Mounting structure

Soulbringer · December 2007

Hello guys,

I wanna ask you how do you think what material is better for making mounting structure - wood or steal.
We have to compare them not only for durability but also for their price.

Thank you

crewzer · December 2007

Re: Mounting structure

Soulbringer,

I don't know if there's a single best answer to your question. Steel frames are strong, aluminum frames are lightweight, and wood frames are inexpensive. My first rack was on my camper, and it was mostly aluminum, with some wood for tilt legs. My present rack structure is all pressure-treated wood. I suspect the rack for my next home's PV array (~3 kW to ~3.6 kW) will be mostly aluminum, though with wood risers.

Here are some brochures and pictures that may give you some ideas: http://store.solar-electric.com/panel-mounts.html

Personally, I like the rack pictured in this Sharp module cut-sheet: http://lib.store.yahoo.net/lib/wind-sun/Sharp123.pdf :cool:

Hope this helps,
Jim / crewzer

P.S. How's the row-spacing math exercise going?

System2 · December 2007

Re: Mounting structure

I just built an aluminum rack to hold 8 GE 110 Watt PV', + one BP of different dimension. One piece, to be suspended / hinged at the top and tilted out at the bottom as required. Worked great, BUT there was far more flex than I had expected. So, added to each of the 6 angle aluminum uprights, a treated 2X4. WOW! What a difference. No flex now! So for me, this works great. The aluminum holds everything in place and the treated 2X4's keep it stiff.
Wayne

BB. · December 2007

Re: Mounting structure

Just watch building things with Pressure Treated Lumber... You should be using stainless hardware/screws, or building hardware that is specifically rated for pressure treated lumber.

Regular painted and/or minimally galvanized/plated hardware can rust out in a matter of a few years.

From a Lowes Hardware website:

Although it may be well known that corrosion-resistant fasteners should be used when building outdoor furniture, it may come as a surprise that new pressure-treated wood requires fasteners with higher levels of protection. Your best bet is to use stainless steel fasteners whenever possible. Or, look for hot-dipped galvanized fasteners with a G-185 rating (1.85 ounces of zinc is used for every square foot of metal). Previous choices, such as G-60 or G-90 fasteners, will break down over time if used with the new preservatives.

-Bill

System2 · December 2007

Re: Mounting structure

I used Unistrut http://www.unistrut.com , it's strong, lightweight and inexpensive. Home Depot sells galvanized unistrut for about $15 per 10' section of P1000T http://www.unistrut.com/Browse/cat_detail.php?S=S02&P=P1000T .

Cheers,

Bad Apple

System2 · December 2007

Re: Mounting structure

Good point as usual BB. I used the special epoxy coated screws available here especially for the new treated lumber. Kept wondering if the coating was being cut off as they went through the mounting holes in the aluminum though. I would have used stainless, but the only stainless I could easily get my hands on were too soft and twisted off way too easily for my liking.
Cheers
Wayne

crewzer · December 2007

Re: Mounting structure

I've used both stainless and (what I think is) G-185 hardware on my pressure treated (PT) lumber racks. The G-185 stuff is coarse and ugly, but, so far, so good.

The original question and ensuing responses are a sterling example of why we need lots of information -- both technical and environmental -- from our visitors. I have a PM from soulbringer indicating his location to be "Latitude 41.27, Longitude 23.14". The only place where those coordinates intersect on dry land is in southwestern Bulgaria (Petrich?). The sources and availability of supplies and materiel may be a little different from that to which we are accustomed here in the U.S.

HTH,
Jim / crewzer

System2 · December 2007

Re: Mounting structure

I think it's safe to assume that if someone has access to PV panels, inverters, batteries, etc. that they can get lumber or steel to mount an array on. Here's the link for Unistrut's European site: http://www.unistrut.co.uk/ , Bulgaria is included in the highlight when you hover over the region selector link.

Others that follow may find it useful as well.

Cheers

System2 · December 2007

Re: Mounting structure

Here is a link from Builder News Magazine re the changes to pressure treated lumber & fasteners:

http://www.buildernewsmag.com/viewnews.pl?id=32

Some excerpts:

"Two years ago the wood preservative industry agreed to phase out the use of the most common wood preservative, chromated copper arsenate (CCA), for most residential construction and consumer uses, bowing to pressure from consumer safety groups concerned with the carcinogenic and toxic effects of arsenates. A production ban went into effect December 31, 2003, with exceptions made for uses such as highway barriers and agricultural fenceposts, certain post-frame columns, permanent wood foundations and marine pilings.
Wood treaters are using several alternative chemicals in place of CCA, including:
- Sodium Borate (SBX) containing disodium octaborate tetrahydrate (DOT), marketed under the names Tim-Bor, PacBor, Advance Guard, and SmartGUARD, for wood used in interior, above-ground applications free of liquid water, such as sill plate.
- Copper Azole (CA), marketed as Wolmanized Natural Select.
- Alkaline Copper Quat (ACQ), sold under the trade names Preserve, NatureWood, AC2, and others.

"- Thicker galvanizing generally extends service life of a product. The treated wood industry recommends using stainless steel and hot-dip galvanized connectors and fasteners.
- Due to the uncertainties in pressure-treated wood chemicals, Simpson recommends using stainless steel fasteners, anchors and connectors with treated wood when possible.
- At a minimum, customers should use G185 HDG (Simpson's ZMAX), batch/post hot-dip galvanized (per ASTM A123 for connectors and ASTM A153 for fasteners), or mechanically galvanized fasteners (per ASTM B695, Class 55 or greater) product with the newer alternative treated woods. Due to the many variables involved, Simpson cannot make an unqualified recommendation of any galvanized or other coating for use with treated wood.
- Because of the many variables involved, Simpson cannot provide estimates on service life of connectors, anchors or fasteners. All users should obtain coating recommendations from their treated wood supplier. (Some suppliers have clarified their recommendations to include G185 HDG.)
Uncoated and painted products should not be used with treated woods.
When using stainless steel or hot-dip galvanized connectors, the connectors and fasteners should be made of the same material."

JB

PS: The Borate treated lumber is not to be used for exterior (or wet/damp) applications. The Borate treating will wash off if the lumber gets wet. This is strictly for interior applications, & is usually identified by a light green color.

mike95490 · December 2007

Re: Mounting structure

Soulbringer wrote: »

Hello guys,

I wanna ask you how do you think what material is better for making mounting structure - wood or steal.
We have to compare them not only for durability but also for their price.

Can't beat steal for the price ! It does carry a 8-30 year warning, depending if you get caught !

Soulbringer · December 2007

Re: Mounting structure

Thank you for your replies :-).
The location is south-west region Petrich(Bulgaria).
The climate is with calm winter and hot summer. I want to install 20 kW PV plant with 30 degrees tilt angle.
The panel is 1600x800x50 mm (16kg).
There will be two rows with 13 panels ground-mounted.

P.S Crewzer I found one formula:

w=D x (Ф-Y),

Where is: w - geometry of vertical shading device (see pictures above), Φ - solar azimuth, Ψ - plane azimuth

http://pvresources.com/en/shading.php

Do you know for Ф and Y what I have to write?

crewzer · December 2007

Re: Mounting structure

Soulbringer,

See if the attached drawing helps.

Regards,
Jim / crewzer

On Edit: See newer post below with corrected drawing

niel · December 2007

Re: Mounting structure

soulbringer,
if you are going to keep 1 tilt angle year round, assuming it to be aimed south, you may benefit by taking the angle higher as your latitiude is between 41 and 44 degrees for bulgaria and should at least be that many degrees. i believe jim can correct me, but he often says latitude +15 degrees so the best angle would be around 57.5 degrees and certainly should not be less than the latitude or you will lose much more winter power. the summer will be fine at the higher angles because of the longer timeperiods the sun is out, but obviously lower power will be had in the summer without an optimal angle and it is not as bad as that which will be lost in the winter with the higher angles. i have found winter to be the most critical because of the shorter hours of sunlight and how much lower to the horizon the sun sits. unless you add to the complexity of your mount with adjustable angles it should be 1 higher compromised angle. this will of course lengthen the distances between the pv strings needed to prevent shadowing.

crewzer · December 2007

Re: Mounting structure

I actually considered this issue yesterday while working the drawing. As niel suggests, increasing the tilt to latitude + 15 degrees (~56 degrees) will greatly improve winter energy harvests, although there will be some summer loss. Additionally, monthly energy harvest should be more consistant.

However, increasing the tilt angle will require increasing the inter-row spacing. So, as always, the "best" solution usually lies somewhere in considering all parameters for a particular site.

HTH,
Jim / crewzer

crewzer · December 2007

Re: Mounting structure

SB,

I’m posting my response to your e-mail so that we can share this technical discussion with other forum members.

Just to be sure that I understand you. This angle 13 degrees is when the sun is in it`s lowest position 21 December.

13 degrees is the approximate angle of Sun elevation at ~0900 and ~1500 local solar time for your location on December 21. The Sun will be higher between those times. For example, at local solar noon (~1225 local time), the elevation angle will be ~25.3 degrees.

Do you think it`s better if we want to spend some more ground space we have to increase this angle to 17 or 18 degrees? Then the distance will be a little bit shorter and more modules can fit in.

“Increasing” the Sun angle to 17- or 18 degrees in the calulation will certainly allow you to reduce the inter-row spacing. However, the lower section of all PV rows except the front row will be partially shaded until ~1000 and after ~1400 local solar time on December 21, as well as for many days both before and after Dec. 21.

And what program do you use to see the sun`s minimum elevation for my place 41.27N 23.14E.

I have some Sun elevation charts in some of my reference books. An excellent on-line source is this site operated by the U.S. Naval Observatory. Remember to convert coordinate values from decimals (i.e., 41.27 N, 23.14 E) to degrees and minutes (41º 16’ N, 23º 08’ E) when you make your location entry.

Also, the USNO calculator results are based on local time instead of solar time. You can determine the offset by looking at the difference between local noon vs. solar noon.

Hope this helps (HTH),
Jim / crewzer

crewzer · December 2007

Re: Mounting structure

Soulbringer,

I beleive that the geometry and related discussion in some of my previous posts is likely flawed. I'll post an update (correction?) this evening.

With apologies,
Jim / crewzer

Soulbringer · December 2007

Re: Mounting structure

Thanks to everyone,

Now I am sure that I will use steal for mounting structure. My place is 41.27N/23.14E degrees and I think that the tilt angle of 30 degrees for all year is optimal for my location. Crewzer your drawing is showing very well the inter-row distance. I found another web site (http://susdesign.com/sunangle/index.php). I think that if I use 17-19 degrees for sun angle for all year it`s optimal. If you have suggestions guys let me now.

Take care
Ivelin

Soulbringer · December 2007

Re: Mounting structure

And I forgot to mention that the modules won`t be laying on a ground because of the grass and dust. I think 0.7 m above ground is perfect.

crewzer · December 2007

Re: Mounting structure

Ivelin,

My original drawing contained flawed geometry. Specifically, I forgot to account for the Sun’s changing azimuth angle during the day. From ~September 22 to ~March 20, this path causes the arrays shadow’s north-facing ground vector to decrease as the morning progresses, reached a minimum length at solar noon, and then lengthen again as the afternoon progresses.

The most pronounced variation is on December 21. However, it’s not nearly as bad as my original drawing indicated. I’ve deleted the old drawing and posted a corrected version here.

Additional discussions on the related geometry (the “pelekinon”, or "axe") are available from these two sites:

http://www-groups.dcs.st-and.ac.uk/~history/HistTopics/Sundials.html
http://users.hartwick.edu/hartleyc/sundial/sundial.html

Best of luck with your project,
Jim / crewzer

nigtomdaw · December 2007

Re: Mounting structure

I went for steel, 4 fold, strength, security, cost, workability. Yes I had to learn to stick weld, but painting with red lead oxide, then a quality metal paint should see maintanance levels of 7-10 years , IMHO .

Happy Christmas and a Prosperous New Year To all !

BB. · December 2007

Re: Mounting structure

There is also "cold galvanize" paint... I used to use it for stopping rust on the steel parts of a plane I used to own that was based right on the coast (lots of fog and salt spray). Should be applied without any primer (IIRC). And does not need painting/sealing.

I have also used a "rust converter" that contains phosphoric acid to kill rust. It actually converts (thin) rust into a black surface that is very stable and paintable (must be free of grease and thick rust).

Cold galvanized paint will look like a heavily galvanized cyclone fence (don't expect to get a nice finish--lumpy and grainy). The rust converter does not make the surface rough (if you have wire brushed/sanded off the chunks of rust) and can look just fine for your steel work after you have used a good quality rust resistant paint...

You should be able to apply the acid process first (scrap, soap, rinse, acid, soap/rinse, dry) then put the cold galvanizing paint on for the final finish (depending on your weather conditions/environment).

Both do a really good job of preventing/killing rust.

-Bill

Mounting structure

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