PhD Thesis - Energy Systems Research Unit - University of Strathclyde

One way how these limitations can be overcome is by extending cogeneration toinclude micro-scale cogeneration. The idea of micro-scale cogeneration or microcogenerationdiffers from district heating or centralised cogeneration, in that, microscalecogeneration typically refers to single systems capable of supplying heat andpower to individual buildings such as households, small commercial premises, hotelsetc. which may or may not be connected to the centralised grid network [18]. Sizesfor micro-cogeneration are considerably smaller than typical cogeneration or districtheating systems, running into only few kW el . Although the official definition givenin Directive 2004/8/EC on the promotion of cogeneration based on a useful heatdemand in the internal energy market, the relevant EU directive on cogeneration [16],qualifies micro-cogeneration as systems with an electrical power output of less than50 kW el , the definition supplied by Pehnt et al. in [18] better explains and qualifiesmicro-cogeneration for the kind of research conducted in this thesis. Pehnt et al.discuss how given the type of load thresholds found in the domestic environment, alimit of 15 kW el is more adequate to qualify systems specifically meant for use inresidential single and multi-family households. For the scope of this thesis this lowerlimit is therefore used as the basis for system sizing.In terms of market penetration, micro-cogeneration in residential buildings is stillconsidered to be at an early stage of market development [21]. A full country bycountry analysis is outside the scope of this research, however in general it has beenobserved that micro-cogeneration penetration rates have shown a steady growth incountries where the market forces and local legislation have proved to be adequate tosupport micro-cogeneration, examples of which are Germany and Japan (for anexample outside the EU) [18]. The problem with micro-cogeneration is that althoughthe energy flow process concept is similar to large scale cogeneration, the associatedscaling down required to accommodate the highly intermittent and variable energydemands of residential buildings, presents new and challenging design problems.In terms of technology numerous books (e.g. [17, 18, 21]), journal papers (e.g. [22,23]) and research studies (e.g. [24]) have analysed at length the advantages and6