Micro combined heat and power

Micro combined heat and power or microCHP is an extension of the now well established idea of cogeneration to the single/multi family home or small office building.

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Overview

Recognizing that in a number of situations, consumers need both heat and electric power, combined heat and power (CHP) systems (also called cogeneration) came into being. Taking advantage of the fact that all fuel-based electrical generating systems have a maximum efficiency dictated by the laws of thermodynamics, CHP systems provide a source of heat from the so-called waste heat of the electrical generating process. The waste heat of generation consists of combustion exhaust gases which can quite sufficiently provide heating to systems with low temperature demands. Waste heat can also be used to produce additional electricity in combined cycle configurations, but this is not always practical. As such, CHP systems, which have been steadily gaining popularity in industrial communities, which are able to increase the overall energy content utilization of fuels.

For example, in a generate-only system, such as a traditional power plant delivering electricity to consumers, about a third of the heat content of the primary energy source, like coal, natural gas or uranium, reaches the consumer, although the efficiency can be a bit lower for very old plants and significantly higher for new plants. In contrast, a CHP system typically converts at least two-thirds and often as much as 90% of the heat from the primary energy source to useful purposes, like the production of electricity, steam, hot water or space heating. While industry has benefited significantly from CHP systems, a number of features of CHP which make them attractive to industry have acted as barriers for this technology to trickle down to use by individuals.

The vast majority of cogeneration systems use natural gas for fuel. This is due to the fact that natural gas is cheap, burns cleanly, is available in most areas and is easily transported through pipelines, which already exist for many homes. In addition, natural gas can be burned in gas turbines, which are used in the vast majority of large cogeneration systems and many small systems due to their high efficiency, small size, clean combustion, durability and low maintenance requirements. In addition, gas turbines designed with foil bearings and air-cooling operate without lubricating oil or coolants. Finally, the waste heat of gas turbines is mostly in its exhaust, whereas the waste heat of the main alternative for small systems, reciprocating engines, is split between its exhaust and cooling system.

The future of combined heat and power, particularly for homes and small businesses, will be greatly affected if natural gas prices continue to climb. While the waste heat from biomass, solar thermal, coal, diesel, heavy fuel oil and nuclear power plants can be used for CHP, such energy sources are far less convenient, more difficult to transport and more expensive for home use. Nuclear power, in particular, is impractical at small scales. Except for nuclear and solar power, these other energy sources also burn significantly less cleanly than natural gas, except where expensive pollution controls are used. Finally, of all of them, only diesel can be used in gas turbines or reciprocating engines, which are cheap, small and efficient, making them the choice for most small CHP installations.

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 electricity 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 electricity 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 metering 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 States, 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-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.

Technologies

Micro-CHP systems are currently based on several different technologies:

• Internal combustion engines
• Stirling engines
• Steam engines
• Microturbines
• Fuel cells

Market status

The largest deployment of micro-CHP is in Japan at this time. Over 50,000 units are in place with the vast majority of the ECO-WILL type that incorporate the MCHP engine generator unit produced by Honda. The Honda MCHP unit is a high-endurance engine with extremely low noise output and solid-state grid interface, making it as advanced as the company's hybrid automobiles.

It is estimated that about 1,000 micro-CHP systems are in operation in the UK as of 2002. These are primarily Whispergen Stirling engines, and Senertec Dachs reciprocating engines. The market is supported by the government through regulatory work, and some government research money expended through the Energy Saving Trust and Carbon Trust, 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% subsidy 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. [1] Of the 24 million households in the UK, as many as 14 to 18 million are thought to be suitable for micro-CHP units.

Recently, Climate Energy of Massachusetts in the United States has introduced its Freewatt[2] micro-CHP system to the US market. The Freewatt system also incorporates the Honda MCHP engine generator technology. It is claimed that this system will produce about 50% of the electric power needs of a typical US home from the fuel now used to heat the home, thereby doubling the value of the heating fuel purchased by the homeowner and significantly reducing the carbon footprint of the home. This product has already received numerous awards, including breakthrough product of the year from Popular Mechanics Magazine, and is expected to be widely available over the United States over the next year or so. Estimates are that this product could be used in about 50 million homes in the United States. One major United States natural gas distribution company, KeySpan, has closely evaluated this product and is offering a substantial monetary incentive for its customers to purchase this product and help in its energy conservation program.

See also

• Distributed generation
• District heating
• Geothermal power in Iceland
• Cogeneration
• Virtual power plant