Tuesday, 22 November 2016

Heating our homes with bio-gas - how much can we grow?

Ecotricity have released a report claiming that their green gas mills (making bio-gas from grass) could generate enough gas to heat 97% of our homes by 2035 [1]. Is bio-gas a practical solution for low carbon heating? It would certainly be less hassle than converting the grid to hydrogen (see Converting the gas grid to hydrogen, starting with Leeds) or installing expensive insulation. Let's look at some numbers.

According to Ecotricity:

  • One gas mill supplies 3500 homes (now) or 5000 homes in the long term, because gas use is falling - a reduction of 30% per home.
  • 5000 gas mills could supply 97% of households (maximum scenario).

Gas use per home is falling - median gas use reduced by 30% in 10 years to 2014 [2] - but it is a bit of a stretch to assume this will continue. The reductions so far are due to factors including installing cavity wall and loft insulation and more efficient heating systems and there is a limit to the number of homes left to insulate at low cost. It would take heroic measures to achieve another 30% savings. In any case, clearly we can't just rely on green gas - we need more energy efficiency too.

We must also remember the number of households is increasing. At the moment we have about 27 million but by 2035 we can expect the population to have grown by 15% [3] so we could easily have 31 million households. 5000 gas mills, each supplying 5000 households, would cover 25 million. This is 80%, not 97% - but it is still a lot!

  • One gas mill needs 3,000 acres of grassland = 1,200 ha
Therefore 5000 mills needs 6 million ha
  • Growing grass will not harm our food production because it can be grown as a break crop in normal agriculture crop rotation

The break crops that farmers use depend on a number of factors including soil and climate type. However, in the extreme case we can imagine everyone using grass. Crop rotations last four to six years of which one or two years is a break crop. We can imagine grass being grown one in four years, on average.

As of 2010 there were 5.66 million ha of agricultural land and temporary grassland in the UK. One quarter of this would be 1.4 million ha - enough for 19% of the population in 2035.

We have another 5.3 million ha of permanent pasture, currently supporting livestock. We can eat less meat but we are a net importer as it is. I don't think it would be sensible to repurpose 87% of this land to grow grass for gas instead. Maybe we could use a third of it - so 1.7 million ha. That gives us a total of 3 million ha. That could give us a total of 2700 mills, supporting 43% of the population.

In summary, carbon neutral bio-gas from grass could plausibly heat 43% of households in 2035, assuming:

  • we use it as a break crop on all agricultural land
  • we repurpose 1/3 of our permanent pasture from livestock to bio-gas
  • efficiency savings on heating bring another 30% reduction in gas use per household.

This is closer to Ecotricity's medium scenario (2500 mills) than their maximum scenario and but it is still a sizable contribution to our heating demand. Even half this would be useful.

Ecotricity does not mention costs at all, but there clearly are some. Anaerobic digestion currently receives up to 4.3p/kWh subsidy under the non-domestic Renewable Heat Incentive and as of September this year 17% of heat qualifying for the RHI was from injected bio-methane [5]. If this is a fair assessment of Ecotricity's costs (which it may not be) then it looks like bio-gas is broadly similar in cost to converting the grid to hydrogen (but less risky).

[1] Green Gas - the opportunity for Britain (Ecotricity) November 2016
[2] National Energy Efficiency Data-framework Summary of Analysis 2016 (www.gov.uk)
[3] Population projections (ONS)
[4] Tariffs and payments: non domestic RHI (www.ofgem.gov.uk)

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