90
The Last Resource
the same effect on the cost of water, while a change from a 4 per cent
to a
5 per
cent interest rate could wipe out the improvement.
However, nancial juggling apart, in the end the designer must make
a compromise to arrive at the best possible total cost for a given
output of water.
Experience has shown that capital costs per unit of production
become smaller as the capacity of the plant increases. This particular
trend was seen in the submerged tube installations and is being
repeated with the multi—stage ash process. An important point
here is that the ash process offers so much more scope for exploit—
ing this ‘capital gain’. The 2- 6 m.g.d. distillation unit built by Westing—
house at Key West ranks as the largest single unit in the world at
the time of writing; but installations with capacities of 100 m.g.d.
or more may be installed in Southern California, Israel, the USSR
and New York. This fty—fold increase could cut the daily cost per
gallon by more than half—it is gures like these, theoretical though
they may be, that have led to considerable optimism about the
prospects for cheap water by desalination.
Another source of optimism is the possibility of combining
nuclear power generation with desalination. This helps to overcome
restrictions imposed by considerations of the total cost on the full
use of the available energy. The major British companies involved
in desalination have long been aware of the fact that distillation is
more economical when the production of fresh water is combined
with the generation of power. All major installations built by Weir,
apart from the one on Guernsey, are of this dual purpose type.
Water produced in this way has a price comparable to that of water
from more conventional sources in some parts of Western Europe—
about 53. per thousand gallons.
Why is it cheaper to have dual purpose plant? The answer lies in
the very simple principle laid down by the nineteenth—century
engineer N. K. S. Carnot (1796—1832), which states that to operate
any kind of heat engine, heat must be rejected. He worked out
mathematically that the efciency of this process of taking heat,
making it work, and then rejecting the balance depends upon the
difference between the initial working temperature and the nal
temperature at which the heat is rejected, divided by the initial
temperature. In combined power—generation and distillation, these
differences, and hence eîciency, remain the same but the hot water
coming from the turbo—generators is channelled into distillation
The Last Resource
the same effect on the cost of water, while a change from a 4 per cent
to a
5 per
cent interest rate could wipe out the improvement.
However, nancial juggling apart, in the end the designer must make
a compromise to arrive at the best possible total cost for a given
output of water.
Experience has shown that capital costs per unit of production
become smaller as the capacity of the plant increases. This particular
trend was seen in the submerged tube installations and is being
repeated with the multi—stage ash process. An important point
here is that the ash process offers so much more scope for exploit—
ing this ‘capital gain’. The 2- 6 m.g.d. distillation unit built by Westing—
house at Key West ranks as the largest single unit in the world at
the time of writing; but installations with capacities of 100 m.g.d.
or more may be installed in Southern California, Israel, the USSR
and New York. This fty—fold increase could cut the daily cost per
gallon by more than half—it is gures like these, theoretical though
they may be, that have led to considerable optimism about the
prospects for cheap water by desalination.
Another source of optimism is the possibility of combining
nuclear power generation with desalination. This helps to overcome
restrictions imposed by considerations of the total cost on the full
use of the available energy. The major British companies involved
in desalination have long been aware of the fact that distillation is
more economical when the production of fresh water is combined
with the generation of power. All major installations built by Weir,
apart from the one on Guernsey, are of this dual purpose type.
Water produced in this way has a price comparable to that of water
from more conventional sources in some parts of Western Europe—
about 53. per thousand gallons.
Why is it cheaper to have dual purpose plant? The answer lies in
the very simple principle laid down by the nineteenth—century
engineer N. K. S. Carnot (1796—1832), which states that to operate
any kind of heat engine, heat must be rejected. He worked out
mathematically that the efciency of this process of taking heat,
making it work, and then rejecting the balance depends upon the
difference between the initial working temperature and the nal
temperature at which the heat is rejected, divided by the initial
temperature. In combined power—generation and distillation, these
differences, and hence eîciency, remain the same but the hot water
coming from the turbo—generators is channelled into distillation