Thermodynamic cycle combinations: more efficient and cost effective

January 8 2012 - TI Staff

This is the first of a multi-part series that focuses on how the various thermodynamic power cycles can be better integrated, combined and interlocked to give us a new overall improved system of better and more efficient energy use. The cycles working in harmony and linked together can change the way we use energy – electric power, automobiles, transportation, and freight devices. 

A surging development of a new energy source, its production, availability and use, dubbed the “Game Changer” is taking place in most of the countries around the world. This new energy source is shale natural gas; an ideal fuel for today’s advanced industrial gas turbine. The gas turbine is ready to burn this new, clean burning fuel and play a very significant role in the emerging interwoven power system.

A new approach to power usage

Effective mingling and meshing together of the various existing thermodynamic power cycles which have been used over the years can pave way for a new approach to power usage. This has already been evolving before our very own eyes to a certain extent. The overall amalgamation can be more efficient, environmentally friendly and cost effective.

For a better system to evolve at a faster and controlled rate, the newly found gas must become dominant and override coal and crude oil. However, we must carefully handle our new gas. For example, five elements are required to use a battery powered hand held drill/screwdriver: (1) the power plant which supplies power, (2) the transmission line and transformer grid, (3) the battery, (4) the charger and (5) the tool itself. What is placed into use in this process is not only the Rankine cycle but the rest of the elements as well.

This can be better explained by the thermodynamic power cycles, namely, the Carnot cycle, the Otto cycle, the Diesel cycle, the Rankine cycle, the Brayton cycle, the Combined cycle, and the Integrated cycle or the Rice cycle. 

The Carnot Cycle

The Carnot cycle, first proposed in 1819 by Nicolas L. Carnot, is idealistic and theoretical. But it gives a benchmark and target to shoot at for all the other cycles.  Though this cycle has been proven to be the most efficient for any given set of temperatures, it is not easy to use by today's machinery.

In this cycle, a gas is compressed isentropically, and heat is added at constant temperature (isothermal) as the gas expands and the volume increases. The gas continues to expand adiabatically (isentropically), and heat is rejected as the volume decreases and the pressure increases to start the cycle all over again.

Look out for a review of the other thermodynamic cycles and practical applications in modern day gas turbines in the next part of the series.

Ivan G. Rice was past chairman of the South Texas Section of ASME (1974 - 75), past chairman of the ASME Gas Turbine Division (now IGTI) (1975 - 76).  A Life Fellow Member of ASME and Life Member of NSPE/TSPE, he has authored many articles and ASME papers on gas turbines, inter-cooling, reheat, HRSGs, steam cooling and steam injection.