Accurate and reliable measurements of gaseous emissions from animal buildings are critical for development and evaluation of mitigation strategies. Natural ventilation systems in dairy barns are commonly used in regions with mild climate due to lower investment costs and low energy demand. Quantifying gaseous emissions from naturally ventilated barns, however, has additional challenges primarily because of the complexity of ventilation rates (VR) determination. Ventilation rates in a naturally ventilated barn are directly dependent on atmospheric conditions. Uncertainties due to meteorological instabilities, therefore, further complicate estimation of VR. Although unreliable, under certain conditions, indirect methods are widely used for determining VR in naturally ventilated buildings because they are relatively easier and cheaper than direct methods. The main goal of this study was to evaluate two common indirect methods (CO2 and H2O mass b alances) against a direct method as well as identify factors influencing these indirect methods.
These studies were conducted on a naturally ventilated dairy barn on a commercial dairy operation in central Washington. Air velocities were continuously measured with 16 3D ultrasonic anemometers at the barn’s air inlets and outlets. For the CO2-balance method, CO2 concentrations were measured using a photoacoustic IR multigas monitor adjacent to each anemometer. Air temperature and relative humidity were recorded using RH-temperature probes to determine VR via the H2O-balance method. To determine the effects of temperature difference between indoor and outdoor air, data were first sorted with respect to temperature differences, from lowest to highest. The mean VR within 0.5 °C intervals were then computed to reduce data points to a reasonable number for regression analyses as well as for figure plotting purposes.
Indirect methods yielded better VR via the 24-h data averaging than with shorter averaging times (1, 2, and 12 h). The mean VR based on 24-h averaging ranged from 7.9×103 to 7.5×104 m3 min-1 across all study periods and methods. The CO2–balance method tended to overestimate VR, while the converse was true with the H2O–balance method. The cows’ CO2 production rate was estimated at 0.171 m3 h-1 hpu-1 based on the 24-h data averaging. Barn VR increased with wind speed. Wind direction significantly affected barn VR regardless of the method. Barn VR by the H2O–balance and direct method were significantly different below absolute humidity differences of 0.3 g[H2O] m-3. Similarly, significant differences in VR occurred between direct and indirect methods when indoor and outdoor temperature differences were less than 4°C. Both indirect methods were inadequate during milking time. Estimation of VR by either the H 2O or the CO2 mass balances should be done with caution due to these inherent limitations.
Determination of VR in naturally ventilated livestock barns is still a complicated and expensive exercise. Our research will continue development of cost-effective and simpler methods of estimating VR in these facilities.
P.M. Ndegwa, Associate Professor at Biological Systems Engineering, Washington State University, PO Box 646120, Pullman, WA 99164, USA firstname.lastname@example.org
X. Wang, Ph.D. candidate at Washington State University, H.S. Joo, Post doc at Washington State University, G.M. Neerackal, Ph.D. candidate at Washington State University, C.O. Stockle, Professor and department chair at Washington State University, H. Liu
The authors acknowledge partial financial support from the Agricultural Air Research Council, the National Dairy Board, USDA-NRCS through grant #69-3A75-11-210, and the Washington State Agricultural Research Center. Authors also thank the owners, on which this study was conducted, for their cooperation and assistance.
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