The latest airplane fan engines being discussed have exhaust temperatures of around 900o F for minimum fuel burn and maximum thrust for their high PRs. This level of exhaust temperature limits the steam temperature in a CC to about 850o F which is far lower than needed for maximum 1100/1100o F steam.
To overcome this shortcoming, the Combo 5 arrangement has been suggested by the author whereby 5 are units and 5 HRSGs produce 850o F steam at 1800 pig and only one of the 5 units is equipped with a supplementary fired super heater in front of its HRSG to superheat the all the steam to 1100/1100o F whereby the exit of the single supplementary fired super heater exhausts at the same 900o F level so as not to generate any additional steam which would reduce CC efficiency (Fosterpeg principle).
Thus, the higher steam temperature and lower TSRs allow the bottoming cycle to reduce the steam flow to the condenser and also generate more electrical power. The CC efficiency is thus greatly improved. The first law of thermodynamics is applied. The new fan engines with PRs up to 52 and relatively low exhaust temperatures of around 900o F have an avenue to provide high CC efficiencies – same as for the HDs.
Reheating for the new fan engines
As discussed earlier, the older GGs such as the 30 PR LM 5000 had a rather low expansion ratio of about 2.5. However, the latest fan engines with higher TITs and PRs under discussion when converted to industrial units would have expansion ratios of about 4 which are more suitable for rehearing. The GT 24 and 26s reheat at a PR of about 16. The most effective PR would be about 6 for a 36 PR GT (square root) whereas for the 52 PR R -R Treat 1000 would be about 7. The GT 24 and 26s reheat too early and the R-R Treat would reheat somewhat too late. But an expansion ratio of 4 would be an improvement over the old LM 5000. The higher expansion ratio of the Treat is primarily due to the much higher TIT and PRs involved.
A study by GE and R-R, and now P&W (MHI) could pin down what the prospects of RH might be for the newer fan engines. R-R over the years has made several studies on RH for their aero engines for The California Energy Commission. These studies were spearheaded by Jack James of the TCEC and Graham Reynolds of R-R. Such old studies should be updated in light of the new Treat 1000 52 PR fan engine. Coupled with reheat, R-R should consider CLSC.
Inter-cooling and reheat for the new fan engines
The GE LMS-100, a 100 MW IC gas turbine, has been quite successful in generating electrical power as backup for solar and wind. Now, a mechanical drive design is being considered for LNG plants. This unit has a cycle efficiency of about 44 percent using the older GG. It would be somewhat higher in efficiency and output with the newer 80 E1 GG.
The new fan engines by R-R, GE and P&W (MHI) with higher TITs and PRs offer a way to increase power output and cycle efficiency further. What would the improvements be for these new fan engines for the IC mode? The output could be more than 150 MW at 48 percent efficiency. In the CC configuration, the output would go way up, and if some of the IC heat loss could be converted to power, the CC efficiency could approach that of the HD simple cycle units. A lot of study work is suggested by the author for such units. It all depends on the supply of natural gas through fracking and the price of this gas.
MHI has a great opportunity to become a major player in the industrial aero gas turbine business and thus supplement its large HD sales.
Since Pratt & Whitney introduced the geared turbofan aircraft engine, the question has been what is the next generation aeroderivative? Ivan Rice explores that question in this series.