# An Innovative Parabolic Trough Collector Design with a Twin-Cavity Receiver
## 1 Introduction  
The parabolic trough collector (PTC) is the best representative of concentrating solar thermal systems and can be utilized in an extended range of applications. A PTC can operate within a wide temperature range, from low temperatures even up to 600 \({}^{\circ}\)C, using a large variety of working mediums, from simple water up to sophisticated thermal oils, molten salts, and nanofluids, for ensuring the greatest performance in each individual case. The most well-known areas where a PTC could contribute are electrical power generation, solar cooling (with sorption machines), heating desalination, oil recovery, and dehumidification [1, 2, 3]. The vital need for enhancement of PTCs\({}^{\prime}\) thermal efficiency has led, over the years, to the development of several different methods and techniques. Typical methods examine the flow regime in the tube, and there has been a great effort to affect and modify positively this regime in order to achieve greater thermal efficiency. This modification happens with the integration of flow inserts as metal fins and foams, as well as twisted tapes and perforated plates, with the use of sophisticated working mediums such as nanofluids and with the use of specially treated tubing systems such as ribbed and corrugated tubes. [PERSON] et al. [4] studied the effect of a wavy metal strip on the interior of a PTC tube, and it was revealed that thermal losses were reduced by 18%, which led to greater thermal performance. In another work, [PERSON] et al. [5] examined a PTC with a novel insert with a star form integrated inside the absorber-flow tube and found that the maximum possible enhancement did not exceed 1%. In the study of [PERSON] et al. [6], copper foam was appliedinside the PTC receiver tube, and this had a significant effect on the convection regime between the receiver and the working medium. In particular, the use of copper foam led to a remarkable enhancement of the Nusselt number, which in turn led to enhanced thermal performance. Moreover, [PERSON] et al. [7] introduced parallel perforated plates at the inner part of the receiver of a PTC in order to improve thermal efficiency. More specifically, the diameter and the distance between the plates, as well as the slope of the latter were optimized, and it was found that the greater the distance between the plates the higher the Reynolds number and thus the intensity of the convective regime. In the study of [PERSON] et al. [8], a twin-twisted tape was inserted in a PTC, and the flow state was examined. It was found that there is an important effect on the formation of the flow from the geometrical characteristics and the orientation of the tapes (swirling direction). Furthermore, [PERSON] et al. [9] studied a nanofluid-based PTC with wavy configurations at the inner walls of the tube. From the simulation, it came out that in comparison with a typical receiver tube, the thermal performance of the proposed PTC could be slightly enhanced. [PERSON] et al. [10] examined the effect of a couple of twisted tapes inserted in the absorber of a PTC by using a hybrid nanofluid. In this analysis, various tape diameters and several swirling orientations were tested. It was revealed that the performance of the collector increased when the twin tapes achieved a specific swirling orientation and when the concentration of the nanoparticles in the working medium took higher values. In the study of [PERSON] et al. [11], the thermal performance of a PTC was set under investigation by applying Al\({}_{2}\)O\({}_{3}\)/H\({}_{2}\)O and Fe\({}_{2}\)O\({}_{3}\)/H\({}_{2}\)O nanofluids and varying the particle concentrations. It was found that the utilization of nanofluid improves the efficiency of the collector, especially when Al\({}_{2}\)O\({}_{3}\)/H\({}_{2}\)O is applied and when the concentration of the nanoparticles increases. As far as the highly concentrating nanofluids, there are many systems that have been examined numerically, and remarkable thermal efficiency enhancements have been observed.