A guide to calculating the carbon dioxide debt and payback time for wind farms

Selected Extracts excluding Appendices

Why is payback time important?

Apart from providing electricity, the government sees wind farms as an important mechanism for reducing the UK’s carbon dioxide (CO2) emissions. Their contribution to combating climate change is often advanced by wind-power developers and their advocates as a major argument for approving planning applications.

As with any manufacturing process, building and erecting the turbines creates CO2 emissions and a wind farm must pay back its CO2 ‘debt’ before it can claim to contribute to these national objectives. It is obviously vital that this debt is known and considered at the planning stage.

This guide examines aspects of the contribution made by wind farms to reducing CO2 emissions. As well as estimating the CO2 emitted during the construction phase, it considers the longer-term accrued emissions debt arising from the impact of a wind farm on its location.

This will vary considerably from site to site. A wind farm on a grassland site in southern England may have a relatively low debt and short payback time whilst one built on a peat-rich blanket bog will have a much higher debt and longer payback time.

What is a carbon dioxide debt?

It is broadly accepted that wind turbines do not emit CO2 at the point of generation. However, in common with all types of power station, it is emitted during their construction and, through damage directly inflicted on the construction site, over a much longer period. The total debt will vary from site to site but will comprise some or all of the following;

• Emissions arising from fabrication (steel smelting, forging of turbine columns, the manufacture of blades and the electrical and mechanical components);
• Emissions arising from construction (transportation of components, quarrying, building foundations, access tracks and hard standings, commissioning);
• The indirect loss of CO2 uptake (fixation) by plants originally on the surface of the site but obliterated by construction activity including the destruction of active bog plants on wet sites and deforestation;
• Emissions due to the indirect, long-term liberation of CO2 from carbon stored in peat due to drying and oxidation processes caused by construction of the site.

It is important to recognise that peat is a major store of carbon accumulated from dead plant remains over many millennia. It is held in perpetuity because the bog’s wetness and acid conditions prevent the access of oxygen and inhibit the growth of bacteria which would otherwise rot the vegetation. Draining peat for construction reverses both these long-term processes: the soil is exposed to the air, the carbon is converted to CO2 and released slowly to the atmosphere.

Several papers from the wind industry in Denmark and the UK have addressed the first two points with estimates of payback time ranging from about six to 30 months.

However, the industry rarely, if ever, considers the last two. This is a fundamental omission as their contribution to the overall CO2 debt, in particular the last, can be far greater than all the others put together. This paper outlines a procedure for quantifying it.

The purpose of this guide

The guide has been prepared to enable anyone with access to the Environmental Statement (ES) that forms part of a Planning Application (PA) for a wind farm to estimate its CO2 debt. (If some of the requisite information proves to be unavailable, this ought to provide grounds for postponing consideration of the application and the commissioning of further assessment.)

The results of the calculations described should be submitted to planning authorities or Public Inquiries as part of the arguments used in assessing the merits and demerits of an application.

Calculating total CO2 debt and site payback time

Set out below are:

• a list of the variables and constants required to carry out the calculation;
• a worked example (Whinash windfarm, Cumbria) using data from its PA, Environmental Impact Assessment (EIA) and ES;
• details of a spreadsheet which automatically calculates payback time given the site-specific data. It also allows you to explore the effect of turbine size, load factor, peat depth, etc on payback times.

The calculations are based on three scenarios derived from a detailed study by Richard Lindsay of the EIA that accompanied AMEC and British Energy’s first Lewis Wind Power application.1 The study was commissioned by the RSPB and is available on its web site.

Lindsay showed that excavations and ditching associated with wind-farm construction cause peat to dry out over time due to a progressive fall in the water table. Once dry, the surface layers of peat oxidise and its stored carbon is converted to CO2 and released into the atmosphere. This causes a gradual further fall in the water table and the cycle repeats. After some years, the surface level can fall by many feet. He found that ditches could affect the water table as far as 250 metres away on a particularly wet site.

The scenarios used in this calculation are defined as follows:

• The low scenario (the one most often used explicitly or implicitly by developers) generally assumes that peat damage occurs only at the site of turbine bases and borrow pits or beneath access tracks and that the damage extends no more than five metres from the sides of these constructions;
• The medium scenario allows for damage to peat extending outwards for 50 metres from the sides of turbine bases, access tracks and borrow pits;
• The high scenario allows for damage to peat extending outwards for 100 metres from the sides of turbine bases, access tracks and borrow pits.

On typical lowland sites devoid of peat, the low scenario would apply. For many upland sites on dry or shallow peat, the medium scenario best describes how the peat might behave over time while for wet, peat-rich sites with active blanket bog, the high is more likely. Knowledge of the site under consideration should guide you to the most likely scenario. Local Wildlife Trust or nature conservation groups should be able to advise.

The calculations are not detailed or absolute forecasts of how a given site will behave following intrusive construction activity but are, rather, approximate quantifications based on well-understood properties of peat soils, recognised constants and conversion factors and industry-standard publications.

1 Lindsay, Lewis Wind Farm Proposals: observations on the Environmental Impact Statement, RSPB, May 2005.

Performing the calculation

The following information is needed. Some factors vary from site to site but others are constant.2 The site dependent factors are:

• The number and installed capacity of the turbines
• The claimed load (or capacity) factor
• The size of the turbine bases, hard standings and quarries (‘borrow pits’)
• The length, width and depth of the aggregate laid for access tracks
• The width of any drainage ditches cut alongside the access tracks
• The extent of tree felling, if applicable
• The average depth of peat on the site.

The above should all be available in the Environmental Statement. Constants applicable to all sites are:

• The number of hours in a year (8,760)
• The carbon fixed by growing bog (19 gm/m2/year)
• The mass conversion factor, carbon to CO2 (multiply by 44 and divide by12)
• The carbon content of dry peat (55 kg/m3)
• The weight of rock used for access tracks and ballast (2 t/m3)
• CO2 displaced at a power station by wind-power generation (0.43 tCO2/MWh) 3
• CO2 emitted by quarrying and crushing rock (0.2 tCO2/m3)
• CO2 emitted during turbine manufacture (1,189 tCO2/MW turbine capacity)
• CO2 emitted per 15 m x 15 m x 1.5 m turbine base (248 tonnes/base)
(includes cement, aggregate, steel, rock and sand)

Appendix 2 explains how some of these were derived. The others are deemed noncontroversial.

2 Installed capacity is measured in megawatts (MW), electrical production in megawatt hours (MWh), length in metres (m), area in square metres (m2 ) or hectares (1 hectare = 10,000 m2) and weight in grammes, kilogrammes and tonnes. Load Factor (LF) or Capacity Factor (CF) is the expected annual electricity output as a percentage of the maximum theoretically possible; tCO2 = tonnes of CO2. Thus, tCO2/MWh = tonnes of CO2 per megawatt hour and so on.

3 Many planning applications use a figure twice this (0.86 tCO2/MWh) but this is no longer held by the government to be correct, a view endorsed by a ruling by the Advertising Standards Authority in December 2005 – details from REF on request. Any use of the higher figure should be challenged.

A worked example: Calculating CO2 payback time for the Whinash wind farm

The Whinash site, which was refused consent following a Public Inquiry in 2006, would have occupied a rounded ridge between the Lake District and Yorkshire Dales National Parks. The total area of 763 hectares is composed of blanket bog in relatively poor condition, purple moor grass/rush pastures and upland heath. Compared to many sites in Wales and Scotland, it has a shallow peat covering and patchy areas of blanket bog. It is deemed to be a medium scenario site and the extent of peat degradation is thus assumed to be 50 metres. The data below are taken from the developer’s PA, ES and Supplementary Environmental Information.

Number of turbines = 27

Installed capacity per turbine = 2.5 MW

Total installed capacity = 27 x 2.5 = 67.5 MW

Anticipated load factor = 35%

Thus, electricity generated = 67.5 MW x 8,760 hours/year x 35% = 206,955 MWh/year

and CO2 ‘saving’ = 206,955 x 0.43 t/MWh = 88,990 t/year

Access tracks = 16.7 km long x 5 m wide x 0.5 m deep

Average peat depth = 0.48 metres (rounded to 0.5 m)

Site lifetime = 25 years

How much CO2 would be emitted by the project?

CO2 emitted as a result of deforestation
CO2 emitted = None. No tree felling would have been necessary at
Whinash – but see Appendix 1
CO2 emitted during fabrication, transport and assembly of turbines
CO2 emitted = 1,189 t/MW x 27 x 2.5 = 80,258 tonnes
CO2 emitted during manufacture of turbine bases
CO2 emitted = 248 tCO2/base x 27 = 6,696 tonnes
CO2 emitted by quarrying and crushing aggregate for access tracks and turbine bases
volume of access tracks = 16,700 m x 5 m x 0.5 m = 41,750 m3
volume of base ballast = 500 m3/base x 27 = 13,500 m3
volume of hard standings = 50 m x 20 m x 0.5 m x 27 = 13,500 m3
thus, total aggregate volume = (41,750 + 13,500 + 13,500) = 68,750 m3
and CO2 emitted = 68,750 x 0.2 tCO2/m3 = 13,750 tonnes

CO2 emitted due to loss of fixation by damaged bog

The low scenario is always applied to calculation of fixation loss. The area of bog surface lost is as follows:

weighted track length = 16,700 - (27 x 5 x 1.5) = 16,498 m (See note 1 at foot.)

area damaged by access tracks = 16,498 x 25 = 412,438 m2 (25 = damage + ditch + track + ditch + damage)
area damaged by turbine bases = 27 x (5 + 20 + 5)2 = 24,300 m2
area damaged by hard standings = 50 x 20 x 27 = 27,000 m2
area damaged by borrow pits = 0 m2 (off-site quarries proposed for Whinash)
thus, total area lost = 412,438 + 24,300 + 27,000 + 0 = 463,738 m2
lost annual sequestration = 463,738 x 19gm/m2/year = 8.81 tonnes carbon
lost site lifetime sequestration = 8.81 x 25 years = 220 tonnes carbon
thus, fixation lost = 220 x 44/12 = 808 tonnes CO2

CO2 emitted by peat oxidising over time

Weighted track length = (16,700 - (27 x 50 x 1.5)) = 14,675 m (See note 1 at foot.)

Peat damaged by access track construction:
= (14,675 x (50 + 15 + 50)) x 0.5 = 843,813 m3

Peat damaged by turbine base construction:
= (50 + 20 + 50)2x 0.5 x 27 = 194,400 m3

Peat damaged by quarrying ‘borrow’ pits:
=0 m3 (off-site quarries proposed for Whinash)

Total volume of damaged peat = 843,813 + 194,400 + 0 = 1,038,213 m3.

Thus, CO2 emitted = 1,038,213 x 55kg C/m3 x 44/12 = 209,563 tonnes

Total site CO2 cost

emitted by deforestation = 0

emitted by turbine fabrication = 80,258

emitted by concrete manufacture = 6,696

emitted by aggregate extraction = 13,750

lost due to fixation loss = 808

emitted by peat oxidation = 209,563
total emissions = 311,075 tonnes

What is the payback time?

Payback time = 311,075 ÷ 88,900 tCO2 displaced/year = 3.5 years

Notes:
1 The areas damaged by access tracks overlap with the areas damaged by turbine base excavations. To allow for this, the length of access track is reduced (weighted) in this calculation. The procedure used is outlined in Appendix 2. No peripheral degradation is allowed for around hard standings.

2 This calculation may differ slighty from the spreadsheet as the latter allows for displacement losses.

About the aauthor: Dr MMike HHall, FFRSC, FFIBiol

After graduating in physics and biological sciences, Dr Hall followed a career in organic chemistry and medical research as a lecturer at Swansea University (now part of the University of Wales) and, later, in the pharmaceutical industry. He has published over 100 papers.

His contributions to chemistry and biology were recognised by his election as a Fellow of the Royal Society of Chemistry and a Fellow of the Institute of Biology.

On moving to Cumbria in 1988, he became involved in many aspects of conservation and is a member of the Cumbria Wildlife Trust’s Conservation Committee. He manages an SSSI for CWT which combines unusual features of both raised and valley mires, providing direct experience of peat bogs.

The pressure for wind farms in Cumbria rekindled an interest in energy matters which combined readily with his biological interests and led directly to the preparation of this paper.