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Research Papers

Spectral Reflectance Patterns of Photovoltaic Modules and Their Thermal Effects

[+] Author and Article Information
J. P. Silva

CIEMAT–DER, Complutense 22, Madrid 28040, Spainjosepedro.silva@ciemat.es

G. Nofuentes, J. V. Muñoz

Grupo de Investigación y Desarrollo de Energía Solar, Escuela Politécnica Superior, University of Jaén, Campus de las Lagunillas s/n, 23071 Jaen, Spaingnofuen@ujaen.es

J. Sol. Energy Eng 132(4), 041016 (Oct 14, 2010) (13 pages) doi:10.1115/1.4002246 History: Received July 15, 2009; Revised July 15, 2010; Published October 14, 2010; Online October 14, 2010

Abstract

Determination of the working temperature of photovoltaic (PV) modules is an essential task in research and engineering projects. It acquires more relevance in the current environment, characterized by increasing figures of installed PV power, module efficiency, solar applications, and operational configurations. However, most of the current procedures for temperature determination of PV modules are simply based on empirical correlations, carried out at conditions defined by some specific standards, with the corresponding lack of accuracy when modules work under real conditions. Thus, the present work looks into a formal procedure for temperature determination by conducting a power balance between the dynamic incoming and outgoing power fluxes. Some additional parameters are included when compared with classic expressions. In particular, the spectral reflectance of the tandem glass-semiconductor is measured to determine the reflected fraction of solar irradiance. The relationship between reflectance and equilibrium temperature is determined for a representative group of PV modules, and the influence that the working point exerts on the module temperature has also been taken into account. Finally, the influence of spectral distribution on module temperature has been quantified by simulations carried out by using a spectral model. In this way, determination of absolute temperature is achieved within a $±2°C$ range, regardless of module characteristics and climatic or operational conditions. In addition, temperature differences between PV modules that work under the same external conditions can be predicted within $±0.5°C$. To summarize, a thermal model suitable for different PV modules and working configurations is presented. Some new parameters are introduced in the calculus process, and the influence of the most relevant ones has been quantified. In this way, the present work is aimed at making a contribution to the study of PV module temperature.

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Figures

Figure 1

Thermal scheme of an opaque PV module based on an electrical analogy. The heat-converted fraction of solar irradiance is supposed to be concentrated in solar cells at temperature TC. Heat fluxes are transferred through the front and rear module surfaces (at TF and TR) to/from the sky, the wind, the ambient, and the ground (respectively, at temperatures TS, TW, TA, and TG).

Figure 2

Scheme of the experimental setup

Figure 3

Details of outdoor setup: (a) ultrasonic anemometer, (b) tested high-efficiency PV modules, (c) bare type-T thermocouples, welded to medium and big size copper sheets, (d) heat flux sensor and protected type-T thermocouple, (e) isolated and bare temperature sensors, attached at the rear side of the PV modules

Figure 4

Experimental measurements; comparison between temperature readings

Figure 5

Comparison between some of classical standard methods for temperature determination. Error of temperature estimation lies within ±5°C during a cloudless day.

Figure 6

Comparison between some of classical standard methods for temperature determination. Error of temperature estimation lies within ±10°C during a cloudy day.

Figure 7

Estimation of absolute temperature: thermal model fitting for OC and MPP working points during a cloudless day

Figure 8

Estimation of absolute temperature. Thermal model fitting for OC and MPP working points during a cloudy day.

Figure 9

Estimation of temperature difference between OC and MPP working points during a cloudless day

Figure 10

Estimation of temperature difference between OC and MPP working points during a cloudy day

Figure 11

Patterns of spectral reflectance of standard and enhanced single-crystalline PV modules

Figure 12

Patterns of spectral reflectance of multicrystalline PV modules

Figure 13

Patterns of spectral reflectance of amorphous-silicon PV modules

Figure 14

Patterns of spectral reflectance of CIS and CdTe PV modules

Figure 15

Module temperature minus ambient temperature versus reflected fraction of solar irradiance at 800 W m−2 and 1000 W m−2 (considered ambient temperature TA=20°C). All the differences between module temperatures and ambient temperature when the considered specimens are left at OC lie on the uppermost straight line. These differences obviously decrease when modules (bundled according to each cell technology) operate at MPP, as shown in the figure.

Figure 16

Estimated decreasing of module temperature and the corresponding increase in module efficiency in PV modules working at MPP, versus OC, at G0=800 W m−2 and G0=1000 W m−2

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