在单轴太阳能跟踪器上进行直接跟踪误差表征【中文9212字】
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在单轴太阳能跟踪器上进行直接跟踪误差表征【中文9212字】,太阳能,跟踪,进行,直接,误差,表征,中文
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Energy Conversion and Management 105 (2015) 1281 1290Direct tracking error characterization on a single-axis solar trackerFabienne Sallaberry a,b, Ramon Pujol-Nadal c, Marco Larcher d, Mercedes Hannelore Rittmann-Frank da CENER (National Renewable Energy Center), Solar Thermal Energy Department, C/ Ciudad de la Innovacin, 7, 31621 Sarriguren, Navarra, Spainb Public University of Navarra (UPNA), Projects and Rural Engineering Department, Campus Arrosadia s/n, 31006 Pamplona, Navarra, Spainc University of Balearic Islands, Physics Department, Ctra. Valldemossa km 7.5, 07122 Palma de Mallorca, Spaind Institut fr Solartechnik SPF, Hochschule fr Technik HSR, Oberseestrasse 10, CH-8640 Rapperswil, Switzerlanda r t i c l e i n f o Article history:Received 24 April 2015Accepted 30 August 2015Available online 14 September 2015Keywords:Tracking error Single-axis tracking Acceptance anglea b s t r a c t The solar trackers are devices used to orientate solar concentrating systems in order to increase the focus- ing of the solar radiation on a receiver. A solar concentrator with a medium or high concentration ratio needs to be orientated correctly by an accurate solar tracking mechanism to avoid losing the sunrays out from the receiver. Hence, to obtain an appropriate operation, it is important to know the accuracy of a solar tracker in regard to the required precision of the concentrator in order to maximize the collector optical efficiency. A procedure for the characterization of the accuracy of a solar tracker is presented for a single-axis solar tracker. More precisely, this study focuses on the estimation of the positioning angle error of a parabolic trough collector using a direct procedure. A testing procedure, adapted from the International standard IEC 62817 for photovoltaic trackers, was defined. The results show that theangular tracking error was within 0.4 for this tracker. The optical losses due to the tracking werecalculated using the longitudinal incidence angle modifier obtained by ray-tracing simulation. The acceptance angles for various transversal angles were analyzed, and the average optical loss, due to the tracking, was 0.317% during the whole testing campaign. The procedure presented in this work showed that the tracker precision was adequate for the requirements of the analyzed optical system.。 2015 Elsevier Ltd. All rights reserved.1. IntroductionThe method to determine the precision of a solar tracker used in solar thermal collectors has not yet been standardized. Nowadays, existing testing standards for solar collectors consider a solar tracker as a part of the collector 1 . Thus, the losses of efficiency due to tracking imprecision are not quantified in the global collec- tor efficiency test.The International standard IEC 62817 2 enables to certify solar trackers for photovoltaic applications considering both accuracy and durability. However, this standard accuracy test is not applica- ble to solar thermal concentrator tracker, particularly to single-axis solar tracker for linear solar concentrator.The Spanish committee AEN CTN 206/SC 117 3 redacted a pro- posal to the international committee IEC 117 4 , for the standard characterization of parabolic-trough collector (PTC) solar trackers which led to the creation of a working group for a new standard draft approved in November 2014 5 . Corresponding author at: CENER (National Renewable Energy Center), Solar Thermal Energy Department, C/ Ciudad de la Innovacin, 7, 31621 Sarriguren, Navarra, Spain. Tel.: +34 948 25 28 00.E-mail address: fsallaberry (F. Sallaberry).According to Mousazadeh et al. 6 , solar trackers are classified according to their orientation (one or two axes) and their actuation (active or passive, and open or closed loop). Depending on the type of collector, different solar tracking systems rely on different track- ing strategies. For example, for the Fixed Mirror 7 the receiver is the only moving component, while for the PTC 8 , the whole system (mirror and absorber) tracks the sun direction at the same time. The present paper is focused on a small-sized PTC with active loop.In order to identify the tracking error of a solar tracker, devices similar to the sun-sensor on a closed-loop actuation tracker can be used. However, the characterization of the tracking error requires a highly accurate electronic device. Since 1987, when Bhatnagar et al. 9 experimentally measured the average tracking error of a parabolic concentrator with a single-axis tracker at different solar hours using the sun-sensor of the collector, the tracking error isbeing studied. In that study, the tracking error was estimated from the design of the sensor and was 0.93 at noon.The tracking error has also been investigated in several recent studies. In the work of Daz-Flix et al. 10 , the absolute tracking error distribution of a heliostat was theoretically evaluated using Monte-Carlo simulations. Assuming several error sources on theheliostat position, the tracking errors were found to be up to 0.7/10.1016/j.enconman.2015.08.081 0196-8904/ 。 2015 Elsevier Ltd. All rights reserved.Contents lists available at ScienceDirectEnergy Conversion and Managementjournal homepage: www. elsevier. com/locate/enconman1282 F. Sallaberry et al. / Energy Conversion and Management 105 (2015) 1281 1290Nomenclatureaa0 ac as bc/r cc csDgtrackgoptha hd hi hL hThtrackq rsolar absorptance ()solar absorptance at normal incidence () inclination of the solar tracker ()solar altitude angle () collector tilt ()rim angle ()concentrator azimuth angle (respect to south) () solar azimuth angle (respect to south) ()optical losses due to tracking error (%)optical efficiency of the collector ( ) acceptance angle ()defocus angle () incidence angle ()longitudinal incidence angle () transversal incidence angle () angular tracking error ()solar reflectance ( )CPVdabs dglass EWf GbT Gbn Hb IAMk Kbconcentrating photovoltaicabsorber tube diameter (mm) glass tube diameter (mm)East Westfocal length (m)direct solar irradiance on the aperture plane (W/m 2) direct normal irradiance (W/m 2)direct normal solar irradiation (MJ/m 2)K b,simK b,theorspecular scatteringdeviation transmittance ( )mirrors (mrad) or standardsa Caperture width (m)geometrical concentration ratio defined as the ratio of the aperture area to the absorber area ( )LLED NS PTCuincidence angle modifier ( )glass tube extinction coefficient (m 1)incidence angle modifier relative to the direct incidence radiation ( )incidence angle modifier relative to the direct incidence radiation, obtained by simulation ( )incidence angle modifier relative to the direct incidence radiation, obtained by theoretical calculation ( ) collector length (m)Light Emitting Diode North Southparabolic trough collector wind speed (m/s)with a circularly symmetric Gaussian distribution. In the study by Sun et al. 11 , a beam characterization system was used to evalu- ate the tracking error of two heliostats from a central tower solar plant with an estimated accuracy of about 2% for the positioning angle measurement. Zheng et al. 12 analyzed the tracking error on an Linear Fresnel Reflectors collector, and the effect of different factors such as the reflectors positioning, the rotation axis position, the driver accuracy, the tracking software algorithm, the coordi- nates and the structure error.In an earlier study, solar tracking using an inclinometer on a double-axis solar tracker was directly characterized 13 . Addition- ally, a testing procedure was defined to estimate the long-term tracking error due to the positioning of a small-sized solar tracking collector 14 . The maximum optical loss due to tracking was of 8.5%, but the average long-term optical loss calculated for one year was about 1%.For a PTC, a single angle tracking, namely the elevation angle, must be examined in order to determine the solar tracker preci- sion. Various methods are available to control the solar tracker ele- vation, such as optical device 15 , artisanal shadow device 16 , and angular sensor (encoder or inclinometer) 13 .There are different optical devices commercially available to characterize the tracking error. In 2009, Davis et al. designed a commercial device 17 with a high accuracy sensor using image processing to estimate the pointing error of double-axis solar trackers. In 2010, Minor and Garca also presented a solar tracking system based on image processing acquired by a webcam 15 ,which was able to measure the tracking error of a double-axis tracker with an accuracy of 0.1 . In 2012, Missbach et al. 18 pre-sented the results of a sun-sensor by Black Photon company, show- ing highly accurate measurements (standard deviation of 0.01%) on a double-axis tracking system for concentrating photovoltaic (CPV). But all these devices are applicable only for double-axis trackers and not for single-axis trackers.The acceptance angle is commonly provided by the manufac- turer of a solar concentrating system. This value is very useful to identify the requirements of the solar tracker mechanism, but does not provide information on the amount of optical losses in real operating conditions.In this study, a single-axis solar tracker, used on a small-size PTC, is characterized. The paper is organized as follows: in Section 2 the components are presented; Section 3.1 describes the method- ology to obtain the angular tracking error. In Section 3.2 the incidence angle modifier (IAM) is presented by a ray-tracing simu- lation. In Section 3.3 the optical losses due to the tracking errors are calculated using the angular errors estimated in Section 3.1 and the IAM curve obtained in Section 3.2 . The results, presented in Section 4, show that during the testing period the 95 th percentiletracking accuracy was 0.33 and the mean weighted optical lossesleading to a reduction of the collector efficiency was 0.317%. Finally, the conclusions are compiled in Section 5.2. Materials2.1. Solar collector and solar trackerThe solar collector referred to in this study is a small-size PTC with a single-axis solar tracker, model PolyTrough 1800 manufac- tured by the company NEP Solar AG 19 . It had been tested in the SPF laboratory (Institut fr Solartechnik 20 ) according to the European standard EN 12975-2 21 , recently replaced by the International standard ISO 9806 1 .In this solar tracker, the algorithm calculated the sun position at different times; hence it was classified as an active open-loop type actuator. However, no encoder was used, but there was a Hallsensor to detect the motor position. The precision of the tracker was supposed to be 0.025 .This collector, shown in Fig. 1, was tested with an East West (EW) orientation. The study by Larcher et al. 22 and the testing report from SPF 20 provide more details about the solar collector and the tests performed by SPF laboratory. A similar NEP collector PolyTrough 1200 with smaller aperture was tested by Miller et al. 23 at the Australian laboratory CSIRO according to different test- ing methodologies (standards 21,24 and Ref. 25 ). In these studies, the thermal efficiency curves of different models were compared. However, the angle positioning errors of the tracking systems were not studied.F. Sallaberry et al. / Energy Conversion and Management 105 (2015) 1281 1290Fig. 1. Picture of the solar collector PolyTrough 1800.1 0:017 10.8951283Table 1Characteristics of the concentrator 22 .Parameter Symbol ValueFocal length f 647 mmAperture width a 1845 mmCollector length L 10.347 mMirror solar reflectance (anodized q 0.885aluminum with PVD aluminum layerand protective coating)Specular scattering mirrors r 4.4 mradAbsorber tube diameter d abs 34 mmAbsorber tube wall thickness 1.5 mmSolar absorptance at normal incidence on a0 0.9427the absorber fin (measured at SPF)Angular absorptance dependence (adopted a l I 1:8a0from 26 ) cos hiGlass tube diameter d glass 56 mmGlass tube wall thickness 2.5 mmGlass tube refraction coefficient on both 1.473sidesCover tube transmittance (measured at sSPF)Glass tube extinction coefficient k 15 m 1Concentration ratio C = (L a)/ (p dabs )17.3 Fig. 2. Scheme of the rotation angles: tracking angle ac, collector inclination bc,collector azimuth cc, the longitudinal and transversal angles hL and hT.Rim angle / r 71 Table 1 presents the dimensions and the physical properties of the NEP PolyTrough 1800.2.2. Digital inclinometerA tilt angle sensor, model A-2T manufactured by US Digital 27 with connection RS-232C for communication, was used to measure the tilt angle of the solar tracker. This inclinometer was a digital gravity angle sensor that measures inclination on a single axis. The device accuracy was 12-bits.3. Proposed testing procedureThe tracking error characterization proposed was based on the standard IEC 62817 2 testing procedure. This standard is applica- ble mainly to double-axis CPV solar trackers. Thus, the proposed testing methodology needs to be adapted for solar thermal PTC trackers, which are mainly single-axis.3.1. Tracking error characterizationFor the testing procedure, in order to define the incidence angles of the solar radiation on the collector, first it was necessary to describe three position angles for the collector, two of which were fixed (the collector inclination bc and the collector azimuthcc) and one was moving in order to track the sun (the trackingangle ac). The angle ac was by definition 0 when the collector aperture was orientated to the zenith, and it represented the rota-tion angle on the longitudinal axis; the collector inclination bc was also by definition 0 when the collector aperture was orientated tothe zenith, and it represented the rotation angle in the transversal axis; the collector azimuth cc was by definition 0 when the collec-tor was orientated to the South, and it represented the rotation angle respect to the South. Fig. 2 illustrates the rotation angles defining the collector position.At first, the solar tracker was positioned on the testing bench or platform. It was crucial to determine the real position of the solar collector, because some errors on the collector positioning could lead to additional tracking errors. The azimuth of the collector ccwas set on the testing bench with a precision of 0.1 . Before thetesting sequences, the inclination of the collector bc was measured by a spirit level with an accuracy of 0.057 and was found to be 0.To monitor the rotation angle ac, the digital inclinometer, shown in Section 2.2 , was located on an aluminum profile which was fixed to a component used for the reflectors holders avoiding torsional or tolerance effects (see Fig. 3).The digital inclinometer was connected by cable to a computer, whose output was recorded every ten seconds. The direct normal irradiance Gbn was measured by a pyrheliometer (model CHP 1from Kipp h gopthT ; hL b;sim T L g 0o; 0o cosh bT.1for the suns position, the solar elevation as and the solar azimuthcs were calculated using the solar algorithm reported by Blanco- Muriel et al. 30 , with an accuracy of 0.5 min of arc (0.0083 ).Then, the incidence angles hT and hL were calculated as reported elsewhere 31 .The theoretical acceptance angle can also be defined in a para-bolic trough collector according to Eq. (2) 41, where / r is the rim angle of the parabola and C is the geometric concentration ratio.sin/ r The proposed testing method, based on the standard IEC 628172 , requires data recorded for a minimum of five days with a min-ha pC 2imum direct normal irradiation Hb of 2400 W h/m 2 per day. In addition, the data should be separated into different data bins: one for a low wind (for wind speed u lower than or equal to 4 m/s) and one for high wind speed (for wind speed u
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