Theoretical and experimental studies for improving the design of the solar field and organic Rankine cycle turbine in a small linear Fresnel reflector solar thermal power plant

Providing sustainable, cost-effective and environmentally friendly energy for consumer societies and industrial economies has been a major concern for industrialized and developing countries. For that reason, there is a renewed interest in the generation of energy from various solar technologies. Among others, Concentrated Solar Power (CSP) technologies has the potential to meet such demands. However, most recent solar energy harnessing technologies require substantial energy to attain efficient power production with compact plant size and the least payback time. Linear Fresnel coupled with organic Rankine cycle solar thermal power plant may prove to be a promising choice due to its capacity to overcome techno-commercial constraints related with conventional reflector based CSP Technologies. Theoretical and experimental studies for improving the design of a solar field and organic Rankine cycle turbine in a small Linear Fresnel Reflector solar thermal power plant is performed in this study. In the initial stage, the design and optimization of the 3D optical model of the LFR solar field is presented in an attempt to minimize the drift and variation in ray concentration and improve the optical performance. In the solar field optimization, key variables such as the mirror curvature, width, length, the distance between consecutive mirror centre lines and height were selected. Subsequently, a Monte Carlo Raytracing and thermal analysis were performed to investigate the impact of the optimized mirror elements on the optical performance of the solar field. A comparative analysis between two LFR configurations, Central LFR (CenLFR) and Compact LFR (ComLFR) is put forward by adopting a similar approach. Furthermore, a small-scale organic Rankine cycle turbine used for low-temperature applications capable of generating electrical power was theoretically and experimentally investigated. A single-stage axial turbine expander deploying R365mfc, and the new environmentally friendly Novec649 organic working fluids were selected. Modelling of the turbine and comparative analysis of the two working fluids is performed adopting a simple CFD approach proposed. The effect of the range of inlet definition variables such as temperature, pressure, rotational speed and key thermodynamic properties of the fluids on the work output and isentropic efficiency as well as the influence of rotor tip clearance (rotor gap) on the turbine power were investigated and analysed. In the closing stage, the shading analysis of the solar field and environs is performed using different approaches. In this context, shading resulting mainly from structures such as buildings and vegetation is considered. The analysis considers sun and shadow effects that can be easily and dynamically improved or even animated within the program to evaluate the timing and effect of obstructions and the resulting consequence on the optical performance of the solar field. The numerical approaches were validated with optical and thermal experimental data gathered from a linear Fresnel plant erected in Almatret, Spain. Results show a good correlation between the numerical approach and experimental study. Findings from the solar field study show that optimising key mirror elements such as the curvature, width, length, receiver height from the mirror plane, and the distance between two consecutive mirror centrelines can significantly impact the LFR solar field optical performance. This leads to an improved concentration factor which can enhance the energy conversion efficiency of LFR plants and greatly minimize the cost of thermal storage, which results in a low Levelized cost of electricity (LCOE) and offers LFR the economic potential to compete with other CSP power plants. Next to that, results of the comparative analysis show minimized drift in ray concentration and the computed energy efficiency for separate mirror elements, and the overall solar field show improved optical performance for the central configuration. Despite blocking and shading effect minimized in the compact configuration, findings show lower optical efficiency, mainly due to the receiver being fixed and its distance away from the primary mirrors. In both solar field studies, it was observed that losses are greatly influenced by the solar field orientation. As per the ORC turbine, it was observed that the inlet turbine temperature and pressure have the greatest effect on the power, work output and isentropic efficiency. The selection of an organic working fluid and its application in ORC turbine is a crucial aspect mainly due to the dependence of its categorization on the temperature of the heat source, defined by the fluid thermodynamic and thermophysical. As expected, the computed peak power output is generated by the "ideal" turbine expander design with zero clearance of blade tips. Exceeding the 200 μm rotor gap results in a sharp detrimental effect on the turbine performance. Shading analysis was found to be a fundamental step in the phase of design, installation and operation of a solar field. Shading of any form can have a negative influence on the performance of an entire solar field. Such estimations are significant, especially when designing collectors for places where the available land strip does not align with a particular orientation, as in the case of the north-south configuration.