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51
be about 14.6 m
3
and the flue gas temperature leaving the boiler will be about
200 °C. For domestic natural draught gas-fired boilers, excess air may be 60%,
the flue gas temperature will be 165 °C and the CO2 content will be around 7.5%.
Thus the measured temperature on the test day is sufficiently close to the
theoretically expected figure to say that the heating system meets its design
specification.
Electrical power generation
Electricity is generated by alternators driven by steam turbines in power
stations. The largest alternators produce 500 MW of electrical power at 33 kV.
The steam is produced in a boiler heated by the combustion of coal or residual
fuel oil, which could otherwise only be used for making tar. The oil is heated to
make it flow through distribution pipework.
Nuclear power stations produce heat by a fission reaction and the active
core is cooled by pressurized water (pressurized water reactor (PWR)), car-
bon dioxide gas (high temperature gas-cooled reactor (HTGR)), liquid so-
dium (fast breeder reactor) or heavy water (Canadian deuterium (CANDU)
system). This fluid then transfers its heat to water, boiling it into steam to drive
conventional turbines.
Smaller alternators are driven by methane combustion in gas turbine en-
gines or by diesel engines. A large modern power station has four separate
boiler-turbine-alternator sets, producing a total of 2000 MW at a maximum of
38 % overall efficiency.
Approximately half the input fuel's energy is dissipated in natural-draught
cooling towers or sea water, depending on the plant location. Steam leaves the
turbine at the lowest attainable subatmospheric pressure so that as much power
as possible is extracted from it as it passes through the turbines. The tempera-
ture of the cooling water may be as low as 35 °C, which is of little practical use
unless a mechanical heat pump is employed to generate a fluid at 60-90 °C. The
heat could then be pumped to dwellings. Power stations are normally sited
away from centres of population and heat transport costs are high.
During the next 25-100 years, the United Kingdom is going to have to
make more efficient use of its indigenous hydrocarbon reserves, extend nuclear
power generation capacity and develop alternative production methods such as
tidal, wave, solar, wind, geothermal and hydroelectric plants.
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