Combined cycle power plants

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A single gas turbine connected to a generator can generate electricity with a fuel-to-electricity conversion efficiency of perhaps 38% using the best of today's technology. New developments, such as those falling under the auspices of the US DOE's ATS programme aim to push the simple cycle efficiency as high as 41% without cycle adaptation, 43% with adaptation such as recuperation. This is still marginally lower than a modern coal-fired power plant can hope to achieve.

Part of the reason for this lower efficiency resides in the fact that the exhaust gas leaving the gas turbine is still extremely hot; that is, it still contains a significant amount of energy which has not been harnessed to generate electricity. There are a wide variety of applications in which this exhaust heat can be used to generate hot water or steam for use in some industrial process, or for heating purposes. This forms the basis of a gas turbine co-generation system, a topic which will be covered in a separate chapter.

There is a second strategy which can be employed. The exhaust heat can be captured in a steam boiler - normally called a heat recovery steam generator (HRSG) - where it generates steam which is used to drive a steam turbine and create additional electricity. This is the basis for the combined cycle power plant.

Combined cycle plants may employ one, or several gas turbines. Normally each gas turbine is equipped with its own waste-heat boiler designed to capture the exhaust heat as efficiently as possible. In a power plant with more than one gas turbine, each may have its own steam turbine, or the units may be grouped so that several gas turbines supply steam for a single steam turbine.

A combined cycle power plant can be constructed from already available components, but the most efficient plants will employ gas turbines, HRSGs and steam turbines that have been matched to one another. While turbines are manufactured and then shipped to power plants site, the HRSG is built at the site. Two types of HRSG are in common use, horizontal and vertical. In a horizontal HRSG the exhaust gas from the gas turbine

> To atmosphere

> To atmosphere

Solar Power Plant Block Diagram
Figure 4.4 A block diagram of a combined cycle power plant

passes through it horizontally and the water/steam pipes which collect the heat are hung vertically in its path. The vertical HRSG reverses these arrangements. Vertical HRSGs are most popular in Europe where space for power plant development is restricted. Horizontal units are most popular in the USA.

Power plants based on the combined cycle configuration have become the workhorses of independent power producers all across the world. With individual heavy-frame gas turbines available in unit sizes up to 265 MW, such plants can be based on modules of around 300-400 MW. Actual power output can be increased by adding some additional heat generation within the HRSG, a procedure called supplementary firing.

Using the combined cycle configuration, power stations can be brought into service rapidly, with the gas turbine operating first in simple cycle mode, while the waste-heat recovery boilers and steam turbines are added later. Generating capacity can easily be increased incrementally too, by adding additional gas and steam turbines. Such plants boast efficiencies of up to 57%.

New generation combined cycle power plants will soon reach 60% efficiency. This is the efficiency expected by GE Power Systems from its H-System, the product of its project funded under the US DOE ATS programme. Such units are designed specifically for combined cycle operation, and the gas and steam turbines are closely coupled to ensure the maximum performance.

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