The CFD Modeling of Solar Dish System: History
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Computational fluid dynamics (CFD) is a powerful numerical analysis approach in solving various engineering and environment problems. It is a simulation technique that uses numerical equations and digital computers for iterative methods to model and to predict various heat, mass, and momentum transfer and fluid flow problems for the optimization of designs. Concentrated solar power (CSP) is a promising technology for harnessing and utilizing a clean and sustainable source of energy. CSP consists of a solar concentrator that collects and intensifies the heat energy from the Sun and a solar receiver that converts the heat generated to produce electrical energy.

• computational fluid dynamics
• concentrated solar power
• solar dish

## 1. Introduction

Concentrated solar power (CSP) is a promising technology for harnessing and utilizing a clean and sustainable source of energy. CSP consists of a solar concentrator that collects and intensifies the heat energy from the Sun and a solar receiver that converts the heat generated to produce electrical energy. Despite its recent adoption and acceptance from both the scientific communities and energy investors, solar power still accounts for a very small percentage of total energy use [1]. The challenge in the optimal design of CSP is to make the technology more cost-efficient and environmentally friendly. With the advancement of numerical modeling software and computational hardware, most of the design problems in CSP technology have recently been solved using computational fluid dynamics (CFD).
Previous reviews have reported the application of CFD in the field of solar energy. Al-Abidi et al. [2] reviewed the various phase-changing materials (PCMs) in solar photovoltaic (PV) ventilation and the application of CFD in the evaluation of its heat transfer behavior. Liu et al. [3] recently reviewed the different models and equations used to describe the PCM composites. Yadav and Bhagoria [4] conducted a review on the assessment of the heat transfer and the fluid flow of numerous solar air heaters using CFD. A solar air heater is a technology that converts solar energy into thermal energy and is usually integrated into solar PV panels. Kasaeian et al. [5] provided a comprehensive review of previous experimental, analytical, and simulation studies (which include the CFD approach) related to solar chimney applications. Moreover, Nadda et al. [6] presented a review of studies that investigate the application of jet impingement in increasing solar PV efficiency. The investigation approaches include experimental, CFD, and mathematical modeling. Chen et al. [7] published a bibliometric review on the development of concentrated solar power technologies but did not mention any use of CFD modeling. Only a few reviews were found which discussed the application of CFD modeling in concentrated solar power technologies and focused on a parabolic trough. As an example, Yılmaz and Mwesigye [8] performed a comprehensive review on several computational approaches that include CFD in the design of parabolic trough solar collectors. Olia et al. [9] and Bellos et al. [10] also presented a review on the application of nanofluids with CFD modeling in the enhancement of the thermal performance of parabolic trough technology. However, for solar dish technology with CFD modeling, no prior bibliometric review has yet been presented.

### 1.1. Concentrated Solar Power

Concentrated solar power (CSP) is based on the principle of photons coming directly from the Sun being concentrated and collected instead of just being scattered by the Earth’s atmosphere. Direct normal irradiance is the technology’s major energy source [11]. Throughout the years, CSP technology has been proven to have a significant share in the production of renewable sources of sustainable electricity [12][13]. The heat energy from the Sun is collected through optical systems or solar concentrators such as highly reflective mirrors and then concentrated in a receiver that contains heat transfer fluid (HTF). The HTF in the receiver is heated, causing a rise in temperature, by absorbing the concentrated radiation. Then, the heated HTF is used for either electricity production through thermal cycles or as a heat source for industrial and commercial applications [13]. There are four main types of CSP technology, namely, parabolic trough collector (PTC), solar tower, parabolic dish (or solar dish), and linear Fresnel reflector (LFR). These four are classified into two groups based on the manner of focusing the solar energy: (1) the line focusing systems, as in the case of PTC and LFR, or (2) the point focusing systems, which are for solar tower and solar dish [14]. Figure 1 shows the illustration of these four types of CSP technologies.
Figure 1. Concentrated solar power technologies.

### 1.2. Computational Fluid Dynamics

Computational fluid dynamics (CFD) is a powerful numerical analysis approach in solving various engineering and environmental problems. It is a simulation technique that uses numerical equations and digital computers for iterative methods to model and predict various heat, mass, and momentum transfer and fluid flow problems for the optimization of designs [4]. CFD is an accepted methodology by the scientific and engineering communities to design various types of renewable energy technologies [15]. There are three main stages of the CFD study, as shown in Figure 2: pre-processing, processing, and post-processing. The pre-processing stage includes geometry creation, material assignment, mesh generation, and assignment of load and boundary conditions. After that, the governing equations such as the Navier–Stokes equation and the set of algorithms are computed and automatically processed by computers. Finally, the obtained results are visualized and interpreted during the post-processing stage.
Figure 2. Computational fluid dynamics stages.

## 2. The CFD Modeling of Solar Dish System

### 2.1. The Semantic Network of Keywords and Authors

In this section, the contents of published journal articles relevant to the topic of computational fluid dynamics modeling of solar dish are analyzed. A review paper by Hachicha et al. [14] presented content analyses on the numerical models of the other types of CSP, which are central receiver or solar tower and parabolic through concentrator (PTC), but not on the solar dish type of CSP. This research concentrates on the CFD modeling of the solar dish. Out of 80 published articles yielded from the Scopus database related to CFD modeling of CSP as of 1 December 2020, only 23 of these are specific to the solar dish system.
Figure 3 shows the semantic network of keywords and researchers of articles specific to the solar dish system with CFD modeling with their corresponding published year. There are three categories identified and subject to content analysis, namely, solar receiver with helical absorber tube, volumetric solar receiver (VSR) with porous absorber, and the rest which did not belong to either of the two were categorized as others. They were classified based on the type of solar receiver used in the solar dish system. Table 1 shows the extracted data of the solar collector and receiver, which include the collector size and reflectivity, the solar irradiance used, the type of receiver configuration, the size of the receiver cavity (including porosity for volumetric solar receiver), the tube size for tubular absorber, and the receiver material.
Figure 3. The semantic network of the keywords and authors based on the Scopus search keywords “computational fluid dynamics” and “solar dish” or “parabolic dish” (1 December 2020).
Table 1. Solar dish and receiver data.

### 2.2. Solar Receiver with Helical Absorber Tube

Then, Daabo et al. [29] studied the effect of coil pitch and tube diameter on the working fluid’s exit temperature and found that the conical-shaped receiver was more efficient than the other two geometries tested. Meanwhile, Uzair et al. [28] studied the influence of wind flow on the heat loss from the receiver while varying the orientation of the solar dish concentrator by employing computational fluid dynamics in ANSYS CFX. The heat loss analysis was different from most studies which focused on the heat loss due to natural convection. The results showed that the dish orientation indeed significantly affects the flow structure near the receiver, which consequently contributes to heat loss.
Zhang et al. [20] performed combined optical and thermal numerical modeling to optimize the performance of a solar conical receiver with a built-in helical pipe. The simulation was able to obtain the optimal values of conical angle, loop number, and insulation thickness to maximize the optical and thermal efficiency of the cavity receiver using TracePro optics software and ANSYS CFD software, which were also compared to published experimental data. Then, Daabo et al. [39] presented an optical and thermal simulation of a micro-scale cylindrical cavity receiver for the Brayton gas power cycle while varying the solar radiation. The flux distribution in the receiver cavity was predicted using OptisWorks software, and the obtained data became the basis for determining the energy absorbed by the compressed air as HTF through CFD simulation in ANSYS. The results, which were validated through experiments, showed the best configurations of the cavity receiver which can be used for domestic applications of concentrated solar power. Moreover, Craig et al. [16] employed the combined CFD and ray-tracing software in estimating the heat losses of a tubular cavity dish receiver at various inclination angles. The heat source profile was obtained using the Monte Carlo ray-tracing method in SolTrace software and was used as input in ANSYS Fluent through user-defined functions. It was concluded and validated through previous literature that the thermal efficiency was mainly affected by the convective heat losses.

### 2.4. Other Types of Solar Receiver Modification

Yang et al. [25] proposed a forced airflow concept in reducing the convective heat loss in the solar dish receiver. The geometry considered was a hallowed cylindrical cavity zoom with a U-shaped air channel and modeled in Fluent and ICEM software for CFD calculation. There were also two types of air circulation modes compared, the clockwise and anticlockwise circulation, and the results showed that the system can be improved better using the anticlockwise mode. Soltani et al. [23] studied the optical and thermal performance of a helically baffled cylindrical cavity receiver both theoretically and experimentally. First, an optical simulation was conducted for solar flux distribution in the collector and receiver using SolTrace MCRT software to minimize the computational cost before proceeding to the thermal simulation of real flux distribution in ANSYS Fluent. The combined optical and thermal simulation increased the accuracy of the numerical simulation, and the results showed a good agreement with the experimental data with a maximum deviation of only about 2%. The results also concluded that the receiver aperture distance from the focal point of the concentrator has a significant effect on the thermal performance of the CSP system. Lastly, Khalil et al. [22] presented a three-dimensional CFD laminar model using ANSYS Fluent to predict the fluid flow and heat transfer behavior of a thermo-plate conical cavity receiver made of welded sheets of 0.5 mm Inconel-625 with pressurized water as the heat transfer fluid (HTF) and a serpentine fluid path. Since the Reynolds number exceeds the laminar flow domain, it was planned to test the flow field using a turbulent model.

### 2.5. Design of Solar Dish Technology

Figure 4. The maximum thermal efficiency of different types of solar dish receiver: (1) Craig et al. [16], (2) Zhang et al. [20], (3) Daabo et al. [30], (4–6) Daabo et al. [31], (7) Barreto et al. [18], (8) Barreto et al. [21], (9) Aichmayer et al. [26], (10) Aichmayer et al. [33], (11) Soltani et al. [23], (12) Giovannelli & Bashir [27], (13) Li et al. [36].
The largest solar dish collector ever modeled in CFD was by Zhang et al. [21], which has an aperture diameter of 5.06 m. It was also noted from the articles that the optimal distance of the receiver cavity from the collector aperture is not necessarily the focal distance of the parabolic collector. Moreover, optical modeling using the Monte Carlo ray-tracing method for the radiation flux distribution was usually performed first before proceeding to the actual radiation model using CFD. The most popular CFD software for this topic is ANSYS Fluent, while for the optics simulation, it is the OptisWorks software.

This entry is adapted from the peer-reviewed paper 10.3390/solar2020014

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