With the Carnot formula, efficiency can be expressed as temperatures. For gas turbines, Tmax is the temperature of the hot gases leaving the combustion chamber gases and Tmin is the ambient temperature.
Assume we burn fuel at stochiometric conditions and get a flame temperature of 2500 K. Assume also that our turbine is designed to withstand that temperature. Further assume that our engine is working at a pressure ratio of 100, which is very high but not unreasonable. Finally, assume that internal losses are negligible. Our highly hypothetical gas turbine results in an efficiency of approximately 65%.
In comparison, where are we today? The best-performing, simple, open-cycle, single-shaft machines show an efficiency of approximately 40%.
IMPROVING THE WORKING CYCLE
Few things could be more exciting to gas turbine developers than finding improvements in efficiency that have not been invented before. Many ideas have seen the light over the years but only a few have resulted in commercial products. One issue that has been on engineers’ agenda for a long time is the development of modified working cycles. However, efficiency improvement measures often go hand-in-hand with increased complexity and cost.
For example, the simple, open-cycle, single-shaft gas turbine comprises a compressor and a turbine on one shaft. It is simple in its design but also has a limited efficiency capability. This machine can be modified into a two-shaft machine with a high-pressure rotor coaxial to the low-pressure rotor. The modification will improve efficiency but will also increase engine complexity and cost. However, this two-shaft engine is available on the market today and is an example of what can be done to improve efficiency. There are many other successful working-cycle modifications such as compressor intercooling, recuperating exhaust gases, sequential combustion, etc., but these will not be discussed here.
Some efficiency improvement efforts have made significant progress in recent years. From the Carnot efficiency formula above, we find that by increasing the temperature span between the heat source and heat sink we will increase efficiency. According to the second law of thermodynamics, heat can only go from a higher temperature to a lower. The heat sink cannot be made to go to a lower temperature, according to the second law, but the heat-source temperature can be increased, for example by firing more fuel. Firing more fuel raises the temperature and increases the temperature span.
Market restructuring and interconnection issues