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The risk of vacuum loss in linear solar heat collection elements (HCEs) is one of the major practical challenges that are facing the parabolic trough solar concentrator (PTSC) systems in concentrating solar power (CSP) plants. Despite the long-life span of CSP systems, HCEs are frequently replaced due to permeated hydrogen (due to saturation of getters) or leaked air (due to failures in end metal-to-glass welding or cracked glass shells). This study proposes a low-tech solution for boosting the performance of PTSCs with partial or lost vacuum through rotating the HCEs. A carefully validated integrated 3D optical-thermal model is developed and used to quantify the energetic and exergetic performances of PTSCs with rotated and fixed HCEs for both cases of lost and maintained vacuum. Both steady and unsteady simulations are reported using high-precision ground measurements of direct irradiance and meteorological parameters. As the rotational speed increases from 0 to 20 rad/s, the energy efficiency of PTSCs with new and damaged HCEs is improved by ∼47 and 52%, respectively, at a Reynolds number of 4000 and an inlet temperature of 350 °C. By rotating a damaged HCE at only 10 rad/s, maximum improvements of ∼26 and 53% in the useful heat gain are obtained, compared to non-rotating evacuated and non-rotating damaged HCEs, respectively, at the same operating conditions. Overall, the advantage of the proposed concept is more pronounced when the PTSC is operating at relatively low flow rates, high temperatures, or under high irradiance levels. [Display omitted] [ABSTRACT FROM AUTHOR]

This study investigates the performance enhancement of parabolic trough concentrators (PTCs) with transparent insulation material shell (TIMS) of different diameters and thicknesses integrated into evacuated and non-evacuated heat collection elements (HCEs). The results show that non-evacuated TIMS-HCEs have higher energy and exergy efficiencies by up to 62.4 and 63.2%, compared to conventional evacuated HCEs, and by up to 109.2 and 110.9%, compared to conventional non-evacuated HCEs. At high flow rates and low fluid temperatures, the energetic performance of a conventional evacuated HCE is only 6.3% higher than that of the modified non-evacuated one. The modified design also enhances the circumferential temperature uniformity and shows a marginal drop in efficiency when the vacuum is lost. By simulating the PTC performance during four typical days, and without vacuum, the proposed design increased the daily useful heat gain by up to 1.36 and 5.64 kWh, compared to conventional evacuated and non-evacuated HCEs, respectively. Hence, it is proposed as a low-tech alternative to HCE evacuation and as a method of boosting the performance of PTCs operating at low flow rates and high temperatures. [Display omitted] • Receiver of parabolic trough collector is enhanced using transparent insulation. • An MCRT-FVM model is developed for evaluating conventional and modified designs. • Without vacuum, the modified design outperforms a conventional evacuated one. • Energy and exergy efficiencies are enhanced by up to 62.4 and 63.2%, respectively. • The modified design shows better temperature uniformity with no consumed power. [ABSTRACT FROM AUTHOR]

Enhancing the thermal efficiency of parabolic trough collectors (PTCs) is essential for establishing CSP as a sustainable technology. This study proposes a simple procedure for evaluating, predicting, and optimizing the energetic and exergetic performances of PTCs operating with nanofluids. A coupled optical-thermal model is developed to simulate the turbulent flow of three common synthetic oils (Therminol VP-1, Syltherm 800, and Dowtherm Q) mixed with different nanoparticles (Al 2 O 3 , CuO, and SiO 2) with different concentrations, under typical operating conditions of PTCs. The simulation results are fed to a soft-computing algorithm to develop prediction models that act as fitness functions in the multi-objective optimization process. For the considered range of input parameters and by assigning equal weights for the two optimization objectives (energy and exergy efficiencies), optimal design conditions corresponded to a PTC operating with CuO/Dowtherm Q nanofluid (volumetric concentration of 0.243%), at a direct irradiance level of 1000 W/m2, an inlet temperature of 240.793 °C, and a Reynolds number of 2.915E+05. These conditions led to energy and exergy efficiencies of 69.913 and 32.088%, respectively. The proposed procedure is described in detail to facilitate its adaptation and extension to other nanofluids, operating conditions, or other concentrating solar collectors. Image 1 • A procedure is proposed for optimizing the performance of PTC with nanofluids. • Optical, thermal, prediction and optimization models are developed and validated. • 3840 combinations of nanofluids and operating conditions are studied. • Optimal solutions of the multi-objective optimization problem are presented. [ABSTRACT FROM AUTHOR]