On the optimum integration of solar energy into natural gas combined cycles

Concentrating solar power plant projects have been rapidly increasing over the last few years driven by the advancements in the solar technology. While the early power plants were mainly based on parabolic trough collectors using thermal oil as heat transfer fluid, a significant number of large scale power plants either built recently or in the development phase use linear Fresnel collectors and solar tower technology. Moreover, molten salts and water/steam are often preferred as heat transfer fluid. The operational issues associated with the variable nature of solar energy could be overcome by integrating the solar input into a fossil-fuelled power plant. In this paper the integration of solar energy into the bottoming part of a three pressure levels natural gas combined cycle having a rated power output of 400 MW is analyzed. The three main concentrating solar power technologies, namely parabolic trough, linear Fresnel and solar tower are considered in the search for the optimum integration. Detailed models of the combined cycle and solar field are built in the Thermoflex® environment and the performance of different systems configurations is evaluated. Results show how the position of solar heat addition affects the heat absorption in the heat recovery steam generator (HRSG) and, in turn, the overall plant performance. A proper integration makes the solar heat available at moderate/high temperatures while simultaneously generating heat sinks at high temperatures in the HRSG. The best integration options increase the average temperature of heat absorption in the HRSG and, by improving the thermal matching with the flue gases, decrease the irreversibility of the combined cycle. Accordingly, high solar heat to power conversion efficiencies in the 40-50% range are achieved even at moderate solar heat temperatures due to the favorable synergies in the integrated plant.