1 SDLE SunFarm design & characteristics
1.1 Overview
The SDLE SunFarm was established for outdoor testing of long lived materials, compo-nents, and systems53,54. It is a highly instrumented outdoor test facility which is not commonly found in academic research. It is located on CWRU west campus, and is about one acre in size. There are 16 electrical sites on the SunFarm including 14 high precision dual axis trackers and two sites of fixed tilt racking, shown in FigB.1. A total of 148 full-sized crystal silicon modules bought on the open market from 24 different manufacturers in sets of six or eight were exposed on both trackers and fixed racking.
On the trackers 8000 PV material samples will be exposed under 1X, 2X, 4X, and 5X suns illumination with front surface mirror concentrators.
1.2 Samples
Samples being exposed on the SDLE SunFarm are divided into two major groups: PV modules and PV material sample coupons.
Full-sized crystalline PV modules. In order to better understand power degradation
mechanisms and determine power degradation rates (Rd) 148 full-sized crystalline sil-icon modules from 24 different manufacturers around the world are being exposed on SunFarm to investigate their performance under real-world working conditions. The majority of the population are polycrystalline silicon modules, only two brands are mono-crystalline silicon modules. 24 manufacturers and their nameplate power are listed in appendix A. The 60 crystalline PV module samples studied in this thesis are part of this
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Figure B.1. Top half shows the blue print of SDLE Sunfarm and the distri-bution of 16 electrical sites. Bottom half shows an operating tracker and the electrical cabinets behind trackers. On the tracker frame showing in the figure, top half features six PV modules mounted horizontally, and the bottom half features 48 sample trays mounted in 12 by 4 rows.
Figure B.2. A mechanical drawing of a sample tray, on the left.A 3D draw-ing of 5X front surface mirror concentrator on the right.
total population. These 60 modules are from 20 different manufacturers with 3 samples from each brand.
PV material samples. In order to better understand PV modules degradation
mecha-nisms, we need to know how each component of PV modules degrade over time. PV material and component samples are made and exposed on the SDLE SunFarm Back-sheet samples, front Back-sheet samples and transparent conductor oxide (TCO) samples are cut into 1 × 1.5 inch coupon and held with sample trays(Fig.B.2).With the use of front surface concentrator (Fig.B.2), PV material samples can be exposed at 1X, 2X, 4X, 5X sunlight intensities as well as real-world climate conditions. Material samples will be taken off periodically for optical characters measurements.
1.3 PV mounting system
Two different PV mounting system are being used on the SunFarm. Fourteen dual-axis trackers which are commonly used for high-concentration photovoltaics (HCPV) can
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keep the module plane normal to sunlight. Two sites are fixed tilt rack, which is com-monly used for roof PV installation or utility power plant. Fixed racks are facing south and tilted at 22.3◦
Fixed rack. Power produced from a PV array is proportional to the direct sunlight it
receives. Typically, fixed PV arrays are tilted to an angle equal to the latitude of the arrays’
location, which is the average elevation angle of the sun through out a year. Here in Cleveland since we usually have more cloudy days in winter, the fixed rack on SDLE SunFarm was installed at a shallower angle; latitude of the SDLE SunFarm is 41.5◦, the tilt angle of the fixed racking is 22.3◦. The 30 meters of fixed tilted rack are divided into two identical electrical sites. Eighteen PV modules are exposed on each site.
Trackers. Dual-axis solar trackers orient PV modules normal to direct sunlight at all
times. They are often seen in concentrated photovoltaic (CPV) applications especially HCPV systems, which enable the optical components in concentration system. In flat-panel PV applications, trackers can maximize the performance of PV modules by min-imizing the indecent angle of sunlight to the module plane. The 14 dual-axis trackers on the SDLE SunFarm were manufactured by Feina Tracker, Spain. Each tracker con-sist of three parts, foundation pole, tracker head, and tracker panel, shown in Fig.B.3.
Tracker head is 10 feet off the ground, driven by two DC motors in both horizontal and azimuth directions. The motion of the motors is controlled by a Tracker Control Unit (TCU) inside the 4 to 5 inch electrical box behind each tracker.
Tracker panels are 16’ 4” (5m) width× 13’ 1” (4m) length, shown in FigB.4, which can hold up to 12 PV modules in landscape mode. In order to enhance the capability of testing various modules, components, and materials ten flexible unistrut were placed on the tracker panel to fasten modules and sample trays to the tracker.
Figure B.3. The photo was taken when SunFarm was under construction.
Relative position of fixed rack and tracker was shown in the image, fixed rack is in the front of SunFarm in order to avoid shading issue. Relative position of tracker head, tracker foundation, and two electrical cabinets behind each tracker are shown on the right hand side.
Figure B.4. A mechanical drawing of tracker frame is shown on the left hand side. Distance between horizontal unistrut is 0.5m in order to mount sample trays. On the right, there is a drawing of a tracker frame fully loaded with 12 full size PV modules.
1.4 SunFarm electrical design
Two sites of fixed tilted rack, plus 14 dual-axis trackers formed the 16 electrical sites of the SDLE SunFarm. Two electrical cabinets behind each site separate the power device
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and datalogger system (FigB.3). 110V AC is connected to the power cabinets to power the tracking system as well as Ethernet switches. In the data cabinet, a data logging system consist of Campbell CR1000 datalogger, multiplexer, and battery monitor sensor readings. The SDLE SunFarm have 122 individual power plants, including 120 individual PV modules connected with microinverter and 2 strings of 8 PV modules connected with string inverter. These 122 power plants, which can generate about 32 kW of electricity at peak, all tied to grid through a reversing relay.
1.5 Metrology platform
Power Data. A metrology platform is built for SDLE SunFarm data monitoring, include
power, insolation, and weather monitoring. For power monitoring, either inverters or I-V curve tracers were used. Two trackers with eight full-sized modules on each used Solectria PVI1800 string inverters. Another 10 trackers as well as two tilt rack sites used two brands of micro-inverters, Enphase and Power-One. On the other two trackers a Daystar multi-tracer is used to take I-V curves of full-sized modules and mini-modules with one minute time intervals. A portable I-V curve tracer was used on clear days to take I-V curves on demand.
Insolation data. Redundant insolation sensors were placed around the SDLE SunFarm
in order to get accurate irradiance data and align the trackers. Four Kipp & Zonen pyra-nometers of three different models (CMP6, CMP11, CMP21) were placed on the hori-zontal, tilt rack, and tracker planes. A Kipp & Zonen pyrheliometer (CHP1) was used to measure direct illumination. Multiple split-cell reference cells, Li-cor Li-200 pyranome-ters, and Apogee SP-212 full spectra radiance sensors were placed in the tracker plane to help align the tracker frames’ orientations. Another four Apogee SP-212 full spectra
irradiance sensors and apogee SU-100 UV sensors were mounted on the sample trays to measure the concentrated solar irradiance.
Climate data. Two Vaisala WXD520 weather stations were placed on the SunFarm to
record wind speed, wind direction, rainfall, rain intensity, rain duration, and humidity.
An anemometer was connected to the Master Control Unit of trackers to monitor the wind load on the trackers. A snow cup was used to measure the precipitation. T-type thermocouples were used for backsheet temperature monitoring.
Data acquisition system. The data acquisition system consists of 17 networked
Camp-bell Scientific CR-1000 dataloggers, with each datalogger connected to an AM 16-32 multiplexer, extending the capacity of datalogger to 32 differential measurement chan-nels. The Campbell dataloggers monitor thermocouple and sensor outputs. Enphase micro-inverters use envoy unit to collect data from each individual micro-inverter. Sim-ilarly, Solectria string inverters use Solenview system to collect data. Minute by minute data can be downloaded from their web servers.
1.6 SunFarm Network
Cleveland’s climate, a humid continental, is not typical for PV degradation research. In order to study PV modules’ performance under different climatic conditions, a global SunFarm network was established among nine PV outdoor test beds across the world.
These test beds include four Ohio SunFarms: SDLE SunFarm, Cleveland, Ohio; Lakeview 1MW power plant, Cleveland, Ohio; Replex SunFarm, Mt. Vernon, Ohio; and AEP Dulan test center, Columbus, Ohio (Fig.B.5).
Within the United States, we cooperate with two Q-Lab SunFarms in Arizona and Florida, which are in mid-latitude desert climate and humid subtropical climate area,
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Figure B.5. Upper left corner is SDLE SunFarm with tilt fixed rack in the front and duel axis trackers on the back. Bottom right is Replex SunFarm at Mount Vernon Replex Plastics, with fixed rack, single axis tracker and dual axis trackers. Bottom left is AEP SunFarm atDolan Technology Cen-ter. Mirror Augmented PV (MAPV)system are on the bottom half of the tilt racks and flat back surface mirrors were mounted tilted towards the modules. The top half of the tilt rack has a non-augmented PV system.
respectively. On an even larger scale, we established three SunFarms abroad with in-ternational collaborators: Underwriter Lab Sunfarms in Taitung and Lujhu, Taiwan, and SunFarm at the Indian Institute of Technology Gandhinagar, Ahmedabad (IITGN). These nine SunFarms span a large the range of environmental conditions across the globe.
Similar data collection methods were applied to each SunFarm. In order to better ma-nipulate the Big Data that streams back daily, and manage the sensors that go on each site, a data acquisition system Energy CRADLE was established.