05 Feb 2019
The carbon payback times for wind turbines are much shorter than previously thought, according to international research carried out at the largest community wind farm in the UK.
German student Katharina Lutz found the turbines at Beinn Ghrideag had a payback time of just 47 days - a drastic reduction on the previous, widely accepted, estimate of 2.3 years.
'Carbon payback' is a term referring to the length of time it takes for the negative environmental impact from the construction of a wind farm on peatland to be offset by the positive environmental impact of generating clean energy instead of burning fossil fuel. When peat is disturbed for large concrete foundations to be laid for turbines, greenhouse gases are released into the atmosphere.
Student Katharina carried out her research at Beinn Ghrideag near Stornoway in the Autumn, with the academic support of mentor Alasdair MacLeod from Lews Castle College UHI and financial support from community wind farm owner Point and Sandwick Trust.
Her report, entitled 'COâ‚‚ Emissions from Wind Turbines on Peatland: A Practical Calculation', said: "Estimates for the payback time for turbines on peatland have been quoted as 2.3 years (Pearce, 2009). This is based to an extent on an influential report by Nayak and collaborators at the University of Aberdeen who developed a carbon dioxide calculator, variations of which are still used to estimate the impact of a wind farm proposal (both by supporters and opponents)".
The report summarised: "Installing large wind turbines on peatland has the disadvantage of exposing carbon long trapped in the ground to decomposition, and some estimates suggest a turbine can take several years of operation to compensate for the associated carbon dioxide emissions.
"The main reason for these projected high carbon payback times is the assumption that all displaced peat becomes fully oxidised and that the construction not only causes drainage within the immediate construction area, but that the effect can extend outwards by a distance of up to 400 m.
"However, existing turbines on a wind farm on the Isle of Lewis, Scotland, on a site which is typical of the region, were found to have a carbon payback time is only 47 days when the actual impact following several years of operation is established.
"The result is based on a flora analysis and accurate peat depth and composition measurements, and references three years of electricity generation data. The calculation of CO2displacement associated with turbine electricity production uses the fossil fuel emission factor for the United Kingdom in 2016 rather than the overall grid emission factor in order to accurately reflect the generation type displaced by the wind turbine output."
When concrete foundations are being laid, peat is usually disposed of by being spread thinly over the surrounding area, with the assumption that all the displaced carbon will end up as atmospheric carbon dioxide. However, Katharina found that less drainage takes place than had been thought - and that extensive drainage is not necessary as the hole can be backfilled.
Her mentor, college lecturer Alasdair MacLeod, explained that peat was "90 per cent water bioweight". When wet, peatbog contains no oxygen and very few microbes, "so the carbon in it is protected" from degradation. However, "once you drain it, you expose it to the oxygen so the microbes then have oxygen available and the peat will be metabolised", causing CO2 to be released.
He stressed the "huge assumption everybody makes" was that it was necessary, in order to create a hole in the ground for a wind turbine, to remove all the peat, then put in the hard standing, and have drained all round, to a distance of perhaps 100 metres. "So not only are you oxidising the peat you removed, but you're also drying out a massive area round it".
"What Katharina found was that when you need a wind turbine, you're making a hole in the ground (but) you're not really draining what's around. You only need the hole open long enough to backfill it. In addition to that, what was in the hole was spread very thinly around the surrounding area and that slowly works its way back into the bog, so the peat that was originally there didn't have a chance to dry out and just worked its way back into the bog."
Alasdair, a senior lecturer in energy engineering, was full of praise for his "meticulous" student.
"She was very, very good. Very diligent, scientific. No assumptions made - everything had to be supported. She was a very good researcher and she had good expertise in the biological side of the project which is the moorland, flora, fauna, species and so further. I helped with the technical and scientific side - the energy side - and she learned a lot from that too. I taught her how to calculate the energy value in peat and the implications, such as how fast it grows."
Community wind farm developer Calum MacDonald, who developed Beinn Ghrideag for Point and Sandwick Trust, said: "This is a hugely important piece of research, especially for wind farm projects in the Highlands and Islands. There is mounting evidence that the climatic conditions we have means that peat disturbance is not so great and peat recovery is generally faster in the Highlands and Islands than elsewhere in the UK and Europe.
"It's great to have proper scientific input into an area that has hitherto been almost by rule of thumb.
"We are very proud of the reclamation we did at Beinn Ghrideag and we think it's an exemplary model for the industry to follow.
"Community-owned projects, of course, will always be more conscientious and environmentally-minded because it's our land and our future that we are working on."
Beinn Ghrideag consists of three turbines, each generating 3MW, making it the largest community-owned wind farm in the UK in terms of output. It has won several awards from the renewables, charity and social enterprise sectors, including being named Best Community Project at the Scottish Green Energy Awards in 2015.
Alasdair MacLeod said the research indicated "many turbines could be installed in Lewis without any significant effect on the peat environment".
He added: "If you object to the turbines, you can't object to them on the basis of what they do to the peat. Any objection has got to be something else. You may hate them because of the visual intrusion but you can't hate them because they damage the peatland because they don't.
"It's significant because previous assumptions were that it always took three to five years to pay back and people said you can't put wind turbines on peat - but you can. The reality is, when you put any wind turbines up, you don't drain the surrounding peatland. There's no need to drain it."
Point and Sandwick Trust | www.pointandsandwick.co.uk