Environmental Assessment

In this section, an environmental assessment of the novel tri-generation system operating within a UK energy system context is presented. The environmental assessment compares the 1.5 and 2.0 kWe tri-generation system with an equivalent base case system comprising grid electricity, natural gas fired boiler and electrically driven VCS. The evaluation metric used in the environmental assessment is the annual CO2 emission. This is determined through the multiplication of the annual natural gas and electrical demand by their respective emission factors and summing the result. The emission factors of natural gas and electricity are varied, in a reasonable range, to carry out a sensitivity analysis of the environmental performance. Using electrical emission factor data published by Brander et al. (2011), the environmental performance of the tri-generation system in the context of different countries is presented. The assumptions used in the environmental assessment are the same as those presented in the economic assessment in Sect. 8.2.2. The emission factors used are based on a UK energy system context, and are as follows:

  • • Average natural gas emission factor: 0.184 kg CO2 kWh-1 (EST 2014)
  • • Average electricity emission factor: 0.555 kg CO2 kWh-1 (AMEE 2014)

Table 8.2 presents the environmental assessment results. The 1.5 and 2.0 kWe trigeneration systems produce a respective 51.3 and 50.2 % reduction in annual CO2 emissions compared to the equivalent base case system.

Figure 8.5a shows the annual CO2 emissions of the 1.5 and 2.0 kWe tri-generation systems and equivalent base case systems with respect to natural gas emission factor. Over the investigated natural gas emission factor range of 0.050.3 kg CO2 kWh-1, the tri-generation system always has a lower annual CO2 emission. Both the tri-generation and base case system have a natural gas requirement. However, the greater proportionate natural gas demand in the tri-generation system means its annual CO2 emission reductions are more sensitive to changes

Annual emissions (kg CO2)

1.5 kWe tri

4030

1.5 kWe base

8282

2.0 kWe tri

5756

2.0 kWe base

11,567

Table 8.2 Environmental

assessment results

Annual CO2 emission comparison between th

Fig. 8.5 Annual CO2 emission comparison between the 1.5 and 2.0 kWe tri-generation systems and equivalent base case system with a natural gas emission factor, and b electricity emission factor in the natural gas emission factor. Consequently, as the natural gas emission factor is increased, the relative reduction in annual CO2 emissions compared to the equivalent base case system is diminished. The 2.0 kWe tri-generation system is more sensitive to changes in the natural gas emission factor than the 1.5 kWe trigeneration system due to a lower electrical efficiency.

Figure 8.5b shows the annual CO2 emissions of the 1.5 and 2.0 kWe trigeneration systems and equivalent base case systems with respect to electrical emission factor. The tri-generation system has no electrical demand, and thus only the base case system is affected by the electrical emission factor. The tri-generation system has a lower annual CO2 emission compared to the equivalent base case system when the electrical emission factor is greater than 0.2363 kg CO2 kWh-1 for the 1.5 kWe case and 0.2305 kg CO2 kWh-1 for the

2.0 kWe case.

Figure 8.6 shows the annual CO2 emissions of the 1.5 and 2.0 kWe equivalent base case system in a range of different counties using electrical emission factor data published by Brander et al. (2011). The annual CO2 emissions of the respective tri-generation systems (horizontal lines) are plotted to indicate the countries in which the novel system is currently environmentally viable. The 1.5 and

  • 2.0 kWe tri-generation system is feasible in all the countries investigated except France and Norway as these countries have an average electrical emission factor of less than 0.1 kg CO2 kWh-1. France and Norway have an energy system that is largely characterised by the use of nuclear and renewables. As a result, the average electrical emission factor is low. Figure 8.6 shows that the 1.5 and
  • 2.0 kWe tri-generation system is most environmentally viable in Australia and China. Australia and China generate a large proportion of their electricity from coal, which has a high emission factor per kWh of electricity generated, and thus strengthens the environmental benefit of adopting the novel tri-generation system. Based on the data presented in Figs. 8.3 and 8.6, Denmark is currently the only country investigated where the novel tri-generation system is both economically and environmentally viable. Interestingly, the countries where the tri-generation system is not economically feasible due to a low electrical unit cost are in general the countries in which the system is most environmentally feasible i.e. Australia and China. This is primarily due to cheap electrical generation from easily accessible, more polluting fuels such as low grade coal.

Next, Sect. 8.3.1 contextualises the presented environmental assessment results and concludes the section.

Annual CO2 emission comparison between the 1.5 and 2.0 kW tri-generation systems and equivalent base case system with respect to country of operation

Fig. 8.6 Annual CO2 emission comparison between the 1.5 and 2.0 kWe tri-generation systems and equivalent base case system with respect to country of operation

 
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