Evaluation of a Combined Cycle Setup for Solar Tower Power Plants

As our awareness of the dangers of global climate change grows, the development of new renewable energy technologies is of primary importance in the effort to reduce emissions of carbon dioxide and other greenhouse gases. Rapid increases in the price of oil and worries about the stability and security of the extraction of fossil fuels have lead to renewed interest in the development of local energy sources, thereby reducing dependence on foreign sources. Amid the plethora of option available, one of the more promising solutions is the increased use of concentrating solar thermal power. Solar thermal power generation shows great potential for supplying the base electricity needs of numerous countries in the sun-belt regions, stretching from the tropics to the Mediterranean. This has lead to a recent renewal in the development of such systems, most notably with the construction of the PS10 and PS20 central receiver power plants in Spain. However, all currently existing solar thermal power plants have been based around the use of Rankine or steam turbine cycles, which are limited in the efficiencies they can achieve. In order to make better use of the Sun’s energy, higher efficiency cycles should be used. Recent developments in the field of high temperature solar receivers [13] have made it possible to imagine the use of solar thermal energy to drive gas turbine units, potentially allowing the use of combined cycle setups in solar thermal power plants. In order to evaluate the potential improvement in efficiency that can be achieved by using combined cycles in solar thermal power plants, a detailed simulation of such a setup will be performed. By taking into account the requirements of thermal energy storage and the design of the necessary heat exchangers, an objective evaluation of the performance of the combined cycle can be obtained. As a key element in the energy conversion process, particular attention will be given to the modelling and simulation of the high temperature receiver. Due to the highly variable nature of the solar flux, the performance of the receiver will be evaluated over a range of operating points. In order to obtain a detailed breakdown of the sources of losses and irreversibilities within the system, a complete exergy balance will be performed on the power plant. Combined with the standard energy balance, this analysis will allow identification of the key areas for improvement in the design of solar thermal power plants. Finally, in order to determine whether such a setup is economically viable, the construction, operation and maintenance costs will be evaluated in order to predict the levelised energy cost associated with electricity production in a combined cycle plant.