This article explains how to make your own PV mounting rack, how to install it, and how to wire up the whole works. This is in response to many requests for this info.
Face It SOUTH
The critical consideration in mounting PV modules is the yearly path of the Sun. The PV modules must receive maximum sunlight. Consider shading from trees and buildings. The decision of where to mount should be made only after careful consideration of all your options. The PV modules, in most nontracking situations, should face South. The closer the plane of the rack is to facing true South, the better overall performance the PVs will deliver. Only consider mounting surfaces that are within 15° of facing true South (within 10° is much better). Any surface further off will require more complex, asymmetrical mounting racks. If you don't have a roof or wall that is suitable consider ground mounting. Since PVs produce low voltage DC current, keep the wire lengths to the battery as short as practical.
Where you are going to put your PVs determines the type of rack you need. Roof mounting (on either pitched or flat roofs), wall mounting, and ground mounting are all possibilities. So consider the variables and pick the best for your situation. These racks can be used in all three types of mountings.
So Which Way is South?
Determine South with a good compass and someone who knows how to use it. Be sure to allow for the difference between magnetic North and true North. This difference is called magnetic declination. In California for example magnetic North is some 19° East of true North. If you don't know your magnetic declination, then go to the library and look it up.
Mounting Racks-- your PVs hold on the World
The obvious purpose of the rack is to attach the panels to a fixed surface. At first glance this seems simple enough, but consider wind, snow, falling ice and temperature variations, not to mention possible leaks in the roof!
We are going to talk about a simple to build rack that can hold up to four panels. This rack uses inexpensive hardware store parts. It mounts on roofs, walls, or on the ground with the appropriate foundation. In all mounts, the rack is adjustable for panel elevation, and allows seasonal optimization of the racks tilt. This rack approach was developed by Electron Connection Ltd. for its customers. Its design and application are so simple that I'm sure many others are using just about the same technique.
The Rack Materials
The rack is constructed out of slotted, galvanized, steel angle stock. This stock is available at most hardware stores. Our local store sells National Slotted Steel Angle (stock #180-109) for about $7.00 each retail. This stuff is 6 feet long, with two perpendicular sides each 1.5 inches wide. The stock is about 1/8 inch thick, with a heavy galvanized coating. Its entire length is covered with holes and slots that will accept 5/16 inch bolts. We have had no problems with corrosion or electrolysis with this galvanized stock after three years in the weather. We haven't yet tried this material on a seacoast, and would welcome feedback from anyone who has. To the left is a drawing of a typical length of this steel angle.
You can shop around locally, and may encounter different sizes and lengths. Six foot lengths are long enough to mount 4 of just about any type of module. We use this angle on Kyocera, Sharp and Canadian Solar modules without having to drill any holes in either the angle or the PV modules. Working with this stock is like playing with a giant erector set.
The only tools you really need are:
- Wrenches
- Hacksaw to cut the angle
- Drill for making holes in the surface holding the rack
- Size of modules
- Number of modules
- Mounting location
- Environment of mounting location
Let's consider the rack shown in the photo on the next page as an example. This rack holds four 85 Watt Kyocera PV modules and is bolted to the almost horizontal metal roof of a mobile home. Each PV module is 25.7 inches wide and 39.1 inches long. The mounting holes on the bottoms of the PV modules match the hole cadence in the slotted angle. This particular rack used 9 of the 6 foot lengths of the steel angle. Four lengths comprise the framework for the modules. Three lengths make up the legs and bracing, while two more lengths are used as skids on the roof. Strictly speaking, the skids are not essential, but do add rigidity and relieve stress on the mounting points on the sheet metal roof. We don't want any leaks.
A rack could be built with the about half the materials. The top and bottom pieces of the rack holding the panels, the brace on the legs, and the skids could all be deleted. If this were donethen the rack would be roughly equivalent to most commercial models. In our opinion, PV modules should be mounted as securely as possible. Many commercial racks use the PV
modules' frames as a structural members in the whole module/rack assembly. This rack does not do this. Many commercial racks use 1/8 inch aluminum angle. This rack uses steel of the same thickness; it is much stronger.
This rack lives in snow country, with lots of high winds. Consider that the rack holds some $2,000. 00 worth of PV modules. We figured that the additional $35. the extra bracing costs to be worth it in terms of security. It's comforting to be inside during a howling snow storm and know that when its all over the PVs will still be there. Don't skimp on materials for your rack. Use extra bracing to make it as strong as possible. Remember that it holds over a thousand dollars worth of PV modules. The 9 pieces of slotted angle cost us about $65., and are well worth it.
Laying Out the Rack
You could design the entire rack on paper after first making all measurements of the critical dimensions on the modules. This takes time, and is subject to measurement inaccuracies. We have a simpler idea, with no measuring required. Let's treat the entire project like an erector set. We assemble the entire rack on the ground first, even if it must be disassembled to be finally installed. This assures no surprises upon final installation.
Lay a thick blanket or sleeping bag on a flat, smooth surface. Place all the modules, face down on the blanket and lay on the side angle pieces that connect the panels. See the diagram. Note that no measurement is required. Simply align the mounting holes in the module frames with the holes on the angle. We usually leave any extra angle on these pieces, rather than trimming it off. It comes in handy. On this particular rack the 4 Kyocera modules mounted perfectly, with no trimming of the 6 foot side rails necessary. The distance between the mounting holes on the modules determines the width of the rack.
Cut two pieces of angle to form the top and bottom rack rails. These should be trimmed exactly to fit inside the framework created by the side rails. The net result is all four panels are encased by a perimeter of steel angle. Use 1/4 inch bolts about 1 inch long, washers, lockwashers, and nuts to secure the modules to the framework. The bolts on the corners of the framework go through the module, the side rail, and the top (or bottom) rail. The result is very strong.
If you don't have four panels to put on the rack right now, you can use several pieces of angle stock in place of the missing panels. We strongly recommend building the four panel version. If you don't, then system expansion is going to be harder. Also building a smaller rack costs about as much when the waste on the 6 foot lengths of angle is considered. So build for the future, and see how easy it is to add a panel or two once their rack is already in place.
The Skids
We usually leave the skids uncut six foot lengths. The skids form the base for roof, wall or ground mounting. If the rack is to be wall mounted the situation is much the same except the skids are vertical instead of horizontal. In all cases, one end of the skid is connected directly to the module frame rails by bolts. This forms a rotating hinged point for rack elevation adjustment. This hinge line points East and West (so the rack faces South) in horizontal applications, and up in vertical the Fall increase the PV output by about 5 to 8%. This is really not a very great increase in performance, but the success or failure of an AE system depends on attention to detail. We personally consider that a 5% increase in our PVs performance is well worth the twice yearly expenditure of 15 minutes of our time to adjust the rack.
On roofs that are not horizontal (and most aren't), the legs get shorter as the roof gets steeper. A good overall, nonadjustable, mounting angle is your latitude. If you live at 40° latitude, then mount the rack so that the angle between the rack's face and horizontal is 40°.
The table shows the proper leg lengths for South facing roofs and a variety of latitudes. This table assumes the use of 6 foot rack rails and skids. The top of the table contains roof angles from 0 degrees (flat) to 60 degrees from the horizontal. The left side to the table shows latitude in 5 degree increments. The actual leg lengths in feet are in the body of the table.
Consider someone living at 38° latitude with a 25° slant on his roof. The table shows a leg length of 1.36 feet. Note that this table shows leg length decreasing as the roof's angle approaches the latitude. Once the roof's angle becomes greater than the latitude, the legs are attached to the bottom of the rack rather than the top. Instead of raising the top of the rack to face the Sun, we raise it's bottom.If you're into math, the formula used to generate this table is based on the Cosine Law. Here is a solved and generalized equation that will give leg lengths for all situations regardless of rack or skid dimensions, latitude or roof angle.
L= length of the Leg in feetThe geometry is much the same for wall mounting, but the skids are vertical. In any case, don't be afraid to mount the skids however you must, adjust the rack's elevation, and cut the legs to fit. This approach while, low tech, gets the job done applications.
R= length of the Rack in feet
S= length of the Skid in feet
P= the angle of the roof's plane to the horizontal in degrees
A= your latitude in degrees
The Legs
The actual length of the legs varies depending on:
- 1. Where the rack is mounted
2. Your latitude - 3. Whether or not you want adjustability
4. The slant or pitch of a roof - Let's consider the simplest case, that of mounting on a flat roof or on the ground. In this case, the skids are horizontal and level with the ground. Figure 4 illustrates the geometry of this situation for adjustable racks for latitudes around 40°.
- In the adjustable rack at 42° latitude, the legs are 3 feet, 425 inches long. Altitude adjustment is accomplished by unbolting the legs and repositioning them along the rack rails and mounting skids as shown in Figure 4. On a horizontal surface these 3+ foot legs allow adjustment of the angle between the rack and horizontal from 32° for Summer use, to 57° for Winter use. Twice yearly adjustments during the Spring and again in every time.
A roof is a difficult place to do a good job. The steeper the roof, the more difficult the installation. On steep roofs we prefer to assemble the whole rack, complete with PV modules (already wired together), legs and skids on the ground. Then transfer the whole assembly (about 50 pounds) to the roof for final mounting. We have successfully used the skid mounting technique on metal, composition shingle, composition roll, and shake roofs from 15° to 45° of pitch.
Don't mount the PV modules themselves directly on the roof's surface. PV modules require air circulation behind them to keep them cool. If you are blessed with a pitch that equals your latitude and a South facing roof, please resist the temptation to mount the modules directly on the roof. The high Summer temperatures underneath the modules will greatly reduce their performance and can cause the actual PV cells to fail. So leave at least 2 to 3 inches behind the modules for air circulation.
Use at least 4 bolts (5/16 inch diameter) to secure the skids to the roof. Use large fender washers inside the roof, and lockwashers on the outside. Liberally butter the entire bolt, washer and hole in the roof with copious quantities of clear silicone sealer. When everything is tightened down and the silicone sealer has set, we have yet to have any problems with leakage.
Ground Mounting
If you are ground mounting, take care to pour or bury a massive cement foundation for securing the skids. Ground mounting exposes the PV modules to all sorts abuse. They may be hit by everything from baseballs to motor vehicles. So pick your spot wisely, and provide lots of mass to hold the rack to the ground. Cement blocks, or poured cement strips are best.
Wiring the PV Modules Together
PV modules are usually set up for 12 volt operation. The module contains between 32 to 44 PV cells; each cell is wired to the next in series. Thus the voltage of all the cells is added to produce a nominal 14 to 20 volt output for recharging batteries in 12 VDC systems. Each PV module is a self-contained polarized power source. Each module has a Positive terminal and a Negative terminal, just like a battery.
The PV modules can be wired in parallel which adds their current, or in series which adds their voltage. Systems using 12 VDC will wire the modules in parallel, which systems using 24 VDC or higher will wire the modules in series. Figure 5 illustrates the basic idea of either series or parallel wiring of PV panels.
Use good quality heavy gauge copper wire (THHW or THHN insulation) to make series or parallel connections between the individual PV modules. Solder all possible connections. Most modules use mechanical ring type connectors to connect the wiring to the actual panel. If you use these connectors, solder the wire to them, don't just crimp the wires into the connector. Use shrink tubing instead of tape on all wire to wire connections. Be sure to use polarization indicators on all wires. We use red tape at the ends of all positive wiring.
Wiring the PV arrays to the battery is straight forward, using only two lines. These two wires carry the entire current of the array. Total wire length (consider both wires) and array current determine the wire gauge size necessary.
This wiring diagram does not contain any regulator for the PV system. Although many systems do not require a regulator for the PVs, we always recommend the use of a charge controller. A good rule of thumb is: IF your PVs don't charge the batteries at more than a C/20 rate, AND if the system is ALWAYS being used, then you do not need regulation. When in doubt, add a regulator. In other cases, wire the regulator into the system following the manufacturer's instructions.
This article gives you the basic information so you can figure out what to do for your own particular system. If after reading this, you don't feel comfortable the concepts involved, please seek the aid of someone to help. Proper positioning, mounting and wiring of your PVs is essential if they are deliver their maximum power.
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