Calculating carbon savings from wind farms on Scottish peat lands: a new approach

1. Executive Summary

Proposals for large scale wind farm developments in Scotland have raised concerns about the reliability of methods used to calculate carbon savings associated with wind farms. Guidance produced by SNH in 2003 in a technical guidance note has been used by many to determine carbon payback in the absence of any more detailed methods to determine the impacts on soil carbon stocks.

Wind farms tend to be sited on peat lands which hold large stocks of poorly protected carbon and so have the potential to greatly increase overall carbon losses. Scotland contains the majority of peat soils in the UK (defined here as soils with a surface organic horizon greater than 50 cm in depth) and the estimated stock of carbon held in Scottish soils is approximately 2800 MtC or 11000 MtC0 ₂ equivalent. Scotland has a responsibility to ensure the stability of this carbon where possible and ensure that developments do not cause a significant loss of this reservoir.

In this work an approach has been developed to calculate the impact of wind farm developments on the soil carbon stocks held in peats. This provides a transparent and easy to follow method for estimating the impacts of wind farms on the carbon dynamics of peat lands.

The loss of carbon fixing potential from plants on peat land is small, but nevertheless is calculated for the area from which peat is removed and the area affected by drainage. This calculation uses the annual gains due to the carbon fixing potential of the peat land and the time required for any habitat restoration.

The carbon stored in the peat itself represents a much larger potential source of carbon loss. During wind farm construction, carbon is lost from the excavated peat and from the area affected by drainage. The amount of carbon lost due to removal or drainage of the peat is estimated using default values from the Intergovernmental Panel on Climate Change ( IPCC, 1997) as well as by more site specific equations derived from the scientific literature. Carbon gains due to habitat improvement and site restoration are calculated similarly.

When flooded, peat soils emit less carbon dioxide but more methane than when drained. In flooded soils, carbon dioxide emissions are usually exceeded by plant fixation, so the net exchange of carbon with the atmosphere is negative and soil carbon stocks increase. When soils are aerated, carbon emissions usually exceed plant fixation, so the net exchange of carbon with the atmosphere is positive. To calculate the carbon emissions attributable to the removal or drainage of the peat, emissions occurring if the soil had remained in situ and undrained are subtracted from the emissions occurring after removal.

The extent of the area affected by drainage around each construction at the site strongly influences the total volume of peat impacted by the construction of the wind farm. Therefore, where sufficient measurements are available to describe the hydrological features of the area around the wind farm area, these should be used, perhaps together with a detailed hydrological model, to simulate the likely changes in peat hydrology. If insufficient measurements are available, the greatest likely extent of area affected by drainage should be estimated and used.

When flooded soils are drained, a loss of soil carbon continues until a new steady state is reached, when inputs are approximately equal to the losses. For peats, this steady state is close to 0% carbon, so if the site is not restored after decommissioning of the wind farm, it is assumed that 100% of the carbon will be lost from the drained volume of soil. Restoration of the site could potentially halt the processes of carbon loss, allowing carbon dioxide emissions to be limited to the time before the habitat and hydrological conditions are restored. The amount of carbon lost is then calculated from the annual emissions of methane and carbon dioxide, the area of drained peat, and the time until the site is restored. However, for this option to be used, the site restoration plan should demonstrate a high probability that peat hydrology will be restored across the site ( i.e. the water table at the surface for over 50% of the year), disturbance of the remaining peat will be minimised, and peat forming vegetation will develop in areas from which peat was removed or drained.

Lowering the water table by drainage may also reduce the potential for dissolved and particulate organic carbon retention within the soil. Losses of carbon dioxide due to leaching of dissolved and particulate organic carbon are calculated as a proportion of the gaseous losses of carbon from the different sources in the soil. The Scottish Government has established a rigorous procedure for identifying existing, potential and construction induced peat landslide hazards. This should lead to reduced likelihood of peat landslides occurring. It is assumed that published good practice has been followed to reduce the risk of peat landslides. Therefore this potentially large source of carbon loss is omitted from these calculations.

The presence of extensive areas of forestry on and in the vicinity of the wind farm site can significantly reduce the yield of wind energy, so it may be necessary to clear existing forestry from are area surrounding the site prior to wind farm development. The losses of carbon from tree biomass depend on the fate of wood products following felling. Forestry may be felled earlier than planned due to the wind farm development, so limiting the nature and longevity of wood products. If a forestry plantation was due to be felled with no plan to replant, the effect of the land use change is not attributable to the wind farm development and should be omitted from the calculation. If, however, the forestry is felled for the development, changes in timber, residues and soil conditions are attributable to the wind farm. Losses from the timber are calculated from the area of forestry to be felled, the average carbon sequestered per year, and the lifetime of the wind farm. The change in residue and soil carbon stocks on tree removal depends on subsequent land management, especially the drainage regime. The changes in soil carbon stocks following felling are calculated using the equations for drainage and site restoration already described.

Government policy ( SPP6) is to deliver renewable energy in a way that "affords appropriate protection to the natural and historic environment without unreasonably restricting the potential for renewable energy development" http://www.scotland.gov.uk/Publications/2007/03/22084213/0

This report provides a method to determine potential carbon losses and savings associated with wind farm developments on peat land taking into account peat removal, drainage, habitat improvement and site restoration.