Collins, M.R., and Harrison, S.J., "Test of Measured Solar Heat Gain Variation with Respect to Test Specimen Tilt", SHRE Transactions, Vol. 107 (1), pp. 691-699, 2001. Copyright 2001, merican Society of Heating, Refrigerating and ir-conditioning Engineers, Inc. (www.ashrae.org). Reprinted by permission from SHRE Transactions 2001, Volume 107, Part 1. This material may not be copied nor distributed in either paper or digital form without SHRE s permission.
TEST OF MESURED SOLR HET GIN VRITION WITH RESPECT TO TEST SPECIMEN TILT STRCT This report describes measurements made at the Queen s University Solar Calorimetry Laboratory of solar heat gain coefficient and U-factor of a window and shade combination at 45 o and 90 o tilt angles. The test sample was one sheet of 6.4 mm (1/4") double strength clear glass, and a shade consisting of aluminum foil painted flat black on both sides, mounted on the interior of the window. The test requirements were specified in a document entitled Test of Measured Solar Heat Gain Variation with Respect to Test Specimen Tilt, prepared by the NFRC Solar Heat Gain Test Procedure Task Group. Results indicate that there was a 5% decrease in the measured solar heat gain coefficient, and 22% increase in the system U-factor, between vertical and 45 o tilt angles. INTRODUCTION The determination of the Solar Heat Gain (SHG) through fenestration is required for the evaluation of fenestration energy performance, estimating building energy loads, and assessing occupant comfort levels. SHG is the product of the solar heat gain coefficient (SHGC or F) and the incident solar irradiance (G), i.e., SHG = F G (1) F is defined for a single layer as F = τ + N α (2) where τ and α are the transmitted and absorbed solar radiation, respectively, and N is the inward-flowing fraction of absorbed solar radiation. To validate new SHG models for complex fenestration, SHG tests will need to be performed on full-scale window and shade systems. However, current test practices often attempt to reduce incident angles by tracking the sun in both azimuth and altitude (McCluney 1994). Similarly, NFRC 200 requires that the solar heat gain coefficient be determined at an incident angle of zero degrees. Therefore, tests must be performed in non-vertical orientations. Calorimeter tilt angle may significantly affect the convective flow around the shade and window, which in turn may affect radiation exchange. s such, the degree to which calorimeter tilt affects the solar heat gain coefficient for a window and interior shade combination should be investigated. EXPERIMENT s a result of the previous need, the NFRC Solar Heat Gain Test Procedure Task Group initiated a program to examine the effects of calorimeter tilt on measured values of solar heat gain coefficient. Identical window and shade samples were sent to participating labs, which would determine the solar heat gain coefficient of the specimen under specified conditions
using the method of their choice. One such lab was Queen s University Solar Calorimetry Laboratory, located in Kingston, Ontario, Canada. Test Sample The test sample and mounting requirements were described in communication with the NFRC (NFRC 1998). Figure 1 shows a schematic of the mounting details. The test sample was obtained from the NFRC. It consisted of a 91.0 mm by 91.0 mm (36" by 36") pane of 6.4 mm (1/4") double strength clear glass, and a 84.0 mm by 84.0 mm (33" by 33") interior shade made from a single flat sheet of aluminum foil. The aluminum foil was attached to a 25.4 mm by 12.7 mm (1" by 1/2") fir frame around the perimeter edges to support it and keep it taught. The wood frame and both sides of the aluminum foil were painted flat black to have a solar absorbtance of 0.90 or greater. The glazing was mounted in a 88.9 mm by 88.9 mm (35" by 35") aperture in a modular surround panel made from 76.2 mm (3") thick polyisocyanurate insulation with an 17.5 mm (1/2") exterior plywood facing. The surround had a total R-factor of 3.88 K m 2 /W (22.03 h ft 2 F/tu). 12.7 mm (1/2") groove was cut into the aperture to hold the glazing at a distance of 25.4 mm (1") from the exterior face of the surround panel. The glazing perimeter was sealed with silicone caulking and white tape, both for weather protection and to reduce solar loading in this area. The shade was mounted in a manner that minimized disturbance of the airflow and conductive heat transfer to the mask wall. This was accomplished by inserting four wooden dowels, two in each of the bottom and top of the shade, into the wooden frame of the shade. The dowels were in turn connected to mounting blocks, which were secured to the interior of the mask wall. These dowels provided a rigid mounting system which held the shade at a distance of 70 mm (2.75") from the interior face of the glazing and 25.4 mm (1") between the edge of the surround and the shade. Experimental Procedure Tests were to be performed within specified NFRC guidelines (NFRC 1998). Measurements were to be performed at two different tilt angles, vertical and 45 o using the same test specimen and installation. ll measurements were to be made at irradiation levels above 631 W/m 2 (200 tu/hr ft 2 ) and between incident angles of 20 o and 30 o, which could be achieved by off azimuth tracking. Testing was performed using Queen s Solar Calorimeter located at Queen s University (Fig. 2). full description of the calorimeter and its systems can be found in Harrison and Collins (Harrison and Collins 1999).
To calculate the energy input into a calorimeter due to energy flow through a glazing system, careful metering of the input and output energy flows is required. This includes energy removed by the flow loop, energy added by any internal fans and pumps, and losses through the calorimeter walls. Energy input, Q input, is then calorimetrically determined by Q Q Q Q + Q + Q input = (3) flow fan where Q flow, Q fan, Q pump, Q walls, and Q mask denote: the energy removed by the calorimeter flow loop; the electrical power supplied to the calorimeter s internal fan and pump; and heat lost through the walls and mask, respectively. The energy balance of the calorimeter can be seen in Fig. 3. pump walls mask Tests were performed using a method originally used to characterize solar collectors (Harrison and arakat 1983). The instantaneous energy 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 interior/exterior temperature difference Q input = F G U ΔT, (4) f where f is the area of the specimen. U f represents the windows overall heat transfer coefficient, and ΔT i,o is the temperature difference across the window. f i o f The efficiency of a glazing system can be described as the ratio of instantaneous gain to incident solar radiation. = Q f input η (5) The time averaged thermal efficiency, η, can be graphically represented in the same manner as the instantaneous efficiency curve. Therefore, for a series of tests, a plot of thermal efficiency versus ΔT i,o /G can be developed. y using a linear regression on these points, the window system can be characterized: the slope represents the systems U-factor, and the y-axis intercept is the solar heat gain coefficient. G Once the system was given time to respond to a new set of test conditions, steady-state conditions were determined based on accepted calorimetric procedures (Harrison 1993). In order to achieve steady-state conditions, the heat transfer fluid was circulated through the absorber plate until it remained constant within ± 0.3 (0.17 F) and ± 1 W/ (6.14 tu/h F), for 15 minutes prior to each period in which the data were taken, and for the 15 minutes in which data were collected. n attempt was made to avoid early morning and late evening tests. Rapid changes in solar intensity produced transient effects during these periods. RESULTS In addition to SHGC and U-factor results, it was required that calorimeter operational data be presented (NFRC 1998). Tables 1 and 2 show much of this data for the 90 o and 45 o calorimeter tilt angles respectively. Data shown represents the results of each test, averaged from instantaneous measurements taken at 30 second intervals over the 15 minute data acquisition period. For all tests, flow rate through the absorber plate was constant at 2.15E-5 m 3 /s (0.34 gpm), while electrical
power to the calorimeter, Q fan + Q pump, remained at 4.2 W (14.2 tu/h). Incident solar radiation and net gain for each test have been included in Tables 3 and 4. Complete data summaries can be found in Collins and Harrison (Collins and Harrison 1999), while instantaneous data logs are available from Queen's Solar Calorimetry Laboratory. Thermal and SHG test results are presented in Table 3 to 5 and Figs. 4 and 5. Table 3 and Fig. 4 show the results of tests performed at a vertical orientation, while Table 4 and Fig. 5 show results for the 45 o orientation. The uncertainty associated with individual results was calculated using the method presented by Kline and McClintock (Kline and McClintock 1953). summary of all tests is presented in Table 5. The uncertainties in final SHGC and U-factor results were determined using a method described by Harrison and Dubrous (Harrison and Dubrous 1993). DISCUSSION There was only a slight change in calculated solar heat gain coefficients between vertical and 45 o tilt angles (0.71 and 0.67 respectively). It is likely that the effects of temperature stratification and/or more efficient trapping of warm air between the shade and glass may be responsible for decreasing the SHGC by reducing the inward-flowing fraction at a 45 o tilt angle. This theory can be supported in a number of ways. First, any changes in SHGC with calorimeter tilt can almost certainly be attributed to changes in the inward-flowing fraction. For the 0.828 m 2 (9 ft 2 ) sample size tested here, 0.706 m 2 (8.5 ft 2 ) was covered by a shade which allowed no transmitted component of the incident solar irradiation. Referring to Eqn. (2), if it assumed that the absorbtivity of the shade doesn't change, it can be shown that any difference must therefore be due to changes in the inward-flowing fraction. Further evidence can be found by examining the percent of incident solar radiation that is removed by the absorber panel. etween cases where all other metered energy levels are similar, the absorber removes 53% of the incident solar energy while in a vertical orientation, compared to 48% after it has been tilted. This reduction is likely due to less efficient removal of heat in from the glass and shade to the calorimeter when tilted at 45 o (i.e., a reduction in the inward-flowing fraction). Finally, evidence of stratification and/or a trapped hot air layer can be found by examining shade temperatures. In cases of similar irradiation level and interior and exterior temperatures, the shade temperature increased on average by 6 (10.8 F) when the system was tilted to 45 o. Despite the decrease in measured SHGC with increased calorimeter tilt, differences are likely to be much less significant when testing a commercially available shade system. In comparison to the test sample, a commercial shade and glazing would be less absorbing, and more reflecting and transmitting. n increase of the transmitted irradiance, combined with a decrease in the absorbed irradiance, would reduce the effect of changes in the inward-flowing fraction of absorbed solar energy in the context of the total SHGC. For the worst case scenario examined here (highly absorbing with low transmission), a similar 1 m 2 (10.8 ft 2 ) specimen tested at 45 o under 500 W/m 2 (241.4 tu/h.ft 2 ) of solar irradiation would result in an under-prediction of SHG by only 20 W (14.1 tu/h) or 5% of a value obtained at a vertical calorimeter orientation. Measured differences in SHGC for commercial systems tested at different tilt angles will almost certainly be less pronounced.
Date Test Oct 16-98 Oct 16-98 Oct 23-98 Oct 23-98 Oct 23-98 C Oct 29-98 Oct 29-98 Oct 29-98 C Oct 29-98 D Jun 10-99 Jun 10-99 Jun 11-99 Jun 11-99 Local Time Start Finish 15:00 15:15 14:00 14:15 14:15 14:30 13:45 14:00 13:15 13:30 14:00 14:15 13:30 13:45 12:40 12:55 10:55 11:10 15:15 15:30 16:00 16:15 15:30 15:45 15:55 16:10 Calor. zimuth 1 Start Finish 46.0 46.0 37.0 41.0 39.4 43.4 32.0 36.0 24.1 28.4 34.9 38.7 27.4 31.5 13.7 18.1-15.1-10.7 77.2 80.4 86.1 86.8 80.4 83.4 84.9 87.6 Solar Incident 1 Start Finish 22.0 20.9 28.8 27.0 25.0 23.0 28.1 26.5 30.7 29.4 24.8 23.1 27.6 26.2 30.8 30.1 30.6 31.2 45.9 43.3 3 35.3 43.4 40.7 39.3 36.6 1 0 o indicates due South, while 90 o indicates West. 2 Some measurements lost due to failed thermocouple. Table 1: Operating data collected from individual 90 o tests. tm. Pres. Kpa ("Hg) 102.9 102.9 101.7 101.7 101.7 101.7 102.6 (30.3) 102.6 (30.3) 102.6 (30.3) 102.6 (30.3) Wind Velocity kph (mph) 2.0 (1.2) S 2.0 (1.2) S 10.0 (6.2) S 10.0 (6.2) S 10.0 (6.2) S 3.0 (1.9) N 3.0 (1.9) N 3.0 (1.9) N 3.0 (1.9) N 14.8 (9.2) NE 14.8 (9.2) NE 14.8 (9.2) NE 14.8 (9.2) NE Ti 20.7 (69.3) 20.8 (69.5) 29.3 (84.8) 29.6 (85.2) 29.6 (85.3) 21.4 (70.5) 21.6 (70.9) 21.7 (71.0) 21.1 (69.9) 22.8 (73.0) 22.8 (73.1) 21.9 (71.5) 21.9 (71.5) To 13.7 (56.7) 13.8 (56.9) 13.7 (56.7) 13.2 (55.8) 12.9 (55.2) 13.0 (55.4) 12.7 (54.8) 11.8 (53.3) 10.3 (50.6) 24.0 (75.2) 24.9 (76.8) 25.9 (78.6) 26.2 (79.2) Ti,flow 6.7 (44.1) 6.7 (44.0) 18.5 (65.3) 18.4 (65.2) 18.4 (65.2) 6.4 (43.6) 6.4 (43.5) 6.4 (43.4) 6.0 (42.8) 14.5 (58.1) 14.0 (57.2) 12.8 (55.0) 12.8 (55.0) To, flow 11.5 (52.8) 11.5 (52.7) 22.7 (72.8) 22.6 (72.7) 22.6 (72.7) 11.6 (52.9) 11.6 (53.0) 11.6 (52.8) 11.2 (52.1) 19.1 (66.4) 18.9 (66.1) 17.9 (64.3) 17.9 (64.3) Ti,mask 20.7 (69.3) 21.1 (70.0) 29.2 (84.6) 29.3 (84.8) 29.2 (84.5) 21.9 (71.4) 21.9 (71.5) 21.8 (71.3) 21.2 (70.1) 24.5 (76.2) 24.7 (76.5) 24.0 (75.2) 23.9 (75.1) To,mask 26.3 (79.3) 26.4 (79.5) 23.0 (73.4) 23.0 (73.4) 22.7 (72.9) 2 (82.3) 27.9 (82.2) 26.6 (79.9) 24.6 (76.3) 36.4 (97.5) 38.1 (100.6) 3 (100.4) 38.6 (101.5) TCOG 30.1 (86.2) 30.2 (86.3) 29.6 (85.2) 30.1 (86.1) 29.8 (85.6) 29.6 (85.3) 29.9 (85.8) 29.0 (84.2) 27.4 (81.4) 39.3 (102.8) 41.3 (106.4) 39.5 (103.1) 41.8 (107.3) TCOSh 2 47.1 (116.7) 4 (118.4) 46.4 (115.5) 46.8 (116.3) Qwalls W (tu/hr) (-3.8) (-3.8) -1.4 (-4.7) -1.2 (-4.0) (-3.6) -1.3 (-4.3) -1.4 (-4.7) (-3.7) -0.9 (-3.2) -5.9 (-20.1) -5.9 (-20.1) -8.8 (-30.1) -9.5 (-32.5) Qflow W (tu/hr) 422.8 (1442.8) 429.4 (1465.2) 377.8 (1289.1) 378.2 (1290.4) 377.3 (1287.4) 464.1 (1583.7) 473.1 (1614.4) 468.7 (1599.4) 466.9 (1593.2) 416.3 (1420.5) 442.3 (1509.1) 469.3 (1601.2) 468.9 (1599.6) Qmask W (tu/hr) -0.5 (-1.7) -0.5 (-1.7) 0.6 (2.0) 0.6 (2.0) 0.6 (2.0) -0.6 (-4.3) -0.6 (-1.9) -0.4 (-1.5) -0.3 () (-3.7) -1.2 (-4.2) -1.3 (-4.4) -1.4 (-4.6)
Date Test Oct 16-98 Oct 16-98 Oct 16-98 C Oct 16-98 D Oct 24-98 Oct 24-98 Oct 24-98 C Oct 24-98 D Oct 24-98 E Oct 24-98 F Nov 4-98 Nov 4-98 Jun 10-99 Jun 10-99 Jun 10-99 C Jun 11-99 Jun 11-99 Jun 11-99 C Jun 11-99 D Local Time Start Finish 13:20 13:35 12:45 13:00 12:15 12:30 11:45 12:00 14:45 15:00 14:15 14:30 13:45 14:00 13:15 13:30 12:45 13:00 12:30 12:45 12:30 12:45 12:00 12:15 13:15 13:30 12:45 13:00 12:15 12:30 12:15 12:30 12:45 13:00 13:15 13:30 13:45 14:00 Calor. zimuth 1 Start Finish 1.1 5.2-8.9-4.3-17.9-13.1-27.1-22.2 25.9 29.4 19.3 23.0 11.9 15.9 4.0 8.3-4.3 0.2-13.2-8.3-5.1-0.8-13.7-9.0 39.8 45.9 24.3 32.4 6.3 15.5 6.2 15.5 24.3 32.4 39.8 46.3 52.2 57.5 Solar Incident 1 Start Finish 22.9 23.7 20.5 22.1 20.9 21.2 20.8 20.8 29.0 30.8 26.0 27.6 23.4 24.7 21.5 22.4 20.1 20.8 19.4 19.7 20.1 20.7 19.6 19.8 19.4 17.7 22.3 21.0 23.7 23.2 23.8 23.3 22.4 21.1 19.5 17.7 15.6 13.4 1 0 o indicates due South, while 90 o indicates West. 2 Some measurements lost due to failed thermocouple. Table 2: Operating data collected from individual 45 o tests. tm. Pres. Kpa ("Hg) 102.9 101.5 101.5 101.5 101.5 101.5 101.4 (29.9) 101.4 (29.9) 101.4 (29.9) 101.4 (29.9) Wind Velocity kph (mph) 5.0 (3.1) S 3.0 (1.9) S 3.0 (1.9) S 3.0 (1.9) S (5.0) SW (5.0) SW (5.0) SW (5.0) SW (5.0) SW (5.0) SW (5.0) W (5.0) W (5.0) W (5.0) W (5.0) W 6.0 (3.7) W 6.0 (3.7) W 6.0 (3.7) W E 6.0 (3.7) W Ti 18.6 (65.4) 19.1 (68.6) 19.3 (66.7) 20.4 (68.8) 27.8 (82.0) 28.2 (82.8) 28.6 (83.4) 28.9 (84.1) 29.0 (84.2) 29.2 (84.6) 21.5 (70.7) 21.6 (70.9) 24.7 (76.4) 24.7 (76.5) 24.9 (76.9) 23.2 (73.7) 23.6 (74.5) 23.8 (74.9) 24.1 (75.4) To 13.7 (56.6) 13.5 (56.2) 13.3 (56.0) 13.1 (55.6) 16.1 (61.0) 15.5 (59.9) 14.7 (58.4) 14.4 (57.9) 14.2 (57.5) 14.1 (57.3) 5.2 (41.3) 4.7 (40.4) 23.3 (73.9) 23.0 (73.4) 23.5 (74.3) 22.5 (72.5) 23.6 (74.5) 24.0 (75.2) 24.7 (76.5) Ti, flow 6.7 (44.1) 6.7 (44.0) 6.6 (43.9) 6.6 (43.9) 20.1 (68.3) 19.9 (67.9) 19.6 (67.3) 19.7 (67.5) 19.6 (67.4) 19.7 (67.5) 11.4 (52.5) 11.9 (53.4) 14.2 (57.6) 14.3 (57.8) 14.5 (5) 12.8 (55.1) 13.2 (55.7) 13.4 (56.0) 13.4 (56.1) To, flow 11.5 (52.7) 11.5 (52.7) 11.7 (53.0) 11.7 (53.1) 23.4 (74.0) 23.4 (74.1) 23.4 (74.1) 23.5 (74.4) 23.5 (74.3) 23.7 (74.7) 16.5 (61.7) 16.8 (62.2) 19.9 (67.9) 20.0 (60.7) 20.3 (68.5) 18.3 (65.0) 18.8 (65.9) 19.1 (66.3) 19.2 (66.6) Ti,mask 20.1 (68.1) 20.4 (68.6) 20.7 (69.3) 21.4 (70.4) 28.4 (83.2) 28.7 (83.7) 29.0 (84.3) 29.2 (84.6) 29.3 (84.7) 29.5 (85.1) 23.9 (65.0) 23.7 (74.5) 26.8 (80.3) 26.8 (80.2) 26.9 (80.4) 25.1 (77.2) 25.6 (78.1) 25.9 (78.5) 26.2 (79.1) To,mask 24.1 (75.4) 24.4 (75.9) 25.1 (77.2) 25.8 (78.4) 22.1 (71.7) 22.6 (72.7) 21.7 (71.0) 22.0 (71.5) 21.4 (70.6) 21.4 (70.5) 18.2 (64.7) 18.1 (64.5) 36.6 (97.9) 36.0 (96.7) 36.8 (98.2) 35.7 (96.3) 53.8 (96.2) 36.4 (97.5) 36.9 (98.5) TCOG 28.2 (82.7) 28.2 (82.8) 28.5 (83.4) 29.0 (84.3) 27.8 (82.0) 29.0 (84.2) 28.3 (82.9) 28.3 (82.9) 27.6 (81.7) 27.5 (81.4) 26.0 (78.8) 26.3 (79.4) 38.7 (101.7) 38.7 (101.6) 39.0 (102.1) 38.5 (101.3) 38.5 (101.3) 38.1 (100.6) 38.5 (101.3) TCOSh 2 52.7 (126.9) 53.0 (127.4) 55.2 (131.4) 55.4 (131.7) 55.9 (132.6) 53.0 (127.5) 53.8 (128.8) 54.3 (129.8) 54.7 (130.6) Qwalls W (tu/hr) -1.5 (-5.0) -1.7 (-5.7) -1.6 (-5.4) -1.5 (-5.0) (-3.6) -1.3 (-4.5) -1.3 (-4.5) (-3.6) (-3.7) -0.8 (-2.8) -33.0 (-112.9) -28.3 (-96.5) -2.9 (-9.7) -2.4 (-8.3) -1.9 (-6.5) -3.2 (-11.1) -3.2 (-11.1) -3.4 (-11.6) -3.7 (-12.7) Qflow W (tu/hr) 427.1 (1457.3) 434.8 (1483.6) 453.2 (1546.4) 456.5 (1557.8) 292.8 (999.1) 314.8 (1074.2) 343.7 (1172.9) 351.8 (1200.2) 352.7 (1203.5) 365.3 (1246.5) 453.7 (154) 438.5 (1496.2) 514.5 (1755.7) 515.5 (1758.9) 527.2 (1798.9) 491.5 (1676.9) 503.5 (1718.1) 516.8 (1763.3) 530.6 (1810.4) Qmask W (tu/hr) -0.4 (-1.3) -0.4 (-5.7) -0.4 (-1.4) -0.4 (-1.4) 0.6 (2.0) 0.6 (1.9) 0.7 (2.3) 0.7 (2.3) 0.7 (2.5) 0.8 (2.5) 0.5 (1.8) 0.5 (1.8) -0.9 (-3.1) -0.8 (-2.9) -0.9 (-3.1) -1.0 (-3.3) -0.9 (-3.2) -1.0 (-3.3) -1.0 (-3.4)
Date Test ΔT i,o G W/m 2 (tu/h ft 2 ) Table 3: Individual test results for 90 o tilt angle. ΔT i,o /G m 2 K/W (h ft 2 F/tu) Uncertainty m 2 K/W (h ft 2 F/tu) Gain η Uncertainty Oct 16-98 7.0 (12.6) 847.36 (268.61) 0.0083 (0.0469) 0.00014 (0.0008) 0.62 0.02 Oct 16-98 7.0 (12.6) 843.19 (267.29) 0.0083 (0.0471) 0.00014 (0.0008) 0.64 0.02 Oct 23-98 15.6 (28.1) 811.02 (257.09) 0.0192 (0.1093) 0.00023 (0.0013) 0.58 0.02 Oct 23-98 16.4 (29.4) 837.23 (265.40) 0.0195 (0.1109) 0.00023 (0.0013) 0.56 0.02 Oct 23-98 C 16.7 (30.1) 852.76 (270.32) 0.0196 (0.1113) 0.00023 (0.0013) 0.55 0.02 Oct 29-98 8.4 (15.2) 888.98 (281.80) 0.0095 (0.0538) 0.00015 (0.0008) 0.65 0.02 Oct 29-98 9.0 (16.2) 897.53 (284.52) 0.0100 (0.0567) 0.00015 (0.0008) 0.66 0.02 Oct 29-98 C 9.9 (17.7) 906.07 (287.22) 0.0109 (0.0617) 0.00015 (0.0009) 0.65 0.02 Oct 29-98 D 10.8 (19.4) 896.96 (284.33) 0.0120 (0.0681) 0.00016 (0.0009) 0.65 0.02 Jun 10-99 -1.2 (-2.1) 732.47 (232.19) -0.0016 (-0.0091) 0.00014 (0.0008) 0.69 0.02 Jun 10-99 -2.1 (-3.8) 761.53 (241.40) -0.0027 (-0.0156) 0.00013 (0.0008) 0.71 0.02 Jun 11-99 -4.0 (-7.2) 735.41 (233.12) -0.0054 (-0.0307) 0.00015 (0.0008) 0.77 0.02 Jun 11-99 -4.3 (-7.7) 743.41 (235.66) -0.0058 (-0.0328) 0.00015 (0.0008) 0.76 0.02 Date Test ΔT i,o G W/m 2 (tu/h ft 2 ) Table 4: Individual test results for 45 o tilt angle. ΔT i,o /G m 2 K/W (h ft 2 F/tu) Uncertainty m 2 K/W (h ft 2 F/tu) Gain η Uncertainty Oct 16-98 4.9 (8.8) 864.23 (273.96) 0.0056 (0.0321) 0.00016 (0.0009) 0.62 0.02 Oct 16-98 5.6 (10.1) 890.78 (282.38) 0.0063 (0.0357) 0.00017 (0.0010) 0.61 0.02 Oct 16-98 C 6.0 (10.7) 907.54 (287.69) 0.0066 (0.0372) 0.00017 (0.0010) 0.62 0.02 Oct 16-98 D 7.3 (13.1) 923.96 (292.89) 0.0079 (0.0449) 0.00019 (0.0011) 0.62 0.02 Oct 24-98 11.7 (21.0) 695.95 (220.61) 0.0168 (0.0951) 0.00036 (0.0021) 0.52 0.02 Oct 24-98 12.7 (22.9) 768.17 (243.51) 0.0165 (0.0940) 0.00036 (0.0020) 0.51 0.02 Oct 24-98 C 13.9 (25.0) 816.65 (258.88) 0.0170 (0.0967) 0.00036 (0.0021) 0.52 0.02 Oct 24-98 D 14.5 (26.2) 859.84 (272.57) 0.0169 (0.0960) 0.00036 (0.0020) 0.51 0.02 Oct 24-98 E 14.8 (26.7) 877.95 (278.31) 0.0169 (0.0960) 0.00036 (0.0020) 0.50 0.01 Oct 24-98 F 15.2 (27.3) 889.31 (281.91) 0.0171 (0.0969) 0.00037 (0.0020) 0.51 0.02 Nov 4-98 16.3 (29.4) 925.98 (293.53) 0.0176 (0.1001) 0.00038 (0.0021) 0.56 0.02 Nov 4-98 17.0 (30.5) 938.70 (297.57) 0.0181 (0.1025) 0.00011 (0.0021) 0.54 0.02 Jun 10-99 1.4 (2.4) 946.00 (299.88) 0.0014 (0.0082) 0.00011 (0.0006) 0.67 0.02 Jun 10-99 1.7 (3.1) 957.18 (303.42) 0.0018 (0.0103) 0.00011 (0.0006) 0.66 0.02 Jun 10-99 C 1.4 (2.6) 969.29 (307.26) 0.0015 (0.0083) 0.00011 (0.0006) 0.67 0.02 Jun 11-99 0.7 (1.2) 926.00 (293.54) 0.0007 (0.0042) 0.00011 (0.0006) 0.65 0.02 Jun 11-99 0.0 (-0.1) 937.29 (297.12) -0.0000 (-0.0002) 0.00011 (0.0006) 0.66 0.02 Jun 11-99 C -0.2 (-0.3) 951.53 (301.63) -0.0002 (-0.0011) 0.00011 (0.0006) 0.67 0.02 Jun 11-99 D -0.7 (-1.2) 961.06 (304.65) -0.0007 (-0.0039) 0.00010 (0.0006) 0.68 0.02 Test Table 5: Fenestration performance test results. Daytime U-Factor W/m 2 K (tu/h ft 2 F) SHGC 90 o tilt angle 7.11 ± 0.63 (1.25 ± 0.11) 0.71 ± 0.01 45 o tilt angle 8.69 ± 0.58 (1.53 ± 0.10) 0.67 ± 0.01
The U-factor of the system increased from 7.11 to 8.69 W/m 2 K (1.25 to 1.53 tu/h ft 2 F) as the calorimeter was changed from a vertical to 45 o tilt angle. In addition, each of these U-factors are greater then that of a single glazing, which has a U- factor of 6.30 W/m 2 K (1.11 tu/h ft 2 F) (SHRE 1997). oth of these observations can again be attributed to temperature stratification and/or more efficient trapping of warm air between the shade and glass with increasing calorimeter tilt. For example, consider a single glazing, and single glazing and shade combination. In the case of the single glazing, heat flux through the glass can be determined based on the ambient to indoor temperature difference, the thermal resistance on the glass, and the indoor and outdoor air film coefficients. If we now consider the single glazing and shade combination, heat flux through the glass can be determined in the same way, except that the temperature difference will be between the ambient and hot air trapped between the shade and glass. The heat flux in the second case will also be increased by radiative flux from the hot shade to the glass. Intuitively we know that a greater heat flux through the glazing will occur in the second case. However, because the U-factors in both cases are based on the same ambient to indoor temperature difference, the larger flux of the second case translates into a larger system U-factor. y this same reasoning, if the shade and air between the shade and glass become hotter as the calorimeter changes from a vertical to 45 o orientation, then the U-factor can be expected to increase. Evidence of this can be found by examining shade temperatures. s previously presented, in cases of similar irradiation level and interior and exterior temperatures, the shade temperature increased on average by 6 (10.8 F) when the system was tilted to 45 o. While the required minimum irradiation level was easily achieved, the incident angle requirement was difficult to obtain for the vertical orientation. To produce results with solar incident angles between 20 o and 30 o for a vertical orientation, tests needed to be performed in the early morning or late evening. In both instances, there are problems with rapidly changing irradiance levels that prevent steady state results from being obtained. lthough some results could be produced in the winter months (when the sun is low enough in the sky to avoid this problem), some summer testing was unavoidable. In those instances, the 20 o to 30 o incidence limitation was relaxed, and data was collected at incident angles between 35 o to 45 o. lthough the optical properties of glass do not change significantly between 0 o and 60 o incident angles, it had to be assumed that the optical properties of the shade behaved in a similar manner. However, to avoid the effects of transients caused by rapidly changing irradiation levels, it was considered an acceptable risk that did not seem to affect the results in any measurable way. ecause the fenestration consisted of a single glazing, the results were highly dependent on wind speed. Specifically, low wind speeds would result in small air-film coefficients and lower window U-factors then those determined at high wind speeds. Ideally, calculation of the exterior air-film coefficient would be useful in determining which tests were performed with sufficient wind to exclude the exterior air-film coefficient as a factor. However, the presence of a hot air and shade layer between the glass and the interior makes calculation of the interior and exterior air-film coefficient difficult and inaccurate. In the absence of an accurate measure of h o, tests conducted during periods of low wind speed were simply omitted from the results.
CONCLUSIONS For the tilt angles examined, solar heat gain coefficient did not change significantly. The SHGC calculated in a vertical orientation was 0.71, while 0.67 was determined for a 45 o calorimeter tilt. It is suspected that this difference was primarily due to changes in the inward-flowing fraction. System U-factor seemed to increase with calorimeter tilt from 7.11 W/m 2 K (1.25 tu/h ft 2 F) at vertical orientations, to 8.69 W/m 2 K (1.53 tu/h ft 2 F) at 45 o orientations. While close to U-factor data provided by the NFRC of 6.98 and 6.70 W/m 2 K (1.23 and 1.18 tu/h ft 2 F) (NFRC 1998), the U-factor in both cases was greater then that of a single glazing. It was theorized that the trapping of hot air between the shade and the glass is responsible for increased energy loss through the glass, which appears as an increase in U-factor for the overall system. Similarly, the increase in temperature of this trapped layer with calorimeter tilt is suspected to be responsible for the increase in U-factor between vertical and 45 o calorimeter orientations. In the context of energy calculations, the error produced by determining SHGC in a tilted calorimeter is likely to be small. Errors can be attributed to changes in the inward-flowing fraction that, in the context of a more transmitting and less absorbing shade, will have a limited effect on the overall SHGC for a commercially available shading product. In the "worst case" scenario tested here, the measured reduction between vertical and a 45 o tilt was only 5%. Perhaps more significant is the 22% increase in U-factor when the calorimeter was tilted to 45 o. Caution should be exercised to avoid the use of daytime U-factors calculated in a tilted calorimeter. Significant over-prediction of heat loss or gain would result. RECOMMENDTIONS To fully understand the effects of calorimeter tilt on the solar heat gain coefficient of a fenestration system, a more comprehensive test series is required. The differences in measured solar heat gain coefficient should be examined for a real shade and glazing system, at a number of tilt angles. It is already known that tilt affects the inward-flowing fraction of the shade layer (Collins and Harrison 2000). What is not known is to what degree these changes affect the overall solar heat gain. NOMENCLTURE rea, (m 2, ft 2 ) F Solar Heat Gain Coefficient, dimensionless G Solar Irradiance, (W/m 2, tu/h ft 2 ) N Inward-flowing fraction, dimensionless Q Power, (W, tu/h) R R-factor, (m 2 K/W, ft F h/tu) SHG Solar Heat Gain, (W/m 2, tu/h ft 2 )
SHGC Solar Heat Gain Coefficient, dimensionless T Temperature, (, F) U U-value, (W/m 2 K, tu/ft F h) τ Transmissivity, dimensionless α bsorptivity, dimensionless η efficiency (dimensionless) Subscripts COG Center of glass COSh Center of shade f Fenestration fan Calorimeter fan flow bsorber flow i Indoors input Net input to calorimeter mask Mask or surround panel o Outdoors pump Calorimeter pump walls Calorimeter walls REFERENCES SHRE (1997), SHRE Handbook of Fundamentals. tlanta: merican Society of Heating, Refrigeration, and ir Conditioning Engineers. Collins, M.R., and Harrison, S.J. (2000), The Effects of Calorimeter Tilt on the Inward-Flowing Fraction of bsorbed Solar Radiation in Venetian linds. Under review for publication in SHRE Transactions. Collins, M.R., and Harrison, S.J. (1999), Test of Measured Solar Heat Gain Variation with Respect to Test Specimen Tilt. Contract Report to NFRC. Harrison, S.J. (1993), The Determination of Fenestration Solar Heat Gain Coefficient Using Simulated Solar Irradiance. CNMET, Report by Queen s University. Harrison, S.J., and arakat, S.. (1983), Method of Comparing the Thermal Performance of Windows. SHRE Transactions, 89 (1), 3-10. Harrison, S. J., and Collins, M. R. (1999), Queen s Solar Calorimeter Design, Calibration, and Operating Procedure. Presented at NORSUN 99, Solar Energy Society of Canada, Edmonton, Canada. Harrison, S.J,. and Dubrous, F.M. (1993), Uncertainties in the Evaluation of Window SHGC and U-Values Measured Using an Indoor Solar Simulator Facility. SHRE Transactions, 98 (2), 638-645.
Kline, S. J., and McClintock, F.. (1953), Describing Uncertainties in Single-Sample Experiments. Mechanical Engineering. McCluney, R. (1994), ngle of Incidence and Diffuse Radiation Influences on Glazing System Solar Gain. Proceedings of the Solar 94 Conference of the merican Solar Energy Society, San Jose, California. NFRC (1998), Test of Measured Solar Heat Gain Variation with Respect to Test Specimen Tilt, NFRC Memorandum, prepared by the NFRC Solar Heat Gain Test Procedure Task Group. 70.0 mm 25.4 mm luminum Foil 84.0 x 84.0 mm Fir Frame 25.4 x 12.7 mm Glass 91.0 x 91.0 mm 6.4 mm thick Wood Doweling 25.4 mm Mask Wall 76.2 mm thick Wooden Mounting lock 25.4 mm all sides 12.7 mm Plywood 12.7 mm thick ack View Side View Figure 1: Schematic of shade and window installation.
a) Mask Wall Solar bsorber Panel Test Specimen ir Flow ctive Thermal Guard Insulation Liquid to ir Heat Exchanger Circulating Fans Supply Water Return Water affle b) Figure 2: Queen s Solar Calorimeter. a) Photo of the calorimeter and b) cross-sectional schematic (not to scale).
Control Volume Q flow Q input Q walls Q pump Q fan Q mask Figure 3: Calorimeter energy balance for standard test procedures. 1 0.9 0.8 0.7 Efficiency (η) 0.6 0.5 0.4 0.3 0.2 SHGC = 0.71 +/- 0.01 U = 7.11 +/- 0.63 W/m 2 K 0.1 = 1.25 +/- 0.11 tu/fft 2 h 0-0.0070-0.0020 0.0030 0.0080 0.0130 0.0180 0.0230 ΔT/G (Km 2 /W) Figure 4: Results of 90 o Tests. Efficiency (η) 1.00 0.90 0.80 0.70 0.60 0.50 0.40 0.30 0.20 SHGC = 0.67 +/- 0.01 U = 8.69 +/- 0.58 W/m 2 K 0.10 = 1.53 +/- 0.10 tu/fft 2 h 0.00-0.0070-0.0020 0.0030 0.0080 0.0130 0.0180 0.0230 ΔT/G (Km 2 /W) Figure 5: Results of 45 o tilt tests.