The SGS GmbH (Societe Generale Surveillance) is a worldwide company for product testing, certification and verification. The service field includes agriculture, automotive, consumer testing, environmental, industrial, life science, minerals and chemicals. The SGS SOLAR TESTHOUSE, as a part of SGS GmbH was established in the summer of 2009 in Saxony, a region where many of the german solar technology manufacturers are located. The business field of the Solar Test House is Photovoltaic (PV) module testing & certification for performance tests, durability, safety and compliance with legal regulations for IEC standards. The testing of PV modules consists of optical investigations, mechanical stress tests (wind and snow), climate simulation (heat, humidity and frost), hail impact, electrical properties (sun simulation) and safety tests. The testing facilities are allowing large size PV modules up to 2.2m x 2.6m.
1.2 PV module testing
The PV module testing follows the IEC standards as specified below:
ï‚· IEC 61215:2005 crystalline silicon terrestrial photovoltaic (PV) modules - design qualification and type approval
ï‚· IEC 61646 ed. 2.0: 2008: thin-film terrestrial photovoltaic (PV) modules - design qualification and type approval
ï‚· IEC 61730 ed.1: 2004: photovoltaic (PV) module safety qualification - part 1: requirements for construction, part 2: requirements for testing.
The IEC 61646 standard for thin-film PV modules concerns many aspects identical to the international standard IEC 61215 for crystalline modules. The main difference between the two standards is the additional test procedures for light soaking adapting to the special properties of thin-film technologies. Part of IEC 61730 describes the testing requirements for photovoltaic modules in order to provide safe electrical and mechanical operation during their expected lifetime. Specific topics are provided to assess the prevention of electrical shock, fire hazards, and personal injury due to mechanical and environmental stresses.
Since the reliability of most of the PV modules are guaranteed for 25 years, the modules need to be tested in various environmental conditions. Some of the modules might fail because of overheating, short circuit, improper isolation, water penetration into glass cover, or EVA [1] browning. The IEC 61215 performance test describes the requirements of several test items. It includes mainly of sun simulator tests (flash, steady state, UV and light soaking), climate tests (changing of climates, coldness, warmth, humidity) and mechanical load tests (hail, wind pressure, snow) as well as electrical insulation tests. The tests are passed for following pass criteria: the degradation of maximum output power does not exceed the prescribed limit after each test nor 8 % after each test sequence; no sample has exhibited any open circuit during the tests; there is no visual evidence of a major defect; the insulation test requirements are met after the tests; the wet leakage current test requirements are met at the beginning and the end of each sequence and after the damp heat test; specific requirements of the individual tests are met. [2]
1.3 Solar cell
A solar cell is a semiconductor diode which is made up of a thin (approx. 200 µm), flat wafer material (in case of crystalline silicon). The material is typically crystalline or polycrystalline. Solar cells absorb and convert the sun's light energy into electrical energy.
1.3.1 Fundamentals of solar cell
Sunlight is composed of portions of solar energy called "photons". When the sunlight particles are absorbed in the active layer of a solar cell, the light energy excites the holes and electrons into higher energy states. These photons, then emitted by a radiator have different wavelengths. The lower the wavelength has the higher the energy of the photons and excited of the electrons. Figure 1.1 shows the description of valence band and the conducting band of a semiconductor. When the light is absorbed in a solar cell, the electrons are excited [3] from the valence band and lifted up to the conducting band.
Figure 1.1 Photoexcitation of electrons from the valence into the conduction band of a semiconductor (left panel). Semiconductor under constant illumination (right panel) [4]
The energy of the photons with particular wavelength is given by the formula 1.1;
1.1
h is Planck's constant, c is the speed of light and λ is wavelength. The unit of wavelength is in nanometer (nm). If the energy of the photons is smaller than the energy gap, then no electron will be able to cross the band gap. When the energy of the photons is larger than the energy gap, electrons will reach the conducting band, as shown in Figure 1.2. All energy exceeding the band gap energy will be given as heat to the lattice - this energy is lost for photovoltaic energy conversion.
Figure 1.2 Band gap between valence and conduction band [5]
At room temperature, most of the electrons remain in the ground state. With light absorption, energy is transferred to some electrons exciting them into the conducting band, while leaving holes in the valence band
The excited electrons overcome the band gap and leave a hole in the conducting band. The holes behave like electrons but have a positive charge and exist in the valence band. Under an applied voltage bias they would travel in opposite directions like electrons.
A crystalline silicon solar cell is composed of mainly two semiconductor materials, n-type semiconductor (with excess electrons) and p-type semiconductor (with excess holes). The doping atom has either three (e.g. boron) or five (e.g. phosphorous) valence electrons, which are one less or one more than silicon's four. When p-type and n-type semiconductor materials come in contact with one another, the holes and electrons begin to diffuse in opposite directions, due to the concentration difference on either side of the junction. This diffusion is shown in Figure. 1.3. The n-type region has a high electron concentration and the p-type region has a high hole concentration. The electrons diffuse from the n-type region to the p-type region. Similarly, holes flow, by diffusion, from the p-type region to the n-type region. The holes in the n-type material and the electrons in the p-type material become minority charge carriers and recombine with the oppositely charged majority carriers. Minority carriers which reach the edge of the space charge region are swept across it by the electric field, which drops across the junction.
Figure 1.3 The majority charge carrier's movements after joining a p- and n-type semiconductor
The movement of electrons from the valence band to the conduction band creates an electron hole pair. The movement is caused by energy from photons that bring the electrons from a lower energy level to a higher energy level across the energy band gap. Current, which is the sum of electrons and holes that travel in opposite directions through the circuit, but in opposite directions is generated by this movement of electrons.
Figure 1.4 Electrons and Hole's position after being excited by a photon with sufficient energy
When the external voltage is applied to a p-n junction, the process is called bias. The external voltage, in this case, is in the form of a battery source supplied to a p-n junction. In forward bias, the positive terminal of the voltage source is connected to the p-type material and the negative terminal of the voltage source is connected to the n-type material. The positive potential repels holes to move toward the junction while the negative potential repels electrons to move toward the junction which neutralizes charge carriers on both sides of the barrier. This, in turn, decreases the width of the barrier. In p-n junction, under forward bias conditions, majority carriers diffuse into the neutral region where they will reconnect.
Figure 1.5 P-N junction in solar cell [6]
The current in a p-n junction consists of electrons in the conduction band and holes in the valence band, as shown in figure 1.5. The current flow may be caused due to diffusion of the charge carrier or by drift. This is a result of a concentration gradient or an applied electric field, as shown in figure 1.6
Figure 1.6 Forward bias P-N junction. In the regions outside the depletion regime carriers move by diffusion and are accelerated across the junction by the internal electric field
The purpose of an external voltage in the forward-bias p-n junction is to reduce the barrier potential across the junction and to allow the majority of charge carriers to cross the junction. An electron leaves the conduction band of the p-type material and moves to the conduction band of the n-type material (vice versa for holes in the valence band).
1.3.2 How a PV cell works
A conventional p-n-type solar cell scheme is shown in Figure 1.7. The p-n-junction is located near the surface of the solar cell where these two p-n materials are in contact. When the solar cell is exposed to sunlight, photons create minority charge carriers throughout the entire volume of the light absorber. By diffusion, these carriers are driven towards the junction where they become swept into regions where they become majority charge carriers. If no external load is connected to the device, the cell produces a photo-voltage. If a load is present, this electric potential difference drives a current through it.
3.3 I-V characteristics
A PV solar cell can be considered as a current source when used in parallel with a diode in an electrical circuit. The performance and current generation of a solar cell depend on the intensity of the sun light. When there is no light, a PV solar cell behaves like a diode. On the contrary, if the cell is exposed to light, the incident photons generate a current being proportional to the light intensity.
Figure 1.8 shows three I-V characteristics of a solar cell diode. In the first case, there is no voltage applied to a p-n junction solar cell diode. The result is a zero bias. In the second case, a positive voltage is applied to a p-n junction. This can be done in the form of an incident light source striking the solar cell, producing a positive current. It is also referred to as a forward bias. In the third case, a negative voltage is applied to a p-n junction. This may be due to a lack of incident light striking the solar cell. The result is called reverse bias and the current produced during this process is called dark current
Figure 1.10 shows the current produced by a PV solar cell. In most cases, it depends on the type of semiconductor material that has been used to manufacture the solar cell. The solar cell is characterized by the maximum open circuit voltage (Voc) which is at zero output current and a short circuit current (Isc), which is at zero output voltage. In both cases the cell does not deliver any electrical power.
The power output can be computed by the product of current and voltage, as equation
1.2
In formula 1.2, the power output will depend on the current generated by a solar cell and this current is affected by the intensity of solar radiation.
Figure 1.11 shows an example of a solar cell with current and voltage output in relation to the radiation intensity.
1.3.4 Types of solar cells
There are two primary types of PV technologies that are currently available commercially. These types include crystalline silicon and thin film materials. In crystalline silicon technologies, individual solar cells are cut from large single crystals or from ingots of crystalline silicon. In thin film solar cell technologies, the solar cell materials (mostly direct semiconductors) are deposited on glass or thin metal that mechanically supports the cell or module.
In the solar cell industry, crystalline silicon is the most common semiconductor material used for solar cell manufacturing and accounts for more than 80% of the solar cells on the market. Thin film account for only 20%.
Table 1.1 Characteristics and types of solar cells
Type
Mono-crystalline
Poly-crystalline
Thin-film
Cell
Characteristics
Mono-crystalline solar cells are produced from a very pure semiconducting material that is extracted from melted silicon and then cut into thin plates or ingots. They have a higher level of efficiency than other types of solar cells.
Poly-crystalline solar cells are produced from liquid silicon which is poured into blocks that are subsequently cut into plates or ingots. During the solidification process crystal structures of different sizes are formed and during solidification crystal defects are formed.
An amorphous solar cell is produced when the silicon film is deposited on glass or another substrate material. This type is often called thin layer solar cell because the layer thickness amounts to less than 1µm. The efficiency of amorphous solar cell is much lower than that of the other two solar cell types.
Cost
More expensive than other technologies
Less expensive than Monocrystalline
Least expensive of the technologies
1.4 PV System
Figure 1.12 shows a simplified diagram of a PV system which consists of a PV generator with several PV modules (connected in parallel or in series). The inverter, which converts the direct current (DC) to alternative current (AC), can be fed into the national grid. The batteries are used to store the DC voltage. The charge regulator regulates the charging current to the battery in order not to overcharge or deep discharge the battery. In practice, PV cells are grouped into modules, which are sealed units of convenient size for the purpose of handling. Modules, in turn, can be grouped into an array to customize the electrical output for desired voltage or current.
1.5 Solar Spectrum
In the space, the sun spectral profile referenced to Air Mass Zero (AM0) is close to the black body spectral profile at T = 5800K. Air Mass (AM) is the measure that is used to define the ratio of the mass of atmosphere through which solar radiation passes to the mass it would pass when the sun is directly overhead (Zenith).
On the earth's surface, AM1.0 is at sea level when the sun is at zenith. When the sun is at 480 from zenith , it is AM1.5. It is AM2.0 when the zenith angle θz is 600.
For general estimation with zenith angle: 0 < θz < 700
1.3
On the earth, it is modified by the filtration and diffusing effect of the atmosphere and by the reflection on the earth's surface. Depending on the altitude and latitude, two solar spectra are generally defined and employed as standard spectra for solar cell characterization, the AM1.0 and AM1.5. The solar radiation varies significantly with location, atmospheric conditions, time of day, earth sun distance etc, as shown in Figure 1.13. In order to do a comparison between the performances of solar cells tested at different locations, a terrestrial solar radiation standard has been defined and all measurements are referred to at this standard. The solar irradiation at AM1.5G is the power density from the integrated solar spectrum. For AM1.5G, this is approximately 970 W/m2. The value of the irradiance is close to the maximum received at the earth's surface and implies the standard spectral distribution. This value is approximately at STC (Standard Test Condition) at 1000W/m2 for solar cells in which most of the solar cells are tested. For indoor characterization and reliability tests, most commercial solar simulators are designed to emit illumination of wide range wavelength close to this AM1.5G spectrum by means of appropriate light sources, optics and special filter arrays.
In the PV industry, STC corresponds to the irradiance of 1000W/m2 and the spectrum of sunlight incident on a clear day at AM1.5 with the sun at an angle of 41.19 ° above the horizon and a temperature of 25oC. The output power specified for a solar cell is measured under these standard test conditions and denoted in units of Wp [7] . Figure 1.14 shows the spectral power density of a black body at T = 5762K. The extraterrestrial spectrum is the AM0 (no air mass considered) and the terrestrial spectrum is the AM1.5G at the earth's surface. The change from extraterrestrial spectrum radiation AM0 to AM1.5 radiation is caused by the atmospheric attenuation which is due to scattering and absorption of the radiation by clouds, air molecules, dust particles or aerosols in the atmosphere. In solar simulators special lamps as high-pressure short-arc Xenon lamps (T = 5800 K), are used which their emission spectrum approximates best to the sun spectrum (black body). There are suitable filters allow to simulate AM0, AM1.0 and AM1.5 specifications.
1.6 Characterization of PV Module and Solar Simulator
According to the IEC standard 60904-3 [8] , the PV module performance rating is determined by measuring the maximum power of a PV module in a laboratory under standard test conditions (STC, 1000 W/m2, air mass AM1.5 radiation and sample temperature 25oC). The other procedure known as NOCT (Normal Operating Cell Temperature) rating is used. This method is the actual operating cell temperature when the irradiance is 800 W/m2, ambient temperature is 20°C, the wind speed is 1 m/s at a module tilt-angle 450, and the cell (or module) in an electrically open circuit state. In this procedure the module first equilibrates with a specified ambient temperature and maximum power is measured at a nominal operating cell temperature. The NOCT rating has lower power value than the Wp rating, but it is probably more relevant measurement for a PV module.
Customized solar simulators are used in the PV module testing laboratories and industry. The purpose of using solar simulator is to simulate the natural sunlight from the atmosphere (at AM1.5G conditions). The testing in this case is not depending on the weather conditions but the adjustment of the radiation range and module temperature. There are three ratings of solar simulators and within each of these criteria the 'A' is the top rating and 'C' is the lowest rating. As shown in Table 1.2 the classification of simulators has three criteria levels: spectral match, irradiance homogeneity and stability over time. 'AAA' level is described in Figure 1.14.
Fig. 1.14 Solar simulator classification levels (3-level rating)
Table 1.2 Classification of solar simulators for PV-power measurements [9]
Quality Indicator
Method
Classification
A
B
C
Spectral match to AM1.5 reference spectral irradiance (IEC 60904-3)
Ratio of contributions to intensity in 6 wavelength ranges in nm
(400-500-600-700-800-900-1100:
solar simulator/AM1.5G reference)
0.75-1.25
0.6-1.4
0.4-2.0
Non-uniformity of irradiance
Monitoring of irradiance distribution in the test area. Calculation from minimum and maximum of irradiance
<2%
<5%
<10%
Temporal stability of emitted light
(LTI = long term instability )
Monitoring of irradiance at a fixed point in the test area. Calculation from minimum and maximum light intensity
<2%
<5%
<10%
1.7General description of given tasks
The main task of the thesis is classifying the sun simulators. There are three types of measurements need to be tested in four chambers. The tests are "Spectral match to AM1.5 reference", "Non-uniformity of irradiance" and "Long term instability test, LTI" as shown in Table 1.2. The test facilities are a spectrometer and an X-Y scan table equipped with pyranometers.
The first task is to measure the spectral radiance distribution of the sun simulators and to compare the ratio to the AM1.5 spectrum by using a spectrometer. The simulators require a defined spectral match with ratio to the AM1.5 reference spectral distribution from 400nm to 1100nm wavelengths.
The second task is to measure the uniformity and long term instability of the irradiance. The non-uniformity of irradiance test is to define the homogeneity of whole test field in sun simulators. The test field is the same or larger size of the largest PV module. The purpose is to make sure the irradiance is uniform under all lamps. The X-Y scanner table is equipped with pyranometers which can scanning to the whole testing field inside of the chambers, then evaluate the non-uniformity and light intensity under the different kinds of halogen lamps. Those three measurements have been prepared in 4 sun simulators called flasher, steady state, UV and light soaking sun simulators.
Chapter 2 Theoretical background
2.1 Photometric physical parameters and solar spectrum
ï¬ Planck's law
The relationship between temperature (T) and wavelength (λ) of a body emitting radiation is described by Planck's law. It explains the relationship (of the spectral radiance) of electromagnetic radiation (at all wavelengths) emitted from a black body with an absolute temperature T. Planck's law is thus alternatively called black body radiation law and its relationship between temperature ad wavelength. As a function of wavelength (λ) and temperature (T), Planck's law can be written as,
2.1
Where, I is the spectral radiance which is energy per unit time per unit surface area per unit solid angle per unit wavelength, λ is the wavelength in nanometers, T is the absolute temperature of the emitting body (K), h is the Planck constant (6.62606896Ã-10-34 J·s), c is the vacuum speed of light (299,792,458 m/s), e is the base of the natural logarithm and k is the Boltzmann constant (1.380 6504 Ã- 10-23 J /K). From the equation 2.1, if the wavelength (λ) and temperature (T) of the lamps used to produce light in a spectrum simulator are known then their (ideal) spectral radiance can be computed under the assumption that the lamp emits the light only by thermal heating of a body. i.e. a lamp filament in an ordinary bulb.
Figure 2.1 shows, the blackbody radiation of the solar spectrum at T=5500K and the daylight spectrum. It is noted that at higher temperatures the maximum radiance of the spectrum shifts to shorter wavelength and vice versa.
Figure 2.1 Planck's blackbody radiation and daylight spectrum [10]
ï¬ Blackbody Radiation
Blackbody radiation is the electromagnetic radiation which is given out by a blackbody material. Blackbody materials are substances which absorb all electromagnetic radiation that comes in contact with it, including light and heat. In reality a perfect blackbody material does not exist in nature but substances, such as ash, soot [11] , and granite material come close to blackbody properties. When a blackbody material is heated it emits thermal radiation in a form of heat. Planck's law describes the distribution of blackbody radiation as a function of wavelength and temperature as shown in Figure 2.2.
Figure 2.2 The Black body radiation according to Planck's law for different temperatures. Also shown is Wien's displacement law describing the spectral shift of the radiance maximum to higher wavelength for lower temperature [12]
ï¬ Wien's displacement law
Wien's displacement law is derived from Planck's law when the wavelength has the maximum intensity. The result is the inverse relationship between the wavelength of the peak of the emission (λmax) of a black body and its temperature (T). The law states that the product of maximum wavelength (λmax) and temperature (T) of a blackbody is a constant:
2.2
which yields;
2.3
where: T is the temperature of the blackbody in Kelvin (K), b is a constant of proportionality, called Wien's displacement constant and equals, λmax is the wavelength maximum emission of the blackbody radiation.
ï¬ Stefan-Boltzmann law
Stefan-Boltzmann law states that the total energy radiated per unit surface area of a blackbody is proportional to the fourth power of the temperature of the body. It is a law that shows relationship between the temperature of the radiating body and power emitted.
This relation is given in the equation below;
2.4
Where; E is the total power density radiated per unit area, T is the absolute temperature and σ is Stefan-Boltzmann constant (5.67Ã-10-8W m-2 K-4). Equation 2.4, the power density emitted (W/m2) from a light source, for example, when the lamp temperature is known, the emitted radiation can be calculated.
2.2 Light intensity simulation with different lamps
The light intensity on a surface varies inversely proportional to the square of the distance between the light source and the observer (measuring point).This phenomenon is described by the so called inverse square law. Artificially it is difficult to produce a light source with the simulator that exactly matches the standard AM1.5 spectrum regarding the spectral distribution and while maintaining an integrated intensity of 1000W/m-2. There is usually a considerable amount of variation between the spectrum of the laboratory lamp and the standard AM1.5 spectrum. Most of the lamps used in the simulator are gas filed lamps because for normal filament lamp, when is heated above T=3500K the filament wire which is a metal will melt. The standard light source lamps for simulators emit in a similar wavelength regime but with characteristic emission peaks as described below for the different lamp types.
ï¬ Xenon lamp
Xenon short arc lamps are used as a light source for a photometric application and are commonly used in the sun simulators. The spectrum of wavelengths ranges from the ultraviolet over visible to the infrared region between 200nm to the mid infrared as shown in Figure 2.3. In the lamp, the light is produced by the electrical discharge of xenon noble gas at high pressure. The lamp emits a neutral white light at high color temperature of 6100K, which is close to the sun surface temperature of 5800K. The life-time of xenon lamp is approximately around 1000 hours. Xenon lamps operate at higher colour temperature (6100K) than the halogen lamps (3200K). Since most xenon light bulbs may be dimmable in a certain range, controlling the light intensity is possible by altering the lamp current, for example, using as a flasher light. There are 3 common types of xenon lamps, named single beam xenon HID (High Intensity Discharge lamps) lamp, bi-xenon lamp and hi/low pressure xenon lamp.
Figure 2.3 Spectral distribution of a commercial 150W xenon lamp (e.g. Osram XBO 150) and solar radiation [13]
ï¬ Tungsten-halogen lamp
Tungsten lamps provide a wide range continuous highly luminous intensity spectrum from UV light to near infrared and far infrared. The lamp has a tungsten metal filament with smaller glass tube filled with halogen gas with a typical colour temperature of 3200K. Due to the wide range of its spectrum, the lamps can be used for most wavelength spectral measurements and also for warm in-house lighting. When the lamp glass tube is filled with halogen gas the lamp is called tungsten-halogen lamp. Tungsten-halogen lamp is visible to near infrared applications as shown in figure 2.4.
Figure 2.4 Spectrum of tungsten-halogen lamp [14]
ï¬ Mercury (Hg) lamp
The mercury lamp is one of the most popular types of discharge lamps that use mercury. They provide a range of spectral radiation from the ultraviolet through the near infrared range. The mercury lamp emits a neutral white light at high color temperature of 5900K and it has very strong peaks in the ultraviolet and visible region as shown in Figure 2.5. Due to this characteristic mercury lamps are not useful in the spectral measurements range above infrared and do not provide a black-body like spectral distribution in the visible and UV range. Mercury lamps require no warming up period for starting or restarting and reach near full brightness within seconds.
Figure 2.5 Mercury lamp emission spectrum compared with AM1 spectrum [15]
ï¬ Metal halide
Metal halide bulbs belong to a family of lamps known as HID lamp. It is a high quality lamp which uses a chemical compound of halogen with a more electropositive element from the group I or II of the periodic system of elements. It has excellent life time about 3000 hours and a colour temperature of approximately 4500K. The metal halide lamp has high intensity, colour temperature and operational life than halogen and xenon lamps. The examples of HID lamps includes: mercury vapour lamps, metal halide lamps and high pressure sodium lamps.
ï¬ HMI (Hydrargyrum Medium-arc Iodide/Mercury Iodide: HgI2)
A HMI lamp is 2 to 5 times more efficient than a normal incandescent lamp, which means it uses less operating power. The colour temperature is around 6000K, as similar to the sun surface temperature. Figure 2.6 shows the standard spectral radiance of a metal halide lamp. It has extreme seal technology, high luminous flux and a lifetime about 500 hours. They are mercury halide gas discharge arc length lamps with a wider spectral wavelength range.
