TEI's 2-kW PV Installation -- Training Array
In 2010, The Evergreen Institute purchased a 2-kW PV system for its installation classes. This is training system that will be installed many times over the years by eager students.
We decided to purchase and install a grid-connected system with battery backup, arguably the most difficult of all PV systems to install. Students come for four days to install the system. During the PV installation class, they learn a ton about system sizing, wire sizing, ampacity, ground, the National Electric Code as it pertains to PV systems, and much more. They gain a huge amount of practical knowledge that prepares them to install systems on their own homes or on homes and businesses of others.
We began by hiring a local excavator to dig the foundation for concrete piers to which we would attach the rack. He dug six holes below the frost line to avoid frost heave. These holes are 2 x 2 by 30 inches deep.
We then installed 12-inch Sonotubes – paperboard concrete forms. We reinforced them with 2 x 4s so they would not topple over when concrete was poured into them. We inserted a couple bars of No. 3 rebar in each pier after the concrete was poured.
My friend Pete Veronesi (shown here) and my son Skyler and I set up the Sonotubes two weeks before class, so we could pour concrete before the rack was installed. Concrete needs about 28 days to cure, but you can place a rack on a concrete foundation a couple weeks beforehand. Be sure to consult with a professional structural engineer regarding time from pour to installation.
We poured the concrete and two weeks later students showed up to help install the rest of the system. One group quickly went to work on a battery box (below) to hold the 8 six-volt flooded lead acid batteries. Wired in series they produce 48-volt DC that’s fed into the inverter in power outages. The inverter converts the 48-volt DC to 240-volt AC that feeds the critical loads during a power outage.
The battery box must be lined with plastic to prevent acid that retains acid should a battery crack open, a rare occurrence. We purchased a 4 x 8 sheet of ABS plastic for $50, cut it to size, then caulked the seams to create a watertight seal.
Batteries were then installed and wired in series later in the workshop. Wiring batteries in series increases their voltage. Eight six-volt batteries produce 48-volt DC electricity that is fed into the inverters during outages. Remember that in a grid-connected system with battery backup, the batteries only power critical loads and then only when the grid goes down.
Indoors, several students and I began to install the inverter. Here I am screwing in a mounting plate for the two Outback 3648 GVX inverters used in this system. Note, the G in GVX stands for Grid. Be sure to ordr th GVX inverters for interactive grid-tied systems.
Tom Bruns from Dimension, Inc. levels mounting plate for the inverter.
Two students lift the first inverter into place. Inverters are heavy and expensive, so be careful. In our system, we wired two 120-volt inverters in series to produce 240-volt AC electricity.
Tom Bruns and Dan Masterson, two of my students, attach the inverter to the mounting plate using a cordless drill.
Two inverters have been installed. All the DC wires will enter on the right side and connect to busses and circuit breakers sized to protect the wire from overcurrent. All the AC currents will originate on the left side of the inverters.
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We next mounted the charge controller. Charge controllers are required in all battery-based systems: off-grid systems and grid-connected systems with battery backup. They regulate the charging of the batteries, preventing overcharging, which can damage batteries. This charge controller is already wired. That is, this picture is out of sequence, but I didn’t have a photo of the charge controller right after we mounted it.
The battery box needs to be placed as close to the charge controller and inverter as possible to reduce line loss – more specifically voltage drop. We used 2/0 wire to connect the batteries to the system. Large diameter wire helps reduce voltage drop. Remember the batteries are wired at 48 volts, so you need to do everything you can to reduce line loss.
Notice that the battery box (shown below) has a hinged lid. We installed a lock to prevent unauthorized entry. We also installed a two-inch PVC vent pipe to remove hydrogen gas (not shown here).
To mount the rack, we first installed some steel C-channel. We began by drilling holes in the concrete into which we inserted anchor bolts. The C-channel was then attached to the anchor bolts. That’s Rocky Huffman of Sustainable Energy Systems in Augusta, KS. He’s become a great ally and has helped The Evergreen Institute get off the ground by his many extremely generous donations of materials, including insulation and salsa!
Given that this site is extremely windy, we decided to fabricate some brackets to attach the C-channel to the concrete pier. We all felt a lot more comfortable with this arrangement. Our plans called for a single anchor bolt per pier, and that just didn’t seem adequate.
Here are two anchor bolts embedded in the concrete pier. We ran the bolts through hole in a bracket we fabricated from steel.
Student lines up holes on bracket with those on C channel.
The original design called for a single anchor bolt attached to the C channel, as shown below. Note that lower nut in this assembly is the leveling nut. It is used to help us ensure a level mount.We decided to build a bracket and attach the C channel via two anchor bolts (shown above)
To attach the adjustable legs of the rack and the frame to which the PV modules will be bolted, both of which we made from Unistrut, we first installed L-feet which we purchased from the folks at Unistrut. They were bolted into the C-channel. The Unistrut legs were then bolted to the L-feet.
Here, you can see that we have attached the rails to the steel C channel. The PV modules will attach to the rails.
Modules are laid on the ground in proper position for wiring. They’re traditionally placed face down to prevent overheating – they get pretty hot in the sun and hence difficult to handle safely.
Joe Steenbergen drills holes in the aluminum frame of the PV modules to attach the grounding lugs. Notice the wood block Joe used to prevent the drill bit from damaging the back of the module.
Students Joe Steenbergen, now with Victor Energy, and Tom Bruns of Dimension, Inc. started installing PV modules. Modules were bolted to the rack from behind. We ran bolts through holes in the Unistrut into the aluminum frames of the PV modules. This took some time.
We assembled all the modules, then lifted the rack into place one section at a time.
Array is tilted into place. Notice that because this array is heavy, we divided into three sections to make it easier to lift.
The photo below shows the array raised into place. Because this is an adjustable array and because it was first installed in the summer, the tilt angle is rather low to catch more of the high-angled summer sun.
The adjustable back legs (bottom) attach to the Unistrut (rails holding modules) by bolts. The nuts can be loosened to allow a change in the angle of the leg during adjustment.
Students hold the modules while another student bolts the module to the rack from the bottom. Top down mounts with midclamps and end clamps are generally a lot easier.
In this photo, you can see the L-foot that attaches to the back leg. When we adjust the tilt angle of the rack, we must remove the top bolt to raise or lower the leg. Although it sounds simple, you need at least four or five people to support the rack and the legs so it doesn’t come crashing down.
Two students, Darla Pennington and Lucy Mullis, run leads from PV modules into the fused combiner box. The combiner box allows the modules to be wired in parallel.
One problem with a grid-connected system with battery back up is that you must wire the system at fairly low voltages – no higher than 160 volts – to prevent frying the charge controller. In this system, we had to wire the 12 modules in six series strings of two modules each to keep from exceeding the input voltage of the Outback charge controller. Unfortunately, this meant the system was only wired to 88.8 volts.
This photo (below) shows the completely wired combiner box. Note that all the positives are wired into fused disconnects. All the negative leads are wired into a common bus bar. From the fuses and bus bar, one positive and one negative wire run underground in conduit to the charge controller.
After the modules are wired in series, the ground wire is run. See the copper wire. It is attached to ground lugs attached to the aluminum frame of each module and the metal Unistrut of this homemade rack. The ground must be continuous. It runs to the combiner box, then to a ground rod placed near the array.
With the array installed, we next ran conduit through a ditch that ran to the house. The ditch only needs to be 18 inches deep. Conduit protects the wire from moisture. We ran number 4 copper wire to reduce line loss – that is, voltage drop.
Here’s the conduit that runs from the classroom building to the array (below). We ran a second section of conduit to house an AC circuit from the breaker box to power an outlet on the array, a great addition in case you want to run power tools when working on the array or when working in the vicinity in the future. I ran an extension cord to my chicken coop for lighting and heating water in the winter to prevent waterers from freezing.
We run caution tape twelve inches below the surface to warn excavators of the potential danger.
The tilt angle of the rack was determined using a magnetic angle finder available for about $10 (currently) from most hardware stores.
To stabilize the rack, we connected the back legs via pieces of Unistrut.
This is a second set of students, proudly standing in front of the array for the system they have just successfully wired.
This array is installed by students who sign up for TEI's PV Design and Installation class. We leave the array up until a week or so before class, then tear most of it down so students can practice wiring and assembling an array.