Temperature coefficients

Ozcan Bazkir, Scientific and Technological Research Council of Turkey (Türkiye Bilimsel ve Teknolojik Araştırma Kurumu, TÜBİTAK), Tunus Caddesi No:80, 06100, Kavaklıdere, Ankara

PV module temperature measurements

Photovoltaic (PV) devices are sensitive to temperature due to their temperature dependency of band gap energy. Change in temperature, affects the performance of electrical parameters like short-circuit current, I_sc; open-circuit voltage, V_oc; maximum power, P_max. In general, for PV devices, the nominal values of these parameters are determined at Standard Test Conditions (STC) so as to ensure their standardization. In IEC 61215 standard, the STC is defined as 1000 W/m² irradiance, 25 ^oC device temperature and AM1.5 spectral irradiance distribution.

However, the performance of PV devices in real applications can be completely different from the performance at STC due to the effects they can be subjected to. Depending on the seasons and geographical locations, PV devices can be exposed to various conditions like variable wind-speed, ambient temperature, spectrum, irradiance level and angle of incidence of the solar radiation. All these conditions have effect on the temperature of a PV module. This, impacts the performance parameters and hence the energy efficiency of PV devices. In order to calculate energy yield correctly, the performance parameters obtained at STC have to be extrapolated to any temperature. This can be done via knowledge of the temperature coefficient for each parameter. Temperature coefficients of PV devices are generally defined as the percentage change of the electrical parameters with change of temperature. Accurate determination of temperature coefficients has crucial importance for wide range of applications from estimation of degradation rate of PV modules to their use in utility scale installations. In order to commission and to predict the output of large-scale systems besides the electrical and meteorological data, system operators often use translation equations given in Equation1.

[math]\displaystyle{ I_{sc,c}\ ={\ \ I}_{sc}\ \left[1+\alpha\left(T_2-T_1\right)\right] }[/math] (1.a)

[math]\displaystyle{ V_{oc,c}\ =\ \ V_{oc}\ \left[1+\beta\left(T_2-T_1\right)\right] }[/math] (1.b)

[math]\displaystyle{ P_{max,c}=P_{max}\left[1+\kappa\left(T_2-T_1\right)\right] }[/math] (1.c)

Where I_sc,c, V_oc,c and P_max,c are corrected electrical parameters ; α (%/⁰C), β (%/⁰C) and κ (%/⁰C) are respectivelly the relative temperature coefficients of short-circuit current, open-circuit voltage and maximum power.

For electricity generation applications, the most useful parameter for the performance analysis of PV devices is the maximum power. Inaccurate determination of this parameter for different temperature levels can result in the wrong prediction for the energy yield of a system. Especially for large PV systems if the temperature difference is big compared to STC temperature, wrong estimation of the expected output becomes significant and with a potentially crucial financial impact.

REFERENCES

IEC 60891, Photovoltaic devices – Procedures for temperature and irradiance corrections to measured I-V characteristics

IEC 61215-2, Terrestrial photovoltaic (PV) modules - Design qualification and type approval - Part 2:Test procedures

IEC 60904-9, Photovoltaic devices – Part 9: Solar simulator performance requirements

IEC 60904-10, Photovoltaic devices – Part 10: Methods of linearity measurement

IEC 60904-2, Photovoltaic devices – Part 2: Requirements for photovoltaic reference devices