26. Effects of Mirror Geometry on the Optical Efficiency of a Linear Fresnel Reflector (LFR)

The linear Fresnel reflector (LFR) is a concentrating solar thermal power (CSP) technology, which benefits from a relatively simple and low-cost design. It typically comprises of a single receiver tower and several linear mirror elements. In comparison to more conventional solar concentrators, such as the parabolic trough or solar power tower, the LFR has a lower optical efficiency. However, there are numerous parameters, which influence an LFR’s optical performance: mirror width, mirror spacing, mirror number, mirror curvature and receiver height. In many LFR designs, these parameters are not properly analysed with the solar field configuration generally being taken from a known solar power plant or prototype. One of the main reasons behind this is the complexity associated with ray tracing, which would be needed to investigate fully the optics of the system, including surface reflections and shadowing. Comprehensive optical analyses using Monte Carlo ray tracing techniques are both computationally and time intensive, but there is a growing set of ray tracing tools designed for CSP systems. This provides opportunities to achieve gains in efficiencies by investigating system performance for different mirror configurations and sun positions. In this study, SolTrace, a specialised software tool developed by the National Renewable Energy Laboratory (NREL), is used to evaluate the FRESDEMO pilot LFR plant located at the Plataforma Solar de Almería, Spain. The FRESDEMO plant comprises of 25 mirror element rows and covers an area of 2100 m². A number of simulations of the FRESDEMO plant have been carried out before, but this study investigates potential gains in optical efficiency that could have been achieved through varying mirror numbers and widths. Furthermore, the performance of flat mirrors versus variable curved mirrors is investigated. Optical errors are established and compared for a range of different configurations. An outcome from the study is a set of alternative design scenarios to increase the optical efficiency of the LFR, and practical implications of these changes are evaluated.