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SE—Structure and Environment
J. agric. Engng Res. (2001) 79 (1), 107}116 doi:10.1006/jaer.2000.0678, available online at http://www.idealibrary.com on SE*Structure and Environment
Validation of Ventilation Rate Measurement Methods and the Ammonia Emission from Naturally Ventilated Dairy and Beef Buildings in the United Kingdom T. G. M. Demmers; V. R. Phillips; L. S. Short; L. R. Burgess; R. P. Hoxey; C. M. Wathes Silsoe Research Institute, Wrest Park, Silsoe, Bedfordshire MK45 4HS UK; e-mail of corresponding author: [email protected] (Received 28 October 1999; accepted in revised form 4 November 2000; published online 23 January 2001)
Measurements of ammonia emission, especially from cattle buildings, are needed to assess the environmental impact of ammonia. There are no data available for cattle buildings in the United Kingdom because most buildings are naturally ventilated and methods for long-term measurement of emission from naturally ventilated buildings have not been developed. Two measurement methods, based on either the release of a tracer gas or on the pressure di!erence across ventilation openings, were validated in a full-scale cross-section of a naturally ventilated livestock building against a known release rate of a gaseous pollutant at high and low wind speeds. A good correlation between the measured and the actual release rates was found for the tracer gas method with an average recovery rate of 108%. The method based on pressure di!erence failed to estimate the ventilation rate correctly when only measured pressure coe$cients were used, because the measurements of mass #ow rates in and out through all openings of the building failed to balance. The traditional approach, based on measured values for the external pressure coe$cients and an estimate of the internal coe$cient balanced the #ow through the building by de"nition, but failed to estimate the actual emission rate correctly. Current knowledge of the discharge coe$cient for the opening designs of the building is insu$cient for the pressure di!erence method to be used to estimate the ventilation rate. Using the tracer method, measurements were carried out between February and May 1996 in a straw-based beef house and a slurry-based dairy cow house with cubicles and scraped passage ways. The buildings were space boarded and had a covered ridge. The ammonia emission was estimated to be 3)5 and 8)9 kg NH per livestock unit per year for the beef and dairy buildings, respectively. 䊚 2001 Silsoe Research Institute
1. Introduction Ammonia deposition is known to damage ecosystems, either directly (van Eerden, 1982) or indirectly, through acidi"cation following nitri"cation of ammonia (van Breemen et al., 1982). Agriculture, and cattle in particular, is considered to be the major source of atmospheric ammonia in the UK. However, accurate measurements of ammonia emission rate from the various sources are rare (Demmers, 1997). UK cattle buildings are usually naturally ventilated because of the mild climate and are characterized by numerous ventilation openings of varying design with the function of each, i.e. air inlet or outlet, variable in time. 0021-8634/01/050107#10 $35.00/0
Methods for measuring ventilation rate in naturally ventilated airspaces are limited and simple fan wheel anemometers, traditionally used in estimating ventilation rate in mechanically ventilated livestock buildings cannot be used. Two validated methods for measuring the ventilation in naturally ventilated livestock buildings are described and applied to measure the ammonia emission from two typical cattle buildings.
1.1. ¹racer gas method to estimate ventilation rate The constant release tracer method is one of a number of methods used to measure the ventilation rate of a
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Notation c tracer concentration, m m\, (ppm) C , *C wind pressure coe$cient, di!erence in N N wind pressure coe$cient C discharge coe$cient B g gravity constant, m s\ H level of neutral plane, m LN H height of opening, m h height of building, m P, *P pressure, pressure di!erence, N m\ Q ventilation rate, m s\ q dynamic pressure, Pa v air velocity, m s\ v average air velocity, m s\ v wind speed at meteorological height, m s\
livestock building (Demmers, 1997; Demmers et al., 1998). Using Eqn (1) the ventilation rate Q in m s\ can be calculated as a function of time t in s from the rate of tracer release uN in m s\ and the indoor tracer concentration cG in m m\ after correction for the background concentration of tracer cC: uN (t) Q(t)" cG (t)!cC (t)
x y o u N
length of building, m width of building, m air density, kg m\ rate of tracer release, m s\
Subscripts boj bottom of opening j e external or background i internal i/e internal or external j opening j t thermal buoyancy toj top of opening j w wind
perature criterion has been shown to be less reliable than the concentration criterion (Demmers et al., 1998, 1999) due to local variations in temperature caused by solar radiation. Therefore, this criterion will not be considered in this paper.
(1)
1.2. Pressure di+erence method to estimate ventilation rate
Tracer gas methods rely on complete mixing of the air space (Sherman,1990), which is very unlikely in livestock buildings, and a single measurement of the tracer concentration is therefore inadequate. However, measurement of the tracer concentration at the ventilation openings of the building is a viable alternative, although this requires instantaneous identi"cation of the function of the opening as either an air inlet or outlet. The ventilation rate can then be calculated from the average concentration at the air inlets corrected for the average concentration at the air outlets. Three criteria are available to distinguish between inlets and outlets; (i) pressure di!erence over the opening, (ii) temperature gradient over the opening and (iii) concentration gradient of the tracer over the opening. The "rst is the most direct and depends only on the wind for the driving force of ventilation if the pressure coe$cients are used. However, in calm weather this criterion cannot be used, because the di!erence due to buoyancy is generally too small to be measured. Both the temperature and concentration gradient use a criterion above which the opening is deemed to be an inlet and below which it is an outlet. The criterion is higher than the background values to allow for sensor accuracy and drift. In both mechanically and naturally ventilated buildings, the tem-
A method based on the pressure di!erence across the opening can also be used to estimate the ventilation rate and is based on natural ventilation theory (Bruce, 1986; Van't Ooster, 1994). The two driving forces responsible for natural ventilation are the pressure di!erence in N m\ due to thermal buoyancy *PR and wind *PU, as functions of the length, width and height of the building x, y and h, respectively, in m: *PR (h)"g(oC!oG)(HLN!h) *PU(x, y, h)" ov (CNC(x, y, h)!CNG)
(2) (3)
where o in kg m\ is the air density at the relevant position, g the gravity constant in m s\, v the wind speed in m s\ at 10 m height and CN the wind pressure coe$cient. The height of the neutral plane HLN in m is the height h at which there is no pressure di!erence due to thermal buoyancy. The #ow through any opening can be found by applying Bernoulli's law, assuming the air to be incompressible: 1 1 *P(h)" o v(h) (4) C 2 G C B where CB is the discharge coe$cient, which depends on the geometry of the opening only, oG C is the air density
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inside o for air #owing out of the building or the air G density outside the opening o for air entering the buildC ing and v is the air velocity. The average air velocity v through any opening j can be found by integration over the height H in m of that opening from the bottom H to @M the top H : RM 1 &RMH vN " v(h) dh (5) H H H &@MH
The equations for velocity through a vertical opening acting entirely as an inlet or outlet are given in Eqns (6) and (7) for, respectively, thermal buoyancy and wind. Equation (8) represents the combined e!ect of buoyancy and wind. For openings that do not act entirely as an inlet or outlet, the inlet and outlet areas are described by separate equations. These and equations for other speci"c openings have been derived by Van't Ooster (1994) and Demmers (1997):
2 C "o !o " 2 g "o !o " G C G ("H !H " vN "! B C LN RMH H 3 H o !o o H C G G C !"H !H " ) (6) LN @MH "C !C " o NCH NG C vN "v C (7) H B C !C o NCH NG G C 2C 1 1 v "! B ("o v (C !C ) H C NCH NGH 3 H 2g(o !o ) o H C G GC #2g(o !o )(H !H ) C G LN RMH !"o v (C !C )#2g(o !o ) C NCH NGH C G ;(H !H )" ) (8) LN @MH According to Brockett and Albright (1987), the height of the neutral pressure level is independent of the wind e!ect and can therefore be calculated through iteration,
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using the equation describing thermal buoyancy [Eqn (6)] and assuming continuity of mass #ow. The ventilation rate of a building is then the sum of the air#ows at all the outlets or inlets. The major bene"t of this method is that information is obtained on the direction and magnitude of the air#ow rate at any opening or group of openings of the building. In principle, this information allows detailed analysis of the contributions of di!erent sections of the building to gaseous emissions. Although the driving force for ventilation is always the combined e!ect of thermal buoyancy and wind, the latter is by far the largest component at wind speeds over 3)0 m s\ (Wise, 1977). At these wind speeds, the pressure coe$cients can be measured accurately. The pressure measurements are less reliable at low wind speeds ((2 m s\). However, the average pressure coe$cients measured under windy conditions can be used in Eqn (8), even at low wind speeds, provided the wind direction is the same and the reference wind speed is known.
2. Materials and methods 2.1. Experimental facility A cross-section of a naturally ventilated livestock building was constructed in the Silsoe Structures Building (Robertson & Glass, 1988; Hoxey & Richards, 1993) (Fig. 1). Two parallel, internal walls 4 m apart were erected in the centre of the building. Validation of the two methods described above was carried out in this cross -section using a known rate of emission of ammonia. The building was 12)9 m wide, 4)1 m high at the eaves and 5)3 m high at the ridge. The slope of the roof was 103. The ventilation openings in the cross-section consisted of an open ridge 0)3 m wide with #ashings (0)12 m high) and sidewall openings "tted with space boarding with 25 mm
Fig. 1. Mean pressure coezcients measured around and in the cross-section of the Silsoe Structures Building (dynamic pressure, q*10 Pa). Also shown are the area source, A, the line source, B and the point source, C. The heaters are located over the area source
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gaps between 125 mm boards, each 1)8 m high, "tted below the eaves. Electrical heaters (each nominally 250 W m\, total input 5)6 kW) were installed at ground level in the cross-section to simulate the sensible heat output of animals at a realistic stocking density. The heaters were arranged on either side of a central passage way. The pollutant, carbon monoxide prediluted with air, was injected into the cross-section through an area source made of perforated lay-#at tubing, 1)4 m wide, situated underneath the heaters. The carbon monoxide simulated ammonia emitted from a urine "lm on the #oor. A tracer gas could also be injected into the air space, either through a point source or a line source. An array of tracer gas point sources distributed over the area source of pollutant will compare best with the release of the pollutant. However, in livestock buildings point sources within reach of animals, might not be practical, therefore a line source above animal level was also considered. The point source was a perforated ball of 250 mm diameter, located 0)4 m above the #oor level, while the line source was a 4 m length of perforated lay-#at tubing of 400 mm diameter, which extended across the width of the cross-section at ventilation opening height (3 m). Both sources were placed in the centre of the windward half of the cross-section. Methane was used as the introduced tracer and was diluted with air prior to entering either source to create a mixture with a density almost equal to that of air. Sampling points for the concentration of both gases were located in the centre of the ventilation openings, in between two boards or half-way up the #ashings for the sidewall vents and the ridge vent, respectively. Additionally, the gas concentrations were measured in the centre of the cross-section. The reference point was upwind of the building. Carbon monoxide and methane concentrations were measured using an infrared analyser. Both analysers were calibrated before and after each experiment. Sample locations were selected using a sample sequencer operating at a frequency of one channel per 90 s. The average concentration over the last 30 s of each period was logged. Pressure measurements were made using probes developed by Moran and Hoxey (1979) mounted on the sidewalls (#ush with the solid wall or 5 cm out from the space boarding), at the roof (5 cm up from the roof ) and 5 cm inside the openings. A reference measurement of static and dynamic wind pressure was made at ridge height 25 m upwind of the building. The measurements were taken with individual static pressure sensors located as close to the probe as possible at a rate of 1 Hz. To prevent measurement errors due to buoyancy the sensors were placed wherever possible at the same height as the probes.
Temperature and humidity were measured at the same locations as methane and carbon monoxide with a temperature and humidity probe. Weather conditions were monitored 25 m upwind of the building. Measurements were made of temperature and humidity at 1)5 m above ground and solar radiation, wind speed and direction at ridge height. Data from the analysers and weather station were logged with a data logger, which also controlled the sample sequencer. Data from the pressure measurements and the sonic anemometer were stored separately. Measurements were carried out under windy (wind speed over 4 m s\) and calm weather conditions (wind speed under 2 m s\). In both cases, the wind direction was approximately normal to the side of the building. The experiments were repeated using both point and line sources of tracer gas.
2.2. ¸ivestock buildings The two cattle buildings were ventilated naturally. Each had space boarding on all sides and a ventilated ridge in the form of a &cranked crown' ventilator. The space boarding comprised 129 mm wide boards spaced 30 mm apart. The beef building held 99 animals in straw-bedded holding pens either side of a central feed passage. The dairy building held 90 dairy cows in cubicles with straw bedding, the passage ways were scraped every 2 h. The constant tracer method was applied to both buildings using carbon monoxide as the tracer. Ten sampling points were located in the ventilation openings of each building. The ammonia concentration was measured using a chemiluminescent nitric oxide analyser, preceded by stainless-steel thermal converters. Measurements of weather conditions and data analysis were the same as those at the experimental cross-section. Measurements were taken from February to May 1995.
3. Results 3.1.
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Table 1 Average methane and carbon monoxide concentrations measured in a cross-section of the Silsoe Structures Building; measurements at high (6 ' 0ⴞ0 ' 3 m sⴚ1) and low (2 ' 0ⴞ0 ' 4 m sⴚ1) wind speed; data given for methane tracer released from point and line sources and carbon monoxide (pollutant) from an area source; the standard deviation of each concentration was below 0 ' 4 mg mⴚ3 (no. of measurements, n* *11) Concentration, mg m!3
Location
High wind speed
Low wind speed Point source
Background Windward wall top half Windward wall bottom half Leeward wall top half Leeward wall bottom half Ridge 0)5 m below ridge Centre of cross-section
¸ine source
Point source
¸ine source
CH4
CO
CH4
CO
CH4
CO
CH4
CO
1)4 3)7 2)5 20)7 15)0 19)3 18)9 15)7
0)2 1)7 1)0 9)7 6)9 10)5 11)1 9)9
1)4 2)1 3)1 13)9 12)0 15)2 15)5 17)7
0)2 0)6 1)3 7)6 7)2 6)6 6)9 6)6
0)9 1)1 7)0 13)8 14)3 14)9 14)0 15)9
0)2 0)3 3)4 17)6 19)1 15)3 12)5 13)5
0)9 0)9 2)9 15)2 15)1 14)3 13)1 17)9
0)2 0)2 4)2 18)8 19)1 13)1 12)5 13)0
building. There was little di!erence in the general distribution of methane between the point and the line sources. However, the concentration distribution of carbon monoxide released from the area source was clearly di!erent from the patterns of methane concentration, especially at the high wind speed, and resembled the temperature distribution [data not shown (Demmers, 1997)], with signi"cantly higher concentrations at the leeside wall compared with the ridge outlet. The latter was caused by short circuiting of fresh air from the wind ward side to the ridge, partially due to the thermal buoyancy of the heaters. The concentration pattern of the point and particularly the line sources were less a!ected by this as these sources were located within the central recirculating area. The identi"cation of the function of the openings as either an air inlet or outlets was not straightforward from these data. Clearly, the leeside wall and the ridge were both outlets, as both the methane and carbon monoxide concentrations were high. The windward wall was an air inlet, with tracer concentrations expected to be close to the background levels. However, the tracer gas concentrations at the windward wall were signi"cantly higher than the background concentration. The raised concentration arose from mixing of incoming air with internal air behind the space boarding. This was con"rmed using smoke. Hence, a minimum di!erence between the background and inlet concentration, or concentration criterion, of 7)5 mg m\ was needed to identify the whole windward wall as an inlet compared to just 2)5 mg m\ for part of the opening. The ventilation rate was calculated using Eqn (1) with the concentration criterion to identify the function of the
openings. To validate subsequent estimates of ammonia emission, the emission of carbon monoxide was estimated using measurements of its concentration and the calculated ventilation rate. This estimate of carbon monoxide emission was compared with the actual release rate (Table 2). The ventilation and emission rate estimates for the perimeter method are presented for two concentration criteria. For each combination of source location and wind speed, the lowest criterion was set to identify at least one of the sampling points on the windward wall as an inlet. The highest criterion was set to identify both sampling points on the windward wall as inlets. The choice of concentration criterion a!ected the estimate of ventilation rate. Using the lower concentration criterion, i.e. taking the elevated methane level measured at the lower part of the windward wall as indicative of an outlet, resulted in a higher estimate of ventilation rate at low wind speed. At high wind speed, although the concentration of methane at the bottom half of the windward wall was also high, and hence, this section of the opening identi"ed as an outlet, the estimate of ventilation rate decreased. This was due to the variation in mean concentration by allocating the high tracer concentration to either the inlet or outlet category. The concentration criterion had the opposite e!ect on the estimated emission rate, compensating for the error made in the ventilation rate. The level of compensation depended on the relative distribution of the tracer gas (methane) and the pollutant (carbon monoxide). In general, the agreement between the estimated emission rate of carbon monoxide and the actual release rate was satisfactory. The calculated emission rate on average overestimated the actual release rate by no more than
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Table 2 Comparison of the estimated with the actual emission rate of carbon monoxide; the estimated emission rates derived from the measured ventilation rates and pollutant concentrations measured at the perimeter of a naturally ventilated cross-section of the Silsoe Structures Building; both point and line sources used for tracer release; measurements at high and low wind speed conditions; where appropriate mean and standard deviation given
¸ow wind speed Point source Line source High wind speed Point source Line source
Concentration criterion, mg m!3
Ventilation rate, m3 h!1
Pollutant emission rate, g h!1
Pollutant recovery, %
1)5 3 1)5 2
3500$250 2900$200 4550$50 3900$400
30$3 30$3 30$1 30$1
107 106 108 105
1)5 7)5 1)5 3
11 500$2050 13 000$2750 10 000$1400 11 500$1750
185$18 229$25 177$14 192$14
99 122 94 102
8%, except at high wind speeds using point source of tracer gas and a high threshold criterion (7)5 mg m\). The emission rate was then overestimated by 22%. This was caused by the di!erence in concentration pattern of methane and carbon monoxide especially at the lower part of the windward wall and suggests that positioning of the sampling point was critical and that a high concentration criterion should be avoided. Overestimates of the emission rate could be caused by errors in either the ventilation rate or the concentration of carbon monoxide at the outlets of the cross-section. The ventilation rate was overestimated because the concentration of the tracer at the inlet was not equal to the background concentration. The emission rate was correspondingly underestimated. Correction by using the background concentration instead of the inlet concentration did not give an improvement. Changing conditions inside the building during the sampling cycle of 12 min needed to sample all locations are most likely to have contributed to the inaccuracy. The point source was to be preferred over the line source as it gave slightly more consistent estimates of the actual emission rate, when used with the lower concentration criterion. The better location of the point source, above the area source, was an important contributing factor.
3.2.
coe$cients from all measurements with a dynamic pressure q above 10 Pa are given in Fig. 1. All data sets with q lower than 10 Pa were ignored, because the low accuracy of the pressure transducers caused a large scatter of data points. The standard error for the average C was N typically 0)01. The low standard error is mainly due to the limitation of wind angle and wind speed. The measured pressure coe$cients C compare well N with those measured in previous work by Hoxey and Richards (1993) and Richardson et al. (1995) on the same building without ventilation openings. The presence of the space boarding on the windward wall did therefore not have a large in#uence on C values. The #ashings of N the ridge vent did a!ect the C distribution on the roof, N with lower values of C towards the ridge on the windN ward half and lower values up to half way down the roof on the leeside of the ridge. The space boarded opening on the leeside again had little impact on the measured C values compared with a solid wall. The N internal pressure was lowest on the windward wall of the cross-section, but the pressure variation inside the building was small. The C values indicate clearly that the windward wall N was an air inlet with a C di!erence across the wall of N 0)73. The ridge was an air outlet, as expected, with a difference in C across the ridge of !0)35. The pressure N measurements suggested that at wind speeds over 4 m s\, the leeside wall was a minor air inlet, with a di!erence in C across the opening of just 0)06. HowN ever, this pressure di!erence did not correspond with the observed air#ow pattern, using smoke. Detailed analysis of the C data for this opening (with q '10 Pa) showed N that the opening alternated between acting as an outlet and inlet (standard deviation and standard error were 0)17 and 0)002, respectively). On average, the opening
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acted as an air inlet according to the static pressure data. Both the windward wall (di!erence in wind pressure coe$cient *C "0)73; standard error, SE"0)006) and N the ridge openings (*C "!0)12, SE"0)003) show N a similar variable pressure pattern, but were consistently identi"ed as an inlet and outlet, respectively. The height of the neutral plane was calculated using Eqn (5), the equations for the buoyancy e!ect alone, and the equation for the law of conservation of mass. The measured C values including the internal C values, N N temperature, humidity and wind speed at ridge height were used to estimate the rate of ventilation due to buoyancy alone [Eqn (5)] and to buoyancy and wind combined [Eqn (8)] of each opening (Table 3). The ventilation rate due to buoyancy alone accounts for the in#uence of wind by using the apparent height of the neutral plane, which depends on the wind speed and pressure coe$cients. The estimates of ventilation were calculated using C values measured by Pearson and Owen (1994), B i.e. 0)6 for the ridge and 0)75 for the space boarding. Using the estimated ventilation rate the emission rate was calculated for each opening and the building as a whole. The estimated carbon monoxide emission rate was compared with the actual release rate and is given as a percentage of recovery. The lower openings of a building are generally inlets, when only buoyancy e!ects are considered, as is indeed the case for the low wind speed conditions. Due to the wind e!ect the apparent height of the neutral plane can change signi"cantly, resulting in a minor outlet at the leeside wall for the high wind speed conditions. The ventilation rate based on the combined e!ects of buoyancy and wind implied that the leeside wall opening acts as an air inlet for both the low and high wind speed conditions. This is in contrast to the smoke experiments
which suggested the opening was acting at least partially as an air outlet. The mass balance of air#ow over the cross-section as a whole was calculated from the ventilation rate of each individual opening, but it was not equal to zero. As a result, the percentage recovery of the carbon monoxide was extremely poor. Using the traditional method, i.e. solve the neutral plane level and the C value through NG iteration assuming that the mass balance for the building is valid, C values of !0)15 and !0)12 were found for NG the low- and high-speed conditions, respectively. Again the recovery of carbon monoxide was poor at 194 and 149%. The calculated C value, half the measured value, NG appeared unrealistic. As the discrepancy in the mass balance over the building could not be explained by the variation in the pressure measurements both inside and outside the building only, two other possible explanations were identi"ed. The "rst is the pressure discharge coe$cient, as measured by Pearson and Owen (1994). Their measurements were repeated and proved to be correct. However, their experimental conditions might not be a true representation of the wind-induced air#ow through an opening. In their method, all the air#ow was forced through the space boarding and the air#ow rate and pressure drop across the space boarding were recorded. When space boarding is part of a building the air does not necessarily pass through it and may #ow around the building as if the wall was solid. A better approach would be to measure the air#ow through an opening exposed to wind in a wind tunnel. This could result in a lower C . B Secondly, the assumption that all the dynamic pressure translates to static pressure, from which alone the #ow through an opening results, could be incorrect. Inside the building there are signi"cant air velocities of for example,
Table 3 Estimates of ventilation and carbon monoxide emission rates using natural ventilation theory compared with the actual release rate for each opening; measurements in the naturally ventilated cross-section of the Silsoe Structures Building at high and low wind speed normal to the building =indward wall
¸eeside wall
Ridge
Cross section R
High wind speed Ventilation rate by buoyancy alone, m h\ Total ventilation rate, m h\ Emission rate, g h\ Recovery, %
!50 26 600 62
1100 6600 124
!1050 !10 200 !121
0 23 000 !65 !35
¸ow wind speed Ventilation rate by buoyancy alone, m h\ Total ventilation rate, m h\ Emission rate, g h\ Recovery, %
450 5800 10
1000 2000 20
!1450 !3000 !40
0 4800 11 38
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0)4 m s\ approaching the leeward wall, which add a dynamic element (0)1 Pa) to the internal pressure. The same is, however, true for the external pressure on the windward wall. Incorporating the dynamic pressure in the static pressure coe$cients would greatly contribute to the complexity of the model.
3.3. Ammonia emission from livestock buildings The estimated annual ammonia emission rate from the two cattle buildings, measured using the constant release tracer method with a concentration criterion of 0)6 mg m\ is given in Table 4. The lower value for the concentration criterion was made possible by an improved sampling strategy, whereby rising air inside the building was prevented from reaching the sampling point, placed between two boards. The annual emission is based on a winter housing period of 190 days per annum. Comparison with literature values expressed per livestock units, lu (500 kg of live weight) shows that the value derived for the dairy unit compares well with the 8)3 kg lu\ yr\ given by Van't Ooster (1994). The value for the straw-based beef unit is lower than for slurry-based systems possibly indicating that straw-based housing systems give lower ammonia emissions than traditional slurry-based systems. Compared with all values, the value given by Oldenburg (1989) for all types of German cattle buildings seems unrealistically low, which may have been due to restricting measurements of ventilation rate and ammonia concentration to the ridge.
so that any increase in tracer concentration above background indicates an air outlet of the building, though this will not identify the situation where air re-enters the building. However, the criterion must also allow for mixing of air when the sampling point is directly behind the opening. The method was di$cult to apply at high wind speeds, where use of a concentration criterion of 7)5 mg m\ resulted in a large overestimate of the ventilation rate. A concentration criterion of 2)5 mg m\ was more appropriate because one or more openings were correctly identi"ed as inlets. A concentration criterion well above background level will identify inlets with an elevated tracer concentration correctly, but will result in an overestimate of the ventilation rate. Provided the distribution of the tracer and the gaseous pollutant are similar, this overestimate is compensated for in the emission rate estimate by the same argument. However, when the distribution of the tracer and pollutant are signi"cantly di!erent, the overestimate is either overcompensated for or exacerbated. For naturally ventilated livestock buildings a concentration criterion between the background level plus the analyser detection limit to a maximum of 2)5 mg m\ is recommended to detect the function of an opening. Preventing the sampling of mixtures of external and internal air just behind the space boarding will greatly improve the reliability of this criterion. The actual level will depend to some extent on the data set. Errors in the selection of inlets resulting in an overestimate of the ventilation rate will occur and are impossible to eliminate.
4.2.
4. Discussion 4.1. Identi,cation of air inlets and outlets The identi"cation of an opening as either an air inlet or an air outlet proved to be complex. In principle, the concentration criterion should be kept as low as possible,
The accuracy of the constant release tracer gas method with sampling locations at the perimeter of the building was higher than expected. The recovery of released carbon monoxide was within 8% of the true rate, for the
Table 4 Annual ammonia emission rates from cattle buildings estimated using the continuous tracer release method with a concentration criterion of 0 ' 6 mg mⴚ3 as compared with published values Cattle type Beef Beef Dairy Dairy All
Housing type Straw bedded Slurry-based Cubicle; slurry with scraped #oor Slurry-based with fully slated #oor All types
lu, livestock unit (500 kg live weight).
Country United Kingdom The Netherlands United Kingdom The Netherlands Germany
Annual emission, kg NH3 lu!1 yr!1
Source
3)0 4)7 8)9 8)3 1)1
This study VROM (1996) This study van't Ooster (1994) Oldenburg (1989)
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lower concentration criterion. In comparison, the estimated ammonia emission rate from two mechanically ventilated livestock houses using the same method was 13% lower than a reference method using fan wheel anemometers to measure the ventilation rate (Demmers, 1997; Demmers et al., 1999). An error of 10}15% is commonly accepted in the literature for the measurement of ventilation rate by the tracer gas methods for full-scale buildings (van't Ooster, 1994). The accuracy of the tracer gas method in a full scale, naturally ventilated livestock building is likely to be lower unless an equivalent number of sampling positions can be used in relation to the number and size of the openings.
4.3.
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substantially complicate the model and measurements required. The discharge coe$cients of common agricultural ventilation structures as determined by Pearson and Owen (1994) and the theoretical values given by Bruce (1986), appeared to be incorrect when applied to a full-scale livestock building. Values for the discharge coe$cients of space boarding and the open ridge will have to be lower for the estimated ventilation and emission rate to match the actual release rate. Ventilation rate estimates based on natural ventilation theory gave a poor match with tracer dilution measurements. As a result, the percentage recovery of release rate of simulated pollutant were also poor, although the corrections in the pressure and discharge coe$cients could give some improvement. The large di!erences in recovery at high and low wind speeds indicate a dependancy of the error on wind speed. The tracer gas method must be preferred to the natural ventilation theory for estimation of the ventilation and emission rates of naturally ventilated buildings, until a more detailed understanding of the discharge and total pressure coe$cients is available.
4.4. Ammonia emission estimates from livestock buildings The ammonia emission factors for the straw-based units in this study and in literature are generally lower than the emission factor for comparable slurry-based units. This suggests that adoption of straw-bedded housing systems could lower ammonia emissions. However, it is known from litter-based pig housing systems, that the reduction in ammonia mission is mirrored by a rise in nitrous oxide emission (Groenenstein, 1993). The ammonia emission factors derived from this work have been included in the production of an up-to-date ammonia emission inventory (Pain et al., 1998). This inventory estimates total UK ammonia emissions to be 250$60 kt [NH ] yr\. Approximately, 60% of this total is at tributed to cattle production, with housing attributing 52 kt [NH ] yr\. 5. Conclusions Assessment of two methods to measure the ventilation and gaseous pollutant emission rates of naturally ventilated buildings indicated that the constant release tracer gas method gave the most reliable estimates of ventilation rate. The national ventilation theory using the measured pressure coe$cients failed to estimate the ventilation rate correctly. Using a combination of measured external and computed internal pressure coe$cients the ventilation
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rate estimate was balanced by de"nition, but the method again failed to estimate the actual emission rate correctly. More research is needed to measure the discharge coe$cients of space boarding and ridge con"gurations used in naturally ventilated buildings and to measure the pressure coe$cients based on total pressure rather than static pressure alone. Estimates of pollutant emission rate based on the tracer gas method for ventilation rate measurement were within 8% of the actual release rate, when a concentration criterion of 2)5 mg m\ was used to identify the function of openings as either air inlets or air outlets of the building. Ammonia emissions from two UK cattle buildings were estimated using the constant release tracer method using a tracer concentration criterion of 0)6 mg m\ to identify the opening function. The estimated annual ammonia emission factor (per livestock unit of 500 kg live weight, lu) was 3)0 and 8)9 kg [NH ] lu\ yr\ for a straw-based beef building and a slurry-based dairy building, respectively. Acknowledgements We thank our colleagues Dave Matthews and Andreas Sieve for their help in building the experimental crosssection. The work was funded by the Ministry of Agriculture, Fisheries and Food. References Boulard T; Baille A (1995). Modelling of air exchange rate in a greenhouse equipped with continuous roof vents. Journal of Agricultural Engineering Research, 61, 37}48 Brocket B L; Albright L D (1987). Natural ventilation in single spaced buildings. Journal of Agricultural Engineering Research, 37, 141}154 Bruce J M (1986). Theory of natural ventilation due to thermal buoyancy and wind. In: Proceedings of C.I.G.R. seminar on pig, rabbit and small birds species housing, Rennes, France, 8}11 September 1986, Vol. II, pp 1}9. CIGR, Merelbeke Demmers T G M (1997). Ventilation of livestock buildings and ammonia emissions. PhD Thesis, University of Nottingham, United Kingdom Demmers T G M; Burgess L R; Short J L; Phillips V R; Clark J A; Wathes C M (1998). First experiences with methods to measure ammonia emissions from naturally-ventilated cattle buildings in the UK. Atmospheric Environment, 32, 285}293
Demmers T G M; Burgess L R; Short J L; Phillips V R; Clark J A; Wathes C M (1999). Ammonia emissions from two mechanically ventilated UK livestock buildings. Atmospheric Environment, 33, 217}227 Down M J; Foster M P; Mcmahon T A (1990). Experimental veri"cation of a theory for ventilation of livestock buildings by natural convection. Journal of Agricultural Engineering Research, 45, 269}279 Groenenstein C M (1993). Animal-waste management and emission of ammonia form livestock housing systems: "eld studies. In:Livestock Environment IV (Collins E; Boon C R, eds), pp 1169}1175. ASEA, St. Joseph, MI Hoxey R P; Richards P J (1993). Flow patterns and pressure "eld around a full-scale building. Journal of Wind Engineering and Industrial Aerodynamics, 50, 203}212 Moran P; Hoxey R P (1979). A probe for sensing static pressure in two-dimensional #ow. Journal of Physics and Environment: Scienti"c Instruments, 12, 752}753 Oldenburg J (1989). Geruchs- und Ammoniak-emissionen aus der Tierhaltung. [Odour and ammonia emissions from housed livestock.] KTBL-schrift 333, KTBL, Darmstadt Pain B F; van der Weerden T J; Chambers B J; Phillips V R; Jarvis S C (1998). A new inventory for ammonia emissions from UK agriculture. Atmospheric Environment, 32, 309}313 Pearson C C; Owen J E (1994). The resistance to air #ow of farm building ventilation components. Journal of Agricultural Engineering Research, 57, 53}65 Richardson G M; Hoxey R P; Robertson A P; Short J L (1995). The Silsoe structures building: the completed experiment, part I. In: Proceedings of the 9th International Conference on Wind Engineering, New Delhi, India, 9}13 January 1995. New Age International Publishers Limited, Wiley Eastern Limited, New Delhi Robertson A P; Glass A G (1988). The Silsoe structures Building*its design, instrumentation and research facilities. Div. Note 1483, BBSRC, Silsoe Research Institute, Silsoe Sherman M H (1990). Tracer-gas techniques for measuring ventilation in a single zone. Building and Environment, 25(4), 365}374 Van Breemen N; Burrough P A; Velthorst E J; Van Dobben H F; De Wit T; Ridder T B; Reijnders H F R (1982). Soil acidi"cation from atmospheric ammonium sulphate in forest canopy through fall. Nature, 299, 548}550 Van Eerden L J M (1982). Toxicity of ammonia to plants. Agriculture and Environment 7, 223}235. Van:t Ooster A (1994). Using natural ventilation theory and dynamic heat balance modelling for real time prediction of ventilation rates in naturally ventilated livestock houses. Milan, 29 August}1 September 1994. CIGR, Report 94-C-026, Merelbeke VROM; LNV (1996). Wijziging Uitvoeringsregeling ammoniak en veehouderij. [Change of implementing regulation ammonia and livestock industry.] Staatscourant 49, 8 March 1996, Den Haag Wise A F E (1977). Ventilation of buildings: a review with emphasis on the e!ects of wind. Transactions of ASHREA, 83, 135}153
Validation of Ventilation Rate Measurement Methods and the Ammonia Emission from Naturally Ventilated Dairy and Beef Buildings in the United Kingdom T. G. M. Demmers; V. R. Phillips; L. S. Short; L. R. Burgess; R. P. Hoxey; C. M. Wathes Silsoe Research Institute, Wrest Park, Silsoe, Bedfordshire MK45 4HS UK; e-mail of corresponding author: [email protected] (Received 28 October 1999; accepted in revised form 4 November 2000; published online 23 January 2001)
Measurements of ammonia emission, especially from cattle buildings, are needed to assess the environmental impact of ammonia. There are no data available for cattle buildings in the United Kingdom because most buildings are naturally ventilated and methods for long-term measurement of emission from naturally ventilated buildings have not been developed. Two measurement methods, based on either the release of a tracer gas or on the pressure di!erence across ventilation openings, were validated in a full-scale cross-section of a naturally ventilated livestock building against a known release rate of a gaseous pollutant at high and low wind speeds. A good correlation between the measured and the actual release rates was found for the tracer gas method with an average recovery rate of 108%. The method based on pressure di!erence failed to estimate the ventilation rate correctly when only measured pressure coe$cients were used, because the measurements of mass #ow rates in and out through all openings of the building failed to balance. The traditional approach, based on measured values for the external pressure coe$cients and an estimate of the internal coe$cient balanced the #ow through the building by de"nition, but failed to estimate the actual emission rate correctly. Current knowledge of the discharge coe$cient for the opening designs of the building is insu$cient for the pressure di!erence method to be used to estimate the ventilation rate. Using the tracer method, measurements were carried out between February and May 1996 in a straw-based beef house and a slurry-based dairy cow house with cubicles and scraped passage ways. The buildings were space boarded and had a covered ridge. The ammonia emission was estimated to be 3)5 and 8)9 kg NH per livestock unit per year for the beef and dairy buildings, respectively. 䊚 2001 Silsoe Research Institute
1. Introduction Ammonia deposition is known to damage ecosystems, either directly (van Eerden, 1982) or indirectly, through acidi"cation following nitri"cation of ammonia (van Breemen et al., 1982). Agriculture, and cattle in particular, is considered to be the major source of atmospheric ammonia in the UK. However, accurate measurements of ammonia emission rate from the various sources are rare (Demmers, 1997). UK cattle buildings are usually naturally ventilated because of the mild climate and are characterized by numerous ventilation openings of varying design with the function of each, i.e. air inlet or outlet, variable in time. 0021-8634/01/050107#10 $35.00/0
Methods for measuring ventilation rate in naturally ventilated airspaces are limited and simple fan wheel anemometers, traditionally used in estimating ventilation rate in mechanically ventilated livestock buildings cannot be used. Two validated methods for measuring the ventilation in naturally ventilated livestock buildings are described and applied to measure the ammonia emission from two typical cattle buildings.
1.1. ¹racer gas method to estimate ventilation rate The constant release tracer method is one of a number of methods used to measure the ventilation rate of a
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2001 Silsoe Research Institute
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Notation c tracer concentration, m m\, (ppm) C , *C wind pressure coe$cient, di!erence in N N wind pressure coe$cient C discharge coe$cient B g gravity constant, m s\ H level of neutral plane, m LN H height of opening, m h height of building, m P, *P pressure, pressure di!erence, N m\ Q ventilation rate, m s\ q dynamic pressure, Pa v air velocity, m s\ v average air velocity, m s\ v wind speed at meteorological height, m s\
livestock building (Demmers, 1997; Demmers et al., 1998). Using Eqn (1) the ventilation rate Q in m s\ can be calculated as a function of time t in s from the rate of tracer release uN in m s\ and the indoor tracer concentration cG in m m\ after correction for the background concentration of tracer cC: uN (t) Q(t)" cG (t)!cC (t)
x y o u N
length of building, m width of building, m air density, kg m\ rate of tracer release, m s\
Subscripts boj bottom of opening j e external or background i internal i/e internal or external j opening j t thermal buoyancy toj top of opening j w wind
perature criterion has been shown to be less reliable than the concentration criterion (Demmers et al., 1998, 1999) due to local variations in temperature caused by solar radiation. Therefore, this criterion will not be considered in this paper.
(1)
1.2. Pressure di+erence method to estimate ventilation rate
Tracer gas methods rely on complete mixing of the air space (Sherman,1990), which is very unlikely in livestock buildings, and a single measurement of the tracer concentration is therefore inadequate. However, measurement of the tracer concentration at the ventilation openings of the building is a viable alternative, although this requires instantaneous identi"cation of the function of the opening as either an air inlet or outlet. The ventilation rate can then be calculated from the average concentration at the air inlets corrected for the average concentration at the air outlets. Three criteria are available to distinguish between inlets and outlets; (i) pressure di!erence over the opening, (ii) temperature gradient over the opening and (iii) concentration gradient of the tracer over the opening. The "rst is the most direct and depends only on the wind for the driving force of ventilation if the pressure coe$cients are used. However, in calm weather this criterion cannot be used, because the di!erence due to buoyancy is generally too small to be measured. Both the temperature and concentration gradient use a criterion above which the opening is deemed to be an inlet and below which it is an outlet. The criterion is higher than the background values to allow for sensor accuracy and drift. In both mechanically and naturally ventilated buildings, the tem-
A method based on the pressure di!erence across the opening can also be used to estimate the ventilation rate and is based on natural ventilation theory (Bruce, 1986; Van't Ooster, 1994). The two driving forces responsible for natural ventilation are the pressure di!erence in N m\ due to thermal buoyancy *PR and wind *PU, as functions of the length, width and height of the building x, y and h, respectively, in m: *PR (h)"g(oC!oG)(HLN!h) *PU(x, y, h)" ov (CNC(x, y, h)!CNG)
(2) (3)
where o in kg m\ is the air density at the relevant position, g the gravity constant in m s\, v the wind speed in m s\ at 10 m height and CN the wind pressure coe$cient. The height of the neutral plane HLN in m is the height h at which there is no pressure di!erence due to thermal buoyancy. The #ow through any opening can be found by applying Bernoulli's law, assuming the air to be incompressible: 1 1 *P(h)" o v(h) (4) C 2 G C B where CB is the discharge coe$cient, which depends on the geometry of the opening only, oG C is the air density
VE NT I L ATI O N RA TE M EA S U REM EN T ME TH O D S
inside o for air #owing out of the building or the air G density outside the opening o for air entering the buildC ing and v is the air velocity. The average air velocity v through any opening j can be found by integration over the height H in m of that opening from the bottom H to @M the top H : RM 1 &RMH vN " v(h) dh (5) H H H &@MH
The equations for velocity through a vertical opening acting entirely as an inlet or outlet are given in Eqns (6) and (7) for, respectively, thermal buoyancy and wind. Equation (8) represents the combined e!ect of buoyancy and wind. For openings that do not act entirely as an inlet or outlet, the inlet and outlet areas are described by separate equations. These and equations for other speci"c openings have been derived by Van't Ooster (1994) and Demmers (1997):
2 C "o !o " 2 g "o !o " G C G ("H !H " vN "! B C LN RMH H 3 H o !o o H C G G C !"H !H " ) (6) LN @MH "C !C " o NCH NG C vN "v C (7) H B C !C o NCH NG G C 2C 1 1 v "! B ("o v (C !C ) H C NCH NGH 3 H 2g(o !o ) o H C G GC #2g(o !o )(H !H ) C G LN RMH !"o v (C !C )#2g(o !o ) C NCH NGH C G ;(H !H )" ) (8) LN @MH According to Brockett and Albright (1987), the height of the neutral pressure level is independent of the wind e!ect and can therefore be calculated through iteration,
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using the equation describing thermal buoyancy [Eqn (6)] and assuming continuity of mass #ow. The ventilation rate of a building is then the sum of the air#ows at all the outlets or inlets. The major bene"t of this method is that information is obtained on the direction and magnitude of the air#ow rate at any opening or group of openings of the building. In principle, this information allows detailed analysis of the contributions of di!erent sections of the building to gaseous emissions. Although the driving force for ventilation is always the combined e!ect of thermal buoyancy and wind, the latter is by far the largest component at wind speeds over 3)0 m s\ (Wise, 1977). At these wind speeds, the pressure coe$cients can be measured accurately. The pressure measurements are less reliable at low wind speeds ((2 m s\). However, the average pressure coe$cients measured under windy conditions can be used in Eqn (8), even at low wind speeds, provided the wind direction is the same and the reference wind speed is known.
2. Materials and methods 2.1. Experimental facility A cross-section of a naturally ventilated livestock building was constructed in the Silsoe Structures Building (Robertson & Glass, 1988; Hoxey & Richards, 1993) (Fig. 1). Two parallel, internal walls 4 m apart were erected in the centre of the building. Validation of the two methods described above was carried out in this cross -section using a known rate of emission of ammonia. The building was 12)9 m wide, 4)1 m high at the eaves and 5)3 m high at the ridge. The slope of the roof was 103. The ventilation openings in the cross-section consisted of an open ridge 0)3 m wide with #ashings (0)12 m high) and sidewall openings "tted with space boarding with 25 mm
Fig. 1. Mean pressure coezcients measured around and in the cross-section of the Silsoe Structures Building (dynamic pressure, q*10 Pa). Also shown are the area source, A, the line source, B and the point source, C. The heaters are located over the area source
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gaps between 125 mm boards, each 1)8 m high, "tted below the eaves. Electrical heaters (each nominally 250 W m\, total input 5)6 kW) were installed at ground level in the cross-section to simulate the sensible heat output of animals at a realistic stocking density. The heaters were arranged on either side of a central passage way. The pollutant, carbon monoxide prediluted with air, was injected into the cross-section through an area source made of perforated lay-#at tubing, 1)4 m wide, situated underneath the heaters. The carbon monoxide simulated ammonia emitted from a urine "lm on the #oor. A tracer gas could also be injected into the air space, either through a point source or a line source. An array of tracer gas point sources distributed over the area source of pollutant will compare best with the release of the pollutant. However, in livestock buildings point sources within reach of animals, might not be practical, therefore a line source above animal level was also considered. The point source was a perforated ball of 250 mm diameter, located 0)4 m above the #oor level, while the line source was a 4 m length of perforated lay-#at tubing of 400 mm diameter, which extended across the width of the cross-section at ventilation opening height (3 m). Both sources were placed in the centre of the windward half of the cross-section. Methane was used as the introduced tracer and was diluted with air prior to entering either source to create a mixture with a density almost equal to that of air. Sampling points for the concentration of both gases were located in the centre of the ventilation openings, in between two boards or half-way up the #ashings for the sidewall vents and the ridge vent, respectively. Additionally, the gas concentrations were measured in the centre of the cross-section. The reference point was upwind of the building. Carbon monoxide and methane concentrations were measured using an infrared analyser. Both analysers were calibrated before and after each experiment. Sample locations were selected using a sample sequencer operating at a frequency of one channel per 90 s. The average concentration over the last 30 s of each period was logged. Pressure measurements were made using probes developed by Moran and Hoxey (1979) mounted on the sidewalls (#ush with the solid wall or 5 cm out from the space boarding), at the roof (5 cm up from the roof ) and 5 cm inside the openings. A reference measurement of static and dynamic wind pressure was made at ridge height 25 m upwind of the building. The measurements were taken with individual static pressure sensors located as close to the probe as possible at a rate of 1 Hz. To prevent measurement errors due to buoyancy the sensors were placed wherever possible at the same height as the probes.
Temperature and humidity were measured at the same locations as methane and carbon monoxide with a temperature and humidity probe. Weather conditions were monitored 25 m upwind of the building. Measurements were made of temperature and humidity at 1)5 m above ground and solar radiation, wind speed and direction at ridge height. Data from the analysers and weather station were logged with a data logger, which also controlled the sample sequencer. Data from the pressure measurements and the sonic anemometer were stored separately. Measurements were carried out under windy (wind speed over 4 m s\) and calm weather conditions (wind speed under 2 m s\). In both cases, the wind direction was approximately normal to the side of the building. The experiments were repeated using both point and line sources of tracer gas.
2.2. ¸ivestock buildings The two cattle buildings were ventilated naturally. Each had space boarding on all sides and a ventilated ridge in the form of a &cranked crown' ventilator. The space boarding comprised 129 mm wide boards spaced 30 mm apart. The beef building held 99 animals in straw-bedded holding pens either side of a central feed passage. The dairy building held 90 dairy cows in cubicles with straw bedding, the passage ways were scraped every 2 h. The constant tracer method was applied to both buildings using carbon monoxide as the tracer. Ten sampling points were located in the ventilation openings of each building. The ammonia concentration was measured using a chemiluminescent nitric oxide analyser, preceded by stainless-steel thermal converters. Measurements of weather conditions and data analysis were the same as those at the experimental cross-section. Measurements were taken from February to May 1995.
3. Results 3.1.
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Table 1 Average methane and carbon monoxide concentrations measured in a cross-section of the Silsoe Structures Building; measurements at high (6 ' 0ⴞ0 ' 3 m sⴚ1) and low (2 ' 0ⴞ0 ' 4 m sⴚ1) wind speed; data given for methane tracer released from point and line sources and carbon monoxide (pollutant) from an area source; the standard deviation of each concentration was below 0 ' 4 mg mⴚ3 (no. of measurements, n* *11) Concentration, mg m!3
Location
High wind speed
Low wind speed Point source
Background Windward wall top half Windward wall bottom half Leeward wall top half Leeward wall bottom half Ridge 0)5 m below ridge Centre of cross-section
¸ine source
Point source
¸ine source
CH4
CO
CH4
CO
CH4
CO
CH4
CO
1)4 3)7 2)5 20)7 15)0 19)3 18)9 15)7
0)2 1)7 1)0 9)7 6)9 10)5 11)1 9)9
1)4 2)1 3)1 13)9 12)0 15)2 15)5 17)7
0)2 0)6 1)3 7)6 7)2 6)6 6)9 6)6
0)9 1)1 7)0 13)8 14)3 14)9 14)0 15)9
0)2 0)3 3)4 17)6 19)1 15)3 12)5 13)5
0)9 0)9 2)9 15)2 15)1 14)3 13)1 17)9
0)2 0)2 4)2 18)8 19)1 13)1 12)5 13)0
building. There was little di!erence in the general distribution of methane between the point and the line sources. However, the concentration distribution of carbon monoxide released from the area source was clearly di!erent from the patterns of methane concentration, especially at the high wind speed, and resembled the temperature distribution [data not shown (Demmers, 1997)], with signi"cantly higher concentrations at the leeside wall compared with the ridge outlet. The latter was caused by short circuiting of fresh air from the wind ward side to the ridge, partially due to the thermal buoyancy of the heaters. The concentration pattern of the point and particularly the line sources were less a!ected by this as these sources were located within the central recirculating area. The identi"cation of the function of the openings as either an air inlet or outlets was not straightforward from these data. Clearly, the leeside wall and the ridge were both outlets, as both the methane and carbon monoxide concentrations were high. The windward wall was an air inlet, with tracer concentrations expected to be close to the background levels. However, the tracer gas concentrations at the windward wall were signi"cantly higher than the background concentration. The raised concentration arose from mixing of incoming air with internal air behind the space boarding. This was con"rmed using smoke. Hence, a minimum di!erence between the background and inlet concentration, or concentration criterion, of 7)5 mg m\ was needed to identify the whole windward wall as an inlet compared to just 2)5 mg m\ for part of the opening. The ventilation rate was calculated using Eqn (1) with the concentration criterion to identify the function of the
openings. To validate subsequent estimates of ammonia emission, the emission of carbon monoxide was estimated using measurements of its concentration and the calculated ventilation rate. This estimate of carbon monoxide emission was compared with the actual release rate (Table 2). The ventilation and emission rate estimates for the perimeter method are presented for two concentration criteria. For each combination of source location and wind speed, the lowest criterion was set to identify at least one of the sampling points on the windward wall as an inlet. The highest criterion was set to identify both sampling points on the windward wall as inlets. The choice of concentration criterion a!ected the estimate of ventilation rate. Using the lower concentration criterion, i.e. taking the elevated methane level measured at the lower part of the windward wall as indicative of an outlet, resulted in a higher estimate of ventilation rate at low wind speed. At high wind speed, although the concentration of methane at the bottom half of the windward wall was also high, and hence, this section of the opening identi"ed as an outlet, the estimate of ventilation rate decreased. This was due to the variation in mean concentration by allocating the high tracer concentration to either the inlet or outlet category. The concentration criterion had the opposite e!ect on the estimated emission rate, compensating for the error made in the ventilation rate. The level of compensation depended on the relative distribution of the tracer gas (methane) and the pollutant (carbon monoxide). In general, the agreement between the estimated emission rate of carbon monoxide and the actual release rate was satisfactory. The calculated emission rate on average overestimated the actual release rate by no more than
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Table 2 Comparison of the estimated with the actual emission rate of carbon monoxide; the estimated emission rates derived from the measured ventilation rates and pollutant concentrations measured at the perimeter of a naturally ventilated cross-section of the Silsoe Structures Building; both point and line sources used for tracer release; measurements at high and low wind speed conditions; where appropriate mean and standard deviation given
¸ow wind speed Point source Line source High wind speed Point source Line source
Concentration criterion, mg m!3
Ventilation rate, m3 h!1
Pollutant emission rate, g h!1
Pollutant recovery, %
1)5 3 1)5 2
3500$250 2900$200 4550$50 3900$400
30$3 30$3 30$1 30$1
107 106 108 105
1)5 7)5 1)5 3
11 500$2050 13 000$2750 10 000$1400 11 500$1750
185$18 229$25 177$14 192$14
99 122 94 102
8%, except at high wind speeds using point source of tracer gas and a high threshold criterion (7)5 mg m\). The emission rate was then overestimated by 22%. This was caused by the di!erence in concentration pattern of methane and carbon monoxide especially at the lower part of the windward wall and suggests that positioning of the sampling point was critical and that a high concentration criterion should be avoided. Overestimates of the emission rate could be caused by errors in either the ventilation rate or the concentration of carbon monoxide at the outlets of the cross-section. The ventilation rate was overestimated because the concentration of the tracer at the inlet was not equal to the background concentration. The emission rate was correspondingly underestimated. Correction by using the background concentration instead of the inlet concentration did not give an improvement. Changing conditions inside the building during the sampling cycle of 12 min needed to sample all locations are most likely to have contributed to the inaccuracy. The point source was to be preferred over the line source as it gave slightly more consistent estimates of the actual emission rate, when used with the lower concentration criterion. The better location of the point source, above the area source, was an important contributing factor.
3.2.
coe$cients from all measurements with a dynamic pressure q above 10 Pa are given in Fig. 1. All data sets with q lower than 10 Pa were ignored, because the low accuracy of the pressure transducers caused a large scatter of data points. The standard error for the average C was N typically 0)01. The low standard error is mainly due to the limitation of wind angle and wind speed. The measured pressure coe$cients C compare well N with those measured in previous work by Hoxey and Richards (1993) and Richardson et al. (1995) on the same building without ventilation openings. The presence of the space boarding on the windward wall did therefore not have a large in#uence on C values. The #ashings of N the ridge vent did a!ect the C distribution on the roof, N with lower values of C towards the ridge on the windN ward half and lower values up to half way down the roof on the leeside of the ridge. The space boarded opening on the leeside again had little impact on the measured C values compared with a solid wall. The N internal pressure was lowest on the windward wall of the cross-section, but the pressure variation inside the building was small. The C values indicate clearly that the windward wall N was an air inlet with a C di!erence across the wall of N 0)73. The ridge was an air outlet, as expected, with a difference in C across the ridge of !0)35. The pressure N measurements suggested that at wind speeds over 4 m s\, the leeside wall was a minor air inlet, with a di!erence in C across the opening of just 0)06. HowN ever, this pressure di!erence did not correspond with the observed air#ow pattern, using smoke. Detailed analysis of the C data for this opening (with q '10 Pa) showed N that the opening alternated between acting as an outlet and inlet (standard deviation and standard error were 0)17 and 0)002, respectively). On average, the opening
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VE NT I L ATI O N RA TE M EA S U REM EN T ME TH O D S
acted as an air inlet according to the static pressure data. Both the windward wall (di!erence in wind pressure coe$cient *C "0)73; standard error, SE"0)006) and N the ridge openings (*C "!0)12, SE"0)003) show N a similar variable pressure pattern, but were consistently identi"ed as an inlet and outlet, respectively. The height of the neutral plane was calculated using Eqn (5), the equations for the buoyancy e!ect alone, and the equation for the law of conservation of mass. The measured C values including the internal C values, N N temperature, humidity and wind speed at ridge height were used to estimate the rate of ventilation due to buoyancy alone [Eqn (5)] and to buoyancy and wind combined [Eqn (8)] of each opening (Table 3). The ventilation rate due to buoyancy alone accounts for the in#uence of wind by using the apparent height of the neutral plane, which depends on the wind speed and pressure coe$cients. The estimates of ventilation were calculated using C values measured by Pearson and Owen (1994), B i.e. 0)6 for the ridge and 0)75 for the space boarding. Using the estimated ventilation rate the emission rate was calculated for each opening and the building as a whole. The estimated carbon monoxide emission rate was compared with the actual release rate and is given as a percentage of recovery. The lower openings of a building are generally inlets, when only buoyancy e!ects are considered, as is indeed the case for the low wind speed conditions. Due to the wind e!ect the apparent height of the neutral plane can change signi"cantly, resulting in a minor outlet at the leeside wall for the high wind speed conditions. The ventilation rate based on the combined e!ects of buoyancy and wind implied that the leeside wall opening acts as an air inlet for both the low and high wind speed conditions. This is in contrast to the smoke experiments
which suggested the opening was acting at least partially as an air outlet. The mass balance of air#ow over the cross-section as a whole was calculated from the ventilation rate of each individual opening, but it was not equal to zero. As a result, the percentage recovery of the carbon monoxide was extremely poor. Using the traditional method, i.e. solve the neutral plane level and the C value through NG iteration assuming that the mass balance for the building is valid, C values of !0)15 and !0)12 were found for NG the low- and high-speed conditions, respectively. Again the recovery of carbon monoxide was poor at 194 and 149%. The calculated C value, half the measured value, NG appeared unrealistic. As the discrepancy in the mass balance over the building could not be explained by the variation in the pressure measurements both inside and outside the building only, two other possible explanations were identi"ed. The "rst is the pressure discharge coe$cient, as measured by Pearson and Owen (1994). Their measurements were repeated and proved to be correct. However, their experimental conditions might not be a true representation of the wind-induced air#ow through an opening. In their method, all the air#ow was forced through the space boarding and the air#ow rate and pressure drop across the space boarding were recorded. When space boarding is part of a building the air does not necessarily pass through it and may #ow around the building as if the wall was solid. A better approach would be to measure the air#ow through an opening exposed to wind in a wind tunnel. This could result in a lower C . B Secondly, the assumption that all the dynamic pressure translates to static pressure, from which alone the #ow through an opening results, could be incorrect. Inside the building there are signi"cant air velocities of for example,
Table 3 Estimates of ventilation and carbon monoxide emission rates using natural ventilation theory compared with the actual release rate for each opening; measurements in the naturally ventilated cross-section of the Silsoe Structures Building at high and low wind speed normal to the building =indward wall
¸eeside wall
Ridge
Cross section R
High wind speed Ventilation rate by buoyancy alone, m h\ Total ventilation rate, m h\ Emission rate, g h\ Recovery, %
!50 26 600 62
1100 6600 124
!1050 !10 200 !121
0 23 000 !65 !35
¸ow wind speed Ventilation rate by buoyancy alone, m h\ Total ventilation rate, m h\ Emission rate, g h\ Recovery, %
450 5800 10
1000 2000 20
!1450 !3000 !40
0 4800 11 38
114
T. G . M. D E M M E RS E¹ A ¸ .
0)4 m s\ approaching the leeward wall, which add a dynamic element (0)1 Pa) to the internal pressure. The same is, however, true for the external pressure on the windward wall. Incorporating the dynamic pressure in the static pressure coe$cients would greatly contribute to the complexity of the model.
3.3. Ammonia emission from livestock buildings The estimated annual ammonia emission rate from the two cattle buildings, measured using the constant release tracer method with a concentration criterion of 0)6 mg m\ is given in Table 4. The lower value for the concentration criterion was made possible by an improved sampling strategy, whereby rising air inside the building was prevented from reaching the sampling point, placed between two boards. The annual emission is based on a winter housing period of 190 days per annum. Comparison with literature values expressed per livestock units, lu (500 kg of live weight) shows that the value derived for the dairy unit compares well with the 8)3 kg lu\ yr\ given by Van't Ooster (1994). The value for the straw-based beef unit is lower than for slurry-based systems possibly indicating that straw-based housing systems give lower ammonia emissions than traditional slurry-based systems. Compared with all values, the value given by Oldenburg (1989) for all types of German cattle buildings seems unrealistically low, which may have been due to restricting measurements of ventilation rate and ammonia concentration to the ridge.
so that any increase in tracer concentration above background indicates an air outlet of the building, though this will not identify the situation where air re-enters the building. However, the criterion must also allow for mixing of air when the sampling point is directly behind the opening. The method was di$cult to apply at high wind speeds, where use of a concentration criterion of 7)5 mg m\ resulted in a large overestimate of the ventilation rate. A concentration criterion of 2)5 mg m\ was more appropriate because one or more openings were correctly identi"ed as inlets. A concentration criterion well above background level will identify inlets with an elevated tracer concentration correctly, but will result in an overestimate of the ventilation rate. Provided the distribution of the tracer and the gaseous pollutant are similar, this overestimate is compensated for in the emission rate estimate by the same argument. However, when the distribution of the tracer and pollutant are signi"cantly di!erent, the overestimate is either overcompensated for or exacerbated. For naturally ventilated livestock buildings a concentration criterion between the background level plus the analyser detection limit to a maximum of 2)5 mg m\ is recommended to detect the function of an opening. Preventing the sampling of mixtures of external and internal air just behind the space boarding will greatly improve the reliability of this criterion. The actual level will depend to some extent on the data set. Errors in the selection of inlets resulting in an overestimate of the ventilation rate will occur and are impossible to eliminate.
4.2.
4. Discussion 4.1. Identi,cation of air inlets and outlets The identi"cation of an opening as either an air inlet or an air outlet proved to be complex. In principle, the concentration criterion should be kept as low as possible,
The accuracy of the constant release tracer gas method with sampling locations at the perimeter of the building was higher than expected. The recovery of released carbon monoxide was within 8% of the true rate, for the
Table 4 Annual ammonia emission rates from cattle buildings estimated using the continuous tracer release method with a concentration criterion of 0 ' 6 mg mⴚ3 as compared with published values Cattle type Beef Beef Dairy Dairy All
Housing type Straw bedded Slurry-based Cubicle; slurry with scraped #oor Slurry-based with fully slated #oor All types
lu, livestock unit (500 kg live weight).
Country United Kingdom The Netherlands United Kingdom The Netherlands Germany
Annual emission, kg NH3 lu!1 yr!1
Source
3)0 4)7 8)9 8)3 1)1
This study VROM (1996) This study van't Ooster (1994) Oldenburg (1989)
VE NT I L ATI O N RA TE M EA S U REM EN T ME TH O D S
lower concentration criterion. In comparison, the estimated ammonia emission rate from two mechanically ventilated livestock houses using the same method was 13% lower than a reference method using fan wheel anemometers to measure the ventilation rate (Demmers, 1997; Demmers et al., 1999). An error of 10}15% is commonly accepted in the literature for the measurement of ventilation rate by the tracer gas methods for full-scale buildings (van't Ooster, 1994). The accuracy of the tracer gas method in a full scale, naturally ventilated livestock building is likely to be lower unless an equivalent number of sampling positions can be used in relation to the number and size of the openings.
4.3.
115
substantially complicate the model and measurements required. The discharge coe$cients of common agricultural ventilation structures as determined by Pearson and Owen (1994) and the theoretical values given by Bruce (1986), appeared to be incorrect when applied to a full-scale livestock building. Values for the discharge coe$cients of space boarding and the open ridge will have to be lower for the estimated ventilation and emission rate to match the actual release rate. Ventilation rate estimates based on natural ventilation theory gave a poor match with tracer dilution measurements. As a result, the percentage recovery of release rate of simulated pollutant were also poor, although the corrections in the pressure and discharge coe$cients could give some improvement. The large di!erences in recovery at high and low wind speeds indicate a dependancy of the error on wind speed. The tracer gas method must be preferred to the natural ventilation theory for estimation of the ventilation and emission rates of naturally ventilated buildings, until a more detailed understanding of the discharge and total pressure coe$cients is available.
4.4. Ammonia emission estimates from livestock buildings The ammonia emission factors for the straw-based units in this study and in literature are generally lower than the emission factor for comparable slurry-based units. This suggests that adoption of straw-bedded housing systems could lower ammonia emissions. However, it is known from litter-based pig housing systems, that the reduction in ammonia mission is mirrored by a rise in nitrous oxide emission (Groenenstein, 1993). The ammonia emission factors derived from this work have been included in the production of an up-to-date ammonia emission inventory (Pain et al., 1998). This inventory estimates total UK ammonia emissions to be 250$60 kt [NH ] yr\. Approximately, 60% of this total is at tributed to cattle production, with housing attributing 52 kt [NH ] yr\. 5. Conclusions Assessment of two methods to measure the ventilation and gaseous pollutant emission rates of naturally ventilated buildings indicated that the constant release tracer gas method gave the most reliable estimates of ventilation rate. The national ventilation theory using the measured pressure coe$cients failed to estimate the ventilation rate correctly. Using a combination of measured external and computed internal pressure coe$cients the ventilation
116
T. G . M. D E M M E RS E¹ A ¸ .
rate estimate was balanced by de"nition, but the method again failed to estimate the actual emission rate correctly. More research is needed to measure the discharge coe$cients of space boarding and ridge con"gurations used in naturally ventilated buildings and to measure the pressure coe$cients based on total pressure rather than static pressure alone. Estimates of pollutant emission rate based on the tracer gas method for ventilation rate measurement were within 8% of the actual release rate, when a concentration criterion of 2)5 mg m\ was used to identify the function of openings as either air inlets or air outlets of the building. Ammonia emissions from two UK cattle buildings were estimated using the constant release tracer method using a tracer concentration criterion of 0)6 mg m\ to identify the opening function. The estimated annual ammonia emission factor (per livestock unit of 500 kg live weight, lu) was 3)0 and 8)9 kg [NH ] lu\ yr\ for a straw-based beef building and a slurry-based dairy building, respectively. Acknowledgements We thank our colleagues Dave Matthews and Andreas Sieve for their help in building the experimental crosssection. The work was funded by the Ministry of Agriculture, Fisheries and Food. References Boulard T; Baille A (1995). Modelling of air exchange rate in a greenhouse equipped with continuous roof vents. Journal of Agricultural Engineering Research, 61, 37}48 Brocket B L; Albright L D (1987). Natural ventilation in single spaced buildings. Journal of Agricultural Engineering Research, 37, 141}154 Bruce J M (1986). Theory of natural ventilation due to thermal buoyancy and wind. In: Proceedings of C.I.G.R. seminar on pig, rabbit and small birds species housing, Rennes, France, 8}11 September 1986, Vol. II, pp 1}9. CIGR, Merelbeke Demmers T G M (1997). Ventilation of livestock buildings and ammonia emissions. PhD Thesis, University of Nottingham, United Kingdom Demmers T G M; Burgess L R; Short J L; Phillips V R; Clark J A; Wathes C M (1998). First experiences with methods to measure ammonia emissions from naturally-ventilated cattle buildings in the UK. Atmospheric Environment, 32, 285}293
Demmers T G M; Burgess L R; Short J L; Phillips V R; Clark J A; Wathes C M (1999). Ammonia emissions from two mechanically ventilated UK livestock buildings. Atmospheric Environment, 33, 217}227 Down M J; Foster M P; Mcmahon T A (1990). Experimental veri"cation of a theory for ventilation of livestock buildings by natural convection. Journal of Agricultural Engineering Research, 45, 269}279 Groenenstein C M (1993). Animal-waste management and emission of ammonia form livestock housing systems: "eld studies. In:Livestock Environment IV (Collins E; Boon C R, eds), pp 1169}1175. ASEA, St. Joseph, MI Hoxey R P; Richards P J (1993). Flow patterns and pressure "eld around a full-scale building. Journal of Wind Engineering and Industrial Aerodynamics, 50, 203}212 Moran P; Hoxey R P (1979). A probe for sensing static pressure in two-dimensional #ow. Journal of Physics and Environment: Scienti"c Instruments, 12, 752}753 Oldenburg J (1989). Geruchs- und Ammoniak-emissionen aus der Tierhaltung. [Odour and ammonia emissions from housed livestock.] KTBL-schrift 333, KTBL, Darmstadt Pain B F; van der Weerden T J; Chambers B J; Phillips V R; Jarvis S C (1998). A new inventory for ammonia emissions from UK agriculture. Atmospheric Environment, 32, 309}313 Pearson C C; Owen J E (1994). The resistance to air #ow of farm building ventilation components. Journal of Agricultural Engineering Research, 57, 53}65 Richardson G M; Hoxey R P; Robertson A P; Short J L (1995). The Silsoe structures building: the completed experiment, part I. In: Proceedings of the 9th International Conference on Wind Engineering, New Delhi, India, 9}13 January 1995. New Age International Publishers Limited, Wiley Eastern Limited, New Delhi Robertson A P; Glass A G (1988). The Silsoe structures Building*its design, instrumentation and research facilities. Div. Note 1483, BBSRC, Silsoe Research Institute, Silsoe Sherman M H (1990). Tracer-gas techniques for measuring ventilation in a single zone. Building and Environment, 25(4), 365}374 Van Breemen N; Burrough P A; Velthorst E J; Van Dobben H F; De Wit T; Ridder T B; Reijnders H F R (1982). Soil acidi"cation from atmospheric ammonium sulphate in forest canopy through fall. Nature, 299, 548}550 Van Eerden L J M (1982). Toxicity of ammonia to plants. Agriculture and Environment 7, 223}235. Van:t Ooster A (1994). Using natural ventilation theory and dynamic heat balance modelling for real time prediction of ventilation rates in naturally ventilated livestock houses. Milan, 29 August}1 September 1994. CIGR, Report 94-C-026, Merelbeke VROM; LNV (1996). Wijziging Uitvoeringsregeling ammoniak en veehouderij. [Change of implementing regulation ammonia and livestock industry.] Staatscourant 49, 8 March 1996, Den Haag Wise A F E (1977). Ventilation of buildings: a review with emphasis on the e!ects of wind. Transactions of ASHREA, 83, 135}153