Construction Details for a PV Array at Craters of the Moon Monument

On a recent vacation trip we stopped a couple national monuments in which the visitor centers had grid-tied solar electric arrays.  I thought that the arrays showed some nice construction detail, and took quite a few pictures. 

The pictures below show show some of the construction details that I thought might be helpful to people building PV installations.

The pictures are from Craters of the Moon National Monument.  I might add that Craters of the Moon has a lot more interesting things to look at than PV arrays, but I figure those things are covered in a lot of other places :)  The drive up from Twin Falls Idaho along US 93 and then Idaho 33 toward Craters of the Moon was exceptional.

Craters of the Moon has been working on various sustainability projects...
And, in more detail here...

PV array at craters of the moon
The large 50 KW PV array at Craters of the Moon.


The ground mount system is based on galvanized steel pipe.  The pipe members are attached to each other by standard industrial fittings that just slide onto the pipes and then are secured with what are basically large set screws.  This is a very clean, open, and simple mounting system that appears to be very sturdy and looks like it will provide a long life.PV mounts

The pipe rack mounting system has a nice open and clean look -- the shovel is optional.

Here are a bunch of pictures showing details of the mounting system.

PV array ground mount system PV mounting system PV mounting system6

PV mounting system
Front strut to horizontal rail attachment.
PV mounting
Getting the alignment of all the vertical struts right
might be the hardest part of this mounting system.
PV mounting
Braces connecting the front and rear struts.

PV mounting
It looks like all of these T fittings have an
extra set of lugs for a side brace.
PV panel clip
The clips that hold the modules to the rails.
PV mounting

A somewhat similar system made by ProSolar called GroundTrac...

Equipment and Wiring

The PV array at Craters is very large (50 KW), and spread out over several racks.  These pictures are of the smallest of the racks, and are more representative of what you might do for your house.

The PV installation is grid tied.  Large "string" inverters are used to couple the PV modules to the grid.  The "string" means that a string of several modules are connected in a series loop.   In this arrangement, which is used in most grid tied systems, the first module has one wire connected to the inverter and the other wire connected to the 2nd module in the string.  The 2nd module is in turn connected to the 3rd, and so on around the ring.  The last module in the ring has its output cable connected back to the inverter.   Hooking the modules up in this way minimizes wiring, and also allows smaller wire sizes to be used, as the voltages add around the loop and the current stays constant.  The number of modules that should be connected in one strubg depends on the panel voltages, the inverter specs, and the coldest temperatures that the array sees.   Most of these inverters can accept inputs from more than one string of modules.  The companies that supply inverters usually have a calculator on their website that allows you to pick the number of modules for the loop.

The grid-tie inverter used in this installation is an SMA.  Note that the DC disconnect, which provides for disconnecting the PV array at the source, is built into the inverter which makes for a nice compact arrangement and eliminates the need for a separate disconnect switch.  This may not be allowed in all locations.


PV grid tie inverter

Grid tie inverter for the PV modules on this rack and on adjacent rack (the underground conduits on the left carry DC from the adjacent rack).

Note how the equipment support rails tie right into the mount verticals -- nice and simple.

PV inverter back view
Back view of inverter.
PV Equipment mount detail
Tying equipment mount rail to verticals.
PV placard
The required placard.
PV module wiring
PV module wiring -- note the simple series
 connection of the panels.
PV module wiring
Another view of the wiring.
PV module labeling
PV module labeling.  The UL listing is
required for nearly all PV installations.

PV module grounding
PV inverter PV modules and Kristy
Kristy approved.

In addition to the equipment shown in these pictures, there will need to be a connection to the circuit breaker box, and an AC Disconnect switch that is normally located near the circuit breaker box.  The conduit coming out of the bottom of the inverter is headed in that direction.  The AC Disconnect provides a way to insure that the PV array cannot power grid and provides positive protection for anyone working on the system.  The grid tie inverter will automatically disconnect the PV array from the grid in case of a power failure, but the AC disconnect provides an extra level of protection that can be visually checked.


Gary April 12, 2011