At low irradiance levels precipitation furthermore seems to shift the spectral irradiance more towards the high energy photon region of the spectrum [Figure 2].

Introduction

With data made available by the Utrecht Photovoltaic Outdoor Test facility (UPOT) [1] the performance of ten commercially available PV modules during time periods of low irradiance and precipitation was analysed. Measurements were taken between 2013-03-20 and 2013-11-05 and had a granularity of a few seconds.

Results

As was expected precipitation causes module temperature to drop. Subsequently open circuit voltage (V _OC ) increaces [Figure 1].

At low irradiance levels precipitation furthermore seems to shift the spectral irradiance more towards the high energy photon region of the spectrum [Figure 2].

As a result short circuit current (I _SC ) increases for modules which have a higher spectral responsiveness in this region (i.e.

a-Si and CdTe) [4] [Figure 3].

Conclusions

In terms of the PR it is shown that precipitation can increase performance by up to more than 12% [Figure 4].

The lower PR difference for the CIS module can be explained by its overall low performance and indicates that the module is faulty [Figure 5].

It should be noted that a time-lag between cell temperature measured at the back and photon excitation at the front of the module possibly attribute too much performance gain to a shift in spectral irradiance. Accounting for this still yields significantly higher PRs for a-Si and CdTe modules.

References

[1] van Sark W.J.G.H.M., Louwen A, de Waal A, Elsinga B, Schropp R. E.I. UPOT: The Utrecht photovoltaic outdoor test facility. 27th EU PVSEC, Frankfurt, Germany 2012 [2] Williams, S., Betts, T., Helf, T., Gottschalg, R., Beyer, H., & Infield, D.. Modeling long-term module performance based on realistic reporting conditions with

consideration to spectral effects. Proceedings of the Third World Conference on Photovoltaic Energy Conversion, Osaka, Japan 2003

[3] N. H. Reich, B. Mueller, A. Armbruster, van W. G. J. H. M. Sark, K. Kiefer, and C.

Reise. Performance ratio revisited: is pr > 90 % realistic?, Progress in Photovoltaics:

Research and Applications 2012; 20:717–72

[4] ten Kate O.M., Personal communication, 2014-02-11

1 Utrecht University, Copernicus Institute, Heidelberglaan 2, 3584 CS Utrecht, the Netherlands

K.B.D. Esmeijer ¹ , A. Louwen ¹ , W.G.J.H.M. van Sark ¹

Figure 3

mono Si A mono Si B mono Si C polySi A polySi B a Si

CdTe CIGS HIT CIS

Rain No Rain

Rain No Rain

0 50 100 150 200 250

0.0 0.5 1.0 1.5

Irradiance  ^W 



Is c  A 

a  Si

20 50 100 200

18.5 19.0 19.5 20.0 20.5 21.0 21.5 22.0

Irradiance  ^W 



Vo c  V 

a  Si

Rain No Rain

Figure 2

Figure 1 Figure 4

Rain No Rain

Figure 5

Methods

Only timeperiods in which a plane of array irradiance of less than 250 W/m ² for at least 2 consecutive hours was measured were incorporated in the dataset. The average photon energy (APE) [2] was furthermore used to characterize the spectral irradiance and the temperature corrected performance ratio (PR) [3] was used to compare the different modules.

PV performance during low irradiance and

rainy weather conditions