Experimental thermodynamic analysis of air-based PVT system using fins in different materials: Optimization of control parameters by Taguchi method and ANOVA
Introduction
Because solar energy is clean, environmentally friendly, unlimited and free, it is one of most attractive renewable energy source (Cuce et al., 2013, Sobhnamayan et al., 2014, Shukla et al., 2017, Bayrak et al., 2017, Al-Waeli et al., 2017, Lamnatou and Chemisana, 2017). Solar cells have an enormous potential to supply future energy demand of mankind's (Cuce et al., 2013). In practice, approximately 10–20% of solar radiation can be converted to electricity, and rest remains as waste heat or thermal energy and causes panel to overheat (Shukla et al., 2017, Teo et al., 2012, Skoplaki and Palyvos, 2009, Agrawal and Tiwari, 2011, Chauhan et al., 2018, Nadda et al., 2018, Singh and Agrawal, 2015). Electrical efficiency of solar cells reduces when temperature of PV module increases. Every 1 °C temperature rise in solar cells means that electrical efficiency is lost by 0.40–0.65% (Shukla et al., 2017, Chauhan et al., 2018, Nadda et al., 2018, Chandel and Agarwal, 2017, Baloch et al., 2015, Agrawal and Tiwari, 2010). In study conducted in Dhahran, Saudi Arabia, it was stated that when PV panel surface temperature increased from 35 °C to 60 °C, energy efficiency decreased by 35% (Rehman and El-Amin, 2012). Panel surface should be cooled in order to obtain higher efficiency from PV in especially hot summer days. Photovoltaic thermal (PVT) systems have been designed to increase overall energy efficiency of system by reducing operating temperature of PV modules. Also, waste heat removed from panel can be used for heating or drying purposes. Thus, both electrical and thermal energies are obtained simultaneously. Thanks to PVT, two separate physical systems are integrated into a single system and land requirement and system costs are minimized (Chauhan et al., 2018, Wu et al., 2019, Yang and Athienitis, 2014, Joshi and Tiwari, 2007). The aim of our study is to investigate electrical, thermal and exergy efficiencies of PVT using fins in different materials and configurations, and to perform optimization analysis of control parameters that affect thermal and exergy efficiencies. Also, the most ideal number of fins for each fins material were investigated. The best combination of control parameters on thermal and exergy efficiencies was determined by the Taguchi method. Finally, contribution rates of control parameters on both thermal and exergy efficiencies were determined by ANOVA. Experiments were performed according to frequent and sparse configurations of fins made from copper, aluminum and brass materials in both monocrystal and polycrystal panels, and compared to the empty state.
In PVT systems, various applications such as air, water, both air and water, phase change materials (PCMs), heat pipe cooling system and nanofluid have been studied (Chandel and Agarwal, 2017, Wu et al., 2019, Yang and Athienitis, 2014, Bahaidarah et al., 2016, Tiwari et al., 2006, Özakin and Kaya, 2019, Omer and Zala, 2018, Elsafi and Gandhidasan, 2015, Slimani et al., 2017, Ooshaksaraei et al., 2017). It is seen that different designs of air-based PVTs are used. These are: single pass from front surface of panel (unglazed or glazed), single pass from back surface of panel (unglazed or glazed), double pass from on both surface of panel (in same direction or in opposite direction, unglazed or glazed), double pass where air enters from one surface of panel and exits other surface (unglazed or glazed).
PVTs are widely used on exterior facades of buildings to heat homes with excessive heat accumulated in solar cells. Kamthania et al. (Kamthania et al., 2011) investigated performance of air based double facade pass PVT in New Delhi, India in winter season. They observed that room temperature increase 5–6 °C. Tiwari et al. (Tiwari et al., 2006) theoretically and experimentally investigated performance of air-based PVT in New Delhi, India. They have expressed that there is complete agreement between theoretical and experimental results and overall efficiency of PVT system increases by 18%. In the study carried out in Kirkuk, Iraq, thermodynamic analysis of air-cooled PVT using polycrystal panel was performed. They explained that the electrical and thermal efficiencies increased by 20% and 44% respectively when the airflow rate was increased from 0.024 m3s-1 to 0.057 m3s-1, respectively (Ömer and Zala, 2018). Since double-pass systems are more advantageous than single-pass systems, it has been focused on researches in PV field (Elsafi and Gandhidasan, 2015, Slimani et al., 2017, Shan et al., 2014, Hegazy, 2000). Hegazy (2000) examined the effect on thermal, electrical and general efficiencies of four different designs of glazed glass. All designs were glazed. These are: Single pass from on front surface of plate (Model 1). Single pass from on back surface of plate (Model 2). Single pass in same directions from both surface of plate (Model 3). Double pass: Air enters from on front surface of plate and exits from on back surface (Model 4). Under the same operating conditions; they concluded that Model 3 which consumed the lowest fan power is the most suitable design. Elsafi and Gandhidasan (2015) simulated the PVT model system which is added fins and analyzed the effect on the thermal and electrical performance of the fins, in Dahran, Saudi Arabia. They reported that it is 1% and 3% improvement in annually thermal and electricity gains, respectively.
Optimization of parameters is paramount importance to improve efficiency of PVT system. In recent years, studies have been carried out on optimization of PV panels using different analysis methods. Singh et al. (2015) investigated the optimization of parameters (channel depth, channel length, heat transfer coefficient and air velocity) using Genetic Algorithm (GA), at PVT system, and reported that a total exergy efficiency of 16.88% could be achieved. Singh et al. (2015) optimized seven design variables in PVT using Evolutionary Algorithm (EA). They reported being improvement 69.52% and 88.05% in overall exergy and thermal efficiencies as compared to the un-optimized system given in literature, respectively. Thakare et al. (2016) carried out optimization the effect of the water reinforcement profile on the dimensions of the PVT system. They observed being a significant reduction in both the collector area and storage volume. Singh et al. (2015) analyzed the effect on efficiencies of PVT system of some parameters (length and depth of channel, air velocity, thickness of tedlar and glass, inlet fluid temperature) using GA, in India, New Delhi. They examined the changes in overall thermal and exergy yields according to two different optimization approaches. These, 1. What is the change in exergy and thermal efficiencies when the overall exergy efficiency is optimized? 2. What is the change in exergy and thermal efficiency when overall thermal efficiency is optimized? They expressed that there is improvement in overall exergy efficiency and overall thermal efficiency by 4.6% and 13.14% respectively during optimization process. Singh and Agrawal (2016) optimized the design parameters of dual channel PVT module by using fuzzyfied genetic algorithm (FGA). After the optimization process, they reported that solar cell temperature decrease, and the electrical and exergy efficiency of the system increase by 1.13% and 12.21%, respectively. Singh and Agrawal (2015) optimized the design parameters of the PVT system using Genetic Algorithm–Fuzzy System (GA–FS). They explained that overall exergy efficiency of the system optimized using GA-FS is 15.82%, which is 1.27% higher than exergy efficiency of optimized system using GA, and 5.40% higher than exergy of the non-optimized system. Hong et al. (2018) investigated the optimization of control factors (such as isolation transition, slope angle and load resistance) by Taguchi method and ANOVA in studies related to maximum power point tracking (MPPT) of PV cells. They observed that selected factors were significant and confidence level exceeded 99%. Fan et al. (2018) developed optimization strategy for twelve parameters which could be effective on thermal energy and net electricity gain by Taguchi method and ANOVA. The determined optimal design was compared to two different designs. The first design was a series of non-optimal simulation studies, while the second design was used by authors in previous studies (Fan et al., 2017). They were stated that thermal energy and net electricity gain increased under same studying conditions by 21.9% and 20% respectively compared to first design, and 24.7% and 126% respectively compared to second design. Furthermore, uncertainty analysis was performed and it was observed that while thermal efficiency ranged between 48.8 and 56.9% and net electrical efficiency did not change significantly. Hosseinzadeh et al. (2018) investigated effect of control parameters on efficiency of nanofluid-based PVT system by Taguchi method. They explained that most effective parameter on efficiency of PVT system is coolant inlet temperature. In addition, uncertainty analysis was performed, and they were found that uncertainty of experiments for all status was 95% confidence level. Kuo et al. (2019) studied optimization of parameters affecting the electrical and thermal efficiency of the PVT system by using Analytic Hierarchy Process (AHP) and Taguchi method. They explained that electrical and thermal efficiencies of traditional PVT system were 12.74% and 34.06% respectively, whereas, optimal PVT system was 14.29% and 44.96%, respectively.
There are very limited studies on optimization of air based PVT systems in literature. It has been observed that there are no studies on effects and optimization of fins made of different materials in air cooling PVT system on second law efficiency. In this study, electrical, thermal and exergy efficiencies of air- based PVT added of fins made from three different materials, were investigated according to frequent and sparse fin configurations, and it was compared to non-fins state. In addition, ideal number of fins was examined. Taguchi method was used to determine best combination of control parameters affecting thermal and exergy efficiencies. ANOVA was carried out to determine contribution rates of control parameters affecting both electrical and thermal efficiencies.
Section snippets
Experimental setup
Fig. 1a and b shows the mobile experimental setup consisting of PV panels and measuring devices. Experiments were carried out in garden of Atatürk University Engineering Faculty, in July 2016 Erzurum of Turkey. The main skeleton of the air cooling duct shown in Fig. 2a was fabricated of steel. The bottom and sides of the channel were covered with Plexiglas (5 mm). At the corners of the channel where the Plexiglas intersect, sealing was provided with silicone. To ensure thermal insulation, the
Thermodynamic analysis
Fig. 2 shows the cooling channel in which fins are placed according to sparse configuration, and model of airflow through the channel according to this configuration. The air channel has single inlet and single outlet.
At a specific time interval (Δt), equation of mass conservation for control volume was shown in following equation
Economical analyses of the PVT system
We evaluated the performance of the PVT system in terms of energy and exergy efficiency. For a system, however, economic analysis is more important than the energy analysis of that system. There are numerous studies in the literature on the economic analysis of PVTs (Brahim and Jemni, 2017, Evola and Marletta, 2014). Although these studies are carried out under the climatic conditions of a particular geographical region, they provide a reference to all researchers and manufacturers. Erzurum
Results and discussion
In this study, fins were used to remove waste heat accumulated in PVT system and thus to increase efficiency of panel. Electrical, thermal and exergy efficiencies of PVT system were investigated. Also, both Taguchi method and ANOVA were used to optimize control parameters affecting both efficiencies and to obtain most appropriate control parameter combination. The results are detailed below.
Conclusions
The results have summarized below (Note: The percent increase rates are given for copper, aluminum, and brass respectively).
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Considering monocrystal panel, electrical efficiency increased 33–34%, 26–27% and 17–22% for sparse configuration, 45–48%, 33–34% and 21–24% for frequent configuration, respectively; thermal efficiency increased 61–63%, 52%, and 42–43% for sparse fin configuration respectively, 107–109%, 65–68%, and 55–58% for frequent fin configuration respectively; exergy efficiency
Declaration of Competing Interest
We declare that the authors have no conflict of interest or relationship with any organization or person in relation to the subject matter or materials discussed in this article.
Acknowledgements
The authors express their gratitude to financial support provided by Atatürk University Scientific Research Project Unit under BAP/2015-147 numbered project.
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