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Appendix 7 CO ₂ Allocation Approaches for Electricity

Background and introduction

Unlike fossil fuels, electricity does not have a standard kg( CO ₂)/kWh factor. The factor varies from year to year, depending on the primary fuel mix and the generating efficiencies of the power stations.

It was agreed that, for the Scottish Energy Study base-year study, the default factor would be the UK 'basket' emission factor of 0.432 kg( CO ₂)/kWh(delivered). However, it was accepted that there were limitations to this model, as there are for any model.

Figures from a model are fine as long as the basis and boundaries by which they are calculated are explicit and the values are not taken out of context. The danger is when figures from an analysis are then used for something for which they were never intended; one can end up with unrealistic figures that may influence projections and, as a consequence, strategy, policy and targets.

The general advantages of establishing a robust method for calculating a fair and reasonable kg( CO ₂)/kWh(delivered) emission factor for electricity in Scotland include:

• A structured model would reflect fairly the impact on base year CO ₂ emissions from electricity, but not in isolation from other fuels and/or imports & exports to England and NI.
• The model can be used to project the realistic impact of future generation/consumption scenarios on CO ₂ emissions from Scotland.
• This, in turn, would help the Scottish Executive to develop its strategy for a sustainable low-carbon economy in Scotland, using a model for making like-for-like comparisons.
• Once a suitable methodology has been devised, the model can then be used in future years to make like-for-like comparisons with the base year emissions.
• Over the years, one can also use the model to measure actual change(s) and demonstrate the impact from Scottish Executive and other Government decisions/influences.

To prime consideration is to think about how one regards electricity.

• Does one consider electricity as:
  - a product (much like steel or cement, that is made at one place then used elsewhere), or
  - should it be considered a secondary fuel (with the CO ₂ associated with its end-use rather than its point of generation)?
• Does one measure and attribute the impact that the Scottish Executive can have on emissions arising from electricity generation and, importantly, its end-use?

How one 'regards' electricity will have a major bearing on what model one adopts for evaluating the associated CO ₂ generation.

Below are two models. Each has different merits and drawbacks. These are:

A7-1. 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 ⁸⁰; 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.

A7-2. 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. In the Scottish Energy Study we have used a convention that includes the fuel used by CHP in the total fuel consumed by the sector concerned. In the previous study in 1990, the data for CHP were not reported separately from other forms of power generation, 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 losses in the generation process, as they increase the amount of fuel used (and hence CO ₂ produced) for a given output. Losses arise owing to 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 losses of 5.2 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.

A7-3. 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.

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.

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