2.2. M ASS BALANCE MODELS – N ATURAL TRACER GASES
2.2.1. Heat & Moisture balances
During the last few decades comprehensive research has been carried out on the heat and moisture production of different types of livestock. Predictive equations for total heat and sensible heat production on cattle, pigs and poultry suited to Northern European conditions were developed from reviewed literature in 1978 by Stroem. The literature was further reviewed by CIGR working groups in 1984, 1992 and 2002 where the equations were refined and correction factors for interfering sources such as water evaporation arising from spilt water, wet feed and manure were integrated. The equations presented in detail in section 3.3. Although these equations were derived from climate controlled chambers, thus do not represent real conditions, they are the result from decades of research. Chepete & Xin 2004 recently checked the heat and moisture production equations used for broilers as applied in this report and found they were in good agreement with laboratory results.
The total animal heat production will fundamentally depend on the fact that animals are homothermal and full heat producing, because their heat production due to maintenance and production must be dissipated from their bodies. Homothermal refers to an organism that maintains a constant internal body temperature. Consequently, their body weights, production levels, i.e. their feed intake (energy concentration in feed), will influence their total heat production directly. How the heat is dissipated will depend on the physiology of the animals and on the surroundings with respect to air temperature, radiation from cold/warm surfaces, air velocity and bedding conditions. Furthermore,
animal heat production varies diurnally as a consequence of the animal activity influenced by feeding routines and photoperiod (light vs. darkness). Therefore, it is important to define for which condition the animal heat production is referred. In accordance with common practice, 20°C and normal production conditions on a 24-hour basis are selected as benchmarks for all species, this temperature lies within the thermoneutral zone, this is where the temperature can vary without causing changes in heat dissipation from the animals (Pedersen & Sällvik 2002).
Investigations by Pedersen et al. (1985) showed a reduction in total heat of 2.4% per °C increase (within the temperature range from 15 to 25°C) for broilers of 1.5 kg and a higher reduction for smaller animals. On the basis of available literature, it can be concluded that total heat production with respect to ambient temperature can be described by a linear relation. For temperatures above 30°C, no clear relation can be found between ambient temperature and total heat production. However, it is likely that the heat production will increase for animals that are exposed to sudden temperature changes, because of the metabolization of feed. On the other hand, for animals exposed to constant high temperatures, the feed intake is likely to be reduced, thus resulting in a lower heat production. It is therefore assumed that a linear relation will be acceptable also for ambient temperatures above 30°C.
Total heat (Φtot) is composed of sensible and latent heat. Sensible heat (Φs) is dissipated in accordance with the temperature gradient between the animal deep body temperature and the ambient environment. Latent heat (Φl) dissipates from the animal in the form of moisture from the respiratory track and the skin. Animal deep body temperature is an important index of the physiological status of an animal, physical, chemical and biological temperatures are affected by temperature.
Consequently,Φswill therefore be zero when the ambient temperature is equal to the animal deep body temperature, depending on species, age, and ambient temperature level. At house level some of the sensible heat dissipated is used for evaporation of water from wet surfaces, feed, manure, spilt drinking water. This will result in changes in the partitioning between ΦsandΦl at house level, fortunately these factors are theoretically accounted for in equation 3 (section 3.3.1). Factors affecting the Φs used
for evaporation could be flooring system, stocking density, watering, moisture content of the feed, litter, feeding system, animal activity and relative humidity.
To maintain the animal heat balance and body temperature, Φl will increase with increasing temperature to substitute the decrease inΦs. The partitioning between Φs and Φl is furthermore affected by factors such as type of animal, production stage, body surface area, fur type, dryness of skin, and sweating ability (Pedersen & Sällvik 2002).
Part of the sensible heat produced is lost due to transmission through the building (qb).
This is calculated by the mean value of the heat transmission coefficient (U) weighted by the areas of the different construction elements (walls, ceiling, doors, windows), the mean area of all these elements, A; and the temperature difference between outdoor (TO) and indoor air (Ti) (Owen 1994; NBE-CT-79.1979 & Schauberger et al. 2000), (refer equation 4 section 3.3.1). Buildings for all classes of stock, except adult cattle and most calves, are insulated to a high standard, with U values of the order of 0.6 Wm
-2K-1 (Carpenter. 1974). In Europe and the US a heat conductance of U = 0.4 Wm-2°C-1 or better is usually necessary. In all climates such standards are necessary, both for welfare and for performance, in order to minimize solar heat gain.
The heat and moisture balance methods were expected to have many uncertainties.
Heat production of the animals, radiation input, the influence of the building and building materials. This method has more sinks and sources then the CO2 method and therefore more errors. The accuracy of the thermal and moisture balances is expected to be slightly less then the accuracy of the CO2 method (Scholtens and Van´t Ooster 1994).
This was tested in Experiment 3.
The moisture balance is a tool normally used to obtain a minimum ventilation rate, which guarantees acceptable humidity and CO2 levels within the barn. Latent heat dissipated from the animal in the form of moisture from the respiratory tract and the skin. To maintain the animal heat balance and the body temperature, latent heat will increase with increasing temperature to substitute the decrease in sensible heat. In the moisture balance method it is assumed that only ventilation can remove the moisture produced by the animals.