Environmental Assumptions

The greenhouse gas emissions from burning natural gas were taken from the most authoritative source – the IPCC guidelines for stationary combustion.[187] This gives 182.1±6.7g of CO2- equivalent per kWh of fuel combusted (HHV), including the global warming potentials of other emissions (NOx, CH4, etc.).

Combustion is not the only source of greenhouse gasses however, as energy consumption and methane leakage occur at all stages of the fuel production chain – extraction, processing, transmission and distribution. The additional life-cycle emissions from these activities were estimated using SimaPro 7, a Life Cycle Assessment (LCA) software package from PRé.

Since 2004 the UK became a net importer of natural gas, and by 2008 only 67% of the supply mix was indigenous, with 24% Norwegian, 8% Dutch and the remainder being imported as liquefied natural gas (LNG).[256] Natural gas leakage from distribution pipelines is inevitable, and particularly important as methane has a global warming potential 22 times higher than CO2 (over a 20 year time horizon). Leakage rates from distribution were assumed to be 0.5-2.0% based on studies of the UK and other infrastructures.[257-260] These stages were modelled using inventories from the EcoInvent 2.0 database, and assessed using the Impact 2002+ indicator. The emissions from sourcing and distribution were estimated to add 7-9% (14.9±2.3 g/kWh) to the total CO2-equivalent emissions; which was typical among European estimates.96

The CO2 emissions from reforming natural gas were taken to be the same as from combustion, as the number of carbon atoms per kWh of fuel input was invariant. The operating efficiency of each fuel cell technology therefore determined the magnitude of emissions reductions. The emission of other powerful greenhouse gasses (CH4 and NOx) from fuel cell CHP systems have been measured in several studies to be around one-tenth those from combustion.[63, 107, 168, 170, 264-266] This was neglected as the resulting reduction in greenhouse gas emissions was <0.1g CO2/kWh. It should be noted that fuel cells would offer improvements to local air quality, however this was outside the scope of this single-criterion study.

***

The displaced emissions from centrally generated electricity were estimated using detailed data from the Digest of UK Energy Statistics (DUKES),[199] as well as environmental performance reports from five major energy suppliers in the UK. The proportion of total electricity generation, average efficiencies and carbon intensities of each type of plant are given in Table 6.4. Carbon intensities were estimated in SimaPro using EcoInvent 2.0 data for UK or European plants, and are compared to estimates published by the International Atomic Energy Agency in 2000.

Proportion97

Efficiency (HHV)98

Direct CO2

emissions Whole life-cycle CO2 emissions

Source (g/kWh) SimaPro IAEA [267]

CCGT 40.6% 45.6% ± 4.5% 423 ± 24 455 ± 25 434-689 Coal 33.7% 33.8% ± 2.1% 1005 ± 71 1088 ± 71 967-1308 Nuclear 15.6% 35.4% ± 4.3% 0 8 ± 1 9-21 Biomass / Waste99 2.6% 23.5% ± 2.6% 1853 ± 45 51 ± 9 31-61 French Imports100 2.1% - - 85 ± 6 - Wind 1.3% - 0 11 ± 1 9-48 Hydro 1.3% - 0 3 ± 1 4-23 Oil 1.1% 28.8% ± 0.6% 991 ± 65 1126 ± 60 802-901 Pumped Hydro 1.0% 74.5% ± 5.0% - 868 ± 149 - Others 0.7% 18.1% ± 6.7% 1490 ± 377 1613 ± 540 -

Table 6.4: Composition and carbon intensity of the UK electricity mix for 2007. Greenhouse gas emissions are given for each type of plant, both those from direct combustion, and for the whole life-cycle with fuel sourcing and plant capital.

97 _{Based on annual TWh of energy generated, taken from DUKES Tables 5.4 and 5.6.[199]}

98 _{Net efficiencies are given, which include the 18.1TWh of electricity consumed by the power stations themselves.} Gross efficiencies are 1.02-1.14 times higher than those presented. Averages were taken from DUKES Tables 5.6 and 5.10, and the standard deviations came from the range of individual plant performances given by energy suppliers. 99 _{For biomass, whole life cycle emissions are lower than direct emissions from combustion due to the CO2 absorbed} in producing the feedstock.

The average gross plant efficiency in the UK was 40.5% HHV, or 36.0±1.9% when transmission losses and consumption by the plants was included, meaning that 2.78±0.15 MJ of primary energy was consumed per MJ of electricity delivered. Sourcing the fuel and building plants also add to this, making the whole life-cycle energy consumption 3.16±0.92MJ per MJ delivered.

The annual average carbon content of grid electricity was estimated to be 647g per kWh delivered, using the data from Table 6.4 and accounting for 6.6% transmission losses.[199] Of this, 572g were direct emissions from combustion, and 76g were from fuel production and distribution, and from construction of the power plants. The figure for direct emissions is in line with recent government estimates,[31, 269] but is higher than the grid average used in the UK government’s Standard Assessment Procedure (422g/kWh) and the assumed long-term average rate (430g/kWh).[20, 270]

There is substantial debate over what emissions would actually be displaced by micro-CHP, for example in references [271-273]. It is argued that demand reducing measures would displace so-called marginal plant rather than the average generation mix.[272, 273] Marginal (or peaking) plants are those which respond to instantaneous changes in national demand, varying their output during the day to balance supply and demand. It is unlikely that nuclear baseload generators would be turned off because of micro-CHP systems; instead it would be low efficiency coal, oil and gas generators with higher than average emissions, meaning that micro- CHP could offer greater reductions. The numerous government recommendations for the carbon intensity of electricity generation are discussed by Hawkes in [20], with the conclusion that “there is a great deal of uncertainty regarding appropriate CO2 rates for residential consumption and generation, and this is a ripe area for research”.

Similarly, there are difficult choices to be made when considering how emissions savings from micro-CHP will evolve over time. It can be expected that heat and electricity generating systems will change considerably over the lifetime of the fuel cell, and several studies have suggested that the carbon intensity of electricity could reduce by as much as 70% in this time-frame.[274- 276] It is argued that deep and rapid decarbonisation of the grid would have negative implications for fossil-fuelled micro-CHP, however it is only the baseload and average generation mix that is expected to change substantially.[67, 277] Fossil fuelled plants are likely to remain as the marginal generators, as the output of renewables cannot be controlled without excessive storage, and nuclear is inflexible and cannot provide the rapid start-up and ramping

rates required. Fitting these fossil plants with carbon capture and storage (CCS) systems could offer a route to lowering marginal carbon intensity, however it remains to be seen whether CCS can demonstrate the required flexibility without incurring cost and efficiency penalties due to the increase in systems complexity.[278]

Other developments could potentially be realised within the same time-scale as centralised grid decarbonisation, such as the use of fuel cells as regional or national marginal generation as part of a virtual power plant, or the development of lower carbon fuel sources such as bio- methane.[277] These further complicate future trajectory of emissions savings, so it was assumed that the carbon intensity of both the fuel cell and the reference system will remain unchanged over the 10-15 year time period being studied.

Six combinations of emission factors were considered, as shown in Table 6.5. Emissions from the average grid mix were used for the central case, and were similar to the marginal emissions that were estimated to be displaced by micro-CHP in a study by Ilex due to the recent switch from gas back to coal in the UK.[273] CCGT and coal plants were included to investigate the impact of displacing the best fossil-fuelled alternative, and the worst marginal emissions.

Direct emissions Whole life cycle

Natural gas 182.1 ± 6.7 197.0 ± 7.1

Displaced heat 213.0 ± 11.7 230.4 ± 12.6

Grid average 572 ± 28 647 ± 32

CCGT 423 ± 24 455 ± 25

Coal 1005 ± 71 1088 ± 71

Table 6.5: Carbon intensities assumed for the different emissions scenarios. Heat was assumed to be produced with an 85.5 ± 3.5% efficient condensing boiler.

6.4.2. Central Results

The distribution of carbon emissions from the 1,000 properties is shown in Figure 6.14, calculated from the reference scenario with whole life cycle emissions. The average direct emissions from combustion were 5.4 tonnes per year, plus an additional 0.5 tonnes from the fuel life-cycle, which is not typically considered in other studies (e.g. those in Table 3.2). The average direct emissions were in line with other estimates for the UK, which give 5.5-5.8 tonnes per house per year, or 135-145MT for the entire UK domestic sector (~25 million houses).[11-13]

Figure 6.22: Histogram showing the distribution of greenhouse gas emissions in houses without micro-CHP installed. The average balance of CO2 emissions from the reference, PEMFC and SOFC scenarios is shown in Table 6.6 below, which gives the breakdown of emissions between the various components of energy use. The fuel cells reduce carbon emissions by displacing centralised generation, both by reducing on-site consumption and exporting electricity for others to consume.

Reference 1kW PEMFC 1kW SOFC

Fuel cell: 4860 ± 656 4295 ± 409

Boiler: 3916 ± 1964 1638 ± 1751 2172 ± 1876

Purchased electricity: 1978 ± 1125 559 ± 554 492 ± 531

Exported electricity: -2496 ± 594 -2914 ± 708

Net sum: 5894 ± 2926 4562 ± 2739 4046 ± 2806

Table 6.6: Average carbon balance for the 1,000 houses from three different scenarios, showing the CO2 emissions

produced and displaced by each item (in kg per year). Whole life cycle emissions were assumed, with the average grid mix being displaced.

The emissions reductions that could be made by installing each type of fuel cell are shown in Figure 6.23, plotted against annual thermal demand by convention. No defined trend was seen against either thermal or electrical demand; savings increased linearly until around 15MWh annual thermal demand and then levelled out. The 1kW fuel cells tended to run at full capacity in houses with higher thermal demand, and so no further gains could be made. A reasonable logarithmic fit could be produced when reductions were plotted against CO2 emissions from the reference scenario – which was analogous to Figure 6.15 where savings were plotted against traditional energy bills.

0 10 20 30 40 50 60 70 80 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 N um be r of ho us e s

Annual CO₂ emissions with a condensing boiler and grid electricity Mean: 5.9 tonnes (whole life cycle)

5.4 tonnes (direct emissions only)

Figure 6.23: The distribution of emissions reductions made by installing each type of fuel cell. Each data point represents

annual thermal demand and CO2 reductions that were simulated in one of the 1,000 houses.

When displacing the UK average grid mix, the CO2 savings estimated for each fuel cell technology were substantial, averaging 1.3-1.9 tonnes per year, meaning the carbon footprint for energy consumption in each household could be reduced by 25-35%.

The influence of including additional life-cycle emissions is shown in Figure 6.24. The absolute reductions were around 20% lower when only direct emissions were considered, although the percentage reductions were not as strongly affected as the reference emissions were also lower – 5.4 tonnes per household compared with 5.9 tonnes. The average direct emissions for PEMFC and SOFC systems (1.1-1.5 tonnes per year) were comparable to estimates given in other recent simulations of fuel cell micro-CHP in the UK, for example [20].

0 500 1000 1500 2000 2500 3000 0 10 20 30 40 50 Red u ct io n in C O ₂ emi ss io n s (k g)

Annual thermal demand MWh)

PEMFC

Mean annual CO₂ reductions

Whole life cycle: 1333 ± 292 kg Direct emissions: 1070 ± 245 kg 0 500 1000 1500 2000 2500 3000 0 10 20 30 40 50 Red u ct io n in C O ₂ emi ss io n s (k g)

Annual thermal demand MWh)

SOFC

Mean annual CO₂ reductions

Whole life cycle: 1848 ± 317 kg Direct emissions: 1527 ± 271 kg 0 500 1000 1500 2000 2500 3000 0 10 20 30 40 50 Red u ct io n in C O ₂ emi ss io n s (k g)

Annual thermal demand MWh) PAFC

Mean annual CO₂ reductions

Whole life cycle: 2062 ± 351 kg Direct emissions: 1721 ± 301 kg 0 500 1000 1500 2000 2500 3000 0 10 20 30 40 50 Red u ct io n in C O ₂ emi ss io n s (k g)

Annual thermal demand MWh)

Electricity grid carbon intensity: 647 g/kWh (572 g/kWh direct) Emissions from heat production: 230 g/kWh (213 g/kWh direct)

AFC

Mean annual CO₂ reductions

Whole life cycle: 1326 ± 246 kg Direct emissions: 1065 ± 206 kg

Figure 6.24: Comparison of the direct and whole life cycle emissions reductions from PEMFC and SOFC systems, when displacing the average grid mix. Absolute and relative reductions are shown in the left and right plots, respectively.