Tracer gas technique, air velocity measurement and natural ventilation method for estimating ventilation rates through naturally ventilated barns

Abstract

Naturally ventilated barns have the advantage of providing an energy-efficient and low-noise micro-environment for animals.  Such barns are very common, especially for cattle, which are a major source of ammonia and methane emissions as well as other airborne pollutants. Emission fluxes are calculated as the product of the ventilation rate (VR) and the gas concentration.  The VR of naturally ventilated barns are directly influenced by atmospheric conditions.  Therefore, the estimation of VR involves high uncertainties; furthermore, there is no reference method for VR estimation.  For these reasons, three different methods were investigated to estimate VR through naturally ventilated dairy barns: (1) the tracer gas technique, using radioactive isotope Krypton-85 (⁸⁵Kr), (2) the measurement of the air velocity through the barn openings, and (3) the natural ventilation method.  The investigations were performed in two naturally ventilated dairy barns (each with a volume of 5,670 m³) during mild (T≥17℃) and cold weather (T<17℃) conditions.  The VR ranged between 48,000 and 475,000 m³/h.  There was a significant (p<0.01) correlation between all three methods for barn A (Pearson’s correlation coefficient 0.59-0.86).  For barn B only, the tracer gas and the natural ventilation method showed a significant (p=0.03) correlation (Pearson’s correlation coefficient 0.62).  The season (mild or cold) and the VR estimation method both had a significant (p<0.03) effect on the estimated VR for both barns.  The outside wind speed had a significant (p<0.005) effect on the estimated VR for barn A.  For barn B, this effect was not significant (p=0.052).  The tracer gas technique used has the advantage of measuring the tracer with high resolution in time (1 Hz) and space (18 or more measured points).

Keywords: Natural ventilation, tracer gas technique, radioactive isotope ⁸⁵Kr, air velocity, air exchange rate, dairy barn