The gas turbine combined cycle (GTCC) is the most efficient thermal power generating cycle in use today. The power generation industry has extensive experience with GTCC design, engineering, financing, and operations for plants from the MW to the GW scale. GT OEMs have continued to improve GTCC efficiency by developing materials and manufacturing processes that allow increases in firing temperature while minimizing cooling requirements. Today’s most efficient gas turbines have firing temperatures around 2600 F / 1425 C and when used in combined cycle can offer up to 60% net GTCC efficiency. Further advances will push performance to higher levels.
Solar heated thermal cycles that add heat to working fluids at higher temperatures, naturally lead to plant designs with higher efficiencies. Higher efficiencies translates to smaller solar fields for the same power, and potentially lower specific plant costs.
Solar heated gas turbines in combined cycle, where heat is added to air exiting the compressor in place of, or as a preheating step before the combustor, make better thermodynamic use of the solar heat than Rankine cycles with direct or indirect steam generation.
In the 2000-2005 timeframe, the European SOLGATE project contributed to the development of solar heated gas turbines. Presently, the European SOLUGAS project is carrying on with this goal using a Solar Mercury 50, a commercially available 5 MW class gas turbine. Successful development of solar heated gas turbines will yield cost-effective utility-scale, dispatchable power systems that promise to reduce system size and cost relative to other solar thermal designs.
Solar’s Mercury 50 machine has been in commercial power generation service for over ten years. More than 40 units have been sold accumulating more than 350,000 fleetwise operating hours. The machine uses a recuperator to preheat compressed combustion air by cooling the engine’s exhaust stream. This feature improves simple cycle engine efficiency. It also makes this engine an ideal platform for SOLUGAS. The engine is flange-ready to deliver all compressed air to a heat exchanger, and flange-ready to take heated air back to the combustor. The SOLUGAS project will use a solar receiver in place of the recuperator to preheat compressed air. This will result in hotter engine exhaust for use in a future bottoming cycle.
Models were built with THERMOFLEX to analyze the base Mercury 50 performance in detail, and the modified Mercury 50 that uses solar heating. GT PRO was used to quickly compare how both machines perform in combined cycle.
Traditionally, simple cycle engine performance is reported at ISO ambient conditions, burning pure CH4, with no inlet or exhaust losses. The THERMOFLEX model of the engine at ISO conditions is shown below. Summary of ISO performance for the recuperated Mercury 50 engine is:
4575 kW at generator terminals
38.5% LHV efficiency at generator terminals
39.2 lb/s exhaust flow
718 F exhaust temperature
5095 BTU/s in recuperator
SOLUGAS modified design uses a solar receiver in place of the recuperator. In this configuration, the simple cycle performance is:
4844 kW at generator terminals
40.7% LHV efficiency at generator terminals
39.2 lb/s exhaust flow
1173 F exhaust temperature
5095 BTU/s solar heat input
The power increase comes from the 1 psi (7%) reduction in exhaust pressure because the exhaust gases don’t flow through the recuperator. The engine exhaust temperature is higher because it’s from expander exit directly. The LHV electric efficiency increase is due to a combination of the power increase and the “fuel-free” nature of the solar heat input. It’s very important to remember that while the solar heat input is “fuel-free”, it is by no means “free-free”. Neither capital, nor operating costs for the solar system are accounted for in the LHV efficiency.
If solar heated gas turbines can be successfully commercialized at utility scale, they will be used in combined cycles to maximize the efficiency and minimize plant specific size and cost. It’s likely that these systems will be significantly larger, use ground-based gas turbines with reheat combined cycles, and probably use molten salt as an intermediate storable heating media.
However, for this scale system a comparison of GTCC performance was done assuming two-pressure non-reheat combined cycle with a cooling tower. Combined cycle calculations were done using GT PRO to make use of its extensive automation which supplies reasonable assumptions for most every input used in this up-front scoping level study.
The recuperated engine has a significantly lower exhaust gas temperature leaving the gas turbine. For an unfired HRSG, this leads to a combined cycle with a low steam temperature at the turbine, and a relatively high stack temperature. In contrast, the solar heated engine has a high exhaust temperature, potentially suggesting a reheat bottoming cycle although not for this scale plant. The higher exhaust temperature allows higher steam turbine inlet temperature, better heat recovery and consequently better overall efficiency.
Recuperated engine in combined cycle summary performance is:
5295 kW net power
45.4% net LHV electric efficiency
SOLUGAS machine in combined cycle summary performance is:
7582 kW net power
64.3% net LHV electric efficiency
The extra power is mostly from the steam turbine, and the net LHV efficiency boost includes “fuel-free” solar heat input.
GT PRO result for the SOLUGAS engine in a two-pressure non-reheat combined cycle. At ISO, this design makes 7.6 MWnet at LHV net electric efficiency of 64.3%, considering the “fuel-free” solar heat input.