass, a renewable energy source, is biological material derived from living, or recently living organisms, such as wood, waste, and alcohol fuels. Biomass is commonly plant matter grown to generate electricity or produce heat. For example, forest residues , yard clippings and wood chips may be...
, LPG, vegetable oil (such as
rapeseedRapeseed , also known as rape, oilseed rape, rapa, rapaseed and canola, is a bright yellow flowering member of the family Brassicaceae...
oil), woodgas,
solar thermalSolar energy, radiant light and heat from the Sun, has been harnessed by humans since ancient times using a range of ever-evolving technologies. Solar radiation, along with secondary solar-powered resources such as wind and wave power, hydroelectricity and biomass, account for most of the available...
, and
natural gasNatural gas is a gas consisting primarily of methane. It is found associated with fossil fuels, in coal beds, as methane clathrates, and is created by methanogenic organisms in marshes, bogs, and landfills...
, as well as multi-fuel systems. (Nuclear power is hazardous at small scales, due to radiation risks, so it is generally not viable for micro-CHP.) The energy sources with the lowest emissions of particulates and net-carbon dioxide, include solar power, biomass (with two-stage gasification into
biogasBiogas typically refers to a gas produced by the biological breakdown of organic matter in the absence of oxygen. Biogas originates from biogenic material and is a type of biofuel....
), and natural gas.
Engines
External combustion engines, can run on any high-temperature heat source. These engines include the
Stirling engineA Stirling engine is a heat engine that operates by cyclic compression and expansion of air or other gas, the working fluid, at different temperature levels such that there is a net conversion of heat energy to mechanical work....
, and the
steam engineA steam engine is a heat engine that performs mechanical work using steam as its working fluid.The idea of using boiling water to produce mechanical motion has a long history, going back about 2000 years...
. Both range from 10%-20% efficient, and as of 2008, small quantities are in production for micro-CHP products. Other possible heat cycles include the
Organic Rankine CycleThe Organic Rankine cycle is named for its use of an organic, high molecular mass fluid with a liquid-vapor phase change, or boiling point, occurring at a lower temperature than the water-steam phase change . The fluid allows Rankine cycle heat recovery from lower temperature sources such as...
(lower heat),
Ericsson cycleThe Ericsson cycle is named after inventor John Ericsson, who designed and built many unique heat engines based on various thermodynamic cycles. He is credited with inventing two unique heat engine cycles and developing practical engines based on these cycles...
, and Stoddard cycle.
Market status and government policy
The largest deployment of micro-CHP is in Japan at this time, where over 50,000 units are in place, with the vast majority being of the "ECO-WILL" type based on Honda's MCHP engine generator unit. It is estimated that about 1,000 micro-CHP systems were in operation in the UK as of 2002. These are primarily "Whispergen"
Stirling engineA Stirling engine is a heat engine that operates by cyclic compression and expansion of air or other gas, the working fluid, at different temperature levels such that there is a net conversion of heat energy to mechanical work....
s, and Senertec Dachs
reciprocating engineA reciprocating engine, also often known as a piston engine, is a heat engine that uses one or more reciprocating pistons to convert pressure into a rotating motion. This article describes the common features of all types...
s. The market is supported by the government through regulatory work, and some government research money expended through the Energy Saving Trust and
Carbon TrustThe Carbon Trust is a not for dividend company limited by guarantee created by the UK government to help businesses and public organisations to reduce their emissions of carbon dioxide into the atmosphere, through improved energy efficiency and developing commercial low carbon technology...
, which are public bodies supporting energy efficiency in the UK. Effective as of 7 April 2005, the UK government has cut the VAT from 17.5% to 5% for micro-CHP systems, in order to support demand for this emerging technology at the expense of existing, less environmentally friendly technology. The reduction in VAT is effectively a 12.5%
subsidyA subsidy is a form of financial assistance paid to a business or economic sector. Most subsidies are made by the government to producers or distributors in an industry to prevent the decline of that industry or an increase in the prices of its products or simply to encourage it to hire more...
for micro-CHP units over conventional systems, which will help micro-CHP units become more cost competitive, and ultimately drive micro-CHP sales in the UK.
http://www.fuelcellsworks.com/Supppage2451.html Of the 24 million households in the UK, as many as 14 to 18 million are thought to be suitable for micro-CHP units.
In Germany, 3,000 ecopower micro-CHP units have been installed, using the U.S. based Marathon Engine Systems long-life engine. The engine runs on natural gas and propane. The ecopower micro-CHP is also available in the United States. The German government is offering large CHP incentives, including feed-in tariffs and bonus payments for use of micro-CHP generated electricity. The United States federal government is offering a 10% tax credit for smaller CHP and micro-CHP commercial applications.
Advantages
The advantage of having some "ownership" of one's electrical power was discussed above. Actual utility bill savings are probably minimal when looking at life-cycle cost of this approach as compared to a simple natural gas furnace.
There are definite pollution-reduction advantages if the unit is replacing an electric heating system powered by a coal power plant.
Also, like other distributed power systems, the end user can configure the unit as an emergency power source in the event of a power outage.
A big picture advantage of this approach is the ability to distribute power generation, locally, at the end-user rather than a remote power plant. If deployed on a large scale, this can reduce the need for new power plant installations and free-up transmission line capacity for other uses (i.e. solar energy or wind turbine farms). There is also the reduced long-range transmission losses. Avoiding transmission line losses and power plant construction reduces costs, energy consumption and pollution for everyone.
Disadvantages
Some of the concerns of this technology are that
gas fuelsFossil fuels or mineral fuels are fuels formed by natural resources such as anaerobic decomposition of buried dead organisms. The age of the organisms and their resulting fossil fuels is typically millions of years, and sometimes exceeds 650 million years...
have historically had high price volatility. This makes it difficult to forecast the system's operating cost. Also the fuels are non-
renewableRenewable energy is energy generated from natural resources—such as sunlight, wind, rain, tides, and geothermal heat—which are renewable . In 2006, about 18% of global final energy consumption came from renewables, with 13% coming from traditional biomass, such as wood-burning...
with a net release of carbon dioxide. Gas leaks are very hazardous. Units operating on vegetable oil, however, (as has been done in Germany with approximately 1800 installed units as of 2007), use a renewable and very safe fuel. Some vendors will not sell the generator alone, but only packaged with a new gas furnace. Despite what the salesmen say, there is no legitimate reason for this, because a generator and a furnace can run off separate thermostats, thus avoiding issues of compatibility and liability. The structuring of the products in this way may be a concession made in deals brokered with gas companies for rebates.
A practical disadvantage is the "energy balance" of the generator. Often in the summer months for example, electricity is required but only heat for hot water, meaning that either the power production must be limited dynamically by the heat demand, so that residual power demand must be satisfied from the grid, or some of the heat produced must be cooled away and means has to be provided to achieve this. In practice there are few occasions when the equipment produce exactly the required local mix of heat and electricity, so cooling and/or power exchange with other systems are necessary. In practice all this means that not all of the local energy requirements can be met by the CHP equipment alone unless there is to be energy (heat) dumping. Large CHP plants can turn surplus heat in the summer months into cooling by means of complex and expensive absorption refrigeration plants, but this is generally not economical on a small scale.
Another issue is the low electrical efficiency of very small micro CHP in the size range around 1 kWe. A report by the UK Carbon Trust suggested that at this scale there may be no CO2 saving from micro-CHP, compared to using an efficient condensing boiler for one's heat and generating one's electricity from a gas CCGT power station. (By contrast, large CHP plant does make a sizeable saving compared to separate production of heat and electricity).
Large premises are likely to be the most economical advantageous for micro-CHP, especially where heat demand is relatively stable throughout the year. (For example in apartment buildings, hospitals or nursing homes where there are significant need for hot water for bathing).
It follows from the above that in sizing CHP equipment to make the most economical energy saving, the key factors are the
minimum heat load required and any possibilities of storing surplus heat. If this is zero for any appreciable time, then the investment in a micro-CHP system is less financially viable. A possible exception to this might be in
off-the-gridThe term off the grid or off-grid refers to living in a self-sufficient manner without reliance on one or more public utilities.Off-grid homes are autonomous—i.e. do not rely on municipal water supply, sewer, natural gas, electrical power grid or similar utility services...
locations where mains electricity is not available and local generation of power is the only option.
See also
- Distributed generation
Distributed generation, also called on-site generation, dispersed generation, embedded generation, decentralized generation, decentralized energy or distributed energy, generates electricity from many small energy sources....
- District heating
District heating is a system for distributing heat generated in a centralized location for residential and commercial heating requirements such as space heating and water heating...
- Geothermal power in Iceland
Because of the special geological situation in Iceland, the high concentration of volcanoes and geothermal energy are very often used for heating and production of electricity...
- Grid-tied electrical system
A grid-tied electrical system, also called tied to grid or grid tie system, is a semi-autonomous electrical generation or grid energy storage system which links to the mains to feed excess capacity back to the local mains electrical grid...
- Home fuel cell
A home fuel cell, also called micro combined heat and power and microgeneration, is a residential-scaled clean energy system. A home fuel cell is an alternative energy technology that increases efficiency by simultaneously generating power and heat from one unit, on-site within a home...
- Microgeneration Certification Scheme
The Microgeneration Certification Scheme is the first product and installer certification scheme to cover all the microgeneration technologies. The MCS Mark is currently owned by the UK’s Department of Energy and Climate Change...
- Stationary fuel cell applications
Stationary fuel cell applications are stationary fuel cell applications that are either connected to the electric grid to provide supplemental power and as emergency power system for critical areas, or installed as a grid-independent generator for on-site service.-Codes and standards:Stationary...
- Trigeneration
Trigeneration or trigen is the simultaneous production of mechanical power , heat and cooling from a single heat source such as fuel or solar energy...
- Virtual power plant
A virtual power plant is a cluster of distributed generation installations which are collectively run by a central control entity....
Codes and standards
External links