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SEARC OSOTF Design and Operations Manual

12,480 bytes added, 15:20, 11 October 2011
Regular maintenance of the system should be performed to ensure that all connections in the box are secure, and should be checked every time wiring is modified inside the box. In addition, at times of high humidity, desiccant should be utilized to limit the humidity inside the measurement enclosure.
 
== Panel Electrical Measurement ==
The measurement of panel performance is achieved utilizing a purpose built transducer from AYA instruments. The transducer is placed in the system between the panel and DC-DC converter , which allows the Maximum Power Point Tracking (MPPT) of the DC-DC converters to individually optimize the output of each panel , ensuring that the measured output is the maximum possible output for the panel in its current condition.
 
The purpose of the power transducer is to measure current and voltage separately through the use of a current shunt and voltage divider circuit, respectively. This allows for a measurement of panel power output, and also gives an indication of the shape of the I-V curve at varying operating conditions, as the location of the maximum power point on an I-V graph can be determined. This can help to identify temperature, spectral and degradation effects on the PV panels. Essentially, in a traditional I-V plot a vertical shift in the maximum power point indicates a change in insolation, and a horizontal shift represents a change in temperature or other factors effecting cell efficiency.
 
=== AYA Sensor overview ===
A single AYA sensor box is shown in Figure. Each box is capable of measuring the current and voltage outputs for 32 photovoltaic panels, for a total of 64 outputs. The current is measured using an isolated current shunt and voltage is measured using an isolated voltage divider circuit. All outputs are measured through single ended measurements on an AM 16/32B multiplexer.
 
Channel isolation is critical in this application to eliminate noise or potential damage to the data acquisition system due to common mode voltages. Isolation ensures that the power source (in this case PV panels) does not have a path to ground through the datalogger. A good explanation of common mode voltages and the danger they pose to data acquisition systems can be found here (http://www.dataq.com/applicat/articles/isolation.htm)
 
==== Current shunt ====
The current shunt used in each circuit is the CMS2015-SP10 MagnetoResistive Current Sensor purchased form sensitec. This unit is a high accuracy integerated current shunt and galvanic isolator which outputs a 0-2.5V signal proportional to its rated 0-15A input current. This current range was chosen to allow the flexibility to handle high current crystalline solar modules. The absolute error on this unit therefore is 0.075A, giving the following accuracies for a crystalline and amorphous panel at three levels of insolation:
 
 
{| style="border-spacing:0;"
| style="background-color:#000000;border-top:0.035cm solid #000001;border-bottom:0.018cm solid #00000a;border-left:0.035cm solid #000001;border-right:none;padding-top:0cm;padding-bottom:0cm;padding-left:0.191cm;padding-right:0.191cm;font-color:#FFFFFF";| '''Irradiation (G/G<sub>STC</sub>)'''
| style="background-color:#000000;border-top:0.035cm solid #000001;border-bottom:0.018cm solid #00000a;border-left:none;border-right:none;padding-top:0cm;padding-bottom:0cm;padding-left:0.191cm;padding-right:0.191cm;font-color:#FFFFFF";| '''Crystalline (Isc<sub>STC</sub><nowiki>=8.24A)</nowiki>'''
| style="background-color:#000000;border-top:0.035cm solid #000001;border-bottom:0.018cm solid #00000a;border-left:none;border-right:0.035cm solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.191cm;padding-right:0.191cm;font-color:#FFFFFF";| '''Amorphous (Isc<sub>STC</sub><nowiki>=1.55A)</nowiki>'''
 
|-
| style="border:0.018cm solid #00000a;padding-top:0cm;padding-bottom:0cm;padding-left:0.191cm;padding-right:0.191cm;"| <div align="right">'''1/4'''</div>
| style="border:0.018cm solid #00000a;padding-top:0cm;padding-bottom:0cm;padding-left:0.191cm;padding-right:0.191cm;"| <div align="right">3.6%</div>
| style="border:0.018cm solid #00000a;padding-top:0cm;padding-bottom:0cm;padding-left:0.191cm;padding-right:0.191cm;"| <div align="right">19.4%</div>
 
|-
| style="border:0.018cm solid #00000a;padding-top:0cm;padding-bottom:0cm;padding-left:0.191cm;padding-right:0.191cm;"| <div align="right">'''1/2'''</div>
| style="border:0.018cm solid #00000a;padding-top:0cm;padding-bottom:0cm;padding-left:0.191cm;padding-right:0.191cm;"| <div align="right">1.8%</div>
| style="border:0.018cm solid #00000a;padding-top:0cm;padding-bottom:0cm;padding-left:0.191cm;padding-right:0.191cm;"| <div align="right">9.7%</div>
 
|-
| style="border:0.018cm solid #00000a;padding-top:0cm;padding-bottom:0cm;padding-left:0.191cm;padding-right:0.191cm;"| <div align="right">'''3/4'''</div>
| style="border:0.018cm solid #00000a;padding-top:0cm;padding-bottom:0cm;padding-left:0.191cm;padding-right:0.191cm;"| <div align="right">1.2%</div>
| style="border:0.018cm solid #00000a;padding-top:0cm;padding-bottom:0cm;padding-left:0.191cm;padding-right:0.191cm;"| <div align="right">6.5%</div>
 
|-
| style="border:0.018cm solid #00000a;padding-top:0cm;padding-bottom:0cm;padding-left:0.191cm;padding-right:0.191cm;"| <div align="right">'''1 '''</div>
| style="border:0.018cm solid #00000a;padding-top:0cm;padding-bottom:0cm;padding-left:0.191cm;padding-right:0.191cm;"| <div align="right">0.9%</div>
| style="border:0.018cm solid #00000a;padding-top:0cm;padding-bottom:0cm;padding-left:0.191cm;padding-right:0.191cm;"| <div align="right">4.8%</div>
 
|}
Therefore it can be seen that the accuracy of this sensor is acceptable for all crystalline panels for a wide variety of irradiations, however in the case of the Amorphous panel, there are some issues with the resolution of the sensor. It is recommended that for the channels associated with amorphous panels, the CMS2005-SP10 sensor is used, which has a current range of 0-5A, which more closely matches the output of the amorphous panel.
 
A wiring diagram of the current shunt circuit is shown [[image:Current and Voltage diagram.jpg |thumb| Current Shunt and voltage divider circuit in the AYA sensor box]]
 
==== Voltage divider circuit ====
The voltage divider circuit can be used to measure the open circuit or power point voltage of a solar panel. The wiring diagram for the voltage divider circuit implemented into the AYA box is shown in teh accompanyig figure. The operating principle of this unit is described in the equation in the accompanying figure
 
Essentially, the output voltage is proportional to the input voltage, relative to the ratio of the resistors in the circuit. In this case, an input voltage of 200V has been scaled to a 0-5Voutput. The resistors used in the shunt are MFR series purchased from Yageo corporation, with a 1% accuracy and a constant response up to 70°C.
 
Each voltage divider channel has a gain and offset potentiometer, which was adjusted for zero offset in the circuit. The output voltage of this circuit is galvanically isolated from the power circuit using a unity gain AD202 isolation amplifier.
 
==== Data logger measurements ====
All datalogger measurements were made to the CR1000 datalogger through single ended measurements on the AM16/32 B multiplexer. The choice to use single ended measurements was made due to the relatively high voltage range of the input signals, and in order to limit the number of channels utilized on the CR1000. A wiring diagram showing this arrangement is shown in section Wiring Diagram
 
A Major concern with the use of single ended measurements is the difference in ground references between the sensor being measured and the data logger. In this case, this effect is limited by the sharing of ground plane between the data logger and transducer power supply. In addition, the ground plane of the transducer and multiplexer are carried through shielded cable back to the data logger, ensuring that ground plane interference is a minimum. The maximum cable length for the transmission of the ground plane is 60ft for current measurements.
 
=== Wiring Diagrams ===
A wiring diagram showing a single circuit of the data acquisition system associated with measuring panel output is shown, and a .pdf file is also available[[image:Datalogger_wiring_diagram.jpg|thumb|Datalogger wiring diagram showing a single channel of current and voltage input]][[image:Datalogger_wiring_diagram.pdf‎|thumb|Datalogger wiring diagram showing a single channel of current and voltage input]]
 
=== Callibration ===
Ground loop detection
 
The effects of ground loops were determined by measuring the voltage difference between datalogger ground and AYA sensor ground transmitted to the datalogger. This test was performed using a logging Digital Multi Meter as it multiplexed through the full range of sensors for voltage and current. The results are shown below in Figure
 
Common-mode voltage
 
Common mode voltages present in each datalogger channel were measured as the average of the voltage between the output hi and power supply ground and output lo and power supply ground. The results for each channel are shown in the following table.
 
DMM validation
 
The overall accuracy of the system was determined by independently measuring the short-circuit output of each panel and the output of the accompanying AYA transducer at the time it is being measured by the CR1000 datalogger. This test allowed for a determination of the accuracy of the system at each point in the measurement, showing areas where the greatest loss in accuracy is found.
 
 
=== Operation and Maintenance ===
The AYA transducer is measured through measurement of the 0-2.5V output on the current side, and through the measurement of the 0-5V output on the voltage side. Each channel is measured independently through the AM16/32B multiplexer. The code required to scan these channels is outlined in the programming manual.
 
Regular maintenance of the system should be performed to ensure that all connections in the box are secure, and should be checked every time wiring is modified inside the box. In addition, at times of high humidity, desiccant should be utilized to limit the humidity inside the measurement enclosure.
 
== Panel Thermal Measurement ==
All panels were monitored for back sheet temperature at a minimum of one point. If channels were available, two points at the top and bottom of the panel were measured, as there have been recorded differences in temperature over the length of a panel. In order to translate from back sheet temperature to cell temperature, a correlation is available from [Sandia Laboratories http://photovoltaics.sandia.gov/docs/PDF/King%20SAND.pdf] (eq.12).
 
===Thermocouples===
The temperature of the back sheet of each panel was measured utilizing site-fabricated thermocouples. The thermocouples were created by purchasing outdoor compatible duplex insulated SLE (Special limits of error) thermocouple wire. The wires were run from the data acquisition box to each panel, and cut to length. It is critical to label both ends of the wire with a durable label to ensure proper hook-up. Both ends were then stripped and a portable [thermocouple welder http://dcccorporation.thomasnet.com/item/thermocouple-welders/hotspot-i-ii-thermocouple-welders/item-1112?&plpver=10&origin=compare&filter=0&CTypeID=2] was used to create a solid junction at the panel side. For future designs, however, it would be recommended to utilize [shielded thermocouple wires http://www.omega.com/ppt/pptsc.asp?ref=TT-J-TWSH&Nav=temh06] to protect against electrical interference over the long spans of wire required.
 
===Thermocouple mounting===
The thermocouples were mounted to the rear face of each panel using alumized tape. In addition, a thermally conductive paste was used to ensure full thermal contact of the thermocouple with the back face of the panel. The typical installation procedure was to cut a piece of tape approximately 1.5"X1.5", place a pea-sized amount of thermal paste in the center of the tape, and seat the thermocouple junction in this paste. The entire unit was then affixed to the rear of the panel. In terms of placement, the thermocouple was placed at the absolute center of amorphous panels, or in the case of crystalline panels at the center of an individual cell which was closest to the center of the panel, in order to ensure a proper reading of cell temperature.
 
===Multiplexer attachment===
The thermocouples were wired into AM 25T multiplexers. These multiplexers are solid state devices and include integrated cold-point calibration and should be used for all thermocouple measurements. For a wiring schematic of the AM 25T see the wiring diagrams section.
 
= Data Acquisition =
The Data acquisition for the SEARC OTF is managed through hardware and software purchased from [http://www.campbellsci.ca/ Campbell Scientific]. Data is collected every 5 minutes from all sensors and is stored as .csv (comma seperated values) data files. Currently the data is streamed to a computer at the Applied Sustainability Group at Queen's, where it is automatically backed up onto a dropbox cloud server, as well as onto two other physical PC's.
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