1. Introduction
Concentrating Solar Thermal Collectors (CSTC) are able to serve a great range of solar thermal applications and are able to be integrated into domestic hot water systems, desalination and dehumidification set-ups, absorption chiller set-ups (solar cooling systems), and power production applications [
1,
2,
3,
4,
5]. There is a wide agenda that has been developed around the optimization of such systems as concerns the optical and thermal efficiency, with a variety of optimization technics. The most well-known performance enhancement methods are related to the improvement of the convective heat transfer from the receiver to the thermal fluid and they are applied to linear concentrators such as Compound Parabolic Collectors (CPCs) and Parabolic Trough Collectors (PTCs). This enhancement could be achieved with the use of special working mediums such as nanofluids, with sophisticated flow tube geometries (ribbed and corrugated tubes), and with the integration of flow inserts (metal foams, twisted tapes, fins, etc.) along the flow of the working fluid.
There are many studies in which such methods have been proposed for CPCs and PTCs. Korres et al. [
6] investigated a nanofluid-based compound parabolic collector (CPC) and they compared it with the case where only the base fluid is applied in laminar flow conditions. The results were positive since the thermal efficiency was improved sufficiently with the enhancement reaching 2.8%. Rehan et al. [
7] studied two different nanofluids (Fe
2O
3/H
2O and Al
2O
3/H
2O) as working mediums in a PTC at several different concentrations. The efficiency of the collector was increased and the increment was more significant in higher concentrations and especially in the case where Al
2O
3/H
2O is applied.
A PTC with a hybrid nanofluid and a double twisted tape inside the flow tube was examined by Alnaqi et al. [
8]. It was found that there was a specific orientation of the swirling direction where the thermal efficiency was maximized. As regards the hybrid nanofluid, a higher concentration leads to lower thermal losses. It is important to mention that many studies have examined nanofluid integrations numerically, with high particle concentrations and important enhancements came out. However, it should be taken into account that in experimental set-ups has been proved that agglomeration problem occurs and it has a negative impact in such enhancement when the concentrations take very high values [
6,
9,
10]. Liu et al. [
11] conducted experiments in outdoor conditions regarding a CPC and a significant enhancement was found with the use of water/CuO nanofluid. In another work, Bellos et al. [
12] investigated the integration of a wavy-walled flow tube in a PTC, using a nanofluid as the thermal fluid. The simulation indicated a slight enhancement in thermal efficiency compared with a typical flow tube. Lu et al. [
13] used a water-based nanofluid with CuO nanoparticles in a CPC by performing indoor experiments and a very good enhancement was revealed with the use of the nanofluid. Mwesigye et al. [
14] used perforated plates parallel to each other inside a PTC flow tube receiver. The positioning of the plates was optimized in order to increase the convective heat transfer and thus the thermal efficiency was enhanced sufficiently. Liu et al. [
15] inserted two twisted tapes in a PTC and they investigated the flow regime. It was revealed that the swirling of the fluid inside the tube led to the increment of the convective heat transfer coefficient. In the study of Ref. [
16], a PTC with a wavy-type metal strip inserted in the flow tube was investigated. The research showed that there is an important drop in the heat losses which was greater than 15% and thus a significant enhancement was achieved in the thermal efficiency. Similar methods have been applied in studies [
17,
18,
19,
20].
An equally important role in the thermal output, also, is played by the receiver design and positioning as well as the reflector’s geometry. As far as the enhancement technics in this field several studies have been conducted, especially with linear cavity receivers, to be an alternative solution. Most of these studies are referred to as PTCs. Korres and Tzivanidis [
21] are the first who studied the effect of the angular aperture of circular cavity receivers in the absorbed solar irradiation and they developed two semi-empirical relationships in order to calculate the equivalent absorptance of such receivers, considering the entrapment of solar irradiation inside the cavity. In addition, the study of Korres and Tzivanidis [
22] examined the operation of a PTC with an integrated single cavity receiver inside an evacuated glass envelope. This proposal, which appears for the first time in scientific literature, was found to ensure a great thermal performance enhancement of about 12.2% compared to a conventional PTC. Moreover, the same authors investigated the optical and thermal performance of a PTC with a double circular cavity receiver in a vacuum environment [
23] and the thermal performance was enhanced by approximately 16%. It should be mentioned that in studies [
22,
23], the cavity receivers were first optimized. A PTC with a partially-evacuated circular cavity receiver was examined by Avargani et al. [
24] and higher thermal performances were achieved compared to the conventional design. More proposals regarding cavity receivers and special reflector designs could be found in many literature studies [
25,
26,
27,
28,
29,
30,
31,
32,
33,
34,
35,
36,
37].
Considering the lack of literature regarding enhancement methods in CPCs and especially in cavity receivers, the present study is dedicated to this field. In particular, an innovative linear single cavity receiver with a single glass evacuated tube is integrated into a novel asymmetric compound parabolic collector (ACPC) design which has been developed by the authors in a previous study [
25]. The solar collector that appears in the study [
25] has a maximum optical efficiency of 77.68% and it was found to exceed by much, three other similar geometries from literature as far as optical efficiency is concerned. In this study, two different numerical models were developed; one for the conventional geometry of study [
25] and one for the cavity configuration. Both models were examined for inlet temperatures from 20 °C up to 80 °C as in studies [
3,
25], considering water as the working fluid with a typical volume flow rate of 15 lt/h [
3]. Emphasis was given to the comparison of the thermal and optical performance as well as in the fluid’s outlet and the absorber mean temperature. The geometry of the integrated cavity receiver was optimized according to two independent parameters and two possible optimum designs were revealed. The optimization aims to the maximization of the collector’s optical performance. The global simulation results indicated an enhancement of up to 4.4% and 4% in the optical and thermal efficiency respectively. An analytical solution was developed for verifying the numerical results and the maximum deviations were found to be less than 1% in all the compared parameters. The design and the simulations were performed with SolidWorks which is a proper tool for performing both optical and thermal studies. To our knowledge, there is no other study in literature where such receiver designs have been proposed for CPCs. Hence, the proposed solar collector appears for the first time in the international scientific literature. In particular, it combines a linear cavity receiver, enclosed in a single glass evacuated tube, with an asymmetrical concentrator and it comes to substitute conventional systems, providing higher optical and thermal efficiency. This combination is originally unique and its investigation constitutes a significant contribution to global research on the solar thermal systems field.
4. Conclusions
In this work, the integration of a linear cavity receiver in an ACPC, developed by the authors in a previous study, was investigated. This is the first time in literature that such integration is being proposed. Two different cavity receiver designs (SC-ACPC 1 and SC-ACPC 2) were proposed and compared with the previous study’s design (CNV-ACPC). Next, the most important concluding remarks are listed.
The numerical results were verified through an analytical solution developed by the authors with a sufficient agreement to be achieved (deviations less than 0.5% in thermal performance and 5.0% in thermal losses).
Two new geometries were revealed from the optical optimization process.
The proposed designs came out from a detailed optical performance optimization conducted through ray tracing.
The cavity configurations lead to a significant enhancement of the optical performance of the CNV-ACPC geometry up to 4.4% for θΤ range of 23°.
SC-ACPC 1 seems to be more suitable for wide incident angle range applications since it ensures a greater mean enhancement for θΤ in the range of 2° to 25° in contrast to SC-ACPC 2 which is better for θΤ in the range of 2° to 15°.
SC-ACPC 1 was found to be the best solution between the two suggestions according to an evaluation process with an Efficiency Index (EI), having as the main criterion the balance between the maximization of the mean optical efficiency enhancement and the wider possible incident angle range.
The mean enhancement with the use of SC-ACPC 2 for θΤ in the range of 2° to 20° is slightly greater than the respective of SC-ACPC 1 considering the same θΤ range (3.15% against 3.05%).
SC-ACPC 2 ensures 3.2% mean thermal efficiency enhancement against CNV-ACPC. The maximum possible thermal efficiency enhancement, in this case, reaches 4.0% and it appears for Ti = 20 °C.
SC-ACPC 1 configuration appears slightly lower mean and maximum enhancements than the SC-ACPC 2 geometry (2.92% and 3.70% respectively).
In general, the proposed designs ensure sufficient enhancements both in thermal and optical performance. These enhancements could be even greater when low-absorbing coatings with an absorptance of around 80% are applied, as in various studies [
23,
48].
Future work could be conducted for a deeper investigation of the proposed systems. It would be useful for the proposed configurations to be tested in terms of the convective regime inside the flow pipe and for possible enhancements in the field using flow inserts, nanofluids, or even a combination of them. Another interesting aspect would be the investigation of non-cylindrical cavities applied to the receiver geometry and the comparison of them with the cylindrical ones. This is recommended, to determine how the cavity’s design affects the collector’s performance.