Accelerated Environmental Chambers and Testing of PV Modules

11 Accelerated Environmental Chambers and Testing of PV Modules Sean Fowler Q-Lab Corporation, Westlake, Ohio, United States

11.1 Introduction Photovoltaic (PV) modules are unique electrotechnical products. They must be exposed to sunlight to function, and shielding them from other elements of the weather detracts from their primary purpose. Nearly all other electronic devices are designed to keep out humidity and survive heat by performing their primary function within an enclosure, and durability testing of these products reflects this focus. Environmental testing focuses on these factors, and many associate the term “environmental chamber” with devices that control these two parameters. These testing devices are designed and built with varying capabilities to control hot and cold extremes and cycle between conditions, which is a subject of this chapter. According to the International Electrotechnical Commission (IEC) Technical Committee 82 (TC82), which oversees standards for PV modules, an environmental test is one “in which a product is exposed to simulated environmental conditions such as temperature, wind, rain, snow, hail, or humidity.” The tests in the IEC module qualification standards address these conditions as well as mechanical loading, both static and dynamic. In other fields concerning electronic devices, IEC TC104 (TC104) on environmental classification and testing describes these stresses and adds many others, including dust ingress, abrasion due to wind-blown sand and dust, seismic shock, mold growth, vibration from mechanical systems, and others. Two other stresses included in TC82’s list of environmental tests are ultraviolet light and salt mist exposures. Their omission from the committee’s definition of environmental tests implies that they exist apart from environmental testing or as an afterthought, despite the necessary exposure of PV modules to sunlight and in some cases corrosive conditions. It is true that weatheringdthe field that

encompasses ultraviolet light exposuresdand corrosion testing have existed as separate entities from environmental or climatic testing, and each has a long history with its own technical standards. Although currently there is no standard definition of weathering, the one proposed is “photo-induced changes resulting from exposure to the radiant energy present in sunlight in combination with heat (including temperature cycling) and water in its various states, predominately as humidity, dew, and rain.” The salt mist test is a key component of the field of atmospheric corrosion testing. A concise definition of atmospheric corrosion is difficult to formulate, but one can be stated in two parts. “Corrosion is an electrochemical process that returns refined metals to their more natural oxide states; atmospheric corrosion is a degradation process that takes place in a film of moisture on a metal surface, where the film may be so thin that it is invisible to the naked eye.” Weathering tests have not been strongly emphasized in the history of PV module testing, partly because of the general emphasis on heat and moisture in the testing of electrotechnical devices. Another reason is the difficulty in creating multifactor environments that expose large specimens to simulated sunlight in combination with heat and moisture. Typically, chambers that attempt to do this must sacrifice some facet of performance. They may sacrifice accuracy in their simulation of the UV portion of sunlight in some cases. In others, they lack moisture control or the capability to create significant thermal cycling within the exposure. Cost is also a factor. Complex chambers are expensive to design, build, and maintain, reducing the volume of data any organization can obtain within a practical testing budget. In turn, this reduces the statistical significance of the data coming from them. An alternative, economical approach is needed. In recent years, the PV industry has started to adopt some practices from other industries, such as

Durability and Reliability of Polymers and Other Materials in Photovoltaic Modules. https://doi.org/10.1016/B978-0-12-811545-9.00011-2 Copyright © 2019 Sean Fowler. Published by Elsevier Inc. All rights reserved.

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automotive and building materials. In 2017, the PV industry accepted its first international standard for testing and qualifying polymeric materials for use in modules, IEC 62788-7-2. These tests can be performed on much smaller specimens, allowing the use of standardized weathering test chambers. This development is akin to qualification of automotive paints, which are tested independently according to standard methodologies specified by the car and truck manufacturers. The principle is that it is not necessary to test the entire car in order to qualify the paint’s durability. These tests have been inserted into the international qualification regime to supplement the temperature- and humidity-focused tests that have been in use. This chapter will discuss the chambers used for environmental testing of PV modules and their components. The focus will be on describing the technology behind the environmental stress tests used by the industry. Special consideration will be given to standard weathering chambers due to their recent adoption by IEC TC82 in its module qualification regime. Perhaps a better understanding of the tools will improve how test methods are both conceived and carried out. The terms “temperature and humidity chamber,” “environmental chamber,” and “climatic chamber” will be used interchangeably. Weathering and corrosion chambers will be considered as special cases of climatic chambers because they incorporate these basic stresses while introducing additional ones. Sunlight simulation and salt spray add complexity to climatic tests and create unique challenges controlling the parameters taken for granted in traditional climatic chamber testing.

11.2 The Basics of Temperature and Humidity Chambers Most climatic chambers are cuboid shaped, and they are typically specified or categorized by the volume inside the exposure area. Small, benchtop chambers may have capacity in the tens of liters, while large chambers can be sized to test an entire car or truck. Beyond the size, chambers are characterized by the minimum and maximum temperature and relative humidity levels achieved and the rate of change between controlled conditions. Most chambers are constructed with insulated walls to keep external surface temperatures at a safe operating

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level, maximize performance and energy efficiency, and prevent condensation formation on interior chamber surfaces. The amount of insulation required depends on the high and low temperature extremes of the chamber operating specifications. Controlling temperature and relative humidity requires the ability to add and remove heat and moisture from the air. Although this seems obvious, and the essential technologies for achieving these capabilities are old, the systems required to achieve test conditions required by many applications can be surprisingly complex. The next sections will discuss these capabilities in relation to common environmental testing requirements for PV modules.

11.2.1 Air Circulation Air movement within a chamber is controlled by one or more blowers, and the system typically includes a damper system which allows fresh air to mix with exhaust air as needed to achieve programmed conditions. Damper systems vary in complexity. Simple systems may include a single damper between air intake and exhaust to control the ratio of fresh and recirculated air within the system. Conditioning processesdheating, cooling, drying, or moisturizingdmay be applied to the fresh air prior to mixing with exhaust air or after mixing occurs. More complex systems may include multiple blowers and dampers to combine air streams from different conditioning processes for precise control. These streams could be dry and water-saturated air combined in proportion to the relative humidity set point or hot and cold air streams similarly mixed. Regardless of the chamber’s complexity, air flow into and out of the system is equalized to avoid pressurizing or depressurizing the test space, with a minor exception involving a particular dehumidification technique which will be discussed later. When maintaining steady state conditions, relatively little air flow through the test space is required provided the chamber is sufficiently insulated and the devices under test (DUT) are not significant heat contributors. Higher air flow is normally required during cycling between test conditions or to compensate for heat-generating DUT. Variable-speed blower systems allow for both situations, although some chambers operate with a constant high flow rate through them regardless of the situation. The dispersion of air flow is important for maintaining homogeneous conditions in the test space. Air

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baffles, deflectors, and diffusion plates are employed in chamber designs to facilitate uniform air flow through the working space. It is important to recognize that even perfectly distributed air flow does not ensure perfect uniformity. When the incoming air has different temperature and humidity levels than the set point, a necessary condition during transitions or when the DUT generate heat, the areas of the working spaces at air intake and exhaust locations must also be different. This has implications for measurement, control, and calibration of temperature and humidity chambers. These points will be discussed throughout this chapter.

11.2.2 Temperature Control Chamber temperature control can be specified for the air or a specific DUT. Controlling temperature of the DUT seems like an easy solution to many testing problems. The opposite is true. Accurate control requires a responsive system, and the mass of many items tested can add a significant amount of latency. This problem often creates significant instability during constant-temperature conditions. Perhaps a better way to deal with the problem is to record the temperature of the DUT but allow the chamber software to control chamber air temperature; then the user adjusts the set point to achieve the desired DUT temperature. Modern chambers can do this automatically, a point discussed later. In the case of PV modules, the safety and performance series of standards published by TC82, IEC 61730 and 61215 respectively, specify the module temperature in the various temperature and humidity tests [1], but in practice many laboratories control the air temperature of their chambers and monitor the module temperature. When the air temperature is specified, it can be measured in the chamber working space, at the point of entry into the working space, or in the chamber exhaust. Regardless of where the thermometer is located, the control system is typically capable of accommodating a user-input temperature offset to compensate for non-uniform conditions and inherent biases between the control sensor and the working space. Calibration will be discussed later, but first is a description of the techniques for controlling temperature and humidity.

11.2.3 Heating and Cooling Heating is achieved by electrical resistance heaters placed in the airstream and/or in water immersion

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systems as part of humidity generation. Heat transfer is usually achieved by forced air convection, but heat can also be radiated from surfaces in the working space. The latter technique is common in chambers designed for the salt mist test, discussed later, but it can also be employed to prevent water from condensing on relatively cool interior surfaces during high temperature and humidity conditions, when insulation is insufficient. Viewing windows are sometimes heated for this reason. The cooling technology is more complex. At its simplest, cooling can be achieved by blowing laboratory ambient air through the chamber without any refrigeration. Such chambers are often referred to as ovens because they operate at a minimum temperature of room ambient plus some offset, say 10e20
C. The IEC safety qualification standard for PV modules, 61730, includes exposures at 105
C “dry” conditions, which can be performed in a controlled oven. If such a chamber includes humidity control, it can be used for the ubiquitous 85
C/85% relative humidity (RH) damp heat test described in the IEC PV module qualification standards and the 60068-267 standard. Of course, cooling below room ambient temperatures is required by many tests for PV modules and other devices. Several required PV module tests include temperatures as low as 40
C. This is an interesting temperature because it is a common line of demarcation between refrigeration systems using a single compressor and “cascade” systems that use two. A brief explanation of the refrigeration cycle is included here to describe the differences between the two. The refrigeration cycle begins with a gaseous refrigerant being compressed into a liquid state by a mechanical compressor into a part of the system called the condenser. Compressing to a higher pressure heats the gas/liquid mixture, and that heat is taken away by a heat exchanger. A valve then allows the gas to expand into an evaporator, where thermodynamic processes immediately cool it to its boiling point. Another heat exchanger draws heat from the space being cooled. The evaporated refrigerant, at low pressure, is drawn into the compressor to repeat the cycle indefinitely. This describes a single compressor system using a refrigerant with a boiling point of approximately 46
C at normal atmospheric pressure. It is this boiling point that impacts the minimum temperature achieved. Although a single compressor could in theory be combined with a refrigerant with a lower boiling point, the pressures

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required to condense the gaseous refrigerant would be too high for the piping and valves. When lower temperatures are required, a twostage, or cascade, system is used. Conceptually, a cascade refrigeration system is comprised of two independent single-stage systems joined together by a heat exchanger. In this exchanger, the evaporated refrigerant of the first system is used to cool the compressed refrigerant of the second system rather than directly cooling the air in the working space. The second system uses a refrigerant with a boiling point below 80
C and is capable of achieving working space temperatures as low as approximately 70
C, depending on the DUT and chamber design. The disadvantages of a cascade system are higher complexity and lower energy efficiency due to the extra heat exchange process. Of course, these disadvantages result in higher typical costs. However, most manufacturers, but not all, recommend a cascade system for PV module testing despite the fact that the minimum test temperature of 40
C matches the published minimums of many singlestage systems. PV modules are artificially powered for some tests to simulate their power production and subsequent heat dissipation in sunlight. Because numerous modules are often tested in a single

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chamber, this puts a significant thermal load on the chamber. A single-stage system may not achieve the minimum module temperature quickly enough or even at all [2]. The cascade systems will achieve faster transitions, a factor that can be utilized to decrease test times. This point will be discussed later. Even lower temperatures are possible with a threestage system, and liquid nitrogen or carbon dioxide can be injected directly into the working space for even lower temperatures or to provide a very rapid cooling of the DUT. However, these technologies are largely irrelevant for testing PV modules, components, or materials, so they will not be discussed here.

11.2.4 Humidity Control Fig. 11.1 shows the relationship between absolute humidity, temperature, and relative humidity (RH). This is a critical point to consider for the discussion of humidity control in environmental test chambers. The mass of water that can be sustained in vapor form in air increases with temperature. For example, at 85
C, RH of 85% means that air, at normal atmospheric pressure, contains 298 g of water vapor per cubic meter (g/m3). This is its absolute humidity. At the same RH, air at 25
C contains only 20 g/m3 of water vapor, or approximately one order of

Figure 11.1 Dew point chart showing the relationship between temperature and relative humidity for several dew points and their respective absolute humidity levels.

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magnitude less. This difference is important to remember when considering the methods available to add humidity within a temperature and humidity chamber, which are discussed below. Another way of representing absolute humidity is the dew point, a useful concept for understanding the relationships between temperature and RH. The dew point represents the temperature at which water vapor in the air will begin to condense if the air were cooled. In other words, the dew point represents the temperature required for air to be saturated, or at 100% RH, given the amount of water vapor currently present in the air. In the examples above, 85
C/85% RH has a dew point of 80.9
C, while 25
C and 85% RH air has a dew point of only 22.3
C. Both dew points are close to the given temperatures because the RH is approaching 100%. The dew point temperature can never be higher than the current temperature of the (A)

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air, although the two will be equal when the RH equals 100%.

11.3 Methods of Adding Humidity There are four methods for generating humidity inside a chamber. Each has an effect on heat and temperature and can be classified according to whether the process adds or removes heat from the system. The choice of which one to employ largely depends on the desired temperature and RH range for testing.

11.3.1 Steam Generation (Boiler) A boiler is an enclosed water vessel with an immersion heater (Fig. 11.2A). As the water temperature approaches boiling, heated water vapor enters the airstream through a tube. The humidity generated (B)

(D)

Figure 11.2 Examples of humidification devices used in environmental chambers. (A) Boiler type humidifier used in an environmental chamber. (B) Heated water bath humidifier used in a condensation chamber (heating element is beneath the bath). (C) Atomizing spray humidification nozzle. (D) Nebulizer humidification system.

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is truly a vapor and requires no additional heat. The main benefit of this type of humidity generation is the ability to inject large amounts of vapor into the air. Because heat has been added during the process, boilers can readily achieve high RH at high temperatures (higher absolute humidity) than other systems, and they take up relatively little space [3]. The addition of heat can be a disadvantage when high RH at low temperatures is required, because the heat must be removed. Boilers can also cause RH oscillation as the immersion heater is cycled on and off [4]. Another disadvantage is that boilers can quickly collect mineral deposits from the evaporated water unless it has been significantly demineralized.

11.3.2 Heated Water Bath Like a boiler, the heated water bath adds heat as well as humidity to the air, but without boiling the water (Fig. 11.2B). Unlike a boiler, the bath is open to the airstream and has a relatively large surface area to facilitate evaporation as air flows over the water. The heater is controlled according to the needs of the system. The responsiveness of the water bath depends on the water’s surface area to volume ratio. Higher ratios result in faster responsiveness and more humidity released into the air, but lower ratios offer more stability [4]. The main advantage of a heated water bath is that it can balance stability and responsiveness at moderate absolute humidity levels, but it may not provide enough humidity to achieve high absolute humidity. It can also take up more space than boilers or other systems due to the need for a large surface area. Another disadvantage is that low absolute humidity levels can be difficult to achieve because the water bath is constantly releasing water vapor into the air stream, even if it is not desired. Heated water baths are the most forgiving type of humidity generator when it comes to harm caused by minerals in the water [3]. No minerals are introduced into the air stream, and buildup on heaters and other surfaces tends to be very gradual.

11.3.3 Atomizing Spray Atomizing spray systems combine compressed air with water at an atomizing nozzle to create very fine droplets (Fig. 11.2C). Usually, the atomizer is located downstream from the chamber heater, and the heat evaporates the droplets as they pass by in the moving air. This technique removes heat through evaporation, which can be advantageous or not depending on

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the programmed settings. Atomizing spray humidifiers are effective at adding a significant amount of moisture into the air, and they take up very little space. The cooling effect from the droplet evaporation must be compensated for in the heating system, but this means simply increasing the power of the heater. Besides the need for extra heating capacity, the main disadvantage of this method of humidity generation is the possibility of clogging of the spray nozzle with minerals in the water or releasing small particles into the airstream. Supplying compressed air could be another disadvantage in chambers that do not require it for refrigeration.

11.3.4 Ultrasonic Nebulizers Like atomizing spray nozzles, ultrasonic nebulizers remove heat from the system through evaporation (Fig. 11.2D). They are very commonly used for home and healthcare applications where humidity is added to dry room environments for comfort and health reasons. Although normally damaged or destroyed by high temperatures, more robust designs are available for commercial use in environmental chambers. Ultrasonic nebulizers use a piezoelectric material in a circuit that vibrates at a very high frequency, ejecting extremely fine droplets of water from the surface. The nebulizers are kept in a shallow bath of unheated water, and the droplets produced are drawn into the air stream and quickly evaporated. The cooling effect is beneficial when attempting to achieve high humidity at relatively low temperatures, but their use in chambers requires additional heating capacity to compensate for this effect. Another benefit is that this technology does not require the use of compressed air, making it energy efficient and quiet. The main disadvantage of ultrasonic nebulizers is that they have to be spread over a relatively large surface area to generate significant amounts of humidity, much like heated water baths. This drawback tends to limit their use to small chamber sizes.

11.4 Methods of Removing Humidity Removing humidity can be just as important for some test conditions as adding it. The two most common methods for this are refrigeration, used by almost all chambers, and the use of desiccants in two types of systems [5]. It is important to recall the previous discussion concerning absolute humidity

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and dew point. Without the ability to remove humidity from the air, the chamber RH is affected by laboratory conditions. Reducing RH, in this case, can only be achieved by heating the air, but what if you want to achieve low RH and low temperature? Chambers must include one of the humidity-reducing technologies to make this possible. The most common technique for humidity removal is refrigeration because it also cools the air, achieving two goals simultaneously. When air is cooled, it is less able to sustain water vapor and condensation occurs; water condensed in this manner in a chamber is drained away. Typical chambers are capable of achieving a dew point of approximately 7
C. This value is derived from the efficiency of heat transfer in an exchanger and the fact that cooling coils typically must be kept above the freezing point of water to avoid buildup of ice. Based on the minimum dew point, chamber manufacturers publish temperature and RH specifications in a graphical format called a climatogram, showing temperature on one axis and RH on the other, with one or more shaded areas depicting the chamber’s capabilities. See Fig.11.3 for an example. Desiccants are substances that absorb moisture from the air; silica gel bags commonly used in product packaging are a common example. They are used in two methods for humidity reduction in chambers. In the first, called dry air purge,

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compressed air is passed through a container of a desiccant material on its way into the chamber. Two containers are used, so that one is being vented to room conditions to regenerate the desiccant material, while the other is actively removing moisture from the compressed air. The drying effect of the compressed air decreases the humidity, but additionally this technique is often combined with refrigeration in a specific way. Cooling coils are normally kept above the freezing point of water to avoid ice formation on them, which decreases their heat exchanging efficiency and puts stress on the compressor. However, dry air purge creates a slight positive pressure in the system so that ice sublimates from the coils in the dry environment, allowing them to operate a few degrees below 0
C. This feature is commonly called a “low RH package” or something similar. Large chambers may also include a recirculating desiccant dryer, where air is continuously circulated through a system with a wheel made of a desiccant material. This wheel is partially exposed to room conditions or heated to release moisture for regeneration. A third type of dehumidification, nitrogen purge, is used by some chambers, but typically not those involved in testing PV modules. Nitrogen gas does not retain water vapor, so it can be introduced into the working space to push moisture-laden air out. The technique is hazardous to people because it drives oxygen out of the working space, so it is rarely used

Figure 11.3 Climatogram showing the achievable temperature and relative humidity settings for a corrosion chamber with and without a refrigeration module.

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in a walk-in type chamber. For PV module testing, the extreme low humidity levels possible with nitrogen purge are unnecessary.

11.5 Measuring Relative Humidity in Environmental Chambers Measuring relative humidity is surprisingly difficult. Many techniques and technologies have been developed over the centuries, the oldest known tracing back to the Western Han dynasty in China around 200 BC [6]. Early instruments used horse or even human hair in tension in a dial-type gauge. Leonardo da Vinci developed a crude type that was refined in the 1700s. Fig. 11.4 shows an example of a 19th century device still in operation in Salzburg, Austria. Today’s state-of-the-art measurement systems are based on a principle developed by the National Institute of Standards and Technology (NIST). This technique starts with fully saturated air at a known pressure (higher than atmospheric), and temperature. The air is allowed to expand through a valve to normal atmospheric pressure, where the temperature and pressure are again measured and the

Figure 11.4 A 19th century weather station hygrometer in Salzburg, Austria.

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relative humidity of the new environment is known by applying thermodynamic principles. Effectively, the technique involves a very special climatic chamber that generates a precisely known relative humidity, which then is used as a reference for calibration of other devices. RH measurement is a subject worthy of extended discussion, but for the purposes here, a summary of three techniques will be discussed in the context of their application to environmental chambers and PV module and material testing. All three types of sensors, shown in Fig. 11.5, are used for direct measurement and control within the chamber or for calibration. There are certainly other measurement methods, and some of them could be applied to environmental chambers.

11.5.1 Psychrometric Wet Bulb/ Dry Bulb Sensors The earliest fundamental measurement technique for RH, wet bulb/dry bulb sensors, continue to be commonly used today. These sensors rely on two thermometers. One is positioned in the air as any thermometer might be, while the other includes a wick or sock over the end which is kept constantly wetted. Evaporation from the end of the wick depresses the temperature of that sensor, and this difference is used to determine the relative humidity from thermodynamic principles. At 100% RH, there is no evaporation from the wick, so the two temperatures are equal. In practice, these formulae are input into computer software to perform the calculations automatically, or the software may simply include well-developed data tables to relate the two measured temperatures to an RH value. The latter technique is most common because the use of thermodynamic equations must include either a measurement of atmospheric pressure or an assumed value. The data tables assume the atmospheric pressure, and these are accurate in most instances. As with any measurement technique, this technique has strengths and weaknesses. The benefits of psychrometric RH measurement systems are that they can be very accurate, since they rely only on accurate measurement of temperature. These sensors tend to be very responsive and robust, although they do require some maintenance. Wicks need to be replaced periodically, and the volume and purity of water keeping the wet bulb wet must be maintained. Wet bulbs drying out and contaminated wicks are the

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Figure 11.5 The three main types of RH measurement devices used in environmental test chambers.

major sources of measurement error in these systems. In either case, evaporation is inhibited or eliminated, and the RH measurement is biased in the direction of saturation. If a dry bulb becomes wet, the same bias occurs, although this is typically only a temporary condition if it occurs. Another limitation is that the placement of such sensors is limited to locations where water tubing can be easily routed.

11.5.2 Electronic Sensors Certain materials change electrical impedance, either resistance or capacitance, as a function of humidity. These properties can be measured and correlated to the relative humidity of the environment; therefore these are secondary, or indirect, measurement techniques. If the chamber requires RH measurements of air below 0
C, these can be the only practical solution. Additionally, certain situations may make the maintenance of a wet bulb wick difficult or impractical, and electronic sensors are easier to use. However, electronic sensors are susceptible to contamination, to a greater extent than wet

bulb wicks. Although it is possible to protect them with the use of semipermeable materials, this technique slows down the response time of the sensor and may introduce hysteresis into the system. Another problem is that many electronic sensors have difficulty operating at a high RH, and failures of this sort are not easily detectable. Nevertheless, electronic sensors offer sufficient accuracy for most applications and are commonly used.

11.5.3 Chilled Mirror Dewpoint Hygrometer Another fundamental RH measurement technique involves detecting the dew point in the environment. In theory, the chilled mirror technique can be used to detect condensation of any gas, but for the purposes of temperature and humidity chambers, it is used to detect condensation of water and thus the dew point of the air. A chilled mirror hygrometer is composed of a thermal conducting metallic mirror surface, a thermoelectric cooler, and a platinum resistance thermometer embedded in the mirror. Also included

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is an LED, a photodetector, and a separate platinum resistance thermometer. A beam of light from the LED is reflected off the mirror surface and received by the detector. The temperature of the mirror surface is reduced until condensation commences. As dew is formed on the mirror, some of the light is scattered, and the detector output decreases. The system then iteratively adjusts the temperature of the mirror surface until it detects the precise temperature at which equilibrium is achieved, which is when the amount of condensation on the surface is neither increasing nor decreasing. This is the dew point. A separate temperature measurement of the air is all that is needed to accurately calculate the relative humidity. This is perhaps the most accurate technique available in a device small enough to be placed in most chambers, although small psychrometric devices do exist. One drawback is that the mirror surface must remain very clean for the technique to work, making it less practical for use as a chamber’s control sensor. Another is that the iterative determination of the dew point introduces some latency into the measurement, especially in a rapidly changing environment such as a chamber during a transition. However, chilled mirror hygrometers are used for calibration of electronic sensors and to measure the RH for the purposes of determining chamber uniformity.

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system often. Transitions are the next subject discussed.

11.6.1 Transitions Between Test Conditions: Step Versus Linear

11.6 Temperature and RH Control: Putting It all Together

Transitioning between conditions is one of the most important aspects of environmental testing. The IEC qualification standards for PV modules include two different tests featuring temperature and/ or humidity transitions. Yet transitions are one of the most variable parts of any test, and they pose challenges for laboratories and chamber manufacturers alike. There are two types of transitions in an environmental chamber, see Fig.11.6. First is a step transition. A step transition says nothing about how long it should take to attain the condition being transitioned to. It says, go from condition “A” to condition “B.” Many standards in temperature and humidity testing, weathering, and corrosion include these. It seems likely that in some cases the inclusion of a step transition is unintentional. Many readers of these methods interpret a step transition to mean, “Get from A to B as quickly as the chamber can do so, but as long as it gets there by the end of the step, that is acceptable.” Other readers believe the transition should be instantaneous, as in a thermal shock chamber that includes two working spaces, one hot, one cold, and a lifting system to move specimens quickly between them. Some standard test methods provide guidance by giving a time limit on the

Temperature and humidity control systems are founded on principles of proportional, integral, and (sometimes) derivative (PID) control algorithms, discussion of which is beyond the scope of this chapter. Chamber manufacturers expend significant resources perfecting their control software and consider this proprietary information. However, common techniques present themselves and are openly discussed by some manufacturers. Chambers that are designed to operate over a wide temperature and RH range may begin by chilling and drying air rather than rely on ambient conditions in the laboratory. Heat and humidity are then added back into the system for precise control since these processes are easier to administer in small doses. High airflow is required when attempting to accomplish precisely-controlled transitions because latency is minimized and more opportunities exist to condition the air appropriately when it recirculates through the

Figure 11.6 Linear and step transitions between conditions in an environmental test.

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transition. Occasionally, this ambiguity is intentional, particularly when there are factors to performing a test external to chamber capabilities that limit the ability of standards writers to provide specific guidance. Regardless of the reason, step transitions often lead to disagreements between people who test, people who request testing, and manufacturers of the chambers who perform tests. The second type of transition is a linear one where a fixed rate of change over time is directly specified or implied. Effectively, a linear transition serves to slow down transitions compared to a typical step transition. This is because most chamber software is programmed to see a step transition as a “get there as fast as possible” goal and works the heating and cooling hardware near its limits to achieve condition “B” quickly. There are several reasons to slow down a transition. One is to reduce variability between tests and make it clear to all parties how the transition is to occur. By controlling the rate of the transition, differences between chamber heating and cooling capacity are mitigated, as are differences in heat produced by various DUT. The latter difference can be quite significant when testing powered PV modules. Another reason to slow the transition is to avoid unrealistic stresses caused by rapid heating and cooling. A PV module in service can be subjected to wide variations in temperature, but these occur relatively slowly in the natural environment. Designers of the PV module qualification test sequence intended that PV modules be subjected to a thermal cycling test, not a thermal shock test. Yet a third reason to slow transitions is to regulate the time in specific temperature or RH ranges, a consideration especially important in corrosion testing or when the glass transition temperature of a polymer being tested is a concern. In the case of step transitions, chamber manufacturers have a competing challenge to “getting there as fast as possible.” Severe temperature overshoots1 are usually not desirable, so control algorithms are designed to slow down transition rates as the set point is approached. Chamber manufacturers typically make great efforts to minimize the temperature or humidity overshoots while transitioning. However,

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Overshoot includes situations when the condition temporarily goes below the set point during a transition to a lower temperature or RH, not just above the set point during transitions upward.

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this can be a problem when a test method uses a step transition and also specifies a DUT temperature. A massive object takes longer to achieve condition “B” in a transition than the chamber air temperature, but air temperature is what is traditionally measured by a chamber. In response to this issue, many chambers have the ability to place a temperature sensor on or inside a DUT and control the chamber to that sensor. Some modern chambers can be programmed to allow the air temperature to go beyond the set point of the next step to speed up the heating or cooling of the DUT. When the DUT temperature approaches the set point, the air temperature is adjusted to gradually equilibrate. This “controlled overshoot” feature is especially useful in tests that require a minimum or specific “soak” or “dwell” time for the DUT after one transition before moving on to the next one. This is the case in the PV qualification standards.

11.6.2 Characterizing Environmental Chambers: Uniformity, Fluctuation (Stability), and Heating and Cooling Rates Environmental chamber specification sheets include performance parameters such as minimum and maximum temperature and relative humidity, heating and cooling rates, and sometimes uniformity of conditions in the working space. Much of this information is given according to standard characterization techniques described in IEC TC104 standards, and these will be briefly described here. IEC 60068-3-5 is a standard on characterizing temperature chambers, while its companion 60068-36 characterizes humidity chambers. These standards include a test sequence, data collection regime, and calculation criteria to characterize the uniformity of the chamber, referred to as gradient or variation in space, fluctuation of conditions over time, and heating and cooling rates. In the procedure, temperature sensors are placed throughout the chamber in defined areas, at least nine of them, and data is recorded during a test sequence that includes transitions to the high and low temperatures of the chamber’s operating range and operation in stable conditions. This data is used for a variety of characterization calculations and other purposes as well. These are worth discussing. According to the standard, two terms describe uniformity. Gradient represents the difference

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between the maximum and minimum temperatures of all the positions measured at constant-temperature conditions. Variation in space refers to the largest difference between the center and any of the other measured positions. Some chamber manufacturers report the uniformity as plus/minus the standard deviation from the mean of all positions measured. All three methods can be relevant depending on exactly what is being tested inside a chamber. Of course, once specimens are in the chamber, the air flow changes, possibly impacting the uniformity numbers generated under empty chamber conditions. Fluctuation, referred to in some weathering and corrosion standards as operational fluctuation, is calculated as plus/minus two standard deviations from the mean. It is a measure of the stability of conditions while operating at a steady state. To generate this data, the chamber is allowed to stabilize at a constant-temperature setting. Then, data is recorded at regular intervals, usually once per minute or more, for 30 min or longer. This data is averaged and the sample standard deviation calculated. It is important to note that the standard technique performs this test with an empty chamber. When using a DUT temperature control, massive specimens can introduce latency and increase fluctuation. Heating and cooling rates are very important for many applications. The standard method for calculating heating and cooling rates is to perform step transitions from minimum to maximum temperature, in both directions, and measure the time it takes to achieve each one. However, because control algorithms will slow heating and cooling processes as the set points are approached, the standard says to eliminate the time it takes to move through the first and last 10% of the operating range. This provides a rate in terms of Kelvin or degrees Celsius per minute. Some manufacturers also publish transition times between the two temperature extremes, with or without a specimen load. Some publish data showing the transition rates with various specimen loadings by mass. Faster rates require more heating and cooling power, which increases costs. The heat generated by the DUT is significant for PV modules tested while supplied with current. For this reason, testing PV modules typically requires a chamber with a faster cooling rate than specified by the climatic test. However, the rate of change in the PV standards is low relative to the capabilities of many chambers. Linear control is required in some cases to slow down

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the transition to comply with test requirements, which will be described later. Another use for this data is calibration and inputting offsets into control software. This can be necessary due to the location of control sensors. For practical reasons, control sensors are often outside the working space. Even if located within the working space, typically they are not in the center. The data collected in the 60068-3-5 standard sequence can be used to calibrate according to the needs of the test application. In some cases, operators may choose to take the average of all positions in the chamber as the reference temperature against which an offset is input into the controller. In other cases, the center value is used. DUT or product temperature control is another way to address this issue because the control sensor is directly affixed to the specimen or a representative specimen under test, albeit with some associated problems discussed previously. The PV module standards require measuring the temperature of the module, or a single “representative” module in a chamber filled with multiple specimens. The IEC committee devotes an entire standard to characterizing humidity chambers, but the 60068-3-6 standard is nearly identical to its temperature counterpart. However, there is one very important difference. Difficulties with RH measurement were discussed earlier. In recognition of this fact, the IEC procedure includes a single RH measurement in the center of the chamber2. With the RH and temperature measured in this location, the absolute humidity is known. The standard goes on to make the assumption that the absolute humidity is perfectly uniform throughout the chamber. The other temperature data points can be applied to this absolute humidity to calculate the RH in each of those locations. As an example, suppose one is performing this measurement in a chamber running the damp heat test, 85
C at 85% RH, and the center measures exactly these conditions. Elsewhere in the chamber, one

2

Although the standard says the measurement is to be taken at the center of the working space, in some cases it is better to place the RH sensor elsewhere. A good example is a corrosion chamber with the spray nozzle in the center, a very common arrangement. In this case, the center of the chamber is not part of the working space. Moving it somewhere in the usable test space is sound engineering judgment. Words in a standard should not override good practice.

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thermometer records 83
C while at the opposite extreme it is 87
C. Applying the absolute humidity of 298 g/m3 to these temperatures yields RH values of 92% and 79% respectively3. Water vapor diffuses quickly through a mass of air, so the assumption is reasonable. This is a very practical solution to the problem. RH sensors with high enough accuracy are expensive and rather large. Mounting nine of them in a chamber would be exceedingly costly and difficult.

11.7 Salt Mist and Corrosion Chambers The standard qualification sequence does not require any salt mist corrosion test. However, references to it are made for PV modules installed in corrosive environments, such as near the ocean or roads that are salted for snow and ice removal. Salt mist, or fog, testing has existed for more than 100 years. This is sometimes considered a special case of environmental testing, and today corrosion and environmental testing are merging in significant ways. The basic salt fog test, described in ASTM B117, ISO 9227, and IEC 60068-2-11, includes a 5% sodium chloride solution pumped to an atomizing spray nozzle and combined with compressed air to create a fine mist. This process is essentially the same as atomizing spray humidity generators, except with a corrosive solution instead of purified water. In fact, there are test standards for paints that use an atomizing spray nozzle, called a fog nozzle, to generate saturated humidity to assess moisture resistance. Most salt fog testing is done at a constant chamber air temperature of 35
C. In some cases, salt mist tests are conducted on electrotechnical devices as an

3

There is a great deal to be said about stated tolerances in test standards, but this example shows why many of them have no practical basis. The tolerance for damp heat typically has a plus/minus of 5% RH. According to the IEC 60068-2 series of standards, tolerances incorporate all factors of error, including measurement error, fluctuation, and uniformity. If one properly accounts for measurement uncertainty, which is 2% to 3% in ideal conditions [10], then all of the other factors must be limited to about half of the stated tolerance, or about 2.5% RH. This is all but impossible in a real testing situation. This problem unnecessarily consumes a lot of time in the testing industry.

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accelerated moisture ingress test. When in a typical humidity chamber, any water that permeates or penetrates an enclosure may not immediately cause damage to electrical components because of its purity. Adding salt to the environment ensures rapid damage to components should the mist make its way inside. Over time, the salt fog test has been combined with climatic testing, starting with simple two-step tests where periods of fog were alternated with forced-air drying by heated ambient air [7]. Later, saturated humidity steps were added into these test sequences. RH was generated using all of the techniques discussed earlier with the exception of ultrasonic nebulizers. RH control was rudimentary, however, and conditions were either wet or dry without any control in-between. These tests, commonly called cyclic corrosion tests, proved inadequate and were prone to a variety of problems, including poor reproducibility. As a consequence, many of today’s automotive corrosion test methods include much of the language and methods of environmental tests, including RH control and linear ramping between conditions [8]. Environmental testing with salt mist includes several cyclic test methods in IEC 60068-2-52, which are referenced in the PV standard IEC 61701. These methods mirror early attempts to improve the tests by moving specimens from a salt spray chamber into a temperature and humidity chamber. These tests are labor-intensive and prone to variability due to the manual handling of specimens. However, in the past there were no other options because controlling the environment in the same chamber as salt spray had not been reliably achieved. Today, this has changed because of improved designs, but several challenges must be overcome. Controlling relative humidity in a salt spray environment is extremely challenging because all RH measurement technologies are susceptible to temporary or permanent damage when exposed to salt spray. Techniques to mitigate this problem include retractable sensorsdwhich are inserted during climate control and removed from the working space during periods of fogdand sensor placement in the recirculating air stream outside the working space. Temperature and humidity chambers do not normally have significant volumes of liquid water in the working space that must be evaporated before decreases in RH can be attained. In a salt spray environment, this is the primary part of the test. Additionally, salt fog chambers tend to have salt

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Figure 11.7 Painted metal specimens loaded into a horizontally designed salt spray chamber.

deposits accumulating in the working space, which serve as large desiccant blocks which remove humidity from the air. Some chambers include hardware to wash surfaces of deposited salt [9]. Another challenge salt fog chambers face is that they must be designed horizontally so that specimens do not drip onto one another (see Fig. 11.7). Corrosion products from one specimen would contaminate others if this were to occur. This disadvantages the common cuboid shapes that promote uniformity because specimens can only be placed in a single horizontal plane within the large space. Although seemingly a minor issue, the spray hardware obstructs air flow, creating an extra challenge to achieving uniformity. More significantly, corrosion chambers must rely, at least partially, on radiant heat because flowing air through the chamber dramatically disrupts the salt fog dispersion. Salt fog flowing through recirculation systems will damage components such as heaters, blowers, and sensors. These unique challenges for corrosion testers add up to somewhat compromised uniformity, fluctuation, and ramp rates compared to comparably-sized climatic chambers. The benefits of automatic operation and reduced specimen handling should be considered in balance with these trade-offs.

11.8 Weathering Chambers A definition of weathering is provided in this chapter’s introduction. The key factor is that weathering degradation starts with photons of light disrupting chemical structures, followed by heat and moisture processes further degrading the material. Weathering testing is not new to the PV industry.

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Polymeric materials used for backsheets, encapsulants, and edge seals have been supplied by companies with experience in weathering testing from other industries they sell into. These suppliers, along with testing organizations and chamber manufacturers, spearheaded the IEC standards for testing components. Two technologies have been recognized for standardized testing, and they are the focus of this section. Fluorescent UV and xenon arc weathering chambers will be treated as special cases of climatic chambers. What sets weathering chambers apart is the existence of a light source(s) as the central component of the chamber, around which all other climatic factors have to be designed. One consequence of this fact is that the weathering chamber working spaces are mostly planar surfaces rather than the three-dimensional volumes of temperature and humidity chambers.

11.8.1 Fluorescent UV Weathering Chambers In weathering, fluorescent UV chambers are probably the most common type used. The technology was introduced in 1970 and standardized later in the decade. It is most commonly used for testing paints and plastic resins, but many other applications exist. The fluorescent lamps used are the same as standard fluorescent lighting lamps, except that their phosphors fluoresce a broad UV spectrum rather than in the visible region. Two main types exist and have been described in ASTM G154 and ISO 4892-3: UVA-340 and UVB-313. Each emits energy in both the UVA and UVB regions, but their peaks and cut-on wavelengths are different. Fig. 11.8 shows spectral irradiance distributions for these two types along with the ASTM G177 peak natural sunlight standard4. This sunlight spectrum has higher irradiance than what the PV industry considers “one sun.” To illustrate this, at 340 nm, a common control wavelength for weathering devices, the spectral irradiance is 0.73 W/(m2 nm), while the standard preferred by the PV industry has a value of 0.50 W/(m2 nm) at 340 nm. The charted spectral curves clearly show that the UVA-340 fluorescent lamp has a very good 4

The G177 standard includes a published spectral irradiance table only in the UV, but generating a full spectrum chart is a simple matter of inputting the G177 parameters into the SMARTS2 model.

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Figure 11.8 Spectral irradiance curves of standard fluorescent UV lamps versus peak sunlight.

match to peak sunlight in the entire UVB range and most of the UVA range. For wavelengths greater than approximately 360 nm, the irradiance drops off and is deficient compared to sunlight. UVB-313 lamps emit shorter wavelengths than natural sunlight. In addition to low heat radiation, a key benefit of fluorescent UV lamps is their spectral stability. As the lamps age, the irradiance output decreases but the shape of the spectral curve does not. Variable power supplies and feedback irradiance control systems compensate for reduced irradiance, and a lamp life at peak summer sunlight levels can be thousands of hours. The most common chambers can operate at “three sun” irradiance levels at 340 nm, although the lamp life is reduced. The fluorescent UV lamp technology is the most economical of any method used for weathering. Fluorescent UV weathering chambers are unique among environmental chambers. The working space of the chamber is not in the interior of the chamber, but rather on the two outer walls that run parallel to the two banks of lamps. These walls are formed by inward-facing holders for flat specimens and not insulated. Temperature is controlled by a thermometer affixed to a black metal panel, similar to DUT control in climatic chambers. The fluorescent lamps radiate little heat, and heated forced air convection is needed to achieve most set points. The chamber includes a heated water bath with temperature control. During the condensation function, the lamps are turned off and the backs of specimens are cooled by room air. As the temperature increases in the chamber interior, specimens radiate heat to the outside and remain below the dew point of the humid air inside, creating continuous condensation on the specimen

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surface. The technology is very simple, but heated condensation as a stressor is unique in the field of climatic testing. As an independent test, “hot condensation” is one of the first tests performed on new paint formulations or for quality control. Blisters form quickly if the paint lacks resistance to moisture. Tests in fluorescent UV chambers typically alternate between UV exposure in dry, heated conditions and condensation. Cycles are normally programmed in blocks of 4, 8, or 12 h. Because condensation formation can take as long as an hour to occur due to the slow heating of the water bath, a minimum recommended time for a condensation step is 4 h. Water spray is available in these chambers, providing some mechanical washing of the specimen surface and a mild thermal shock to specimens from the relatively cold temperature of the spray. Thick or thermally insulating materials may not become adequately wetted by condensation because heat cannot sufficiently radiate to the outside room, preventing the front side temperature from decreasing much below the dew point. Water spray can mitigate the problem by wetting and cooling the specimen surface. The general principle for using fluorescent UV lamps to conduct weathering tests is that UV light is responsible for the vast majority of photodegradation of polymers, especially those considered durable enough for outdoor use. The energy produced by this system is concentrated in the spectral area of interest for most testing. This efficiency means that there is no need to remove excessive heat produced by the lamps, simplifying the overall chamber design. UV conditioning tests for PV modules represent a special case of fluorescent UV chambers. The qualification tests, known as MQT 10 in IEC 61215-2, describe exposure dosages in the UVB and UVA ranges but not what light source is to be used to achieve them. Although xenon arc and metal halide chambers can be used, fluorescent UV lamps may be the most practical. ASTM E3006 describes a method to achieve the requirements of the UV preconditioning tests in the IEC qualification standards using standard fluorescent UV weathering lamps and control technology. Original exposure dosages required a mix of UVB-313 and UVA-340 lamps or specially designed lamps for the test. Updated requirements have changed the dosage so that the UVB-313 lamp is no longer needed. Fluorescent UV weathering chambers are not large enough for module exposures, but specially constructed chambers have existed for many years. Several manufacturers

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Standard Fluorescen nt UV weathering chamber

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UV condioning test chamber from WeissTechnik

Figure 11.9 Photographs of a standard fluorescent UV chamber (Q-Lab Corporation) and a commerciallyavailable chamber designed to perform UV conditioning tests in the IEC qualification standards for PV modules. Courtesy of WeissTechnik GmbH.

of temperature and humidity chambers have fitted their products with banks of fluorescent UV lamps to perform these tests. See Fig. 11.9 for photos of standard fluorescent UV chamber and one designed for PV module testing.

11.8.2 Xenon Arc Chambers Xenon arc lamps were invented in the 1950s in Germany and began to be used almost immediately for weathering testing. These lamps are made from a quartz tube (long arc) or bulb (short arc) with various pressure levels of pure xenon gas inside them. Long arc lamps are the type used for weathering because of their relatively long life under continuous use. The arc emits a continuous spectrum that approximates sunlight in the UV, visible, and IR spectral regions. Today the technology is the most commonlyused light source for simulation of the full spectrum of sunlight. Xenon arc lamps produce shorter wavelengths than terrestrial sunlight, into the UVC range. Therefore optical filtration is used to accurately simulate the UV spectrum. In addition, xenon arc lamps have very high irradiance peaks in the near infrared region. This energy results in extra radiant heat, beyond what a perfect sunlight simulation would produce, and this must be removed by the air handling system unless the temperature set point is very high. Some optical filter systems use infrared-absorbing coatings or reflecting mirrors to reduce the radiant heat load in the working space. In any case, this heat must be removed by the

system. Xenon arc lamps require cooling to prolong their service life, and this can be achieved by either water or air circulation. The choice of which type to use is dictated by the power density of the lamp. Higher wattages in a smaller package require water cooling because air cannot remove heat quickly enough. When air cooling is sufficient, it is the preferred choice because of the added complexity of water cooling systems. Xenon arc lamps have a limited useful life because aging of the quartz tube and degradation of electrode materials cause the UV output efficiency to decrease. At around 1500 h, the UV output has decreased enough that replacement is necessary. At low irradiance levels the useful life can be extended somewhat. Perhaps the most critical variable in xenon arc testing is the choice of optical filters to use. These remove short wavelength energy from the working space. They can be characterized by a “cut-on” wavelength which is the shortest wavelength with irradiance above a nominal threshold value. Weathering standards have defined three families of filtersdDaylight, Window, and Extended UVdand within each family is a multitude of chamber specific and proprietary types. Daylight filters are designed to simulate direct sunlight, which has a cut-on wavelength of around 295 nm. Window filters simulate sunlight through a vehicle or building window, which filters most or all of the UVB. Extended UV filters transmit some shorter wavelengths than experienced from natural sunlight. Seemingly small differences between optical filters sometimes result in significant

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differences in material degradation. This creates reproducibility problems between laboratories, which is common. For this reason, some newer test methods have been written with a narrowly-defined UV spectral irradiance. The older definitions of Daylight and Window filters attempted to standardize around the many types offered by xenon arc chamber manufactures, but the newer definitions attempt to focus the range. The IEC weathering standard for PV materials uses an optical filter definition developed for ASTM D7869, a standard for automotive paints and exterior components. The IEC standard also uses the same maximum irradiance, 0.80 W/(m2$nm) at 340 nm, which is slightly higher than peak sunlight (see Fig. 11.10). Acceleration comes from the fact that this peak irradiance is run throughout the entire cycle, day in and day out. Testing near the maximum operating temperature also provides acceleration. Climatic control in xenon arc weathering chambers is more challenging than other types of chambers because of the significant radiant heat produced by the lamps. Xenon arc lamps used in weathering chambers range in power from 1500 to 15,000 W, and roughly half of the radiation produced is in the infrared. For this reason, xenon arc chambers control two temperatures in most cases. The first is a black panel temperature or the surface temperature of a thin panel of stainless steel painted black. Two versions of this type of thermometer exist, with the difference being that one of them is affixed to a piece of white plastic to insulate the back side and increase its heat load and temperature. The two are known as

Figure 11.10 Spectra of peak sunlight and the xenon arc spectrum specified in IEC component weathering standards.

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uninsulated and insulated black panel thermometers, with the insulated type also called a black standard thermometer to indicate there are none more black in terms of absorption of thermal radiation. The black panel temperature is a function of the radiant heat load, which is a function of lamp irradiance, the presence of any infrared absorbers or reflectors in the optical filter system, and the chamber air temperature, which is also controlled. The black panel temperature is intended to be an approximation of the hottest specimens in the specimen plane, uninsulated for metallic specimens and insulated for plastics. The two interdependent temperatures are independently controlled by adjusting the ratio of fresh room air to recirculated air and overall rate of air flow that moves through the working space. The overall rate of air flow removes heat from the black panel and controls its temperature, and the ratio of cool room air to conditioned chamber air affects the chamber air temperature. Most xenon arc chambers are equipped with RH control. Humidity is generated with boilers, atomizing spray nozzles, and ultrasonic nebulizers, depending on the chamber size and specific design. Generally, humidity removal is not a concern for xenon arc weathering tests because no standard test methods require low dew points. Typical xenon arc chambers specify air temperatures ranging from a minimum of 38
C to a high as 80
C. The minimum can be achieved by cooling with ambient air and humidity generating techniques that remove heat. Although not common, some chambers have optional refrigeration to run at reduced chamber air temperatures, but not as low as the freezing point of water. Xenon arc weathering chambers are designed in two form factors, rotating rack and flat array (Fig. 11.11). As mentioned earlier, weathering chambers have an essentially planar working space due to the necessity of maintaining a fixed distance between specimens and lamps for irradiance control and uniformity. The flat array form factor does accommodate three-dimensional items, but if the top surface protrudes above the tray significantly, the irradiance will be higher than the chamber setting and the specimen will be hotter, sometimes several degrees hotter. Rotating rack chambers may appear three-dimensional as the specimen rack moves chambers around the central light source(s), but the cylindrical basis of the rack is a planar surface curved onto itself. In short, xenon arc weathering chambers

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Figure 11.11 Examples of rotating rack and flat array xenon arc weathering chambers.

11.8.3 Metal Halide Light Sources Metal halide lamps are quartz tubes filled with trace amounts of mercury and metal halide salts. An electrical current vaporizes the mercury and creates spikes of light, mostly in the UV. When the mercury arc is struck, the heat vaporizes the metal halides, and this process broadens the spectrum of light produced. The precise blend of these salts is a key factor in their spectral output. In the field of PV and material testing, there are two uses for this technology. In the first, high power metal halide lamps designed to radiate mostly in the UV can generate high UV

irradiance for materials testing. Second, the salts can be blended so that the lamp produces a broad spectrum similar to sunlight. Fig. 11.12 shows the spectrum of a metal halide lamp used by NIST in its SPHERE weathering chamber, while Fig. 11.13 shows the SPHERE device itself. Although the NIST device is a unique scientific test apparatus, efforts are underway to commercialize a smaller version of it, but for now this is still a technology for experimentation. There are, however, commerciallyavailable metal halide chambers with a similar light source and aspects of climatic chambers. These purport to produce irradiance levels up to 30 times higher than “one sun” levels. However, their match to natural sunlight is not as accurate as the

6.0 5.0 Irradiance (W/m2/nm)

contain a mostly two-dimensional working space, unlike temperature and humidity chambers. Xenon arc chambers include water spray for testing outdoor materials as the only method for wetting specimens5. In the rotating rack form factor, the spray nozzle is turned on continuously during that portion of the cycle, and specimens are sprayed intermittently as they move past the fixed nozzles. Flat array chamber nozzle systems spray all specimens simultaneously, but mimic the rotating rack behavior by pulsing the nozzles on and off during the spray period.

NIST SPHERE Peak Sunlight

4.0 3.0 2.0

5

Some chambers include spray nozzles that spray the back sides of specimens. The intent is to operate the chamber at high RH and cool the specimens by spraying the back side to create condensation on the front side. This has not been proven effective because the spray water is rapidly equilibrated with the chamber air temperature, so the specimen temperature never drops below the dew point.

1.0 0.0 250 300 350 400 450 500 550 600 650 700 750 Wavelength (nm)

Figure 11.12 Spectrum of a NIST SPHERE metal halide lamp.

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Figure 11.13 NIST SPHERE test chamber.

more common weathering chamber light sources. As noted above when discussing optical filters in xenon arc chambers, small differences in the UV spectrum can have significant impact on degradation and test results. Even though most of the energy produced is in the UV, the irradiance levels produce significant heat. These high irradiance chambers usually have liquid-cooled specimen trays to avoid thermal failures of the specimens. Care must be taken to avoid reciprocity assumptions when testing with these chambers, beyond the spectral mismatch issues that exist between metal halide lamps and sunlight. Little evidence exists that the same results are obtained from equivalent UV dosages of identical spectral output, applied at different rates over time. The material behavior to high irradiance versus real-life levels is highly variable according to its chemical structure and other factors. More complex polymer systems are less likely to adhere to linear reciprocity. Nevertheless, quickly determining how polymeric materials degrade from UV light can be useful to experienced material experts. The second type of metal halide chambers uses full-spectrum lamps designed to simulate sunlight. These are commonly used to test for thermal effects

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from solar radiation rather than for weathering. The spectral stability in the UV as lamps age is not well publicized, and irradiance control is often limited to regulating the distance between lamp arrays and the DUT. Large chambers with several lamps mounted in parabolic reflectors can test fullsize PV modules or whole vehicles. IEC 60068-2-5 includes such a test, which is more closely aligned with traditional environmental testing than weathering. In theory, xenon arc lamps can be used for these exposures, and often they are, but the infrared output is excessive and may not thermally stress the materials the same way that sunlight does. It is also very difficult to make large scale xenon arc chambers for something as large as a PV module. Confusion exists about the IEC standard as well as MIL-STD-810G. Makers of electrotechnical devices, including PV modules, have taken these solar radiation thermal stress tests to be weathering tests. TC104 has corrected this misunderstanding by revising 60068-2-5 to specify that weathering is different and is best conducted in a standard weathering chamber. Test “Sa” covers the traditional solar radiation test while the new “Sb” refers to ISO 4892-2 for weathering. The biggest problem with using metal halide lamps for weathering is that the lamp technology is not standardized. There is no single metal halide lamp due to how the technology works. There are a great many types, similar to the variety of fluorescent lamps manufactured. For weathering, fluorescent UV lamp types have been standardized and can be obtained from multiple sources around the world. No such standardization is yet to occur for metal halide lamps. Furthermore, not enough is known or published about the spectral stability of these lamps at different irradiance levels. Does the spectrum shift at high irradiance versus low? The spectral curves for xenon long arc lamps used for weathering tests experience minimal change between low and high irradiance. How do these lamps age and how long can they maintain the UV spectrum compared to xenon arc lamps? Xenon arc chamber manufacturers have specific lamp life specifications. None of these uncertainties regarding metal halide lamp use in testing is a permanent state. Over time these questions may be answered and specific metal halide lamp spectra standardized. Until then, metal halide lamp use for weathering is still in the realm of experimentation.

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11.9 Climatic Tests in the PV Module Qualification Standards Fig. 11.14 shows an example of an environmental chamber designed for testing PV modules. There are three climatic tests in the qualification standards: thermal cycling, humidity freeze, and damp heat. These are labeled MQT 11, 12, and 13, respectively, in IEC 61215-2. MQT 10, UV preconditioning test, is considered separately from these climatic tests earlier in this chapter. These tests specify module temperature rather than chamber air temperature. It is important to note that these are parts of test sequences within the qualification test regime. Sometimes the stresses cause a failure mechanism not detected until later on in the sequence. Therefore, each one is of limited value on its own. The cycling tests stress soldered connections and adhesion between laminated layers, while damp heat detects moisture ingress that results in delamination between layers and corrosion of metallic interconnects. Each is briefly described with an emphasis on chamber characteristics important to it. For brevity, only 61215 sequences are discussed, although the 61730 safety standard series includes similar test sequences with different performance criteria. The damp heat test is the most straightforward, yet widely considered the most challenging. The module is maintained at 85
C with 85% RH in the surrounding air. It is performed in its own sequence,

Figure 11.14 Environmental chamber for testing PV modules. Photo courtesy of Espec.

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with half of the tested modules then undergoing a hail impact test and the other half a mechanical load test. Most labs dedicate chambers for this test for reasons of economy since the test requires no refrigeration. Thermal cycling is performed during two of the test sequences. In one of them, it as an independent test conducted for 200 cycles prior to a final performance measurement. In the other sequence, it is performed for 50 cycles between a UV conditioning test, discussed later, and a humidity freeze test of 10 cycles. The thermal cycling test includes injecting current through the modules, which can detect solder failures in real time during the test. In principle, it is a simple test: module temperature is adjusted between þ85 and 40
C and maintained at each temperature for a minimum dwell time of 10 min. There is flexibility in the ramp rate between conditions. The test should not take longer than 6 h to complete a full cycle from ambient conditions to 40, up to 85
C, and back down to ambient, including the dwell time. However, the transitions cannot be faster than 100
C per hour. One cycle can be completed in as little as approximately 2.8 h rather than the maximum of 6 h allowed. A 200-cycle test can be completed almost 4 weeks faster with the fastest allowed transition rates. As discussed earlier, faster transitions require more heating and cooling power and thus add to the chamber’s cost. In addition, one chamber manufacturer calculates that the faster transition consumes about 50% more power than the slow one and requires 78% more cooling water. The humidity freeze test is conducted after thermal cycling and before tests of the junction box and cable anchorage, although one of the two modules in this sequence skips those tests and goes to final power output measurement tests. This is a combination of the damp heat and thermal cycling tests. The full cycle is 24 h and 10 cycles are required, although by maintaining the maximum ramp rates it is possible to reduce this by around 10%. From ambient conditions, the modules are heated to 85
C, where the RH is maintained at 85% and the conditions are held for 20 h. RH control is turned off and the temperature is decreased to 40
C. The maximum ramp rate is 100
C per hour just as in the thermal cycling test, but is allowed to proceed at double that rate once the modules are at the freezing point. The minimum dwell point at the low temperature is 30 min, and then the cycle is reversed. Many module manufacturers, in the absence of clear alternatives, often report test results from

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longer-duration tests or multiples of the prescribed cycles. Their intent is to show that their modules have improved durability versus older designs or against their competitors. It is an understandable approach. Yet these tests can become very costly, and experts have questioned whether, for example, there is any benefit to the damp heat test beyond the 1000 h in the existing qualification standards. Is it possible that a different approach to improving durability may be worth diverting some of the resources devoted to ever-longer qualification test sequences? Recently the IEC has published a new series of standards that seek to qualify module components and their materials separately from the entire module construction. This is analogous to what car or window manufacturers do. Longer-duration tests on materials are less costly and can deliver better simulations. Once materials have been qualified to withstand sunlight, heat, and moisture, it may be that module qualification only requires testing for mechanical interactions between the components. The existing tests appear effective for this purpose.

11.10 Conclusions This chapter covered the technology of climatic test chambers, from the traditional temperature and humidity chambers to specialized corrosion and weathering devices. The methods for creating the stress factors of heat, moisture, UV light, thermal cycling, and corrosive mists were detailed and related to standardized tests used by the PV module industry. Some of the specific challenges facing the specialized tests were presented in the hopes that users and developers of test methods understand practical limitations and write new methods accordingly.

References [1] Thermotron Industries, Solar PV Testing: A White Paper [Online], July 2010, www. thermotron.com.

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[2] Cincinnati Sub-Zero, Types of Chamber Cooling Systems [Online]. www.cszindustrial.com. [3] Espec North America, Resources: Methods of Humidity Generation [Online]. www.espec. com/na. [4] Cincinnati Sub-Zero, Understanding Humidity in Environmental Test Chambers [Online]. www.cszindustrial.com. [5] Espec North America, Resources: How to Chambers Lower Humidity? Hudsonville, MI: s.n. [6] Wikipedia.org, Hygrometer, [Online]. https://en. wikipedia.org/wiki/Hygro. [7] Q-Lab Corporation, Introduction of Cyclic Corrosion Testing, Westlake, OH: s.n. [8] Laboratory test methods for the simulation of atmospheric corrosion: lessons from the automotive industry, in: S.P. Fowler (Ed.), U.S. Department of Defense Allied Nations Technical Corrosion Conference, SSPC, Birmingham, AL, 2017. Vols. Paper No. 2017e789123. [9] S.S.L. Fowler, Considerations for relative humidity and temperature control in atmospheric corrosion teststandards, Polymers Paint Colour Journal (July 2014). DMG Events. [10] S. Bell, Good Practice Guide No. 124: The Beginner’s Guide to Humidity Measurement, National Physics Lab, Teddington, Middlesex, United Kingdom, 2012.

Further Reading [1] Espec North America, Resources: How Do Test Chambers Get So Cold? [Online], 2016, www. espec.com/na. [2] Arndt, Regan, Pluto, Dr. Ing Robert, Basic Understanding of IEC Standard Testing [Online]. https://www.tuv-sud-america.com/us-en.