OT - Anaerobic Digestion - could you check my use of these figures please?

[TimW]

According to Farmers' Guardian, a biogas plant with a capacity of one megawatt, "requires 20,000-25,000 tonnes [of maize] a year, accounting for 450-500 hectares of land. I don't know but I assume they are talking electricity, not watts of heat from biogas.

But what we need is a figure for MWh. If a plant has a 1MW capacity does that mean it produces 1MW for most of 24hrs x 7 days x 52 weeks?, so say

8000 MWh ? or what?

4,000 kWh is the amount of electricity consumed by an average household. which is 4MWh?

So something like 2000 households per 500 Ha of land. which is 0.25Ha of land required for AD sourced electricity for an average household

UK has 260 persons per sq km so 2.6 persons per Ha, and an average household is 2.3 persons and only 70% of land is agricultural So we have around 0.6Ha of agricultural land per household in the uk.

We produce less than 60% of the food we eat already. I haven't accounted for agricultural land which couldn't be used to grow energy crops (eg hill pasture and marginal land) I think I wildly overestimated the MWh you might get from a MW capacity But on this basis growing energy crops in the uk is fairly bonkers

Have I made any obvious big mistakes?

Tim W

[Dave Liquorice]

1 MW is a rate of doing work = power. I read the word "capacity", with a journalists eyes, to mean that it can produce up to 1 MW "continuously".

MWHr is total energy and has the same problem unless you say over what time period, 1 MWHr/year is sod all energy. 1 MWHr/second is quite a lot of energy.

91% uptime near enough I should think. Provided you can feed the digester to keep up enough gas output.

I wish... this place chobbles through 18.4 MWHr/year though take out the space heating element and 4 MWHr is pretty close.

Only .25 ha, surprised it's such a small area. We have 0.462 ha according to the Land Registry map. Of course maize is probably one of the higher output bio crops and it wouldn't grow up here. We planted 800+ mixed (birch, sycamore, blackthorn, larch, rowan and others) trees > 15 years ago. If we felled the lot I doubt the resulting logs would keep the wood burner going for a week of evenings...

1 * 24 * 7 * 52 = 8,736 MWHr, doesn't look like it.

More or less, however I see a place for using bio waste, cattle slurry, waste from processed food production, "out of date" super market chuck outs, etc

[Chris Hogg]

I've been through the calculation and can see no obvious faults, but you might add as a final comment that on the basis of the figures you've used, to provide domestic electricity for the whole of the UK from AD would take about 40% of the available agricultural land (0.25/0.6). Your numbers only apply to domestic electricity and don't include that used by industry.

But as I think TNP pointed out, there are more efficient ways of converting sunlight into electrical energy than by growing crops, such as PV. Using your figures, with AD it takes 0.25 Ha to supply an average household with the 4000 kWh that that household uses in a year. Put another way, that's 0.46 kW per 2500 m^2. (4000/365/24), or

0.18 W/m^2 (460/2500). Domestic solar panels in the UK average about 15W/m^2 over a year (from a series of posts here in July 2015: Solar power calculations, please help!). I believe commercial installations in the UK may get a bit less, say 10W/m^2, but I'm not sure. It means that by your numbers, solar panels would provide between 50 and 80 times more electricity per unit area than AD of crops.

As you imply, food is better used for eating, but bear in mind that there's a lot of green waste associated with food production that could be used in AD without compromising that food production.

[Another Dave]

I don't know but you could have a look here:

"... biofuels made from plants, in a European country like Britain, can deliver so little power, I think they are scarcely worth talking about."

Another Dave

[thesilentone]

Your reply is a bit misleading, as the link refers to Biomass and Biofuel not Anaerobic Digestion.

AD offers a very environmentally friendly solution compared to the Biomass sector when competing for land. AD is very efficient and the CO2 off-set/reductions v fossil fuel alternatives are exceptional. But, it is not a silver bullet and we have to eat, so the aspirations for AD in the UK long-term would not exceed 5% of the plough-able hectares used form energy crops. AD along with Wind, PV, Hydro, Biomass, Geo-thermal is a good solution.

[The Natural Philosopher]

In the end what ever it is that is being anaerobically digested has to have grown in the first place.

Ergo it will not overall use less land or be more efficient than basic biomass.

Are you unable to grasp that point?

No, it doesn't.

No, AD along with Wind, PV, Biomass, Geo-thermal, is no solution at all.

Hydro would be, if we had more mountains, we don't so that's that.

[Chris Hogg]

As TNP says, that's a fantasy. I presume you're in the AD marketing business. Biomass in whatever form, whether combusted directly or indirectly as in AD, is only viable if what you're burning or digesting is a waste product from another process and would otherwise have no value. Growing crops specifically for AD is the road to ruin.

[Tim Streater]

He's another one who wants to build five power stations to get the output of one.

[The Natural Philosopher]

What seems to happen is that these people start from the premise that actually renewable energy is the way forward and then invent spurious data to explain how it all *could* work.

Whereas I started Gridwatch and have been banging a drum to explain why it never ever *can* work.