Discussion
5.1. Discussion & conclusion of Experiment 1 results
Realistically, it is not yet certain how to accurately measure air exchange rates from naturally ventilated livestock houses especially under summer conditions, with changing wind speeds/directions and the identification of inlets and outlets a very complicated task indeed (Demmers et al. 2001). Clearly, the models which have been calculated to perform under northern European conditions were pushed to their limits under the hot summer conditions experienced on the 03.08.04 and 14.08.04. Under summer conditions high air exchange rates up to 100/h were estimated by Müller &
Möller (1998). A tracer gas monitoring system capable of measuring the high air exchange rates in summer has been developed by Müller & Möller (1998). This involves measurements from 40 sampling points running simultaneously with a maximum sampling rate of one second, air change rates of up to 1000h-1 are measurable. However, the mass balance models (CO2, heat and moisture) were the only practical methods available for gaining an estimation of the air exchange rates in this project.
Diurnal variation in the production of metabolic heat, water and CO2 will limit the accuracy of these methods for estimating the ventilation rate. However there are no widely accepted formulae for predicting this diurnal variation and, in consequence, averages over 24h were estimated to eliminate this problem (Pedersen et al. 1998).
Pedersen et al. (1998) found the models gave comparable results except when the internal and external differences between CO2, temperature and moisture were too
small. This was often the case during this experiment, but it was found that by correcting for this, reasonable results were still obtained.
In the literature (Pedersen et al. 1998), it is stated that the mass balance models will not give reliable results when the CO2, temperature and moisture difference between the stall and outside environment are < 200 ppm, <2°C and <0,5x10-3 kg water/m3 dry air, respectively (Pedersen et al. 1998). On the 03.08.03, the heat and moisture balance models were unable to calculate reliable results for 67% of the time due to the hot outside temperatures (median 25,8°C) particularly because the difference between inside and outside conditions were less than the recommended minimum and the outside temperatures and moisture levels were higher than inside the stall, resulting in negative AER results. This is because the heat and moisture balance models rely on the concept that heat and moisture is produced solely from within the stall and emitted to the outside environment, not gained from outside conditions, as was the case during these measurements. On the 14.08.04 the CO2 mass balance was unable to obtain realistic results throughout most of the period because of the air circulation fans and very high AERs causing very low inside CO2 concentrations and thus very high AERs. On both days it was found that by incorporating the minimum difference values (i.e. 200 ppm for CO2 differences <200 ppm) into the models a good 24h average was obtained and in some instances realistic hourly AERs. However, a problem with incorporating the recommended minimum values into the equations is that the true values and conditions are not considered and the diurnal variations which happen under real conditions are lost and replaced with a peak AER.
When no corrections were applied to the CO2 model, the AERs were at times too high, but they always remained positive, because the background CO2 concentrations are never higher than stall concentrations. On the 03.08.03 when no corrections were made the calculated AERs ranged from 390,4 and with corrections the range was, 27,8-44,9/h. Without corrections the heat and moisture balance AERs on the 14.08.03 ranged from (-)240,9 -1057,4/h and (-)16132,8-343,3/h, respectively, and with corrections the ranges were improved, from 20,5-52,6/h and 24,1-85,9/h, respectively.
The CO2 model is not sensitive to temperature fluctuations and was able to perform sufficiently under the hot conditions on 03.08.03. In the literature, concentration
differences of 200 ppm, 250 ppm and 500 ppm are recommended for reliable results (Pedersen et al. 1998, Seedorf et al. 1998 and Choiniere et al. 1992). However, on the 03.08.03, only 13% of the CO2 concentration differences were above 200 ppm, the majority were between 150 and 200 ppm. Because of the high AERs, the CO2 level was consistently below 200 ppm, therefore, a 2 concentration limit of 150 ppm was used and was sufficient. However, because of the even higher AERs on the 14.08.03 than the 03.08.03 with the increased thermal buoyancy, air circulation fans and direct transverse winds, the CO2 concentrations were too low, and the calculated hourly AERs were inconsistent with real conditions i.e. peaks in the morning and troughs after midday. Even though the hourly AERs were inaccurate, the 24h mean 30/h would be possible during the afternoon times with high temperatures and direct transverse winds.
The temperatures on the 14.08.03 (median 18,7°C) were low enough for reliable heat and moisture balance results throughout, most of the period, except when the temperature difference was too small (33% of the time), however this occurred during the middle of the day when temperatures and wind speeds were high (transverse winds speeds > 5ms-1 occurred in the afternoon), thus peak AERs would have been expected.
During the winter/spring season AERs of 29.4 /h and 55/h were calculated by Demmers et al. (1998). Kaharabata et al. (2000) used an external tracer gas method to measure AERs in a naturally ventilated dairy barn under summer conditions, a maximum air exchange rate of 48/h was calculated. On the 14.08.03 the highest and lowest 24h means (moisture balance 85,9/h and heat balance 52,6/h) were averaged to give a 24h AER mean of 53,2/h. This seems realistic considering Demmers et al. (1998) and Kaharabata et al. (2000) AER results..
In conclusion, because of the inaccuracy of the models under summer conditions it appears that it is more useful to do an in depth analysis on all 3 models to obtain an estimated AER rather than using only 1 model, as this provided a better 24h mean value and at the same time provides some insight into the influencing variables, as well as providing a back up if:
1) The outside temperatures are too high (heat and moisture balances results will be inaccurate).
2) The CO2 concentrations between inside and outside environments are too low (CO2
balance results will be too high).
Although the calculated AERs can not be validated, it is concluded that when the driving forces behind natural ventilation are analysed i.e. bird age (mass), weather conditions (outside temperatures, wind speeds and directions) are known, and the small differences between inside and outside conditions are corrected for (if necessary), a realistic impression of the hourly diurnal air exchange rates and 24h means were obtained. With this information the mass balance model results can be analysed and then an informed decision can be made regarding which mass balance models provided a realistic mean for the time period.
The assumed 24h mean AERs on the 03.08.03 and 14.08.03, 30,0/h and 53,2/h under the conditions do seem realistic, as was shown with comparative research which used more accurate AER calculation techniques.