Cool Pig Buildings.

Growing pigs produce tremendous amounts of heat (Brown-Brandl, et al. 2004) and when raised in confinement facilities this heat must be removed by the barn's environmental control system (Brumm, 2012). Standard barn ventilation systems that use ambient or outside air to remove this heat has only limited capacity to control rising barn temperaturesduring warm and hot weather. For this reason, numerous studies and papers have been published indicating performance improvements from cooling pig buildings forboth market pigs and breeding animals (especially boars). How best to cool pigs economically is the real challenge to the pig industry.

Most of the cooling systems used in pig barns around the world utilizesome form of evaporative cooling because of the relatively low costs to install and operate the systems. Cooling can be accomplishedby directlywetting pigs or sows in their pens or stalls and then moving room air over the animals to remove heat. Another evaporate method is by fogging (spraying a very fine mist into the inlet air) or having the air enter through a wet evaporative pad in the wall. Both fogging and evaporative padslowers the inlet air temperature while increasing the air's moisture content or humidity.

Evaporative cooling works well when atmospheric conditions are relatively dry (dewpoint temperaturesbelow 10 C). As the moisture content in the atmosphere increases, the effectiveness of evaporative cooling rapidly decreases. When dewpoint temperatures of the inlet air are above 15 C, performance of evaporative cooling systems begins to decline and quickly falls off when dewpoint temperatures reach 20 C and above. In the Midwestern part of the USA, recent climate trends are forhigher dewpoint temperatures during the spring and summer seasons, which is limiting the effectiveness of evaporative cooling systems in pig barns.

A recently completed Minnesota Pork Board/ National Pork Board (MPB/NPB) funded project entitled: Reducing the Environmental Footprint of Swine Buildings (Jacobson, et al., 2011)had the requirement to provide retrofit or remodeling guidelines to reduce energy use and air emissions for pig buildings (especially grow finish) presently being used in the Midwestern U.S. For both new and remodeled buildings, the results focused on structural upgrades such as insulation and mechanical items like improved environmental control, fan and heater maintenance and management, along with manure pit management. However, the project found the environmental impact of pork production might be reduced the most by improving pig performance through more effective cooling systems since most production losses due to poor housing systems occur during warm and hot conditions.Thus the benefits of a successful cooling system in pig production are not only improved animal performance but also lower environmental impacts. 

Current (2011) financial summaries from the University of Minnesota Center for Farm Financial Management indicate electric and fuel (generally propane) usage account for only 2 to 3% ($2.40 per pig) of the production costs for a typical "wean to finish" farm in Minnesota (www.finbin.umn.edu). One of the most important factors in energy consumption is not related to typical efficiencies in heating and ventilating but rather in optimizing the barn environment for pig performance. Curtis (1983) and others (Hahn et al, 1987; Huynh et al, 2004a, Mount, 1975, Brown- Brandl et al., 2000) stress the need to provide an indoor climate conducive to animal performance. Providing this environment requires proper control of indoor temperature, humidity, airflow (rates and velocities), and gas concentrations. Unfortunately, in an effort to reduce building costs, a majority of barns have been built in the U.S. with inadequate insulation and have ventilation systems that do not provide for optimum environmental conditions in the barn, especially cooling.

Baker (2004) provides an overview of all of the parameters impacting the effective environmental temperature (EET) of the pig. In general, drafts (high air velocities at pig levels) and cold surfaces significantly reduce this EET resulting in the need to increase the barn temperature and subsequent heat energy (both fossil fuel and feed). Likewise high outside and barn temperatures can reduce pig performance, both growth rate and feed efficiency.In general, ideal temperatures for grow-finish pigs in non-bedded pens are reported to be between 16 and 21° C (60-70° F) (Figure 1). Factors such as beginning and ending pig weight, group size, pig space allocation, and genotype may be responsible for part of the variation in this ideal temperature range.

Figure 1. Effect of environmental temperature on growing-finishing pig performance (Coffey, et al., 1995).

Nienaber et al (1987) fed pigs from 45 to 90 kgand reported pigs maintained at 25 C gained 82% as much as those housed at 20 Cand required 103% as much feed per unit of gain. Pigs at 31 C gained 58% as much as the ideal situation (20 C) and required 118% as much feed per unit of gain. Lopez et al (1991), with data collected on pigs starting at 90 kg and fed for a 21 day period,reported that pigs maintained at 25 Cgained 90% as much as those housed at20 C and required 101% as much feed per unit of gain. Pigs at 30 C gained 80% as much as the ideal situation (20 C) and required 103% as much feed per unit of gain.

Massabie and Granier (2001) conducted two experiments to determine the effects of air movement and ambient temperature on pig performance and behavior. Treatments included three ambient temperatures (28, 24 and 20°C) combined with two air velocities (still air or 0.56 m/s at day 1 increasing up to 1.3 m/s at day 43). For the hotter environmental temperatures, air velocity improved average daily feed intake (ADFI) and average daily gain (ADG) but lowered feed efficiency (FE) and lean tissue percentage. However, at temperatures near the optimum (20-24° C); air movement had a negative effect on pig performance. ADG was higher but FE declined and lean tissue percentage was lower. This suggests that achieving an optimum temperature through methods other than ventilation air movement(i.e.floor cooling) may have production advantages. Huynh et al (2004b) found that floor cooling significantly increased feed intake and growth rate under summer conditions. ADG was improved by 32 g/d or about 4.5%.

Brown-Brandl, et al (2000) studied manual and thermal induced feed intake restrictions on finishing barrows measuring effects on growth, carcass composition and feeding behavior. Results suggest that high-lean-growth pigs reared in hot environments deposit more fat and less protein than those raised in a "thermoneutral" environment and fed similar amounts of the same diets.

These research results suggest that some improved performance (ADG and FE) can be achieved for grow-finish pigs through environmental control, primarily cooler barn temperatures during warm and hot weather. This same principle of a thermal comfort zone applies to the breeding herd as well. The EET for sows varies from 12 to 22 C and is even lower for boars. Performance of sows, primarily during lactation, is significantly improved if they can be cooled whenever air temperatures are above 20 C. Lactating sows when heat stressed will reduce their feed intake and the reduction in milk production translates to lower weaning weights and poorer sow conditions at weaning andlower subsequent rebreeding conception rates (Black, et al., 1993). 

Thus, the question becomes how best to provide an effective cooling system in pig housing systems and still make it economically (cost) attainable. Nearly all of the operating cooling systems used in pig buildings today(refrigeration cooling is only donein boar studs)involve evaporative cooling.

The simplest and probably most cost effective evaporative cooling is drip cooling of lactating sows in the farrowing crate and the sprinkling of grow-finish pigs in slatted floor confinement barns.For both of these situations the sows or pigs are individually cooled rather than the barn space.In the next level of complexity the air inside the barn is cooled by evaporative cooling pads or fogging systems that have the incoming air pass through a wetted pad or a fogging zone.

These cooling systems depend on the evaporative transfer of heat which can be very effective as long as ambient or outside environmental conditions are relatively dry (dewpoint temperatures < 10 C). During humid conditions or periods of high dew point temperatures, the amount of heat removed from the animal or the air temperature drop at the evaporative pad or point of fogging are limited since the processes depend on evaporation of water. If the room air moving over the pig is nearly saturated then the water on the pig skin will evaporate very little and only a small amount of heat will be removed. This is the challenge that has yet to be obtained in practice, finding a cooling source and system that is not dependenton evaporative transfer of heat and, which because of the large cooling load needed, is economical to install and operate.

The costs of cooling systems for pig buildings can be divided between the initial or capital costs to install the system and the operating costs to run the system. Table 1 list relative values for both of these costs for some typical cooling systems seen in pig barns in the U.S. today and some potential alternative cooling systems in the future.

Table 1. Relative cooling system costs for pig buildings

As indicated above, sprinkling and/or dripping systems are relatively cheap to install, operate at low pressures, and only need simple controls (thermostat and timer) plus have a low operating cost. Fogging generally is slightly more expensive to purchase and install as well as to operate since it runs at higher pressures (often 100 Bars) and needs to be located in the inlet air stream. Evaporative cooling pads are still more expensive than the first two, since they need to be an integral part of the building and the ventilation/environment control system. Operation costs are still quite low, since only the pumping of water and some maintenance of the pad material to prevent clogging and algae growth are involved.

A significant jump in capital expense and especially in operating costs would result with refrigeration cooling or common "air conditioning" in pig buildings. Because of the need to only have a "single pass" or a non-recirculating air system, sizing of refrigeration units for pig barns are very large (a suggested size for a 1000 head finishing barn is about 350 kW of refrigeration) and most importantly the operating costs would be prohibitively large except for some unique facilities such as boar studs. In comparison, a geothermal cooling system using deep thermal wells, would be higher toinstall than the refrigeration cooling but the operating cost for the geothermal system is considerably less.

Geothermal energy is one alternative source of energy for supplying large amounts of cooling that is available worldwide and whose technology is becoming more common for residential and industrial applications. Geothermal systems can capture energy from the earth and with relatively simple engineered systems heat and cool a swine building. Since most swine buildings requires air exchange to remove moisture and heat produced by the pigs, this energy source has the potential to temper the incoming or inlet ventilation air in cold or cool weather which will reduce the need for additional supplemental heat from more conventional sources. More importantly in warm weather, geothermal energy can cool the incoming air, resulting in cooler than outside or ambient barn temperatures and lower ventilation rates without adding moisture to the incoming air (as evaporative cooling pads do). These lower barn temperatures have the added, and likely more significant benefit, of improving animal performance (and subsequent economic impact) during periods of warm outside temperatures. Although the initial investment in geothermal systems is large, the potential return on investment could be significant.

A recently completed monitoring project at a commercial sow, including gestation, farrowing and nursery pig production facility in western Minnesota demonstrated the application of geothermal energy for a pig building (Jacobson, 2011). The farm began operation in early 2009 and incorporates a full scale geothermal system (total of 10 geothermal heat exchangers (figure 1), or five exchangers evenly spaced along each side of the facility with 32 thermal well water loops per exchanger and thus 320 thermalwells that were 75 m deep to provide building heating and cooling. The 47 x 175 mbuilding holds approximately 900 sows and 3700 nursery pigs and consists of two crated gestation rooms, tworeplacement gilt development rooms (one crated and one pen), ten 18 crate farrowing rooms, and eight large pens nursery rooms. Manure is collected in shallow pits (about 75 cm deep) in all rooms with pull plugs that transferred manure to a sump and then to an outside buried concrete pit with a concrete cover shown in the foreground of figure 2 with the building in the background.

Figure 1. Geothermal heat exchanger (one of 10) used to heat and cool the inlet ventilation air located in the hallways on each side of building.

Figure 2. 900 sow geothermal sow/nursery barn with covered manure storage pit.

One measure of performance for this geothermal system can be drawn from the two gestation rooms in the building. These tworooms were chosen since they were NOT heated or cooled by any other source than the geothermal system. The mean ventilation or airflow rate (in cubic feet per minute or cfm/sow) over the 12 month monitoring period for the two gestation rooms at this facility is shown in Figure 3. Also, in this figure is the average ventilation or airflow rate from two gestation barns in Minnesota that were monitored in 2002 - 2003 which represent a more "conventional" gestation barn. As shown, there is a large difference (140 vs. 40 cfm/sow or 240 vs. 70 m³/hr) between the airflow rates for the two sites as a function of the outside or ambient temperature.

Figure 3. Comparison of gestation room ventilation rates (cfm /sow) for geothermal site and a comparable conventional gestation barn in southern Minnesota.

An estimated capital cost for the geothermal system at this 900 sow and 3400 nursery pig facility was $400,000 when installed in 2009. Animal performance values for thisgeothermal(GT) farm versus the Pigchamp™ Benchmarking database average (P-B) for the same time frame (from October, 2010 through June, 2011)areshown in Table 2. As seen in Table 2, most of the geothermal farm's parameters indicate that it exceeded the performance of the P-B averages during this 9 month period.

Table 2. Sow Performance comparison between the Geothermal (GT) and an industry standard as measured by the Pigchamp™ Benchmarking (PB) database

For marketpig production, the previously mentioned"paper" study (Jacobson, et al, 2011) proposed several barn styles or versions that incorporated both non-evaporative and evaporative cooling systems. Theso-called Green Pig Barns (GPB) were sized as 2400 head facilities (all in/all out) with shallow pits or gutters (45 to 60 cm deep) and manure storage located outside of the pig facility.

Versions A and B have partially slatted floors with the solid floor incorporating in-floor heating and cooling provided by "cross-linked polyethylene" orPEX tubing in the floor. Version A uses a geothermal system capable of providing 140 kW of cooling to the floor. Theoretically, this cooling capacity will remove 25% of the heat produced from pigs at the final growth stage. This cooling is anticipated to reduce maximum ventilation requirements by 25% during summer heat periods. Additional cooling of the incoming ventilation air will be provided with evaporative cooling pads. Version B incorporates the use of mechanical cooling (geothermal) of both the solid floor in the pig pens and the incoming ventilation air. A boiler system would be required to provide floor and traditional convective heating in cold weather and for small pigs. This system insures that thermalneutral conditions for the pigs in the barn can be met during the entire season at all growth phases. 

GPB Version A uses a ground source heat pump system for heating and cooling the solid floor. An estimation of the size of the cooling system would include 24 thermal wells and a heat pump to supply approximately 175 kWof cooling for upper Midwest USA conditions. An evaporative pad cooling systemwould be required to further reduce barn air temperatures. As a result of this cooling, the summer ventilation rate in the barn could be reduced by 1/3 from 200 to 135 m³/hr/pig.

GPB Version B uses a complete geothermal exchange system to heat and cool theinlet air and solid floors in the barn. Additional floor heating and air heating would be provided by a boiler with the use of PEX tubing in the floor and fin tubes respectively. Preliminary design of the geothermal system for central Minnesota is estimated to be 96 thermal wells (75 m deep) that supply 450 kW of cooling. 

All versions of the Green Pig Barns are expected to reduce emissions due to the incorporation of cooling systems. Building construction costs per pig space are expected to be 1.3 to 2 times higher than typical construction. These costs are offset by an estimated 3-7% increase in ADG and 5- 10% decrease in FE of pork produced. Other benefits include better pig health and worker environment. Using these assumptions annualized net present value per pig space is between $2.43 and $9.03 with 6.0 -12.8 years to payback over the baseline (non-cooled) building. These economic projections would improve significantly with additional gains in animal performance. It is generally thought that these performance gains are anticipated but there is currently no supporting research data to confidently predict the magnitude of these performance improvements on an annual basis in commercial scale operations.

The results sited in this paper indicate that there are cooling alternatives to the traditional evaporative systems used in pig facilities in the Midwestern USA and other pig growing areas of the world that could result in reduced energy and emissions per pound of pork produced while still being economically viable. A geothermal system was evaluated as one possible method to provide cooling for pig buildings that could provide an effective and economicapproach to cooling pig facilities. 

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