Reinventing hot water?

Expectations are high for cogeneration (the joint generation of heat and power) to support solutions for energy-related problems and climate issues. The sizing, i.e., the rated thermal and electric power, of cogeneration units, is currently largely based on simple static criteria in which the output of a cogeneration units is maximised for a given or estimated heat-demand profile. In this paper, we investigate the validity of this sizing strategy. Firstly, we develop a methodology to objectively evaluate the impact of the use of cogeneration on the overall primary-energy use and greenhouse-gas emissions. To do so, we compare two scenarios; a base case without additional cogeneration in which heat is provided by boilers and all electricity comes from the central power system and an alternative scenario with cogeneration where the heat is delivered by the cogeneration, if necessary supported by back-up boilers, and where the cogeneration also takes over part of the electricity demand formerly delivered by the central power system. The major advantage of our method is that we fully take into account the dynamic interaction between the cogeneration and the central power system by scrupulously simulating the power generation on an hourly basis and a sub-power-plant level. As such, we can detect possible sub-optimal dispatching of the central power system caused by the non-dispatchable and possibly fluctuating output of the cogeneration. Apart from the conventional sizing strategy of cogeneration, we look at alternative options where the interaction with the central power system is improved. A first possibility is to use smaller cogeneration, i.e., reduced-scale sizing, that can deliver a smaller share of the thermal energy but that can operate with a higher annual use. A second possibility considered is the extension of the use of the cogeneration without reducing the scale, which implies that part of the heat cannot be used. This option is called partial-heat-usage sizing. To illustrate our method and its use for optimal sizing of cogeneration, we look at the cogeneration in the service sector in the Belgian energetic context towards 2010. We find that both the reduced-scale sizing and the partial-heat-usage sizing can lead to significantly higher primary-energy savings and emission reductions.