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CCAT MEOW

230 bytes added, 10:15, 8 November 2012
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The PV rack turned out to be one of the most difficult components to rebuild. Although the bandits did not take the PV rack, in their mad dash efforts to remove and steal our valuable PV panels, they did manage to leave the rack system horribly mangled and more-or-less useless to us. Due to a lack of mechanical and structural expertise, our preliminary designs for the new rack weren’t fully functional (each panel weighs roughly 40 lbs. and must be able to safely travel down the highway at speeds of 65 mph; headwinds encountered on the highway must also be considered). In the name of safety and a long lasting system, we decided to work with local professional John Davis at Solar Racks to build the new PV rack. For John’s efforts, we traded him a couple of 60 W monocrystaline PV panels that had been lying around the CCAT house and had not been put to use in some years; because we are a worthy NGO, John also gave us a good deal on materials. By working with John we were able to give several students the opportunity to work in a metal working shop and receive hands-on experience with building solar racks (*ADD PHOTOS*).
The final design for the rack was a stationary flat-mount (i.e. 0 degree tilt) rack. This design was ultimately chosen for three reasons. The first reason is that a flat mount simplified the design and minimized materials cost. Second, we figured that a flat-mount rack would be much safer than a tilted or adjustable rack for the mobile system. The logic here was that if we were to make an adjustable rack, we would run the risk of someone accidentally leaving it 45 degrees during transport (which is likely since CCAT has a very high turn around rate of employees who don’t always know the finer details of how the systems at CCAT work). Additionally, the rope and pulley system used for the adjustable rack on the MEOW V1 was not devoid of problems: the rack required the careful attention of two people to raise and lower the panels and the rack [http://www.appropedia.org/CCAT_MEOW_rack reportedly bowed at critical points]. The third reason we went with the flat mount is that our analysis showed we would only lose 59 kWh/year (6%) in energy production as opposed to a tilted rack system (41°), and would lose 116 kWh/year (12%) with an adjustable rack. We decided the risks and costs of a tilted/adjustable rack didn’t outweigh the benefit of the additional 6%/12% electricity we would receive.
===Wiring the System===
Because CCAT is mainly a teaching and demonstration facility, we decided to take the reconstruction process as an opportunity to give a free PV system wiring workshop to HSU students and community members. The workshop was led by the Arcata based solar company Roger and His Band of Merry Solar Installers (*ADD PHOTOS*). Held over two separate weekends, the workshop series was very successful where over XX 20 participants learned the basics of PV wiring and left with the satisfaction of contributing to a fully functional solar system!
The four panels of the array were all wired in series (i.e. four series with one parallel string). This configuration simplified the wiring process and made the system safer by allowing less DC current to pass through the lines. Initially, we had some concern that the 150 V limit of the charge controller would be surpassed in an all series configuration. If all four panels were to be simultaneously open-circuited the system voltage would total 147 V (36.8 V x 4 panels); if one considers a safety factor of 1.25 (representing a cold morning with full sun) the system voltage could spike to 184 V and possibly fry the charge controller. However, after consulting with Roger and his professional team, they informed us that this situation is highly unlikely due to the flat (0degree) tilt of the array, the local solar resource, and shading profile at the CCAT house.
The four 12V GEL batteries were all wired in series to create the necessary 48 V input that the GVFX48 requires to turn on. We installed all the necessary breakers inside the convenient Outback PS1 combiner box to protect each component from overload current (between the PV panels and charge controller, between the charge controller and batteries, and between the batteries and inverter).
The Table below shows the monthly and annual estimates for the amount of electricity (kWh) the MEOW V2 is capable of producing. The calculations use [http://rredc.nrel.gov/solar/old_data/nsrdb/1961-1990/redbook/sum2/24283.txt NREL data] for the solar resource in Arcata, CA at a horizontal tilt (2nd column) and the monthly shading profile in the CCAT driveway according to a solar path finder reading (3rd column). The calculations are for the 920 W, flat-mount (i.e. 0°) array and consider a total system efficiency of 85% (93% for the inverter, 95% for the batteries, and 96% for wire losses and module soiling).The calculations estimate that the MEOW will produce roughly 870 kWh/year, which could change based on how often the trailer is moved from the CCAT driveway as well as the depth of discharge (DOD) that the batteries are cycled to.
{| class="wikitable"
| 73%
| NA
| '''871.3'''
|}
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