1. Introduction
Policy efforts and support to reduce carbon dioxide emissions worldwide have been recently made due to the problems of increasing energy demands and primary energy depletion [
1]. South Korea has encouraged the development and distribution of renewable energy sources to achieve the obligatory renewable energy supply rate of 30% compared to the expected energy consumption by buildings until 2020 (
Table 1). In particular, the implementation of zero energy building (ZEB) for the energy independence of buildings is attracting attention to reduce energy demands in the building sector. The ZEB implementation requires an increase in the energy efficiency and energy production of the building itself [
2]. Particularly, the energy production of buildings using renewable energy facilities is an important element. Among the representative renewable energy facilities applied to buildings, PV systems are the sources of abundant energy. They are sources of renewable energy with ever increasing installation cases because they are highly applicable to buildings (
Figure 1). However, it is difficult to secure the economic efficiency of PV systems due to limited installation areas in buildings, and installation and maintenance costs. Therefore, studies on the performance improvement of PV systems have been actively conducted to advance their grid parity and increase their penetration rate.
Elements that affect the performance of PV systems include the efficiency of the cell, shadow on the panel, and solar radiation. Studies have been conducted to improve the performance of PV systems considering these elements. Although flexible thin film Si-based solar cells have been beneficial for application and high power conversion efficiencies, the balance between the cell efficiency and robustness of flexible photovoltaic cells remains a pending issue. Bahabry et al. [
5] demonstrated that corrugated architecture silicon cells achieved a bending radius of 140 μm of the back contacts with a power conversion efficiency of 17.2% on a wafer scale (127 × 127 mm
2). Furthermore, the shading effect due to trees, clouds, and buildings affects the performance of the photovoltaic solar panel. Sathyanarayana et al. [
6] investigated the correlation between the short circuit current and performance of the photovoltaic panel under uniform shading conditions. The results indicated that the short circuit current and power output decreased with shading. The performance of the photovoltaic panel can be improved by applying the maximum power point tracking (MPPT) control. However, the classical perturb and observe (P and O) tracker is not adequate under the ramp change of the irradiation level. Abdel-Salam et al. [
7] proposed a modified P and O-based MPP tracking method that showed a higher performance of the tracking speed, efficiency, and tracking accuracy compared to the classical P and O method.
On the other hand,
Figure 2 shows that the power of the PV module decreases as the cell temperature increases. In other words, the cell efficiency can be increased by cooling the PV module. Therefore, many cooling methods have been proposed to improve the performance of the PV module. The cooling methods of the PV module may be divided into passive and active methods.
Active cooling methods are useful under high-temperature conditions that require effective cooling performance of PV modules. Teo et al. [
9] carried out the computational fluid dynamics (CFD) simulation and experiments to find out the cooling performance of the parallel array of air ducts attached to the back of the PV module. As a result, the solar cell achieved a higher efficiency of 12%–14% with cooling compared to the efficiency of 8%–9% without cooling. Moharram et al. [
10] conducted an experiment to demonstrate the cooling effect by spraying water on PV modules. It was found that the proposed system reduced the cell temperature, and the panel surface could be cleaned in hot and dusty regions. D’Angola et al. [
11] analyzed the performance of a PV system with active cooling in which fluid is pumped on the back of the module. In order to find the best compromise between the PV power gain and the pump consumption in terms of flow rate, simulation of a thermal-electric-hydraulic model considering the power losses of the circulation pump was carried out. It was found that the net power gain of the cooling considering the pump consumption was possible through the complete model.
Passive cooling methods have an advantage that no additional power is required. A scale analysis and numerical study on PV modules that involved an open-air channel were conducted by Mittelman et al. [
12]. In addition, Ebrahimi [
13] developed a new cooling technology that used natural vapor as a coolant to extract heat from PV modules. It was suggested that PV modules could be installed in places with natural vaporization such as rivers and canals.
The other cooling method includes a technology that uses a cooling fin attached to the back of a PV module to enhance the convective heat transfer coefficient. Koundinya [
14] proposed a finned heat pipe integrated with a solar module. It was found from the CFD results that the maximum decrease in the solar module was 20 °C. Elsafi [
15] developed a mathematical model to predict the performance of a photovoltaic/thermal (PV/T) system according to different fin configurations. The simulation results indicated that the use of a cooling fin was beneficial to improve the performance of the PV/T system. Nehari [
16] conducted a two-dimensional (2D) numerical simulation to analyze the length of the internal fins inside the PV-PCM system. It was clear that the fins reduced significantly the increase in the PV temperature. In addition, the PV module was more cooled as the length of the fins was 25, 30, 25 mm. Hasan [
17] presented a theoretical and experimental study of the cooling technology using fins attached to the back side of the PV panel. The results indicated that the fins dropped dramatically the temperature of the PV panel by about 5.7%. The technology using cooling fins is neither high-tech nor creative. In addition, the cooling effect could be obtained easily using this technology and can be applied to PV modules that are already installed. However, the simulation analysis of cooling fins considering the airflow around the PV module and solar irradiation simultaneously has been rarely carried out. Therefore, in this study, in order to analyze the fin effect on heat transfer, a simulation model was developed under the condition of airflow around a PV module using CFD.
2. Research Methodology
The performance of a PV module is measured under standard conditions such as standard test conditions (STC) and nominal operating cell temperature (NOCT). The estimation of PV performance is performed under the STC (ambient temperature of 25 °C and irradiance of 1000
). Using the electrical efficiency (
) of STC. The electrical efficiency (
) can be calculated using Equation (1) when the temperature of the PV module is
T [
18]. The temperature coefficient (
) is determined by the material of the PV module. It can be observed that an increase in the temperature of the PV module decreases the electrical efficiency. This indicates that the performance of the PV module can be improved using cooling technology.
Meanwhile, the IEC 61215/61646 norms, which considers more realistic conditions than STC, defined NOCT as the temperature of a PV module at standard reference environment (SRE: ambient temperature (
) of 20 °C, an irradiance of 800
, and wind speed of 1 m/s). Moreover, under the conditions of G irradiance and
, the temperature of the PV module (
) can be calculated using Equation (2) by applying NOCT [
19].
The International Electrotechnical Commission (IEC) provides a standard procedure for measuring NOCT under the following conditions.
- (1)
The test modules are open circuited.
- (2)
Reject all data when the irradiance is below 300 .
- (3)
Wind speeds during the measurement are limited to between 0.25 m/s and 1.75 m/s.
- (4)
Ambient temperature ranges between 5 °C and 35 °C.
The aim of this study was to analyze the module temperature reduction effect of the cooling fins considering the standard conditions for performance assessment to improve the power generation efficiency of a PV module. However, the simulation models of previous studies that analyzed the temperature of a PV module based on CFD seldom considered the airflow around a PV module directly. Therefore, the indoor test of a PV module was simulated under the NOCT condition using the CFD simulation (
Figure 3). The simulation model considered the airflow around a PV module. Using this model, the reduction in the temperature of the PV module according to the shape of the cooling fin was analyzed.
In this research, the prediction of the electrical efficiency was performed using PV temperature obtained from the simulation result. In order to analyze indirectly an unglazed photovoltaic module, a theoretical model, which integrates both thermal and electrical aspects, has been developed by Spertino et al. [
19] However, the calculation of the electrical efficiency is complicated to apply to this simulation. In this paper, the estimation of the electrical efficiency considering the PV temperature was simply carried out with Equation (1) as the thermal analysis on the cooling effect of fins was focused.
5. Conclusions
In general, electrical efficiency decreases as the temperature of a PV module increases. The application of cooling technologies has been examined for the purpose of increasing the electrical efficiency by reducing the overheating of a PV module. However, the active cooling system requires additional power for cooling, and additional facilities incur maintenance cost. On the other hand, the passive cooling system can secure a certain level of cooling performance without additional power and maintenance cost once it is installed. In this study, the cooling effect of a passive cooling technology that can reduce the temperature of the PV module using fins and slits was analyzed, and its contribution in achieving a high electrical efficiency was predicted. The CFD simulation was performed for the analysis. The results of this study are summarized as follows.
(1) It was confirmed that the temperature of the PV module can be reduced by approximately 15 °C using cooling fins. Moreover, the increase in the efficiency caused by the decrease in the temperature was calculated using a linear model that represents the correlation between the temperature and efficiency of the module. As a result, the efficiency of the module without cooling fins was 13.26%, and increased by approximately 7.96% due to the cooling effect of the fins, reaching 14.39% (under the conditions of 600 and 0.5 m/s).
(2) In the simulation results, the parameter study results showed that the cooling performance did not change significantly due to the shape of the fins. In particular, it is necessary to develop fins that do not cause structural problems due to their structural characteristics and can secure the ease of fabrication for PV modules because the increase in the length of the fins may cause serious structural problems under the fast wind speed conditions. Moreover, it is important that the fins can be installed in the existing PV modules without difficulty.
(3) Slits that have a smaller area than the air channel at the bottom of a PV module can have an effect like that of a diffuser. The simulation results showed that it was possible to improve the cooling performance of the fins by promoting the formation of turbulence at the bottom of the module, resulting in the temperature reduction by approximately 0.6 °C. In other words, the cooling performance of fins can be further improved by the installation of slits instead of securing the heat transfer area by increasing the thickness of the fins. Especially, the effect of slits is sufficient in the condition of lower air velocity. The cell temperature further reduces by 8.62 °C due to the slits. Therefore, the application of slits is economical and can have structural benefits because it does not require an increase in the material and weight of fins to secure the cooling performance.
(4) The hot spot phenomenon, which involves overheating while the reverse bias is forced because the solar radiation incident on the cells in a PV module is not uniform, may seriously affect the electrical efficiency and durability of a PV module [
22]. In this study, it was found that the surface temperature of the PV module was relatively uniformly distributed due to the cooling effect of the fins. In other words, it is confirmed that the overheating and non-uniformity of the surface temperature by hot spot can be improved simultaneously by installing fins.