Energy from theSun 303Sunlight can be converted directly into electricity by means of photo-
voltaic (PV) solar cells (see box on page 302). Solar panels on spacecraft
have provided electrical power for spacecraft from the earliest days of
space research nearly fifty years ago. They now appear in a variety of
ways in everyday life; for instance, as power sources for small calcu-
lators or watches. Their efficiency for conversion of solar energy into
electrical energy is now generally between just under ten per cent and
twenty per cent. A panel of cells of area one square metre facing full
sunlight will therefore deliver between 100 and 200 W of electrical
power. A cost-effective way of mounting PV modules is on the surface
of manufactured items or built structures rather than as free-standing
arrays. In the fast-growing building-integrated-PV (BIPV) sector, the
PV fa ̧cade replaces and avoids the cost of conventional cladding. In-
stalled on rooftops in cities, they provide a way for city dwellers to
contribute renewably to their energy needs. Japan hasdone the most to
encourage rooftop solar installations and by 2000 had installed 320 MW
capacity. The USA and Germany follow as countries with large rooftop
programmes,the USA with a target by 2010 of one million roofs and
Germany with programme for 100 000. The cost of energy from solar
cells has reduced dramatically over the past twenty years (see box below);
so much so that they can now be employed for awide range of appli-
cations and can also begin to contribute to the large-scale generation of
electricity.
Small PV installations are also suitable, especially for developing
countries, to provide local sources of electricity in rural areas. About a
third of the world’s population have no access to electricity from a central
source. Their predominant need is for small amounts of power for light-
ing, for radio and television, for refrigerators (for example, for vaccines
at a health clinic) and for pumping water. The cost of PV installations for
these purposes is now competitive with other means of generation (such
as diesel units). Over the twenty years to the year 2000, approximately
1.1 million ‘Solar Home Systems’and ‘Solar Lanterns’ had been in-
stalled in Asia,Africa and South American countries.^54 Solar Home
Systems provide typically 15–75 W from a solar array (Figure 11.14)
and cost in the range of $US 200–1200. Smaller ‘solar lanterns’ (typi-
cally 10–20 W) provide lighting only. Larger installations are required
for public buildings, although they need not be that much larger. Many
small hospitals can benefit from an electrical power source as small
as 1–2 kW. For instance, by 1995, seventy small hospitals in Sri Lanka,
through assistance from the Australian government, had installed 1.3-kW
solar arrays, backed up by 2200 amp-hour batteries, to provide for light-
ing, refrigeration for vaccines, autoclave sterilisation, pumping for hot
water (produced through a solar-thermal system) and radio. Over