A Reassessment of the Alternative Regeneration Cycle

Two prior papers and several patents have considered improvements to a gas turbine engine’s cycle efficiency by using two turbines in series with an intermediate heat exchanger that preheats combustion air. This approach allows heating the combustion air to temperatures higher than those that can be achieved with “conventional regeneration” in which the combustion products are fully expanded across a turbine before any heat recovery. Since heat addition in the combustor of the “alternative regeneration” cycle occurs at a higher average temperature, then under certain conditions the cycle efficiency can be higher than that available from a cycle using conventional regeneration. This paper reconsiders the usefulness of the alternative regeneration cycle with more detailed modeling than has been presented previously. The revised modeling shows that the alternative regeneration cycle can produce efficiencies higher than conventional regeneration, but only for a more limited set of conditions than previously reported. For high-technology engines operating at high temperatures, the alternative regeneration cycle efficiencies can be three to four percentage points better than comparable conventional regeneration cycles. For lower-technology engines, which are more typical of those currently installed, improvements in efficiency only occur at lower values of heat exchanger effectiveness, which limits the usefulness of the alternative regeneration cycle. Also considered is an extension to the cycle that employs a second heat exchanger downstream of the second turbine for the purpose of further preheating the combustion air. In its optimum configuration, this “staged heat recovery” can produce additional small improvements of between 0.3 and 2.3 percentage points in cycle efficiency, depending on the particular cycle parameters assumed.