Future changes in the carbon intensity of grid electricity and its effect on the carbon emissions from domestic electric heating solutions

Transcription

1 Future changes in the carbon intensity of grid electricity and its effect on the carbon emissions from domestic electric heating solutions Philip James, Dr Rodger Edwards University of Manchester, UK In response to the issue of climate change, the UK is committed to meeting its share of an EU target that 20% of the EU s energy should come from renewables by Further, it has set a target in the Climate Change Act to reduce UK greenhouse gas emissions by at least 80% by Meeting these targets will lead to a significant reduction in the carbon intensity of the UK s electricity supply. is paper assesses the carbon emissions factor for grid electricity currently used in the Standard Assessment Procedure (SAP) for calculating carbon emissions from dwellings and the Simplified Building Energy Model (SBEM) used for calculating carbon emissions from non-domestic buildings. is currently used figure is found to be around 20% lower than those based on the most recent data. However, the carbon emissions factor for grid electricity is expected to fall. Based on the future electricity generation energy mix, as projected by studies conducted for the UK Government s Renewable Energy Strategy Consultation, carbon emissions factors have been predicted here for e central estimate is kgco 2 /kwh. Based on this projection, the carbon emissions of heat pumps are assessed if used for the provision of space heating and hot water within both existing and new-build dwellings. Comparisons are made with other domestic heating solutions and current issues with the practical application and energy efficiency maximisation of heat pumps are discussed. e results show that with the predicted carbon intensity of grid electricity in 2010, carbon emissions savings of around 30% can be made by switching from an efficient gas boiler to a heat pump. Further, carbon savings of around 60% could be achieved by 2020 if the carbon intensity of grid electricity is reduced as predicted through the greater use of renewable electricity generation. It is concluded that accurate and regularly updated short- and long-term projections of the carbon emissions factor for grid electricity are needed to better inform decisions based on the carbon emissions from different building services solutions. Keywords: carbon emissions factor, domestic dwellings, electric heating, grid electricity, heat pumps

2 Introduction In response to the issue of climate change, the UK government has signed up to a series of targets for the reduction of greenhouse gas emissions. ese include the UK's Kyoto Protocol commitment to reduce greenhouse gas emissions by 12.5% below 1990 levels over the period (DEFRA 2008); a UK national target to reduce CO 2 emissions by 20% below 1990 levels by 2010 (DEFRA 2008); an EU target for a 20% reduction below 1990 levels in EU greenhouse gas emissions by 2020 rising to 30% if there is an international agreement (European Commission 2008); and a commitment in the Climate Change Act to reduce the UK's greenhouse gas emissions by at least 80% below 1990 levels by 2050 (Great Britain 2008). Given the commitment to these targets policies must be devised to meet them. To this end the UK government has committed to an EU target to source 20% of the EU s energy from renewables by e UK s contribution to the achievement of this target is to source 15% of its energy from renewables by 2020 (BERR 2008). If these targets are met it could result in a reduction in the carbon emissions from the generation of grid electricity. e carbon emissions from, or carbon intensity, of grid electricity can be measured by the carbon emissions factor. is value, as used here, gives the average amount of carbon emitted per unit of electricity generated by all sources that supply the grid. It is measured in kilograms of carbon dioxide per kilowatt-hour (kgco 2/kWh). e carbon emissions factor of grid electricity is integral to the calculation of carbon emissions from buildings. e current figure used in the Standard Assessment Procedure (SAP) for calculating carbon emissions from dwellings is kgco 2/kWh (BRE 2008). e same figure is used in the Simplified Building Energy Model (SBEM) and other approved commercial software used for calculating carbon emissions from non-domestic buildings (ODPM 2006). is figure is assessed here for its accuracy. If the above targets for carbon emissions reductions and the increased use of renewable energy sources are met, the carbon emissions factor of grid electricity should decrease. Projections of the future electricity generation energy mix have been produced for the government s Energy White Paper (DTI 2007) and for the Renewable Energy Strategy Consultation (BERR 2008). Based on these projections predicted values for the carbon emissions factor of grid electricity in 2020 are presented. If the carbon emissions factor of grid electricity falls in future years this has implications when considering the whole-life carbon emissions from a range of building services solutions. Perhaps most important are the implications for the carbon emissions from electric heating solutions. Electric heating may have traditionally been considered a high carbon emissions heating solution, however, with greater use of heat pumps in the UK this could change. erefore, based on the carbon emissions factors presented in this study, the current and predicted future carbon emissions from heating using heat pumps are compared with those from an efficient gas boiler. e carbon intensity of grid electricity As stated, the CO 2 emission factor for grid electricity used by SAP and SBEM is kgco 2/kWh. is figure is derived from research by the Building Research Establishment (BRE). As they state,

3 e expected average annual emission factor for grid electricity of kgco 2/ kwh is based on the expected mix of electricity supply for the average of the Central Growth/Low Price and Central Growth/High Price scenarios between 2005 and 2010 from the DTI energy projections presented in Energy Paper 68 which have been adjusted to take account of expected transmission and distribution losses. Although there will be variations in the actual emission factors at different times of the day, it is appropriate to use an average value for SAP calculations (Pout 2005). erefore, the carbon emissions factor is not based on the actual electricity generation energy mix but on the average of Department of Trade and Industry (DTI) projections for 2005 and 2010 (DTI 2000). e electricity generation energy mix predicted by the DTI in 2000 is shown in table 1. Also shown in table 1 are the actual electricity generation figures for 2005 on a gross supplied basis from the Digest of UK Energy Statistics (BERR 2008a). is data shows a much higher proportion of coal-fired electricity generation, less gas-fired electricity generation, and less generation from renewables than was predicted for 2005 by the DTI projections used to calculate the carbon emission factor for SAP and SBEM. As would be expected, the DTI projections have been updated on several occasions since 2000, with the most recent update in November 2008 (DECC 2008). As shown in table 1, the updated Central projection for 2010 predicts a much higher proportion of coal-fired electricity generation, less gas-fired electricity generation, less nuclear generation, and less generation from renewables than was predicted for 2010 by the DTI projections used to calculate the carbon emission factor for SAP and SBEM. Table 1. Projected electricity generation by fuel type for the Central Growth/Low Price and Central Growth/High Price scenarios in 2005 and 2010 (DTI 2000). Also shown, the actual electricity generation figures for 2005 on a gross supplied basis (BERR 2008) and the updated Central projection for 2010 (DECC 2008). TWh CL 2005 CH 2005 CL 2010 CH 2010 DUKES Gross Supplied 2005 Coal Oil Gas Nuclear Renewable Import Storage Total Updated Central 2010 e carbon emissions per kwh of electricity from coal-, oil- and gas-fired generation are dependent on the amount of carbon released from the fuel burnt and the amount of electricity generated as a result. An adjustment can be made to this figure to account for grid transmission losses, giving the carbon emissions per kwh of delivered electricity. is calculation is summarised in table 2 using the

4 2005 figures for fuel used in electricity generation and the gross electricity supplied. As shown, the carbon emissions per kwh from coal-fired electricity generation are over twice those from gas-fired generation. is is due to the higher carbon emissions per kwh of fuel burnt and the lower efficiency of coal-fired power stations. erefore, the underestimation of the reliance on coal-fired electricity generation in the period means that the carbon emission factor for grid electricity used in SAP and SBEM is too low. Using the actual electricity generation energy mix for 2005 and the updated projection for 2010 from table 1, and using the carbon emissions per kwh of delivered electricity for coal-, oil- and gas-fired generation from table 2, updated carbon emission factors for grid electricity have been calculated and are shown in table 3. In the calculation the electricity supplied from imports and storage is excluded and zero carbon emissions are assumed from renewable and nuclear generation. e updated carbon emissions factors are around 25% higher than the value of kgco 2/kWh used in SAP. As shown in table 3, the updated figures calculated here compare well with estimates from other sources. Table 2. e figures for calculating CO 2 emissions per kwh of delivered electricity from coal-, oil- and gas-fired generation. *Assumes 13% grid losses (MTP 2008). Fuel Carbon 2005 Fuel emissions / used for kwh fuel electricity burnt (kgco 2/ generation kwh) (Pout (TWh) 2005) (BERR 2008a) 2005 Gross supplied electricity (TWh) (BERR 2008a) Carbon emissions / kwh electricity generated (kgco 2/kWh) Coal Oil Gas Carbon emissions / kwh electricity delivered (kgco 2/kWh)* Table 3. Estimates of the carbon emissions factor for grid electricity for 2005 and Also shown, carbon emissions factors calculated for the Market Transformation Programme and by DEFRA. Source Carbon emission factor (kgco 2/kWh) James and Edwards (2009) Market Transformation Programme (MTP 2008) DEFRA (2008a) If the carbon intensity of grid electricity continues to decrease beyond 2010 then it is valid to ask how long it will be before the carbon emissions factor reaches the value of kgco 2/kWh currently used in SAP and SBEM. Projections up to 2030 were made for the Market Transformation Programme (MTP 2008). ese were based on the electricity generation energy mix projected by the central price,

5 central carbon savings scenario used in the Energy White Paper (BERR 2008b) (see table 4). A carbon emissions factor of kgco 2/kWh was predicted for However, the electricity generation energy mix projections used by the MTP do not include the policy implications resulting from the Renewable Energy Strategy Consultation. is consultation and the resulting policies are central to the achievement of the target for 15% of the UK s energy to come from renewables by Studies by Sinclair Knight Merz (SKM 2008) and Redpoint (2008) were conducted for the Renewable Energy Strategy Consultation (BERR 2008). eir projections for the electricity generation energy mix in 2020 are shown in table 4. e carbon emissions factors resulting from these projections were calculated and are also shown in table 4. e calculation uses the carbon emissions per kwh of delivered electricity for coal-, oil- and gas-fired generation from table 2 and assumes zero carbon emissions from renewable and nuclear generation. e results indicate a significant reduction in the carbon emissions factor from grid electricity would occur as a result of the projected increases in renewable electricity generation. e central estimate from this study is the figure of kgco 2/kWh calculated using the SKM medium-renewables scenario. In the calculation of the above figure no account has been taken of future changes in the efficiency of coal-, oil- or gas-fired electricity generation. e MTP study did take account of efficiency improvements in coal- and gas-powered plant and the higher efficiency of newly built plant. is explains the lower carbon emissions factor calculated by the MTP study, as compared with that based on the MTP generation percentages but using the carbon emissions / kwh of delivered electricity for each fuel from table 2. Given that no account is taken of improved coal- and gas-powered plant efficiency, the carbon emissions factors presented for the SKM and Redpoint projections could be considered overestimates. However, given the lower percentage of generation from coal and gas that is predicted by the SKM and Redpoint studies, less new coal- and gas-powered plant will be required. erefore, the higher efficiency of new coal- and gas-powered plant will have less effect on the predicted carbon emissions factors based on these studies.

6 Table 4. Projected carbon emissions factors for grid electricity in 2020 based on projections of the electricity generation energy mix by the MTP, SKM and Redpoint. For MTP the figure given was calculated by that study; the figure in parentheses was calculated from the generation percentages using the carbon emissions / kwh of delivered electricity for each fuel from table 2) MTP SKM Redpoint Low Medium High Coal (%) Oil (%) Gas (%) Nuclear (%) Renewables (%) Average carbon emissions factor (kgco 2/kWh) (0.474) Heat Pumps Heat pumps, as used for domestic heating, extract thermal energy from the surrounding environment and upgrade it to a higher, more useful temperature. A heat pump consists of a compressor and carefully matched evaporator and condenser coils. A refrigerant liquid circulates within the system and evaporates when absorbing heat from the outside environment. It is possible to extract heat from the air at temperatures as low as -20 o C. e resulting refrigerant gas is then compressed by an electric motor, raising its temperature. e heat is passed via a heat exchanger in the condenser into water and can be used to provide space heating and hot water (NHBC 2007). Heat pumps are defined by the source of the heat taken from the outside environment. In a Ground Source Heat Pump (GSHP) heat is absorbed from the ground. Below about 2 metres the ground temperature varies little over the year and is stable at around o C. e system may be closed-loop, where a fluid is circulated through pipes buried in the ground (called a ground loop). e ground loop may be buried vertically or horizontally. When the heat energy has been extracted by the heat pump, the fluid is re-circulated back through the ground loop. It is important to demonstrate that the heat extracted from the ground over a whole year is less than the expected solar gain to that area - this defines the required area of the ground loop. Alternatively, an open loop system may be used where groundwater is taken from one area, the heat extracted by the heat pump, and the cooled water released back into the ground at a different location (NHBC 2007). In an Air Source Heat Pump (ASHP) heat is absorbed from the outside air. Air temperature is less stable than ground temperature. It will also be lowest when space heating demand is highest. erefore, ASHPs typically have a lower Seasonal Performance Factor (the average ratio of energy produced to energy consumed) than GSHPs. As figure 1 shows, the Coefficient of Performance of an ASHP is significantly reduced at lower ambient air temperatures. Normal gas central heating systems typically have a flow temperature of 75 o C and a return temperature of 65 o C, however, heat pumps typically have a flow

7 temperature of 50 o C and a return temperature of 35 o C, therefore, the heating delivery system needs to be designed with this in mind. In new builds, under-floor heating will often be specified as the most effective way to deliver low-grade heat. However, the most up-to-date heat pumps can produce water up to 65 o C thereby reducing dependency on supplemental systems and avoiding problems of legionella in the hot water supply (Sanyo 2008). ough as figure 1 shows, heat pumps are more efficient at lower output flow temperatures. Typical Seasonal Performance Factors for heat pumps may be in the range of 2:1 to 5:1 (NHBC 2007). Figure 1. e efficiency of an Air Source Heat Pump at different ambient air temperatures and with different output flow temperatures (Source: Viessmann (2006)). Comparing the efficiency of heat pumps and gas boilers Figure 2 compares the CO 2 emissions incurred per kwh of heat produced by a highly energy efficient gas boiler and by heat pumps operating at a range of efficiencies. ree carbon emissions factors are used for the grid electricity powering the heat pumps: firstly, the MTP projection for 2010 of kgco 2/ kwh (see table 3); secondly, the current figure used in SAP of kgco 2/kWh; and thirdly, the figure of kgco 2/kWh calculated from the SKM mediumrenewables scenario projection for 2020 (see table 4). e results show that using the predicted 2010 carbon emissions factor for grid electricity of kgco 2/kWh, a heat pump operating at an efficiency of 250% and above will produce less CO 2 emissions per kwh than an efficient gas boiler. At an efficiency of 500% the heat pump produces less than half the CO 2 emissions per kwh of the gas boiler. Using the current SAP carbon emissions factor for grid electricity of kgco 2/ kwh, a heat pump operating at an efficiency above 200% will produce less CO 2

8 emissions per kwh than an efficient gas boiler. At an efficiency of 500% the heat pump produces well under half the CO 2 emissions per kwh of the gas boiler. Using the projected 2020 carbon emissions factor for grid electricity of kgco 2/kWh, the heat pump produces less than three-quarters of the CO 2 emissions per kwh of the gas boiler when operating at 200% efficiency. At 500% efficiency the heat pump produces less than a third of the CO 2 emissions per kwh of the gas boiler. It should be stated that the effect of the storage and delivery of heat on the efficiency of the systems has not been considered here. Figure 2. Carbon emissions per kwh of energy produced for a 93% efficient gas boiler and for heat pumps operating at various efficiencies and using various carbon emissions factors for grid electricity (kgco 2/kWh). e actual efficiency or Coefficient of Performance (CoP) at which a heat pump operates will depend on the system itself, the ambient air or ground temperature, and the output temperature of the hot water being produced. e average efficiency or Seasonal Performance Factor will depend on the amount of output required from the system when operating at different CoPs. e data in figure 1 shows that a heat pump may produce hot water for space heating with an output temperature of 35 o C at an efficiency of 400%, and may produce domestic hot water with an output temperature of 55 o C at an efficiency of 300%. is does not take into account variations in ambient temperature but is reasonable for ambient temperatures of 7-10 o C. Given these efficiencies, the relative proportions of water supplied at the two temperatures will determine the overall Seasonal Performance Factor. A CoP of between 3 and 4 for a heat pump providing space heating and domestic hot water is consistent with experimental results, as is a rising CoP when the proportion of water produced at the temperature required for space heating increases (Stene, 2005).

9 Table 6 shows the space heating and hot water demand for three homes: a standard existing home, a standard new home compliant with the 2006 Building regulations, and an advanced home with best practice building fabric. As the thermal performance of the building fabric improves in the homes the space heating requirement is reduced, however, the delivered energy demand for domestic hot water remains unchanged. As is shown, the average efficiency of the heat pump is highest when the proportion of space heating in the total heat demand is highest. is is because the water for space heating is delivered more efficiently due to its lower output temperature. erefore, the largest percentage carbon savings compared with the gas boiler are made in the standard existing home where the space heating demand is highest. However, for all the home types and using all carbon emissions factors for grid electricity, considerable carbon emission savings are made in comparison with the efficient gas boiler. ese results show that with the predicted carbon intensity of grid electricity in 2010, carbon emissions savings of around 30% can be made by switching from an efficient gas boiler to a heat pump. Clearly, switching from an old, less efficient gas boiler to a heat pump would give even larger carbon savings. Further, if the carbon intensity of grid electricity is reduced through the use of more renewable electricity generation (as projected in the SKM 'mediumrenewables' scenario), then by 2020 carbon emission savings of around 60% could be achieved by switching from an efficient gas boiler to a heat pump. It should be stated that the provision of space heating at 35 o C may not be feasible in existing homes since the radiators will be sized for a higher output temperature. In this case, a higher output temperature may be required from the heat pump and this would reduce its Seasonal Performance Factor. An output temperature of 35 o C may be possible in existing homes if a new heat distribution system is installed which is sized for the requirements of the heat pump. Alternatively, if improvements are made to the home to reduce its heat loss, then the lower output temperature from the heat pump may be sufficient with the existing heat distribution system to achieve the required space heating. Table 5. e carbon emissions arising from meeting the space heating and hot water demand, as calculated by SAP, for three 120m 2 detached homes: a standard existing home, a standard new home compliant with the 2006 Building regulations, and an advanced home with best practice building fabric. Home Delivered heating demand (kwh) Delivered hot water demand (kwh) System and average efficiency Carbon emissions factor (kgco 2 / kwh) Total emissions (kgco 2) Percentage reduction (%) Standard 12, Gas 93% existing Heat pump home % Standard new home Gas 93% Heat pump %

10 Advanced home Gas 93% Heat pump % Conclusions e carbon emissions factor of grid electricity is integral in the calculation of carbon emissions from buildings. e current figure used in SAP and SBEM is kgco 2/kWh. is figure was devised as an average for the period However, using the actual electricity generation figures for 2005 and the updated projections for 2010, the carbon emissions factor was calculated as kgco 2/ kwh for 2005 and kgco 2/kWh for e average of these figures is around 25% higher than the value currently used. is is primarily due to an underestimation of the reliance on coal-fired electricity generation. e carbon intensity of grid electricity is expected to decrease. Projections made for the Market Transformation Programme (MTP 2008) predict a carbon emissions factor of kgco 2/kWh by However, the electricity generation energy mix projections used by the MTP study do not include the policy implications resulting from the Renewable Energy Strategy Consultation. ese policies will be central to achieving the target for 15% of the UK s energy to come from renewables by Based on the electricity generation energy mix projected by studies conducted for the Renewable Energy Strategy Consultation (Redpoint 2008; SKM 2008), carbon emissions factors have been predicted here for e central estimate is for a carbon emissions factor of kgco 2/kWh by If the carbon emissions factor of grid electricity does fall as predicted in future years, this has implications when considering the whole-life carbon emissions from a range of building services solutions. Using the carbon emissions factors presented in this study, the current and predicted future carbon emissions from heating using heat pumps were compared with those from an efficient gas boiler. e results show that with the predicted carbon intensity of grid electricity in 2010, carbon emissions savings of around 30% can be made by switching from an efficient gas boiler to a heat pump. With the predicted carbon intensity of grid electricity in 2020, savings of around 60% could be achieved by switching from an efficient gas boiler to a heat pump. e results presented here show the importance of the carbon emissions factor for grid electricity when making decisions that involve comparing carbon emissions from alternative building services solutions. It is therefore imperative that the most accurate and up-to-date figures are used. Should data on the current electricity generation energy mix or updated projections of the future electricity generation energy mix make the currently used carbon emissions factor unrealistic, then systems should be in place for this to be corrected. Furthermore, accurate and regularly updated long-term projections of the carbon emissions factor for grid electricity should be made available, such that they can inform decisions on issues of whole-life carbon emissions in the built environment and elsewhere.

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