Micro-generators: The prospects for combined heat and power systems on a domestic scale

Energy use in the domestic sector represents a large portion of the primary energy consumption and carbon dioxide (CO₂) emissions. Highly distributed energy systems, employing various micro-generators, may be able to reduce domestic energy demand and CO₂ from this sector. Very small 'combined heat and power' (CHP) systems are one class of microgenerator that can supply heat and power to a single residential home at significantly increased efficiency and reduced carbon emissions compared with separate supply. Such systems are known as micro-CHP (mCHP) or domestic-CHP (DCHP) in the United Kingdom, and are used to replace central heating boilers typically installed in houses. The relative merits of three alternative network-connected mCHP plants have been evaluated; based respectively on an internal combustion engine (IC), a Stirling engine, and a fuel cell. Each of the systems varies in terms of their power generation efficiencies, and supply differing proportions of electricity and heat. They are consequently suitable for various housing types and sizes. All these mCHP plants employ natural gas as the fuel input. The use of any of the three systems will result in lower CO₂ emissions (relative to those from the most efficient condensing boilers)-about 450kg for every thousand hours used in the case of a Stirling engine based system installed in a reasonably well insulated home. They also lead to energy-cost savings for the consumer, up to £46 per MWh (a 61% saving on conventional energy supplies) depending on house type. However, their capital costs are at present more expensive than a conventional boiler, with the fuel cell being prohibitively so. The IC and Stirling engine variants are shown to display the greatest economic and environmental benefit.