Category Polymeric Materials for Solar Thermal Applications

Typical Data for Wind Load (According to EN12975 and to PV Norms such as EN61646 etc.)

Negative loads may lead to destruction of the collector covers. In general this happens because the cover is pulled out of the frame, leading to glass breakage. The cover itself (when made of toughened glass) is usually strong enough to withstand the applied forces if not pulled out of the frame. Concerning the mounting parts the same applies as for the positive loads.

18.4.2.2 Typical Normative Requirements

As stated above, thecurrentversionofEN12975 requires 1000 Pa only. An increase to 2400 Pa for the normal testing seems reasonable, and a high load test of up to 5400 Pa for specially exposed locations is to be expected. Some countries such as France or Ireland have already set up higher limits up to about 3000 Pa, always depending, however, on the installation location.

Although the E...

Read More

Why this is a Problem for Polymeric Collectors or Why this Test does not Reflect the Requirements for Polymeric Collectors

There is no specific problem for polymeric collectors. It may be assumed that it is more difficult to reach the required stability for polymeric covered collectors, especially with the expected higher loads. If the collector casing is made of polymeric materials the mounting parts used to fix the collector on the roof must also be taken into account.

18.4.2.1 Typical Data for Snow Load (According to EN12975 and to PV Norms such as EN61646 etc.)

Most conventional flat plate and evacuated tube collectors will have no problems with the expected 2400 Pa positive load. Even loads of up to the maximum of 5400 Pa will usually not result in any damage. The weakest points are in general the mounting parts (roof hooks)...

Read More

Andreas Bohren and Stefan Brunold

18.4.1

Description of the Specific Test and Test Procedure

The mechanical load tests are intended to assess the resistance against wind and snow loads. This is achieved by means of applying a positive (simulating wind and

snow load) and a negative force (simulating wind load) onto the collector. For flat plate collectors usually a set of pneumatically actuated suction cups are used to apply the forces. Other methods are, however, possible depending on the type of collector. Basically, though, the same tests are applied for all collectors. Although there are some problems in conducting the test for certain types of collectors (e. g...

Read More

Active Devices

If the active devices work properly, the collector should be protected from “high temperatures.” Nevertheless, in general it cannot be ruled out that the overheating protection system might be in the “off” state for some reason, such as during installation, for example, or due to component damage (e. g., motor, photovoltaic module, pump).

Thus, we recommend two separate test procedures:

1) Active device switched on: In this case, the system (collector + active overheating protection) is installed according to the manufacturer’s specification. Then the high-temperature test is performed as described by the current standard. Finally, the collector is inspected for any signs of damage. No severe damage must occur in order to pass the test.

2) Active device switched off: In this case...

Read More

Passive Devices

Passive overheating protection measures employ some mechanism that reduces the collector temperature during non-standard use (stagnation). However, if the collector is operating in normal mode (again), one has to make sure that the passive over­heating protection is “turned off” again, for example, thermotropic coatings need to return to their transparent state or the venting mechanisms need to close again.

Thus, we recommended looking for criteria or test procedures that allow assessing proper functioning of the collector after one or more cycles where the passive overheating protection was “turned on.”

Possible approaches can range from simple visual inspection (also during final inspection) to detailed measurements of the “passive overheating protection components” (e. g...

Read More

Comments on Overheating Protection

Because some materials, especially commodity plastics, cannot withstand elevated temperatures, there have been some attempts to equip the collector with some kind of overheating protection.

Passive overheating protection can be realized using, for example, thermotropic coatings or a passive venting mechanism.

Active overheating protection, in contrast, is achieved by including additional technical equipment that upon exceeding a certain control temperature turns on and reduces or maintains a certain viable temperature level. Examples can be a pump circulating a cold fluid, some kind of roller shutter preventing radiation striking the collector or a forced ventilation system employing convective cooling. Conveniently, these systems may be driven by a photovoltaic panel.

Although passive ove...

Read More

Possible Alternative Procedure

18.3.3.1 General Comments

Some polymers degrade when exposed to UV, water, and heat or a combination thereof. Depending on the exact composition of the used polymeric materials it will take a shorter or longer time before visible or non-visible degradation occurs.

Table 18.2 Value of climate parameter for all climate classes (exposure test 30 days).[18]

Value for all climate classes

Global solar irradiance on collector plane, G (Wm 2) 850

Global daily irradiation on collector plane, H (MJ m-2) 14

Surrounding air temperature, ta (0C) 10

Considering the variety of different materials and their different reaction to ambient loads and taking into account the fact that there is only very limited experience with long-time exposure of polymeric collectors, it is currently neither possible nor ad...

Read More

Why this is a Problem for Polymeric Collectors or Why this Test does not Reflect the Requirements for Polymeric Collectors

The exposure test and the high-temperature resistance test are not only a potential problem for polymeric collectors but also for all other collector types.

Potential defects caused by these tests can be:

• breakage, tearing, bending, lumps, or fatigue of material at the collector casing and covers;

• interference with sealings;

• depositions caused by outgassing of collector materials;

• staining of collector materials caused by degradation.

18.3.3

Read More