1、AN-04-6-3 - Calorimetric Analysis of the Solar and Thermal Performance of Windows with Interior Louvered Blinds Michael R. Collins, Ph.D. Associate Member ASHRAE ABSTRACT To provide validation data for new numerical models of fenestration incorporating shading devices, tests will be required on full
2、-sized window and shade systems. This paper gives data pertaining to a double-glazed window with internal venetian blinds. Using a technique called solar calorimetry, two blinds were tested at each of three blind slat angles and two solar projle angles. One blind was typical of commercially availabl
3、e products and had a significant reflectivity, while the other was paintedflat black and was largely absorbing. The selected blind slat and solar profile angles allowed for the interception of both minimal and maximum amounts of solar irradiation by the shade. In all cases, the shade decreased the l
4、evel of solar heat gain (by between 5% and 40%), with signif- icant reductions occurring for the more reflective blind when it was positioned to intercept the solar energypassing through the window. It is expected that in this configuration, much of the solar energy transmitted through the window wa
5、s reflected back to the outdoor environment. The shade had no signijicant efect on the thermal transmission of the window. INTRODUCTION Mounting a shading device adjacent to the indoor surface of a window, such as a venetian blind, is common practice for providing privacy and controlling daylighting
6、. It is reasonable to assume that the presence of these shading devices will also affect the solar heat gain coefficient (SHGC) and thermal performance (U-factor) of the window system. To date, however, reliable methods of predicting the potential solar and thermal benefits of shades have not been a
7、dequately devel- oped, largely due to the complexity of the system. In fact, the potential benefits of shading devices are simply excluded for Stephen J. Harrison, Ph.D., P.Eng. energy analysis. In particular, the National Fenestration Rating Council (2001) specifies that products be rated with no s
8、hading attachments. The difficulties in analyzing shaded windows are easy to demonstrate. Consider the transmitted, reflected, and absorbed solar or short-wave radiation, as shown in Figure 1. For an unshaded window, each layer is planar, parallel, and specularly reflecting. The progression of an in
9、cident ray of sunlight is easily followed through the system by performing a ray trace. Furthermore, the system is rotationally homoge- neous, allowing for an analysis based on incident angle rather than solar altitude and azimuth angle. With the addition of a shade, the individual layers may no lon
10、ger be planar or spec- ularly reflecting, and the system will no longer be rotationally homogeneous. Now consider the thermal aspects of the system, also shown in Figure 1. For a normal window, there are parallel thermal resistances that account for radiative and convective heat transfer between eac
11、h layer. The system can essentially be examined as a one-dimensional system. When a shade layer is added, radiative and convective heat transfer from the inner glass may or may not occur with the shade, and the thermal resistance network becomes much more complex. In both cases, the system becomes s
12、ignificantly more difficult to analyze when a shade is added. Currently, efforts are in progress to develop reliable numerical methods of characterizing the performance of windows with shading devices. In Canada (Collins et al. 2002a, 2002b), the U.S. (Klems and Warner 1992), and Europe (IEA), indep
13、endent initiatives are progressing that have the ultimate intention of upgrading window analysis soft- ware. Ultimately, to validate those efforts, solar and thermal M.R. Collins is an assistant professor in the Department of Mechanical Engineering, University of Waterloo, Waterloo, Ontario, Canada.
14、 S.J. Harrison is an associate professor and Director of the Solar Calorimetry Laboratory in the Department of Mechanical Engineering, Queens University, Kingston, Ontario, Canada. 474 02004 ASHRAE. Solar (Short-Wave Radiation) T Window T Window and Shade Thermal (Long-Wave Radiation I Convection )
15、Figure 1 Solar and thermal energy transfer in a shaded and unshaded window. Interrefletion has been omitted for dari. Terminology is presented in the nomenclature. tests on full-scale windows and shade systems will be required. To support the Canadian efforts, tests were therefore performed for sele
16、cted cases at the Queens University Solar Calorimetry Laboratory. A double-glazed window was tested in combination with two horizontal and louvered shades of identical geometry but differing optical properties. The samples were tested at combinations of two solar incident angles and three louver ang
17、les. It is the intention of this publi- cation to describe the test procedure and test samples and conditions and to present and discuss those experimental results. A comparison to numerical predictions is in progress (Collins et al. 2004). PROCEDURE Experimental Apparatus The Solar Calorimetry Labo
18、ratory (SCL) is shown in Figure 2. Along with its solar tracker, the calorimeter is located on the roof of the mechanical engineering building at the university (44.14“ lat., 76.49“ long.) and has an unob- structed southern view of Lake Ontario. The calorimeter and its systems are briefly described
19、in the following paragraphs. A detailed description of the calorimeter and its systems and Figure2 Queens solar calorimeter: (a) photo of the calorimeter and (b) cross-sectional schematic (not to scale). calibration and commissioning procedures can be found in Harrison and Collins (1999). To measure
20、 net heat gain through a glazing system, a test window must first be mounted in the mask wall. This wall covers the calorimeter aperture and serves as the interface between the interior and exterior environment (Figure 2). To determine losses, Qmask, thermocouples are used to measure the temperature
21、 difference, ATmask, across sections of the ASH RAE Transactions: Symposia 475 mask wall. Combining the temperature measurements with the wall total U-factor, Urnask, and its surface area, Amask allow the losses to be calculated as Qmask = Amask. ATmask. mask (1) The wall construction of the solar c
22、alorimeter is designed to reduce heat loss using an active thermal guard (Figure 2). A guard heater is activated when the interior surface of the calo- rimeter is hotter than the heater, it is and deactivated when the heater is hotter than the interior surface of the calorimeter. Ideally, by elimina
23、ting the temperature gradient across the wall (ATwaffs z O), it should be possible to eliminate heat flux (ewarrs s O). The total heat loss from the entire calorimeter wall can be estimated by Qwais = Cwails *wails . walls 3 (2) where Awans and Uwalls are the calorimeter wall surface area and U-fact
24、or, respectively. Heat extraction (or addition), and interior temperature control, is primarily accomplished through the calorimeter flow loop (Figure 2). Within the calorimeter, conditioned fluid is added to the internal circulating loop consisting of an air-to- fluid heat exchanger, solar absorber
25、 plate, and a circulation pump. The absorber plate is the primary energy absorption device, while the air-to-fluid heat exchanger aids in removing heat energy from the air, promotes a uniform air temperature within the test cell, and increases the response time ofthe calo- rimeter. To determine the
26、energy removed (or added) by the internal flow loop, a reference heat source or calorimetric ratio method (Harrison and Bernier 1984) is used. This method utilizes an electrical heater installed in series with the calo- rimeter loop. Recognizing that the same mass flow rate exists through the refere
27、nce heater and the calorimeter, and the specific heat of the fluid remains relatively constant, then (3) where Qflow is the energy removed from the calorimeter through the flow loop, P,.,is the power input to the reference heater, and ATyow and ATref are the temperature rises across the calorimeter
28、and reference heaters, respectively. A number of systems provide important weather data for solar heat gain testing. Next to the calorimeter, a weather tower contains instrumentation for measuring wind speed and direc- tion, ambient temperature, and relative humidity. Addition- ally, two Pyranometer
29、s are attached directly to the mask wall for diflse and direct radiation measurement. Four shielded thermocouples meter the uniformity of air temperature within the test cell. The Solar Calorimetry Laboratory maintains a computer- controlled sun-tracking test frame. The device has two track- ing axe
30、s to track the azimuth and altitude of the sun. The solar tracker is controlled by custom software that calculates the suns position in the sky and controls the tracker position using two potentiometers. The tracker software provides excellent and versatile control of the tracker. It is able to fix
31、the solar position to an accuracy of f1 and has the ability to track azimuth only, altitude only, or track at an offset. Experimental Procedure To measure the energy performance of a window, the net heat input into a calorimeter due to energy flow through a glaz- ing system is determined by careful
32、metering of the input and output energy flows. This includes energy removed by the liquid flow loop, energy added by any internal fans and pumps, and transmission losses through the calorimeter walls. Energy input, Qinput, is then calorimetrically determined by - Qinput - Qow- Qfan- Qpump + Qwals +
33、Qmosk (4) where Qfan and Qpump denote the electrical power supplied to the calorimeters internal fan and pump, respectively. The energy balance of the calorimeter can be seen in Figure 3. The solar heat gain of a window is the product of the solar heat gain coefficient, SHGC, and the incident solar
34、irradiance, I, i.e., SHG = SHGC . I, (5) where the SHGC is defined as n SHGC = T+ Ni.ai, (6) i= 1 where z and a are the transmitted and absorbed solar radia- tion, respectively, and N is the inward-flowing fraction of absorbed solar radiation. The subscript i refers to individual layers of the fenes
35、tration system, where a single pane of glass or a shade constitutes a layer. Accounting for the windows Figure 3 Calorimeter energy balance for standard test procedures. 476 ASH RAE Transactions: Symposia z 0.84 projected area, A) and the thermal transmission, U? the steady-state instantaneous energ
36、y flow rate through a glazing system is calculated as the difference between the gain due to solar radiation and the heat loss due to the interiodexterior temperature difference, Qinpuf = SHGC .I. Af- Uf. ATi,o. A,-, (7) where Aq, is the temperature difference across the window. The efficiency of a
37、glazing system can be described as the ratio of instantaneous gain to incident solar radiation (Harri- son and Barakat 1983). a P E 0.09 0.07 0.84 - The time-averaged thermal efficiency, 77, can be graphi- cally represented in the same manner as the instantaneous effi- ciency curve and, for a series
38、 of tests, a plot of 77 versus ATJI can be developed. By using a linear regression on these points, the window system can be characterized, where the slope repre- sents the system U-factor and the y-axis intercept is the solar heat gain coefficient. It should be noted that this test method allows fo
39、r both an accurate and precise prediction of the solar heat gain coefficient using relatively few data points. Unless a significant number of data points are collected, however, the prediction of U-factor would be much less precise. While this method assumes the thermal and solar charac- teristics o
40、f a window system are uncoupled, it is the only method currently available for the experimental determination of window thermal and solar performance. The majority of calorimetric facilities will use modeling software to determine Uffor input into Equation 7. Calorimetric facilities can instead prod
41、uce a plot like the one shown in Figure 6. In either case, it is assumed that Nand U-factor are uncoupled, and the rela- tion between q and AT,! is linear. Since solar calorimetry never truly produces steady-state results, quasi-steady-state conditions were determined based on accepted calorimetric
42、procedures (Harrison 1993). In order to achieve quasi-steady-state conditions, the heat transfer fluid was circulated through the absorber plate until it remained constant within kO.3“C and +1 W/“C for 15 minutes prior to each period in which the data were taken and for the 15 minutes in which data
43、were collected. Results were also analyzed after each test to ensure that other measured data did not demonstrate any trending during the test period. Test Samples and Conditions A single wood-frame window was selected for the exper- iments. It was composed of two 3 mm lites of clear glass with a 13
44、 mm air gap. The overall area of window, A) and glass, A, was 0.720 and 0.473 m, respectively. The window was of particular interest as one of a series of specimens that were extensively tested both experimentally and numerically (Elmahdy 1990). Consequently, there is an abundance of data available
45、concerning its construction, optical and thermal properties, and overall solar and thermal performance. The glass has an R-value of 0.003 1 K.m2/W for each lite. The opti- cal properties of each lite are as given in Table 1. Two commercially available aluminum blinds were selected for the tests that
46、 were considered to be typical ofmany commercially available products. The slats had a width of 25.4 mm, thickness of O. 17 mm, and an arc length and a radius of curvature of 27.3“ and 52.3 mm. The slats were separated by a pitch of 22.2 mm. One blind had a white enameled surface while the other (an
47、 identical product) was painted flat black. The radiative properties ofthe blind material used in the exper- iment were measured using a Gier Dunkle MS-251 solar reflectometer and a Gier Dunkle DB-1 O0 infrared reflectome- ter. They were found to have a solar absorptance of 0.32 and 0.90, and hemisp
48、herical emissivities of 0.75 and 0.89 for the white and black blinds, respectively (ASTM 1996). Finally, the slat conductivity was measured using a guarded hot plate apparatus by testing a “stack“ of blind slats (Machin 1997). In that way, the combined conductivity of the aluminum slat and enameled
49、coating could be determined, and it was found to be 120 W/m.K. During tests, the blinds were installed at a nomi- nal distance (center of slat to inner glass) of 30 mm. The shades were installed using the mounting hardware provided? and the shade bottom was secured at the bottom to prevent sag as the calorimeter tilted. All hardware was carefully placed so as not to receive any direct irradiation. Experiments were performed for each blind at three blind slat angles of +So, O“, and 45“ and at solar profile angles of 30“ and 45“ (Figure 4). A slat angle of O“ represen
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