# An Innovative Parabolic Trough Collector Design with a Twin-Cavity Receiver
## 1 Introduction
However, in real operating conditions, agglomeration issues prevent such an enhancement, and thus the operation of the systems could be affected negatively [12]. Another important aspect, which should be taken into consideration in the PTC thermal efficiency enhancement field, is the design of the receiver and the treatment of its surface. Typical PTC absorbers are usually coated by solar selective materials that ensure significantly high solar absorbance by almost eliminating thermal emissivity. Nevertheless, such coatings are accompanied by special materials and high amounts of energy to be consumed for the production and deposition process. These factors increase remarkably the cost of production and the environmental impact. Hence, it is important to avoid these disadvantages and to find alternative solutions to obtain the desired result. The linear cavity receivers seem to be a novel idea, since they could cooperate with low-cost and environmentally friendly coatings with medium solar absorbance in the range of 0.75 to 0.85 [13; 14] by ensuring significantly higher absorbance due to the solar irradiation entrapment [15]. However, only a few studies have been conducted in the field of linear cavity receivers, and most of them are regarding V-cavities [16; 17; 18; 19; 20; 21; 22; 23; 24]. For instance, [PERSON] et al. [21] conducted experiments and examined the operation of a novel V-type cavity PTC working with thermal oil. The receiver was insulated with thermal insulation, and the aperture of the receiver was covered by glass. The collector appeared to have a sufficiently high thermal efficiency. The linear circular cavity receiver, which is in the main scope of the present work, has not been examined widely. [PERSON] and [PERSON] [25] were the first who developed two semi-empirical formulas for estimating the equivalent absorbance of linear circular cavity receivers by taking into consideration the multiple reflections that occur in the cavity. The same research team [15] examined a small PTC with such a cavity receiver enclosed inside an evacuated tube, something that appeared for the first time in the literature. The proposed collector was found to exceed the conventional PTC configuration optically and thermally by 13.0% and 12.2%, respectively. Moreover, in Ref. [26], a PTC with a partially-evacuated cavity receiver was compared with a conventional design, and it was revealed that the cavity design had greater thermal efficiency. In this work, a new small-scaled PTC with a twin-cavity receiver (PTC-TC) was examined thoroughly and compared with a designed flat receiver PTC (PTC-F). Both designs are novel and are promising solutions, but the twin-cavity idea is a more promising one due to its advanced design. Specifically, the twin-cavity design presents high novelty due to the special design that leads to both high thermal and optical performance, exploiting the advantages of the cavity receivers on a linear concentrating technology. The comparison was conducted regarding thermal and optical efficiency, while the PTC-TC geometry was firstly optimized. In the optimization procedure, several geometrical parameters were taken into account in order to achieve the desired result. Then the two collectors were inserted in the SolidWorks Flow Simulation environment, and two different numerical models were developed. Several different mesh grids were examined in each model in order to ensure independence from the grid results. A similar procedure was performed for the flow rate, and the optimum one was determined for each configuration. Then, the two geometries were compared with each other for a wide range of inlet temperatures from 20 \({}^{\circ}\)C up to 200 \({}^{\circ}\)C. The thermal efficiency and the thermal losses were determined first. It was revealed that the proposed PTC exceeds the PTC with the flat receiver by 8% on average in thermal performance. Syltherm-800 was used as the working fluid in the collectors, while the latter were designed and simulated in SolidWorks.