This article investigates the influence of GT Model and Spool Arrangement on solar hybrid combined cycle (CC) performance. To investigate the way the GT interacts with the solar system, two commercial codes were used. A “user-defined” GT model was first developed on the basis of design conditions performance, then carefully calibrated against manufacturer data to accurately predict its off-design behavior depending on load and ambient temperature conditions. To assess the influence of GT model and spool arrangement on the solarized CC performance, the following GTs were selected: the single-shaft Siemens SGT800 and two two-shaft engines: the heavy-duty GT Siemens SGT750 and the aeroderivative GE LM6000. It was observed that the single-spool SGT-800 assures the lowest power penalty. The SGT-750 appeared to provide the highest fuel saving in the middle of the day, both in winter and summer, and the highest solar fraction as well.
Power generation by Concentrated Solar Power systems is a way to reduce dependency on fossil fuels and at the same time to accomplish the international commitment of CO2 reduction. First attempts to integrate large scale power plants with concentrated solar systems were carried out in the Solar Electric Generating Systems like those operating since 1985 in California, USA. Solar energy is used to produce superheated steam on a 300-500°C temperature level depending on the used thermo fluid (oil, air, molten salt or direct steam). This steam is eventually combined in an hybrid power plant with the one produced in a conventional steam generator, prior to expansion. This solution was also proposed for Combined Cycle (CC) applications under the name of Integrated Solar Combined Cycle systems. In integrated solar cycles, thermal energy is usually introduced into the steam cycle at 500-600°C. A reasonable value for peak thermal to electric efficiency in medium temperature steam cycles is about 37%. In the last decade a large effort was made to develop high temperature and high pressure central receivers in order to increase the temperature level at receiver outlet up to 800°C - 1000°C.This higher temperature level is expected to allow the integration of solar systems with gas turbine (GT) power plants. Significant improvements in solar-to-electricity efficiency can be gained by supplying thermal energy directly to the topping Brayton cycle. A higher efficiency in solar power generation is the most valuable advantage of hybrid solar gas turbine technology, compared to other solar-fossil hybrid solutions.
A hybrid solar gas turbine uses concentrated solar power to preheat pressurized air exiting the compressor before entering the combustion chamber. The solar tower technology allows for very high concentration ratios to achieve the highest tolerable receiver temperature. A two axis tracking heliostat field collects and concentrates the solar radiation onto an absorbing device. The key component in the solar system is the receiver, where the concentrated solar power energy is absorbed and transferred to pressurized air. Hybrid solar gas turbine systems have reached a demonstration level: a 230 kWe unit has been solarized and tested in the EUfunded Solgate project. The technical feasibility of solar energy generation was successfully established also with 65 kW Capstone Microturbine. A crucial point is certainly the development of high temperature solar receivers. Even though prototypes at 30 bar with aperture radiation flux of up to 10 MW/m2 have been shown to reach 1300°C exit temperatures, more work needs to be done to get a reliable operation state.
To investigate the hybridized solar CC power plant, and particularly the way the GT interacts with the solar system, two commercial codes were used. The simulation environment TRNSYS® with the model library STEC (developed by DLR) was used to model the heliostat solar field and the solar tower receiver. The gas turbine model as well as the heat recovery steam cycle were modeled by means of Thermoflex®. A “user defined” GT model was first developed on the base of design conditions performance, then carefully calibrated against manufacturer data to accurately predict its off design behavior depending on load and ambient temperature conditions. Once calibrated, the GT model was integrated in the CC power plant and coupled with the solar system. Pressure and heat losses between compressor and combustion chamber were explicitly taken into account. The hourly performance of the hybrid solar plant was calculated by making TRNSYS® and Thermoflex® to interact each other in an iterative way. This method can be easily adapted to different gas turbines since Thermoflex® permits to set “user defined gas turbine” components.
This modeling tool was applied to study three solarized hybrid GT models operating in a two pressure levels combined cycle (Figure 1). To assess the influence of GT model and spool arrangement on the solarized CC performance the following GTs were selected: the single shaft Siemens SGT800 and two two-shaft engines: the heavy-duty GT Siemens SGT750 and the aero derivative GE LM6000. Four ambient temperature profiles have been selected, one representative for each season in a high solar potential site. For each GT model, the solar field was designed so to obtain a receiver air outlet temperature of 950°C in the hottest hour of the year.
The simulation tool was able to predict on an hourly basis the variation of parameters like the compressor pressure ratio β, the GT inlet flow and the normalized GT free spool rotational speed influencing the hybrid GT behavior (Figure 2 - continuous lines refer to the pure fossil reference GT model). Single spool SGT-800 engine responds to solarization by slightly increasing the compressor pressure ratio, since the inlet air mass flow is governed only by the ambient conditions. When considering SGT-750 two-spool engine with a power turbine, the expansion ratio of the core engine turbine reduces. So the free shaft speed n decreases to guarantee the shaft power balance. This in turns produces a reduction in the inlet air mass flow. Finally, GE LM6000 PF engine combines both control strategies. Because of solar heating, HP shaft speed decreases slightly, leading to a small reduction in the compressor pressure ratio. This combines with the reduction in compressor inlet air flow. Note also that shaft speed variations for this engine are smaller than for SGT-750 model, resulting in a more stable behavior against variable solar input.
The solarized CC performance was then evaluated in terms of power production penalty, fossil fuel saving and solar fraction due to solarization (Figure 3 - typical Summer day). During the night, the pressure drop across pipings and receiver counteracts the beneficial effect of a decreasing ambient temperature on power production. This results in a power penalty in the range between 2% and 4% depending on the GT model. During the day, for hours between 9 and 19, the increase in the ambient temperature is overlaid with the solar heating, thus leading to a more significant power penalty up to 12%. The single spool SGT-800 was found to assure the lowest power penalty while the SGT-750 appeared to provide the highest fuel saving in the middle of the day, both in Winter and Summer, and the highest solar fraction as well. Overall Hybrid CC performances were finally evaluated and compared with the standard CC, in terms of electric power and efficiency reduction, fossil fuel saving, and solar energy to electricity conversion efficiency. The modeling results here reported provide a glimpse into the solar hybrid technologies and their impact on the efficiency standards in power generation. SGT-800 GT was found to assure an annual solar fraction of 34.2 %, the smallest penalty in GT power production because of solarization (-18.4 GWh/year) and the highest solar energy to electricity conversion efficiency of about 52%.