Category Polymeric Materials for Solar Thermal Applications

Possible Alternative Procedures

Additionally to the standard procedures the validity limit of the functions has to be determined.

The examples shown in the chapter “The limit is dependent on the absorber temperature and on the ambient temperature” are related to an idealized relatively sharp limiting behavior. In reality such limiting measures may not have this sharp onset in the drop-down of the efficiency.

18.6.3.1 Determination of the Validity Limit for the Standard Procedures

The test procedure (6.1.4.4, 6.1.5.5) may be expanded to higher fluid inlet tempera­tures (in steps of, for example, 5 °C). In steady-state tests the onset of the limiting

0 10 20 30 40 50 60 70 80 90 100

absorber temperature [°C]

Figure 18.2 Efficiency graph of a solar collector with twin-wall sheet glazing, black absorber (a = 0...

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The Limit is Dependent on the Absorber Temperature and on the Ambient Temperature

This limiting behavior can be observed, for example, in the case of an adequate thermotropic layer that is mounted on the inner side of the glazing of the collector. This type of temperature limitation is only satisfying in the case of non-selective absorbers and double glazed collectors because a good thermal coupling from the thermotropic layer to the absorber and a higher thermal resistance to the ambience is needed.

This behavior is illustrated here for a modeled polymeric collector with an idealized thermotropic layer that switches in the temperature range 55-60 °C (Figure 18.1) and with different transmittances in the opaque state (Figure 18.2) [1].

As the temperature ofthe thermotropic layer is a function ofboth the absorber and the ambient temperature the limit of the absorber tem...

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Problems Regarding Polymeric Collectors

These tests and computations may not reflect the requirements for polymeric (and other) collectors if they have measures to limit the temperature of the absorber.

18.6.2.1 The Limit is Dependent Mainly on the Absorber Temperature

This limiting behavior will arise, for example, in the case of an adequate thermotropic layer that is mounted on the absorber or in the case of ventilation or other limiting cooling measures that are controlled mainly by the absorber temperature.

If the temperature limit is mainly dependent on the absorber temperature then the efficiency curve will more or less abruptly fall to zero at this temperature. This limiting behavior and therefore the area of validity of the formulas (7), (8), (32), (32.1), (D.3), (D.4), (H.1) and (H...

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Discontinuous Efficiency Curves

Robert Hausner

18.6.1

Description of the Specific Test and Test Procedure

Within EN 12975-2, the following tests and calculation procedures are concerned with discontinuous efficiency curves of polymeric collectors with temperature limiting measures, such as thermotropic layers, temperature-controlled ventilation, or other active or passive measures.

In the following chapters of EN12975 second-order dependencies on the temper­ature difference (tm — ta) (tm: middle temperature of the fluid in the collector, ta: ambient temperature) are established (Table 18.3).

A simple linear dependency on this temperature difference is established in Chapter C.2 [Determination of stagnation temperature; Formula (C.1)].

Table 18...

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Typical Data for Ice Stones Test of Different Sizes (According to EN12975 and to PV Norms such as EN61646 etc.)

The destruction limit for flat plate collectors with toughened glass is in the range of about 45 mm diameter and normative impact speed of 30.7 m s—1 Impacts at lower levels usually do not cause any problems.

18.5.2.2 Typical Normative Requirements

So far the impact resistance test is not mandatory for the European certification. It is already mandatory for the Australian (using ice stones) and US certification (using steel balls). In central Europe most countries will reasonably adapt an impact resistance requirement in the range of 20-30 mm (ice stones). This assumption is based on the probability of a hail storm with the corresponding hail-stone size during the expected life time of the collector...

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

The impact resistance test is not only a problem for polymeric collectors but also for other collectors such as evacuated tube collectors. There are no real doubts about the suitability to assess the impact resistance using the current methods. Depending on the norm, certification scheme, or also building insurance codes the required impact energies may, however, vary. Furthermore, the risk of hail damage very much depends on the geographic location of the installation. It is, therefore, reasonable to assume that polymeric collectors – as all other collectors – will reveal a certain hail resistance allowing for installation in a possibly restricted area only.

18.5.2.1 Typical Data for Steel Ball Test of 150 g (According to EN12975)

A drop height of 2...

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Impact Resistance Test

Andreas Bohren and Stefan Brunold

18.5.1

Description of the Specific Test and Test Procedure

Two different test procedures for the assessment of impact resistance are found in the different international test and certification standards. Depending on the standard there are only some minor differences in the application.

One method involves using steel balls dropping from increasing heights on the collector. In the second method laboratory ice stones of different sizes fired with well – defined speed onto the collector. The rating of the impact resistance is based on the maximum impact energy (impact speed, falling height) without damage to the collector...

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Possible Alternative Procedure

The methods for assessing the wind and snow loads can be considered as adequate and there is no reason to introduce new test methods. It is certainly advisable to take into consideration applying these tests only to collectors that have been pre-aged to include the potential weathering induced brittleness of the cover.

18.5

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