The impact of thermal storage on the operational behaviour of residential CHP facilities and the overall CO2 emissions

When evaluating the environmental impact of small-scale cogeneration facilities, two important boundary conditions are often overlooked. Firstly, cogeneration units are mostly considered as stand-alone facilities, although, in reality, they will be part of a system that may also contain a thermal-storage tank and back-up boiler. Secondly, usually mainly static and simplified methods are used to calculate the possible reduction of CO2 emissions. In this paper, these issues are discussed in two parts. The dimensioning of cogeneration facilities to fulfil a certain heat demand and the impact of thermal-storage tanks on the operational behaviour of these units are dealt with. It is shown that the use of thermal-storage tanks prolongs the yearly operation time of a CHP facility and allows the cogeneration unit to operate more continuously. Also, it is clarified how to interpret thermal load-duration diagrams in a correct way. Furthermore, the impact of thermal storage on the overall CO2 emissions is investigated. Hereby, the interaction with the expansion of the central power system and the annual use of the cogeneration units are two important parameters. Using a small thermal-storage device causes the net reduction of CO2 emissions, in comparison with a reference scenario without additionally installed cogeneration, to be almost three times higher compared to the case without heat buffer. Finally, it is shown that the operational behaviour of multiple small-scale cogeneration units can be approximated by the behaviour of one large fictitious unit for the determination of the net reduction of CO2 emissions.