Concentrated Solar Thermal Plant for Future Fuels Production

Abstract: Concentrated Solar Thermal technology (CST) is a very promising renewable energy technology and has a broad range of use. Conventionally, CST systems are mostly used for power generation according to the Rankine cycle and thus often referred to as Concentrated Solar Power (CSP). In this present study, the solar heat is utilized to drive a thermochemical redox cycle of a metal-oxide in order to produce synthetic gas, a combination of hydrogen and carbon monoxide. Later, the synthetic gas is converted into usable liquid fuel whereas the production pathway is CO2 free. This thesis focuses on the process modeling and economic evaluation of solar-driven future fuels production plants. Four future fuels have been selected and modeled using commercial simulation software Aspen Plus®. These 4 future fuels are syngasoline, syndiesel, ethanol and methanol where they can be seen as a very good substitute for current transportation fuels. The heat required at high temperature is delivered using concentrated solar thermal technology with tower configuration for which the heliostat field is designed using in-house software HFLCAL developed by DLR. Syngas is converted into aforementioned fuels using either Fischer-Tropsch or plug-flow reactor. The reactor is modeled taking into account the kinetic of reaction for each fuel, while in case of the absence of kinetic, a stoichiometric approach is implemented. To analyze the hourly plant’s performance, a quasi-steady state analysis is done within MATLAB® environment. The metric used to evaluate the plants are production cost in €/L and overall thermal efficiency. The results show that aforementioned conversion pathway yields higher production costs compared to current market while the lowest production cost is obtained for Methanol at 1.42 €/L. It is shown that solid to solid heat exchanger (STS) efficiency plays a major role in order to make the plant more economically viable. Combining electricity supply of Photovoltaic (PV) and CSP is also shown to be one way to reduce the production cost. If the plant combines PV-CSP is used as the electricity source, syngasoline emerges to be the closest proposed plant to current market fuel production cost with a production cost of 5.99 €/L at the base case scenario which corresponds to 622% relative difference with current market’s production cost and 2.87 €/L at the best case scenario which corresponds to 245% relative difference with current market’s production cost. At the base case scenario, the highest overall thermal efficiency is obtained for the syngasoline plant (4.05%) and at the best case scenario for the ethanol plant (9.2%).