Appendix 5-1 Strathy Wood Wind Farm

Transcription

1 1918 Appendix 5-1 Strathy Wood Wind Farm Carbon Payback Calculation E.ON November 2013

2 Contents 1 Introduction 1 2 Peat Depth 2 3 Peat Carbon Content 3 4 Predicted Net Carbon Change 4 5 Discussion 6 6 References 7 Tables Table 1: Mean peat profile characteristics derived from site survey data 2 Table 2: Summary of core data entered in the carbon payback calculator 4 November 2013 E.ON 1918

3 Document Prepared For E.ON Climate & Renewables Developments Ltd. (E.ON) Document Prepared By Geeta Puri Principal Consultant Document Approved By Jean Curran Operations Director Version Date Reason Final 11/11/2013 Copyright 2013 Atmos Consulting Ltd Rosebery House, 9 Haymarket Terrace, Edinburgh, EH12 5EZ Moray House, Bank Street, Inverness, IV1 1QY Durham Dales Centre, Castle Gardens, Stanhope, County Durham, DL13 2FJ Mynydd Awel, Mold Business Park, Maes Gwern, Mold, CH7 1XN Keystone Innovation Centre Croxton Road Thetford Norfolk IP24 1JD

4 1 Introduction This report summarises the predicted net losses and gains of peat and its associated carbon content for the proposed Strathy Wood Wind Farm. Estimates for peat loss are based upon site design and measured peat depths and Worksheet 5 of the carbon payback calculator has been used to determine carbon dioxide losses and gains from peat, both directly from excavation, oxidation and erosion and indirectly from the simulated impact of drainage on the peat resource at the site. The overall carbon payback period which takes into account the carbon savings and losses for all components of the wind farm and the fuel it is expected to replace has been calculated which includes carbon losses and gains from peat. It is also useful to separately estimate the potential losses and gains of peat as carbon dioxide equivalent units (CO2e). As there are many factors such as wind capacity factors and replacement of fossil fuels and/or other renewable energy sources the direct and indirect impacts of a wind farm on the peat resource can be lost in the consideration of payback time. A full carbon payback calculation has been undertaken with and without restoration and the core data used for this are summarised within this report (Appendix 5-1). Although a carbon packback period has been determined the net change in peat carbon stocks in particular is highlighted in this report. November 2013 E.ON

5 2 Peat Depth The proposed development area contains significant pockets of peat which are up to 3 metres deep in places and classified as blanket peat (Chapter 11 of the ES contains details of the peat depth site survey and Figure 11-2 Peat Depth Surveys, is reproduced here for ease of reference). Although the peat resource is not pristine due to past land use activity, primarily forestry, it does represent a significant and important store of carbon and hence the requirement to determine the potential losses of carbon (and associated habitats). The detailed site survey for peat depth has informed the developer of where areas of peat greater than 1 metre are located in the proposed development area and these have been avoided. The survey of peat depth described in Chapter 11 of the ES followed the guidance Site Surveys for Developments on Peatlands, Scottish Government (2011). A systematic, grid based approach was used wherever possible to determine total peat depths around proposed infrastructure and access tracks using on both a 100 metre and 50 metre grid basis. Table 1 provides a summary of the average profile characteristics of the peat found on site following the site survey of depth and description of profiles from peat cores taken at the time. The peat depth data has been placed into three depth categories and the mean proportions of acrotelmic and catotelmic peat determined from the peat core data for the whole site survey area. Figure 11-2 taken from Chapter 11 of the ES shows the sample points for peat depth along with the design of the proposed development. Table 1: Mean peat profile characteristics derived from site survey data Categories Depth (cm) Fibrous Peat (cm) Colloidal Peat (cm) < 1 metre > 1 metre > 2 metres November 2013 E.ON

6 3 Peat Carbon Content Using peat depth values from the site survey along with infrastructure details such as turbine foundation depth, access track length and design and hardstanding locations (as detailed in Chapter 4, Table 4-1 of the ES ) a total, predicted volume of peat either excavated or affected by drainage was calculated and can be found in Worksheet 5 of the carbon calculator tool and reproduced here in Table 2. The results present potential carbon losses and gains predicted and an overall payback period has been determined, the results of which are summarised in Table 3. Carbon losses include the removal and drainage of peat and gains include the replacement of fossil fuel energy with wind energy and restoration of the peatland post-development. Although total potential carbon losses are relatively small compared to the potential, carbon gains over the lifetime of the wind farm it is important that where possible the permanent loss of peat is avoided. Excavated peat will be managed as detailed in the draft peat management plan (Appendix 11-7) which also recommends the re-use and re-instatement of peat according to its physical properties to support successful revegetation of blanket bog species. Peat carbon content and bulk density values are used to calculate total carbon stock and these are taken from average Scottish blanket and basin peat values derived after consultation between SEPA and the James Hutton Institute (A. Lilly pers comm, 2013.) The results for with and without restoration are helpful to compare the relative impact of restoring the site in terms of peat carbon stocks and hence two spread sheets are provided (D.Nayak, pers comm, 2013). November 2013 E.ON

7 4 Predicted Net Carbon Change The total predicted loss of carbon from peat is 53,878 tco2 during the construction and operation phase of the wind farm. The net gain of carbon from peat is determined from the re-instatement of access tracks, turbine bases and hardstandings etc. which will take place once the proposed development is decommissioned. It is important to note that the figures for carbon losses and gains are predicted values which depend upon the careful management of excavated peat, the avoidance of deep peat for infrastructure and the effective re-instatement of peat after decommissioning of the site. The carbon calculator takes into account core data as summarised in Table 2 which generates the net carbon payback periods for with and without restoration scenarios provided in Table 3 Table 2: Summary of core data entered in the carbon payback calculator Parameter Description Summary Infrastructure: Turbine bases 26 Crane hardstandings 26 Access tracks (new and upgraded) 9.2 and 7.9 km Temporary buildings 2 ha Borrow pits 5 Cable trench Forestry: Area to be felled and replanted 20m x 20m x m x 20 x km x 5.5m x km x 5.5 x ha x m x 80 x 0.5 per BP 550m x ha Capacity factor Site specific measure 33 Fuel mix which wind farm will replace Assuming drainage impact on peat is 20m based upon Chapter 11 Figure for forest felling is 92 ha fossil fuel mix If mix of fossil fuels and renewable energy this will change the payback period Restoration: Re-instatement of peat, recovery of semi-natural vegetation total area Depth Peat properties: Level of decomposition Scottish default vaues for %C and bulk density Drainage impacts 110 ha 3 depth categories produced and mean value for acrotelm and catotelm derived. See worksheet 8 for details 70:30 ratio of acrotelm : catotelm and generic 55%C and 0.25 g/cm 3 bulk density. 20m drainage impact (Chapter 11) November 2013 E.ON

8 Table 3: Carbon payback period (months) with and without restoration With restoration 12 Total tco 2 loss from peat (removed + drained) Without restoration 20 Total tco 2 loss from peat (removed+drained) Overall the predicted carbon loss for peat due to wind farm construction, operation, decommissioning and restoration is 53,878 tco2 with restoration and 91,031 tco2 without restoration. Post restoration carbon fixation is simulated for a maximum of 10 years after decommissioning of the wind farm Overall the carbon payback period with restoration is 12 months and without restoration 20 months assuming the wind farm replaces energy generated from a mix of fossil fuels. November 2013 E.ON

9 5 Discussion The simulated carbon changes in peat during the lifetime and restoration of the site have shown the significant savings in the peat carbon store than where no restoration is undertaken. The carbon payback period for with and without restoration scenarios does not seem particularly sensitive to the predicted difference in peat carbon stocks suggesting other factors such as the fuel mix the wind farm is assumed to be replacing affect the payback time more significantly. The results presented are consistent with carbon payback periods for similar sites on Scottish peatlands and relative to the lifespan of the proposed development this is a short amount of time. As with all sites where a carbon payback period is calculated there remain uncertainties in the model for extent of drainage of peat around infrastructure, the fuel mix that the wind derived energy is replacing and the net carbon exchange between soil and trees when forestry is removed/replanted. November 2013 E.ON

10 6 References Carbon Payback Calculator, D. Nayak et al (2011) Scottish Government Development on Peatlands: Site Survey Guidance (2011) Scottish Government A. Lilley James Hutton Institute personal communication (2013) D. Nayak University of Aberdeen personal communication (2013) November 2013 E.ON