Significance and Use
In many geographic areas there is concern about the effect of falling hail upon solar collector covers. This practice may be used to determine the ability of flat-plate solar collector covers to withstand the impact forces of hailstones. In this practice, the ability of a solar collector cover plate to withstand hail impact is related to its tested ability to withstand impact from ice balls. The effects of the impact on the material are highly variable and dependent upon the material.
This practice describes a standard procedure for mounting the test specimen, conducting the impact test, and reporting the effects.
The procedures for mounting cover plate materials and collectors are provided to ensure that they are tested in a configuration that relates to their use in a solar collector.
The corner locations of the four impacts are chosen to represent vulnerable sites on the cover plate. Impacts near corner supports are more critical than impacts elsewhere. Only a single impact is specified at each of the impact locations. For test control purposes, multiple impacts in a single location are not permitted because a subcritical impact may still cause damage that would alter the response to subsequent impacts.
Resultant velocity is used to simulate the velocity that may be reached by hail accompanied by wind. The resultant velocity used in this practice is determined by vector addition of a 20 m/s (45 mph) horizontal velocity to the vertical terminal velocity.
Ice balls are used in this practice to simulate hailstones because natural hailstones are not readily available to use, and ice balls closely approximate hailstones. However, no direct relationship has been established between the effect of impact of ice balls and hailstones. Hailstones are highly variable in properties such as shape, density, and frangibility. These properties affect factors such as the kinetic energy delivered to the cover plate, the period during which energy is delivered, and the area over which the energy is distributed. Ice balls, with a density, frangibility, and terminal velocity near the range of hailstones, are the nearest hailstone approximation known at this time. Perhaps the major difference between ice balls and hailstones is that hailstones are much more variable than ice balls. However, ice balls can be uniformly and repeatedly manufactured to ensure a projectile with known properties.
A wide range of observable effects may be produced by impacting the various types of cover plate materials. The effects may vary from no effect to total destruction. Some changes in the cover material may be visible when there is no apparent functional impairment of the cover plate material. All effects of each impact must be described in the report so that an estimate of their significance can be made.
Data generated using this practice may be used: (1) to evaluate impact resistance of a single material or collector, (2) to compare the impact resistance of several materials or collectors, (3) to provide a common basis for selection of cover materials or collectors for use in various geographic areas, or (4) to evaluate changes in impact resistance due to environmental factors such as weather.
This practice does not state the size(s) of ice ball(s) to be used in making the impact. Either the person requesting the test or the person performing the test must determine ice ball size to be used in the testing. Choice of ice ball size may relate to the intent of the testing.
If the testing is being performed to evaluate impact resistance of a single material or collector, or several materials or collectors, it may be desirable to repeat the test using several sizes of ice balls. In this manner the different effects of various sizes of ice balls may be determined.
The size and frequency of hail varies significantly among various geographic areas. If testing is being performed to evaluate materials or collectors intended for use in a specific geographic area, the ice ball size should correspond to the level of hail impact resistance required for that area. Information on hail size and frequency may be available from local historical weather records or may be determined from the publications listed in Appendix X1.
The hail impact resistance of materials may change as the materials are exposed to various environmental factors. This practice may be used to generate data to evaluate degradation by comparison of hail impact resistance data measured before and after exposure to such aging.
1.1 This practice covers a procedure for determining the ability of cover plates for flat-plate solar collectors to withstand impact forces of falling hail. Propelled ice balls are used to simulate falling hailstones. This practice is not intended to apply to photovoltaic cells or arrays.
1.2 This practice defines two types of test specimens, describes methods for mounting specimens, specifies impact locations on each test specimen, provides an equation for determining the velocity of any size ice ball, provides a method for impacting the test specimens with ice balls, and specifies parameters that must be recorded and reported.
1.3 This practice does not establish pass or fail levels. The determination of acceptable or unacceptable levels of ice-ball impact resistance is beyond the scope of this practice.
1.4 The size of ice ball to be used in conducting this test is not specified in this practice. This practice can be used with various sizes of ice balls.
1.5 The categories of solar collector cover plate materials to which this practice may be applied cover the range of:
1.5.1 Brittle sheet, such as glass,
1.5.2 Semirigid sheet, such as plastic, and
1.5.3 Flexible membrane, such as plastic film.
1.6 Solar collector cover materials should be tested as:
1.6.1 Part of an assembled collector (Type 1 specimen), or
1.6.2 Mounted on a separate test frame cover plate holder (Type 2 specimen).
1.7 The values stated in SI units are to be regarded as the standard. The values given in parentheses are for information only.
1.8 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.