Pressure Drop as a Function of Air Flow Rate for Roll-Punched Transpired Solar Collectors With Different Porosities

Transpired solar collectors are simple and cost-effective devices suited for preheating ventilation air for buildings. They work by drawing outside air through a south-facing perforated metal absorber. The absorber is heated by solar radiation, heat is transferred to the air as it is drawn through the perforations in the absorber, and the pre-heated air is delivered to the building. Often overlooked in the design of transpired collector systems is the effect on collector efficiency of pressure drop across the absorber. The pressure drop is a function of the air flow rate through the perforations, the overall porosity of the absorber, and the properties of the air. The National Renewable Energy Laboratory (NREL) conducted a series of laboratory tests to characterize the pressure drop as a function of the relevant parameters for six commercially-available absorber configurations; three porosities in aluminum and three in steel. Each absorber material is roll-punched to one of three depths to create absorbers with three different porosities. Pressure drop, air flow rate, air temperature, and air relative humidity data were collected during the tests. The data were fit to a model that can be used to predict pressure drop across the absorber as a function of air properties and flow rate. Use of these correlations is expected to aid designers in ensuring that transpired collector systems are designed for optimal thermal efficiency and cost effectiveness.