Combined Heat and Power Systems for Residential Use
Once available only to large commercial buildings, Combined Heat and Power generation (CHP) systems are now being produced on a scale that is safe, practical, and affordable to homeowners. CHP technologies, sometimes referred to as cogeneration, have provided heat and electrical energy efficiently at commercial and industrial sites for many years. However, after hundreds of successful residential installations in Japan and Europe, several manufacturers are now offering models in the U.S.
A CHP system uses fuel such as natural gas to produce heat and electricity simultaneously. The electricity can be used for any household device such as lights and appliances. Simultaneously, the heat produced can be used for water heating and/or space heating. About 10% of the fuel used is lost as exhaust, much like a high efficiency furnace.
The engines used in the CHP units for producing electricity can be internal combustion or Stirling (also called external combustion) engines. Other types of generation technologies, such as fuel cells, have not reached the commercialization stage. Micro-CHP, as residential-sized CHP systems are usually called, run on propane, natural gas, or even (in the case of Stirling engines) concentrated solar energy or biomass. The byproduct of electricity generation is waste heat—and plenty of it. One 6-kW unit provides 10 gpm of hot water at 140 to 150°F. This waste heat can be used to heat an entire home, water for domestic use, for swimming pools and spas, or even as an energy source for heat-driven (absorption) cooling systems.
CHP systems are extremely efficient, offering combined heat and power generating efficiency of about 90%, compared to about 30 to 40% for electricity from a central power station.
Micro-CHP units range in capacity from about 1 kW to 6 kW and are about the size of a major appliance. Installation may be performed initially by specialists and, after the technology matures, by an experienced plumber, electrician, or HVAC technician. Units come as grid-tied systems which connect to utility power as backup or as stand-alone systems for remote residences.
One unit with a new, small capacity engine simultaneously produces 1.2 kilowatts of electric power and 11,000 Btus of heat in the form of hot water. The system is combined with a high efficiency, natural gas-fueled warm air furnace or boiler for supplemental space heating.
The small engines tend to burn very cleanly - exceeding all emissions requirements for CO2 and NOx. One unit claims to produce less CO and nitrous oxides than a single burner on a kitchen gas range.
The primary challenge for getting the highest efficiency and best economic return on CHP is to fully utilize all of the thermal energy produced when generating electricity. As the technology develops, various operating regimes will be tested to optimize the energy available based on variables such as the loads in the home, the climate and the season.
Source:http://www.toolbase.org/Building-Systems/HVAC/combined-heat-power