Earth Sciences / 41Fast-growing crops, harvested to be burned, are being cultivated in several parts of the world as
‘biomass’ fuel. Willow (Salix species) and similar woody plants can be burned directly, after drying
then chopping and compressing them, which reduces their bulk. Alcohols can be obtained from
plants rich in sugar or starch and either used directly or dehydrated and mixed with gasoline to make
‘gasohol’. Low petroleum prices led to a decline in the number of Brazilian cars being built to run on
‘gasohol’, but in 1999 car manufacturers announced an increase in production in an attempt to boost
car sales. Fiat planned to raise output of these vehicles from 90 in August to 1,300 in September and
Volkswagen planned an increase from 800 to 1,200. General Motors introduced a new model in
September and Ford planned to relaunch its models in the spring of 2000. Methanol, an alternative
liquid fuel, can also be obtained from plant material. Such fuels are renewable because they can be
replaced easily by growing more of the fuel crop, and although they are based on carbon they make
no contribution to the greenhouse effect, because the carbon they release when burned is absorbed
photosynthetically by the plants which replace them; the carbon is recycled. Biomass crops occupy
land, however, and if they are to be grown on the scale needed to supply useful amounts of fuel they
could compete for space with food or fibre crops, and because they sell for a lower price than
conventional crops they may be grown very intensively to maximize yields.
Solar heat is absorbed by a black surface. Manufactured solar collectors exploit this, using the absorbed
heat to warm water, then transferring the heat to a hot water system. Collectors are limited
geographically, because they are most efficient where insolation is greatest and do not work well in
high latitudes. They have been installed on many buildings, usually attached to roofs or walls facing
the Sun, but their high capital cost often makes the energy they provide more expensive than that
supplied by the public utilities. In the tropics, however, direct solar heating can be used to distil
water and for cooking, with real advantages.
Photovoltaic cells can be used in any latitude, because they convert light, not heat, into electrical
power. In years to come this technology may provide useful amounts of energy, but at present its low
efficiency (about 15 per cent) means very large arrays are required and the resulting power is much
more expensive than electricity generated by other means. For many years, scientists and engineers
have been discussing the technical feasibility of constructing truly vast arrays of photovoltaic cells
in geostationary orbit and transmitting the power generated as microwaves to a receiving station on
the surface, where they would be reconverted to electrical power. The amount of energy obtainable
in this way would be great, although it would not be cheap, and at present no one can predict its
environmental consequences.
Sunlight can be concentrated. A device developed in Israel by the commercial arm of the Weizmann
Institute of Science and Boeing uses highly reflective mirrors (heliostats) to track the Sun and reflect
sunlight to another reflector on top of a central tower. This reflector redirects the sunlight to a matrix
of concentrators, which increase the intensity of the light 5000 to 10000 times, compared with the
sunlight reaching the surface outside the facility. The concentrated light is fed to a receiver, called
‘Porcupine’ because it contains hundreds of ceramic pins arranged in a geometric pattern. Compressed
air flowing across the pins is heated and channelled to gas turbines that generate electrical power.
The prototype plant was installed late in 1999.
Wind power is also exploited widely, but it, too, suffers from the fact that although solar energy, as
wind, is abundant, it is variable and very diffuse. The amount of energy captured by a wind turbine
is proportional to the square of the diameter of the circle described by its blades and the cube of the
wind speed (ALLABY, 1992, pp. 194–202) in a 32 km h-1 wind, a 15-metre-diameter rotor linked to
a generator operating at 50 per cent efficiency generates 24 kW of power. Most modern wind generators
have a rated capacity of about 750 kW and are established in arrays (‘wind farms’), each turbine
occupying about 2 ha, the spacing necessary to avoid mutual interference, therefore up to 3000