Stuck in a rut
Gas turbines are stuck in a rut. These reliable workhorses rose to power at the end of the second world war and never really looked back, but they are a victim of their own success. As every engineer knows, you don’t fix what isn’t broken, and so the industry hasn’t – for almost 100 years.
Sure, gas turbine technology has improved in that time: we’ve added in interstage cooling and waste heat recovery systems, we’ve used new materials to allow us to increase cycle temperatures, and advances in computational methods have helped us to refine our designs. However, one thing remains untouched – the fundamental cycle itself – and as a result performance has plateaued. Achieving a step change in gas turbine performance is going to need us to take a step back and re-think our approach, and we may well have to challenge some of the well-established norms of gas turbine design.
What is pressure gain combustion?
If you carry out an exergy analysis of a gas turbine you quickly find that the greatest losses in thermodynamic availability occur across the combustion chamber. It follows that, if we want to see a step change in gas turbine performance, the combustion chamber is the place to start, and pressure gain combustion is a technology doing just that. Pressure gain combustion does what is says on the tin – achieves a pressure gain across the combustion process, rather than the pressure loss realised in more traditional ‘constant pressure’ combustors. As a result, the entropy rise across the combustion process is reduced and the turbine is able to extract more work.
There are a number of ways that this can be achieved in practice, but all rely on constraining the gas during combustion – moving more towards a constant volume combustion process, rather than a constant pressure one. A major consequence of this is that the combustion must be unsteady, often pulsing like the exhaust from an internal combustion engine. This is a major challenge to gas turbine integration and one that we’ll discuss later. However, the potential benefits of pressure gain combustion on cycle performance are significant. Although the predicted gains vary, depending on the gas turbine cycle and modelling assumptions, most authors would agree that fuel reductions of 5 to 10 percentage points are (theoretically) feasible.