Sustainable energy

With enough cheap, nonpolluting energy, all things are possible. The average power needs of a developed country today range from 4-14 kW per person, which translates into an energy consumption of 120-450 GJ/year per person.[41] This energy is used in the ways listed in Table 10.1. The trans­port sector is more dependent than the others on fossil fuels; there are, at

Table 10.1

Energy consumption by sector

Sector

Proportion (%)

Transport

32

Domestic

29

Industry

35

Other (food)

4

present, no viable alternatives to gasoline, diesel, and kerosene for vehicle and aircraft propulsion.

There are, ultimately, only four sources of energy: the sun, which drives the winds, wave, hydro, and photochemical phenomena; the moon, which drives the tides; radioactive decay of unstable elements giving geothermal heat from the Earth’s crust or, when concentrated, nuclear power; and hydrocarbon fuels, the sun’s energy in fossilized form. All, ultimately, are finite resources, but the time scale for the exhaustion of the first three— solar, lunar, and geothermal—is so large that it is safe to regard them as infinite. And it is these three that offer the possibility of power generation without atmospheric pollution. So let us examine each in turn.

We have MacKay (2008) to thank for introducing some simple physics and a great deal of common sense into the discussion of renewable energy. The paragraphs that follow derive from his analysis.

Wind. The problem with wind power, like that of most other renewable energy sources, is the low power density, that is, power per unit area that can be harvested. On land, it averages 2 W/m2; offshore it is larger, about 3 W/m2. The average land area per person averaged across a country with a popula­tion density like that of the United Kingdom is about 3500 m2. That means that if the entire country were packed with the maximum possible number of wind turbines, it would generate just 7 kW per person. Placing them off­shore helps solve the overcrowding problem, but maintenance costs are high.

Solar. Solar energy density depends where you are on the Earth’s surface. In a temperate-zone country it can be as high as 50 W/m2. It can be cap­tured in a number of ways. Solar-thermal systems use the sun’s energy to heat water with an efficiency of perhaps 50%, capturing 25 W/m2. Hot water is low-grade energy, not readily transmitted or converted to other energy forms, so it can help with home heating, but not much else. Photovoltaics convert photons to electrical power with an efficiency of between 10% (cheap solar cells) and 20% (very expensive ones). The solar cells would have to be cheap if used to cover large areas, capturing about 5 W/m2, again requiring a vast area to capture enough energy to contribute much to daily needs. We don’t need water pipes and solar cells to capture the sun’s energy, of course. Plants can do it for us, and then we can burn them or ferment them or even eat them to make use of the energy they stored (energy from biomass). But the capture efficiency of even the most efficient plants is only 1%, and commercial agriculture processing consumes energy in tractors, fermentation vats, and distillation equipment, reducing the effective effi­ciency to perhaps 0.5%.

Hydro. The power that can be extracted from hydroelectric schemes is limited by altitude and rainfall. In mountainous areas where it is practical to catch and store the rain, releasing the water through a height drop to a turbine, it is possible to capture 0.2 W/m2—not a lot but enough to make significant contributions to power generation in Switzerland and to provide Norway with almost all its electricity.

Waves. Energy can be captured from waves by placing something in their path—a fixed barrier with a turbine driven by the water whooshing in and out, for instance. Thus waves carry an energy per unit length rather than an energy per unit area, and it is large—as much as 40 kW per meter. Capturing it, however, is not easy; it is unlikely that any wave machine would trap more than a third of this power. Few countries have long coast­lines (some have none), but for those that do, wave power is an option. But once again the scale of the operation has to be vast to make a real contri­bution: to provide 1 kW per person to a country of 50 million inhabitants needs 3,700 km of barrier. And any wave-driven device takes a considerable battering; maintenance is a problem.

Tides. Tidal power is power from the moon. Tidal power where tides are high can deliver 3 W/m2 by making both the incoming and the outgo­ing tidal flow drive turbines. This is about the same as wind power, but few countries are in a position to capture much of it. For those countries with tidal estuaries or at the mouths of large landlocked seas (such as the Mediterranean), harnessing tidal power is an option. But it is not one that will, by itself, provide as much energy as we need.

Geothermal. Geothermal heat is heat conducted up from the hot core of the planet, augmented by heat from the decay of unstable elements in the crust. This heat leaks out at the surface, but not in a useful form. To gener­ate electricity it is necessary to heat water to at least 200°C, and for most of the Earth’s crust that means drilling down to about 10 km, making it expensive to harvest. In a few places the heat is much closer to the sur­face, so much so that water bubbling up naturally is above its boiling point. Where this is so (Iceland; hot springs in the United States, New Zealand, and elsewhere), extracting geothermal heat is a practical proposition. For most other countries the contribution it can make is small.

The conclusions: no single renewable source can begin to supply energy on the scale we now use it. A combination of all of them might. But think of the difficulties. There is the low power density, meaning that a large fraction of the area of the country must be dedicated to capturing it. If you cover half the country with solar cells, you cannot also plant crops for biofuel on it, nor can you use it as we now do for agriculture and livestock for food. There is the cost of establishing such a dispersed system and, in the case of offshore wave and wind farms, maintaining it (even on land some 2% of wind tur­bines are disabled each year by lightning). And there is the opposition, much of it from environmentalists, that paving the country and framing the coast with machinery would create. MacKay’s (2008) book examines all this in greater depth; for now we must accept that the dream of copious cheap, pol­lution-free energy from sun, wind, and wave is not going to become a reality.