42 / Basics of Environmental Science
turbines, occupying 6000 ha, are needed to match the output of a large conventional power station.
The unreliability of the wind means conventional generating capacity must be held available for use
when the wind speed is too low or so high that the blades must be feathered (turned edge-on to the
wind) to stop the rotors turning. Suitable sites for such installations are limited, and in highly valued
open landscapes they tend to be visually intrusive and arouse strong opposition. Were wind power to
provide a substantial proportion of our energy requirements, there could be a risk that the very large
installations might affect local climates by extracting a significant part of the energy of weather
systems.
The vertical movement of sea waves can also be used to generate electricity. The technology
is well advanced, but wave power suffers from disadvantages similar to those of wind power.
The installations need to be large and both they and the cables carrying the energy they generate
to shore must be able to withstand ocean storms. They must also be located in places where
wave movement is large and reliable, but well clear of shipping lanes. This limits the availability
of suitable sites. An alternative device, which occupies a much smaller area, extracts energy
from the oscillation of waves within a cylindrical structure, and energy can also be obtained
in still waters by exploiting the temperature difference between warm surface water and cold
deep water.
Wind and wave power can probably be used most effectively on a small scale in places that are
beyond the reach of conventional energy distribution systems, such as remote, sparsely populated
islands where demand is modest and the cost of links to the mainland high. Direct solar energy,
captured by solar collectors or photovoltaic arrays, may become more popular if they can be
made more efficient and ways can be found to spread the high capital cost over the lifetime of
the installation. Biomass conversion, exploiting energy captured by photosynthesis in green
plants, is perhaps the most promising of the renewable technologies. It requires land that is
surplus to rival agricultural needs; a criterion that perhaps is now being met in many parts of the
European Union.
Compared with the energy our planet apparently receives from the Sun, the amount we derive from
fossil and nuclear fuels seems trivial. It is tempting, therefore, to suppose that solar energy can be
harnessed to provide environmentally benign power from an original source that is free. Unfortunately,
the technical problems are formidable, the costs high, and the environmental consequences uncertain.
12. Albedo and heat capacity
Walk in a snow-covered landscape on a sunny day and you may feel more comfortable if you wear
dark glasses. Indeed, you may be well advised to wear dark glasses, because the light may be bright
enough to hurt your eyes. Once the snow has melted, and the ground is carpeted with plants, you will
have less need of dark glasses. The light will not be so bright.
We can see objects because of the light reflected from them. Their colour is determined by the
wavelengths of the light they reflect and their brightness by the amount. Freshly fallen snow
reflects 80–90 per cent of the light falling on it, grass 18–25 per cent, and this is why you need
dark glasses when crossing snow: it may be almost as bright as the Sun itself. Grass, on the other
hand, is much duller.
The proportion of light reflected by a surface is called the ‘reflection coefficient’, or more usually
‘albedo’ of that surface. It can be measured and is usually expressed as a fraction or a percentage. As
Table 2.1 shows, albedo varies widely from one surface to another.