Numerical simulation of thermal fluid dynamics of line-focused solar power plants with direct steam generation
Nowadays the renewable energy sector is growing extensively. In the field of the concentrated solar power (CSP) a common technology is the parabolic trough. It is a line-focused solar power plant which reflects the sunlight on a straight absorber tube. The heated fluid in the absorber in the primary circuit is used to produce steam in the secondary circuit. Next the steam drives a steam turbine and generator for electricity production.
The biggest advantage of CSP in contrast to solar photovoltaic is the possibility to use thermal energy storage systems. Therefore they can provide base load electricity even after sunset.
At the moment the most commercially available line-focused solar power plants are operating with synthetic oil as heat transfer fluid. But synthetic oil is very expensive due to higher investment costs of the primary circuit and necessary periodical substitutions of the oil. In the end the additional heat losses of the heat exchanger between primary and secondary circuit and the limited temperature of 400 °C of the oil decrease the overall efficiency of the solar power plant.
Therefore other technologies are being developed. One option is the replacement of the synthetic oil by water and with that the removal of the entire oil circuit. In the so-called direct steam generation (DSG) process water directly evaporates in the absorber under high pressure and drives a steam turbine. Especially the better steam parameters pressure (110 bar) and temperature (up to 500 °C) increase the efficiency of the steam turbine. The challenging task of DSG process is the elaborateness of the system which contains a multiphase flow and needs a carefully thought out control system.
The computer code ATHLET (Analysis of Thermal-hydraulics of Leaks and Transients) from the field of reactor safety analysis will be used for the simulation of the thermal fluid dynamics. It was developed for the investigation of the coolant systems of a nuclear power plant and is well validated. Nevertheless the code is applicable to non-nuclear systems.
This thesis should enable a reliable and accurate simulation of parabolic troughs with DSG. The following contents will be considered:
- Validation of ATHLET for solar power plants by usage of experimental data
- Simulation of different modes of operation of a test facility
- Detailed analysis of the heat transfer und thermal fluid dynamics in the absorber (vaporisation point, transient behaviour)
- Simulations with varying boundary conditions (start-up simulation, changes of the direct normal irradiance)
- Derivation of suggestions for a proper control system