Performance of Solar Cells Using Thermoelectric Module in Hot Sites

Renewable Energy 89 (2016) 51e59

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Renewable Energy

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Performance of solar cells using thermoelectric module in hot sites

* M. Benghanem a, b, , A.A. Al-Mashraqi a, K.O. Daffallah a, c a Physics Department, Faculty of Science, Taibah University, P.O. Box 30002, Madinah, Saudi Arabia b International Centre of Theoretical Physics, ICTP, Strada Costiera, 1134014 Trieste, Italy c Electronics Department, Faculty of Engineering and Technology, University of Gezira, P.O. Box 20, Wadmedani, Sudan article info abstract

Article history: The ambient temperature at Madinah site is between 40 C and 50 C during the summer months and Received 27 June 2015 sometimes is over 50 C. The cell temperature reaches the value of 83 C. This affects the behaviors of Received in revised form solar cells (SC) and decreases their efficiency. The performance of solar cells is presented in this work 17 September 2015 using thermoelectric module (TEM) as cooling system. In fact, we have found experimentally that the Accepted 6 December 2015 efficiency of solar cells decreases with increase in its temperature. The efficiency of solar cells drops by Available online xxx 0.5% per C rise in temperature. So, it's necessary to operate them at lower temperature in order to increase their efficiency. Cooling the solar cells would enhance its performance. The hybrid PV/TEM Keywords: Solar cells performance system is proposed for PV applications in hot sites. © Efficiency 2015 Elsevier Ltd. All rights reserved. Thermoelectric cooler Hybrid PV/Thermoelectric system

1. Introduction module can generate a small amount of electrical power if a temperature difference is maintained between two terminals [2]. His- Solar energy represents a great potential of renewable energy torically, the motivation for using thermoelectric modules to cool source in the world. The solar irradiation and the ambient tem- microelectronic integrated circuits in the computer industry has perature affect the output power of photovoltaic (PV) system. The been used to increase their clock speed below ambient tempera- efficiency of solar panels decreases when the temperature of the tures, which can be advantageous in some situations [3,4]. As in- solar panels increases [1]. The cooling of solar panels improves its tegrated circuit power and power density continue to increase, the efficiency. computer industry may begin to approach the limit of forced-air The application of thermoelectric technology to cool micro- cooled systems and will need to find alternative solutions [3]. electronic circuits is not new. It has been established for some time Thermoelectric technology has been highlighted as a possible so- that the technology can be used in cooling, heating and micro- lution to these problems [5], and there is evidence of ongoing power generation applications, and can offer some distinct ad- research into cooling the whole of a microprocessor with a ther- vantages over other technologies. For example, in cooling or moelectric module, and focus on cooling microprocessor ‘hot spots’ refrigeration, the technology does not require any chlorofluoro- using embedded micro-thermoelectric devices incorporated into carbons or other fluid that may need to be replaced; can achieve the microprocessor die [6]. temperature control to within ±0.1 C; is electrically quiet in Recent research has been investigated for PV cooling system. operation; the modules are relatively small in size and weight; and Water cooling systems have been studied using water spray [7,8].In do not import dust or other particles which may cause an electrical order to cool the building integrated photovoltaic (BIPV) system, a short circuit [2]. thermoelectric module (TEM) system has been developed [9].In A standard thermoelectric module utilizes the Seebeck, Peltier this late, the authors proved that the combined system TEM/PV can and Thomson effects and can operate as a heat pump, providing be operated at a solar panel temperature of 53 C, without loss of heating or cooling of an object connected to one side of the module solar panels power. Thus, solar panels were cooling at the tem- if a DC current is applied to the module terminals. Alternatively, a perature of 10 C, which will improve the efficiency of solar panels. Simulation software has been used to study the performances of solar cells using thermoelectric modules which allowed the increase in the efficiency of solar cell from 6.8% up to 10.92% at 83 C * Corresponding author. ICTP, Strada Costiera, 1134014 Trieste, Italy. E-mail address: [email protected] (M. Benghanem). [10]. Other work has been investigated using thermoelectric http://dx.doi.org/10.1016/j.renene.2015.12.011 0960-1481/© 2015 Elsevier Ltd. All rights reserved. 52 M. Benghanem et al. / Renewable Energy 89 (2016) 51e59 cooling system to improve the efficiency of PV array. The results 3. Thermoelectric Effect showed that the efficiency of solar cells varied from 8.35% to 11.46% without cooling system and reached the values of 12.26% up to Thermoelectric technology is an alternative method of power 13.27% with cooling system [11]. Otherwise, the temperature of the generation. The main building structure is the thermoelectric solar cells can rise up to 70 C, which allow the deterioration of module (TEM) that can directly convert heat to electricity. This solar cells minimizing its life and getting a low efficiency [12]. For phenomenon was first observed by Seebeck [15]. The simple this, the authors tried to remove excess heat generated by solar concept is to apply temperature difference between two terminals cells to get good performance of solar cells. In the other hand, that trigger the generation of small amount of power (Fig. 3). thermal behaviors of a hybrid PV/TEM system integrating a pin heat Alternatively, this thermoelectric module can function as a heat sink were investigated. In particular case, when integrating the heat pump according to Seebeck/Peltier effects. As shown in Fig. 3, two sink under condition of natural convection, the whole PV/TEM electrical insulating ceramic plates enclose several p-type and n- system was cooled better that using the PV only with heat sink type thermo elements that are electrically connected in series and module and the cooling efficiency is better [13]. thermally parallel with electrical insulation. As TEM is bi-functional One of the most problems of using the PV systems in Saudi device which can either operate as Heating/Cooling device [16] or Arabia is the high ambient temperature which can reach the value generate power, this portability feature can well be exploited in of 55 C in summer months. So, this will increase the solar cells BIPV system for cooling PV module and simultaneously generating temperature and affect the performance of PV panels. For this, we extra power [17]. propose the hybrid system solar cell/thermoelectric module, not Thermoelectric module can be used as cooling system using only to cool the solar cell but also to avoid the heat generated by the Peltier effect. The principle is to get a heat flux between the junc- other side of thermoelectric module. tion of two thermo elements P and N. A Peltier cooler allow the transfer of heat from one side of Peltier module to the other depending on the current's direction [18]. We can also use the 2. Data Base of Temperature at Madinah Site Peltier cooler or thermoelectric cooler (TEC) as generator. If, we want to use the thermoelectric as cooler, we apply a voltage across Madinah site (Latitude ¼ 24.46 N and Longitude ¼ 39.62 E) is the device and then we get that one side of the device is hot and the classified as semi-arid area and has a great potential of solar ra- other side is cold. The performance of TEC depends on ambient diation [14], with a daily annual average yield ranges from temperature, design of the heat exchanger, Peltier parameters and 2 2 4.5 KWh/m /day until 8.5 KWh/m /day, received on tilt PV surface. geometry of Peltier module. The data have been recorded in our laboratory at Physics Depart- When we apply a voltage between two different conductors A ment since 2008 until June 2015. From the observed data, we note and B, we will get a heat at the junction. The rate dq/dt of the that the ambient temperature at Madinah site is between 40 C generated heat is given as follow: and 50 C during the summer months as indicated in the Fig. 1. ¼ðp p Þ Sometimes, the ambient temperature is over 50 C as shown in dQ=dt A B I (1) Fig. 2(aec) corresponding to the year 2011, 2013 and 2014 respectively. So, this will increase the solar cells temperature and where I represents the current (from A to B), PA and PB are Peltier's affect the performance of PV panels. For this, we propose the parameters of the conductors. hybrid system solar cell/thermoelectric module which is a solution to improve the performance of solar cells in hot locations like 4. Solar Cells Model Madinah site. Many models have been studied in literature for solar cells [19,20] showing the influence of serial resistance RS, shunt resistance RSH and temperature T on IV characterization. Fig. 4 shows the equivalent circuit of solar cells [21]. Fig. 4 shows that the current I generated by the solar cell is given as follow:

I ¼ IL ID ISH (2)

where IL is the generated photocurrent, ID is the diode current and ISH is the current through the shunt resistance RSH. The output voltage V delivered by the solar cell is given as follow:

¼ , V Vj I RS (3)

where Vj is the voltage across both diode and shunt resistance and RS is the serial resistance. The current ID is given by the Shockley diode equation: qV I ¼ I exp j 1 (4) D 0 nkT

Where I0 is the reverse saturation current, n is the diode ideality factor, q is the elementary charge, k is the Boltzmann's constant, T is the absolute temperature and k T/q is equal to 0.0259 V at a tem- Fig. 1. The ambient temperature at Madinah during July 2014. perature of 25 C. M. Benghanem et al. / Renewable Energy 89 (2016) 51e59 53

a) b)

Fig. 2. (a) The ambient temperature at Madinah site during 14e16 July 2011. (b) The ambient temperature at Madinah site during 18e20 July 2013. (c) The ambient temperature at Madinah site during 18e20 August 2014.

Fig. 3. Basic single stage thermoelectric module.

The current through the shunt resistance RSH is given as follow: In this present work, we have measured the values of RS and RSH before and after cooling the solar cell. We have obtained some V interesting results highlighting the effect of temperature on R and ¼ j S ISH (5) R . RSH SH Using the Eqs. (3)e(5), the relation (2) becomes: 5. Hybrid System Solar Cell/Thermoelectric Module (SC/TEM) qðV þ IRSÞ V þ IRS I ¼ I I exp 1 (6) The hot surface of the TEM is connected to a heat sink to L 0 nkT R SH enhance heat extraction. The other side of the TEM, i.e. the cold The above explicit model has been found more adequate in side, is attached to the back side of the solar cell as shown in Fig. 5. previous work [21] to characterize the solar cells. In a sunny day and for several hours during the day, the cell 54 M. Benghanem et al. / Renewable Energy 89 (2016) 51e59

ReRa Tracer Software (ReRa solution PV measurement systems, ReRa Tracer Software Version 1.5.1.3, The Netherlands). ReRa Tracer is software which allows us to measure the IV curves and analyze the data by means of different calculation techniques. Pyranometer (Kipp & Zonen Delft, CM11, Holland), used for solar radiation measurement. Thermocouple of type K for measuring the ambient temperature and cell temperature. Digital Multimeter (PeakTech, 3695, Germany) connecting with thermocouple for measuring the ambient and cell temperature.

Fig. 4. Equivalent circuit of solar cell. At the ﬁrst step, we have used two identical solar mini-panels as

Fig. 5. Solar cell cooling system using TEM. temperature as exposed to solar irradiations, can reaches up to we can see in Fig. 7a and b. We have used nearly the same condi- 60e80 C. tions for the two solar mini-panels. The results show that we have got the same parameters for the two panels used before cooling (Table 1). 6. Results At the second step of our experimental work, we have measured the ambient and cell temperature before and after cooling. In order In this experimental work, we have highlighted the effect of to ensure good results, we have used repeated measurement for the temperature on performance of solar cells. We have used the same conditions. Also, the instruments used are calibrated in order thermoelectric module for cooling the solar cells as indicated in the to ensure that the instruments meet their requirements. experimental setup of Fig. 6. Fig. 8 shows that the cell temperature is clearly reduced after cooling during the day. This is a very important result which will improve the performance of solar cell as we can see in next 6.1. Experimental Measurements sections. The measurements are performed at Taibah University Faculty of Science, Physics Department, Solar Energy Laboratory, Madinah 6.2. Effect of the Temperature on the Solar Cell Characteristics (KSA). We have measured the IV characteristic of two solar cells, one without cooling system and the other with cooling system The temperature affects the currentevoltage (IeV) character- using thermoelectric module (TEM). For this, we have used the istic of solar cells. In fact, while increasing T, the magnitude of the following instruments: exponent in relation 6, reduces and the value of I0 increases exponentially with T. The apparent effect is to reduce the open- Source measure (Keithley Instruments Inc., 2420, USA, “Cleve- circuit voltage (VOC) linearly with increasing temperature. landeOhio”). Measuring the IV curve of a solar cell is done by The generated photocurrent IL increases with increasing tem- exposing the solar cell at a standard solar radiation and using an perature. This is due to the augmentation in the number of ther- electronic load, the experimental IV curve is traced by given mally generated carriers in the solar cell. some parameters like serial resistance, fill factor and efficiency By using the above factors in relation (6), we can deduce the of the tested solar cell. effect of temperature on solar cell efficiency. In fact, the change in M. Benghanem et al. / Renewable Energy 89 (2016) 51e59 55

Fig. 6. Experimental setup of solar cell cooling system using thermoelectric module. voltage in IeV curve due to the temperature is more evident than the change in current. Then, the overall effect on efﬁciency seems to be similar to that voltage. Fig. 9 shows the effect of temperature on experimental IeV curves.

6.3. Effect of Temperature on Series and Shunt Resistances of Solar Cell

We have measured the serial resistance of solar cell at different temperatures [21]. The results show that the increase in temperature will increase the series resistance and then the output voltage V delivered by the solar cell will be decreased as indicated in relation (3). Fig. 7. (a). IeV characteristic for mini-panel 1. (b) IeV characteristic for mini-panel 2. We have also measured the shunt resistance of solar cell at different temperatures. The current passes through the shunt Table 1 resistor increases as the result of decreasing its resistance, for a The Parameters of two solar mini-panels used before cooling. certain level of junction voltage. As the result the voltage controlled Parameters Solar mini-panel 1 Solar mini-panel 2 portion of the IeV curve begins to drop from the origin producing a considerable decrease in the terminal current I and a small reduc- Size (cm2) 18.8 18.8 2 tion in V . Very low values of R will result in a considerable Solar Irradiation (W/m ) 424 443 OC SH Cell Temperature (C) 34.6 35.1 reduction in VOC. Fig. 10 show the effect of temperature on series Ambient Temperature (C) 24 23 and shunt resistance of solar cell. Isc (mA) 137.85 140.87 We have plot in Fig. 11 the measured values of series resistance Jsc (mA/cm2) 7.33 7.49 before and after cooling. As we can see the values of series resis- Voc (V) 2.459 2.383 Impp (mA) 113.54 118.80 tance after cooling is smaller than the values of series resistance Vmpp (V) 1.913 1.813 before cooling. This is also a good result to get a solar cell with good Fill factor (%) 64.1 64.2 performance. Efﬁciency (%) 11.6 11.5 We note that the series resistance decreases while the cell temperature decreases as indicated in Fig. 12.

We have plot in Fig. 13 the measured values of shunt resistance Table 2 before and after cooling. As we can see the values of shunt resis- Average cost, in $/watt, of small scale PV system in local market. tance after cooling is higher than the values of shunt resistance PV system size (kW) 2 3 4 7 10 before cooling. This is also a good result to get a solar cell with good performance. Cost ($/W) 1.2 1.25 1.45 1.94 1.95 Cost of hybrid PV/TEM system (%) 12 8 6 3.6 3.2 Since the series resistance decreases and shunt resistance 56 M. Benghanem et al. / Renewable Energy 89 (2016) 51e59

Cell Temperature (Tc) Ambient Temperature (Ta) 55 Cell Temperature after cooling (Tcc)

C) 40 o

30 Temperature ( Temperature

09:00 10:15 11:30 12:45 14:00 15:15 Time (Hours) Fig. 10. Effect of series and shunt resistance on the currentevoltage characteristics of a Fig. 8. Evolution of ambient and cell temperature before and after cooling. solar cell.

Fig. 11. Series resistance before and after cooling vs. the time.

Fig. 9. Effect of various temperatures on experimental IeV characteristic. 7. Experimental PV Panel/TEM System Proposed for All PV Applications in Hot Sites increases while cooling the solar cell, we have measured the effi- Fig. 18 shows the new PV panel/thermoelectric cooling system ciency of solar cell before and after cooling as indicated in Fig. 14. which will be applied for all PV panels at hot countries like Saudi We note from Fig. 14, that the efficiency of solar cell increases Arabia. after cooling. The increase in efficiency % per oC decrease in tem- Fig. 19 shows the proposed hybrid PV/TEM system for PV ap- perature is shown in Fig. 15 which show that the maximum in- plications in hot countries. We note that an appropriate PV panel is crease is 1.3% and the minimum is 0.19% with an average of 0.55%. used for powering different thermoelectric modules. This PV panel This is a good result as we can also show in Fig. 16 which we is independent from PV generator sizing for a given application. have represented the efficiency vs. cell temperature. We note that the efficiency is decreasing when cell temperature is increasing. Power generated with combined solar cell/Thermoelectric 7.1. Process of the Proposed System module (SC/TEM) system with controlled solar cell temperature is plotted in Fig. 17. We show that maximum power generated by SC is When the solar radiation is increasing, the PV panel powering 181 mW at 940 W/m2/25 C. This means, that the maximum power the TEMs modules gives more voltage to drive the TEMs. If this generated by SC with cooling system is greater than power gener- voltage is increasing, the cold side of TEMs became colder as shown ated by the same SC without cooling system (see Fig. 18). in Fig. 20. Then, the cell temperature of each PV panel decreases M. Benghanem et al. / Renewable Energy 89 (2016) 51e59 57

Fig. 14. Efﬁciency before and after cooling.

Fig. 12. Series resistance vs. cell temperature.

Fig. 13. Shunt resistance before and after cooling vs. the time.

Fig. 15. The increase in efﬁciency % per C decrease in temperature. and we get the best performances of the hybrid PV/TEM system in hot location. Q,PE PV ¼ P (7) C H ,E P ﬁ 7.2. Economic Analysis Where EP is the ef ciency of the PV system and PEP is the per- centage of energy needs produced by the PV system. The objective is to determine the economic viability of a hybrid PV/TEM system as an alternate energy source in hot countries when Calculus of the area of PV system (PVA): compared to electricity from the grid. The economic analysis will PVC ,H consider a number of factors such as system location and wattage PVA ¼ (8) cost. The following procedure is used to perform the economic I analysis [22]: Calculus the cost of PV system (CPV) including materials and Calculus of energy demand (Q). installation: Estimation of sunlight hours for the studied site (H). Estimation of the lowest solar radiation for the considered site CPV ¼ PVC,1000,CW (9) (I). Calculus of the capacity of PV system (PVC): Where CW is the wattage cost. 58 M. Benghanem et al. / Renewable Energy 89 (2016) 51e59

Fig. 19. Hybrid PV/TEM system.

Fig. 16. Evolution of efﬁciency of solar cell vs. cell temperature.

Fig. 20. Cold side temperature of TEMs vs. output voltage of PV panel powering the TEMs.

Fig. 17. Power generated with solar cell/Thermoelectric module (SC/TEM) system. costs are based on the local market. Table 2 shows the average cost, in $/watt, of small scale PV system. The cost of PV system varies between 1.2 $/W and 1.95 $/W for the above size. The additional cost of hybrid PV/TEM system varies between 3.2% and 12%. So the average of the additional cost in the range of 6% for small scale PV system as indicated in the next example. For, large scale PV system, the additional cost will be smaller as showing in Fig. 21, by using the interpolation function. The comparison includes only systems between 2 and 10 kW, the most common size range for PV systems installed in experimental projects. In this analysis, we assume a 1.45 $/W cost difference for small scale PV systems. This cost difference represents some of the un- Fig. 18. PV panel/Thermoelectric module (TEM) cooler system. certainty in the future capital costs of proposed hybrid PV/TEM systems. Although there is slight increase in cost, the performance of the PV system will be better in hot sites since the cooling system The total cost of hybrid PV/TEM system proposed (CTot): in the hybrid PV/TEM system enhance its performance.

CTot ¼ CPV þ CTEM (10) 7.3. Example Where CTEM is the additional cost of TEM modules needed.The cost of electricity produced by a solar electric system depends on the The cost of 4 Kw 24 V Off Grid PV System with all accessories is installed capital cost associated with PV, as well as the amount of 5800 $ [24]. 12 solar panels of 300 W are needed. So, we need 12 electricity generated by the PV system [23]. The Installed PV system TEMs which the cost is about 12 13 US$ ¼ 156 US$. M. Benghanem et al. / Renewable Energy 89 (2016) 51e59 59

systems.

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