Experimental Study of Photovoltaic Thermal-thermoelectric (PVT-TE) Air Collector

Received Jan 29, 2018 Revised Jul 26, 2018 Accepted Aug 6, 2018 In this study, an experimental study has been conducted to determine the performance of the photovoltaic thermalthermoelectric air collector (PVTTE) hybrid system. Hybrid system consists of photovoltaic panel (PV) and thermoelectric modules (TEs) that can improve the energy efficiency of the system. The results of output temperature (To) and plate temperature (Tp) obtained from the experiment have been used to determine the performance of this hybrid system. Effect of mass flow rate and radiation intensity is also being investigated. Experimental studies were carried out at 0.02 kg/s and 0.09 kg/s which represent minimum and maximum of mass flow rate, and radiation intensities in the range of 268-922 W/m. Keyword:


INTRODUCTION
The evolution of renewable energy sources such as solar energy is a source of energy that provides benefits to the environment and clean energy. In fact, this energy is also an alternative source to the poor or rural people who can not use modern energy sources. Thermal and electrical energy can be generated from solar energy. These two energies are produced in different forms but they can be produced simultaneously if hybrid collectors are used. This hybrid system is made up of a combination of two types of collectors, namely thermal collector and photovoltaic (PV) in a unit called the photovoltaic-thermal (PVT) collector. PVT collector is a device designed to receive solar energy, convert it into thermal and electrical energy, which transfer the thermal energy to the fluid that flows into the collector. A PVT collector consists of PV panel, insulation and frame. Accordingly, PVT collector consists of one or more cover (glass sheets) or a transparent material placed above an absorbing plate with air flowing around it [1][2][3][4][5][6][7].
Heat energy can also be divided into two mediums namely water and air. Both fluids absorb heat and are used as heat transfer mediums in PVT collectors. This is because the high temperatures in the environment and solar radiation can affect the power output of the PV system. If the temperature on the panel increases, the efficiency of the solar panel output power will decrease [8][9][10][11]. By then, cooling of the PV panel is needed to increase the efficiency of the panel. At optimum levels, electrical efficiency can be controlled simultaneously with dropping temperature of panel. Therefore, PVT collectors can generate sustainable energy as overall efficiencies increases.
Passive and active cooling systems have their own limitations. The active cooling system not only requires electricity to operate but also wastes the heat transferred to the environment. To overcome this disadvantages, thermoelectric devices (TEs) that function as fins can absorb wasted heat by attach the device at the back of PV panel. The performance of the TE module can be identify by the temperature at the cold and hot side of the device and also figure of merit. PV panel cooling system has reviewed by [12]   with heat absorption by the device. TE has benefits such as compress in size, lightweight and energy-efficient compression. Additionally, direct current sources (DC) such as fuel cells and PV cells can be used as a power source for TE [13]. When a low power current is applied to the TE device, heat can be transferred through the module from one side to the other. Then, one side of the module will cool down and the other one will heat up. At cold side temperatures, this cooling system occurs using electrons instead of refrigerants as heat carriers and is suitable for outdoor use in combination with solar cells [14]. Accordingly, the TE module can be used for heating and cooling and is ideal for temperature control applications. Furthermore, the TE device can be used as a heat pump for heating during winter. In most cases, PVT collectors typically consist of absorbing plates, PV cells, and heat removal systems and typically PV cells attached to the absorbing plate [15][16][17][18][19]. TE design in this study is the most common for PV-TE system consisting of TE module and is equipped with several components. Among of these, the back PV cell with an aluminum sheet located between them to flow the heat lost on the back surface of the PV cell. A model was developed by [20] and optimization method was introduced for solar terrestrial of TE (STEG). In addition, energy-saving modules using the TE combined with solar system can be achieved by studying the cooling efficiency of the modules [21]. The performance of the module combination depends on several parameters such as flow rate of cooling water, solar cell temperature, and TE heat module temperature. This combined module is used in chamber and the result shows that the temperature differences can be achieved by 16.2% between the ambient air and temperature in chamber. The solar-powered mobile system design combined with TE was studied by [22] using the first and second law of thermodynamics. All the components in the solar still are studied to identify the total heat loss of each component.
For this study, the authors have conducted an experiment to determine the effect of TE devices on energy efficiency for the whole hybrid system. According to the authors' knowledge, it can be concluded that the study of thermal equilibrium on the PVT system is limited and studies on PVT-TE hybrids have not been studied by any previous researchers.

MATERIAL AND METHOD
The thermal photovoltaic hybrid system consists of air collectors and TE modules. The system has been installed and designed at the Solar Energy Research Institute, UKM (Malaysia). The type of PV panel is mono-crystalline silicon solar cells with a total area of 0.52m 2 each. The residual heat absorbed by air collectors can be used to develop a PV-TE hybrid module by affixing the TE device behind the PV module as shown in Figure 1. The addition of electricity produced by the TE converter can be achieved as there is a temperature difference between the cool and hot side of the TE module. The selected elements of TE module are model (TEC1-12706) which are connected in series with the 110-modules specified in Table 1. By increasing the number of TE modules, the quantity of power consumption will also increase. Therefore, high electrical energy will be generated and each parameter will affect it in a different way. Furthermore, thermal performance can be increased by using silicon thermal paste when attach the TE module behind the PV panel. The study was conducted in Bandar Baru Bangi from 3 rd to 25 th August. The experiments were carried out using two airflow rates of 0.02 kg/s and 0.09 kg/s which represents the minimum and maximum value. To get the results, appropriate equipment should be used to obtain the desired parameters for assessing system performance. The measurement of the amount of solar radiation is measured using a pyranometer. Data logger with K-type thermocouples are also used to measure different temperatures including temperatures on the top and bottom of the PV as well as the hot and cool side of the TE module. Then, multimeter type A Fluke 15B + is used to measure current output and PV voltage, as well as electric current of TE modules. Solar radiation is considered to be the main source of energy in collector with the surface of the PV panel. The affected part of solar radiation will be absorbed by the PVT layer and the rest is transferred to the working fluid. Thermal efficiency ( ℎ ) depends largely on the obtained heat ( ) and the temperature output ( ) below the specified airflow rate in the collector channel. This thermal efficiency ( ℎ ) is generally regarded as instantaneous efficiency in the PVT system due to instantaneous operating conditions such as solar radiation, surrounding temperature, wind speed and so on. The basic parameters can be defined as the ratio of useful energy that transfered to the energy of the collector system. This relationship can be shown as [23][24][25][26][27]: and The use of fins attached to the back of the collector shows it is more practical in design and gives superior results as a heat absorber medium. The overall efficiencies depend on the amount of fin per unit length (fin density) as well as the effectiveness of individual fin. According to [14], the proposed electricity conversion efficiency of the proposed PV-TE system is in the range of 1-4%. Therefore, the author can state that it is an exciting new alternative to power generation especially in remote areas where it can improve indoor ventilation for hot and humid areas. For thermoelectric generators operated by solar energy, the following equations are as follows: Here ηTE is the efficiency of the thermoelectric system, ΔT is the temperature difference on both sides of the thermoelectric and α, R and K are Seebeck coefficients, electrical resistance and thermal conductivity of the modules. With this, to achieve high efficiency of ηTE, ΔT should achieve the highest possible temperature and high Seebeck coefficients are also required. However, low electrical and low thermal conductivity need to be achieved. Some assessments have been studied to calculate the electrical efficiency of PV and there are many studies that indicate that temperature of PV mainly affects the energy performance. To determine the efficiency of the photovoltaic panel, it can be expressed as the following equation: The performance of the PVT system can be described by a combination of efficiency [28]. It consists of thermal efficiency (ηth) and electrical efficiency (ηel). This efficiency usually includes a useful thermal acquisition ratio and electricity acquisition from the system to solar radiation towards collector gap within a certain time or period. In this study, the TE module can also produce electrical energy so that the total efficiency known as overall efficiency (ηT) can be used to evaluate the overall performance of the system which can be determined by the following equation:

RESULT AND DISCUSSION
The average solar radiation of the PVT-TE hybrid system on weekdays is recorded from 11.00 to 15.30 to obtain different solar radiation. Based on observation, the maximum air temperature of the outlet temperature is 40oC at mass flow rate of 0.02 kg/s with highest radiation intensity as shown in Figure 2.
From Figure 2, the maximum of To and Tp obtained is 40 o C and 75 o C respectively with solar radiation of 900 W/m 2 . The pattern of the graph is non uniform due to the fluctuation of solar radiation. The average reduction in the temperature of the solar cell is also due to the thermal transfer from the back of the solar cell to the air through the TE module. Therefore, the yield of electricity is higher in low PV cell temperature due to low electron collisions in the area of solar cell depletion.
In Figure 3, the maximum value of To and Tp is increase with increasing of solar radiation. Thus, the maximum value of To is 39 o C and 62 o C for Tp with 860W/m 2 of solar radiation. After 2.00 p.m, it can be seen that the temperature of Tp decrease with decreasing of solar radiation. It can be said that solar cell temperature decreases with increased mass flow rate thus increasing the efficiency of the cell. The influence of mass flow rate affects the efficiency of the system as the mass flow rate through the channel in the collector cause a cooling effect to the photovoltaic cells. Based on Table 2 and 3, it can be seen that the efficiency analysis at different mass flow rates gives rise to thermal efficiency where the increase of 0.310 to 0.597 at mass flow rate 0.02 kg/s and 0.09 kg/s. This thermal efficiency change is in line with the increase of mass flow rate.
Normally, PV cell efficiency will decrease when PV cell temperature increases. The performance of the TE module can be improved when the heat accumulated on the surface of the PV cell is absorbed by the TE and increases the temperature difference between the sides of TE module. Hence, the efficiency of the hybrid system can be improved as the TE module efficiency increase is greater than the reduction of PV cell efficiency. Based on the above results, the TE module's electrical efficiency is higher at lower mass flow rates as the time required for heat transfer from TE module to air is longer.
Based on the above results, the TE module's electrical efficiency is higher at lower mass flow rates as the time required for heat transfer from TE module to ambient is longer. However, the average electrical efficiency of TE module in this study is low from 0.006-0.023 because the performance of selected TE module in this study is lower than that of the more commercial TE modules. The overall efficiency of collectors is also examined by the amount of thermal efficiency and electrical efficiency of the collector. Based on the above results, the correlation between mass flow rates is proportional to the increase in the overall efficiency of the collector.

CONCLUSION
The effects of various mass flow rates on energy analysis have been presented. Therefore, it can be concluded that temperature is the main factor among these parameters that affects the efficiency of the hybrid system conversion. In addition, the parameters studied are also important to select the appropriate values for the convection heat transfer coefficient and the focus ratio to maintain a greater temperature differences between the two sides of TE module.