Damp-heat test

The damp heat test can be made under steady state conditions when 40 °C and 93% relative humidity (RH) are specified, or under dynamic conditions when the temperature is cycled from 25 °C to 40 °C or 55 °C. Clearly, other conditions could be chosen to suit the application. 

Finally, considering that, in general, ZnO is known to be particularly sensitive to a humid environment (see paragraph 9.4.1.2), this feature should also be checked, in more detail, for the case of ZnO films deposited by CVD. In fact, Sang et al. [84] reported that in a humid environment, ZnO B films deposited by LP-CVD showed higher degradation than sputtered ZnO Ga films. On the other hand, Oerlikon Solar (formerly Unaxis Solar) [79] has proven that thin him silicon solar modules using LP-CVD ZnO as TCO layers can successfully pass the standard damp-heat test, provided they are encapsulated in an appropriate manner. 

Reliable stability data of the p-i-n solar cell itself are not easily obtained, especially for non-encapsulated cells or modules. One of these tests e.g. for EN/IEC 61646 certification of modules is the so-called damp-heat test (85°C, 85% humidity, up to lOOOh). Recent studies were performed by Stiebig et al. [50, 51] exposing different types of cells to harsh conditions. One of the most important results was the excellent stability of silicon thin film solar cells. Remarkably, this is also valid for small area modules even without encapsulation [52]. This is of high interest because costs and efforts for module encapsulation strongly depend on the inherent stability of the solar cells. As a more detailed treatment of this subject is beyond the scope of this chapter, the reader is referred to the original papers [50,51]. 

In the damp heat test, the test pieces are subjected either to static temperature and humidity conditions of 40"C at 93% relative humidity or to 12-1- 12 hour cycling between 23" C at >95% relative humidity and 40 C (or 55 C) at 93% relative humidity. For the water spray test a mist of distilled water at 40 C is sprayed over the surface of the plastic at the rate of 125 to 250 ml per hour per square meter of surface, while for the salt spray test... 

The goal of the backsheet is to provide UV and moisture protection, electrical insulation, and durability. To this purpose, standard international committees (such as lEC and UL) have defined a set of tests to be passed before a photovoltaic s module could be placed on the market. The lEC 61215 is one of the most used standards for qualifying a PV module [7]. The aim of these tests is to check the resistance of the PV module to all the variables that can affect and infiuence the life of the module itself such as electrical performance, hail and impact test, weathering, and temperature (UV and thermal cycling). One of the most important tests is the damp heat test, 1000 hours inside a climatic chamber at 85°C and 85% relative humidity. Most of the PV module producers tend to prolong well above 1000 hours of damp heat, to be sure that the module will survive after 25 years giving at least 80% of the initial power. This trend is also taken into consideration into the new version of the standard tests that is considering to have at least 2000 hours of damp heat for qualification. 

Fig. 13.10 Results of FEM simulations of the humidity in PV modules in the encapsulant. Left diagram in front of the impermeable silicon cell (Pos 4 in the inset to the right) calculated with real monitored climatic border conditions at different locations with periodic repletion of one-year data of weathering. Right diagram during an extended damp-heat testing for 3000 h at 85 °C and 85 % relative humidity at different places in the PV module 1 is between the permeable back-sheet, 2 in the gap between two cells, 3 at halfway between edge and centre of a cell, 4 at the centre of the cell...

Fig. 13.13 Normahsed power (standardised power output of the modules related to the virgin data before testing) over damp-heat testing time at 85 °C (left) and 90 °C of seven different PV modules...

Miscellaneous Fastness Tests. The fastness to hot pressiag, ISO 10S-X1 /, test is similar to the fastness to dry heat test except that the time of pressing is 15 s (again at 150, 180, and 210°C), and the test can either be carried out dry when a damp cotton fabric containing its own weight of water is placed on top of the dry test fabric, or wet when the test fabric also contains its own weight of water. 

A water spray test is similar to damp heat except that there is the constant presence of small water droplets. The test is made using essentially similar equipment to that for the salt mist test. 

The ZnO film plays an important role for module stability in accelerated lifetime testing under damp heat conditions, which forms a part of the... 

The free carrier optical reflection of test modules before and after damp heat indicates that the effective carrier density is not much affected [58]. Hence, the degradation of the ZnO sheet resistance is probably more of a carrier transport problem. It is, at present, unclear where electron barriers are located. They may be present at the grain boundaries in general [59]. In this case, the disturbances of the ZnO microstructure (induced by the substrate but also depending on preparation parameters) are only harmful because they allow a faster penetration of the humidity into the film. On the other hand, the disturbed regions may themselves be highly resistive after damp heat exposure, which forces the current to percolate around these... 

Environmental testing—Part 2 Test methods—Test Cx Damp heat, steady state (unsaturated... 

More recently, Roberts has derived acceleration factors for heat-damp testing of small silicone packaged signal transistors. He concluded that the majority of silicone packaged devices which he had evaluated under damp heat conditions were superior to those packaged in epoxy with the proviso that other unknown factors relating to the chip or connection design may have partially contributed to this superiority. 

The switch also passed a variety of environmental tests, including mechanical shock tests and a 20000-cycle endurance test at elevated temperatures (normally 185 °C). Climatic testing involved damp/heat cycling, with checks on dimensional stability. The exceptionally low CTE and moisture uptake were important reasons why the LCP performed well. 

How frequently the oil condition should be tested depends on operating and atmospheric conditions after the commissioning sample, further samples should be taken at three months and one year after the unit is first energized. After this, under normal conditions, testing should be carried out annually. In unfavorable operating conditions (damp or dust-laden atmospheres, or where space limitations reduce air circulation and heat transfer) testing should be carried out every six months. 

Dynamic mechanical tests measure the response or deformation of a material to periodic or varying forces. Generally an applied force and its resulting deformation both vary sinusoidally with time. From such tests it is possible to obtain simultaneously an elastic modulus and mechanical damping, the latter of which gives the amount of energy dissipated as heat during the deformation of the material. 

The progress of the partial saponification can be checked by a simple solubility test. About 1 ml of the solution is withdrawn and the cellulose acetate precipitated with water. The small sample is quickly washed free of acid and dried as much as possible by pressing between two filter papers. Some fibers of the still damp material are placed in a test tube with 15-20 ml oholuene/ethanol (volume ratio 1 1) and heated to boiling in a water bath. If the fibers go into solution, then after about 15 min the whole charge can be worked up as described above. 

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