The useful heat flux provided by the perforated plate of unglazed transpired collectors under no-wind and windy conditions

The air is heated in an unglazed transpired collector (UTC) from three different regions of the perforated plate: from the front of the plate, from the back of the plate, and from the inner surface of the holes. The paper focuses on the relative contribution of each of these three regions, denoted r1, r2, and r3, respectively, to the total increase in the air temperature. A hybrid approach is used: it combines experimental results with results obtained by computational fluid dynamics simulations. Under no-wind conditions, the largest part of the heat received by the air comes from the front of the plate (r1 is about 60%). The second largest part of the heat received comes from the back of the plate (r2 ranges between 25% and 30%). The inner part of the holes contributes to the heat received by the air with a fraction r3 ranging between 10% and 15%. These percentages are rather constant during the day. r1 slightly decreases, while r2 slightly increases along the UTC. The influence of the wind direction on the values of r1, r2, and r3 is not significant. However, the influence of the wind speed magnitude is significant. When the wind speed increases from 0 to 1 m/s, r1 increases from 60% to about 75%, while r2 decreases from 25%–30% to about 15%. For a wind speed of 1 m/s, the values of r1 and r2 are quite the same along the UTC.