Thermo-economic optimisation of large solar tower power plants

Given the growing number of solar tower power plants in operation, under construction and under planning, the assessment and the optimisation of their performance are required both at the energy level and the economic level. In other words, the same way as any other conventional power plant, large solar tower power plants have to convert the incoming solar radiation into as much electric power as possible, with as less money as possible on the long run. First, the relevant local environmental conditions are identified : obviously the solar direct normal radiation and the sun’s position over time, as well as the ambient temperature and the wind velocity and direction. Then the field of heliostat mirrors is created thanks to an algorithm that allows a compromise between the density of mirrors and the interferences occurring with each other. Using the Gemasolar set-up near Sevilla in Spain as a base case, the performance of the heliostat field is simulated over three specific days and interpolated over the entire year. As a result, the annual output of the central receiver is obtained and implemented as the input for a storage system and a conventional heat-to-electricity conversion cycle. A specific operating strategy provides the overall energy performance of the plant. In parallel, the incident flux distribution on the receiver is also simulated to identify peaks and transients, especially due to clouds. Two multi-aiming strategies are investigated and allow to decrease the peaks significantly without affecting too much the total power. Subsequently, the economic performance of a solar tower plant is assessed thanks to a detailed investment cost analysis and with an estimate of financial indicators such as the levelised electricity cost and the project net present value. A thermo-economic optimisation allows then the description of optimal trade-off set-ups in comparison to the Gemasolar base case. The key decision variables are given by a sensitivity analysis that already shows a set of potential efficiency and cost improvements. The optimisation itself leads to even greater potential improvements of 24 points in field efficiency and 9 [¢/kWhel] in levelised electricity cost. Furthermore, with variable ranges limited to the parameters of Gemasolar, the best equivalent plant set-up shows a 40% smaller land area. After that, multi-tower set-ups are proposed and also implemented in the optimisation, which highlights the optimal single- to multi-tower transition size. Finally, a combination of a parabolic trough collector field and a heliostat field in the same plant is studied and turns out to make a further cost decrease possible.