Page 62 - RSE - Results of the Apollon Project

Results of the APolloN PRoject ANd coNceNtRAtiNg PhotovoltAic PeRsPective

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method SHM , the thermal resistance method (TRM) and the Voc method (see again note 29).
The SHM method envisages keeping the module covered before launching the test in order to have a cell temperature
equal to the environmental temperature. Then the module is quickly exposed to the solar light and the transient
value of the Voc measured. The difference between the Voc value measured immediately after the removal of the
cover and the Voc value measured in steady state conditions allows junction temperature estimation. However, the
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thermal dynamics of the solar cells of the CPV module can last about 0.07 sec , therefore, if the thermal transient
measurement isn’t effected within at least less than 0.014 sec (1/5 of the thermal dynamics of the solar cells) the
SHM could underestimate the junction temperature values. Unless a proper ad-hoc mechanical system is used, this
method does not provide an accurate determination of the solar cell temperature.
The TRM consists in the estimation of the junction temperature by considering the temperature drops on the
heat sink and on the solar cell, under a certain direct normal irradiance (DNI). The accuracy of the TRM is strongly
dependent on the accuracy of the thermal resistance value; this value, however, is infuenced by the contact
thermal resistance that is diffcult to measure and simulate, therefore in this case, too, the method is not accurate
in determining solar cell temperature.
IEC62670, third edition, proposes the VocM method based on the comparison between the Voc value in
experimental conditions and the Voc value in standard reference condition (Vocr). It relies on the module voltage
temperature coeffcient b, which, however, is considered independent of irradiance. Further errors could be produced
in case there are some differences between the reference spectrum and the experimental spectrum, because the
spectral mismatch conditions could affect the Voc due to different contributions of the sub cells being part of the
cells. Therefore this method, too, is not precise in determining solar cell temperature. In order to overcome the
disadvantages of the above mentioned methods, under the APOLLON project a new method for the estimation of
solar cell junction temperature has been developed.
The APOLLON Project Method (APM) is similar to the VocM method, but presents with the following improvements:

p it takes into account the variation of b with the current;
p it doesn’t require knowledge of Vocr;
p it takes into account the possible current mismatch among solar cells.

The APM requires a preliminary indoor characterization to be performed before the outdoor test session (see
Figure 68).

FiguRE 68. Scheme of the APM method for the estimation of the solar cell junction temperature in the CPV module

outdoor test indoor test

dark i-v curve measurement

Measuring outdoor identifcation of electrical parameters
i-v curve
in operating conditions
Calculation of dark i-v curve without series resistance voltage drop

Isc
Measuring of voltage temperature coefficient in function of the current

Voc V (Isc, Tin) ß (Isc)
Estimation of junction temperature

30 IEC 62670 Ed.1 –Part 1: Performance measurements and power rating – Irradiance and Temperature.
31 G.Timò, A.Minuto, P.Groppelli, E.Malvisi, G.Smekens, M.Noack, Matthias Sturm and K.Khalaidovski, Proceeding of the 24 European Photovoltaic
th
Solar Energy Conference and Exhibition Hamburg, Germany, 21-25 September 2009, “Thermal simulation and experimental identifcation of electro-
thermal model parameters for a point-focus concentrating photovoltaic module”.
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