Some common thermal management solutions for enclosures include air conditioners, heat exchangers, ventilation and color when evaluating solar loading. This article is written to present the beneficial temperature effects of shading the enclosure’s surface by shielding the direct and reflective radiation from the sun.
Background: Climate and Solar Loading
The amount of solar radiation striking a perpendicular surface located beyond the earth’s atmosphere at the mean earth-sun distance is called the solar constant Esc â‰ˆ 1366 watts/m2 (127 W/ ft2; 433 BTU/(h â€¢ ft2). Absorption by water vapor, dust and ozone, while passing through the earth’s atmosphere will reduce this value. The typical extreme value of a horizontal surface over most of the United States is 1044 W/m2 (97 W/ft2). In the upper altitudes with clear dry climates, as in the southwestern regions of the United States, extreme horizontal surface values of 111 W/ft2 are possible. An examination of Weather Bureau data indicates that maximum ambient air temperatures of 104ËšF (40ËšC) are only exceeded in Arizona, Nevada and California (ref. IEEE C37.26-1986; IEEE Guide for Evaluating the Effect of Solar Radiation on Outdoor Metal-Enclosed Switchgear).
Background: Solar Absorption Color
The percent of solar energy absorbed by an enclosure is dependent on surface color, finish and texture. Low temperature radiation (larger wavelengths) presents a different set of values for surface color. Low temperature values affect an enclosure surface’s ability to absorb internal heat loads and dissipate radiant heat to the sky and surrounding objects.
When selecting an absorption value based upon the surface color of an enclosure, the value should be inflated. Absorption values of the finish will increase with age. Mounting location can magnify the solar gain by as much as 40% when located around reflective ground surfaces and buildings. Location may also block some of the connective cooling effects.
Background: Surface Color Affects the Low Temperature Radiation
Surface finish and color relates to the percentage of radiation emission and absorption to surrounding objects. The graph below illustrates the effects of surface color on low temperature radiation. From a relatively low temperature source (resistive heater), the metallic surface absorbs less of the internal dissipated heat causing an increased temperature rise. An enclosure’s ability to dissipate internal heat in W/ËšF generally is less for shiny metallic surfaces compared to other finishes. An enclosure made of mild steel with white interior absorbs and dissipates heat similar to a fiberglass enclosure with a gray interior.
Solar Shielding Test
Hoffman has conducted a series of tests to evaluate the thermal benefits of shielding enclosures exposed to direct sunlight. The objective was to capture and compare the internal enclosure temperatures of non-vented NEMA Type 4 enclosures. Three enclosures were configured. One without shielding and two enclosures with different amounts of shielding. The first experiment included no internal heat source; the temperature rise is solely attributed to solar load. The second experiment added 100-watt internal heaters. The purpose of adding heaters was to identify the enclosure’s ability to dissipate heat with the added shielding.
CAPTION: Illustration of the test set-up. The enclosures used were Hoffman COMLINE model E-CL605020 (600mm H X 500mm W x 200mm D). They are RAL 7035 (Textured Light Gray) in finish and color. The top solar shield is model E-SSH5020.
The results of a typical peak day are illustrated in the graph below. The curves show the internal time-based temperatures within each of the different enclosures. The shielding provides definite temperature reduction. In both experiments, during peak daytime loading, the enclosure with only a top shield provided about a 5ËšF temperature reduction, while the top and side shielded enclosure provided a 10ËšF temperature reduction. This equates to a relative 25% and 50% respective reduction. This result was also consistent in the test run with internal heaters.
The graph below approximates an enclosure’s temperature rise due to solar loading. The temperature rise is based on absorption color evaluated with worst case parameters. The dashed lines represent a fully shielded enclosure.
The worst case assumptions when evaluating solar loading of an enclosure are that three sides of an enclosure are illuminated, there is no wind and the sky temperature is equal to the ambient.
â€¢ Total surface area of the enclosure must be calculated
â€¢ Color selected
â€¢ Internal dissipated heat known
â€¢ Maximum allowable internal temperature must be determined
The evaluations shown in this text are approximated curves to assist the user in identifying load values and temperature rises. Actual internal temperatures may vary depending on panel layout. For active cooling systems, it is recommended to provide a safety margin by selecting a system with 25% additional capacity.
There are definite cooling benefits to solar shielding enclosures, as illustrated in the graphical data. The shielding application may even be incorporated with active cooling systems to reduce the required size or loading of an air conditioner or heat exchanger. Certainly a benefit of this passive cooling system is the avoidance of untimely maintenance. Passive cooling with a solar shield is a reliable alternative.
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