Biomass as fuel 293But hydroelectric schemes do not have to be large; Table 11.5 dis-
tinguishes between large and small hydroelectric sources. Many units
exist generating a few kilowatts only that may supply one farm or a
small village. The attractiveness of small schemes is that they provide a
locally based supply at modest cost. Substantial growth in ‘small hydro’
has occurred during the last decade or so, from around 20 000 MW in
1990 to about 40 000 MW in 2000.^29 Installations in China account for
about half of this latter figure where the growth has been about twice
as rapid as in the rest of the world. Many more possibilities still exist
for the exploitation of the potential of small rivers and streams in many
parts of the world.
An important facility provided by some hydro schemes is that of
pumped storage. Using surplus electricityavailable in off-peak hours,
water can be pumped from a lower reservoir to a higher one. Then, at
other times, by reversing the process, electricity can be generated to
meet periods of peak demand. The efficiency of conversion can be as
high as eighty per cent and the response time a few seconds, so reducing
the need to keep other generating capacity in reserve. In 1990 about
75 000 MW of pumped storage capacity was available worldwide with
a further 25 000 MW under construction.^30
Biomass as fuel^31
Second in current importance as a renewable energy source is the use
of biomass as a fuel. The annual global primary production of biomass
of all kinds expressed in energy units is about 4500 EJ (=107 Gtoe).
About one per cent of this is currently turned into energy mostly in de-
veloping countries – we have labelled it ‘traditional biomass’. It has been
estimated that about six per cent of the total could become available for
energy crops taking into account the economics of production and the
availability of suitable land.^32 The energy so generated would represent
about seventy-five per cent of current world energy consumption – a sub-
stantial contribution to global energy needs. It is a genuinely renewable
resource in that the carbon dioxide which is emitted when the biomass
is burnt is turned back into carbon, through the process of photosynthe-
sis, in the renewed biomass when it is grown again. The word biomass
not only covers crops of all kinds but also domestic, industrial and agri-
cultural dry waste material and wet waste material, all of which can be
used as fuel for heating and to power electricity generators; some are
also appropriate to use for the manufacture of liquid or gaseous fuels.
Since biomass is widely distributed, it is particularly appropriate as a
distributed energy source suitable for rural areas.