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2 Background and Data Sources

This section provides background information on the approaches used to estimate energy supply and demand, including the rationale for the choice of 2002 as the 'base year', together with information on data sources. The results of these calculations are discussed in Section 3 and subsequent sections.

This section discusses:

• The approach used to estimate consumption
• Whether 2002 was a representative year
• Estimating CO ₂ emissions arising from energy use.

2.1 The approach to estimating energy consumption

To build estimates of energy supply and demand, the Scottish Energy Study has drawn on a range of complementary data sources, together with the knowledge that supply and demand are in balance. The analysis has used:

• 'Bottom-up' estimates of consumption by four energy demand categories, namely:
  - Domestic
  - Transport
  - Industry, and
  - Services (public, commercial and agriculture).
• 'Top-down' estimates of fuel consumption in the following five categories:
  - Solid fuel (predominantly coal, but with some coke)
  - Natural gas
  - Oil-based products (petrol & diesel for vehicles, kerosene, fuel-oil, burn-oil and LPG)
  - Nuclear fuels, and
  - Renewable sources (used to generate electricity and consumed directly).
• Information on fuel transformation, particularly data on electricity generation and supply. Information was used on both the primary fuels used to generate electricity, and supply of electricity to the four demand sectors. Oil refining is also covered in this section.

The parallel approach taken has helped to ensure that, where possible, data sources can be cross-checked.

2.2 Choice of 2002 as the base year

The Base Year ( BY) selected was 2002 because, at the time of carrying out this part of the study, this was the most recent year for which complete data sets were available.

It is important that the selected year is representative, particularly as the data will be used to inform estimates of future energy use. If there are large one-off variations in energy use (owing, for example, to plant outages), these could bias long-term predictions. In order to determine if 2002 is representative, CO ₂ emissions for 2002 were compared with those for 2000 and 2001 ⁵. This drew on the National Atmospheric Emissions Inventory ( NAEI) published data on CO ₂ emissions ⁶. This includes data on CO ₂ as a result of use of energy, along with CO ₂ emissions that arise from processing fuels, industry processes ( e.g. cement), waste incineration and land use and changes in forestry ( LUCF).

The 2000 and 2001 totals and energy-related CO ₂ were similar, but energy-related CO ₂ in 2002 was approximately 3 Mt (6%) less than 2001.

Table 1: Comparison between NAEI Scottish CO ₂ emissions 2000 to 2002

There may be a number of reasons for the reduction in 2002:

• 2002 was a warmer year than 2000 and 2001. In 2002, degree-days ⁷ were about 10% less than in 2001. This will have reduced demand for heating. Interestingly, in all three years degree-days are less than the 20-year average.
• Changes to the electricity generating efficiency and fuel mix due to increased use of renewable energy and a combined cycle gas turbine ( CCGT) plant at Peterhead.
• The data indicated a reported reduction in transport fuel consumption: however, this may be anomalous, as other data suggest an increase in road miles travelled.

Overall, it was concluded that 2002 is a reasonably representative year to use, even though CO ₂ emissions are 6% lower than the two preceding years.

2.3 Estimation of CO ₂ emissions

Energy-related CO ₂ emissions arise from the combustion of fossil fuel. It is straightforward to estimate how much CO ₂ is produced if information is available on the quantity, type and, in some cases, grade of fuel used: a simple emissions factor can be used to relate fuel use to CO ₂ emissions.

The amount of CO ₂ produced to generate electricity depends on the fuel mix used. The operation of nuclear fission and renewable energy plants does not directly give rise to any CO ₂, whereas oil, gas and coal give rise to increasing levels.

In 2002, Scotland had six major power plants: three nuclear (Chapelcross, Hunterston and Torness), two coal stations (Longannet and Cockenzie) and a gas-fired station at Peterhead. In addition, there are a number of hydro stations (some including pumped storage), small-scale renewable generators and CHP installations. As the fuel mix varies, the CO ₂ produced per kWh of electricity generated varies from hour to hour and year to year. However, even taking into account these variations, the mixture of electricity generation plant in Scotland is not the same as in the UK as a whole, with more renewables and nuclear plants and fewer gas-fired stations. Hence, the mix of electricity generation plant in Scotland will have a different emissions factor from that of the UK as a whole. In addition, Scotland exports electricity, particularly to England, but also to Northern Ireland and the Republic of Ireland.

There are a number of different approaches used to allocate CO ₂ to electricity generated or electricity used. All of these are conventions that simplify a process that could otherwise be very complex. As with all simplifications, these conventions can be misleading if they are applied to situations where they are not representative of the actual CO ₂ emissions. Thus, care is needed when choosing which convention to use, to ensure the adoption of policies that give the most cost-effective reductions in CO ₂.

Most of the conventions used have been devised for use across the UK. In this study, we also use a specific Scottish emission factor for electricity. The use of a Scottish factor would show the impact of specific Scottish policy and programme interventions when reporting CO ₂ emissions in Scotland. For example, this would enable:

• Tracking the impacts of future changes in Scottish electricity generation, particularly the:
  - Increasing use of renewables.
  - Impact of retiring coal or nuclear plant.
  - Addition of any new plant.
  - Implementation of carbon capture and storage technology.

• Showing the impact of electricity market changes on Scottish carbon emissions compared to the impact of measures under the Scottish Climate Change Programme.

The following sections consider two conventions which can be used to calculate:

• The total CO ₂ emissions from electricity generation in Scotland.
• The emissions factor for electricity consumption in Scotland.

2.3.1 Model 1: UK standard CO ₂/kWh factor for electricity

The UK convention takes the total UKCO ₂ emissions from electricity generation and divides this figure by total UK electricity consumption. This takes into account all types of generation and includes the effect of grid losses in the calculation.

The average UK figure emissions factor is 0.432 kg CO ₂/kWh ⁸; this has been set for some time and is used in a wide range of policy and carbon reporting calculations. It has reduced considerably since the early 1990s: in 1990 the UK factor was 0.802 kg CO ₂/kWh and 40.102 TWh of electricity was sold in Scotland, so this equates to a total of 17.33 Mt of CO ₂ emitted.

2.3.2 Model 2: Bottom-up Scottish approach

This is based on the generating plant in Scotland and so reflects the fuel mix used to produce power in Scotland using major power plant.

This approach does not, however, account for the impact of electricity from CHP plant. Most of the electricity produced by CHP would be generated from fossil fuel supplied direct to a site and thus would be recorded in the appropriate demand sector as fossil fuel consumption. So in this analysis of CO ₂ we have not included electricity from CHP plants in the total electricity generated.

When calculating CO ₂ emissions it is important to take into account the total losses in the generation process, as they increase the amount of fuel used (and hence CO ₂ produced) for a given output. Total losses included self-consumption at the power station, in transmission and distribution, and from the energy required for pumped-hydro electricity.

In 2002, 45.5 TWh of electricity was generated in Scotland, with total electrical losses of 5.4 TWh. This resulted in the sale of 40.102 TWh of electricity, both within Scotland and as exports. Based on the Scottish generation fuel mix, this results in total electricity-related emissions of 16.28 Mt of CO ₂. (It is worth noting, in comparison, that this is within 3% of the NAEI figure of 16.85 Mt of CO ₂ from power and heat generation.)

Based on the 40.102 TWh of electricity for sale, this gives a factor of 0.406 kg CO ₂/kWh.

2.3.3 Scottish CO ₂ emissions with each approach

To consider the sensitivity of the results to the model used, the following table compares the calculated CO ₂ emissions and emissions factor for electricity, using each approach for 2002.

Table 2: CO ₂ emissions from electricity

The difference between Models 1 and 2 demonstrates the impact of using a UK or Scottish specific approach. At present, the lower CO ₂ emissions estimated using the Scottish approach

reflect the greater proportion of nuclear and renewable power generation in Scotland.

For the purpose of this study ⁹, the UK factor (Model 1) has been used throughout, with additional calculations using a Scottish approach being shown where this has been considered appropriate.

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