Defra project AC0123

Appendix 3

Mechanical versus natural ventilation for dairy cows

The aimof this chapter is to provide a plausible answer to the question whether or not emissions from naturally ventilated buildings significantly differ from emissions from mechanically ventilated buildings. Dairy cows are taken as the starting point but outcomes are possibly also valid for other livestock categories.

1 Model approach

Ammonia emission is influenced by many factors like feed composition (protein content and - digestibility), pH, urease activity, amount of emitting area, temperature, air velocity, management, slurry composition etc. Most of these factors are not influenced by the ventilation system but a ventilation system has a direct effect on ventilation rates and therefore on inside air temperature and air velocity over the emitting surface areas.It is assumed that mechanically ventilated houses have lower ventilation rates than naturally ventilated houses where windspeed and direction have more influence. The greater ventilation rates (increasing ammonia emission) result in lower inside air temperatures (decreasing ammonia emission). Ventilation rates are calculated based on CIGR equations (CIGR, 2002) and (Mosquera et al., 2010). Inside temperatures can be calculated with the ANIPRO model (Ouwerkerk, 1999) that is also based on the CIGR calculations for heat production and CO2 production. The ammonia emission can be calculated with the Ammonia Emission Model Version 2.0. This model is based on Monteny et al. (1998). The hypothesis is that differences in emission between naturally and mechanically ventilated houses are mainly caused by differences in air temperature and air velocity given the same animal- and manure management. So, with the models we can approach differences in ammonia emission through the calculated differences in temperature, ventilation rate and using the dimensions of the house, the air velocity over the emitting surface area.

As input for both models a standardized cubicle house for 100 dairy cows was chosen in the 2+2 design (meaning 2 outer rows and 2 inner rows of cubicles of both sides of a central feeding lane) with the following dimensions (Table 1).

Table 1Dimensions of standardized dairy facility for 100 cows.

Cubicle width / 1.15 / m / Total length / 32.2 / m
# cubicles outer rows / 28 / Total width / 28.2 / m
# cubicles inner rows / 22 / Total area (gross) / 908.0 / m2
Width walking alley 1/4 / 2.5 / m / Total walking (emitting) area / 420.0 / m2
Width walking alley 2/3 / 3.5 / m / Total lying area / 295.3 / m2
Width feeding lane / 6 / m / Total volume / 4146 / m3
Length cubicles outer rows / 2.7 / m / Roof area / 966.3 / m2
Length cubicles inner rows / 2.4 / m / Cross section / 128.8 / m2
Gutter height / 2.0 / m
Roof slope / 20 / o
Ridge height / 7.1 / m
Height difference gutter-ridge / 5.1 / m

The bodyweight of the cows was taken to be 600 kg, milk production was 25 kg milk per day and the cows were 50 days in-calf. The total heat production is assumed to be similar to the heat production of a practical herd.

The roof was assumed to be made of non-insulated roof panels, side walls are completely open, end walls are closed. Slurry storage is situated underneath the slatted floors in the walking alleys. The concrete slatted floors have slatswith a width of 3.5 cm and the beam width is 12.8 cm.

Table 2 presents the default input parameters of the Ammonia Emission Model.

Table 2Starting input parameters of the Ammonia Emission Model.

Animals / Floor / Pits
# Cows / 100 / Area / 420 / m2 / Area / 420 / m2
Urination frequency / 10 / /day / Temperature / 10 / °C / Temperature / 10 / °C
Urea concentration / 7.50 / g-N/l / Air velocity / 0.2 / m/s / Air velocity / 0.05 / m/s
N-content slurry / 3.06 / g-N/l / Urine puddle / pH / 8.50
-size / 1 / m2
Pasturing / 0 / days / -thickness / 0.5 / mm
Integration period / 30 / days / pH / 8.5
Runs / 10

2 Effect of temperature

The effect of temperature on emission, as calculated by the Ammonia Emission Model is shown in table 4. Van Duinkerken et al (2003) report a temperature effect of 2.75% per degree Celsius.

Table 3Ammonia emission of the house in g/h at different inside temperatures

Temperature / Emission NH3
[g/h] / Emission change
∆T=5 / ∆T=10
0 / 121.7
5 / 146.8 / 21%
10 / 172.6 / 18% / 42%
15 / 193.0 / 12% / 31%
20 / 207.5 / 8% / 20%
25 / 217.3 / 5% / 13%
30 / 227.2 / 5% / 9%

3 Effect of air velocity

Effect of air velocity at floor level or in pits is shown in tables 5 and 6. In the Ammonia Emission Model the effect of velocity in pits is linear and independent from the emission effect of air velocity at floor lever.

Table 4Emission of the house at different air velocities

Air velocity in pits
[m/s] / Emission NH3
[g/h] / Air velocity at floor
[m/s] / Emission NH3
[g/h]
0.01 / 155.4 / 0.05 / 148.9
0.02 / 166.3 / 0.10 / 173.2
0.03 / 175.0 / 0.15 / 183.4
0.04 / 183.4 / 0.20 / 193.0
0.05 / 193.0 / 0.25 / 198.6
0.06 / 201.4 / 0.30 / 201.2
0.07 / 208.6 / 0.35 / 204.4
0.08 / 215.8 / 0.40 / 207.0
0.09 / 222.8 / 0.45 / 208.2
0.10 / 231.6 / 0.50 / 211.7

4 Differences in ventilation rates for natural and mechanical ventilation

The goal of ventilation is the removal of heat, moisture and gasses (CO2, NH3, CH4, H2S etc.) from the building. CIGR equations take total heat production, CO2 production and CO2 concentration and animal activity to calculate the ventilation rate in m3 per hour. Minimum ventilation rates roughly range from 200 to 500 m3/hour per cow with CO2 concentrations of 600 and 350 ppm for indoor and outdoor conditions respectively.

Table 5Calculated

Body Mass / Milk production / DIC / Minimum ventilation
[kg] / [kg/day[ / [days] / [m3/hour]
600 / 25 / 50 / 211
700 / 50 / 170 / 525

Beside sensitive heat production,solar radiation, the insulation values of building materials etc.,play a role in the heat balance of the building and the resulting temperature at a given ventilation rate.ANIPRO uses these factors to calculate the inside temperature for a naturally ventilated building using only thermal buoyance as the driving force behind.

Table 6Calculated inside temperatures based in minimal required ventilations raters

Outside temperature
[oC] / Inside temperature
[oC]
10 / 14.8
15 / 19.5
20 / 24.0
25 / 28.3

Assuming the mechanically ventilated buildings' ventilations rates are close to minimal requirement, inside temperatures are about 4 oC higher than outside temperatures.

Ventilation rates in naturally ventilated animal houses were measured by Mosquera et al. (2011) and ranged from around 500 m3 per hour per animal to more than 3000 m3 per hour per animal (average 1591 m3 per hour per animal). It is therefore very likely that air velocity at floor level are also greater than in mechanically ventilated building. At the same time inside temperaturesare close to outside conditions.

3.2 Combined effect of temperature en air velocity for natural and mechanical ventilation

To estimate the combined effect of temperature and air velocity in both naturally and mechanically ventilated building combination of parameters was defined (Table7).

Table 7Parameters for emission calculations

Natural / Mechanical
T-outside / T-inside / T-pits / V-floor / V-pits / T-inside / V-floor / V-pits
5 / 7 / 10 / 0.1 / 0.04 / 9 / 0.04 / 0.01
10 / 12 / 10 / 0.2 / 0.07 / 14 / 0.06 / 0.02
15 / 17 / 15 / 0.3 / 0.10 / 19 / 0.08 / 0.03
20 / 22 / 15 / 0.4 / 0.13 / 24 / 0.1 / 0.04

T-inside is always 2 degrees higher than T-outside for naturally ventilated buildings and 4 degrees higher for mechanically ventilated building.

Air velocity in pits is coupled to air velocity at floor level. Relative differences are equal. The temperature in pits is constant at 10oC when T-outside is 5 and 10 oC (winter) and 15 oC for T-outside is 15 and 20 oC (summer). All combinations of T-inside and T-pits on one hand and V-floor and V-pit on the other hand have been calculated for both mechanically and naturally ventilated buildings.

Result are presented in figure 1 as a relative emission of the emission calculated with the parameters given in table 2. That emission was 175.9 gram NH3/hour.

Figure 1Relative emission under different circumstances. Legend: V-floor/V-pits

It can be concluded that air velocity contributes more to total emission than temperature and that the ammonia emission from naturally ventilated buildings is higher than from buildings with mechanical ventilation based on assumptions regarding ventilation rates and air velocity at floor level.

An estimation of emission based on results from mechanically ventilated buildings would probably underestimate the total (national) emission of ammonia, given the fact that 90% of the animal buildings for dairy are naturally ventilated.

References

CIGR (2002) 4th Report of Working Group on Climatization of animal houses. Heat and moistureproduction at animal and house levels. Eds.:S. Pedersen and K. Sällvik.

Duinkerken, G. van, G. André, M.C.J. Smits, G.J. Monteny, K. Blanken, M.J.M. Wagemans, L.B.J. Šebek (2003), Relatie tussen voeding en ammoniakemissie uit de melkveestal, In Dutch with summery in English, PraktijkRapport Rundvee 25, Praktijkonderzoek Veehouderij, Lelystad, 66p.

Monteny, G.J., S.S. Schulte, A. Elzing, E.J.J. Lamaker (1998) A conceptual mechanistic model for the ammonia emissions from free stall cubicle dairy cow houses. Transactions of the ASAE, 41 (1) pp. 193-201.

Mosquera, J., J.M.G. Hol, C.M. Groenestein (2010) Evaluation of the CIGR method for ventilation rate calculations from animal houses, Evaluatie van de CIGR methode voor de bepaling van het ventilatiedebiet uit stallen, In Dutch with summary in english, Rapport 429, Wageningen UR Livestock Research, Lelystad.

Mosquera, J., J.M.G. Hol, A. Winkel, J.W.H. Huis in ‘t Veld. F.A. Gerrits, N.W.M. Ogink, A.J.A. Aarnink (2011 revised version), Dust emissioin from animal houses: dairy cattle, Fijnstofemissie uit stallen: melkvee In Dutch with summary in English, Rapport 296, Wageningen UR Livestock Research, Lelystad.

Ouwerkerk, E.N.J. van, 1999. ANIPRO klimaat- en energiesimulatiesoftware voor stallen. IMAG Nota V 99-109, Wageningen, 87 pp.

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