The energy performance of an airflow window

Transcription

1 The energy performance of an airflow window B.(Bram) Kersten / id.nr University of Technology Eindhoven, department of Architecture Building and Planning, unit Building Physics and Systems Research goal Airflow windows in buildings are used to improve the indoor thermal comfort and/or reduce the energy use by (air?)conditioning systems. The goal of this study is to find out the efficacy of a thermal airflow window in combination with the effect of different design parameters such as various sunscreen properties and ventilation air volume flows.

2 2 Theoretical background Windows are used in building constructions to provide the building with daylight and to create the possibility to look outside. The big disadvantage of windows in the façade is that they provoke thermal bridges, which create unwished heat flows between inside and outside. Through the years several developments have reduced this disadvantage. Double or triple glazed windows with special glass in between have reduced the thermal conductivity a lot, in comparison to the original single glass windows. Special applied coatings on the surface of the glass are used to reduce the transmitted solar radiation or the infrared radiation from inside out. Beside the energy cost to compensate the unwished penetrated heath, a cold or hot glass surface on the inner side causes more comfort problems such as draft near the window and radiation asymmetry. The airflow window is also an invention to improve the thermal comfort and reduce energy costs. It consists of a double window construction with an air cavity in between. Nowadays the outside window is mostly a double glazed window and the inner window a single glass leaf. On the bottom of the air flow cavity, ventilation exhaust air will be extracted out of the room and transported through the cavity to the exhaust duct, as can be seen in Fig. 1. This results in less temperature difference between the inside glass surface and the room. Part of the undesirable energy will be discharged by the ventilation system or gratefully used elsewhere. Fig. 1 Airflow window without sunscreen The heat flow density (without radiation) through the interior glass leaf can be approached by: And when there is no ventilation airflow into the cavity: With: = Exterior outdoor glass surface temperature = Interior indoor glass surface temperature 2

3 R 1, R 2 h r h cv1, h cv2 R v = the thermal resistances between the cavity surfaces and the exterior glass surface temperatures T 1 resp. T 2. = surface coefficient for radiation. = surface coefficient for radiation. = with this thermal resistance the heat flow added to the air in the cavity from the incoming airflow was calculated. A = Surface area of the cavity (b*h) = Volume air flow ρ = Density of the air c p T in T out = Specific heat capacity of the air at constant pressure = Mean cavity temperature = Inflow air temperature = Outflow air temperature γ T = The gradient factor 0,5<γ T < By complete mixing γ T =1. No mixing (plug flow) γ T =0,5. Due to the advanced insulation properties of double glazed windows nowadays, the airflow window is usually used to work against the undesirable heat flow from outside to inside. To improve the functionality of this technique one can apply a sunscreen in the airflow cavity to catch and reflect the incoming sun radiation (Fig. 2). For the largest part, the absorbed radiation is than transformed into heath, which can be extracted by the ventilation exhaust air. Fig. 3 shows a simplified thermal network of an airflow window with and without sunscreen. Fig. 2 Airflow window with intern sunscreen 3

4 Fig. 3 Thermal network of an airflow window [1]. The solar radiation spectrum has a bandwidth between ca. 280 and 2500 nm. The energy content herein is: Ultra violet: 280 to 380 nm = 5% of energy Visual light: 380 to 780 nm = 50% of energy Short infra red: 780 to 2500 nm = 45% of energy Most of the incoming solar irradiation penetrates clear glass windows by transmission. The incoming solar spectrum on surfaces with different angles is presented in Fig. 4. Fig. 4 Spectral distribution of incident solar irradiation [2]. Radiation with larger wavelengths (>2500nm) is hardly transmitted through glass. Heating bodies or materials, which have absorbed the solar irradiation, emit radiation in this large infrared regime. This heath flux can almost exclusively penetrate the glass windows by conduction after it is absorbed into the glass. Fig. 5 shows the different types of radiation with respect to its wavelengths. Fig. 5 Radiation bandwidth ( 4

5 3 Method With simulations we try to find out the efficacy of a thermal airflow window, inclusive the effect of different design parameters such as various sunscreen properties and ventilation air flows. To simulate this thermal problem we have used the software program COMSOL Multiphysics version 4.1. For these simulations we have made two models of an airflow window, one with and one without a sunscreen in the cavity. The model parameters are described in the Model section. This study is focused on the heat flows through the windows and cavity, so the parapet and other building details are excluded from the study. The airflow window model has a height of 3 meter, so we can neglect the influence of bottom- and top boundaries. Because the simulation program considers all radiation as the same, it is not possible to make a distinction between large infrared radiation and the solar spectrum (UV, light & short IR). This, however, is necessary because they act very different when they fell on a glass plate. Because we are interested in the total penetrating solar radiation, this problem is almost solved as we simulate with interior and exterior air temperatures nearby 0 Kelvin. Instead of an indoor air temperature of 22 ⁰C we simulate with a temperature of 22K, so the temperature difference stays the same but the influence of infrared radiation caused by object temperatures are negligible with respect to the entered solar irradiation. We adjust all other parameters, which are dependent of the temperature (air pressure, density etc.), in such a way that they act as though the temperature (T ) is normal. In this case we can use the glass properties like absorption and scattering coefficients, which apply more to the solar spectrum. We assume there is a solar irradiation of 500W/m 2 on the outside glass plate. 500 W/m 2 divided by pi gives an incident radiation of 159,1549 W/m 2, which we apply to the outside glass window boundary. When solar radiation hits transparent material like a glass plate it undergoes the following interactions: Fig. 6 Radiation and participating media interactions 5

6 The solar energy balance over a single and double glass window are visualized in the figures below: Fig. 7 Thermal window scheme - energy balance (left: single glass window, right: double glass window [2]) In the simulated airflow models we use three clear glass windows. We adjust the glass settings (absorption and scattering coefficients) in such a way that they give nearly the same results for energy reflectance, absorption and direct transmission as were given for a clear double and triple window in the specifications (see appendix). In Fig. 8 the black numbers show the energy rates according to the window specifications and the red ones are the values that result out of the validation simulation with the window model. This simulation is done without a sunscreen and without airflow. Here, we were only concerned with the radiation problem. Note that the distance between the glass plates from the triple window is not the same as in the airflow window model. Fig. 8 Solar radiation flow through a triple glass window The glass coefficients that result in these values are listed in the Model section. 6

7 In the simulations we use different parameters for volume airflow in the cavity and for the absorption coefficient of the sunscreen, to find out what their separate influences are on the heat fluxes. This information can be useful, because these parameters are also related to other aspects. The volume airflow depends mostly on the ventilation rate of the room, which also influences the energy demand. The absorption coefficient of the sunscreen is likely to also have an effect on the amount of daylight that can enter the room. The simulated variations are: Table 1 Simulation variants in this study (total 19) Nr.: Ventilation airflow Sunscreen, absorption coefficient 1-4 0,0 [m/s] (No airflow) κ r = 50 [1/m] 5-8 0,1 [m/s] (Laminar) κ r = 100 [1/m] X ,2 [m/s] (Laminar) κ r = 300 [1/m] ,4 [m/s] (Laminar) κ r = 500 [1/m] 17 0,0 [m/s] (No airflow) No screen 18 0,0 [m/s] (No airflow) Glass validation, (only radiation problem) 19 0,2 [m/s] (turbulent) With turbulence model (low Reynolds number k-ε) No screen κ r = 500 [1/m] 7

8 4 The model The physical modules in Comsol that are applied to solve this heat problem are: Radiation in Participating Media (rpm) Heat Transfer in fluids and solids (ht) Non-Isothermal Flow (nitf) In all of the simulation variants, the airflow in the cavity is assumed to be a laminar flow. To find out what the influence is we also simulated one simulation variant with the Reynolds-Averaged Navier-Stokes (RANS) equations as turbulence model type with a Low Reynolds number k-ε model and the Kays-Crawford heat transport turbulence model. Geometry and mesh properties Dimensions [m]: All the three window glasses (WxH) = 0,004x3 Air cavity between double glazed window (WxH) = 0,016x3 Ventilated air cavity = 0,1x3 Sunscreen in the middle of the air cavity = 0,002x3 In Fig. 9 and Fig. 10 the three 2D-models that were studied are shown. The upper model represents the airflow window without sunscreen. Note that the aspect of these pictures are out off ratio. The axis show the real dimensions in meters. The colours indicate the different material densities. The left two red lines with the small bleu rectangle in between represents the double glazed outside window. The red line on the right side represents the interior single glass window and the big bleu rectangle in the middle is the ventilated air cavity. The arrow shows the direction of the airflow in case of ventilation. The simulations that include a sunscreen in the ventilated aircavity were done with the model shown in Fig. 10. The screen is indicated as the small red rectangle in the middle. The colours in this picture represent the different radiation absorption coefficients of the materials. In the study we simulate with different absorption coefficients for the sunscreen. 8

9 Fig. 9 Visualization of the simulated model without sunscreen, coloring based on material density Fig. 10 Visualization of the simulated model with sunscreen, colouring based on the radiation absorption coefficient of the material Mesh settings: The mesh of the complete construction consists of elements. Below (Fig. 11) the mesh statistics of the model are given and the figure visualizes the lowest half meter. 9

10 Fig. 11 Mesh geometry (lowest half meter) of the simulation mode (lowest half meter ), and mesh statistics (right) Boundary Conditions Convective Cooling: Air temperature outside (left) = 30 K Surface heat transfer coefficient outside = 20 W/m 2.K Air temperature inside (right) = 22 K Surface heat transfer coefficient inside = 7,7 W/m 2.K (incl. radiation) Thermal insulation boundaries = top and bottom of the model In case of air flow through the cavity the inlet and outlet boundary is not thermal insulated, but: Top of the cavity = Outflow Bottom of the cavity (temperature) = 22 K 10

11 Boundary incident radiation intensity: Outside (left) boundary = 159,1549 W/m 2 (= 500W/m 2 divided by π) Inside (right) boundary = Surface emissivity: ε =0 = Diffusive reflectivity: ρ d = 0 (The inside glass boundary does not emit, absorb or reflect radiation, it only transmits the incoming radiation.) Top and bottom boundary = Surface emissivity ε =0 Diffusive reflectivity ρ d = 0,5 Non-isothermal Flow All but one of the studies with cavity ventilation were done with a laminar airflow, so without a turbulence model. As previously discussed, this one variant was simulated with a RANS, low Reynolds number k-ε turbulence model in combination with a Kays Crawford heat transport turbulence model. Top of the cavity Bottom of the cavity = Outlet (Normal outflow velocity, U 0 = 0,1m/s (case1), 0,2m/s (case2) or 0,4m/s (case3) = Inlet (Pressure, no viscous stress, P 0 = 0 Pa) Volume Force into air cavity = Cavity left and right boundary conditions = No slip Material Properties and Initial Condition Thermal conductivity k: Glass = 1 [W/m.K] Sun screen Air Density ρ: = 0,25 [W/m.K] = E-4 * (T )^ E-8 * (T )^ E- 11*(T )^ E-15*(T )^4 [W/m.K] Glass = 2500 [kg/m 3 ] Sun screen = 1150 [kg/m 3 ] Air = nitf.pa*mw_a/(r_const*(t )) [kg/m 3 ] (nitf.pa = absolute pressure, Mw_a= molar mass=28.97[g/mol], R_const=general gas constant) Heat capacity at constant pressure C p : Glass = 800 [J/(kg*K)] 11

12 Sun screen Air = 1700 [J/(kg*K)] = *(T )^ E-4*(T )^ E- 7*(T )^ E-10*(T )^4 [J/(kg*K)] Scattering coefficient σ s : Glass (two inner windows) = 20 [1/m] Glass (outside window) = 25 [1/m] Sun screen = 100 [1/m] Air = 0 [1/m] Absorption coefficient κ r : Glass (two inner windows) = 11 [1/m] Glass (outside window) = 11 [1/m] Sun screen = 50 (case1), 100 (case2), 300 (case3) and 400 (case4) [1/m] Air = 0 [1/m] Initial temperature conditions P ref = 30K = 1 atm. Initial velocity field (in case of ventilation air flow) = Solver Settings Type of analysis: Stationary Heat Transfer (ht) and Non-isothermal flow (nitf) Linear system solver: MUMPS Relative tolerance: 0,001 Pivot threshold: 0,1 Memory allocation factor: 1,2 12

13 5 Results In the tables below the simulated energy flows of the different variants are presented. These are the incoming and outgoing fluxes throughout the complete air flow window model by radiation, conduction and convection. Also the inside average glass surface temperature is given. In order to keep the overview, the internal energy flows are disregarded. Because the simulation models are 2 dimensional, the total energy fluxes are given in W/m. The values are positive when the fluxes go from left (outside) to right (right). In the simulations an incoming solar irradiation of 500W/m 2 was assumed. Note that the 2D models are three meter high, so the total irradiation on the outside glass surface is 1500W/m. Table 1 Energy flows by no airflow into cavity Energy from outside [W/m] Removal by ventilation [W/m] Energy inwards [W/m] Mean inside glass surface temperature [⁰C] No sunscreen, no ventilation airflow 0 [m/s]: Net radiation Net conduction Sunscreen κ= 50 [1/m], no ventilation airflow 0 [m/s]: Net radiation Net conduction Sunscreen κ= 100 [1/m], no ventilation airflow 0 [m/s]: Net radiation Net conduction Sunscreen κ= 300 [1/m], no ventilation airflow 0 [m/s]: Net radiation Net conduction Sunscreen κ= 500 [1/m], no ventilation airflow 0 [m/s]: Net radiation Net conduction

14 Table 2 Energy flows by 0,1m/s airflow into cavity Energy from outside [W/m] Removal by ventilation [W/m] Energy inwards [W/m] Mean inside glass surface temperature [⁰C] Sunscreen κ= 50 [1/m], ventilation airflow 0,1 [m/s]: Net radiation Net conduction Sunscreen κ= 100 [1/m], ventilation airflow 0,1 [m/s]: Net radiation Net conduction Sunscreen κ= 300 [1/m], ventilation airflow 0,1 [m/s]: Net radiation Net conduction Sunscreen κ= 500 [1/m], ventilation airflow 0,1 [m/s]: Net radiation Net conduction

15 Table 3 Energy flows by 0,2m/s airflow into cavity Energy from outside [W/m] Removal by ventilation [W/m] Energy inwards [W/m] Mean inside glass surface temperature [⁰C] Sunscreen κ= 50 [1/m], ventilation airflow 0,2 [m/s]: Net radiation Net conduction Sunscreen κ= 100 [1/m], ventilation airflow 0,2 [m/s]: Net radiation Net conduction Sunscreen κ= 300 [1/m], ventilation airflow 0,2 [m/s]: Net radiation Net conduction Sunscreen κ= 500 [1/m], ventilation airflow 0,2 [m/s]: Net radiation Net conduction Sunscreen κ= 500 [1/m], ventilation airflow 0,2 [m/s]: Simulated with Low Reynolds number k-ε turbulence model. Net radiation Net conduction

16 Table 4 Energy flows by 0,4m/s airflow into cavity Energy from outside [W/m] Removal by ventilation [W/m] Energy inwards [W/m] Mean inside glass surface temperature [⁰C] Sunscreen κ= 50 [1/m], ventilation airflow 0,4 [m/s]: Net radiation Net conduction Sunscreen κ= 100 [1/m], ventilation airflow 0,4 [m/s]: Net radiation Net conduction Sunscreen κ= 300 [1/m], ventilation airflow 0,4 [m/s]: Net radiation Net conduction Sunscreen κ= 500 [1/m], ventilation airflow 0,4 [m/s]: Net radiation Net conduction Even though the outdoor temperature is 8⁰C higher than indoor, the net conducted energy of the outside glass surface is in all cases from glass surface to outside air, because the glass temperature will be higher than the temperature of the air. 16

17 To recapitulate the main results from above, the performances of the simulated variants were visualized in the figures below. Fig. 12 shows the total energy flux that enters the room and in Fig. 13 only the incoming part of energy is given, which is directly ceded from the inside glass surface due to temperature difference between interior glass surface and intern air. Fig. 12 Total energy penetration through the simulated variants of airflow windows Fig. 13 Incoming part of energy which is directly ceded from the inside glass surface In the annexes data of velocity field, radiative and temperature distribution into the air flow window models are presented. 17

18 6 Conclusion & Discussion Conclusion As expected, the airflow window is an effective way to prevent overheating and/or reduce energy demand by air-conditioning systems. The potential of a ventilated air cavity is less without application of a sunscreen. When part of the solar radiation is captured by such a screen, the ventilation exhaust air will carry off part of this energy. A large volume flow of 0,4m/s (=144m 3 per meter width) does not seem much more effective than a small airflow of 0,1m/s (=36m 3 per meter width). This differs only ca. 65W/m, while the difference between no ventilation and an airflow of 0,1 m/s is ca. 425W/m (by sunscreen k=500 [1/m]). The effectiveness of the sunscreen is approximately proportional to its absorption coefficient, but still less effective without a ventilated air cavity. Without airflow the penetrated energy is still 739 W/m (sunscreen k=500 [1/m]). With an airflow of 0,4m/s it s only 178,2 W/m. Without a sunscreen and airflow the transmitted radiation is approximate 65% of the total solar irradiation on the exterior surface by absorption and scattering into the glass plates. By applying a sunscreen with absorption coefficient of 500 [1/m], the transmitted radiation reduced to ca. 12 to 21%, depending on the volume air flow. This also means that the incoming daylight will be severely reduced. Discussion As can be seen in Fig. 13, the directly ceded energy from the interior glass surface to inside air is growing when the sunscreen absorbs more sunlight, except under the conditions of the highest ventilated airflow (0,4m/s). This can be explained as follows. In case of low volume flows the air cavity temperature rises, as well as the temperature of the interior glass window. In case of 0,4 m/s the glass temperature is not rising, but drops when more solar radiation is captured in the blinds, even though it results in higher cavity air temperatures. This can be explained by the higher volume flow, because it reduced the rising air temperatures and in case of less transmitted solar radiation by the blinds, the interior window absorbs less radiation so the net temperature of this window drops a little bit. In practice there are more parameters that will influence the effectiveness of the airflow window, but are not taken into account in this study. The airflow window in this study is three meters high. The height influenced the mean air temperature in the ventilated cavity, because it determines the amount of absorbed solar radiation that is ceded to the airflow. And the temperature in this air cavity influenced the energy conduction to the indoor air. Beside, sun blinds have another property that will influence the energy balance. Their scattering coefficient determines the solar reflection, but is not varied in our study. Also, the scattering and absorption coefficients of the glass windows are not varied in our study. Also, other cavity widths will influence the air temperature and intern flow profile, therefore altering the energy flow. As we mentioned, the absorption coefficient of the materials depend of the radiation wavelength, but are assumed to be equal in the study. Furthermore the performance of the airflow window is investigated at a stationary peak load condition. The overall performance also depends on dynamic conditions during all seasons. 18

19 Bibliography 1 M.h. de Wit, [2009]: Heat, air and moisture in building envelopes. Course book Eindhoven University of Technology: blz K. Maatouk, Non-gray radiative and conductive heat transfer in single and double glazing solar collector glass covers. International Journal of Thermal Sciences 45 (2006)

20 Annex 1: Used window properties Radiative intensity [W/m2] and temperature [⁰C] distribution through the airflow window. - Sunscreen and no ventilation airflow 0 [m/s]: - Sunscreen with absorption coefficient κ= 50 [1/m] and no ventilation airflow 0 [m/s]: 20

21 - Sunscreen with absorption coefficient κ= 100 [1/m] and no ventilation airflow 0 [m/s]: - Sunscreen with absorption coefficient κ= 300 [1/m] and no ventilation airflow 0 [m/s]: 21

22 - Sunscreen with absorption coefficient κ= 500 [1/m] and no ventilation airflow 0 [m/s]: - Sunscreen with absorption coefficient κ=50 [1/m] and no ventilation airflow 0,1 [m/s]: 22

23 - Sunscreen with absorption coefficient κ=100 [1/m] and no ventilation airflow 0,1 [m/s]: - Sunscreen with absorption coefficient κ=300 [1/m] and no ventilation airflow 0,1 [m/s]: 23

24 - Sunscreen with absorption coefficient κ=500 [1/m] and no ventilation airflow 0,1 [m/s]: - Sunscreen with absorption coefficient κ=50 [1/m] and no ventilation airflow 0,2 [m/s]: 24

25 - Sunscreen with absorption coefficient κ=100 [1/m] and no ventilation airflow 0,2 [m/s]: - Sunscreen with absorption coefficient κ=300 [1/m] and no ventilation airflow 0,2 [m/s]: 25

26 - Sunscreen with absorption coefficient κ=500 [1/m] and no ventilation airflow 0,2 [m/s]: - Sunscreen with absorption coefficient κ=500 [1/m] and no ventilation airflow 0,2 [m/s]: Simulated with Low Reynolds number k-ε turbulence model. 26

27 - Sunscreen with absorption coefficient κ=50 [1/m] and no ventilation airflow 0,4 [m/s]: - Sunscreen with absorption coefficient κ=100 [1/m] and no ventilation airflow 0,4 [m/s]: 27

28 - Sunscreen with absorption coefficient κ=300 [1/m] and no ventilation airflow 0,4 [m/s]: - Sunscreen with absorption coefficient κ=500 [1/m] and no ventilation airflow 0,4 [m/s]: 28

29 Visualizations of radiative intensity, temperature distribution and air velocity into the models. - Sunscreen and no ventilation airflow 0 [m/s]: - Sunscreen with absorption coefficient κ= 50 [1/m] and no ventilation airflow 0 [m/s]: 29

30 - Sunscreen with absorption coefficient κ= 100 [1/m] and no ventilation airflow 0 [m/s]: - Sunscreen with absorption coefficient κ= 300 [1/m] and no ventilation airflow 0 [m/s]: - Sunscreen with absorption coefficient κ= 500 [1/m] and no ventilation airflow 0 [m/s]: 30

31 - Sunscreen with absorption coefficient κ=50 [1/m] and no ventilation airflow 0,1 [m/s]: 31

32 - Sunscreen with absorption coefficient κ=100 [1/m] and no ventilation airflow 0,1 [m/s]: 32

33 - Sunscreen with absorption coefficient κ=300 [1/m] and no ventilation airflow 0,1 [m/s]: 33

34 - Sunscreen with absorption coefficient κ=500 [1/m] and no ventilation airflow 0,1 [m/s]: 34

35 - Sunscreen with absorption coefficient κ=50 [1/m] and no ventilation airflow 0,2 [m/s]: 35

36 - Sunscreen with absorption coefficient κ=100 [1/m] and no ventilation airflow 0,2 [m/s]: 36

37 - Sunscreen with absorption coefficient κ=300 [1/m] and no ventilation airflow 0,2 [m/s]: 37

38 - Sunscreen with absorption coefficient κ=500 [1/m] and no ventilation airflow 0,2 [m/s]: 38

39 - Sunscreen with absorption coefficient κ=500 [1/m] and no ventilation airflow 0,2 [m/s]: Now simulated with Low Reynolds number k-ε turbulence model. 39

40 - Sunscreen with absorption coefficient κ=50 [1/m] and no ventilation airflow 0,4 [m/s]: 40

41 - Sunscreen with absorption coefficient κ=100 [1/m] and no ventilation airflow 0,4 [m/s]: 41

42 - Sunscreen with absorption coefficient κ=300 [1/m] and no ventilation airflow 0,4 [m/s]: 42

43 - Sunscreen with absorption coefficient κ=500 [1/m] and no ventilation airflow 0,4 [m/s]: 43

44 Annex 1: Used window properties The window specifications, which we used for validating the model window properties, are obtained from: 44

45 45