4th International Conference On Building Energy, Environment

A Calculation Model for The Energy Performance Assessment of Fattening Pig Houses

E. Fabrizio 1, A. Costantino 1 and L. Comba 11Department of Energy, TEBE Research Group

Politecnico di Torino, Torino 10129, Italia

SUMMARY Climate control in animal houses is a relevant issue. In fact, it provides appropriate breeding conditions, but it requires a considerable amount of energy with respect to the overall process consumption. For this reason, studies aimed at its management and optimization are needed. In this work, the main features and the potentialities of a model for the computation of the energy use for climate control and indoor environmental condition of pig houses is presented. In particular, the case of fattening pig houses equipped with mechanical ventilation system is considered. The calculation model is based on a customization of the simple hourly method of ISO 13790 Standard and it determines thermal and electrical energy uses for heating and ventilation purposes, respectively. The hourly indoor environmental conditions provided by the model can be also used to evaluate the quality of the indoor environment of the pig house. This work is part of the EPAnHaus project, which aims at defining an energy performance certification of livestock houses through measurements and development of numerical simulation models.

INTRODUCTION Energy performance of buildings is one of the main topics in the European Union (EU) policies that, through its legislation, has established rigorous requirements aimed to promote and to improve the energy performance of new and existing buildings (European Parliament 2012). Until now, those requirements concern only buildings destined for human use, without considering buildings for the animal production even if their energy consumption for climate control (heating, cooling and ventilation) is not negligible. At the same time, EU has set out important requirements in livestock sector, especially in relation with animal welfare (European Council 2007, 2008), without considering energy related to aspects such as the energy use for climate control in buildings addressed to intensive animal farming.

Climate control plays a fundamental role in animal production, because it allows to guarantee animal health and, at the same time, to maximize the growth rate and the milk or egg production of reared animals (Esmay and Dixon 1986), but it entails energy consumption. This aspect is of particular importance in case of livestock houses where animals that are more sensitive to the climate condition variations are reared, such as broilers, pigs and laying hens. In these houses, high energy consumption values for heating, ventilation and cooling are expected if climate conditions are fully mechanically controlled. In particular, in a pig house for the fattening of pigs, between 34 and 37 kWh∙m-2∙year-1 of electrical energy are used for ventilation and cooling tasks, value that represents about 50% of the total electrical energy consumption of the rearing farm (Costantino et al. 2016).

Despite the importance of this energy share, currently neither calculation methods nor commercial tools for the estimation of the energy consumption due to climate control into animal houses are available. Such a tool would be useful for investigating the opportunities of the energy savings present in this sector, by the increment of use of renewable energies, the adoption of new envelope technologies and the use of new and more performing heating and ventilation systems.

In this paper, a calculation model for the estimation of the energy consumption for climate control in a fattening pig house is presented. This intensive animal production is chosen as object of the model because swine represents the most important meat production in Europe with about 23 million of tons of pig meat produced in 2015 (Eurostat 2016). Furthermore, high ventilation flow rates, evaporative cooling and the presence of high heat loads caused by the high animal stocking density (not common in buildings destined for human use) require ad hoc modeling solution.

The presented model is based on the simple hourly method descripted in the ISO 13790 Standard. The chosen calculation method is considered the most adequate to the purposes of the work (Costantino et al. in press), because:

- it considers the dynamic behavior of the building;- it adopts a time step small enough to take into

account the cooling needs also in cold periods;- it can be easily implemented into spreadsheet or

other computational environment (such asMATLAB), simplifying the modelling of the fanperformance.

The needed boundary conditions (e.g. set points temperatures and heat emissions of the animals) were retrieved from animal physiology manuals (Rossi et al. 2004).

The model provides as results the energy consumption for heating (only when needed) and ventilation (for both Indoor Air Quality control and cooling). Those values are expressed in kWh∙m-2∙year-1 and in Wh∙kgmeat-1. The former unit of measurement is generally used for evaluating the energy performance of building for human uses. The latter is a unit of measurement useful for the farmers and suitable for further analysis based on LCA (Life Cycle Assessment) approach.

In the present work, a practical application to a typical fattening pig house with forced ventilation is also shown.

METHODS The presented calculation model is developed to consider a pig house with the following specifications: - one room with different open boxes for the pig rearing;- a slatted floor that allows the manure to be collected in asewage pit down the floor;

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- a mechanical negative pressure ventilation system withexhaust fans placed at the pit level (pit fans) that remove theair from the sewage pit. The fan layout can be considered asa tunnel ventilation system, since fans are usually placedalong one of the shorter walls of the house.Pig houses with similar characteristics represent the vastmajority of pig houses for fattening pigs across Europe.Assuming these specifications, the indoor environment can beconsidered as one unique thermal zone.

The modeling procedure is based on the following steps: a) specification of the thermo-physical properties of the houseenvelope;b) specification of the initial occupancy and weight gainschedules of the animals;c) specification of the sensible and latent heat production ofthe animals during their life cycle;d) specification of the ventilation schedules (base ventilation,cooling ventilation) and control logic (usually based on thecontrol of the sole indoor air temperature);e) solution, for each time step, of the air heat balance andcalculation of the heating/cooling load or the free runningtemperature;f) solution, for each time step, of the moisture balance;g) calculation of the heating energy requirement;h) calculation of the electricity requirement for ventilation;i) calculation of the global energy consumption and otherperformance indicators (e.g. energy costs)

The model is intended to perform a simulation over a complete year (8760 hours) under typical weather conditions (e.g. TRY-Test Reference Year data).

Modelling of the house

The thermal modelling of the house is done by applying the simplified dynamic model given in the international Standard ISO 13790 (Energy performance of buildings—calculation of energy use for space heating and cooling). It is based on an electric equivalent network made up of five resistances and one capacitor (Marchio et al. 1997). The testing of this model can be found in Roujol et al. (2003) where the BESTEST method and experimental data were used. Even though this simulation method is less detailed than other simulation tools (e.g. EnergyPlus), especially as regards the effect of the solar radiation, this model was adopted since it performs an hourly calculation of heating and cooling loads and it can be implemented into a spreadsheet. This feature makes it easy to modify and highly customizable by implementing other calculations.

The main assumption of the thermal zoning is that the entire pig house is considered as a unique thermal zone of a rectangular shape. Input data (step a) regard the geometrical properties of the house envelope (wall, floor and window dimensions) and the thermo-physical properties of the building components (U-values of envelope components and the optical properties of the windows).

Boundary conditions and internal loads

The main production data required by the model are the number of reared pigs, their stocking density, the duration of each batch (up to 200 days), the empty days between consecutive batches, the initial and final age of the animals during the batch and their final live weight. From the production input data, all the necessary schedules are created by the model. First the pig daily age is computed, then the yearly schedules (8760 values) of every input variable

regarding the occupancy such as pigs weight, sensible heat production and vapor production are computed as a function of the daily age of the animals. All the necessary relations between the daily pig age and the various quantities were determined creating appropriate functions from tabled data by regression analysis (step b). As an example, the equation of the pig weight W as a function of the pig age d is

𝑊 = 0.5829 𝑑 − 22.938 [kg] (1)

which is valid for 90 < d < 300 days of age and for the European heavy fattening pig as reported by Rossi et al. (2004).

The house is usually divided into different boxes that contain up to 10 pigs (Lindley and Whitaker 1996). The optimal range of indoor air temperature depends on the rearing system, on the age of the pigs and on the housing system. In particular, the floor type of the boxes considerably affects these temperature thresholds, as shown in Figure 1. Three main types of floors can be identified:

- slatted floor;- continuous floor;- straw floor.

The higher air flow near the animals allowed by the slatted floor, leads to higher air temperature set points if compared with the ones needed in the cases of continuous floor or straw floor.

Figure 1. Indoor air temperature set points during pig growth, for different types of floors in fattening pig houses (elaboration from Rossi et al. 2004)

Air temperature set points as a function of the pig weight can be easily derived for the three floor cases as a regression. In particular, for slatted floor the air temperature that should be maintained is equal to

𝑇 = 36.734 𝑊−0.1474 [°C] (2)

From the values present in Rossi et al. (2004) the sensible heat emission per pig (step c) is computed as a biquadratic function of body weight W and air temperature T as

Φs,p = 4.265574 + 2.303499 𝑊 + 5,275411 𝑇 0,006278 𝑊2 − 0,183321𝑇2 − 0,029035 𝑊𝑇 [W] (3)

Similarly, the vapour emission per pig is computed as a biquadratic function of body weight W and air temperature T starting from the values provided by Rossi et al. (2004) as

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�̇�v,p = 47,44281 + 1,19069 𝑊 − 6,47397 𝑇 − 0,00923 𝑊2 +

0,20842 𝑇2 + 0,04901 𝑊 ∙ 𝑇 [g

h] (4)

Once defined the pig age and the weight through Eq. (1) and knowing the number of pig present in the house np, all the boundary conditions of the calculations referring to the indoor heat and vapour loads can be obtained.

The production features inputted are the number of pigs and age (days) of the piglets and the duration (days) of the batch. The outdoor boundary conditions are the hourly values of global solar irradiance on the various surfaces of the house, the outdoor air temperature and the outdoor air humidity ratio. In the case of yearly energy assessment, these data come from the TMY database.

The pig houses can be naturally or mechanically ventilated. In the last case, which is considered in the present paper, houses are equipped with fans for both Indoor Air Quality (IAQ) control and cooling purpose. Ventilation for IAQ control is called base ventilation and its purpose is to reduce the contaminants produced by pigs maintaining their concentration below threshold values. Base ventilation flow rate is usually determined as a function of pig weight and number. In the developed calculation model, the volumetric air flow rate required per pig is computed (step d) as a cubic function to the pig weight W as

𝑉�̇� = 1,8 ∙ 10−5 𝑊3 − 0,0046𝑊2 + 0,512 𝑊

− 4,792 [𝑚3

ℎ]

(5)

The energy balance

The energy model is a customization of simple hourly method of the ISO 13790 (2008) Standard. The model is a lumped parameters model (Figure 2) where the thermal behavior of the building is described by means of an equivalent resistive-capacitive electrical network. More in detail, the circuit scheme is made of 6 components: 5 resistances and 1 capacitor connected as reported in Figure 2. The electrical current represents the thermal flow, while the voltage of each circuit node represents the temperature of the modeled house elements. The capacitor Cm represents the thermal capacity of all the building and it is considered on the tm node of temperature. The temperature nodes are the supply air temperature tsup, the external air temperature te,the indoor air temperature tair, the envelope internal surface temperature ts and the building mass temperature tm.

The heating/cooling capacity acts on the air temperature node, while the solar and internal heat gains (that are split between the convective and the radiative parts) act on both the envelope internal surface and the building mass temperature. As shown in Figure 2, the 5 resistances are related to the thermal conductance due to heat transfer coefficient through ventilation (Hve), windows (Htr,w), opaque components (Htr,op, furtherly split between Htr,em and Htr,ms) and the heat transfer coefficient between the internal surfaces of walls and the air temperature (Htr,is). The indoor air temperature can be computed as

𝑡air =𝐻tr,is∙𝑡s+𝐻ve∙𝑡sup+Φia+ΦH/C,nd

𝐻tr,is+𝐻ve [°C] (6)

The full set of equations (step e) for the determination of the conductances and of the heat balance are reported on Appendix C of the ISO 13790:2008 Standard.

Figure 2. Schematic representation of the 5R1Cmodel

The model solves the air heat balance determining the ambient load (heating/cooling load) needed for maintaining the set point temperatures reported in Figure 1. If a heating load is required, the provided hourly value is stored to compute the total heating energy needs QH. If a cooling load is needed inside the house for maintaining the set point temperature, the model considers this hourly cooling load qc to be satisfied by free cooling with outdoor air. This free cooling is provided by fans that regulate their air flow rate. The outdoor air flow rate Vcv that may be necessary to reduce the indoor air temperature Ti to the set point temperature can be computed as

�̇�cv =𝑞c

1.01∙(𝑇i−𝑇a,out)∙

1

ρif 𝑇i − 𝑇a,out > ∆𝑇 [

m3

h] (7)

The outdoor air temperature Ta,out has to be sufficiently lower than the indoor air temperature. The temperature differential T between the indoor air temperature and the outdoor air temperature (e.g. 0.5 °C – 2 °C) is set by the user. This value plays a relevant role in the assessment of the ventilation of the house since it affects the maximum cooling ventilation flow rate. In fact, small values of T lead to the increment of the maximum outdoor air flow rate and, consequently, the maximum air velocity into the house, while large values of T tend to reduce both the previous quantities. In any case, a small T value is favorable since it allows longer periods of free cooling activation. The outdoor air ventilation flow rate considered in the air heat balance is increased (it is the sum of the base ventilation flow rate and the cooling ventilation flow rate) and the indoor temperature conditions are updated.

The humidity balance

The model solves also the humidity mass balance (step f), at each time step, considering the vapor production of the animals, the humidity ratio of the ventilation air entering the house and the humidity ratio of the indoor air. From the humidity ratio of the indoor air and the indoor air temperature, the relative humidity is computed by standard psychrometric formulations. The properties of moisture storage of walls,

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animals and straws on the floor are neglected. Similarly, the moisture production of the sewage that is present on the floor and in the pit under the floor is not considered since the ventilation principle prevents the moisture produced by the sewage to affect the humidity of the part of the house where animals are reared. In fact, the air flows from the vents in the house and through the slatted floor into the sewage pit. Then it is evacuated through exhaust fans.

Modelling of the ventilation energy use

Following the base ventilation flow rate (always present and dependent on the animal weight) and the cooling ventilation flow rate, the exhaust airflow rate that has to be provided by the fans is computed. The energy use of each fan is then determined as

𝐸f = (𝑉V

𝑆𝐹𝑃) ∙ 100 [kWh] (8)

where Ef is the energy use in the time step for ventilation, Vv is the ventilation flow rate of the time step and SFP is the Specific Fan Performance, which relates the electric energy consumption of a fan to its volumetric flow rate as a function of the static pressure. Curves of SFP are provided by certified measurements of commercial fans.

The hourly values of heating load and electricity power consumption for ventilation are integrated during the year in order to have the heating energy need and the electricity need for ventilation (step i). Once assigned the specific cost to each fuel and to the electricity supply, financial indicators can be computed. The indoor air temperature, effective temperature and relative humidity can be plotted for the entire year. Other long-term indicators like HSI (Heat Stress Index) can be computed from the provided results, being valuable information to asses and verify the indoor environmental quality in the analyzed pig house.

EXAMPLE OF APPLICATION The application of the model to a case study is presented in this paragraph. The considered pig house is located in the province of Torino (North-West of Italy) and it has 280 m2 of useful floor area. The house is 17.4 m large, 15.75 m width and 3.5 m eight at ridge level. Its useful volume is about 890 m3. The main thermo-physical properties of the analyzed house are reported in Table 1.

Table 1. Main thermos-physical properties of the analyzed house

Building element U-value

[W∙m-2∙K-1]

κ-value

[KJ∙m-2]

Walls 0.9 70

Floor 1.0 89

Roof 0.7 50

Windows 5.0 [ - ]

The house is used to fatten “heavy pigs”. For this purpose, piglets of about 47 kg are carried to the pig house and they are reared until the final live weight of 150 kg is reached. To reach this target weight, about 180 days are needed: pigs enter in the house at 120 days of age and exit at 300 days of age. When the animals reach the target weight, they are sold to various slaughterhouses.

The input data regarding the pigs are reported in Table 2.

Table 2. Main data regarding the analyzed pig farming

Parameter Value Unit of measurement

Animal stocking density 0.9 pig∙m-2

Number of pigs 247 pigs

Age of entry 120 days

Age of exit 300 days

Batch duration 180 days

Empty days between batches 20 days

Batches per year 1.8 batch∙year-1

This pig house has also been adopted for the model validation comparing in field measurements of indoor environmental conditions and energy use for ventilation with the computational results. Merely two batches can be completed in a year. During the first days of the batch and in case of cold weather, the house has to be heated. The hourly heating load profile during a batch carried out in winter period is reported in Figure 4 and the net yearly energy use for heating is equal to 32 kWh∙m-2∙y-

1. In Figure 5 the hourly profile of the base ventilation flow rate and of the cooling ventilation flow rate (referred to a batch in winter period) are reported. The base ventilation increases as a function of the increase of the pig weight.

Figure 3. Hourly heating load profile during a batch (180 days)

Cooling ventilation is activated to maintain the set point temperature when there is a cooling need and it has a maximum value which is about 7 times higher than the maximum base ventilation flow rate. The effect on the indoor air temperature can be seen on the profiles of Figure 6, where the indoor air temperature and the indoor relative humidity profiles are reported with the outdoor air profile and the set point temperatures (upper and lower set point temperatures are respectively 0.2 °C greater and 0.2 °C lower than the set point of Eq. 2).

The indoor air temperature can be maintained equal to the set point temperature during the first part of the year. When the outdoor temperature and the internal load of animals increase, this target is not fulfilled. Indeed, even though there is the

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activation of the cooling ventilation the indoor air temperature results to be higher than 30°C for several hours during the year, with negative effects on animal health and growth. The relative humidity ranges, for most of the time, between 40% and 80%. A peak of about 100 % is due to the sudden temperature reduction consequent to the empty period.

Figure 4. Hourly ventilation air flow profile during a batch (180days)

The ventilation energy requirement (supplied by electric energy) for both base and cooling ventilation has to be determined once the type and number of fans are defined. Varying the type of fan and its regulation, different values of energy use for ventilation can be obtained. In particular, in case of small fans (each rated at 14400 m3∙h-1 and a SFP of 22 m3∙Wh-1 at 0 Pa of static pressure), four fans are requested and it is possible to estimate the energy use for ventilation equal to 33 kWh∙m-2∙y-1.

A favorable reduction of the energy use for ventilation can be achieved, for example, adopting two larger fans (39000 m3∙h-

1 of maximum airflow and a SFP of 39 m3∙Wh-1 at 0 Pa of static pressure) which will decrease the energy use for ventilation until 19 kWh∙m-2∙y-1. This solution, however, is not feasible since the operation during hours with low ventilation requirements will be compromised by on-off functioning.

A promising solution is the adoption of one variable flow fan as first fan that operates following the variation of the base ventilation flow rate. Other smaller constant air flow fans activate only in case of high ventilation requirements. This may reduce the consumption to 18 kWh∙m-2∙y-1, value that has to be seen as the minimum energy requirement for ventilation.

The energy values (32 kWh∙m-2∙y-1 for heating and 33 kWhe∙m-

2∙y-1 for ventilation) are in line with reference benchmark values for those type of animal houses (Costantino et al. 2016).

Some retrofitting measures may however be taken into consideration for this case study, namely a better thermal insulation of roof and walls (as it reduces the energy requirement for heating and it does not increase significantly the electricity use for ventilation) and the adoption of an evaporative cooling system to reduce the heat stress during hot periods.

In particular, the adoption of evaporative cooling (through a pad of 10 cm with a saturation efficiency of 0.73) may lead to the reduction of the indoor air temperature (see Figure 7) and to a slight reduction of the electric energy use for ventilation (31 kWh∙m-2∙y-1 instead of 33 kWh∙m-2∙y-1).

Figure 5. Hourly profiles of air temperature (left) and relative humidity (right) in the analyzed pig house

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Figure 6. Hourly profiles of air temperature (left) and relative humidity (right) in case of the adoption of evaporative cooling

CONCLUSIONS The basic features of a model for the determination of the long-term energy performance and indoor environmental conditions for a pig house were explained and the model was applied on a simple case study.

This model may be useful for the design of new pig houses because it allows the choice of the best solutions in term of building envelope and climate control system. The presented model may also be useful for evaluating the effectiveness of retrofit measures applied to existent pig houses, from both energy and financial points of view.

ACKNOWLEDGEMENT This work was done through the financial support of the SIR 2014 “EPAnHaus – Energy performance certification of livestock houses” project, grant number RBSI141A3A funded by MIUR [Italian Ministry of Education, University and Research].

REFERENCES Costantino A., Fabrizio E., Biglia A., Cornale P. and Battaglini

L. 2016. Energy use for climate control of animal houses:he state of the art in Europe. Energy Procedia, vol. 101,pp. 184-191.

Costantino A., Ballarini I. and Fabrizio E. in press. “Comparison between simplified and detailed methods for the calculation of heating and cooling energy needs of livestock housing: a case study”, Proceedings of BSA 2017 - 3rd Building Simulation Applications Conference, Bozen, Italy, 8-10 February 2017, in press.

Esmay M.L. and Dixon J.E. 1986. Environmental control for agricultural buildings. Westport, CT: The AVI Publishing Company, INC.

European Council 2007. European Council Directive 2007/43/C of 28th June 2007 on Laying down minimum rules for the protection of chickens kept for meat production.

European Council 2008. European Council Directive 2008/120/EC of 18th December 2008. Laying down minimum standards for the protection of pigs.

European Parliament 2012. European Parliament and Council Directive 2012/27/UE of 25th October 2012 on energy efficiency.

Eurostat 2016. Agriculture, forestry and fishery statistics. 2016 Edition. Belgium: European Union.

ISO [International Standard Organization]. 2008. Energy performance of buildings. Calculation of energy use for space heating and cooling. ISO 13790:2008.

Lindley J.A. and Whitaker J.H. 1996. Agricultural buildings and structures. St Joseph,MI: ASAE.

Marchio D., Millet J.R. and Morisot, O. 1997. Simple modeling for energy consumption esti-mation in air conditioned buildings. Proceedings of Clima 2000, Brussel, Belgium.

Rossi P., Gastaldo A., Ferrari P. 2004. Ricoveri, attrezzature e impianti per l’allevamento dei suini. Verona: Edizioni L’informatore Agrario. [in Italian].

Roujol S., Fleury E., Marchio D., Millet J.R. and Stabat, P. 2003. “Testing the energy simula-tion building model of Consoclim using Bestest […]”. Proceedings of the 8th IBPSA Conference, pp. 1131–1138.

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