CHP System Evaluation

A typical US house has four main categories of loads that need to be met with some energy source:

• Electric loads in the US are conventionally supplied by electric utilities through a complex transmission and distribution system to bring electricity from a central plant to each residence.
• Heating loads in the US are conventionally met with natural gas fired, forced-air furnaces and forced-air heat pumps.
• Space cooling is usually provided by utility electricity - powered air conditioning heat pumps.
• Domestic hot water heating is generally provided by a natural gas fired heater or electric resistance heater, and a hot water storage tank of about 30 gallons.

Installing a combined heat and power (CHP) unit in a house provides a more efficient, integrated method of meeting the electric, space heating, water heating, and with the addition of additional equipment, even the space cooling loads of the house, when compared with a baseline (conventional) system. For practical reasons, commercially available CHP units grid-connected CHP unit would most likely be thermally-driven, maintaining the set temperature of a large hot water storage tank at a given set temperature. Electricity would be bought from the utility when the house’s demand exceeds the unit’s output, and would be sold back to the utility when the CHP output exceeds the required load. In Japan and some parts of Western Europe, there are a few thousand residential CHP units in operation. These parts of the world have very high electric rates (about 20-25 cents/kWh) and this is very beneficial to the economics of a CHP system. However, drivers in the US include a strong demand for consistent electric power quality and reliability (despite an aging electric infrastructure undergoing increasingly frequent brownouts and blackouts), increasing electricity prices, as well as increasing uncertainty in fuel and electricity prices. In order to successfully address these demands, a residential CHP product would need to be able to provide power while the electric utility does not. Although use of a properly sized CHP system reduces emissions and operating costs for meeting the loads in a home, it also tends to increase the monthly fuel bill. Thus, the ideal system would be fuel flexible to minimize vulnerability to the price fluctuations of any one fuel (e.g. natural gas).