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Research Papers

Intercomparison of Temperature Sensors for Outdoor Monitoring of Photovoltaic Modules

[+] Author and Article Information
Marko Jankovec

e-mail: marko.jankovec@fe.uni-lj.si

Marko Topič

University of Ljubljana,
Faculty of Electrical Engineering,
Tržaška cesta 25,
SI-1000 Ljubljana, Slovenia

Contributed by the Solar Energy Division of ASME for publication in the Journal of Pressure Vessel Technology. Manuscript received July 16, 2012; final manuscript received January 7, 2013; published online April 29, 2013. Assoc. Editor: Santiago Silvestre.

J. Sol. Energy Eng 135(3), 031012 (Apr 29, 2013) (7 pages) Paper No: SOL-12-1178; doi: 10.1115/1.4023518 History: Received July 16, 2012; Revised January 07, 2013

Solar cells' temperature is a very important parameter that affects performance of photovoltaic (PV) modules since main electrical parameters of PV cells and modules are temperature dependent regardless the technology. The present study evaluates and compares different sensor types and mountings for long term outdoor temperature monitoring of PV modules along with a standardized method for determination of cell's temperature from open-circuit voltage. For that purpose, a special multicrystalline silicon PV module with miniature in situ Pt1000 temperature sensors was used for reference temperature measurement. On the back side of the PV module different temperature sensors were attached, including thermocouple (TC), platinum Pt1000 (PT) and digital temperature sensors DS18B20 (DS). All sensors except one were covered by a 1 cm thick insulation block. The whole setup was mounted on the outdoor PV testing site and all temperatures were monitored for several days with selection of different environmental conditions. On the basis of measurement results, deviations of different temperature sensors are investigated and compared to temperature calculated from open-circuit voltage measurement according to standard EN 60904-5. Among sensors attached at the back side, covered PT and TC sensors deliver the best results in range of 1–2 °C of lower temperature in average; while the covered DS sensor gives additional 1–2 °C underestimated temperature values. The worst measurement results demonstrate the PT sensor without insulation. All temperature sensors exhibit similar and adequate time response regarding the thermal capacitance of the PV module. DS sensors, although showing somewhat worse results, offer great advantages if several temperatures have to be acquired simultaneously and require very simple data acquisition equipment. They feature comparable measurement accuracy than commonly used Pt1000 temperature sensors if they are covered by insulation with 10 mm thick walls in lateral direction to avoid micro-environmental changes.

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Figures

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Fig. 1

Locations of laminated PT sensors in the PV module

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Fig. 2

Locations and attachment of sensors in the center of the cell A

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Fig. 3

A photo of sensors attached at the white back sheet at the center of the cell A. Location of the laminated PT sensor is marked by a dot on the back sheet.

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Fig. 4

Temperature difference of temperature under the center of cell A according to two other locations versus temperature difference of the temperature at the center of cell A and air temperature (regression lines added)

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Fig. 5

Junction temperature difference between Voc methods and the temperature of at the back side at the center of cell A in three cases, where different parameters were used in the Voc method. Pairs of lines show the 95 percentile range of each data sets.

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Fig. 6

Temperature, irradiance, and air flow data acquired during a clear sky day

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Fig. 7

Temperature and irradiance data at different times of the day in Fig. 6

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Fig. 8

Response of temperature sensors and temperature calculated from Voc during covering and uncovering of the PV module

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Fig. 9

Air and sensors temperatures during the shading of the PV module

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Fig. 10

Temperature deviations of each sensor measurement method according to the reference temperature, which was calculated from Voc

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Fig. 11

DS sensor (DS18B20) covered by a XPS block with lateral insulation thickness of 3 mm (left hand side) and 10 mm (right hand side)

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Fig. 12

Average temperature deviations between DS and PT sensor at the center of the cell A using different wall thicknesses of XPS insulation on DS sensor

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