Reflective mirrors effect on the performance of the hybrid PV/thermal water collector

Highlights

• In this paper, the influence of reflective mirrors on the performance of the hybrid PV/thermal water collector was achieved by building experimental set-up for this purpose.

• Two prototype built to compare the thermal and electrical performance of the hybrid solar collector, three variable designed had been studied, which includes the effect of the reflectance mirrors, glass cover and the slope angle of the lower reflector on the performance of hybrid PV/thermal water collector.

• The existence of the reflectance mirrors lead to increase the temperature of the solar cell, therefore, its value was (92.7°C) when using lower and upper mirrors with a glass cover, and (76.1°C) when using only a lower mirror, and (71.35°C)without reflecting mirrors.

• The existence of the reflectors increases the total values of the thermal and electrical efficiencies. The daily total efficiency in average was a result due to use two reflectors was (81.03%), this means a positive effect on the total efficiency, and the effects of the glass cover on the total efficiency is big, the daily average of the total efficiency with and without glass cover was (58.95%) and (49.97%) respectively.

Abstract

Hybrid PV/Thermal collector is the collector which produced electricity and hot water or hot air at the same time and working as solar cells and solar heaters. This system has been designed to increase the electrical efficiency of the solar cells by withdrawing heat from the solar cells. The present study has been carried out in Kirkuk city (latitude 35.46 and longitude 44.39), water was used as a mean to gain heat from the solar cell by passing it in a thermal exchanger, placed at the lower part of the solar cell, two proto types built to compare the thermal and electrical performance of the hybrid solar collector, the study includes making experiments for the months of (February, March, and April 2017, with a different volume flow rates, three variable designed had been studied, which includes the effect of the reflectance mirrors, glass cover and the slope angle of the lower reflector on the performance of hybrid PV/thermal water collector. The existence of the reflectance mirrors lead to increase the temperature of the solar cell, therefore, its value was (92.7 °C) when using lower and upper mirrors with a glass cover, and (76.1 °C) when using only a lower mirror, and (71.35 °C) without reflecting mirrors. The cell surface decreased to (52.75 °C) of the uncooled solar cell. The existence of the reflectors increases the total values of the thermal and electrical efficiencies. The daily total efficiency in average was a result due to use two reflectors was (81.03%), this means a positive effect on the total efficiency, and the effects of the glass cover on the total efficiency is big, the daily average of the total efficiency with and without glass cover was (58.95%) and (49.97%) respectively, these results are accorded with the results of the previous articles.

Introduction

A huge increase in energy consumption has been noticed in the last years, driven by an increase in population and/or in the energy consumption per person in various countries around the world. Solar energy considered one of the important energies which can use because it is renewable energy and infinite source of energy, this energy can be harnessed with minimum detrimental effects on the environment (Ahmed, Ahmed, & Ali, 2014). Electricity generation from solar energy is very important in world due to the recent regulation laws about renewable energy usage (Ahmed Joudi, Hussein, & Ahmed, 2014). Using solar energy is considered one of the most important applications today to generate electricity, these cells operate to convert solar energy to direct current, but the disadvantage of these cells is the rise of its temperature during operation in hot and sunny climate especially in the Middle East region, which characterized by a hot climate during the year (Ahmed, 2016), the rise of the cells temperature causes a decrease in its efficiency. Therefore, the researchers tried to improve the performance of the solar cells by withdrawing heat in cell, and using it in heating, and for other industrial purposes (Ahmed and Mohammed, 2017a, Ahmed and Mohammed, 2017b; Teo, Lee, & Hawlader, 2012).

PV/thermal collectors are the collectors that supply electricity and hot water and air at the same time, they combined two systems together, which are the solar cells and solar heater, and this system is basically designed to rise the electrical efficiency of the solar cells by withdrawing heat from these cells. The hybrid thermal collectors operate by the same mechanism as the thermal collectors, except by replacing the absorbing panels by solar cells, they constitute almost of the same components of the thermal collectors.

Many researches had been conducted by researchers to develop and improve this technology for the past 30 years, and we mention here a summary of some of these studies and researches developing and improving the efficiency of these thermal and electrical systems, Kostic, Pavlovic, and Pavlovic (2010) studied the effect of using reflectors made of aluminum on the performance of hybrid thermal collectors, they found out that the ideal angle of the reflectors depends on the time of the year; the produced thermal and electrical energy increased considerably when using the reflectors in an ideal method, the ideal angle was calculated to be 66°. Aste, Del Pero, and Leonforte (2012) studied improving the performance of hybrid thermal collector using two types of thermal exchangers which placed in the back of the solar cell, and four electrical connecting methods of solar cells. This study shows that it is possible to optimize PV/T efficiency by using the best collector's thermo-electric configuration collector's. As can be noticed from obtained results, harp absorber guarantees better performance that serpentine absorber in all electrical configurations analyzed. Ozgoren, Aksoy, Bakir, and Dogan (2013) studied the effect of water cooling system on the PV/T system to improve its efficiency and found that using the technology of active cooling increasing the electrical efficiency that reached its optimum electrical efficiency about 13.6%. Baccoli et al. (2015) presented and analyzed a mathematical model of a flat thermal collector by using an outside reflector on the lower edge of the collector. The mathematical model had been developed to be used in estimating solar radiation which passing through a transparence cover of the collector with or without a reflector.

El Hocine, Touafek, Kerrour, Haloui, and Khelifa (2015) presented a mathematical model to identify the thermal performance of the hybrid thermal collectors used in heating, the mathematical model showed more precise results in comparison with the previous mathematical models, they found that the efficiency of the hybrid thermal collector used in heating water estimated be 54.51%, and the thermal efficiency of the collector which is used to heat air was 16.24%, and the electrical efficiency of the hybrid collector was 11.12%. Kim and Kim (2012) studied the effect of using glass cover on the efficiency of hybrid thermal collector used to heat water, the researchers showed that using the glass cover improve the thermal efficiency by 14%, and the electrical efficiency reduced by 1.4% at the same time, therefore, using the glass cover is undesirable choice when the priority is to produce electrical power. Naik and Palatel (2014) used a plane reflector fixed on the upper side of the hybrid thermal collector which is used with an angles ranging between (85–100°) in Calicut (11.25 N, 75.78 E) (India), which has a tropical climate, the study showed that the plane reflector with a slope of 100 was the best for increasing the thermal and electrical efficiency. The electrical efficiency increased with the presence of plane reflector by 54%, while the thermal efficiency increased by 21%.

Tabet, Touafek, Bellel, Khelifa, and Adouane (2014) studied the effect of using plane reflector to improvement the performance of photovoltaic thermal air collector in Algeria. Mathematical model was presented to show the effect of the reflector. The study showed that there was an improvement in the performance of the collector when using this reflector. Kostic and Pavlovic (2015) studied the effect of using four mirrors fixed on the upper, lower, right, and left of the solar cell the rate of solar radiation incident the solar cell. Analytical model was used to determine the ideal slope of the angle of the reflector. Mutombo, Inambao, and Bright (2016) analyzed the performance of the hybrid (PV/T) collector used to heat water by thermo-syphon phenomena. The results showed that the total efficiency of the PV/T collector was 38.7% compared with 14.6% for the solar cell, while the tank temperature with capacity of 150 l reached 37 °C. They found the total efficiency of the PV/T collector was higher by 24.1% in comparison with the normal solar cell. Bahaidarah, Tanweer, Gandhidasan, and Rehman (2015) presented experimental and analytical study of the impact of the mirrors placed at an angle on the performance of the hybrid solar collector, the study showed increased energy produced about 22.8% when cooling the solar cell without using mirrors and increased energy produced by 31.5% when cooling the solar cell and the use of the reflective mirrors at the same time. Ghadiri, Sardarabadi, Pasandideh-Fard, and Moghadam (2015) using Nano-fluid type (Fe₃O₄-water) with concentrations ranging between for (1%–3%) to improve the performance of the hybrid solar collector, the researchers found that when using Nano fluid, an increase in the efficiency of the hybrid collector has been achieved by (3%).

Hussein, Numan, and Abdulrahman (2017) using the Nano particle type (Zn-H₂O) to enhancement the efficiency of the hybrid PV/T collector, a small heat exchanger was fixed at the back of the solar cell, and five Nano-fluid concentrations used (0.1,0.2,0.3,0.4,0.5%) in this study. The results showed that using the Nano-fluid leads to improve the electrical and thermal efficiency of the collector, the best concentration was 0.3% and the best volume flow rate of the used liquid was (2 l/min). Zohri, Fudholi, Ruslan, and Sopian (2017) studied the improvement of the performance of the hybrid collector by using groves (V) shape mounted at the rear side of the solar cell to increase the heat transfer from the cell to the cooling flow, a mathematical model was used for this purpose, the highest value of the exit air temperature was 75.96 °C, and the highest thermal and electrical efficiency was 80.01% and 24% respectively. Aste, Del Pero, and Leonforte (2017) achieved a comparative study to show the effect of the glass cover and the type of the heat exchanger on the performance of the hybrid collector. Results showed that the glass cover in cooled seasons was important to increase the thermal efficiency and removing it in summer leads to increase the electrical energy.

This article includes experimental study to improve and compare the performance of the hybrid solar collector used to heat water by building two prototypes of the hybrid PV/T collector, and studying a number of designing and operational variables to indicate the optimal performance of such systems in the Middle East. This study focuses on increasing the efficiency of the solar cell by increasing the rate of solar radiation using the external reflective mirrors and examine their effect on the solar cell and cooling the cells and making comparisons and studying its effect in the climate of Kirkuk city (latitude 35.46°, the altitude 44.39°), in addition the study of the effects of the glass cover on the performance of PV/T collector.