Evaluation and Control of Radiant Heat Load in Open-type Dairy Barns

Abstract

Thermal stress on dairy cattle is induced not only by high ambient air temperature but also by radiant heat gain. In an open-type barn, the inside air temperature can be decreased as low as the outside air temperature by improving air change rate through openings. The radiant heat inside a barn, however, results from a high temperature of surrounding roofs and walls, therefore be controlled only by changing roof or wall thermal characteristics.
Thermal environment in two types of dairy barns (A) and (B) was experimentally investigated. The thermal resistance evaluated between the inside and the outside roof surfaces was 0.8m2h°C/kcal for (A) barn and 0.005m2h°C/kcal for (B). In the clear summer daytime the inside roof surface temperature of (B) barn was dominantly high, and this caused the amount of longwave radiant heat from the inside roof surface 7 times as large as that in (A) barn. Heat transmission through the roof in (B) was also 5-8 times as large as in (A). As being predicted from largeness of the openings, the inside air temperature was within 2°C higher than the outside in both barns.
Two simple methods to reduce inside radiant heat were examined in (B) barn. They are 1) extending thermal-radiation-reflective aluminized film under the roof and 2) evaporative cooling of the roof due to watering on the outside surface. The aluminized film method did not decrease the inside roof surface temperature, but produced a nearly null value of the net radiation exchange inside. The watering method decreased both the inside roof surface temperature and the radiant heat down to the same level as observed in (A) barn.
The effect of reducing radiant heat load on the thermal regime of cattle was evaluated by measuring hair temperature. During cyclic watering on the roof of (B) barn, the change of the hair temperature was closely related to the change of the inside roof surface temperature. As the roof temperature dropped by 15°C according to watering, the hair temperature decreased by 2-3°C.
Equations based on energy flow through a roof were computed to predict the relationship between the degree of radiant heat load and roof thermal characteristics. In the computation the temperature difference between the inside roof surface and the inside air was used as a measure of radiant heat load instead of the actual amount of radiation exchange. The temperature difference increased almost linearly as the absorbed solar radiation of the roof increased, but decreased rather hyperbolically as the roof thermal resistance became large. The temperature difference changed to a considerable extent in the range of the thermal resistance from 0.01 to 1m²h°C/kcal. Out of this range the change of the temperature difference was not significant. Without watering on the roof, thermal resistance of 1m²h°C/kcal was obtained as a design value for reducing radiant heat load when the outside roof surface had the absorptivity to solar radiation as large as 0.8. Half the value of absorptivity reduced the design value of the thermal resistance by one-tenth. Watering on the roof reduced these design values significantly. The water requirement for evaporative cooling was not more than the amount of evaporation equivalent to solar heat gain.