1. introduction 2. test facility

24th European Photovoltaic Solar Energy Conference and Exhibition, 21 – 25 September 2009, Hamburg, Germany

Climate Impacts on the Efficiency of a p-Si PV Module and Annual Output under Real Working Conditions W. Durisch1), Jean-Claude Mayor1), King-hang Lam2), S. Dittmann3), D. Chianese3) 1)

Paul Scherrer Institut, PSI CH - 5232 Villigen PSI, Switzerland phone: 0041 (0) 56 310 26 25, fax: 0041 (0) 56 310 21 99 email: [email protected], [email protected] 2)

The University of Hong Kong, Pok Fu Lam Road, Hong Kong, China email: [email protected] 3)

SUPSI-ISAAC, CH - 6952 Canobbio, Switzerland phone: 0041 (0) 58 666 63 78, fax: 0041 (0) 58 666 63 49 email: [email protected]

ABSTRACT: This work covers extensive outdoor testing of a pc-Si module from Photowatt (PW1400-24V). All tests were performed at PSI’s Solar Test Facility. The location of PSI represents a typical site in the Swiss Midland. The module was fixed on a sun tracker and tested under clear sky conditions as well as under a cloudy sky. During testing, the global-normal irradiance varied between 105 and 1084 W/m2, the cell temperature between 18 and 71°C, and the relative air mass between 1.3 and 7.9. About 900 current/voltage characteristics were automatically acquired, leading to the efficiency as a function of irradiance, cell temperature and air mass. The data were used to develop a new efficiency model to calculate the efficiency under all relevant operating conditions. The model contains six parameters. They were determined by applying non-linear fitting techniques. Applying mathematical transformations reported on earlier, the measurements and the efficiency model can be compared and validated in two-dimensional representations. The module shows an acceptable efficiency behavior over the irradiance range. The STC efficiency (referred to the active cell area) was found to be 13.0 %, corresponding to an STC module output power of 147.4 W (according to manufacturer: 160 W). An efficiency maximum of 13.3 % was found at 628 W/m2. The efficiency linearly decreases with temperature. Its temperature coefficient was found to be –0.0468 percentage points/°C. Regarding the impact of air mass, the efficiency exhibits a maximum value of 13.2 % at an air mass of 5.72. The module shows very good red light sensitivity in the late afternoon. Using measured meteorological data from a sunny site in the South of Jordan, the electricity production for the Photowatt module was calculated. The yearly output of South-oriented, 30°-inclined modules was found to be 303 kWh/(m2 cell area). For sun-tracked modules, the annual output amounts to 416 kWh/(m2 cell area).

Keywords: Polycrystalline silicon module - 1: Climate impacts on efficiency - 2: Efficiency model – 3

1. INTRODUCTION Polycrystalline silicon, pc-Si, plays a significant role in the world's photovoltaic cell and module production. In 2008 about 48 % of solar cells produced worldwide were made on the basis of polycrystalline silicon. It is expected that pc-Si modules continue to keep this dominant position for many more years. Cell and module efficiencies are increasing steadily and specific module prices will further decline. Therefore the economic assessment of modules needs to be updated continuously. For this purpose the annual module output is required. This in turn implies knowing the impact of climatic parameters on the efficiency. This work covers extensive outdoor testing of a pc-Si module from the manufacturer Photowatt, BourgoinJallieu, France under varying climatic conditions, as well as the development of a semi-empirical efficiency model to predict the efficiency under all relevant operating conditions. In conjunction with meteorological data, the

efficiency model allows precise calculation of the annual electricity output and the economical assessment of PV installations at a selected site.

2. TEST FACILITY The test facility includes a sun-tracker, on which the module to be investigated is fixed, Fig. 1. The electrical data of the module are measured using PSI’s I/V data acquisition system located in the nearby laboratory. The cell temperature ϑ as well as the ambient temperature ϑa, the global-normal irradiance Gn into the module plane and the wind speed are also measured and acquired. The relative air mass, AM, at the time of the individual I/V tests is calculated from the corresponding sun position. The cell efficiency η is determined from the electrical data in the maximum power point, irradiance and cell area. Each I/Vtest results in data set for η, G, ϑ and AM. A large number of such sets is required to develop models for the efficiency η in the form of η = f (Gn, ϑ, AM).


evaluated, leading to the efficiency as a function of irradiance, cell temperature and air mass.

3. RESULTS A typical test result of a current/voltage (I/V) and power/voltage (P/V) measurement run on Photowatt’s module PW1400-24V is shown in Fig. 2.

Figure 1: PSI’s photovoltaic outdoor test facility. A suntracker continuously directs test modules towards the sun. Six pyranometers (Kipp & Zonen, CM21) are installed on top of the tracker for precise measurement of the irradiance impinging onto the test modules. Current and voltage data of the module are transferred to the nearby PV lab.

3.

TESTING

The module tested in this work is a pc-Si module from Photowatt, France (PW1400-24V). It contains seventy-two polycrystalline silicon cells, 125.5 by 125.5 mm. The manufacturer’s specifications are as follows: Rated output power 160 W, short circuit current 4.8 A and open circuit voltage 43.2 V, respectively. All tests were performed at PSI’s PV test facility, representing a typical site in the Swiss Midland. For investigating the impact of irradiance, cell temperature and relative air mass on the cell efficiency, the module was fixed on a sun-tracker. The electrical data of the module was acquired with a current/voltage (I/V) measuring system developed by PSI [1]. It consists of a dynamic load, electronic load-control, precise digital multimeters and a notebook interfaced to the measuring instruments. Appropriate precision resistors are used to measure the current. A surface temperature probe (Pt-100) was mounted on the rear side of the module to measure the cell temperature. The signals from the multimeters are transmitted via an IEEE bus to the notebook. Specially developed software visualizes the data measured after each test. The test results are stored into a database and can immediately be printed out in form of a condensed report, containing graphs and all relevant data.

Figure 2: Current/voltage and power/voltage characteristic of Photowatt’s module PW1400-24V under clear-sky conditions at noon time. The I/V characteristic in Fig. 2, consists of about 200 measured current/voltage data pairs, taken within about 12 seconds, at a global-normal irradiance Gn of 1000 W/m2, a relative air mass of 1.35 and an ambient temperature of 22.9 °C. The measured points are interconnected with straight lines. From the I/V characteristic the power/voltage (P/V) characteristic is calculated, showing the maximum output power of 123.3 W at the maximum power point (mpp) voltage of 29.2 V. The test was performed at the end of a series of measurements to determine the temperature coefficient of the efficiency. In order to cover a large temperature range, the rear side of the module was covered with a plastic foil. Therefore the cell temperature of 69.5 °C is higher than expected by the Ross-approximation (cf. bellow). Under these test conditions the cell efficiency η was found to be 10.9 %. Using the η/Gn/ϑ/AM sets evaluated from all the I/V tests performed under varying climatic conditions, the parameters p, q, m, r, s, and u in the following semiempirical efficiency model [2] could be determined by applying non-linear least squares fitting methods. η = p[qGn/Gno + (Gn/Gno)m][1+ rϑ/ϑo + sAM/AMo + + (AM/AMo)u]

Tests were performed under cloudless sky conditions from noon time until sunset. During this time the air mass increased from 1.3 to 7.9, whereas the global-normal irradiance decreased from approximately 1100 to about 300 W/m2. In order to get data at low irradiance values, tests were performed under cloudy skies. Tests were also performed during the warming up phase of the module after exposure to high irradiance values at noon time. In this way the cell temperature could be varied within 18 and 71 °C. In total, about 900 I/V scans were acquired and

where Gno = 1000 Wm-2

(1) ϑo = 25 °C

AMo = 1.5

This model takes into account that efficiency is zero at zero irradiance. It also takes into account the fact found empirically that the efficiency decreases in good linear approximation with temperature, under constant irradiance and under constant relative air mass. The model also takes into account a non-linear dependence of the efficiency on air mass, also found empirically. Knowing the parameters


p, q, m, r, s, and u, the model allows calculation of the efficiency under all relevant operating conditions.

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For the module in question, the model parameters were found to be: p = 17.97 +/- 0.56 m = 0.1559 +/- 0.0026 s = -0.9752 +/- 0.0020

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Cell efficiency η, %

The model also allows studying the impact of one single climatic parameter whereas the other parameters are held at constant values [3].

Poly-Si Module PW1400-24V Efficiency vs. air mass

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SN 01400 0082565 2

At irradiance Gn = 1000 W/m and cell temperature ϑ = 25 °C Transformed measurement points, 916 Efficiency Model η max= 13.2 % at AM = 5.72

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q = -0.2308 +/- 0.0049 r = -0.08467 +/-0.0002 u = 0.9893 +/- 0.0010

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Parameter sets for other modules are found in [3]. Applying transformation techniques reported on earlier [4], measurements and model can be compared and validated in two-dimensional representations, Figs. 3 to 5.

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Figure 5: Efficiency of Photowatt’s module PW1400-24V vs. relative air mass at constant irradiance and at constant temperature. Fig. 3 shows an acceptable efficiency behavior over the irradiance range, i.e., a fairly well part-load behavior. The STC efficiency (referred to the active cell area) is found to be

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ηSTC = p(q + 1)(2 + r + s) = 13.0 %


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corresponding to an STC output power of 147.4 W. This value is 7.9 % lower than specified by the manufacturer. An efficiency maximum of 13.3 % is found at 628 W/m2. At an irradiance of 100 W/m2, the cell efficiency is 11.4 %.

Poly-Si Module PW1400-24V SN 01400 0082565 Efficiency vs. irradiance At cell temperature ϑ = 25°C and air mass AM = 1.5 Transformed measurement points, 916 Efficiency Model

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η max= 13.3 % at 628 W/m

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In Fig. 4 a linear decrease of the efficiency with temperature is found, at constant irradiance and at constant relative air mass. The temperature coefficient of the efficiency (percentage points, pps, per °C) is found to be

Global-normal irradiance Gn, W/m

αSTC = (∂η/∂ϑ)STC = p(q + 1)r/ϑo = -0.0468 pps/°C Figure 3: Efficiency of Photowatt’s module PW1400-24V vs. irradiance, at constant temperature and at constant air mass.

According to Fig. 5, the module tested shows a very good behavior with respect to the relative air mass. In contrast to other modules types, where the efficiency is decreasing with increasing air mass [2, 4], the module shows a well-adapted sensitivity for the red-shifted solar spectrum in early morning and late afternoon.

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Poly-Si Module PW1400-24V Efficiency vs. temperature

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SN 01400 0082565 2

At irradiance Gn = 1000 W/m and air mass AM = 1.5 Transformed measurement points, 916 Efficiency Model Temperature coefficient: αSTC= (dη/dϑ)STC = -0.0468%/°C

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The value of the temperature coefficient (in absolute terms) is somewhat smaller than for monocrystalline silicon solar cells [3]

4. ANNUAL OUTPUT

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Figure 4: Efficiency of Photowatt’s module PW1400-24V vs. temperature at constant irradiance and at constant air mass.

Using the above semi-empirical efficiency model together with meteorological data from Al Quwairah, a sunny site in the South of Jordan [5], the annual electricity output of the module PW1400-24V for this site can now be calculated according to the method presented in [3]. It is suggested that the Ross approximation [6] should be used to determine the cell temperature ϑ under real operating conditions: ϑ = ϑa + h⋅G.


In this approximation, ϑa and G denote the ambient temperature and the global irradiance onto the module, respectively. The Ross-coefficient h was determined experimentally to be 0.029°C/Wm-2 for the module investigated. Strictly speaking, the above efficiency model together with the numerical values given for the parameters p, q, m, r, s, and u apply to sun-tracked modules only, i.e., in the Ross approximation, G has to be substituted by Gn. In the case of fix-installed modules, it is assumed that Gn in the efficiency model (1) can be replaced by Gi, i.e., the global irradiance onto the inclined module. This implies that reflection losses due to slant insolation onto the module in the morning and evening hours are negligible. Taking into account this assumption, calculation of the electricity output was carried out for sun-tracked and for South-oriented, 30°-inclined modules. At the site selected in the South of Jordan, continuously sun-tracked modules receive an annual global irradiance of 3547 kWh/m2 [5], leading to an annual electricity output of 416 kWh/(m2 cell area). The output referred to the module’s output power amounts to 3202 kWh/kWSTC. The mean annual cell efficiency was found to be 11.7 %. In the case of South-oriented, 30°-inclined modules, the annual global irradiation amounts to 2523 kWh/m2 [5], leading to an annual electricity output of 303 kWh/(m2 cell area) and of 2326 kWh/kWSTC. The mean annual cell efficiency in this case is 12.0 % and the cell area required for 1 kWSTC is 7.7 m2 (corresponding to a module area of 9.1 m2). Note, the above performance data refer to a working (active) cell area of 1.134 m2. For the module performance, the module area of 1.338 m2 has to be taken into account. Annual electricity output calculations according to other methods, in particular the Performance Matrix method, are found in [7].

5. CONCLUSIONS Based on extensive outdoor testing a semi-empirical efficiency model for Photowatt’s module PW1400 was developed to calculate the efficiency under all relevant operating conditions. Mathematical transformations were applied to study the individual influences of the cell temperature, irradiance and relative air mass on the efficiency. From this it is concluded that the module offers an acceptable efficiency behavior over the irradiance range and a very good behavior over the relevant air mass range. The STC efficiency (referred to the cell area) was calculated from the efficiency model to be 13.0 %. The module’s output power was found to be 7.9 % lower than specified by the manufacturer. The module tested exhibits a more favorable temperature sensitivity than modules based on monocrystalline silicon solar cells. Annual output calculations led to the conclusion that at good sites in sunny countries, the annual electricity yield of South-facing, 30°-inclined PW1400 modules is a good 2.5 times higher than in the Swiss Midland (approximately 1000 kWh/kWSTC).

Based on the annual output calculations presented and knowing the investment cost per m2 module area, life time of module as well as the long-term stability of the efficiency, the specific electricity generation cost can be calculated. However, further comprehensive efficiency measurements, modeling and precise predictions of annual output data for other modules are required to produce the basis for the techno-economical optimum module type selection for a given site.

REFERENCES [1]

W. Durisch, D. Tille, A. Wörz, W. Plapp: Characterization of photovoltaic generators, Applied Energy 65,273-284 (2000). [2] W. Durisch, B. Bitnar, A. Shah, J. Meier: Impact of air mass and temperature on the efficiency of three commercial thin-film modules Nineteenth European Photovoltaic Solar Energy Conference, Proceedings, ISBN 3-936-338-15-9, 2675-2677 (2004). [3] W. Durisch, B. Bitnar, J.-C. Mayor, H. Kiess, K.-H. Lam, J. Close: Efficiency model for photovoltaic modules and demonstration of its application to energy yield estimation, Solar Energy Materials & Solar Cells 91 (2007) 79-84 [4] W. Durisch, O. Struss, K. Robert: Efficiency of Selected Photovoltaic Modules Under Varying Climatic Conditions, Renewable Energy, First edition 2000, Elsevier, 779-788 (2000). [5] W. Durisch, J. Keller, W. Bulgheroni, L. Keller, H. Fricker: Solar Irradiation Measurements in Jordan and Comparisons with Californian Data, Applied Energy 52, 111-124 (1995). [6] R. G. Ross: Interface Design Considerations for Terrestrial Solar Cell Modules, Conference Record of the 12th IEEE Photovoltaic Specialists Conference, November 15-18, 1976 Baton Rouge Louisiana , 801-806 (1976). [7] R. Kröni, S. Stettler, G. Friesen, D. Chianese, R. Kenny, W. Durisch: Energy Rating of Solar Modules, Final Report BFE, PV P+D, DIS 47456/87538, 24 pages, February 2005.

ACKNOWLEDGEMENT The authors would like to thank the Swiss Federal Office of Energy, BFE, Berne, Switzerland for financial support of this project. Dominik Müller, Groupe Solvatec SA, CH4132 Muttenz, Switzerland, we thank for providing the test module.