STEADY STATE AND DYNAMIC THERMOPHYSICAL PARAMETERS OF TRANSPARENT BUILDING COMPONENTS

Transcription

1 STEADY STATE AND DYNAMIC THERMOPHYSICAL PARAMETERS OF TRANSPARENT BUILDING COMPONENTS 1. ABSTRACT In modern architecture the buildings are often designed with large glass facades or windows. Due to these transparent building components the natural light and the solar energy can be used and the intervisibility to the environment can be ensured. In dependence on the weather and using conditions high thermal losses or high solar gains can be caused. The thermal comfort and the energy demand depend very sensitively on the transparent components. To ensure a high and consistent comfort the complex thermal effects have to be regarded and optimized. This includes the heating system, the cooling system, ventilation or the complete air conditioning as well. Each thermal calculation requires the knowledge of the relevant parameters. The main thermal parameters are the heat transfer coefficient U and the solar heat gain factor g. Usually steady state values are used and provided as manufacturer's instructions. The use of constant values is limited and enables only rough estimations of the energy transfer. More detailed calculations demand the consideration of the dynamic effects and dynamic parameters. These are determined by the weather and using conditions. The steady state parameters as well as the dynamic parameters can be measured. Both measuring techniques will be described in the paper. Results of measurements on inclined roofs will be illustrated. An aim of intelligent building design is the reduction of the heating and cooling energy demand. The building has to be energetically optimized. Large transparent components can cause high solar gains, low thermal comfort or high cooling energy demand. To prevent these the transmittance of windows can be controlled. An example is given in the paper. 2. INTRODUCTION Many modern buildings are characterized by large windows or glass facades. But the thermal behaviour of buildings is considerably influenced by large transparent components. On the one hand windows cause high thermal losses in cold seasons and on the other hand they can cause high solar gains. A good building design requires optimized window constructions. The maintenance of the thermal comfort often requires an air conditioning. Alternatively, intelligent and optimized application of windows can be used. The aim is the reduction of the energy demand and the reasonable use of daylight. The common steady state parameters of transparent building components are the heat transfer coefficient U stat and the solar heat gain factor or solar transmittance g stat. These coefficients enable the estimation of average energy transfers or energy demands. The detailed calculation requires the consideration of the dynamic energy transfer depending on the unsteady boundary conditions. The boundary conditions are the outside climate, the room climate and the using conditions. In contrast to opaque building components the energy transfer of windows depends on the solar radiation. The typical influence of real weather data will be illustrated below. The thermal dynamic can be included in dynamic parameters. These parameters can be measured or calculated regarding the provided material parameters are known and all thermal effects can be described. Some measurement techniques will be described in this paper. The saving of energy requires a sensible application of windows. Some structural engineering measures can be the modification of the window type, the window area or orientation, the control of the transmittance and the shading. An important reduction of the cooling energy demand can be achieved with an intelligent control of the transmittance and reflectance. Roller blinds, venetian blinds or smart windows with manual or automatic control can be used. The influences on the thermal behaviour of a typical room can be shown in this paper. Andreas Donath, Department of Applied Physics, Brandenburg Technical University Cottbus, Germany, P.O. Box: , D Cottbus, Germany

2 3. THERMOPHYSICAL PARAMETERS OF TRANSPARENT BUILDING COMPONENTS 3.1 Steady state parameter Generally the thermal behaviour of transparent components are described with the heat transfer coefficient U stat and the solar heat gain factor g stat. Both values are defined for specific steady state boundary conditions. These conditions do not correspond in detail with the natural conditions and mainly suit the purpose of comparability of different products. A detailed calculation of the steady state value for windows and doors enables the ISO / DIS 15099, (2000, 2003). In contrast to complex calculations the given U-value (manufacturer's indication) is often used in a simple way. But the defined boundary conditions at the test or calculations differ from the real weather conditions. This U stat -value does not contain the real dependency on the solar radiation, the convection on the surfaces and the long-wave radiation outside and inside. Furthermore the influence of the outside and inside temperatures are not regarded. The influence of the solar radiation can be implemented in the solar heat gain factor g (see below). Then the largest deviations are caused by the radiative heat transfer between the outside surface and the surrounding, particularly the sky. This can be considered in an effective outside temperature. The effective temperature depends on the air temperature, the weighted radiation temperature of surrounding surfaces and the sky with their view factors, the relevant surface emissivity and the convection on the surface. The temperatures of the surrounding surfaces are very manifold and different for each building. The next Figure shows the ambient air temperature, the sky temperature and the resulting effective temperature of a vertical window under the climate of the Lowland in Northern Germany. The view factor to the sky is 0.5 and the convective heat transfer coefficient is fixed as h conv = 18 W / m 2 K. Figure 1: Ambient air temperature, sky temperature and effective temperature with regard to a vertical surface Climate data: Germany - Lowland Glass emissivity: ε glass = The effective temperature is the temperature that enables the description of the heat transfer by convection as well as by radiation with the same heat transfer coefficient assuming that no low sky temperatures occur. For calculations of the steady state U-value averaged values can be taken into account. The monthly averaged effective temperature is 1.4 K K lower then the air temperature. On the base of weather data the relevant values of other climates can be estimated. All further detailed dependencies of the heat transfer coefficients should be described in real dynamic calculations and can not be described in a simple U stat value. The solar heat gain factor g of a building component describes the ratio of energy entering the room versus the solar radiation on the face on it. The g-value is defined as the sum of direct solar gains and the secondary heat gains under the influence of a constant irradiation. Simple calculations consider the g-value within an effective heat transfer coefficient of the transparent component U eff,g : U eff, g = U g - g s w (1) were U g = steady state heat transfer coefficient of the glass s w = solar gain coefficient The solar gain coefficient s w describes the averaged solar gains under real solar irradiation, i.e. it depends on the weather and using conditions and the building construction details. The necessary simplifications and approximations can cause inaccuracies in calculations.

3 3.2 Dynamic parameter The described thermal parameters depend on the dynamic boundary conditions. Therefore the parameters are in reality dynamic as well. The Figure 2 shows the heat transfer coefficient of a double glassed window (U glass = 1.4 Wm -2 K -1 ) and illustrates the momentary heat fluxes and U-values in dependence on the outside air temperature, the sky temperature and the solar radiation. Figure 2: Outside boundary conditions, heat flux and momentary heat transfer coefficient of a vertical, south orientated window for some days in winter, U stat,vert = 1.35 m 2 K/W, inside air temperature 20 C, Left: influenced only by the outside temperatures Right: influenced by the outside temperatures and additionally by the total solar irradiation The real outside boundary conditions cause an unsteady heat transfer. If the effect of the outside temperatures is considered the heat flux density attains high values in periods of low ambient and sky temperatures (see Figure 2, left side). The differences between the steady state U stat value are caused by the changing of the convection within the enclosure between the glass panes and by the real long wave radiation between the outside window surface and the sky. By additional consideration of the solar irradiation (Figure 2, right side) the heat flux and the heat transfer coefficient are considerably modified. This is effected by the direct solar gains into the room and the changed secondary heat transfer caused by solar energy absorption in the glass panes. The calculation of the momentary effective heat transfer coefficient in summer leads to very high or low values, particularly in periods with temperature differences between inside and outside near zero (Figure 3). But, although the heat transfer coefficient is high, the heat transfer is low in such periods.

4 Figure 3: Dynamic momentary heat transfer coefficient of a window in summer The dynamic g-value describes the momentary relative gains by solar radiation. At transparent components with real boundary conditions these gains are influenced by the climatic conditions. The transparency depends on the incidence angle of the solar radiation. The momentary solar gains and the resulting solar heat gain factor are given in the next Figure for an example. Figure 4: Solar irradiation, solar energy gains, heat losses and resulting momentary solar heat gain factor of a vertical window under real climatic conditions (heat gains: positive heat losses: negative) steady state values: U stat = 1.4 W/m 2 K τ solar = 46 % 4. MEASUREMENTS OF THERMOPHYSICAL PARAMETERS 4.1 Steady state parameter The common used method to determine the heat transfer coefficient is the Hot Box method. The investigation of windows is described in EN 674 and EN 675 (1999). More detailed determination of the heat transfer coefficient enables the standard EN ISO (2000). The building component is exposed to a well defined climate at both sides. Mostly the building component is implemented in the vertical position, also components that will be installed on the building with an inclination, e.g. roof windows (pren ISO ). Building components with different inclinations can be measured with a modified Hot Box (see Figure 5). Figure 6 shows calculations and measurements of the steady state heat transfer coefficient of a roof window in dependence on the inclination and clarifies the dependence on the slope angle. Large double climate rooms enable the investigation of complete building components. The measurement on a roof with window using the Hot Box method shows Figure 7.

5 Figure 5: Measuring principle of the Hot Box method with variable inclination of the specimen Figure 6: Heat transfer coefficient of a doubleglassed window in dependence on the inclination Figure 7: Hot Box method on an inclined roof Some results of a 45 inclined roof with window document the evidence of the measurements. The roof is a typical rafter construction with insulation. Roof area (total): 6.0 m 2, Window area: 0.8 m 2 heat transfer coefficient of the glass area, vertical: U stat,vertical = 1.4 W / m 2 K heat transfer coeff., window with frame, vertical U stat,vertical = 1.56 W / m 2 K heat transfer coefficient of the glass area, inclined U stat,incl = 1.55 W / m 2 K heat transfer coeff., window with frame, inclined U stat,incl. = 1.8 W / m 2 K heat transfer coefficient of the roof (without window) U roof,eff = 0.25 W / m 2 K heat transfer coefficient of the roof (with window) U roof,eff = 0.5 W / m 2 K Some more details of the energy transfer on roof windows are described in (Donath 2002). Measurements of the g-value require the estimation of the direct solar gains and the secondary solar gains. The modified Hot Box method with a solar simulator would provide an opportunity of direct measurements. These measurements are difficult and not common practice. More common are outdoor test facilities to evaluate the component under real weather conditions. In general the test equipment is a Black Box or calorimeter used for direct measurements of the solar energy gains (ASHRAE Standard ). For the characterization of passive solar building components in Europe a test environment was chosen. The developed test cells enable measurements under identical conditions and provide comparable results (see 4.2).

6 4.2 Dynamic parameter The measurement of dynamic parameters requires special methods and special equipment. One possibility is the modification of steady state measurements, e.g. the Hot Box method with dynamic temperatures. But the difficulties remain to implement the natural conditions in these measurements. The real boundary conditions can be included at outdoor measurements. The Figure 8 shows a cross-section of an outdoor test cell. The investigated building component is one external wall of an air-conditioned room. The test component is affected by the weather conditions and by a controlled room climate to simulate different using conditions. There is a network of equivalent test cells in Europe. Comparable outdoor test-sites are used in 11 European countries. All these test-sites are equipped with a weather station and work with the same test procedures and the same data analyses. (Donath et al. 1995, Vandale et al. 2002). Figure 8: Longitudinal crosssection of an outdoor test cell The test procedure enables the simultaneous determination of different thermophysical parameters, e.g. the heat transfer coefficient and the solar transmittance (Gutschker 1996). Investigations of local values as well as overall values of the component are possible. Besides these measurements on separate building constructions investigations on existing buildings enable the estimation of parameters under real boundary conditions. Often long-term measurements are carried out to acquire different conditions. These measurements are subject to incidental boundary conditions, but with additional calculations the interesting dynamic effects can be determined. The Figure 9 shows measuring values of one day on a vertical, south orientated window. The averaged values are given in the caption. Figure 9: Measurement of the energy losses of an attic with roof window room: 13.5 m 2 floor area window: 1.2 m 2, north, 45 averaged values: outside temperature: 12.8 C inside temperature: 20.8 C solar radiation on roof surface: 324 W/m 2 heating power: 35.6 W The next Figure shows the measured heating energy demand of an attic with roof window. Shown is the heating energy consumption depending on differences between the room and the ambient temperature. The measuring points are mean values over periods of 24 h at a room temperature of 20 C. The energy losses and the energy gains of the window are nearly balanced. The fluctuations of the measuring values are caused by dynamic thermal effects. At higher outside temperatures and high solar radiation the room temperature will exceed 20 C. Solar energy can be stored in the roof and the room construction and the energy demand is below the averaged heating energy demand.

7 Figure 10: Heating energy demand per square metre floor area with and without roof window room area: 13.5 m 2 walls: light weight constructions U wall,eff = 0.3 W / m 2 K window: U w,stat = 1.4 W/m 2 K 5. IMPLEMENTATION OF TRANSPARENT COMPONENTS INTO ENERGY CALCULATIONS The transparent components of a building act between the environment and the room as a variable heat resistant and as time dependent heat source of the room. Calculations include this with suitable resistances and an energy source into the thermal models. The relevant boundary conditions are: U-value: outside ambient temperature and outside radiation temperature, room temperature, solar irradiation, convection outside and inside (minor effect), g-value: solar irradiation (direct and diffuse), angle of incidence The simplest implementation of transparent components in the calculation can be done with a constant g-value and the U-value in dependence on the effective outside temperature (see 3.1) or the outside air and sky temperatures. With this simplification monthly or yearly mean values can be calculated. The deviations between these stationary calculations and dynamic simulations are low for large time periods. Whereas short time effects and momentary values can not be investigated or result in inaccuracies. The Figure 11 illustrates monthly energy losses of south orientated window. Figure 11: Monthly energy losses of a window, steady state calculations with constant heat transfer coefficient and dynamic calculation

8 The dynamic calculation considers the heat transfer with convection, radiation and conduction as complex thermophysical effects. These can be implemented in networks of resistances and capacitances that can be mathematical described with heat transfer equations and numerical solved. The momentary energy demand corresponds to the momentary heat losses and can be understand as power of the required heating. The decisive heating period in the winter is in the months November to April (Central European climate). In the example the energy losses of the room by the window in this period for the steady state calculation are 103 kwh/m 2. The dynamic calculation results in 110 kwh/m 2. This comparison shows the possible deviations. The limit of simple calculations is arrived. In summer the steady state values are lower then the dynamic calculated values. The deviations are higher then in winter, but the energy losses of the room in summer are comparatively low. The momentary energy demand regarding the real boundary and using conditions of a room is shown in Figure 12. To understand the high fluctuations the outside air temperature and the solar radiation on the window are given in the diagram. Figure 12: Momentary heating energy demand of a room with south window under using conditions A transparent building component can be equipped with shading devices. This is necessary to reduce high solar gains, i.e. to prevent an overheating of the building and glare or high irradiation in rooms. The use of shading devices requires a special consideration in the thermal models and calculations. The models must be expanded. Constant shading factors only yield incorrect results and the manual or automatic control can not be described. The Figure 13 illustrates the control of the shading device (0 = opened, 1 = closed) in dependence on the solar irradiation on the vertical surface. Using the shading device the room temperature can be reduced. Without solar blind the room temperature would attain 26 C and the room would have to be cooled. With these calculations the room climate can be estimated and the building components (e.g. the automatic control of solar blinds) can be optimized. Figure 13: Dynamic control of a shading device, solar radiation, controlled shading, ambient and room temperature of some days

9 6. CONCLUSIONS The main parameters of transparent building components are the heat transfer coefficient and the solar heat gain factor. Generally steady state values are given as manufacturer's instructions. These values enable the comparison of different products and simple averaged calculations of the energy demand of buildings. If real boundary conditions are taken into account or temporary effects are of interest the thermal effects have to be described with dynamic processes and dynamic thermophysical parameters. The assumption of steady state values often causes deviations in the calculation results of the energy demand or the thermal behaviour. Different measurement techniques enable the estimation of steady state or dynamic parameters. Some techniques are described in the paper. The methods vary from Hot Box measurements in the lab to measurements in outside test facilities and complete buildings. Transparent building components have to be implemented into the energy concept of a building in specific way. They influence a room or a building as heat resistance between inside and the environment and as heat gain. These effects can be integrated into thermal calculation models. The manner of the implementation depends on the problem. The more detailed the results should be the more complex the thermal effects have to be regarded. As an example the controlling of a solar blind is described in the paper. Only dynamic calculations enable the simulation of the thermal behaviour of the room and the optimization of the building components. 7. ACKNOWLEDGEMENT The author would like to thank Cyntia Lanzke, Alexander Kraus and Ulrich Maschke, for their help to perform the measurements. 8. REFERENCES ASHRAE Standard , Method of Measuring Solar-Optical Properties of Materials Donath, A The Real Energy Transfer on Roof Windows, Building Physics in the Nordic Countries, Proceedings of the 6 th Symposium, Trondheim, June 17-19, 2002, pp Donath, A., Gutschker, O., Rogaß, H., Maschke, U Test of passive and active solar building components, International Symposium Non-Destructive Testing in Civil Engineering (NDT-CE), Berlin, Germany, September 26-28, 1995, pp Gutschker, O Estimation of Thermal Parameters by Solving the Heat Conduction Equation. System Identification Competition, Joint Research Centre, European Commission, ECSC-EC- EAEC Brussels, Luxembourg, 1996, pp European Standard EN Measuring procedures for the determination of the thermal transmittance (U value) of multiple glazing (guarded hot plate), CEN 1999 European Standard EN Measuring procedures for the determination of the thermal transmittance (U value) of multiple glazing (heat flow meter method), CEN 1999 European Standard EN ISO Thermal performance of windows and doors - Determination of thermal transmittance by hot box method - Part 1: Complete windows and doors, CEN 2000 European Standard pren ISO Thermal performance of windows and doors - Determination of thermal transmittance by hot box method - Part 2: Roof windows and other projecting windows, CEN ISO / DIS , Thermal performance of windows, doors and shading devices - Detailed calculations. International Organization for Standardization. Vandale, L., Wouters, P., Bloem, H IQ-Test: Improving Quality in Outdoor Thermal Testing. 3 rd European Conference on Energy Performance & Outdoor Climate in Buildings, October , Lyon, France, pp