Building Services Engineering 5th Edition Handbook

108 Heating
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)), carbon dioxide gas (high-temperature
gas-cooled reactor (HTGR)), liquid sodium (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 engines 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. Figure 4.26 shows the energy flows
in a conventional power station.
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
sub-atmospheric pressure so that as much power as possible is extracted from it as it passes
through the turbines. The temperature 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 ◦ C–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 UK 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.
Combined heat and power
Existing power stations generate electricity only, at as high an efficiency as possible. Combined
heat and power (CHP) stations produce less electricity and more heat but improve overall fuel
efficiency to about 50%, as indicated in Fig. 4.27.
Future CHP plants will be smaller than the present electricity-only plants and will be sited close
to centres of industry and population. Coal-fired boilers will be used where practical. Fuel and ash
will be mechanically handled and flue gases filtered to remove dust and impurities (Horlock, 1987).
Flue
12%
High-pressure steam
Boiler
Turbine
Three-phase 50 Hz
38%
Fuel 100%
Alternator
50%
Pump
Feed pump Condenser Cooling tower
4.26 Conventional 2000 MW power station.