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allowed. 8.3.4 Experimental Determination of Collector Incident Angle Modifier 8.3.4.1 Nonconcentrating and Stationary Concen- trating Collectors. The testing of the solar collector to determine its incident angle modifier can be done by one of two methods. However, during each test period, the orientation of the collector shall be such that the collector is maintained within ±2.5° of the angle of incidence for which the test is being conducted. For those collectors (e.g., evacuated tube collectors) for which the incident angle effects are not symmetrical with direction of incidence, it will be necessary to measure the incident angle effects from more than one direction, as illustrated in Appendix G. Method 1: This method is applicable for testing indoors using a solar simulator, or outdoors using a movable test rack (altazimuth collector mount) so that the orientation of the collector can be arbitrarily adjusted with respect to the direction of the incident solar radiation. The collector is oriented so that the test incident angles between it and the direct solar radiation for the four test conditions are, respectively, approximately 0, 30, 45, and 60 degrees. It is recommended that these data be taken during a single day. For some collectors with unusual optical performance characteristics, other incident angles will be more appropriate. For each data point, the inlet temperature of the heat transfer fluid shall be controlled as closely as possible (preferably within ±1°C [±1.8°F]) to the ambient air temperature. The four separate efficiency values are determined in accordance with Section 8.3.3. Method 2: This method is applicable for testing outside using a stationary test rack where the collector orientation cannot be arbitrarily adjusted with respect to the direction of the incident solar radiation (except for adjustments in tilt). For each data point, the inlet temperature of the transfer fluid shall be controlled, if possible, to within ±1°C (±1.8°F) of the ambient air temperature. The efficiency values are determined in pairs, where each pair includes a value of efficiency before solar noon and a second value after solar noon. The average incident angle between the collector and the solar beam for both data points is the same. The efficiency of the collector for the specific incident angle shall be considered equal to the average of the two values. Efficiency values are determined in general accordance with the method described in Section 8.3.3. As with Method 1, data should be collected for incident angles of approximately 0, 30, 45, and 60 degrees. For some collectors with unusual optical performance characteristics, other incident angles will be necessary. |
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jiangguofeng
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愛與雨下: 金幣+1 2013-06-11 13:00:42
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8.3.4 集熱器入射角修正系數(shù)的實驗測定 非聚光式和穩(wěn)態(tài)聚光式集熱器。 測定太陽能集熱器入射角修正系數(shù)的試驗可以通過兩種方法的任一種方法來完成。 然而,在每個試驗期間，集熱器的方向應(yīng)該維持在實施試驗的入射角的±2.5°之內(nèi)。 對于真空管類集熱器，其入射角效果與入射方向是不對稱的,有必要從多于一個方向測量入射角效果,如附錄G所示 方法1： 本方法適合于室內(nèi)使用太陽能仿真器的試驗或室外使用可移動試驗架（地平式集熱器支架）的試驗，這樣集熱器的方向可以根據(jù)太陽輻射方向任意地調(diào)整。擺正集熱器的方向，使在四種試驗條件下集熱器和太陽直接輻射之間的試驗入射角分別為大約0、30、45和60度。 這些數(shù)據(jù)最好在一天之內(nèi)采集。 對于某些具有特殊光學(xué)性能特征的集熱器，選擇另外的入射角可能更為合適。 對于每個數(shù)據(jù)點,熱傳送液體的入口溫度應(yīng)該盡可能嚴(yán)格地調(diào)整至周圍大氣的溫度（最好是在±1°C [±1.8°F]之內(nèi))。 按照8.3.3項下的方法測定四個單獨的效率值。 方法2： 本方法適合于室外使用固定試驗架的試驗，集熱器的方位不能根據(jù)方向任意地調(diào)整（除傾斜調(diào)整外）。 對于每個數(shù)據(jù)點,傳送液體的入口溫度應(yīng)該調(diào)整在周圍大氣的溫度的±1°C (±1.8°F)之內(nèi)。 成對地測定效率值,每一對包括太陽正午以前的效率值和太陽正午后的效率值。 對于它們的數(shù)據(jù)點，集熱器和太陽光束之間的平均入射角是相同的。 采用特殊入射角的集熱器的效率應(yīng)該視為等同于兩個值的平均數(shù)。 效率值的測定大體上按照8.3.3項下所述的方法進(jìn)行。 如同方法1的情況一樣,搜集的入射角數(shù)據(jù)大約為0、30、45和60度。 對于某些具有特殊光學(xué)性能特征的集熱器，可能需要選擇其它的入射角。 |
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lmb147
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8.3.4.2 Single-Axis Tracking Collectors 8.3.4.2.1 Incident Angle, Longitudinal Axis. The collector is oriented such that the projection of the beam irradiance onto the concentrator aperture plane is parallel to the longitudinal axis of the receiver when the concentrator optics are focused. The angle of incidence is the angle formed by the intersection of a normal to the aperture plane and the beam irradiance. The incident angle modifier shall be measured in accordance with Method 1 in Section 8.3.4.1. If the irradiance source is a solar irradiance simulator, the collimation specified in Section 7.3.3 must be met for the concentration ratio of the collector being tested. 8.3.4.2.2 Incident Angle, Perpendicular to Tracking Axis. This test is used to determine the acceptance angleof the collector, which, in turn, can be used to determine therequired tracking accuracy for the collector, as illustrated in Appendix G.The fluid inlet temperature must be controlled as closely as possible (preferably within ±1°C [±1.8°F]) to the air temperature surrounding the collector throughout the test period. Any one of the following three test methods can be used. Method 1 (North-South Axis): When tested outdoors and mounted for tracking in a north-south axis, the test should bemade at approximately solar noon. The collector is adjusted to a position 7 ½° to 10° ahead of the sun and is then to remain stationary during the remainder of the test, which is to be 60 minutes for a 7 ½° lead or 80 minutes for a 10° lead. During this period, direct normal irradiance shall exceed 800 W/m2 (254 Btu/h·ft2) and remain as constant as possible. The collector may be placed at the latitude angle, at a tilt of 90°minus the sun's maximum altitude for the day and location, or may be placed horizontally according to the manufacturer's intended use. The fluid temperature rise shall be taken and recorded at one-minute intervals, or more often if needed, during the time the sun is within five minutes of the plane of focus of the collector Method 2 (East-West Axis): When testing outdoors and mounted for tracking in an east-west axis, the test shall be made in a continuous run from 10:00 a.m. to 2:00 p.m. Apparent solar time for the location. The fluid inlet temperature shall be adjusted to a level such that (tf,i – ta) can be held constant throughout the test. The collector is to be adjusted for an angle equal to the sun altitude at 11:00 a.m. for the location and time of year—see Tables E.7 through E.12 for altitude angles. The collector is to remain in this position throughout the test period. During the entire period, the direct normal irradiance shall exceed 800 W/m2 (254 Btu/h·ft2) and shall be as constant as possible. The fluid temperature rise shall be taken and recorded at one-minute intervals, or more often if needed, from 10:00 a.m. to 2:00 p.m. Method 3 (Solar Irradiance Simulator): When testing indoors in a solar simulator, the tracking collector is to be mounted in the axis position for which it is designed. The tracking angle is to be adjusted between 7 ½° and 10° from the normal to the simulator beam. The fluid inlet temperature shall be adjusted to a level such that (tf,i – ta) can be held constant during the test. The simulator radiation level is to be adjusted to a minimum value of 800 W/m2 (254 Btu/h·ft2) and held constant during the entire test period. The collector shall then be moved in a tracking mode at the rate of 15° per hour until it reaches an angle from the normal to the simulator beam equal to the angle at which the test was started. The fluid temperature rise shall be taken and recorded at one-minute intervals, or more often if needed, throughout the test. 8.4 Computation of Collector Time Constant According to the definition of time constant given in Section 8.2.2, it is the time required for the temperature of the collector outlet to rise to 63.2% of its steady-state temperature. Regardless of which experimental method in Section 8.3.1 is used, at the start the incident solar radiation is equal to zero and the inlet fluid temperature is held sufficiently close to the ambient air temperature so that (tf,i – ta) ≈ 0.Therefore, by monitoring the entering and exit fluid temperatures as a function of time, the time constant is the time required for(8.21) 8.5 Computation of Collector Thermal Efficiency There are four bases upon which collector thermal efficiency values may be computed. All of these bases use the same test method and the same test-data information is recorded. The four bases result from the fact that users have two choices for the area used in the calculation—either gross area (Ag) or absorber area (Ar)—and they have two choices for the temperature used in the calculation—either the inlet temperature (tf,i) or the average absorber fluid temperature (tf). For example, an efficiency may be calculated using the value of the gross area, Ag, and the average of the inlet temperature, tfi, for the duration of the data point using the following equation  8.22) For those collectors that do not accept diffuse radiation, G in the denominator of Equation 8.22 is only the direct component in the aperture plane, Gbp. Similarly, an efficiency may be calculated by using the absorber area, Ar, and by substituting the average collector temperature, tf, for the average collector inlet temperature, tf,i. With these values, the equation is revised as follows: . (8.23) Regardless of which basis is used to calculate efficiency values, at least 16 data points shall be obtained for the establishment of the efficiency curve, and an equation for the curve shall be obtained using the standard technique of a leastsquares fit. The curve shall not be extrapolated beyond the limits of data. 8.6 Computation of Collector Incident Angle Modifier Regardless of which experimental method in Section 8.3.4 is used, values of the thermal efficiency of the collector shall be determined corresponding to each value of incident angle. For conventional nonconcentrating and linear singleaxis concentrating collectors, only four angles of incidence are needed, i.e., 0, 30, 45, and 60 degrees. (It is noted that a rating standard using this test method may require that Kατ be measured for a different set of angles of incidence.) Since the inlet fluid temperature is held sufficiently close to the ambient air temperature so that (tf,i – ta) ≈ 0, the relationships between Kατ and the efficiency, according to Equations 8.14 and 8.15,are: |
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