CCGT Technology

Marchwood Power Station operates a combined-cycle gas turbine (CCGT) process to produce electricity.

CCGT Cycle DiagramSchematic of the CCGT process operated at Marchwood Power Station (click to enlarge)

Natural gas is piped 22km to Marchwood from an offtake of the National Transmission Network at Lockerley in rural Hampshire. At the power station, gas is mixed with filtered, compressed air and burned. The hot combustion gases expand, driving the gas turbines. This in turn drives generators to produce electricity for the national grid. Marchwood Power Station has two gas turbines.

The hot exhaust gases from this process contain significant amounts of recoverable energy. The gases are passed through the heat recovery boiler to produce steam. The high pressure steam is then used to drive a steam turbine which generates further electricity for the grid. The waste gases are expelled to the atmosphere via the stack.

The spent steam is then passed through a condenser. The ‘condensate’ is then cycled back through the heat recovery boiler. The condenser requires large quantities of cooling water which is drawn from the River Test and re-discharged to the river, slightly warmer, after use. The intake and outfall structures are those remaining from the old power station, refurbished as part of the construction phase. The quality and temperature of the water discharged back to the river is carefully controlled as part of the Environmental Permit.

The term ‘combined cycle’ refers to the use of two processes to produce electricity. The burning of natural gas is the first. The recovery of heat from the waste gases to produce steam to drive a steam turbine is the second. Together, these processes capture much more of the energy in the gas. Modern CCGT plants of this kind represent advanced generating technology as they offer high operating efficiencies and low emissions. Marchwood operates at around 58% efficiency and is one of the most efficient power stations in the UK.