When is 60 percent efficiency not 60 percent efficiency?

Combined cycle gas turbine power plants are currently the most efficient way to convert fossil fuels into electricity. For several years, the Holy Grail of power generation was to reach, and exceed 60 percent thermal efficiency at the generator terminals. The path to this accomplishment involved significantly improved gas turbine efficiency, higher combustor firing temperatures, state-of-the-art steam cycles and modern hydrogen-cooled generators.

Bragging rights go to manufacturers who can beat the 60 percent efficiency barrier. All combined cycle plants combine a gas turbine (Brayton) cycle with a steam (Rankine) cycle by using the relatively high exhaust heat from the gas turbine to generate steam. While the gas turbine cycle is an open combustion cycle, using air as the working fluid, the steam cycle is a closed water cycle using the exhaust from the gas turbine as the heat source.

Therefore, the steam turbine can expand the steam to pressures well below the atmospheric pressure (vacuum pressure). The vacuum pressure is directly related to the condenser temperature, which in turn depends on the cooling medium used. If it is water, relatively low temperatures and pressures can be achieved using evaporative cooling or even chillers, while for air cooled condensers, the ambient conditions impose a limitation on the lowest cycle temperature. 

This obviously impacts the output power of the steam turbine and the combined cycle plant efficiency as a whole.

Lately, there is some confusion about reporting the capabilities of different configurations of different manufacturers. Let’s take a step back and look at the salient points of efficiency definitions: ISO 3977 (Gas turbines – procurement) specifies the design parameters for standard conditions. These are, for the gas turbine, 15°C, 60 percent relative humidity, and ambient pressure of 101.3 kPa (14.7 psia).

From an energy accounting standpoint, it is important to define whether the net or gross plant output is referenced.

Power plants also use significant parasitic energy to drive fans, pumps, fuel compressors and other devices, which have their own varying efficiencies. Thus, the net output is the power at the generator terminals reduced by the plant power consumption mentioned before. This parasitic power cannot be neglected in a total plant efficiency comparison.

Efficiency definitions also have to consider whether the higher or lower heating value of the fuel has to be considered. Especially with natural gas which contains a large amount of hydrogen atoms and produces significant amounts of water in combustion products, the difference can be in the order of 5 percent, depending on the fuel.

To compare cycles, it generally makes sense to use the lower heating value, because the gas turbine exhaust stack temperature is usually kept high enough to avoid the formation of liquid water. Unfortunately, natural gas is sold on the basis of higher heating value so it is easy to create confusion.

More in March/April 2013 issue of Turbomachinery International