1. Introduction
Renewable energy resources, such as solar energy, are considered an effective means to address the energy crisis, due to fossil fuel depletion, increased energy demand, and the environmental impact of CO2 emissions. Solar energy can be utilized for a wide range of applications, from domestic hot water and domestic heating and cooling to industrial applications and electricity production by ensuring lower conventional sources consumption and more useful energy production which leads to sustainability. The simplest way to exploit solar energy potential is by employing solar collectors, which can transform solar energy into heat, electricity, or both.
There are multiple types of solar collectors manufactured [
1] based on the required applications, namely flat plate collectors (FPC), evacuated tube collectors (ETC), parabolic trough collectors (PTC), compound parabolic collectors (CPC), Fresnel lens collectors (FLC), PV/T Collectors, and others.
FPC have been examined by many researchers. Kalogirou [
2] investigated possible configurations in order to optimize the system performance through modelling and simulation. The slope of the collector, several riser and tube diameters as well as distances between the top of the collector to the bottom side of the storage tank were analyzed. Korres and Tzivanidis [
3] performed a computational fluid dynamic (CFD) analysis on a flat plate collector with a serpentine flow system and examined the effect of the inclination angle and the effect of the natural convection in the air. Subiantoro and Ooi [
4] developed analytical methods for the calculation of the heat losses from the top of the collector. Wang et al. [
5] performed a comparative analysis of the convection and radiation heat losses on the surface of FPC for different altitudes and studied the effects of the environmental conditions such as air pressure and density. He et al. [
6] conducted an experimental analysis on the effect of nanofluids on flat plate collectors’ efficiencies and calculated the optimum concentration of nanofluids.
The performance of parabolic trough collectors has been studied extensively, both experimentally and analytically, by numerous researchers. Valenzuela et al. [
7] presented a method for the experimental analysis of parabolic trough collectors, in order to determine the thermal and optical performance. Tzivanidis et al. [
8] conducted a thermal and optical analysis of a parabolic trough collector under several operating conditions using both numerical and CFD analysis. Zou et al. [
9] applied Monte Carlo ray-tracing methods to investigate the effect of the sun shape and incident angle on the optical performance of the collector. Shajan and Baiju [
10] used a secondary reflector to homogenize the heat distribution on the receiver. Xu et al. [
11] evaluated the performance of parabolic trough collectors under transient conditions and validated the results with experimental data.
Compound parabolic collectors have also been investigated through numerical methods, simulating processes or experimentally. Su et al. [
12] performed a theoretical ray-tracing analysis on the compound parabolic collector. Santos-Gonzalez et al. [
13] developed a one-dimensional numerical model for the thermal performance of the collector, which was then validated by experimental results. Tchinda [
14] examined the heat transfer within a CPC with a flat one-sided absorber, while Antonelli et al. [
15] proposed a two-dimensional CFD methodology for the estimation of thermal losses for both a circular and flat receiver. Korres et al. [
16] studied the thermal performance of both pure thermal oil and thermal oil with nanofluids with computational simulations. Finally, Korres and Tzivanidis [
17] investigated experimentally and numerically the operation of an asymmetric compound parabolic collector.
Regarding the evacuated tube collectors, Korres et al. [
18] carried out an experimental and numerical analysis of a U-type evacuated tube collector array with mini compound parabolic concentrators in order to calculate the thermal efficiency of the system. Nitsas and Koronaki [
19] developed a mathematical model for the energy and exergy efficiency of an evacuated tube collector, which was validated by experimental data. Ismail et al. [
20] performed a comparative study of a evacuated collector with a circular and a rectangular absorber with heat transfer fluids enhanced with nanofluids.
Concentrating technologies were implemented not only in solar thermal systems but also to photovoltaic cells. For PV applications, concentrating systems were found to achieve higher flux intensity and as a result higher electricity production than the cells without concentrators. Li et al. [
21] analyzed numerically and experimentally a novel asymmetric compound parabolic concentrator which can achieve uniform flux distribution on the PV cell. Renzi et al. [
22] conducted an experimental analysis of a commercial CPV system under real outdoor operating conditions. Sangani and Solanki [
23] examined the gain in output power by using a 2-sun V-trough concentrator with a commercially available PV module.
More recently, PV/T systems have been proposed. In PV/T collectors, PV modules are combined with heat recovery units in order to simultaneously provide heat and electricity from the same aperture area, [
24] therefore fulfilling both thermal and electricity needs either for domestic [
25] or larger scale applications [
26], while at the same time increasing the efficiency of the PV module by reducing its temperature. There are two main types of hybrid solar collectors: flat plate and concentrating.
Numerous studies have been undertaken which examined PV/T collectors both numerically and experimentally. Regarding the flat plate hybrid solar collectors, Jonas et al. [
27] simulated the performance of both a covered and uncovered collector and validated the results by experimental data. Lämmle et al. [
28] built two novel PV/T collectors with overheating protection and tested the effect of this protection on the temperature of the absorber and the efficiency of the system. Lämmle et al. [
29] studied the effect of low-emissivity coatings on heat losses and electrical production, by numerical models and experimental studies and compared the results with a collector with the same design but without the coating. Herrando et al. [
30] modelled numerous alternative absorber-exchanger designs through a three-dimensional computational finite element method and performed a comparative techno-economic analysis of the proposed designs with a reference commercially available PV/T collector. Finally, Guarracino et al. [
31] conducted a coupled thermal and electrical analysis of a sheet and tube hybrid photovoltaic/thermal collector under dynamic conditions by developing a 3D numerical method.
The use of concentrating collectors has been found to lower the number of PV cells, thermal absorber materials, and heat losses and raise the operating temperatures [
32]. Koronaki and Nitsas [
33] investigated experimentally the performance of five asymmetric hybrid solar collectors connected in series and developed a mathematical model based on the experimental data to evaluate the performance of the system under different operating conditions. Proell et al. [
34] constructed and carried out an experimental analysis on a stationary flat plate concentrating CPC PV/T solar collector. In this study, the angle dependent electrical and thermal performance of the collector was measured. Nilsson et al. [
35] performed a long term evaluation of an asymmetric compound parabolic reflector system with two truncated parabolic reflectors. This system was designed for high altitudes and data for the estimation of the annual thermal and electrical output were presented. Nasseriyan et al. [
32] conducted a 2D CFD analysis of an asymmetric CPC collector which was validated by experimental results. The effect of different parameters on the thermal and the electrical production were studied.
Most CFD analyses model the effect of the air or the inert gas between the glass cover and the reflector, in the case of concentrating collectors, or between the glass cover and the absorber, in the case of FPC, by developing the necessary mesh grid. For example, at Refs. [
15,
32] the air was modeled while performing a 2D analysis. The same methodology was used at 3D analysis at Ref. [
30] for a flat plate PV/T collector and at Ref. [
36] for a CPC.
However, modeling the air at the interior of a gap requires significantly high computational resources being conducted, considering that in most cases a great percentage of the mesh elements corresponds to such regions. This is very inconvenient in cases in which the main goal is to calculate the thermal performance, and the air gap function is of low relevance. Hence, the need arises for an alternative method to fill this research gap effectively and efficiently. To this end, in the present study, a novel numerical method was developed which enables the calculation of the collector’s thermal performance without modeling the air function inside the gap. Thus, the proposed method offers a very promising sustainable solution, since its implementation could save valuable computational time and resources.
This study presents a combined thermal and optical, three-dimensional analysis of an asymmetric compound parabolic collector (ACPC) with an integrated hybrid photovoltaic/thermal (PV/T) receiver. Initially, the authors calculated the Incident Angle Modifier () for a wide range of incident angles and the ray-tracing results were verified using three different simulation tools (Tonatiuh, COMSOL, and SolidWorks). The optimal tilt angle of the collector was determined for seven months of the year by conducting a detailed ray-tracing analysis for the mean day of each month considering whole day operation. The specific optimization process was followed to take advantage of the maximum possible solar irradiation in each month and, thus, to ensure the highest possible optical performances with the same geometry, which significantly enhances the sustainability of the proposed system. In the thermal analysis part, a novel numerical modeling method was developed and proposed by the authors for the numerical simulations. This method was then established via two different simulation tools: COMSOL and SolidWorks. The numerical models which were built in the aforementioned programs were verified with each other and validated through experimental results taken from the literature regarding the examined collector. Finally, the thermal performance of the collector was investigated for the mean day of September at solar noon by adopting the optimal tilt angle for that month according to the optical analysis, considering inlet temperatures from 20 °C up to 80 °C.
In conclusion, the innovative simulation method presented in this study could offer notable benefits; firstly, it can be applied to numerous similar geometries. In addition, in terms of feasibility, it allows for the calculation of the optimum tilt angles of the collector for seven months of the year, thus serving as a guide for installation and exploitation of solar energy. This presents an added advantage in the case of this specific collector considering its commercial availability and applications. Finally, it should be mentioned that the modeling method developed requires less computational time than the conventional CFD methods to generate the desirable results, thus adhering to the fundamental principles of sustainability.