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Modeling Energy Balance and Airflow Characteristics in a Naturally Ventilated High Tunnel

Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan www.asabe.org

Citation:  Transactions of the ASABE. 60(5): 1683-1697. (doi: 10.13031/trans.12080) @2017
Authors:   Muzi Zheng, Brian G. Leib, David M. Butler, Wesley Wright, Paul Ayers, Douglas Hayes
Keywords:   Air flux, Energy balance, High tunnel, Natural ventilation.

Abstract. High tunnels (HTs) are used worldwide for greater crop sustainability and profitability, but producers are finding it difficult to control the trapped heat inside HTs. Clearly, proper ventilation management is crucial for obtaining marketable yield and quality, but the ability to predict HT ventilation based solely on external climate parameters is limited. This project analyzes daytime ventilation rates in a Gothic-type HT located in eastern Tennessee. A numerical energy balance model was developed to calculate air flux, and the estimated values of air flux were compared with measured values from sonic and hot-wire anemometers. Uniquely, this study takes into account the sensible and latent heat exchanges between inside and outside conditions, the radiative and convective transfer mechanisms, and the effects of external wind conditions on the incoming air flux. Moreover, relationships between the external weather conditions and internal microclimate were developed so that air flux, inside temperature, and door opening level could be predicted with the energy balance model using only external weather data. The energy balance model revealed that the HT plastic surface reflected nearly 20% of the solar radiation, that 1% of the solar energy was stored in the air (considered negligible), and that 5% of the solar energy was stored in the soil, while the majority of available solar radiation, 74%, was removed through natural ventilation. There was good agreement between the predictive energy balance model and the direct air flux calculation for ventilation rate (R2 > 0.70). At the windward end of the HT, the airflow entering the door was mostly perpendicular to the plane of the door opening, which indicated that the fluctuation of outside wind direction had limited effect on the airflow patterns through the door. As a significant indicator of the energy balance model‘s usefulness, the predicted inside air temperature was validated as similar to the measured values. Thus, the results showed strong evidence that the coupling of the energy balance model and air flux calculation based on external weather parameters can be a valuable method to predict inside microclimate conditions and can be used to estimate the door opening level.

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