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What is it?

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

Micro-CHP systems

In many cases industrial CHP systems primarily generate electricity and heat is a by-product; micro-CHP systems in homes or small commercial buildings are controlled by heat-demand, delivering electricity as the by-product. When used primarily for heat in circumstances of fluctuating electrical demand, 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 dynamic demand management and net-metering are 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 smart 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.

Engine types and technologies

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

• Internal combustion engines

• Stirling engines

• Steam engines

• Microturbines

Fuels and engine types

The majority of cogeneration systems use natural gas for fuel, because natural gas burns easily and cleanly, 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 engine and gas turbine systems. 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 bearings 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 engines, 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 use, 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: biomass, LPG, vegetable oil (such as rapeseed oil), woodgas, solar thermal, and natural gas, 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 biogas), and natural gas.

Engines

External combustion engines, can run on any high-temperature heat source. These engines include the Stirling engine, and the steam engine. 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 Cycle (lower heat), Ericsson cycle, and Stoddart cycle.

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