Solar II

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On p42 we listed four solar biomass options:

1. "Coal substitution."

2. "Petroleum substitution."

3. Food for humans or other animals.

4. Incineration of agricultural by-products.

We'll estimate the maximum plausible contribution of each of these processes in turn. In practice, many of these methods require so much energy to be put in along the way that they are scarcely net contributors (figure 6.14). But in what follows, I'll ignore such embodied-energy costs.

Figure D.1. Two trees.

Energy crops as a coal substitute

If we grow in Britain energy crops such as willow, miscanthus, or poplar (which have an average power of 0.5 W per square metre of land), then shove them in a 40%-efficient power station, the resulting power per unit area is 0.2 W/m2. If one eighth of Britain (500 m2 per person) were covered in these plantations, the resulting power would be 2.5kWh/d per person.

Petroleum substitution

There are several ways to turn plants into liquid fuels. I'll express the potential of each method in terms of its power per unit area (as in figure 6.11).

Britain's main biodiesel crop, rape

Typically, rape is sown in September and harvested the following August. Currently 450 000 hectares of oilseed rape are grown in the UK each year. (That's 2% of the UK.) Fields of rape produce 1200 litres of biodiesel per hectare per year; biodiesel has an energy of 9.8 kWh per litre; So that's a power per unit area of 0.13 W/m2.

If we used 25% of Britain for oilseed rape, we'd obtain biodiesel with an energy content of 3.1 kWh/d per person.

Sugar beet to ethanol

Sugar beet, in the UK, delivers an impressive yield of 53 t per hectare per year. And 11 of sugar beet makes 108 litres of bioethanol. Bioethanol has an energy density of 6 kWh per litre, so this process has a power per unit area of 0.4 W/m2, not accounting for energy inputs required.

Figure D.2. Oilseed rape. If used to create biodiesel, the power per unit area of rape is 0.13 W/m2. Photo by Tim Dunne.

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