d 5.1.111: suitability of pv testing methods for arctic ...sgemfinalreport.fi/files/d5.1.111.pdf ·...

RESEARCH REPORT VTT-R-05735-14

D 5.1.111: Suitability of PV testing methods for arctic conditions; existing methods and development needs Authors: Atte Löf, Riku Pasonen, Marja-Leena Pykälä

Confidentiality: Public


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Report’s title

Suitability of PV testing methods for arctic conditions; existing methods and development

needs Customer, contact person, address Order reference

Cleen Oy Project name Project number/Short name

SGEM –Smart Grids and Energy Markets 85060 - CLEEN/SGEM V

2014/ENE Author(s) Pages

Atte Löf, Riku Pasonen, Marja-Leena Pykälä 32 Keywords Report identification code

Solar energy, PV testing methods, PV in arctic conditions VTT-R- Summary

Electrical energy produced by PV system depends on several external factors. The common practice of the PV industry is to rate PV modules at Standard Test Conditions. However, conditions vary a lot in different locations of the world and different PV modules react differently to altered environmental conditions. The most critical factors influencing the power output of the PV module are temperature and irradiance and other factors are for example spectral response and incidence angle. That is why it is critical that PV modules are tested the same way everywhere and testing methods for different conditions must be studied to provide more comprehensive rating information of PV modules. Testing on cold climate and arctic conditions has gained less interest so far.

During this project an outdoor testing site has been specified and will be built according to DERlab technical guidelines on long-term photovoltaic module outdoor testing setup. Location of the testing site will be Espoo. Outdoor testing site includes three 250 W PV panels, three inverters and measurement devices for radiation, ambient temperature, humidity and wind speed and direction. In addition, A Photovoltaic Power Profile Emulation software will be utilized in the simulation of the actual output of the PV modules. This functionality of the emulator can be used, for example, to transition from cloudy winter conditions to summer sunny conditions and to see how ambient temperature affects to the output power of PV panels.

The purpose of this report is to introduce existing PV testing methods and discuss their suitability for arctic conditions. In addition, suggestions for arctic condition testing methods are proposed. These testing methods will be tested in the future studies in the PV outdoor testing site.

Confidentiality Public

Espoo 19.12.2014 Written by

Atte Löf Research Scientist

Reviewed by

Kari Mäki Senior Scientist

Accepted by

Tuula Mäkinen Head of Research Area

VTT’s contact address

PL 1000, 02044 VTT

Distribution (customer and VTT)

VTT, Cleen Oy /SGEM portal

The use of the name of the VTT Technical Research Centre of Finland (VTT) in advertising or publication in part of

this report is only permissible with written authorisation from the VTT Technical Research Centre of Finland.


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Preface

This work was carried out in the Smart Grids and Energy Markets (SGEM) research program, work package 5. The work has been funded by Finnish funding Agency for Technology and Innovation (TEKES) and the project partners. Espoo 19.12.2014 Atte Löf


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Contents

Preface ................................................................................................................................... 2

Contents ................................................................................................................................. 3

Abbreviations ......................................................................................................................... 4

1. Introduction ....................................................................................................................... 5

1.1 Scope ....................................................................................................................... 5

2. Artic conditions and photovoltaic modules ........................................................................ 5

2.1 Temperature ............................................................................................................. 6 2.2 Daylight hours ........................................................................................................... 8 2.3 Angle of incidence .................................................................................................... 9 2.4 Air Mass ................................................................................................................. 11 2.5 Weather conditions ................................................................................................. 13 2.6 Solar radiation ........................................................................................................ 14

3. Existing testing standards and methods for PV panels .................................................... 16

3.1 Standards relevant to power measurements ........................................................... 16 3.2 Standards for PV module performance testing and energy rating ........................... 18 3.3 Technical Guidelines on Long-term Photovoltaic Module Outdoor Tests ................ 19

4. Testing environments for arctic conditions ...................................................................... 20

4.1 Outdoor testing environment ................................................................................... 20

4.1.1 PV panels ................................................................................................... 21

4.1.2 Inverters ...................................................................................................... 21

4.1.3 Radiation measurements ............................................................................ 22

4.1.4 Temperature and humidity measurements .................................................. 23

4.1.5 Wind speed and direction measurements ................................................... 24 4.2 Hardware simulation testing environment ............................................................... 25

4.2.1 Magna-Power Electronics DC power supply ............................................... 25

4.2.2 Magna-Power Electronics Photovoltaic Power Profile Emulation Software . 25

4.2.3 ET instrumente controllable power electronic load module .......................... 27

4.2.4 Computer algorithm and measurement principle ......................................... 27

5. Development needs for PV testing methods in arctic conditions ..................................... 28

6. Conclusion and future work ............................................................................................. 30

References ........................................................................................................................... 30


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Abbreviations

a and b Empirical coefficients

AM Air Mass

AOI Angle of incidence

CF Capacity factor

CPV Concentrated Photovoltaic

cSi Polysilicon

DERlab European Distributed Energy Resources Laboratories

H Monthly average daily global radiation

H0 Monthly average daily extraterrestrial radiation

HOMER simulation tool

Irref Solar cell reference Parameter

Imp, Isc Solar cell parameter values: maximum power point current, short-circuit current

IEC International Electrotechnical Commission

MPP Maximum Power Point

n Monthly average daily hours of bright sunshine

N Monthly average daylight

NASA National Aeronautics and Space Administration

Pnom Rated power

PPPE Magna-Power Electronics Photovoltaic Power Profile Emulation

PV Photovoltaic(s)

SGEM Smart Grids and Energy Markets

STI, LTI Temporal instability

STC Standard Test Conditions

Tref Solar cell reference Parameter

TC82 Technical Committee 82

Vmp, Voc Solar cell parameter values: maximum power point voltage, open-circuit voltage

α and β Solar cell parameter values (EN 50530)

αs and αt Azimuth angle of sun and azimuth angle of surface

δ Declination angle

φ Latitude of the location

γt Inclination angle

θhor Angle of incidence on a horizontal plane

θtilt Angle of incidence on a tilted surface

θz Zenith angle of sun


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1. Introduction

The installation level of photovoltaic (PV) systems is increasing world widely. The European PV Industry Association estimates that PV could provide 12 % of the European electricity consumption in 2020 [1]. However, this rapid increase of PV installation has a downside. Different types of PV modules have been developed and comparing efficiency of different PV modules is not as easy as it seems. Testing the solar PV modules in various climatic and geographic locations for a longer period of time is essential to determine the real annual energy output of the PV modules.

Electrical energy produced by PV system depends on several external factors. The common practice of the PV industry is to rate PV modules at Standard Test Conditions (STC) where temperature is 25 °C, irradiance is 1000 W/m2 and Air Mass (AM) is 1.5. However, conditions vary a lot in different locations of the world and different PV modules react differently to altered environmental conditions. The most critical factors influencing the power output of the PV module are temperature and irradiance and other factors are for example spectral response and incidence angle. That is why it is critical that PV modules are tested the same way everywhere and testing methods for different conditions must be studied to provide more comprehensive rating information of PV modules. Long-term testing of PV modules has been rapid especially in Germany, but testing on cold climate and arctic conditions has gained less interest so far. International Electrotechnical Commission (IEC) Technical Committee 82 (TC82) has developed and published a number of measurement and qualification standards for PV systems [27].

1.1 Scope

The purpose of this report is to show how different conditions affect to PV module efficiency and also introduce existing testing methods for PV modules and evaluate how those testing methods are suitable for arctic conditions. In addition, purpose of this study is to build a real PV module outdoor testing environment where arctic condition testing is going to be done in the future. Also a simulation environment is going to be built to support the PV panel studies from the real PV module testing environment. These testing environments are used to see if any development is needed for PV testing methods especially for arctic conditions.

2. Artic conditions and photovoltaic modules

The performance of a PV module depends on many things such as solar radiation, angle of incidence, ambient temperature, humidity and wind speed and direction. Additionally, arctic conditions may include frost, snowfall and icing. Irradiance and the panel temperature have the greatest impact on PV module performance. Irradiation decreases when moving from equator towards North but the conversion efficiency of PV modules increases because the ambient temperature decreases. The latitude of the location also has impact on the PV module performance, because in the North the length of the day varies much and the angle of incidence is different than in South. In addition, the weather conditions are not the same in North and South. Therefore, determining the performance of PV module in arctic conditions should be studied.

This chapter presents how the latitude of observed location influences the air temperature, daylight hours, angle of incidence, air mass and solar radiation. The information is presented in figures and tables and it is calculated for six different latitudes which are 30, 40, 50, 60, 70 and 80 degrees in northern hemisphere. Different cities near chosen latitudes are presented in Table 2.1 and geographic coordinates in Europe are presented in Figure 2.1.


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Table 2.1. Cities near chosen latitudes.

City Country Latitude Longitude

Longyearbyen Norway 78°13'N 15°39'E

Utsjoki Finland 69°54'N 27°01'E

Helsinki Finland 60°10'N 24°56'E

Frankfurt Germany 50°07'N 8°41'E

Madrid Spain 40°24'N 3°41'W

Cairo Egypt 30°03'N 31°14'E

Figure 2.1. Geographic coordinates of Europe [28].

2.1 Temperature

Ambient temperature, cloud patterns and wind speed have an influence on PV module temperature. The module temperature has a large effect on PV modules efficiency. Energy production efficiency starts to drop when the temperature of the PV module reaches hot


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temperatures. Experiments have proofed that the production will drop steadily when the panel temperature increases above the standard test condition [2]. In the test setups PV modules may be cooled to keep the temperature at a specified value. In colder temperatures the PV modules energy production works more efficiently. However, in the coldest regions in North the length of the day varies much and days are shorter than in Southern regions during winter time. Table 2.2 and Figure 2.2 present monthly average air temperatures in six different latitudes, which are gathered from NASA atmospheric data center.

Table 2.2. Monthly average air temperatures in different latitudes [3].

Air temperature (°C)

Lat. 30N Lat. 40N Lat. 50N Lat. 60N Lat. 70N Lat. 80N

JAN 13.40 5.34 -3.89 -6.33 -12.10 -21.00

FEB 14.20 5.83 -3.14 -6.95 -11.50 -21.20

MAR 17.10 8.78 1.14 -2.99 -8.36 -21.00

APR 21.40 13.80 8.61 3.40 -4.19 -14.90

MAY 25.10 19.00 14.70 9.74 2.23 -5.61

JUN 27.90 23.60 17.40 14.40 8.74 -0.73

JUL 28.90 25.90 19.40 17.10 12.00 0.79

AUG 29.10 25.60 19.10 15.70 9.77 0.22

SEP 27.60 22.10 14.10 10.80 5.01 -2.62

OCT 23.50 16.90 8.61 5.39 - 1.14 -10.00

NOV 19.00 11.00 1.82 -0.56 -7.74 -16.00

DEC 14.60 6.43 -2.93 -4.82 -11.00 -19.60

Figure 2.2. Monthly average air temperature in different latitudes.

Figure 2.2 shows how the average temperatures of different latitudes vary throughout the whole year. In South the summer time is really hot and therefore it can be assumed that PV modules will not operate the most efficient way during summer time in Southern Europe. In North the ambient temperature will not rise really high and therefore the Northern environment could provide almost optimal operating temperature conditions for solar panels. That is why the operating temperature of PV module should be taken into account when testing PV modules.

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2.2 Daylight hours

The length of day depends on the declination angle of the Earth and latitude of the observed location. In Nordic conditions the day is longer in the midsummer and shorter in the winter. Therefore, during winter the total amount of available solar radiation is low and during summer it is very high. For example, in June, the average length of the day in Rovaniemi is almost 23 hours when in December it is only one hour. Figure 2.3 and Figure 2.4 illustrates how Earth’s declination angle changes during the year.

Figure 2.3 Earth orbit and declination angle [4].

Figure 2.4. Declination angle during the year.

The spring equinoctial is on March 22, summer solstice is on June 22, autumn equinoctial is on September 22 and winter solstice is on December 22. The length of the day in different locations can be calculated with two equations. The declination angle in any day of the year is calculated with Cooper equation [4, 5]:

𝛿 = 23,45 ∙ sin (360° ∙284 + 𝑛

365) (1)

Where n is the day number for the year (Jan 1=1, Dec 31=365). The declination angle is used for the equation for hours of daylight [4, 5]:

𝑁 =2 ∙ arccos(− tan𝜑 ∙ tan 𝛿)

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Where φ is the latitude of location and δ is the declination angle. These equations have been utilized to calculate the length of the day during the whole year in different latitudes and the results are presented in Figure 2.5.

Figure 2.5. Daylight hours during year in different latitudes.

As seen in the figure, the length of the day varies quite a lot depending on the location of the city and the time of the year. In arctic conditions the length of the day in summer is much longer than in Southern locations and in winter it is much shorter. In addition, the monthly averaged daylight hours are calculated to the Table 2.3.

Table 2.3. Monthly average daylight hours in different latitudes.

Daylight hours (h)


JAN 10.30 9.51 8.39 6.47 0.80 0.00

FEB 10.95 10.47 9.80 8.75 6.46 0.42

MAR 11.82 11.73 11.62 11.44 11.11 10.01

APR 12.74 13.08 13.54 14.26 15.72 20.87

MAY 13.51 14.22 15.20 16.85 21.62 24.00

JUN 13.90 14.79 16.07 18.34 24.00 24.00

JUL 13.72 14.52 15.66 17.61 23.38 24.00

AUG 13.05 13.53 14.19 15.25 17.56 23.40

SEP 12.15 12.22 12.32 12.47 12.75 13.64

OCT 11.23 10.88 10.40 9.65 8.11 2.89

NOV 10.46 9.75 8.75 7.07 2.17 0.00

DEC 10.10 9.20 7.93 5.65 0.00 0.00

These results show that the length of the day is very different depending on the latitude of the location and the time of the year and therefore it should be also taken into account when testing PV modules performances.

2.3 Angle of incidence

The solar angle of incidence on a horizontal surface is a direct function of the sun height and it is illustrated in the Figure 2.6.

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Figure 2.6. The solar angle of incidence.

This angle is also called the zenith angle and for Northern hemisphere it is defined as [6, 7]:

𝜃ℎ𝑜𝑟 = 𝜃𝑧 = 90° − 𝜑 + 𝛿 (3)

Where θhor is angle of incidence on a horizontal surface, φ is latitude of the location and δ is the declination angle. The equation has been utilized to calculate angle of incidence in different time of the year in different latitudes and the results are shown in Table 2.4 and Figure 2.7.

Table 2.4. Monthly average angle of incidence in different latitudes.

Angle of Incidence


JAN 37.57 47.57 57.57 67.57 77.57 87.45

FEB 28.03 38.03 48.03 58.03 68.03 78.03

MAR 16.42 26.42 36.42 46.42 56.42 66.42

APR 5.42 15.42 25.42 35.42 45.42 55.42

MAY 0.08 8.36 18.36 28.36 38.36 48.36

JUN 0.00 7.10 17.10 27.10 37.10 47.10

JUL 2.23 12.04 22.04 32.04 42.04 52.04

AUG 11.99 21.99 31.99 41.99 51.99 61.99

SEP 23.97 33.97 43.97 53.97 63.97 73.97

OCT 34.88 44.88 54.88 64.88 74.88 84.88

NOV 41.80 51.80 61.80 71.80 81.80 89.96

DEC 43.35 53.35 63.35 73.35 83.35 90.00


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Figure 2.7. Angle of incidence in different latitudes.

The calculation of the angle of incidence on a tilted surface is more complicated. The surface azimuth angle αt describes the deviation from the south. The inclination angle γt describes the surface tilt or slope of the surface. Figure 2.8 visualizes these angles. [7]

Figure 2.8. Different angles on tilted surface [7].

The angle of incidence is the angle between the vector s in the direction of the sun and the normal vector n perpendicular to the surface. The solar angle of incidence on a tilted surface is [7, 8]:

𝜃𝑡𝑖𝑙𝑡 = cos−1(cos 𝛾𝑡 ∙ cos𝜃𝑧 + sin 𝛾𝑡 ∙ sin𝜃𝑧 ∙ cos(𝛼𝑠 − 𝛼𝑡)) (4)

Where θtilt is the angle of incidence on a tilted surface, γt is inclination angle, θz is zenith angle of sun, αs is azimuth angle of sun and αt is azimuth angle of surface. The smaller the angle of incidence is the higher the sun will shine in the sky and provide more energy to PV modules. If the angle of incidence is zero then the PV module production is at its maximum and directly faced to the sun. Figure 2.7 shows that there are huge differences between the angle of incidence in the Southern and Northern latitudes. The sun will shine higher in South throughout the whole year.

2.4 Air Mass

The Air Mass (AM) is the path length which light takes through the atmosphere before reaching Earth’s ground. It quantifies the reduction in the power of light when it passes

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through the atmosphere. The more atmosphere solar radiation passes through on its way to ground, the less solar energy we can expect to get. The AM is defined as [6]:

𝐴𝑀 =1

cos𝜃𝑧 (5)

Where θz is the zenith angle. When the sun is directly overhead, sunlight passes through the least amount of atmosphere and in that case, the sun is 90° above the horizon and the zenith angle is 0° and therefore the AM is 1.0. Other way to calculate AM is by measuring the length of the shadow cast by a vertical pole. Figure 2.9 represents how AM can be calculated in that way.

Figure 2.9. Calculating AM from the shadow of a vertical pole.

The AM is the length of the hypotenuse divided by the pole height and therefore the AM is [6]:

𝐴𝑀 = √1 + (𝑠

ℎ)2

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Where s is the length of the pole shadow and h is the height of the pole. The above mentioned equations for air mass assumes that the atmosphere is a flat horizontal layer, but because of the curvature of the atmosphere, the air mass is not quite equal to the atmospheric path length when the sun is close to the horizon. At sunrise, the angle of the sun from the vertical position is 90° and the air mass is infinite, whereas the path length clearly is not. An equation which incorporates the curvature of the earth is [6]:

𝐴𝑀 =1

𝑐𝑜𝑠𝜃𝑧 + 0,50572 ∙ (96.07995 − 𝜃𝑧)−1,6364 (7)

Where θz is the zenith angle. For measurement and comparison of solar efficiency the standard AM is 1.5 and it is chosen because it represents the average AM at solar noon for optimally tilted PV arrays. With longer path lengths, there is more scattering and absorption of solar radiation by atmospheric constituents. Similar to solar radiation the AM varies with latitude of location, time of the day and the season of the year. Figure 2.10 shows AM in different latitudes in different time of the year which were calculated by utilizing equation seven.


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Figure 2.10. Air Mass in different latitudes.

As seen in the figure the AM is very big during winter in higher latitudes and sun has to pass thick layer of atmosphere. In Southern latitudes the AM is below five throughout the whole year. The monthly averaged AMs are calculated to the Table 2.5. Table 2.5. Monthly average Air Mass.

Air Mass


JAN 1.58 2.05 3.04 6.21 34.20 37.92

FEB 1.38 1.68 2.24 3.53 9.16 36.63

MAR 1.19 1.36 1.65 2.19 3.41 8,84

APR 1.07 1.16 1.32 1.58 2.05 3.05

MAY 1.02 1.07 1.17 1.33 1.60 2.08

JUN 1.01 1.04 1.12 1.25 1.46 1.83

JUL 1.01 1.06 1.14 1.28 1.52 1.94

AUG 1.05 1.12 1.25 1.46 1.83 2.56

SEP 1.14 1.27 1.50 1.90 2.72 5.11

OCT 1.31 1.56 2.01 2.96 6.06 27.83

NOV 1.53 1.95 2.80 5.28 27.23 37.92

DEC 1.66 2.20 3.41 7.84 37.92 37.92

2.5 Weather conditions

In arctic conditions snow is one factor that affects to PV module performance. In winter time, snow can pile up over the solar panel and reduce its performance. Solar panels can operate after a light snowfall, but if few centimetres of snow pile up, solar panels will no longer be able to produce electricity. If the panels are mounted steeply enough then snow will just fall off eventually. However, the proper angle for a system is usually determined by other factors such as making the panels face the sun directly. The summer sun is high in the sky and so panels are sometimes mounted on a shallow angle and left that way for the whole year. Although the panels will not be producing electricity when they are covered in snow, remember that winter is the least efficient season for solar panels. The difference between Southern and Northern European regions is huge because in South it snows on rare occasions unlike in North where it rains snow every winter.

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Wind will also affect to PV module performance by cooling the panels on the days when the air temperature is high. The amount and speed of wind depends on the location were the solar panels are installed. Usually it winds more in coastlines than in inner land and therefore the difference of the amount of wind between Northern and Southern Europe is not easy to compare.

The effect of relative humidity on the performance of the PV module is studied in many papers. Humidity is higher over oceans and coastal areas and lower over land areas. In the Arctic atmosphere, humidity is low and in some arctic places the air is almost as dry as in the deserts. In winter, humidity is low because surface temperatures are cold and very little water evaporates into the atmosphere. Results of studies of the effect of relative humidity in the solar panels efficiency have proved that low relative humidity will increase the performance of the PV module [9, 10].

2.6 Solar radiation

The accurate information of the solar radiation intensity is essential for the long-term evaluation of the PV module performance. For accurate estimation of PV module performance, it is important to know the global solar radiation throughout the year for the location that is observed. The amount of solar radiance varies at Earth’s surface. Latitude of location, time of the day and the year, cloud cover, and air pollution can cause variations in solar radiance. Figure 2.11 presents the yearly global irradiation in European countries.

Figure 2.11. Yearly sum of global irradiation in Europe [11, 12]. The scales on the left are: * Global Irradiation from < 600 to 2200 kWh/m2: yearly sum of global irradiation incident on optimally-inclined south-oriented photovoltaic modules and ** Solar Electricity from < 450 to 1650 kWh/m2: yearly sum of solar electricity generated by optimally-inclined1 kWp system with a performance ratio of 0.75.


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Different models have been used to estimate solar radiation. Available meteorological, geographical and climatological parameters such as sunshine hours, air temperature, latitude, relative humidity, and cloudiness have been utilized for estimating the amount of solar radiation. The Angstrom equation and its derivations have been widely used to estimate the monthly average daily global radiation. The angstrom equation can be defined as [13, 14]:

𝐻

𝐻0= 𝑎 + 𝑏 (

𝑛

𝑁) (8)

Where H is the monthly average daily global radiation, H0 is the monthly average daily extraterrestrial radiation, n is the monthly average daily hours of bright sunshine, N is the monthly average daylight, and a and b are empirical coefficients.

Utilizing the Angstrom equation requires measurements of monthly average daily hours of bright sunshine and therefore the monthly average daily global radiation is gathered from NASA data center where it is possible to get lot of information regarding to solar power and weather conditions in different locations. Table 2.6 present typical solar radiations in different latitudes and in Figure 2.12 the values from Table 2.6 are presented as a chart were it is easy to compare solar radiation values. Table 2.6. Average daily global irradiation in a month in different latitudes [3].

Solar radiation (kWh/m2/day)


JAN 3.24 1.81 1.02 0.34 0.00 0.00

FEB 4.33 2.73 1.77 1.10 0.24 0.00

MAR 5.49 3.97 2.83 2.46 1.13 0.38

APR 6.72 5.40 3.91 3.99 2.76 2.04

MAY 7.45 6.62 5.05 5.35 4.15 5.08

JUN 8.15 7.68 5.08 5.57 5.19 6.47

JUL 8.01 7.58 4.94 5.33 4.58 5.86

AUG 7.31 6.79 4.55 4.08 3.33 3.47

SEP 6.27 5.34 3.01 2.59 1.81 1.07

OCT 4.78 3.44 1.83 1.19 0.53 0.06

NOV 3.60 2.00 1.05 0.50 0.03 0.00

DEC 2.96 1.48 0.79 0.20 0.00 0.00


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Figure 2.12. Monthly average solar radiation in different latitudes.

From the figure it is easy to see that in Southern latitudes the average solar radiation is much higher than in Northern latitudes.

3. Existing testing standards and methods for PV panels

PV modules are an international business and if each country had its own set of standards for PV modules it would be really time consuming and expensive to participate in different markets. If a manufacturer would have to pass different qualification test sequences according to each region standard it would be confusing. The result would be chaos and therefore one set of worldwide standards have been drafted to make testing of PV modules easier and cost effective. Worldwide standards allow developers of new technologies to know what tests shall be performed and what specifications have to be fulfilled before they can commercialize their products. IEC is the leading global organization that develops and publishes international standards. TC82 “Solar photovoltaic energy systems” technical committee has developed and published a number of module and component measurement and qualification standards for PV modules. The European Distributed Energy Resources Laboratories (DERlab) has also developed technical guidelines for PV module outdoor testing. This chapter presents important standards and methods for PV module testing.

3.1 Standards relevant to power measurements

Standards developed for PV module power measurements are the IEC-60904 series of standards and the IEC-60891 which provides details on how to translate performance as a function of temperature and irradiance. This first set of standards was originally written for crystalline silicon modules but in the latest editions, these measurement standards have been modified to incorporate methods for measurement of thin film PV modules. [16] Figure 3.1 illustrates the standards relevant to PV module power measurements and what is the connection between them.

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Figure 3.1 Standards relevant to power measurements [17].

Table 3.1 presents a short description of the above presented standards. Table 3.1. Standards relevant to power measurements [17].

Scope IEC Standard Notes

Light source

IEC 60904-3: Measurement principles for terrestrial

photovoltaic solar devices with reference spectral irradiance

data

Defines the standard spectrum for STC

IEC 60904-9: Solar simulator performance requirements

Defines the characteristics of the solar simulators into classes A, B or C relating

to: – spectral distribution match

– irradiance non uniformity on the test plane

– temporal instability (STI and LTI) Measurement procedures for these

characteristics are included

Reference devices

IEC 60904-2: Requirements for reference solar devices

Includes selection, construction details and recommended packaging depending on

their use

IEC 60904-4: Procedure for establishing the traceability of

the calibration of reference solar devices

Includes different calibration procedures to get traceabilty to SI units

Test and reference devices

IEC 60904-5: Determination of the equivalent cell temperature

(ECT) of photovoltaic (PV) devices by the opencircuit

voltage method

Helps solve problem of determination of the temperature of a PV device

IEC 60904-8: Measurement of the spectral response of a photovoltaic (PV) device

Standard method for the determination of this basic characteristic

IEC 60904-10: Methods of linearity measurement

Methods for determining the linearity of the electrical characteristics of PV devices

vs. irradiance and temperature


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Light source and PV devices

IEC 60904-7: Computation of the spectral mismatch

correction for measurements of photovoltaic devices

Involved in the calculation are: – the experimental spectrum of the light

source – the standard solar spectrum (EN 60904-

3) – the spectral responses (absolute or

relative) of both test and reference PV devices

How to measure I-V curves

IEC 60904-1: Measurement of photovoltaic current-voltage

characteristics

Standard methods for measuring I-V curves, depending on the light source (natural or simulated: steady-state or

pulsed solar simulator)

How to translate I-V curves

IEC 60891: Procedures for temperature and irradiance corrections to measured I-V

characteristics of photovoltaic devices

From experimental to targeted irradiance and temperature

3.2 Standards for PV module performance testing and energy rating

One of the most relevant standards for PV module performance testing is developed by the IEC TC82. The standard for PV module testing is the IEC 61853 “Photovoltaic Module Performance Testing and Energy Rating” and it consists of four parts, which are presented in Table 3.2.

Table 3.2. IEC 61853 Photovoltaic Module Performance Testing and Energy Rating [18].

Scope IEC Standard Notes

Power rating at: AM = 1.5; AOI = 0; 7 irradiance levels;

4 temperature levels

IEC 61853-1: Irradiance and temperature performance measurements and power

rating

Describes requirements for evaluating PV module performance in terms of power

(W) rating over a range of irradiances and temperatures. (Published January 2011)

Power rating adjustments to:

AM = any; AOI = any

IEC 61853-2: Spectral response, incidence angle and module

operating temperature measurements

Describes test procedures for measuring the effect of varying angle of incidence

and sunlight spectra as well as the estimation of module temperature from

irradiance, ambient temperature and wind speed. (Q3/2013)

Energy rating calculations

IEC 61853-3: Energy rating of PV module

Describes the calculations for PV module energy (Wh) ratings. (Ongoing)

Time period and weather conditions

IEC 61853-4: Standard days Describes the standard time periods and

weather condition that can be used for the energy rating calculations. (Ongoing)

The IEC 61853 standard was created to make a more accurate comparison between different photovoltaic modules performances in different conditions. The first part of the standard was published in January 2011. This standard specifies the performance measurements of PV modules at 22 different sets of temperature and irradiance conditions, as shown in Table 3.3, using either a solar simulator (indoor) or natural sunlight (outdoor). There are several possible indoor and outdoor techniques, and this standard allows many of them. Validation of these techniques for repeatability over time within the same laboratory and for reproducibility among multiple laboratories is extremely important for the successful


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implementation of this standard. The power rating measurements at various temperatures and irradiance levels are more challenging under prevailing outdoor than under controlled indoor conditions. [18] Table 3.3. Availability of IEC 61853-1 performance parameters at various irradiances and temperatures [19].

Irradiance Spectrum Module Temperature

15 °C 25 °C 50 °C 75 °C

1100 W/m2 AM1,5 NA

1000 W/m2 AM1,5

800 W/m2 AM1,5

600 W/m2 AM1,5

400 W/m2 AM1,5 NA

200 W/m2 AM1,5 NA NA

100 W/m2 AM1,5 NA NA

A table of each of the parameters Isc, Pmax, Voc and Vmax, shall be made according to the table above. In outdoor testing environment the outdoor temperature cannot be much affected but to obtain different irradiance levels it is possible to use mesh screens with different percent transmittance. Example screens are shown in Figure 3.2.

Figure 3.2. Meshed screens for different radiation levels. [18]

3.3 Technical Guidelines on Long-term Photovoltaic Module Outdoor Tests

DERlab has developed technical guidelines on long-term photovoltaic module outdoor testing. It provides instructions on the test setup, requirements for the testing location, accuracy requirements for the measurement equipment, maintenance of the equipment and recording time intervals for PV module testing under outdoor conditions. The main scope of the technical guidelines is the measurement of the specific energy yield per year of PV modules under outdoor conditions. Long-term measurements consists of continuous recording of meteorological and electrical data for at least one year, which can be utilized in modelling, performance evaluation and energy yield evaluation in different outdoor conditions. Data evaluation and analyses are not covered in the DERlab technical guidelines, [20]. Figure 3.3 shows the general test setup for long-term module performance testing according to DERlab technical guideline.


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Figure 3.3 Exemplary plan of a standard testing setup [20].

The PV modules are installed in an optimal tilt angle related to the geographical latitude of the test site and azimuth angle is towards the equator. The measurement equipment for energy yield measurements consists of ambient and module temperatures, irradiance in the module plane and current and voltage measurements at the Maximum Power Point (MPP). PV modules have to operate at MPP at all the time except during I-V-curve measurements. In addition, global horizontal irradiance and wind speed can also be measured, but it is not mandatory. The recommended recording time interval is 15 seconds or shorter and the interval for averages has to be 1 min, 5 min or 15 min [20].

4. Testing environments for arctic conditions

4.1 Outdoor testing environment

Outdoor testing environment is going to be built to the roof of building in Biologinkuja 5 in Otaniemi, Espoo and it is based on DERlab technical guidelines on long-term photovoltaic module outdoor testing setup, which is shown in Figure 3.3. Figure 4.1 shows the location of outdoor testing environment on a map.

Figure 4.1. Location of PV outdoor testing environment in Espoo.

Outdoor testing setup consists of three PV panels, three one-phase inverters, radiation, temperature, humidity and wind speed and direction measurement devices. In following subchapters devices will be described in more detail.


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4.1.1 PV panels

QCell 250W PV panels are going to be installed to the testing system. QCell panel is a Polycrystalline Silicon panel and its peak efficiency is 15%. Table 4.1 shows detailed information of the QCell solar panel.

Table 4.1. QCell QPRO-G3-250 (250W) Solar panel. [21]

PERFORMANCE AT STANDARD TEST CONDITIONS

STC: 1000 W/m2, 25 ˚C, AM 1.5 G SPECTRUM

Nominal Power 250 W

Average Power 252.5 W

Short Circuit Current (Isc) 8.71 A

Open Circuit Voltage (Voc) 37.49 V

Current at Pmpp 8.21 A

Voltage at Pmpp 30.76 V

Efficiency (Nominal Power) ≥ 15 %

PERFORMANCE AT NORMAL OPERATING CELL TEMPERATURE

NOCT: 800 W/m2, 47 ± 3 ˚C. AM 1.5 G SPECTRUM

Nominal Power 250 W

Average Power 186.0 W

Short Circuit Current (Isc) 7.03 A

Open Circuit Voltage (Voc) 34.90 V

Current at Pmpp 6.44 A

Voltage at Pmpp 28.89 V

TEMPERATURE COEFFICIENTS

Temperature Coefficient of Isc +0.04 %/K

Temperature Coefficient of Pmpp -0.42 %/K

Temperature Coefficient of Voc -0.30 %/K

PROPERTIES FOR SYSTEM DESIGN

Maximum System Voltage 1000 V

Permitted module temperature on continuous duty

-40°C up to +85°C

4.1.2 Inverters

AEconversion INV-250-45EU (230V/50Hz) micro inverters which are developed for PV and storage systems, shown in Figure 4.2, are going to be installed to the testing system. These micro inverters include communication and monitoring solutions and these micro inverters are used to convert the generated direct current into grid-compliant alternating current.


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Figure 4.2. AEconversion INV-250-45EU (230V/50Hz) micro inverter. [22]

Table 4.2 shows detailed information of the inverter.

Table 4.2. Information of AEconversion INV-250-45EU (230V/50Hz) micro inverter. [22]

AEconversion INV-250-45EU specification

DC Max Power 250 W

DC Max Voltage 45 V

DC Operating Voltage 18 – 45 V

DC MPPT operating voltage 20 – 40 V

DC Max Current 11 A

AC Power Max 240 W

AC Voltage 230 V

AC Voltage Range 184 – 264 V

AC Frequency 50.0Hz

AC Frequency Range 47.5 - 51.5 Hz

AC Nominal Current 1.0 A

AC Power Factor > 0.99

Peak Inverter Efficiency 93.5 %

CEC Efficiency 91.4 %

Nominal MPPT Efficiency 99.8 %

Operating Temperature -25°C up to +70°C

4.1.3 Radiation measurements

Radiation measurements are going to be measured with Kipp&Zonen CMP 11 Pyranometer, shown in Figure 4.3, which is built for PV installations and for routine global solar radiation measurement research on a plane and level surfaces.


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Figure 4.3. Kipp&Zonen CMP 11 Pyranometer. [23]

The CMP 11 is fully compliant with ISO 9060:1990 specification for a First Class pyranometer. The sensing element is coated with a highly stable carbon based non organic coating, which delivers excellent spectral absorption and long term stability characteristics. The Pyranometer gets the needed power from the amount of incoming radiation. Table 4.3 shows specifications of CMP 11 Pyranometer. [23]

Table 4.3. Kipp&Zonen CMP 11 Pyranometer specifications. [23]

CMP 11 Pyranometer specifications

Spectral range 285 to 2800 nm

Sensitivity 7 to 14 µV/W/m²

Response time < 5 s

Zero offset A < 7 W/m²

Zero offset B < 2 W/m²

Directional error (up to 80° with 1000 W/m² beam)

< 10 W/m²

Temperature dependence of sensitivity (-10 ºC to +40 ºC)

< 1 %

Operational temperature range -40 °C to +80 °C

Maximum solar irradiance 4000 W/m²

Field of view 180 °

4.1.4 Temperature and humidity measurements

Vaisala HMT120 will be installed to the testing system to measure ambient temperature and humidity near the PV panels. HMT 120 is designed for humidity and temperature monitoring in cleanrooms. It can be also installed outdoors when using Vaisala Radiation Shield. HMT120 is shown in Figure 4.4.


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Figure 4.4. Vaisala HMT120 temperature and humidity measurement device.[24]

Operating range of Vaisala HMT120 for ambient temperature is from -40 to +80 °C and for humidity from 0 to 100 %RH. [24] PV panel temperature will be measured with a copper-constantan thermocouple to get accurate measurement values from the surface of the panel. Temperature range for copper-constantan thermocouples is from -200 to +200 °C.

4.1.5 Wind speed and direction measurements

Ideally installation of a wind speed sensor (anemometer) should be, according to IEC 61215, at 1.2m distance on the East or West side of the system and at 0.7m above the upper edge of the system. This also shown in Figure 3.3. SMA Anemometer, in Figure 4.5, will be installed to the testing system for wind speed and direction measurements.

Figure 4.5. SMA Anemometer.[25]


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Accuracy of SMA Anemometer is +-5% and measuring range is from 0.8 m/s to 40 m/s. Maximum measuring speed for shorter periods is 60 m/s. Ambient temperature conditions during the operation are from -25°C up to + 60°C. [26]

4.2 Hardware simulation testing environment

A Hardware simulation testing environment was built so that customizable testing setup of PV panels can be fine-tuned. Hardware simulator consists of Magna-Power Electronics programmable DC power source, ET instrumente GbmH controllable power electronic load, Measurement computing sensor modules for voltages and temperatures, and voltage divider & shunt setup for converting measured voltages suitable for the sensors. Figure 4.6 displays connection diagram of the simulation setup.

DC

10kΩ

1kΩ

ABC

Earth

10kΩ

1kΩ

0,5Ω

10kΩ

1kΩPower electronic load

Power source

U1 U2

U3

Figure 4.6. Connection diagram of the simulation setup.

Current is measured as a voltage difference over a 0,5 Ω resistor.

4.2.1 Magna-Power Electronics DC power supply

Magna-Power Electronics XR Series III 6 kW DC power supply is used for the voltage input in the simulation setup to represent PV array. Maximum voltage of the unit is 100 V and maximum current 100 A. Actual values are limited to smaller to simulate 100 W panel. Figure 4.7 displays control panel of the power supply. Power supply is controlled with Photovoltaic Power Profile Emulation software (PPPE) described in following chapter.

Figure 4.7. Instrument panel of the Magna-Power electronics power supply.

4.2.2 Magna-Power Electronics Photovoltaic Power Profile Emulation Software

PV modules produce a DC output with non-linear characteristics varying as a function of temperature and irradiance. The voltage and current characteristics of PV modules fluctuate with temperature and irradiance of the panels. Irradiance is mainly affecting to output current


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and temperature to output voltage of PV module. Magna-Power Electronics Photovoltaic Power Profile Emulation (PPPE) software was utilized in the simulation of the actual output of the PV modules. PPPE software enables to emulate the non-linear characteristics of a PV module and vary these characteristics over a time as a function of temperature and sunlight. [15] PPPE Software generates non-linear voltage and current profiles based on the EN 50530 “Overall efficiency of grid connected photovoltaic inverters” standard. These profiles can be sequentially sent to the power supply, allowing variations in solar parameters to change the power supply output characteristics over user-defined intervals. In addition, an interpolation function allows automated profile generation between curves, for smooth transitions from one temperature and irradiance condition to another. This functionality can be used, for example, to transition from cloudy winter conditions to summer sunny conditions. Figure 4.8 illustrates the main screen of PPPE software. [15]

Figure 4.8. Main screen of Magna-Power simulator software [15].

There are three different methods to generate a power profile in PPPE software [15]: • Reference Parameters: If a reference solar cell and array characteristics are known, profiles can be generated using solar cell parameter values: Tref, Irref, Vmp, Imp, Voc, Isc, β, and α. A drop-down is provided to select polysilicon (cSi) or thin film technology, to autopopulate the β and α values in accordance with the EN 50530 standard. After populating the reference values, generating a new curve is as simple as specifying a temperature and irradiance value for each new curve. • 4-Parameter: The simplest profile generation method, profiles can be generated using just the maximum power point voltage, maximum power point current, open-circuit voltage and

short-circuit current:): Vmp, IMP, Voc, and Isc.


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• Up to 50-manual points: A manual curve can be used with PPPE software, either by manually entering voltage and current points, or importing values from a comma separated value (.csv) file. A maximum of 50-point can be entered, however, the power supply will perform a piecewise linear approximation between these points during operation, ensuring high resolution.

4.2.3 ET instrumente controllable power electronic load module

Power electronic 1000 W DC load module manufactured by ET instrumente is used for the loading the PV simulator. Figure 4.9 displays control panel of the load module. This control panel displays voltage, current and selected control mode. Module is controlled externally via C-code written to Matlab software.

Figure 4.9. Instrument control panel of the load module.

Maximum current of the load module is 60 A and maximum voltage is 60 V, with limitation of power to 1000 W. U/I characteristic of the load are presented in Figure 4.10.

Figure 4.10. U/I characteristic of the ESL 1000 load module.

4.2.4 Computer algorithm and measurement principle

Measurement and control of the load module is done with Matlab software. Flowchart of the code is presented in Figure 4.11.


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Load driversAsk time and date

when measurement ends

Adjust load current

incrementally

Start measurement

sycle of 30s

End measuement

cycle

Read data to vector

Filter data for noise and modify with calinbration

settings

End time is larger than current

time=?

no

yes

End measurement

Calculate voltage, current and power

Find maximum power anc

corresponding current and voltage

Write Pmax,U,I temperature

measurements and time toTXT-file

Figure 4.11. Flowchart of the Matlab script.

Measurement script in matlab is built so that measurement can be left running continuously and data is not destroyed if there is a failure in computer running the script. Results are saved to txt-file results of which are easily imported to various applications.

5. Development needs for PV testing methods in arctic conditions

The arctic conditions bring into consideration ambient temperature down to -50 C, include frost, heavy winds, snowfall and icing. In Northern countries arctic conditions occur in the wintertime when also the angle of incidence is high, daylight time is short and air mass is high in Helsinki area and air mass is increasing to the north. Irradiance and the panel temperature have the greatest impact on PV module performance. Irradiation decreases when moving from equator towards North but the conversion efficiency of PV modules increases because the ambient temperature decreases. These factors have been described in detail in Chapter 3. Current standard IEC61853-1 defines the normal conditions at air mass AM 1.5 and normal incidence irradiance, see Table 5.1.

Table 5.1. Summary of reference power conditions at AM 1.5 [19]

Condition Irradiance W/m2 Temperature C

STC Standard Test Conditions 1000 25 of cell

NOCT Nominal Operating Cell Temperature

(Determined acc. to IEC 61215 or IEC 61646) 800 20 of ambient

LIC Low Irradiance Condition 200 25 of cell

HTC High Temperature Condition 1000 75 of cell

LTC Low Temperature Condition 500 15 of cell

NOTE The conditions provided in this table may be measured directly as part of the performance Matrix defined in Clause 8 of IEC61853-1


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The standard defines different cell temperature values, the lowest value 15 C could represent the spring and autumn conditions in Finland, latitude 60N. The low irradiance condition 200 W/m2 in the table represents more or less summertime as the spring and autumn conditions lie within 100 and 150 W/m2 and during the winter less than 50 W/m2, see Table 2.6. The air mass is only five months less than the value 1.5 selected for IEC61853-1 reference conditions. During the winter months the air mass value ranges from 1.9 to 7.8 at latitudes close 60N, see Table 2.5. Low irradiance values, high angle of incidence and high air mass values originate from the short daylight time during the wintertime. No method is available to take weather conditions like wind, rain, relative humidity, frost snowing and icing into account. These conditions except the wind will probably decrease the obtainable energy as the reflection of a snow layer will increase it. Real outdoor testing will result in a mixture of the conditions mentioned above. The standardized test method is important in comparing different type solar cells from different manufactures. The testing or simulating at the mixture of the conditions at certain latitudes would give information for selecting the best PV panels and accepting the performance obtainable. According to DERlab guide Technical guidelines on long-term photovoltaic module outdoor tests, [20] measurement data shown in Table 5.2 is collected from the outdoor testing site for comparison between different test cases.

Table 5.2. Long-term PV module outdoor testing measurement data.

Meteorological data

Equipment Measurement Parameter

Pyranometer Radiation in tilted module plane (Gtmod)

Temperature sensor Ambient temperature (Tamb)

Temperature sensor Module(cell-) temperature (Tmod)

Pyranometer Global horizontal radiation (Ghor)

Anemometer Wind speed

Pyranometer Diffuse radiation (Gd)

Pyranometer Direct radiation (Gb)

Pyranometer Reflected radiation (Gref)

Wind vane Wind direction

Hygrometer Air humidity

Electrical data

MPP meter Current at MPP (Impp)

MPP meter Voltage at MPP (UMPP)

MPP meter SC current (ISC)

MPP meter OC voltage (UOC)

MPP meter Characteristic U-I-curve

The outdoor testing site and simulation software, described in chapter four, will be utilized in developing new methods for PV module outdoor testing. Weather conditions like wind, rain, relative humidity, frost snowing and icing will be studied. In addition, the effect of angle of incidence, short daylight time and high air mass to PV module output are going to be very

important part of the studies. Also cold ambient temperatures, down to -30 C, can be studied in the area of Espoo.


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6. Conclusion and future work

In some parts of Finland there is almost as much radiation available as in Northern parts of

Germany where huge amount of solar power have already been installed. During summer

the amount of radiation is higher and during winter it is lower in Finland compared to

Germany. In Northern parts of Finland the issues in solar power are during winter when the

time of daylight is short and the angle of incidence and air mass are high which means that

there is not much radiation and solar power available during winter. One solution for this

problem is to build adjustable solar panel racks which can be used to change the tilt angle of

the system for example between the summer and winter. This will not solve the lack of

daylight problem but it will give little more energy during winter because the panel tilt angle

can be changed to steeper angle and it will get more radiation into it. In addition, in

wintertime the load in the grid is at highest when there is not much solar power available and

in summer there is much solar power available but not so much load. This is not the problem

in the grid point of view but if the produced energy in summer could be stored and used

during winter it would be profitable in terms of energy use but at the moment that is not

economically viable.

Testing PV panels in cold climate has not been covered extensively yet and therefore VTT

has decided to build their own outdoor condition testing site for solar panels. VTT is also

looking to focus more research on aspects of arctic solar energy in near future. This outdoor

testing site is used together with Magna-Power simulation software to study PV panels in

arctic conditions, especially the effect of cold temperature and lighting conditions in

wintertime. In future work the outdoor testing site will have a big role in developing new

methods for PV module outdoor testing that will include important aspects for arctic

conditions.

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