In this paper, we will present a method for estimating the stored thermal energy in a building wall that is adjacent to an unglazed transpired collector. We also discuss how this value should be incorporated in the collector efficiency calculations. An unglazed transpired collector is made of a relatively thin, dark, perforated metal wall that is installed approximately 14 inches (35.5 cm) away from a south facing building wall to create an enclosed plenum. Typically, the outside air is drawn into the collector by fans that are located on the top of the collector. These types of solar collectors are used to preheat the intake air using solar energy before the air enters existing HVAC systems. They are generally used in situations and buildings where large ventilation volume flow rates are required. Most of the studies related to unglazed transpired collectors deal with estimation of air temperature rise due to solar gain and recaptured heat loss from the adjoining building wall. In the past, studies have neglected the amount of thermal energy that is stored in the building wall. However, as shown in this study, the stored thermal energy is of significant amount, and if incorporated correctly in the collector efficiency calculations, it would lead to higher efficiency values.

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