Turning most UK grasslands over to biomethane production would all but end livestock grazing, and make the UK almost entirely dependent either on meat and dairy imports, or on animal feed imports for domestic factory farms.
At the end of last year, Ecotricity announced that they had found an alternative to fracking: green gas from grass, "grown on marginal farmland, of which Britain has enough to heat almost every home".
Ecotricity's bioenergy vision looks far more attractive than many others: it does not involve importing any feedstock, converting food to energy, or cutting down any trees.
Instead, Ecotricity wants to see millions of hectares of low-biodiversity grasslands restored to species-rich flowering meadows which would support wildflowers and the pollinators and many other species that depend on them.
With 97% of England's and 90% of Scotland's species-rich grassland having been lost since the 1940s, this seems an enticing idea. The mown grass would be turned first into silage, then into biogas, and then be upgraded to biomethane, which would be fed into the National Gas Grid.
Biofuelwatch usually investigates destructive bioenergy investments, often by corporations or start-up companies with little regard for people or the environment. We decided to investigate Ecotricity's proposal for a very different reason: we wanted to find out whether such an attractive proposition, made by a company with an excellent environmental track record, might indeed be able to deliver genuinely low-carbon and sustainable bioenergy.
The land take: 'just' 10 million hectares
The biggest problem with bioenergy is always its land footprint: Generating one unit of energy from plants requires many times more land than generating it from any other source of energy, including wind and solar power. We therefore asked how much land would be required to replace all natural gas currently used to heat UK homes at present.
Just one peer-reviewed study seems to have been published on this topic and its key land use figure closely resembles information contained in Ecotricity's successfully planning application for a 'green gas mill' near Winchester.
By using the figure in the study and Government figures for UK natural gas use for heating, we calculated that replacing all of that gas with biomethane from grass would require at least 10.2 million hectares of land.
Ecotricity claims that, by 2035, they could replace all gas used for home heating by using 5 million hectares of land for grass, but their estimate relies on the assumption that domestic gas use will fall by around 50% between now and 2035.
But this is a very optimistic assumption, given that the Government has all but abolished support for the home energy efficiency measures and solar roofs. The only driver behind falling domestic gas use which they have not removed is fuel poverty.
This area, 10.2 million hectares, is equivalent to 92% of the UK's grassland area, and is more than double the UK's area of cropland.
What about the livestock?
Turning most UK grasslands over to biomethane production would all but end livestock grazing, at a time when UK meat and dairy consumption are rising. It would make the UK almost entirely dependent either on meat and dairy imports, or on animal feed imports for domestic factory farms.
Intensive livestock farming, including growing soya and other feed, is a major cause of deforestation, particularly in Latin America. Excess nitrogen from factory farms and dense cattle ranches pollutes water and harms biodiversity as well as the climate (via nitrous oxide emissions).
However biodiverse and climate friendly the UK impacts of 'green gas from grass' might be, the indirect greenhouse gas emissions and impacts on biodiverse ecosystems would be anything but benign.
Ecotricity's vision of flowering meadows grown for biomethane in the UK seems highly unlikely, too: to maximise yields and thus income, farmers will have to select the highest-yielding grass species, use nitrogen fertilisers which, even if they are made from biogas residues, will still reduce species richness, and they will likely have to use herbicides, too. Maximising grass silage yields is not compatible with managing grasslands for wildlife.
So, the indirect impacts on biodiversity and greenhouse gas emissions are likely to be severe.
The fugitive methane problem
What about the direct greenhouse gas emissions from biomethane plants? Nitrogen fertilisers are the main reason why atmospheric levels of the powerful greenhouse gas nitrous oxide (N2O) are rising worldwide. And compared to most 'energy crops' for biofuels or biogas, biomethane from grass would result in lower N2O emissions.
Grass requires much less nitrogen fertilisers than crops, and residues from biogas digestion can replace some nitrogen fertilisers, cutting nitrous oxide emissions from fertiliser production, though not eliminating such emissions from soils. Furthermore, the proposed biomethane plants would be small enough to keep transport distances and thus transport fuel use down.
Unfortunately, however, producing biogas and upgrading it to biomethane carries a different and potentially serious climate risk: that of methane leaks. Methane is 28 times as powerful as carbon dioxide over a 100-year period, so any significant methane losses will not only cancel out any greenhouse gas savings from 'green gas from grass', but could turn it into a high-carbon fuel.
Methane can leak from silage clamps, from biogas digesters and from the plants in which the biogas is upgraded. Methane leaks from biogas digesters range from 0.1% to 6%. The technology which Ecotricity wants to use to upgrade the biogas is designed for 1-1.5% methane leakage, though a Swedish study published in 2003 reported 10% of methane leaking from an upgrading plant.
Methane leaks, of course, are a major concern for fracking, too, with one study suggesting a 12% leakage rate from US fracking. Any fuel associated with significant methane leaks - even a 'green' one - is bad news for the climate.
Leaks from biogas and biomethane production can be minimised with good practice, but without any requirement to monitor them, there are no guarantees of that happening. We are concerned that Ecotricity has not acknowledged methane leakage as a serious concern to be addressed, either on their website or in their successful planning application for a first plant of this type.
The limits of photosynthesis
Ultimately, Ecotricity's 'green gas' vision must fail because it comes up against the fundamental limits of photosynthesis: no plant converts as much solar radiation to chemical energy as sugar cane growing in the tropics, which can convert up to 2.4% of the energy from the sun. In the temperate UK climate, the maximum is 1.3%. Much of that energy is then lost during conversion to bioenergy.
For example, biogas contains around 45% carbon dioxide, all of which is vented straight into the atmosphere during biomethane production. Solar PV is of course most efficient closer to the equator, yet even in the UK, it easily achieves a conversion rate of 9%.
This is the reason why bioenergy will always require far more land than any other source of energy, including wind and solar power.
It's also why sustainable large-scale bioenergy will always remain wishful thinking. There is a case to be made for biogas from genuine waste, such as sewage or food waste (as long as it does not compete with composting), but these sources will only ever be able to supply a very small fraction of current energy use.
But we shouldn't be too tough on Ecotricity. Clearly, it would be far better if the company was to continue investing in wind turbines alone. Sadly, this choice has been taken away from them by the Government, which has removed subsidies from virtually all new onshore wind as well as solar capacity, and which has skewed planning rules in England against wind power and in favour of fracking.
At the same time, the Government has made the most obvious solution to UK dependence on gas - insulating our leaky, inefficient housing stock - impossible by removing support for that, too, while failing to comply with or enforce EU and UK energy efficiency laws that require all new buildings to be "nearly zero-energy buildings" by 2020.
Almuth Ernsting is a co-director of Biofuelwatch and has been reseasrching and campaigning against large-scale industrial biofuels and biomass electricity since 2006.
More information: Please see Biofuelwatch for a fully referenced briefing on this topic.