Calculating carbon payback for wind farms
Pegasus Group's Jolyon Orchard describes the Scottish government tool for calculating how long it takes for wind turbines to offset the carbon associated with their manufacture and construction
Wind farms in Scotland are often constructed on peat lands which hold large stocks of poorly protected carbon. During the construction of a wind farm, carbon is lost from the excavated peat and from areas affected by drainage.
As Scotland contains the majority of the UK’s peat soils, proposals for large-scale wind farm developments have raised questions about carbon losses and gains during their construction and operation. A need has also arisen for a standardised methodology for calculating the carbon payback period of wind farms.
The carbon payback period is an estimate of how long it will take a renewable energy project to offset the greenhouse gases emitted as a result of its construction (the “carbon cost”) and begin displacing grid-based electricity generated from non-renewable sources (“the carbon saving”).
The amount of displacement is calculated using the principles of counterfactual analysis, which provides a comparison between what actually happened and what would have happened in the absence of the intervention.
Using this approach it is possible to estimate how long a wind farm needs to be operational before it repays its own carbon impact, and also the total carbon mitigated over its life time.
In 2011, the Scottish government published the second version of an excel-based tool for calculating the carbon payback period of new wind farm developments.
This tool, along with accompanying guidance, provides a method for calculating carbon emission reductions associated with wind farm developments on peat lands using a full life-cycle analysis approach.
In Scotland, it is mandatory for developers to use this tool as part of planning applications for wind farms and calculate the carbon payback of the wind turbines.
From 1 April 2011, the Scottish Environment Protection Agency (SEPA) has been auditing carbon assessments submitted with Section 36 wind farm applications that use the current version of the carbon calculator.
The carbon calculator is able to estimate the emissions arising from the manufacture, transportation and assembly of wind turbines.
The calculation is based on turbine capacity (394–8147t CO2 per MW), although the authors of the calculator stress that site specific values should be used if these are available. This would require the following information to be made available:
| Emission source | Required information |
| Embodied carbon of wind | Tonnes of pre-stressed concrete, steel, aluminium, copper and glass reinforced plastic |
| Embodied carbon of wind | Tonnes of concrete and steel |
| Embodied carbon of access | Tonnes of aggregate (sand and |
| Transport emissions | Number and distance of vehicle |
The carbon calculator is also used to estimate the electricity generated during the lifetime of the wind farm, and the emissions reductions associated with this.
It requires information to be entered such as the number of turbines, their expected lifetime, power ratings and capacity factors, for example.
The electricity generated by a new wind farm is assumed to displace grid-based electricity, which is expected to shift the generation mix away from traditional fossil-fuelled sources.
The tool is not a requirement for wind farm planning applications in England. However, it does provide a robust methodology for calculating the carbon payback period associated with a proposed wind farm (whether constructed on peat lands or not), and can be considered a best practice approach.
The carbon payback period could be included as part of the environmental statement for a wind farm and provide a useful indication of its potential environmental benefits.
The average wind farm will pay back the energy used in its manufacture in 3-5 months of operation. However, the carbon emissions generated from the construction of a wind farm must also be considered and can be significant especially if this involves disturbance of peat lands.
Planning permission for a wind farm which involves constructing 103 turbines on peat land in the Shetland Islands was recently granted planning permission and has been estimated to have a carbon payback period of less than one year.
This article was written as a contribution to the EIA Quality Mark’s commitment to improving EIA practice.
Jolyon Orchard is a senior sustainability consultant at Pegasus Group