Thursday, 20 June 2019

How is a house like 3/4 of a cow?

One way to decarbonise our home heating is to use anaerobic digestion (AD) to process biomass (such as grass) into biogas. So we can grow grass as cattle feed or to heat our homes. How do cows and houses compare in terms of grass demand? Here is a back of the envelope calculation - and the conclusion is: one house is about 0.75 cows. Based on this (rough) figure, if we repurposed all our land that currently supports sheep and cattle to grow grass for biogas, we could supply about 13 million households, a little less than half the current number in the UK (27 million) [1].
The grass these cows are eating could be used to heat our homes instead.

A hectare in Devon is not the same as a hectare in Scotland - using forage as a unit compensates for this.
Most estimates I have seen are based on yields per hectare and available land but I am not sure how they take into account location. A hectare of Devon meadow is not the same as a hectare in the Scottish lowlands. I suspect the grass in Devon grows faster and can support more cattle, largely because of the weather. Whether you are growing forage for cows or for biogas, the same weather constraints apply.

Forage area includes feed crops as well as pasture.
Also, cattle eat feed crops as well as grass. Feed is often more digestible than grass and has a higher proportion of protein but the energy content is similar - at least for the grain based feed that is mainly carbohydrate rather than oil. So my back of the envelope calculation includes the feed crop area as well as the grass - if you grow grass that the cattle eat directly, or hay that is cut for silage or barley for cattle feed, it all requires land to grow and it all gets eaten.

Here is the logic whereby one house comes to 0.75 cows.
  • One cow eats about 11 kg forage (dry matter) each day [2] so 4 tonnes/year.
  • AD using wet grass produces 90 m3 methane per tonne [3]
  • Adjusting for moisture content and converting to energy, this comes to 3,900 kWh/tonne grass (dry).
  • Hence the 4 tonnes forage eaten by one cow could be 16,000 kWh methane
  • A typical gas bill is 12,000 kWh/year [4] - equivalent to 0.75 cows

The collective appetite of all cows and sheep in the UK is equivalent to around 13 million homes.
The population of cows and sheep in the UK is also an area of uncertainty. It varies through the year for a start, as in June there are a lot more lambs - but they don't eat so much anyway. My calculations are based on the population in December [5] so this may be a bit of an underestimate. I have also factored in the variation in appetite due to age, by using equivalent livestock units (1 unit = 1 adult cow) [6]. In total the UK population comes to 7.2 million from cattle and 2.3 million from sheep (1 sheep = 0.1 cows). Their collective appetite is equivalent to 13 million homes.

Choices for our homes - install a heat pump (big hassle) - or change our diet.
For heating a low carbon home there are two main choices that require no new technology: heat pumps powered by wind or other low carbon energy, or biogas. A heat pump is a big hassle to retrofit into existing homes (see Why I haven't installed a heat pump yet) whereas biogas is a zero hassle option if you already use gas. Green gas is already available from some energy companies - I am currently with Ecotricity. However, scaling this up for a significant number of homes would be a massive undertaking and requires us to make changes in other ways - i.e. eating less meat and dairy products. We have a choice.

Further thoughts.

In practice it would not be easy to grow grass for energy on all our current grazing land. Some of it is inaccessible to harvesting machinery. 13 million homes is therefore an upper bound.

Pasture is naturally fertilised by cattle manure and if you grow grass for cutting it needs fertiliser. However most of this fertiliser could come from the slurry remaining after AD has taken place (commonly called digestate).

Note that some of our cattle feed is imported from abroad, so my calculation includes some land in other countries - this probably more than offsets the fact that the population of animals is larger in June and dairy cattle eat more.

There are other ways to convert biomass into biogas, for example by very high temperature processes (such as pyrolysis), and it is possible that some of these will be somewhat more efficient than AD. However those technologies are not yet mature. Grass is very suited to AD because of its high moisture content but for other processes that moisture would first have to be removed. If we went down those routes we should grow woody crops rather than grass. This gives us an alternative choice of landscape in the future.

[1] Families and Households 2017 (
[2] How much forage does a beef cow consume each day (The beef site) 2012
[3] Grass as a feedstock for Anaerobic Digestion (Germinal) 2018
[4] Typical domestic consumption values (OFGEM)
[5] Livestock numbers in the UK Dec 2017 (
[6] Eurostat Statistics explained: livestock units (Eurostat) 2013

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