PhD Thesis - Energy Systems Research Unit - University of Strathclyde

Similarly to the effect on PE SEPARATE and the system’s PES caused by improving gridefficiency, improving the grid emission factor results in a reduction in emissionsproduced in separate generation, Emissions SEPARATE , with a subsequent reduction inthe micro-trigeneration system environmental advantage. In this case, considering apossible improvement in the grid emission factor of about 55% (from 1.088 to 0.5 kgCO 2 per kWh), it can be observed how the system’s annual savings in Scenario 1 Lowquickly drops to approximately 1.9% in both the current and high efficiencyelectrical cases. For other scenarios, such as Scenario 1 High , results indicate thatalthough the extent of the reduction in annual savings is similar, (from 51.5% to -1%), a grid emission factor of 0.5 kg CO 2 per kWh yields a negative percentage ofannual savings. This means that for the 3 household building, 0.5 kg of CO 2 per kWhappears to be the environmental limit beyond which the system loses anyenvironmental advantage over using separate generation. Again from a Malteseperspective, this is currently not a problem as the grid network emission factor iswell above this limit. However, when considering the development of a policypromoting micro-trigeneration in residential buildings this is an important aspectwhich must be taken into serious consideration as it might lead to an overestimationof the potential annual CO 2 savings which might be achieved.Considering the case of 6 household building with low efficiency building fabric(Scenario2 Low ) a similar behaviour can be observed. Figure 5.10 in fact shows thatimproving the grid emission factor from 1.088 to 0.5 kg CO 2 per kWh results in adrop in the system’s annual CO 2 savings to approximately 6%. An importantdifference however is the fact that, whereas in the 3 household building the microtrigenerationsystem’s annual CO 2 savings are practically annulled, in the 6household building the system’s annual CO 2 savings are still relatively significant.This suggests that, although the same system plant size is being used for bothbuildings, the fact that the system is better matched with the demand in the largerbuilding enables it to retain its environmental advantage vis-à-vis grid emissionfactor improvements.206