Residential temperature management
WALTER BOSSCHAERTS, DIMITRI GOETHALS, OLIVIER SUY
Mechanics Department, RoyalMilitaryAcademy
Renaissancelaan 30, 1000 Brussels
BELGIUM
Abstract: This paper can be seen as a case study on the use of renewable energies for the ventilation of a residential building. First, based on the ground plan and material properties, a TRNSYS model is made. This model simulates the dynamic behavior of temperatures in all rooms with climatologically data as input. CFD calculations give a closer look on two particular subsystems of the house, more specific on the behavior of the airflow through an active façade and the ventilation of a warm room with cool air. These results can be used in combination with the TRNSYS model in order to examine their influence on the behavior of the total system.
Key-words: active façade, ventilation, numerical simulation, comfort, residential
1Introduction
Nowadays renewable energies are as frequently used in big projects (e.g. a site of wind turbines) as they are in individual projects.
An example of the latter is given in this article. Natural ventilation in buildings uses renewable energies in order to obtain an acceptable air quality for the occupants. Obviously, the thermal comfort has to be optimized avoiding undesired air flows. Natural ventilation has always been very important in cold and moderate climates while mechanical ventilation can be applied to each type of climate. In the present effort a case study is presented: it concerns a house located near Bordeaux, France.
2Dynamic simulation of a residential house
The thermal simulation of the house (figure 1) was calculated using TRNSYS, a commercial software package.
A weather file, containing meteorological data such as the ambient temperature, humidity factor and total vertical and horizontal intensity of the sunlight, provides the software with the necessary input.
The irradiation processor calculates the direct and diffuse radiation in user defined surfaces (e.g. windows), using the irradiation data from the meteorological file.
This allows studying the impact of the orientation of a house and adapting it eventually according to the yearly based energetical needs.
The fictive sky temperature calculator is indispensable because it takes into account the heat transfers via the long-wave radiation of the external surfaces to the atmosphere.
The geometry of the rooms as well as the thermal properties of the used construction elements and the characteristics of the HVAC elements, all entirely according to the wishes of the client, are entered as parameters into the program.
Due to urban regulations the frontal façade (and also the important glazing of the living) is oriented southwards. In order to study the influence of the flooring properties on the daily temperature variations in the living room, two cases with different flooring (between the basement and the ground floor) were calculated and compared. The first is made out of an excellent insulator type SIPOREX (therefore the insulation between the two levels is excellent but due to the reduced mass, the thermal inertia is rather small).
The second type of flooring is made out of concrete and tiles (Table 1).
Figure 2 shows a graph with the evolution of the temperature in the cellar (wine cellar), in the living room, in the room 12 and the ambient temperature.
The difference between the two cases is most remarkable during the summer months.
Figure 2 shows that a heavy floor induces slower temperature variations which are more damped. This effect is even more visible during the hot periods. Therefore the necessary cooling load in summer time is lower for the floor only made out of concrete.
It should be noticed here that the temperature fluctuates a lot in the living room because of the presence of the huge glazing surface.
Increasing the comfort level by lowering the fluctuations and through cooling of the living with fresh air can be done with the installation of an active façade in the living and the set-up of a night time cooling with fresh external air.
In order to estimate the required power of the fan, the losses in the active façade should be determined.
This will give an indication of the amount of photovoltaic cells to be installed in order to operate the fans to obtain the desired comfort without the consumption of “conventional” energy.
In the continuation of this article, the ventilation problem will be covered.
3Numerical simulation of an active façade
Two particular cases are simulated numerically: the isothermal ventilation flow in an active façade without solar control and the flow in the same façade, with a canvas solar control.
An active façade is formed by 2 separated transparent walls, with a cavity in between, for ventilation purposes.
This system enhances the thermal insulation between the building and the outside world, thanks to the convective airflow in the cavity.
This isothermal study should be seen as a preliminary study, where the façade, equipped with a screen, is hit by the solar radiation at a given incidence. The façade is cooled by a forced ventilation flow blowing through the cavity.
Few is known about the behaviour of an airflow in an active façade and a numerical simulation could improve the macroscopic models of heat transfer, so that an energy balance can be made, as a function of the mass flow through the cavity.
Our objective is to determine the pressure drop in the cavity, in order to determine the required fan properties.
Next, the system can be integrated in the dynamic simulation of the house, so that its influence on the temperature changes in the living room can be studied.
3.1Model
Figure 3a represents a vertical section of the façade (side view). Its height is 2.7m and the distance between the two transparent walls is 0.27m.
The air enters through 2 wire meshes, 9cm each (at the bottom of the façade), and the outlet measures 10cm (on top of the façade).
The fan is located at the outlet. The flow rate varies between 30m³/h-150m³/h.
3.2Active façade without solar control
The results of the calculation (obtained with Fluent CFD code) are shown in figures 3b (without screen), 3c (close up) and 3d (with screen).
The picture learns us that the two jets, coming from both inlets, collect into a single jet, that goes up vertically in the cavity.
The pattern strongly depends on the chosen turbulence model.
In the laminar case and also with the k- SST model, the jet remains concentrated and sticks to the left wall.
In other cases, the jet spreads over the full width of the cavity.
The k- model seems to be the best choice, not only because it is a good compromise between accuracy and computational cost, due to the fact that the boundary layer can be modelled using wall functions, but also several sources recommend this choice [2].
Finally the stream deflects towards the outlet, before being evacuated by the fan.
The evaluation of the calculated static pressure drop between the inlet and the outlet is done with respect to an inviscid and incompressible flow as reference. This permits to quantify losses in the façade.
3.3Active façade with solar control
The screen is placed in the cavity, in order to protect it against the influences of the exterior environment.
It is obvious that the position of the screen influences the flow in the cavity.
The numerical simulation of the flow in the second case is represented in figure 3d.
It shows that the screen divides the flow. Few streamlines cross the screen. Instead the streamlines situated in the lower left of the cavity bypass the screen and rejoin at the outlet.
The simulations show that the cross permeability of the screen increases the flow rate at the left side of the screen.
4Numerical simulation of a cold ventilated room
The results of the time dependent 2D calculation of the ventilation flow in the living room during the night is represented in figure 1. The living room is initially set at 25°C (298K), while the temperature of the air entering the room is 20°C (293K). Air enters the room through an opening of 20 cm with a uniform inlet speed of 0.5 m/s.
The simulation takes into account buoyancy effects due to temperature differences (in order to capture this effect correctly the grid size was carefully studied especially near the inlet). Those effects are responsible for the high downwards deflection of the air in the vicinity of the inlet.
The results were obtained using a k- turbulence model in a 2D approach. This gives a good estimation of the temperature distribution (and thus comfort level) in the room, especially in the neighbourhood of the air entrance.
The figures 4a, 4b and 4c show clearly the stratification of the air due to the difference in the temperature and the rather low mixing. This is further illustrated in figures 4 d and e showing the speed distribution.
5Conclusion
In this contribution a case study is presented where different paths indicate how one can obtain good comfort without the consumption of “conventional” energies, even in warm climates: the use of forced ventilation during the night in combination with a good choice of construction material and the introduction of a ventilated double façade.
References:
[1] Source book active façades, Belgian Building Research Institute, June 2002
[2] Lars K. Voigt Comparison of turbulence models for numerical calculation of airflow in an annex 20 Room, Report, International Centre for Indoor environment and Energy, DTU 2000
Case 1: Insulated floor / Case 2: Concrete floorCellar wall / 2.933 W/(m2K) / Id.
Exterior wall / 0.427 W/(m2K) / Id.
Interior wall / 1.525 W/(m2K) / Id.
Cellar flooring / 0.770 W/(m2K) / Id.
Flooring levels 1,2 / 0.644 W/(m2K) / 1.333 W/(m2K)
Roof / 0.369 W/(m2K) / Id.
Table 1: Thermal conductivity of the walls
Fig. 1: Map of the house
Fig. 2:
Evolution of the temperature in the different rooms of the house in January and June.
Fig. 3: Numerical calculation of an active façade
Fig. 4: Representation of a ventilated room: temperatures after (a) 160 s, (b) 6 min et (c) 12 min; (d) and (e) streamlines and velocity vectors