Elsevier

Energy

Volume 172, 1 April 2019, Pages 57-67
Energy

Numerical investigation and optimization of an experimentally analyzed solar CPC

https://doi.org/10.1016/j.energy.2019.01.119Get rights and content

Highlights

  • A new optical efficiency relationship is proposed.

  • The previous ray tracing agree with the present one (2.9% and 1.8% mean deviations).

  • The simulation results diverge 4.2% on average from the experimental one.

  • The present simulation exceeds the previous theoretical solution in accuracy.

  • The novel design enhances the optical efficiency by 2.4% regarding a daily operation.

Abstract

In this study a compound parabolic collector (CPC) taken from literature was investigated optically and thermally through simulation. The collector was tested at different transversal and the longitudinal incident angles and the results showed us a total agreement between the present and the previous study where TracePro software was used. In addition, a new optical efficiency relationship was proposed and it was found that diverges significantly from the commonly used relationship as the reflector's shape losses and/or the absorber's diameter increase. Also, the CFD analysis results were validated from the previous study experimental data (4.2% mean divergence in thermal efficiency), something that reveals how sufficiently the real problem has been approached. Finally, an optimization process was followed in order to improve the collector's optical performance, while the novel CPC resulted from the optimization was compared with the initial design for the typical conditions of the 11th of June in Athens from 08:00 to 16:00, in order to examine the effect of different solar irradiation intensities in the comparison process. It was revealed that the novel design exceeds the initial one in all the examined hours. The design and the simulation of the collector were conducted via Solidworks Flow Simulation software.

Introduction

Concentrating and evacuated tube collectors are nowadays of considerable interest since they are able to cover a significant range of hot water supplies from domestic and industrial applications up to massive electricity production. The continuous and the rapid development of the specific technologies has set them on the foreground of solar thermal systems. The particular collectors have been investigated experimentally, numerically and through CFD analysis by researchers regarding their thermal and optical characteristics [1]. For instance, Haiping et al. [2] investigated the operation of a novel thermo-photovoltaic with compound parabolic reflectors adjusted on it both numerically and experimentally, while Patil et al. [3] examined alternative receiver designs for parabolic trough collectors.

There are few studies on Compound Parabolic Collectors (CPCs). For example, Li et al. [4] examined optically and thermally a CPC with a U pipe evacuated tube and developed a mathematical model to validate the thermal efficiency of the collector arisen from experimental tests. Korres et al. [5] investigated experimentally and numerically the operation of a U-type evacuated tube collectors’ array with mini compound parabolic concentrators and managed to estimate the thermal efficiency and the outlet as well as the receiver temperature in each module. The same author [6] analyzed the operation of a CPC using the nanofluid Syltherm-800/CuO and the base fluid Syltherm-800 and found that the thermal efficiency enhancement by using the nanofluid is up to 2.76%. A mini compound parabolic collector with an optically optimized reflector and a cylindrical absorber was investigated by Korres and Tzivanidis [7,8] and it was revealed that the specific design exceeds in optical performance two respective geometries from literature. Korres and Tzivanidis [9], also, conducted a whole year analysis comparing the particular configuration to an identical with an asymmetric reflector under the same operating conditions and they found that the asymmetric configuration provides highest optical performance in most cases and generally for the whole year.

Furthermore, Korres et al. [10] examined the optical and the thermal performance of a mini CPC with a flat receiver, while Bellos et al. [11] carried out a detailed analysis of a compound parabolic collector with an evacuated tube where two different working mediums were tested and the optical performance was optimized by modifying the reflector's geometry. Adsten et al. [12] examined six different set ups with special asymmetrical reflectors so as to determine the optimum case that provides the highest optical efficiency while Souliotis et al. [13] analyzed asymmetrical CPC for integrated solar systems with one tank inside the collector. In addition, Kessentini and Bouden [14] developed a numerical model to study the thermal performance of a double tank integrated collector storage system (ICS) accompanied by asymmetric CPC reflectors. The truncation effect on the performance of a CPC was, also, evaluated analytically by Ustaoglu et al. [15], while Lu et al. [16] investigated experimentally the utilization of water/CuO nanofluid in a CPC and it was found that 1.2% nanoparticle concentration is able to improve the thermal efficiency of about 30%

Parabolic Trough Collectors (PTCs) have, also, been investigated through simulating process, numerical methods or experimentally. For instance, Tzivanidis et al. [17] investigated a parabolic trough collector regarding its thermal and optical performance conducting both CFD and numerical analysis for several different operating conditions, while Cheng et al. analyzed a PTC optically [18] and developed a new modeling method for concentrating systems [19] by using the Monte Carlo ray-tracing method. Abad et al. [20] conducted an experimental analysis on a parabolic trough collector and they filled the absorber with metal foam in order to improve the heat transfer and to increase the efficiency of the collector, while Kaloudis et al. [21] applied Al2O3/water nanofluid in a PTC and found that a concentration of 4% increases the thermal efficiency of the collector up to 10%. Marrif et al. [22]. found that water performs better than therminol in such systems, while Akbarimoosavi and Yaghoubi [23] concluded that the higher the thermal conductivity of the absorber's material the more the thermal efficiency. Moreover, Tsai and Lin [24] optimized a variable focus parabolic trough concentrator and compared it to the classical PTC and the semi-cylindrical configuration. Furthermore, Bellos et al. [25] simulated a PTC by using thermal oil and Al2O3/thermal oil nanofluid and found that the use of the nanofluid enhances the thermal efficiency by 4.25%. In addition, a wavy-tape was inserted in the flow tube of a PTC application by Zhu et al. [26] and it was found that the Nusselt number was enhanced significantly, while the heat losses were reduced by at least 17.5%.

Regarding the Evacuated Tube Collectors (ETCs) Iranmanesh et al. [27] examined experimentally the effect of graphene nonaplatelets/distilled water nanofluid on the thermal performance of an ETC. More specifically, it was found that the thermal efficiency increases with the concentration of the nanoparticles in the water. Moreover, Liang et al. [28] analyzed a U-pipe ETC experimentally and validated the results through a mathematical model they developed. Particularly, they compared a finned absorber configuration to a filled one and they found that the filled material enhances significantly the collector's thermal performance. In addition, Papadimitratos et al. [29] applied a Phase Change Material (PCM) in a heat pipe application and they managed to enhance the thermal efficiency up to 26% as far as the normal operation is concerned.

In this work the thermal and the optical performance of a CPC with a U-pipe evacuated tube taken from literature (Li et al. [4]) was investigated through CFD analysis and ray tracing process. The main goal was to optimize optically the examined collector. This optimization was conducted by modifying the reflector's geometry and, thus, two novel reflector's designs were revealed, while the secondly proposed design was considered to be the most suitable for the examined case. Furthermore, the selected design was compared with the initial one for the typical conditions of the 11th of June in Athens from 08:00 to 16:00, in order to examine the effect of different solar irradiation intensities in the comparison process. It was revealed that the novel design exceeds the initial one in all the examined hours. The simulation results were, also, validated from the experimental data that was available from the previous study, while the heat transfer fluid of the CFD analysis is Therminol 55. Moreover, a new detailed optical efficiency relationship for CPC applications was proposed in this study and it was found that diverges significantly from the commonly used relationship as the reflector's shape losses and/or the absorber's diameter increase.

Section snippets

Examined model

The examined model is a compound parabolic collector taken from literature (Li et al. [4]) with a U-type evacuated tube and a geometrical concentration ratio of 3.06. The collector's design as well as the main dimensions of the reflector and the evacuated tube (in mm) is presented on Fig. 1.

In Fig. 1 the geometry of the reflector and the evacuated tube is depicted in detail. Regarding the last one it is clear that it consists of a double glass pipe with an aluminum fin snapped at the interior

Optical analysis

In this section the optical performance of the examined collector is going to be investigated in several different operating conditions. Particularly, the collector's operation is going to be examined at various incident angles of solar rays both in longitudinal and in transversal direction.

Thermal analysis

The collector was tested at five different inlet fluid temperatures in five different dates from t = 08:00 to t = 16:00 (solar time) in Shanghai (latitude: 31.14oN) and it was oriented towards the south by changing the slope after the first three days. The working medium is used is Therminol 55. Table 4 gives the conditions of the real problem which have, also, been introduced to the simulation process.

The sky temperature was calculated via Eq. (14) [4], while the wind heat transfer coefficient

Results

In the particular section all the results regarding the optical and the thermal analysis of the examined CPC are presented. The proposed correlation for calculating the optical efficiency of a CPC is being compared with the commonly used simplified relationship. In addition, emphasis is given in the validation of the optical efficiency arises from the present simulation with the respective from the previous study where TracePro software was used and the thermal efficiency of the system through

Conclusions

In the specific work an already experimentally analyzed CPC by Li et al. [4] was investigated and optimized thermally and optically respectively. The main concluding remarks were pointed out in order to provide the readers with a clear overview about the important outcome of the specific study.

First of all, the new expression for the optical efficiency calculation was proposed in the present study it was compared to the commonly used expression and it was found that the greater the absorber or

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