Marginal Greenhouse Gas Offset for Renewable Energy in the UK

The reduction in Greenhouse Gas (GHG) emissions associated with the generation of electricity from intermittent renewable sources such as wind, wave and tidal power is typically estimated from the average annual emissions associated with the entire power system [1-5]. However, as the UK government continues to encourage development of renewable energy generation, negative headlines are appearing in the media concerning wind farms being paid not to produce as a result of power network constraints and questions are being raised on the future level of backup fossil generation required to handle the variability of renewable energy. This raises doubts over the accuracy of carbon payback calculations and the greenhouse gas intensity of the electricity that is offset by intermittent renewable energy.

In order to develop a real picture of the GHG offset associated with renewable energy generation this study examines historic power generation data from the UK grid to identify the effect on the network GHG emissions of the current small penetrations of wind, wave and tidal power output. The resulting marginal GHG offset is then combined with life cycle emissions data for renewable energy converters to calculate the carbon paybacks. The model will build upon the work of Hawkes [6] that examined the marginal CO2 emissions from demand-side interventions to produce an estimate of 0.69 kgCO2/kWh. This is significantly higher than the figures recommended for use in carbon payback calculations by the UK Department for Energy and Climate Change, most recently quoted as 0.50 kgCO2-e/kWh [7]. A recent study of the Pelamis wave energy converter found the global warming potential to be 27 gCO2-e/kWh (assuming a design life of 20 years) [2], which would correspond to a carbon payback of either 9 or 13 months with the above figures.

The model developed here will feed into further work to develop more accurate estimates of the true carbon footprint of intermittent renewable energy. This will enable future renewable energy scenarios to be modelled to inform government policy and energy industry plans for renewable generation and network development.



1. H. J. Wagner, C. Baack, T. Eickelkamp, A. Epe, J. Lohmann, and S. Troy, "Life cycle assessment of the offshore wind farm alpha ventus," Energy, vol. 36, pp. 2459-2464, 2011.
2. C. Thomson, G. Harrison, and J. Chick, "Life Cycle Assessment in the Marine Renewable Energy Sector," presented at LCA XI, Chicago, USA, 2011.
3. Varun, I. K. Bhat, and R. Prakash, "LCA of renewable energy for electricity generation systems—A review," Renewable and Sustainable Energy Reviews, vol. 13, pp. 1067-1073, 2009.
4. C. A. Douglas, G. P. Harrison, and J. P. Chick, "Life cycle assessment of the Seagen marine current turbine," Proc IMechE Part M: J. Maritime Environment, vol. 222, pp. 1-12, 2008.
5. R. P. M. Parker, G. P. Harrison, and J. P. Chick, "Energy and carbon audit of an offshore wave energy converter," Proc. IMechE Part A: J. Power and Energy, vol. 221, pp. 1119-1130, 2007.
6. A. D. Hawkes, "Estimating marginal CO2 emissions rates for national electricity systems," Energy Policy, vol. 38, pp. 5977-5987, 2010.
7. N. Hill, H. Walker, J. Beevor, and K. James, "2011 Guidelines to Defra/DECC's GHG Conversion Factors for Company Reporting: Methodology Paper for Emission Factors." UK: Department for Environment, Food and Rural Affairs, 2011.