Solar biomass

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All of a sudden, you know, we may be in the energy business by being able to grow grass on the ranch! And have it harvested and converted into energy. That's what's close to happening.

George W. Bush, February 2006

All available bioenergy solutions involve first growing green stuff, and then doing something with the green stuff. How big could the energy collected by the green stuff possibly be? There are four main routes to get energy from solar-powered biological systems:

1. We can grow specially-chosen plants and burn them in a power station that produces electricity or heat or both. We'll call this "coal substitution."

Solar heating: 13 kWh/d

Wind: 20 kWh/d

Figure 6.9. Solar photovoltaics: a 10 m2 array of building-mounted south-facing panels with 20% efficiency can deliver about 5 kWh per day of electrical energy. If 5% of the country were coated with 10%-efficient solar panels (200 m2 of panels per person) they would deliver 50 kWh/day/person.

2. We can grow specially-chosen plants (oil-seed rape, sugar cane, or corn, say), turn them into ethanol or biodiesel, and shove that into cars, trains, planes or other places where such chemicals are useful. Or we might cultivate genetically-engineered bacteria, cyanobacteria, or algae that directly produce hydrogen, ethanol, or butanol, or even electricity. We'll call all such approaches "petroleum substitution."

3. We can take by-products from other agricultural activities and burn them in a power station. The by-products might range from straw (a by-product of Weetabix) to chicken poo (a by-product of McNuggets). Burning by-products is coal substitution again, but using ordinary plants, not the best high-energy plants. A power station that burns agricultural by-products won't deliver as much power per unit area of farmland as an optimized biomass-growing facility, but it has the advantage that it doesn't monopolize the land. Burning methane gas from landfill sites is a similar way of getting energy, but it's sustainable only as long as we have a sustainable source of junk to keep putting into the landfill sites. (Most of the landfill methane comes from wasted food; people in Britain throw away about 300 g of food per day per person.) Incinerating household waste is another slightly less roundabout way of getting power from solar biomass.

4. We can grow plants and feed them directly to energy-requiring humans or other animals.

For all of these processes, the first staging post for the energy is in a chemical molecule such as a carbohydrate in a green plant. We can therefore estimate the power obtainable from any and all of these processes by estimating how much power could pass through that first staging post. All the subsequent steps involving tractors, animals, chemical facilities, landfill sites, or power stations can only lose energy. So the power at the first staging post is an upper bound on the power available from all plant-based power solutions.

So, let's simply estimate the power at the first staging post. (In Chapter D, we'll go into more detail, estimating the maximum contribution of each process.) The average harvestable power of sunlight in Britain is 100 W/m2. The most efficient plants in Europe are about 2%-efficient at turning solar energy into carbohydrates, which would suggest that plants might deliver 2W/m2; however, their efficiency drops at higher light levels, and the best performance of any energy crops in Europe is closer to 0.5 W/m2. Let's cover 75% of the country with quality green stuff. That's 3000 m2 per person devoted to bio-energy. This is the same as the British land area

Figure 6.10. Some Miscanthus grass enjoying the company of Dr Emily Heaton, who is 5'4" (163 cm) tall. In Britain, Miscanthus achieves a power per unit area of 0.75 W/m2. Photo provided by the University of Illinois.

wood (commercial forestry) ¡rape rape to biodiesel maize sugar beet

I short rotation coppice calorific value | energy crops calorific value miscanthus to electricity

I switchgrass •I corn to ethanol

—I wheat to ethanol I-Ijatropha

^ tropical plantations (eucalyptus)

Figure 6.11. Power production, per unit area, achieved by various plants. For sources, see the end-notes. These power densities vary depending on irrigation and fertilization; ranges are indicated for some crops, for example wood has a range from 0.095-0.254 W/m2. The bottom three power densities are for crops grown in tropical locations. The last power I sugarcane (Brazil, Zambia) density (tropical plantations*)

assumes genetic modification, fertilizer application, and irrigation. In the text, I use 0.5 W/m2 as a summary figure for the best energy crops in NW Europe.

=| tropical plantations*

currently devoted to agriculture. So the maximum power available, ignoring all the additional costs of growing, harvesting, and processing the greenery, is

0.5 W/m2 x 3000 m2 per person = 36 kWh/d per person.

Wow. That's not very much, considering the outrageously generous assumptions we just made, to try to get a big number. If you wanted to get biofuels for cars or planes from the greenery, all the other steps in the chain from farm to spark plug would inevitably be inefficient. I think it'd be optimistic to hope that the overall losses along the processing chain would be as small as 33%. Even burning dried wood in a good wood boiler loses 20% of the heat up the chimney. So surely the true potential power from biomass and biofuels cannot be any bigger than 24 kWh/d per person. And don't forget, we want to use some of the greenery to make food for us and for our animal companions.

Could genetic engineering produce plants that convert solar energy to chemicals more efficiently? It's conceivable; but I haven't found any scientific publication predicting that plants in Europe could achieve net power production beyond 1 W/m2.

I'll pop 24 kWh/d per person onto the green stack, emphasizing that I think this number is an over-estimate - I think the true maximum power that we could get from biomass will be smaller because of the losses in farming and processing.

I think one conclusion is clear: biofuels can't add up - at least, not in countries like Britain, and not as a replacement for all transport fuels. Even leaving aside biofuels' main defects - that their production competes with food, and that the additional inputs required for farming and processing often cancel out most of the delivered energy (figure 6.14) - biofuels made from plants, in a European country like Britain, can deliver so little power, I think they are scarcely worth talking about.

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Getting Started With Solar

Getting Started With Solar

Do we really want the one thing that gives us its resources unconditionally to suffer even more than it is suffering now? Nature, is a part of our being from the earliest human days. We respect Nature and it gives us its bounty, but in the recent past greedy money hungry corporations have made us all so destructive, so wasteful.

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