Design, Modelling and Optimisation of Optical Systems for High-Temperature Concentrating Solar Applications

Abstract

Emerging high-temperature solar thermal and thermochemical systems aim to operate efficiently at temperatures above 1000 degree C, necessitating solar concentration ratios typically above 1000 suns. In research and development applications, the required high concentration ratios can be obtained by dedicated on-sun and indoor facilities such as solar furnaces and high-flux solar simulators (HFSSs), respectively. For large-scale solar thermal and thermochemical plants, the requirement of high concentration ratios imposes great challenges on optical concentrators. Point-focusing optical concentrators, including solar dishes and central receiver systems (CRSs), provide higher concentration ratios in the range of 500-10,000 suns than the concentration ratios of 2-100 suns provided by line-focusing optical concentrators such as parabolic troughs and linear Fresnel mirrors. Solar dish systems are capable of producing the required high concentration ratios above 1000 suns, however, their relatively small power output from each module limits the system economics. CRSs with heliostats as the primary optical concentrators present higher power output but restricted concentration ratios up to 1000 suns. Hence, optical concentrating systems that offer both high concentration ratios and high power output need to be explored.

In this work, design, modelling and optimisation of the primary and secondary solar concentrators are performed for the advancement and realisation of economically-feasible concentrating solar technologies. Two types of systems are investigated: (i) experimental-scale HFSSs-based systems and (ii) commercial-scale CRSs. For maximising the design freedom, two-dimensional analytical ray-tracing and three-dimensional collision-based Monte-Carlo ray-tracing programs are developed in house for simulating optical systems involved in this work.

For the HFSSs-based experimental system, reflective optics in the shape of flat, ellipsoidal, hyperboloidal, paraboloidal and the compound parabolic concentrator (CPC) are found to be capable of modifying the characteristics of the HFSS radiative output beam such as concentration ratio, ray distribution and axis direction. The modification of the HFSS output beam characteristics enables the application of HFSSs to meet specific requirements of experimental testing on material, device prototypes and so on. For commerical-scale CRSs, we investigate and optimise their optical, energetic and economic performance at two power levels: a polar-field system and a multi-aperture system with multiple sub-fields. Receiver performance is evaluated for receiver temperature ranging from 600 K to 1800 K. The working temperature thresholds at which the energetic and economic performance benefit from the addtion of a CPC are identified as 900 K and 1200 K, respectively. The increase of the number of apertures from one to four increases the maximum net receiver power from 110 MW to 320 MW. The use of more than four apertures results in only limited further gain of the net receiver power but significantly decreased overall optical efficiency and solar-to-thermal efficiency.

Optical concentrators are of paramount significance for the realisation of economically-feasible solar thermal or thermochemical technologies due to the substantial costs of optical concentrators which occupy about 30-50% of the total plant costs. Besides, optical approaches, particularly system-level design, have the great potential to assist in solving the challenges imposed by high-temperature thermal and thermochemical applications.