Performance prediction of solarized CC power plants operating in a load-following strategy

G Franchini, A Perdichizzi, S Ravelli
The paper reports on simulations and performance prediction of an Integrated Solar Combined Cycle (ISCC) plant operating in a load-following strategy. The ISCC plant was assumed to operate in an isolated or weakly interconnected grid, matching the electricity demand profile of a mid-size community in South Africa (Upington area). Two different profiles, with the same peak level, have been taken as reference for Industrial and Domestic demand, respectively, in a typical winter/summer day. The
more » ... er block is based on Siemens SGT-800 gas turbine (rated 50.5 MW at ISO conditions) coupled to a dual pressure HRSG producing steam for the bottoming Rankine cycle. Additional steam is produced by a solar loop, sized for about 20% annual solar fraction, according to solar radiation availability. Performance of two different Concentrated Solar Power technologies was evaluated: i) Parabolic Trough Collectors (PTC) with thermal oil and ii) heliostats and Central Receiver System (CRS) with molten salt as heat transfer fluid (HTF). In the PTC case, steam is produced at slightly superheated condition, due to the maximum oil temperature of 392°C imposed by thermal oil degradation. The solar tower configuration ensures higher steam temperatures thanks to the usage of molten salt. The load following strategy prescribed that the GT load is adjusted hour-by-hour to match the instantaneous power demand, including both the solar contribution to the total amount of energy generated and the influence of ambient temperature and relative humidity on ISCC power output. A computing procedure was developed by integrating commercial codes (Thermoflex for the power block and TRNSYS for the solar field) to predict thermodynamic performance at nominal and part load conditions, including the off-design behavior of all main components (GT, HRSG, steam turbine, air condenser). Simulation results demonstrated that both the investigated ISCC configurations are suitable for load following operation, with a significant fossil-free electricity production positively affecting the power plant efficiency. In addition, features of PTC vs. CRS were included in the discussion.