PhD Thesis - Energy Systems Research Unit - University of Strathclyde

The same conclusions discussed for the system’s PES can be extended to the annualCO 2 savings (ES). Reducing the system’s operational hours by reducing the usefulthermal load (e.g. micro-trigeneration feeding a smaller building, lower occupancyetc.) results in a lower ES. Also, if the net electrical imports are included in thecalculation of the annual CO 2 savings, reducing the thermal load by improving thebuilding fabric results in a reduction in ES. Finally, decreasing the net electricalimports by improving the appliances’ electrical efficiency increases the ES;otherwise if the net electrical imports are excluded from the calculation the annualES is the same for both electrical efficiencies.In terms of plant configuration, due to the parasitic thermal demand required to coverfor energy losses of the additional chilled water tank, the plant system in Scenario3 High has the lowest ES value. On the contrary the additional flat plate solar waterheater, which offsets part of the CHP unit thermal load, renders the plant system usedin Scenario 4 High the scenario with the highest ES value.Finally exporting all the electricity and importing all the electricity from the gridresults in a situation where although the annual CO 2 savings are the same if the netimports are not included (the cogenerated electricity in Scenario 2 High is similar tothat in Scenario 5 High ; the system therefore behaves in a similar fashion), if the netimports are included Scenario 5 becomes the scenario with the lowest ES.5.4.2 Effect of improving the grid emission factorTo model the reduction in emitted CO 2 for varying degrees of grid networkimprovements, the grid emission factor was varied between the current value of1.088 and a future 0.5 kg CO 2 per kWh delivered at end-use. Using the scenarios inScenario 1 Low (3 household building with low efficiency fabric) as an example,Figure 5.8 shows how the emissions emitted by the micro-trigeneration and separategeneration, Emissions SEPARATE , vary with improving grid emission factor (e Grid ). Asdone with the primary energy savings both plots including and excluding the netelectrical imports are shown. Figure 5.9 then shows how the ES varies withimproving grid emission factor (e Grid ).203