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Thermoelectric Feedback model of Photovoltaic panels Hot Spots
1 Thermoelectric Feedback model of Photovoltaic panels Hot Spots Dymytrov Y.Y., Kubov V.I., Ziulieiev D.D Petro Mohyla Black Sea National University, Mykolaiv, Ukraine Kubova R.M. - Witte Moscow University, Moscow, Russia My name Dymytrov Yurii, I represent Petro Mohyla Black Sea National University and our research Thermoelectric Feedback model of Photovoltaic panels Hot Spots. For more than 5 years, in the Black Sea National University has been studying the characteristics of solar photovoltaic panels. In the process of studying the surface temperature, depending on the electrical load, were identified anomalies, for the explanation of which it was necessary to develop its own model. 2017 IEEE 37th International Conference on ELECTRONICS AND NANOTECHNOLOGY (ELNANO) April 18-20, 2017, Kyiv, Ukraine National Technical University of Ukraine "Kyiv Polytechnic Institute"
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Experimental Installation
2 Experimental installation consists of two identical solar panels and anemometer. On the rear surface of the panels there are digital temperature sensors. Switching load, allow us to measure dependence of current on temperature. On the right figure -the scheme of power balance on solar panels. This scheme includes solar radiation, reradiation of heat into space, power - released into electrical load. Electricity acts as a heat transfer agent(coolant). In the case when the power in the load is zero, all power is released in the panel as the heat. This is true both for the case of a short circuit in the load mode and for the case of idling without load. This is force us to make more careful temperature measurement of the panels surface. WS – Sun Radiation, QR – Heat Radiation to Environment, PL – Electrical Power, QG – Heat Radiation from Environment, Tb – Panels Temperature, T0 – Environment Temperature. Photo Voltaic Panel: S=0.2m2; 36-cells of SPC. Tnom=25C; 1.5AM: P=25W; Voc=21.5V; Isc=1.61A
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IR-thermometer Bosch PTD1 (frontside measuring)
Hot Spots on the PV-panels surface 3 IR-thermometer Bosch PTD1 (backside measuring) Thermovisor MobIR M8 (frontside measuring) #1 SC #1 OC #1 SC The temperature so-called “hot spots” were noticed on the panels. The presence of these spots were confirm by measurements with the contactless thermometer of the rear surface and the thermal imager from the front surface. The spots are related to the current flow. The spots practically disappear in the absence of current - the right upper fragment # 1 OC (one O C). The average surface temperatures of the top and bottom panels almost coincide, as in the case of # 1-SC, # 2-SC, and in the case of # 1-OC, # 2-SC, As it was expected. So it was added a temperature sensor in the center of “hot spot” in one of the panels. #2 SC #2 SC #2 SC :00-13:12 :45
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Diurnal variations examples
4 Diurnal variations examples PV-panel mode: #1 – Short Circuit; #2 – Open Circuit #1 – Open Circuit; #2 – Open Circuit Photocurrent Temperature Wind #1-SC #2-OC Here can be seen Examples of synchronous recording of the photocurrent, surface temperatures of the solar panels and of the environment, and wind speed. On June 13, the wind was weaker and the panel was warmer. On June 13, both panels operated in idling mode, without flowing current and temperature coincided. On June 9, battery No. 1 worked in short-circuit mode - and in a hot spot the temperature increased greatly, but in a cold one (middle) - it fell down a bit. #1-OC #2-OC T1hot, T1middle, T2middle, Tenvironment 1 minute 15 minute SC-mode
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Experimental Data: Heating (Insolation, Wind)
5 Experimental Data: Heating (Insolation, Wind) PV-panel OC-mode Photocurrent (SC-mode), A Wind, m/s Heating (Toc –Te), oC On the left figure - the dependence of the heating of the panel in idle mode relative to the level of insolation determined by the value of the photocurrent. On the right figure - dependence of heating on wind speed for a fixed range of insolation ; v0=3.3m/s
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Experimental Data: Hot Spot Temperature
6 Experimental Data: Hot Spot Temperature PV-panel SC-mode relative to PV-panel OC-mode Hot Spot Temperature (SC-mode), oC Here you can see The temperature in the hot spot depends on the environment temperature - on the left. The temperature in the hot spot depends on the temperature on the unloaded panel (average surface temperature) - on the right. Environment Temperature, oC Reference Temperature (OC-mode), oC
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Experimental Data: Hot Spot Temperature Anomaly
7 Experimental Data: Hot Spot Temperature Anomaly T1hot, T1middle (SC-mode) relative to T2 (OC-mode) Heating (SC-mode), oC Reference Heating (OC-mode), oC Anomaly (SC-mode), oC On the left graph marked with red heating in the hot spot and with the blue heating in the middle of the panel in the short-circuited mode. On the left graph - Heating in the hot spot and in the center of the panel that in the short circuit mode, dependencies on the heating of the unloaded panel. The average surface heating coincides with the heating of the unloaded panel. The temperature anomalies (deviation from the temperature of the unloaded-the average surface temperature) is on the right. Pay attention to the nonlinear nature of the anomaly.
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PV-panel simplified model in Short Circuit mode
8 PV-panel simplified model in Short Circuit mode Generator Load Experimental results do not correlates with the classical photovoltaic panel’s model.(Top picture) In the short-circuit mode, almost all the panel’s power allocates on the weakest element (marked red). So, it is heated much more strongly than the entire surface on average (35 times more). This contradicts the measurement results. There are several spots, not one. Heating is large, but not 35 times greater than average. The overheating of the weakest Cell is 35 time greater relative to average heating of whole Panel, independently on this Cell’s weakness.
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PV-cell model with Thermal Feedback
9 Classic model PV-cell model with Thermal Feedback Thermal Feedback Voltage, Current Heating ; Proposed model The experimental results forced us to reconsider the classical model of the solar cell - just a current generator and a diode. The generator current is proportional to the insolation and the area of the cell. The new model includes a thermal part that takes into account the balance of power. This thermal part includes the powers coming from the sun, given out (or received) through the electrical circuit and power dissipated as heat in the cell. The dissipated power causes heating, determined by the temperature resistance of the element to the environment. Heating causes a change (increasing) in the photocurrent. This shows as a feedback between current and temperature.
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PV-panel model with Thermal Feedback
10 PV-panel model with Thermal Feedback 36 cells Rload – Electrical Load eff – Cell’s Efficiency; eff = 13.73% % S – Cell’s Square; Te – Environment Temperature; T01- T36 – Cell’s Rh1- Rh36 – Cell’s Thermal Resistance; Fs – Sun Flux 1000W/m2 The model of the solar panel is composed of 36 series-connected elements. Red highlighted the electric part of the model, green - the thermal. Rh - thermal resistance of cells with the environment. On picture the case, when the panel is made from the identical elements.
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PV-cell model characteristics depending on Heat transfer coefficient
11 PV-cell model characteristics depending on Heat transfer coefficient Current, A Power, mW Calculated dependences of the current, power and temperature of the panels cell for different values of the heat transfer coefficient ( which is determined by different wind speed). The current changes are practically not noticeable. The open circuit voltage and maximum power change noticeably. But the temperature changes are more than noticeable. The more heat transfers, the lower the heat of the cell. Heat transfer coefficient a=13 W/K/m2, 20 W/K/m2, 40 W/K/m2 Temperature, oC Ki=5∙10-4/oK PV-cell Voltage, mV
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PV-panel with different cells Efficiency model
12 PV-panel with different cells Efficiency model characteristics depending on Temperature Current Feedback Without Feedback Ki=0 With Feedback Ki=5∙10-4/oK Current, A Power, mW eff = 13.73% % eff = 13.73% % Here modeled the panel in which several elements are weaker than others. The efficiency of the main mass of the elements is 14%, while the weaker ones are somewhat less. The weakest has an efficiency of 13.73%. Calculated dependences of the current, power and temperature of the panels cell in the absence of current feedback and if there is one. Changes in current, power and voltage are practically not noticeable. But the temperature distribution changes radically. The temperature of the weakest element falls from 155 Celsius degrees to 80 degrees, and the average panels temperature in both cases is about 45 ° C. Cells Temperature, oC PV-panel Voltage, V PV-panel Voltage, V
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13 PV-panel Cells model Temperature smoothing depending on Temperature Current Feedback Without Feedback Ki=0 With Feedback Ki=5∙10-4/oK Cells Temperature, oC Cells Efficiency, % The temperature distribution of the panel cells is the left scale, due to the degree of their efficiency (weakness) - the right scale, in the short-circuit mode. The case without feedback, - red values, and with feedback - green. Without feedback, the weakest element had the highest temperature, but with the feedback, the temperature distribution of the elements were smeared, and the temperatures reflect the degree of weakness of the elements. Cells Number
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Hot Spot Temperature Anomaly
14 Hot Spot Temperature Anomaly Experimental Data Reference Heating (OC-mode), oC Anomaly (SC-mode), oC Model with single weak cell The calculated dependences of the temperature anomaly for different values of the model parameters are on the right. On the left, for comparison, are experimental dependencies. In the model it is possible to change the degree of heterogeneity of the efficiency of the elements (we do not know it), the coefficient of heat exchange with the environment (we do not know it), the temperature coefficient of the temperature and current coupling (we do not know it). Those, we do not know three factors at once with the necessary degree of accuracy. But the fact that our model reflects the non-linear nature of the dependence of the temperature anomaly on heating, gives us hope for future success. Heat Transfer Coefficient: 13, 20, 40 W/K/m2 Global Cells Efficiency: 14.0%. Week Cell Efficiency: 13.8%, 13.85% Ki= 5∙ ∙10-4/oK
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15 CONCLUSIONS The analysis of experimental data of the hot spots temperature anomalies on the surface of PV-panels leads to hypothesis supposed the determinative influence of temperature-current feedback on the temperature smoothing over the PV-panel surface. The LTspice model of single PV-cell is designed. This model designed with consideration of the cell’s power balance and corresponding temperature changes. The designed model of multi-cell panel of series-connected cells base on developed single cell model. The designed model reproduces the main characteristics of the experimental panel, including current and power dependencies on voltage. Furthermore, the model reproduces temperature changing of PV-panel as a function of electric load, and temperature distribution on the surface of the panel in the short circuit mode. We continue to collect data for different conditions in the hope of developing criteria for the selection of individual unknown components. In the end, note that much work remains to be done to clarify the currently uncertain parameters of the model, but we continue the data set and look forward to success. Thank you for your attention.
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