Master Programme in Energy Systems Examiner: Ulf Larsson
DEPARTMENT OF TECHNOLOGY AND BUILT ENVIRONMENT
SIMULATION OF FtX VENTILATION TECHNIQUE IN A TYPICAL SWEDISH HOUSE
Adrián Ferreras Pascual June 2010
Master’s Thesis in Energy Systems
PREFACE
Along this project we have received a lot of help and information from many people.
Firstly we would like to thank Peter Hansson, who has been my supervisor and has helped me to carry out this project. Without his advices and help it wouldn´t have been possible to realize my Thesis.
In the same way, I would like to thank Mathias Cehlin for the help he provided me related to IDA software, and also to the rest of the professors I asked my doubts.
Besides, thanks all these people I met this year who listened to me when I was stuck with the Thesis Project and supported and offered their help in those moments.
Finally, I would like to thank to Ulf Larsson for making things easy to present the Thesis Project and also to Juan Artieda for being my opponent.
SUMMARY
This project discusses the effect of reducing the amount of fresh air supplied mechanically to a typical Swedish house, remaining constant the amount of air removed from the building. The new ventilation technique studied through this project is used by the so-called FtX ventilation systems, whose main difference comparing to FTX ones is that the supply airflow rate is reduced.
This reduction of the supply airflow rate is simulated by means of commercial software called IDA Indoor Climate and Energy, which provides the energy consumption in the dwelling so as the temperature in the building and the different airflows.
Firstly, the model of the house is built in IDA taking into account the regulations established by the Swedish Building Code and the materials and size of a typical construction in Scandinavian region. The building is located in Bromma, near Stockholm.
The reason to situate the building in this place is that IDA´s database contains the meteorological data registered in this location during a whole year, so yearly simulations can be carried out using real meteorological data.
After simulating every model, results are gathered and compared.
On the one hand, an energy study is carried out for a whole year in order to determine the optimal quotient between the supply airflow rate and the exhaust airflow rate which minimizes the energy consumption. This study indicates that when the supply airflow rate is 0.95 times the exhaust airflow rate, the energy consumed by the building is minimal.
On the other hand, an economical study is done. To calculate the economical cost of the energy purchased by the building, the tariffs of a company which sells district heating and electricity in Stockholm are used. Considering these data and the energy consumption for the whole year, it is calculated the economical cost in each case. The optimal ventilation rate, in monetary terms, is (Qs/Qe) ≅0.9. In this case, the energy cost amounts to 13,880 SEK.
TABLE OF CONTENTS
Page
1. Introduction 1
2. Purpose and limitations 3
3. Methodology and theoretical framework 5
3.1. Swedish Building Code 5
3.2. Location and climate 5
3.3. The house 6
3.4. Considerations about the energy consumption 7
3.5. Ventilation requirements 8
3.6. Heating system 10
3.7. Air infiltration 10
3.8. Models of the building in IDA 12
3.8.1. Basic model 13
3.8.2. Multi-zone model 19
3.9. Model simulation 21
3.10. Energy balance 21
4. Results 23
4.1. Whole year simulation for basic model 23
4.2. Basic model behaviour for typical winter and summer days 30
4.3. Economical study 38
4.4. Comparison between the basic model and the multi-zone one 40
5. Discussion 45
5.1. Whole year simulation for basic model 45
5.2. Typical winter and summer days for basic model 46
5.3. Economical analysis 47
5.4. Comparison between the multi-zone and the basic model 48
6. Conclusions 49
7. References 51
Page APPENDIX
APPENDIX A- Materials for the different enclosing parts 53
APPENDIX B- Plans of the house 55
APPENDIX C- ψ-value of the different thermal bridges 59 APPENDIX D- Schedule of occupants, equipment and lighting 63 APPENDIX E- Leaks through the building enclosing parts 69
APPENDIX F- Values from simulations 71
F.1. Basic model results for whole year simulation 71 F.2. Basic model results for winter and summer days 73 F.3. Multi-zone model results for whole year simulation 77
APPENDIX G- Economical study 79
1 INTRODUCTION
One of the most important objectives of design for the buildings is the thermal comfort of the occupants. Scandinavia is a typical cold region of the world, thus it is really important to ensure a suitable indoor climate in the buildings. Currently, 35% of the energy consumed in Sweden is due to residential and service sector. It is supposed that around 20 or 25% of the energy use is because of space heating and domestic hot water production.
These facts highlight that heating and ventilation systems are extremely important in this region. Besides, isolation is also significant regarding to building construction. As better isolated houses are as less energy will be demanded by space heating systems.
One of the most typical ventilation systems installed in Sweden is the so-called FTX system, exhaust and supply air ventilation with heat recovery. This system extracts polluted air from rooms such as kitchen or bathroom and supplies fresh air to bedrooms and living room. The heat contained in the exhaust air is released to the fresh air by a heat exchanger installed in the attic of the house. This system allows saving energy and the saving can amount to 50% of the energy consumed by the same house without heat exchanger.
Recently, some researchers have proposed a new ventilation technique similar to FTX system. The main difference between both ventilation systems is that the new one reduces the amount of air supplied to the dwelling, remaining constant the exhaust air flow.
2 Purpose and limitations
The objective of this project is to study a new ventilation technique called FtX system and compare its results with the ones got implementing the FTX system. Both ventilation systems are going to be studied in a typical Swedish house located in Stockholm. The study is carried out in Stockholm because IDA, which is the software used to simulate the building, contains the meteorological data registered there during one year, 1977, in this location.
Once the model of the dwelling is built, both ventilation techniques will be implemented in the building and each model will be simulated for the whole year, to get a global view about what is the building´s heat balance in each case. Besides, some particular dates will be simulated to see the building´s behaviour during different seasons.
The results obtained will be used to evaluate the effectiveness of each ventilation system, so as to compare the energy savings produced by changing the ventilation technique.
The aspect studied through this project is the energy consumption for space heating, so the indoor temperature during summer time is not that important, what it means, that no cooling devices are installed to keep the temperature inside between certain boundaries.
The model of leaks in IDA is based on theoretical equations and no measurements were done to check if the behaviour described by equations corresponds to the real situation in a house as the proposed one.
The computational requirements of the multi-zone model makes difficult to simulate its behaviour with high accuracy for long periods of time. So, the yearly simulations of the multi-zone model may be less precise than the basic model.
The building is located in Stockholm, but Sweden is a large country and the meteorological differences between the northern and the southern region are quite important. So, the results got by this model can´t be extrapolated to any location in Sweden. So, for different regions or places such as Kiruna or Malmo the behaviour of the building with regards to the energy consumption could be different.
3 Methodology and theoretical framework
The first step to accomplish with the project is doing an exhaustive review of the Swedish Building Code (BBR), and other sources which enables to know the Swedish building style for dwellings.
Once it is got the background about Swedish construction and Swedish laws referred to buildings the new process is started. The next step is building a model in the computer where the different ventilation systems will be implemented and simulated. In this project the software chosen to build the model and simulate the ventilation techniques is IDA Indoor Climate and Energy 3.0.
When the model of the house is perfectly defined and detailed the different ventilation techniques are simulated in it. The software will provide the results which then will be analyzed determining which system is more profitable, taking into account the surrounding conditions.
Next, it will be explained the model and how was it built.
3.1 Swedish Building Code
It is the set of documents which rules how a construction must be carried out in Sweden.
The main aspects of the Swedish Building Code [1] related to this project will be presented next:
• The maximum value for the average heat transfer coefficient must not exceed 0.5 W/m2⋅K.
• The supply airflow must be equal or higher to 0.35 l/s⋅m2.
• The specific energy consumption of the house must not exceed 110 kWh per m2 of floor area per year, taking into account the region where the building is settled.
This energy can be used for space heating purposes and also for tap water.
3.2 Location and climate
The building is located in Stockholm, Sweden, N59.35º. To be more precise the house is situated in Bromma. The reason to carry out the study of the house in Stockholm is because IDA software contains in its database every position parameter necessary to do a simulation of a building in Stockholm. This data were taken from ASHRAE Fundamentals 2001.
Besides, it also contains the meteorological data registered during 1977 in a weather station in Bromma.
To simulate the building´s behaviour it was necessary to define the ground temperature.
For this purpose, it was taken the annual mean temperature in Stockholm, 6.6ºC, as the ground temperature for the whole year [2].
The location of the building is urban. This fact will determine the wind speed in the building surroundings, but this will be explained in detail subsequently.
Moreover, the location of the building will also determine other aspects of the study, such as the amount of energy which can consume the building.
3.3 The house
Below, it will be presented the general features of the house modelled:
• Heated floor area → 130.83 m2.
• Shallow foundation, slab on ground.
• One floor, height 2.6 m.
• Ventilated attic.
• Building´s height 5.
• Roof pitch angle → 27º
The materials which compose the different enclosing parts are explained in detail in Appendix A. The U-value of each surface of the building is shown in Table 1:
Enclosing part U-value [W/m2⋅K] Area [m2]
Floor & Ground 0.09 130.83
Walls 0.173 93.77
Windows 1.2 21.11
Doors 1 7.48
Ceiling 0.104 130.83
TOTAL U⋅A [W/K] 74.37
Table 1 U-value for the enclosing parts
According to [1], if the dwelling doesn´t accomplish with the requirements given in Clauses 9:2 and 9:3, what involves that the floor area doesn´t exceed 100 m2; the window area should be, at least, 10% of the floor area. Besides, there is no requirement for cooling. The values for the house proposed also accomplish with these maximum values established in [1].
Besides the U-value of each enclosing part, it is also necessary to calculate the thermal bridges in the junctions of different surface. In table 2 it can be seen the ψ-values for the different junctions:
Junction ψ[W/m⋅K] L[m]
Shallow Foundation - External Wall 0.0581 47.06 External Wall – External Wall 0.0352 10.4 External Wall – Window Frame 0.02 56.3 External Wall – Ceiling and Roof 0.03 47.06
External Wall – Outside Door 0.02 20
TOTAL Ψ⋅L [W/K] 6.04
Table 2 ψψψψ-value for the different thermal bridges
To determine the ψ-value for the different junctions between surfaces it was used two different sources. On the one hand it was used a software called UNorm, and on the other hand it was used information provided by ISOVER [3]. Every ψ-value is explained in detail in Appendix C.
According to [1], the average heat transfer coefficient Um is calculated in accordance with the following equation (1):
= ∑ ⋅ ∑ ⋅ ∑=. . . = 0.21"/$%
⋅
& (1)Where,
o Ui → Is the heat transfer coefficient for the individual part of the building i (W/m2K).
o Ai → Is the surface area of the individual part of the building i in contact with heated indoor air (m2).
o ψk → Is the heat transfer coefficient for the thermal bridge (W/m⋅K).
o lk → Is the length of the linear thermal bridge k (m).
o χj → Is the heat transfer coefficient for the thermal bridge j acting at one point (W/K). In this study its effect was neglected.
o Aom → Is the total surface area of the enclosing parts of the building in contact with heated indoor air (m2).
The design of the house is based on an example of a typical Swedish home. The dwelling is composed of three bedrooms, one living room, one kitchen together with the dining room, one bathroom, one store, one laundry room and a hall and a corridor plus one wardrobe. The plans of the house can be seen in Appendix B.
3.4 Considerations about the energy consumption
As it was mentioned previously the maximum specific energy consumption of the house must not exceed 110 kWh per m2 of floor area per year, in the region of Sweden where Stockholm is situated. This means that the amount of energy available during the whole year is:
' = 110 ∗ 130.83 ≅ 14,333 ."ℎ/0123
This amount of energy can be used for space heating and also to heat tap water. As in this project the main objective is determine the energy consumed by the house for space heating, it will be calculated the amount of heat available for this purpose. So, firstly it will be calculated the amount of heat consumed by a family for tap water during a year, and then in will be determined the energy which can be used for space heating.
According to [4], the hot water consumption in the single-family houses is 1.8 kWh per day and per person. So the total amount of tap water consumed in terms of energy during the year will be:
45678975:;< = 1.8 ∗ 365 ∗ 4 = 2,628 ."ℎ/0123
This is the amount of hot water consumed, but taking into account that the heat for majority of the houses in Sweden is provided by district heating system, it will have to be applied a factor to reflect the efficiency of this system, which is very high but lower than 100%. According to [5], The efficiency of district heating is around 98%, so the amount of energy consumed for tap water is:
497< =2,628
0.98 ≅ 2,682."ℎ 0123
Once it is known the heat used for tap water, it is automatically calculated the heat available for space heating. This amounts to 11,651 kWh per year. This amount includes the energy (electricity) consumed by fans and pumps that the HVAC system requires to work.
3.5 Ventilation requirements
As it is mentioned in [1], the air flow supplied to the building must be equal or higher to 0.35 l/s⋅m2. The building is provided with a constant air volume flow system, so called CAV (Constant Air Volume). This system ensures a constant amount of supply and exhaust air in every zone of the building. These amounts of supply and exhaust air should be defined by the designer, and must be equal or higher to 0.35 l/s⋅m2, in the case of the supply air.
The supply air must meet certain conditions before entering the building. This involves the need of energy for supply air conditioning. The amount of energy needed for supply air conditioning can be reduced by installing an ERV system (Energy Recovery Ventilation system). This kind of ventilation technique reduces the need of heat by recovering it from exhaust air for warming or cooling of supply air. In Sweden, the typical way to recover heat is by using heat exchangers [6]. The system installed in the
house is a direct recuperative system. The temperature efficiency of the heat exchanger is 0.7 when the supply air is equal to the exhaust air flow, according to [2] and [6].
As the purpose of this project is study the behaviour of the building when it is varied the relation between the supply air flow and the exhaust air flow, it is necessary to study also the answer of the heat exchanger when this quotient is changed. So, taking into account that when the supply air flow is equal to the exhaust air flow the efficiency of the heat exchanger is 0.7 (η0), when the exhaust air flow is higher than the supply air flow, the efficiency of the heat exchanger will be increased. This equation and the explanation is based on [7],. This is due to the exhaust air contains the same amount of energy, because the flow has not been altered, but as the supply air flow has decreased, it will be possible to heat the supply air to a higher temperature, closer to the desired one. For instance, if the supply air flow was very close to zero, it would be possible to heat the supply air almost to the exhaust temperature because the exhaust air flow would contain the energy enough to heat it, and the efficiency of the heat exchanger would be very close to 1. So, it is possible to say that the heat exchanger efficiency varies with the relation between supply and exhaust air flows as it is shown in equation 2:
@ = @ + B
⋅
C1 − @ E⋅
C1 −FFGHIJKLMNHGOE (2)
In figures 1 and 2 can be seen how the efficiency of the heat exchanger varies with the air flows and also with the α parameter, which can take values between zero and one, and determines the importance of the relationship between air flows in the heat exchanger. As lower α, as smaller the effect of varying the quotient of air flows.
0 0,2 0,4 0,6 0,8 1 1,2
0 0,5 1 1,5 2
η
Qsupply/Qexhaust
Efficiency α = 1
0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1
0 0,5 1 1,5 2
η
Qsupply/Qexhaust
Efficiency α = 0.6 α = 0.6
Fig. 2 Efficiency variation with air flows, α=0.6 Fig. 1 Efficiency variation with air flows, α=1
3.6 Heating system
The house is heated by water radiators. The energy for the heating system comes from district heating, so the efficiency of the system is quite larger than if the energy would be produced by an oil boiler. In this case the efficiency of the district heating is supposed to be around 98% [5].
The heating system is also the responsible of heating the supply air until the desired temperature when the exhaust air doesn´t contain heat enough to warm it. The efficiency of the heating coil in the ventilation system is supposed to be 100%.
3.7 Air infiltration
As it was mentioned previously, the building studied is equipped with a ventilation system which provides fresh air to the house. Nevertheless, it exists other ways which also contribute to the amount of fresh air which enters into the building. This other way is an unintentional air flow and is referred to as air leakages. Air can infiltrate from outside to inside or suck out air from the house through air leakages, depending on the pressure difference between indoors and outdoors.
The amount of air leakage is determined by the pressure difference and the air-tightness of the building envelope. The pressure difference will be determined by the sum of wind pressure, stack pressure and mechanical ventilation components.
The pressure difference because of the wind between inside and outside each envelope part of the building will be determined by the equation 3:
∆QR = CST− ST:E ∙VN%∙WX (3) Where:
o Cp → Is a wind pressure coefficient empirically determined [-].
o Cpi → Is a wind pressure coefficient for the interior of the building [-].
o ρa → Is the air density [kg/m3].
o v → Is the wind speed, taking into account the building height and where is the building located [m/s].
The values for the external wind pressure coefficient were taken from [9] and are shown in table 3, while Cpi is supposed to be constant for every surface and its value is -0.3, which means that air will be sucked into the house from outside.
LOCATION Wind Angle: 0º Wind Angle: 45º
Face 1 0.4 0.1
Face 2 -0.2 0.1
Face 3 -0.3 -0.35
Face 4 -0.3 -0.35
Roof, front -0.35 -0.45
Roof, rear -0.35 -0.45
Table 3 Wind pressure coefficients
The wind speed in the building´s surroundings will be determined by equation 4:
Y = Z= ∙ . ∙ [< (4)
Where:
o Uz → Is the wind speed at the building height [m/s].
o Um → Is the wind speed at the weather station at a height of 10 m [m/s].
o k, a → Are constants which value depends on where the house is located.
These values were also taken from [9].
Besides the wind pressure and as it was said previously, it must be taken into account the stack pressure (∆Ps). This means the pressure difference between inside and outside the building generated because of the temperature difference, what involves an air density difference. In practice this pressure difference can be calculated by equation 5, following the recommendations of [8]:
∆Q\ = 0.043 ∙ ∆] ∙ ℎ (5)
Where:
o ∆T → Is the temperature difference between inside and outside [ºC].
o h → Is the building height [m].
Finally, the mechanical ventilation can introduce a pressure difference over the building envelope (∆Pv). In this case the system installed is a balanced ventilation, but it doesn´t introduce any pressure difference when the supply air flow is equal to the exhaust air flow.
The global pressure difference between inside and outside the building will be calculated by the equation 6:
∆Q = ∆QR+ ∆Q\+ ∆QW (6)
Once it is known the pressure difference in each envelope part, it must be calculated the air flow through the leakages in each surface. According to [8], this can be calculated thanks to the equation 7:
1 3
2 4
θ
N
S
^: = _`∙ a:∙ ∆bc d (7) Where:
o qi → Is the air flow through the leakage of surface i [m3/s].
o lf → Is the maximum air flow rate measured at 50 Pa [l/s⋅m2]. Nowadays, [1]
doesn´t suggest any specific value for lf but in the year 2003, 0.8 [l/s⋅m2] was recommended, so it will be used in these calculations.
o Ai → Is the area of the surface i [m2].
o β → Is a parameter which represent if the air flow is laminar or turbulent. In this case and following the recommendations of [8], the value used was 0.7, which is nor turbulent, neither laminar.
3.8 Models of the building in IDA
As it was mentioned previously, in order to study the behaviour of the house, it was used a software called IDA Energy and Indoor Climate. This program studies the indoor climate of individual zones within a building, as well as calculates the energy consumption for the entire building [10].
Firstly, a brief introduction about the program will be presented, and then it will be explained the models built in IDA, the implementations of the different ventilation techniques in the models and their simulations.
IDA consists of three different levels of user interface, the simplest called wizard, the medium level called standard and the advanced level [10]. Through this project standard and advanced levels were used, nevertheless, the majority of simulations were carried out using the standard level, while the advanced one were only applied to study some small details. The standard level is quite easy to operate, while the advanced one is more complicated. In the last one, the simulation model is defined in form of connected component models, whose meaning is defined by equations [10]. On the one hand, the advanced level allows the user to take a more detailed view of what is happening in the simulation model he/she built. On the other hand, the advanced level requires a deep knowledge of the software and the simulation which is going to be carried out, and many times it is difficult to control every parameter necessary to simulate the model properly.
One of the main advantages in the advanced level is that it enables to examine all equations, parameters and variables, so the time evolution of the variables can be studied [10].
Below, it will be explained the models built in IDA and the simulations carried out in each one of them.
3.8.1 Basic model
The first model of the house built in IDA consists of one single zone. This size of the zone is the same as the one presented in the explanation of the house, so the area of the zone 130.83 m2. The dimensions are 14.52x9.01 m. The height of the first floor is 2.6 m and the attic is ventilated. The height of the building is 5 m, as it was mentioned previously.
The building is located near Stockholm, in Bromma. This was the place chosen because IDA database contains meteorological data for a whole year (1977) in this location. The other place in Sweden were the simulations could have been carried out were Kalmar and Goteborg, but both of them are southern Stockholm, so it was thought that the best option was this one. The meteorological database contains air temperature, relative humidity, wind direction and wind speed, direct normal radiation and diffuse radiation. Besides, for each location IDA has a default data for the parameters mentioned previously which are used to carry out simulations during typical summer and winter days. The rest of parameters which define a location are the latitude, the longitude, the elevation above the sea level and the time zone.
The materials of the external walls are the ones defined in Appendix A, and the U-value for each enclosing part is shown in table 1, as well as its global dimensions.
In order to define the materials which compose each enclosing part of the building, IDA is equipped with a Default tab where the user can define the default configuration for each construction of the building, floor, ceiling, roof, walls and so on. The materials for each enclosing part can be chosen from already defined surfaces saved in the database, or alternatively, the user can define a new resource. When the user defines a new resource, it will have to be defined the materials and their thickness. The components of the new resource can also be loaded from the database or defined by the user. To introduce a new material, it must be defined its heat conductivity, its density and its specific heat. In this model the contribution of some materials, as the polythene layer was neglected when the U-value was calculated because of its scarce importance.
Besides the U-value for each enclosing part, it has to be introduced the loss factor for thermal bridges. Its calculation is automatic taking into account the values collected in table 2 and equation 1. The global loss factor for thermal bridges is 6.04 [W/ºC]. As the model has only one zone the loss factor for thermal bridges doesn´t have to be distributed among different rooms.
Once the enclosing parts of the building are defined the next step is defining their function. In the case of external walls and the floor, the program assigns them automatically as external surfaces. The case of the ceiling is different. As the attic of the building is ventilated, the ceiling makes the function of the roof; even the roof is also defined. In order to represent this, there is the option in IDA of connecting one surface to another. So, in this case, the ceiling of the zone is connected to the roof. By this way, it is possible take into account that the attic is ventilated.
It is also necessary to insert the different components of the walls, such as doors or windows. For this purpose, IDA enables the user introduce different objects in each surface. The basic objects inserted in the walls are windows and doors, whose size and position is defined by the user. There are thirteen windows in the house and four different window´s sizes. There are seven windows whose dimensions are 1x1.2 m, one window, 0.65x1.2 m, three windows, whose size is 0.65x0.65 m, and two more whose dimension is 2.2x1.3 m. All of them are triple glazed window and their U-value, as it was mentioned in Table 1, is 1.2 W/k⋅m2. There are also three doors, two of them whose size is 1.3x2.2 m, and one whose dimension is 0.8x2.2 m. Their U-value is 1 W/k⋅m2. In this section, it has to be defined the opening schedule of doors and windows. In this project, it was established that all the windows and doors remain closed during all the simulation time.
The main reason is that the heat balance is easier to do if there is no heat transmission through opened doors and windows. Normally in Sweden, doors and windows are closed, and that´s why ventilation systems are installed in the building, to avoid natural ventilation. If during winter windows were opened, a large amount of energy would be lost through them, so ventilation systems help to avoid this energy waste.
Each zone of the building, in this case there is only one zone, contains three different loads, occupants, equipment and lights. Its behaviour must be defined by the user. The occupant load represents the number of people which is in a room. It has to be defined the metabolism, energy released by the body, and the insulation occupants wear, clothing [2].
Besides, it has to be determined the time while people are in the zone.
The equipment represents every electrical appliance which is used in the building. In this case, all the electrical appliances which usually are used in different rooms in a house were considered to work as a unique electrical appliance releasing heat. According to [11], the electricity consumption of electrical appliances in a house is defined by equation 8.
' = 2500 + 800 ∙ eº g1hg_1 [l7<9jk6] (8) Taking into consideration what [11] says, as the house is bigger than 120 m2, four persons are supposed to live in the dwelling. So, the amount of electricity consumed by the
appliances and internal lighting system is 5700 kWh per year. Supposing that around 70%
of the energy consumed by electrical appliances turns into heat, the amount of heat released by the electrical appliances and lights during the year is around 3,990 kWh per year, or 10.93 kWh per day. So, in the simulations the data related to equipment and lights will indicate the amount of heat released by them, but they will not refer to the amount of electricity consumed by them. To calculate the electricity consumed by these devices, it will have to divide the amount of heat released by 0.7. This is important when calculating the economical costs of the energy, but it is not so important when doing the energy balance of the house. It must be defined a schedule for the working time of the electrical appliances. In this schedule, besides indicating the working time, it is determined the amount of energy consumed in each instalment schedule.
Finally, it is defined the lighting in each zone. In this case, the lights of the whole building are considered as a single one whose power is the sum of the lights installed in a normal house. As it happened with the other loads, a lighting schedule should be defined.
The schedules related to occupants, equipment and lighting can be seen in Appendix D.
IDA enables the option of defining the internal masses in the room. These ones can be wall masses or convective internal masses, furniture. In the model built for this project it was only considered the furniture. It was considered that 30% of the floor area was occupied by furniture, so its area was 39 m2. For furniture, it was used the default configuration implemented in IDA, so the heat transfer coefficient is 6 W/k⋅m2.
Apart from doors and windows other objects can be inserted in the enclosing parts of the building and also in the internal walls, which in this model are not considered because there is only one zone. One of these objects is leak.
Leaks define how tight a construction is. A perfectly built house shouldn´t have any leak, but this is impossible. So, leaks are also introduced in the model. Through the different leaks outside air will infiltrate into the house and inside air will exfiltrate to the outside, depending on the pressure difference in each surface. Working with IDA, leaks can be characterized by two alternative ways. The first one consists of defining the leak area at 4 Pa. The unit of the leak area is m2. The second way is setting a power low coefficient (k) and a power low exponent (β). These two parameters characterize the equation which will rule the behaviour of the leak. A similar equation to the one used by IDA was mentioned previously in chapter 3.7, equation number 6. Nevertheless, the equation 9 is the one used by IDA:
n: = o:∙ _`∙ a: ∙ ∆bc d = . ∙ ∆Qd (9) Where:
o mi → Is the mass flow of air through leaks [kg/s].
o ρi → Is the density of the air [kg/m3].
o k → Is the power low coefficient.
The different leaks inserted in each surface and its coefficient value is explained more in- depth in Appendix E.
As it was said in chapter 3.7, the pressure difference through an enclosing surface is partly determined by the wind speed and wind direction, it is called wind pressure. In order to characterize the effect of the wind, IDA enables to introduce pressure coefficients for each enclosing surface of the building. These coefficients were calculated following the rules explained in chapter 3.7, so according to [9]. The pressure coefficients introduced in the program, depending on the wind direction, for each surface of the building are collected in table 4:
ANGLE Face 1 Face 2 Face 3 Face 4 Roof
0º 0.7 0 0.1 0 -0.05
45º 0.4 0.4 -0.05 -0.05 -0.15
90º 0 0.7 0 0.1 -0.05
135º -0.05 0.4 0.4 -0.05 -0.15
180º 0.1 0 0.7 0 -0.05
225º -0.05 -0.05 0.4 0.4 -0.15
270º 0 0.1 0 0.7 -0.05
315º 0.4 -0.05 -0.05 0.4 -0.15
Table 4 Pressure coefficients
Taking into account these pressure coefficients IDA calculates the wind pressure on each enclosing part. The wind speed and direction is taken from the climate files that IDA contains in its database. Besides, it should be determined the wind profile of the location.
This step is necessary because the wind speed measured in the weather station is not the same as in the place where the house is situated. So, thanks to equation 4, the wind speed is calculated in the location of the building. As the wind profile chosen is urban, according to [9], k and a parameters take the values shown in table 5:
Terrain coefficient K a Urban 0.35 0.25
Table 5 Constants for interveining terrain
It can also be introduced shading objects whose height and position have to be determined by the user. In this case, it is supposed not to be any object which shades the building.
The last object used in the model is the water radiator. Those devices are inserted on wall surfaces. According to [10], the heat emitted by water radiators is calculated by equation 10:
Q = & ∙ _ ∙ p]q (10)
Where:
o l → Is the device length.
o dT → Is the temperature difference between the water and the zone air.
o K, N → Are constants characterizing a device of a certain height.
Normally, the user needs only to designate surface area and the design water flow.
Alternatively, an easier possibility of defining the water radiator is inputting the power given by the device at the specified temperature conditions. In this case, IDA calculates K and the design mass flow automatically.
In the Controller Setpoints box the user can define the comfort temperature for each zone.
This comfort temperature will determine the minimal temperature in the room which in turn defines the energy consumption of the water radiators. In this model, the comfort temperature was established between 25º and 27ºC. The minimum comfort temperature value was established to be 25 degrees because of the Fanger´s comfort indices.
According to the results got in IDA, percentage of dissatisfied people when the temperature was between these two values was the minimum. Nevertheless, in Sweden, usually the indoor temperature is set around 22 degrees.
The hot water supplied to the water radiators is produced by the Primary System.
According to [10], it consists of seven components; however this project is mainly focused on two of them, the boiler and the controller for hot water supply temperature. To define the boiler, it is necessary determining its efficiency and its maximum heating capacity. In this case, as the hot water supplier is district heating system, the efficiency of the boiler is supposed to be 98% [5]. The maximum capacity of the boiler is assumed to be unlimited, so in this case, it is established that its value is 99,999 kW. To characterize completely the boiler, the user can designate the pump efficiency, the outlet pressure at full pump speed and the boiler and circuit mass, but these parameters are not that important. On the other hand, the controller for supply heating water temperature determines the temperature of the heating water depending on the ambient temperature.
The diagram which rules this model is shown in figure 3:
Fig. 3 Heating water temperature
As it was impossible to define a jump at 10ºC, it was establish that the supply temperature at 10º was 55ºC and at 11º was 20ºC. This decrease of the heating water temperature is due to the fact that during summer time, even ambient temperature was high enough, the water radiators still consumed a lot of energy. By this way that problem was avoided.
Cooling units are not taken into account through this project because the main interest is the energy consumed to warm the zones. So, no changes were done to the chiller and its controller, they use the default parameters implemented in IDA.
To complete the HVAC system the model contains the ventilation system, which is one of the main parts of this project.
Firstly, it is described and characterized the behaviour of the Air Handling Unit. It consists of eight components: the supply air temperature setpoint controller, the exhaust fan, the heat exchanger, the heating coil, the cooling coil, the supply fan, the schedule for operation of both fans and the schedule for the operation of the heat exchanger. It will be mentioned briefly the operation of most of them, and only the behaviour of the heat exchanger will be explained more in-depth.
The supply air temperature is fixed constant by the setpoint controller. This temperature is supposed to be 16ºC, but due to the supply fan set after the coils, the supply air temperature is increased by the number of degrees specified by the user [10], in this case, one. So, the supply air temperature is 17ºC. According to [2], the pressure drop in the supply air duct is 980 Pa, while the pressure drop in the exhaust air duct is 820 Pa.
The exhaust and supply fan are characterized by their efficiency, 0.6, and by the pressure they have to provide to the air. This pressure represents the losses in the supply and exhaust ducts. The SFP of fans is a little bit higher than the one established in the
Swedish Building Code. Probably if the efficiency of fans would be a bit higher, it will accomplish with the rules.
The heating and cooling coils are the responsible of warming up or cooling down the supplied air when the exhaust air doesn´t contain heat enough to get the desired supply temperature. The efficiency of both coils is 1.
The schedule for operation of both fans and the schedule for operation of the heat exchanger is the same. The three systems are always on.
Finally, it will be explained the heat exchanger of the Air Handling Unit. As the ventilation system is an ERV (Energy Recovery Ventilation System) the heat exchanger becomes a heat recovery unit. To characterize the heat exchanger its efficiency should be determined. Besides, it should be indicated a temperature limit for exhaust air stream to avoid freezing. In this case, and as it was mentioned in chapter 3.5, the temperature efficiency of the heat exchanger is 0.7 when the supply air flow is equal to the exhaust air flow, [2] and [6]. In case that the quotient between supply and exhaust air flows varies, the efficiency of the heat exchanger will also be altered following the rule characterized by equation 2, and taking into account that α parameter takes 1 as its value. The behaviour of the efficiency of the heat exchanger is shown in figure 2.
On the other hand, it will be explained the behaviour of the Air Handling Unit in each zone. In this case, as there is only one zone, this fact takes no importance, but in multi- zone building it determines the behaviour of the ventilation system in each room. The type of ventilation system is a constant air volume type, so the amount of fresh air provided to each zone is fixed. Taking into consideration [1], the minimum amount of fresh air supplied to a dwelling must be 0.35 l/s⋅m2. This is another parameter requested by IDA. In this model this value is 0.4 l/s⋅m2, a little bit higher than the minimal amount proposed by [1]. The Air Handling Unit creates well-mixed ventilation. This parameter can be changed in the Gradient Calculation box by Displacement or Fixed Gradient, but in this project a well-mixed gradient was chosen. The gradient calculation selected influences, for instance, the air removed via exhaust air or leaks, when the option chosen is not well-mixed zone. Finally, it should be defined the quotient between the supply airflow and the exhaust airflow. Through this project, it is tried to study the effect of changing this quotient, so this parameter will be altered from one to other simulation.
3.8.2 Multi-zone model
In this chapter, it will be only presented the changes between the basic model and the multi-zone model, so some common parts for both of them will not be mentioned here.
The building´s dimensions and location is the same as in the basic model. Also, the
materials of the enclosing parts are identical. In this case, as the house is separated in different zones it has to be defined the internal walls which separate rooms. Those walls are not that important in the calculation of the energy balance of the building, but they are also explained in Appendix A. The U-value of internal walls is around 0.6 W/m2⋅K.
The global loss factor for thermal bridges is the same in both models, but in the multi- zone one it has to be indicated the value of this factor for each room. This is due to using IDA when a zone is defined it requests for the loss factor for thermal bridges in each room. In Appendix C there is a table which shows the value of the loss factor for thermal bridges.
The function of each enclosing part is defined as in the basic model. The components of external surfaces are inserted as in the basic model. The unique difference is that in the multi-zone model it has to be inserted doors in the internal walls to connect one room to other.
The occupant, equipment and light loads should be defined for each zone. Nevertheless, the global contribution of equipment and lighting for the energy balance of the building is the same in both models. The occupants’ pattern was a little bit more difficult to model, but it is quite similar in both cases. The schedules related to occupants, equipment and lighting in the multi-zone model can also be seen in Appendix D. Besides, the internal masses are almost the same for both models.
Now, it will be explained the main difference between the two models which regards to leaks. In basic model, the leaks were characterized by a power low coefficient (k) and a power low exponent (β), as it happens in the multi-zone model. These coefficients were calculated in the previous case according to equation 8, however in the multi-zone model the power low coefficient value is divided by 2. This change was introduced to get more similar results in both models. This fact will be shown in Results chapter and discussed later.
The pressure coefficients for each enclosing part of the building are the same in both models. All these pressure coefficients are gathered in table 4. The pressure due to the wind acting at any point on an enclosing part is automatically calculated by IDA, taking into account the pressure coefficients and the wind speed and direction in the place where the house is located. The wind profile chosen in this model is the same as in the previous one, urban, so the parameters are also identical, table 5.
The Primary System behaviour is the same as in the basic model. The unique difference between both models is that in the multi-zone one the radiators are laid out in the different zones which make up the house. The controller´s schedule for the heating system is also the same in both models.
Finally, it will be explained the role of the Air Handling Unit in the multi-zone model. Its behaviour is basically the same as in the previous model. The common components for the whole building are modelled as in the basic case. In each zone, the behaviour of the Air Handling Unit is also equal to the basic model. The only difference between them is that in each simulation of the multi-zone model, the relation between supply and exhaust air flows has to be altered for every zone.
3.9 Model simulation
There are two different kinds of zone models in IDA, the climate one and the energy one.
The climate model is very detailed, while the energy one has a lower level of accuracy and it is based on a mean radiant temperature. The problem of the climate model is that is available only for rectangular zones [10]. The zone model determines the type of simulation carried out by IDA.
Besides the type of zone model, it also has to be defined the type of simulation. This is determined by the Simulation Data tab. The first choice which should be done is selecting between a periodic or a dynamic simulation [10]. Choosing the first one, a certain day is simulated a number of times until the system has stabilized. The simulation date is fixed by the user.
A dynamic simulation starts at a particular date and ends at another date; both dates are chosen by the user. When a dynamic simulation is going to be carried out a new tab called Startup is added to the Simulation Data dialog. In this tab, it is selected if the type of integration is going to be periodic or dynamic. A periodic one means that the chosen date is simulated a number of times until the system has stabilized, while a dynamic integration involves that a selected number of days are simulated before the proper simulation starts. Approximately, two weeks of dynamic integration should be enough for realistic representations [10].
Finally, the user has to define the Tolerance and the Maximum Timesteps to carry out the simulation. These are determined in the Advanced tab. The standard value of Tolerance, which determines how accurately equations are to be solved, is 0.02, but in case of monthly or yearly simulations, this tolerance makes the simulation be too slow. So, the tolerance can be relaxed to between 0.1 and 0.3, getting good results [10]. In both models it was established that the Tolerance for monthly and yearly simulations was 0.2. In case of periodic simulations, the tolerance is set as the default value.
3.10 Energy balance
Taking into account the model of the house built by the user and the input parameters, IDA carries out the simulation specified. The energy balance executed by the program consists of the following energy flows, gathered in table 6:
• Heat from air flows and cold bridges.
• Heat from occupants.
• Heat from electrical appliances.
• Heat from controlled room units.
• Heat from windows.
• Heat from walls and floors (structure).
• Heat from lighting.
• Heat from daylight passing through the window pane.
It is going to be explained the heat flow through the floor surface.
There are different techniques related to building construction. The foundation of a house can be a shallow foundation or a deep foundation. In this model, the foundation of the building is a shallow foundation.
Depending on the type of foundation the heat transferred through the floor is calculated in two alternative ways. If the house is built on a crawl space, the heat flow will be calculated taking into account the ambient temperature.
On the other hand, if the house is built on a shallow foundation the heat flow is calculated taking into account the ground temperature. This temperature is supposed to be the annual mean temperature of the location where the building is. The heat flow is shown in figure 4:
Fig. 4 Heat transfer through the floor
To calculate the heat transfer in this way, the soil thickness should be big enough to ensure that its temperature is not affected by the ambient temperature. So, the thickness of the soil is 5 m.
4 Results
Firstly, it will be presented the results got simulating the basic model for the whole year.
Besides, it is also shown the building´s behaviour during specific dates, such as a typical summer day in Stockholm and a winter one. Finally, it will be exposed an economical study of the different ventilation possibilities, taking into account the district heating and electricity prices in Sweden.
On the other hand, a brief comparison between the basic model and the multi-zone model results will be done, with the aim of knowing the difference between both models.
4.1 Whole year simulation for basic model
The main objective of the project is seen the consumption done by the energy systems of the house, taking into account the different ventilation airflows. The simulation of the house´s behaviour during the whole year is done for eight different quotients between the supply air flow and the exhaust airflow. All the data got by means of simulation are gathered in Appendix F.
Below it is shown, in figure 5, the annual energy consumption for space heating in case of different ventilation rates:
Fig. 5 Energy consumption for space heating during one year
The energy consumption for space heating is the amount consumed by the HVAC systems. It consists of the fuel (energy provided by means of district heating) consumed by the boiler to feed water radiators and the heating coil of the Air Handling Unit, plus the electricity consumed by the pumps of the Primary System and the fans of the Air Handling Unit. The energy consumption curve has been adjusted by an equation, shown in figure 5. The R2 value is 0.9985. According to the equation, the minimal energy
y = 17707x2- 33447x + 24738 R² = 0,9985
8900 8950 9000 9050 9100 9150 9200 9250 9300 9350
0,7 0,8 0,9 1 1,1
kWh
Qsupply/Qexhaust
Energy consumption for space heating
Single zone
consumption occurs when the supply airflow is 0.945 times the exhaust airflow. This value, calculated by the equation, amounts to 8943.4 kWh per year, while simulating the model, the energy consumption was 8948 kWh per year.
Below, it will be exposed in figures 6 and 7 the energy provided by district heating and the electricity consumed by the HVAC systems:
Fig. 6 Energy provided by district heating during one year
In this case the curve got thanks to the simulations done in IDA is also adjusted by an equation. The equation says that the minimum value is reached when the quotient between supply and exhaust airflow is 0.97, and the district heating energy consumed is 7472.8 kWh per year. Simulating, this value amounts to 7476.4 kWh.
Fig. 7 Electricity consumed by the HVAC systems during one year
The electricity consumption pattern is linear, so as higher the quotient between the supply airflow and the exhaust airflow, as more amount of electricity is used by the HVAC systems.
Finally, it is presented the energy consumption done by every component of the HVAC systems. Firstly, it will be described, by means of figures 8 and 9, the energy which
y = 17632x2- 34162x + 24020 R² = 0,9992
7400 7500 7600 7700 7800 7900 8000 8100
0,7 0,8 0,9 1 1,1
kWh
Qsupply/Qexhaust
District heating consumption
Single zone
1300 1350 1400 1450 1500 1550
0,7 0,8 0,9 1 1,1
kWh
Qsupply/Qexhaust
Electricity in HVAC systems
Single zone
comes from district heating and that is used by water radiators and by the heating coil installed in the Air Handling Unit.
Fig. 8 Energy provided to the water radiators during one year
The district heating energy consumed is the one shown in figure 8 divided by 0.98, which is the efficiency of district heating according to [5].
Fig. 9 Energy provided to the heating coil during one year
As it can be seen in figures 8 and 9, the main responsible of the district heating consumption is the Primary Systems, what it means, the water radiators. The energy consumed by the heating coil of the Air Handing Unit is really small compared to the consumption of water radiators.
Next, it will be indicated, in figures 10 and 11, the electric consumption done by the fans and pumps which makes up the HVAC systems. Firstly, it is presented the electricity consumed by fans and subsequently the electricity used by pumps.
7200 7300 7400 7500 7600 7700 7800 7900
0,7 0,8 0,9 1 1,1
kWh
Qsup/Qexh
Energy consumed by radiators
Single zone
0 5 10 15 20 25
0,7 0,8 0,9 1 1,1
kWh
Qsup/Qexh
Energy consumed by AHU
Single zone
Fig. 10 Electricity consumed by fans during one year
It can be seen that the consumption is linear, so as higher is the relation between the supply airflow and the exhaust airflow, as more electricity is consumed. This fact is logical due to the amount of air transported through the ventilation ducts is bigger as higher is the quotient, so more energy is required by the fans.
Fig. 11 Electricity consumed by pumps during one year
In the case of the pumps, the lowest consumption is done when the relationship between the supply and the exhaust airflow is around 0.95 and the value is 10.4 kWh per year. So, its contribution to the electric consumption during the year is quite small.
So far the results concerning to the energy “purchased” for space heating has been presented. Besides this energy, energy is also consumed in the building by other systems apart from the HVAC ones. The energy consumed by devices such as electrical equipment or lights is also simulated, and in fact, the energy released by these systems must be taken into account at the time of calculating the global energy balance of the house. The consumption of electricity done by equipment is almost constant in all the simulations, and it amounts to 3023 kWh per year, approximately. The difference
1200 1220 1240 1260 1280 1300 1320 1340 1360 1380
0,7 0,8 0,9 1 1,1
kWh
Qsupply/Qexhaust
Fans
Single zone
10,2 10,4 10,6 10,8 11 11,2 11,4 11,6
0,7 0,8 0,9 1 1,1
kWh
Qsupply/Qexhaust
Pumps
Single zone
between the case of maximum consumption and the minimum one is 1.3 kWh per year, so this difference can be neglected and it can be supposed that the electricity consumed by the equipment and lighting systems is constant.
The amount of heat released by the electrical appliances and the lights installed in the house is around 3800 kWh per year.
The last two heat sources which contribute to warm up the house are the energy released by the occupants and the heat gained thanks to solar direct and indirect radiation. Both of them are also almost constants. The first one amounts to 1925 kWh, while the second one contributes with approximately 4620 kWh per year. The difference between the case of maximum heat released and the minimum one is 3.1 kWh and 4.3 kWh per year in each case.
To summarize the data mentioned previously, in table 6 are gathered the type and contribution of each sources as well as its maximum deviation because of the variation of the relationship between supply and exhaust airflows.
Heat Source Heat released [kWh] Maximum Deviation
Equipment 3023.5 ± 0.03%
Lights 787.7 ± 0%
Solar direct and indirect radiation 4619.4 ± 0.07%
Occupants 1923.87 ± 0.11%
Table 6 Heat released by different sources
Hitherto, every energy source of the building has been presented. Then, it will be exposed the energy flows which remove heat from the house. The heat is transported through the enveloped by different ways. These ways are summarized by IDA in four different categories: heat transmission through the enclosing parts and cold bridges, heat transmitted through the windows, heat lost due to ventilation and heat lost because of infiltration and openings. All these heat flows are negative because they represent heat lost from the building, so a minus symbol will precede every value.
Concerning to the first two categories, the heat which outflows the building is almost constant, independently of the quotient between the supply and exhaust airflows. The amount of heat and the deviation are collected in table 7.
Heat Flow Heat released [kWh] Maximum Deviation Surface transmission and cold bridges -7136.7 ± 0.25%
Transmission through the windows -3104.8 ± 0.08%
Table 7 Constant heat outflows during one year
The value of the other two heat flows depends on the relationship between the supply and exhaust airflows. In figures 12 and 13, it can be seen the heat lost due to ventilation and because of infiltration and openings.
Fig. 12 Heat flow due to mechanical ventilation during a year
Fig. 13 Heat flow due to infiltration and openings during a year
So far, it has been presented all the heat and energy flows of the building. These flows ensure a temperature inside the house around 25ºC.
Finally, it will be studied the air flows. Due to the relation between supply and exhaust airflows varies in each simulation, the amount of air which infiltrates and exfiltrates the house will change from one case to other. The ventilation flows which take part in the basic model are the mechanical inflow and outflow and the inflow and outflow through the external walls (unintentional infiltration flows). From figure 14 to 17, it can be viewed the different flows due to the quotient between supply and exhaust airflows. The values presented next are the mean ventilation rates for the whole year.
-6000 -5500 -5000 -4500 -4000
0,7 0,8 0,9 1 1,1
kWh
Qsupply/Qexhaust
Heat flow mechanical ventilation
Single zone
-2500 -2000 -1500 -1000 -500 0
0,6 0,7 0,8 0,9 1 1,1
kWh
Qsupply/Qexhaust
Infiltration & Openings
Single zone
Fig. 14 Mean value of air removed mechanically during one year
The mechanically removed air is constant, 53.7 l/s⋅m2, so it doesn´t depend on the relationship between the supply and exhaust airflows. The air removed accomplish with the requirements of Swedish Building Code which establishes that the minimum amount of air removed is 0.35 l/s⋅m2. In this case, it is around 0.4 l/s⋅m2.
Fig. 15 Mean value of air supplied mechanically during one year
The supply airflow rate increases linearly as higher is the quotient between QSupply and QExhaust, due to the exhaust airflow is constant, as shows figure 14.
53 53,5 54 54,5 55
0,7 0,8 0,9 1 1,1
l/s
Qsupply/Qexhaust
Mechanical outflow
Single zone
40 42 44 46 48 50 52 54
0,7 0,8 0,9 1 1,1
l/s
Qsupply/Qexhaust
Mechanical inflow
Single zone
