Advanced Cycles
Options for Improving the Joule Cycle
Although the CC cycle already has a high efficiency, it has the disadvantage that two cycles are involved and much of the elegance of the simple Joule cycle is lost. In relation to this it is interesting to explore whether the Joule cycle cannot be improved in a similar way as the Rankine cycle has been improved over time. Although both the Rankine cycle and the Joule cycle have made use of the benefits of higher pressure ratios and higher temperatures, little has been done in the Joule cycle to make use of features such as reheat and heat recuperation to improve the cycle efficiency.
Cycles can be made more efficient by preheating the medium of the cycle (water in case of the Rankine cycle, and air in case of the Joule cycle) with low level heat that comes available during the cycle. In the Rankine cycle this is, for example, accomplished by using extraction steam from the expansion turbine to preheat the boiler feed water. In the Joule cycle such preheat is only possible in gas turbines with a low-pressure ratio of 7-10 where the exhaust gas of the turbine has a higher temperature than the exhaust air from the compressor. In that case the sensible heat in the exhaust gas from the turbine may be used to preheat the air leaving the compressor in a so-called recuperator. In more advanced industrial turbines and aero-derivative turbines with pressure ratios of 15 and higher, the turbine and the compressor have about the same outlet temperature, which makes recuperation impossible. This is a serious disadvantage of more advanced gas turbines and is due to the fact that virtually all gas turbine compressors feature adiabatic rather than isothermal compression. The same reason limits the use of reheat in gas turbine cycles. The very high pressures of 70-200 bar, which would be necessary for a realistic reheat cycle, prohibit the use of adiabatic compression.