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Research Papers

Experimental Study to Characterize the Performance of Combined Photovoltaic/Thermal Air Collectors

[+] Author and Article Information
Véronique Delisle

Natural Resources Canada, CanmetENERGY, 1615 Lionel-Boulet, Varennes, QC, J3X 1S6, Canadaveronique.delisle@nrcan.gc.ca

Michaël Kummert

École Polytechnique de Montréal, P.O. Box 6079 STN Centre-ville, Montréal, QC, H3C 3A7, Canadamichael.kummert@polymtl.ca

J. Sol. Energy Eng 134(3), 031010 (May 31, 2012) (13 pages) doi:10.1115/1.4006576 History: Received January 17, 2012; Revised February 29, 2012; Published May 31, 2012; Online May 31, 2012

Combined photovoltaic/thermal (PV/T) collectors show great potential for reaching the objective of net-zero energy consumption in buildings, but the number of products on the market is still very limited. One of the reasons for the slow market uptake of PV/T collectors is the absence of standardized methods to characterize their performance. Performance characterization is a challenge for PV/T collectors because of the interaction between the thermal and electrical yield. This study addresses this particular issue for PV/T air collectors used in either closed-loop or open-loop configurations. In particular, it presents the potential of the equivalent cell temperature method to determine the temperature of the PV cells in a PV/T air collector and validates models to predict the thermal performance and cell temperature for this particular type of solar collector. Indoor and outdoor experimental tests were performed on two c-Si unglazed PV/T modules. The indoor part of this procedure provided the thermal diode voltage factor and the open-circuit voltage temperature coefficient, two parameters that are essential in the calculation of the equivalent cell temperature. The outdoor procedure consisted of acquiring simultaneous electrical and thermal measurements at various inlet temperatures and flowrates. For the collector used in a closed-loop configuration, thermal efficiency models using the fluid inlet, outlet, or average temperature in the calculation of the reduced temperature provided similar results. For an open-loop configuration, a thermal efficiency model as a function of the fluid outlet flowrate was found to be more appropriate. Using selection of variable methods, it was found that a multiple linear regression model using the fluid inlet temperature, the irradiance, and the fluid outlet temperature as predictive variables could be used to estimate both the PV module back surface average temperature and the equivalent cell temperature. When using the PV temperature predicted by these models in the electrical efficiency model, both PV temperatures showed similar performance. In collectors where the PV back surface temperature is not accessible for temperature sensors mounting, the equivalent cell temperature provides a valuable alternative to be used as the PV temperature. The PV/T collector thermal and electrical performance in either closed-loop or open-loop configurations was found to be encapsulated with a series of five-plots.

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Copyright © 2012 by American Society of Mechanical Engineers
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Figures

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Figure 1

PV/T collector testing loop schematic

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Figure 2

PV/T collector mounted on the outdoor combined PV and thermal testing rig

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Figure 3

PV maximum power point for various irradiance levels as a function of (a) TPV_back,AVG and (b) ECT

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Figure 4

Thermal efficiency in closed-loop as a function of (a) (Ti  − Ta )/G, (b) (To  − Ta )/G, and (c) (Tfm  − Ta )/G

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Figure 5

Thermal efficiency in open-loop as a function of the outlet flowrate

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Figure 6

Pareto diagrams identifying the most important variables in the prediction of (a) ECT and (b) TPV_back,AVG

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Figure 7

Comparison of the measured and predicted electrical efficiencies using TPV calculated with the three-variable multiple linear regression model

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Figure 8

Five-plot system representing the PV/T collector performance in open-loop configuration

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Figure 9

Five-plot system to represent the PV/T collector in closed-loop

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Figure 10

Three-plot system adapted from the IEA to represent the PV/T collector in a closed-loop configuration

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