New installation

Tricksailing

It's been a while since I've visited and have a new project in mind. Fell off my boat into a house at a sunny 23 degrees North. I'm installing a new steel roof over a part of the garage and figured I should take the opportunity to install solar. The piece of roof I'd like to install on is about 4.7 x 4 meters with about  10 degree slope facing WSW (240 degrees). I was thinking of mounting to square section 1.5" square steel tubes running across the 4m width and supported at the ends so as not to penetrate the roof. They'd hold the panels about 15 cm above the roof. My first question is how does this sound - especially given that we get hurricanes here with 100mph+ winds. A second question is if I should leave a gap between each panel to reduce any pressure changes across the panel array and how big a gap I need.

mcgivor

With a span of 4 meters there is a potential for flex or oscillation in high winds using a single tube, a double tube gusseted  ladder design would be far more ridgid, even using lighter gauge steel, but usually involves weldind. Unistrut is another option  if available, all bolt together with angle brackets  
http://www.unistrut.us/index.php?WP=S00_Fitting
Space between panels for expansion should be around 5mm.

Tricksailing

Thank you very much for the response mcgivor.
It looks like I didn't explain well enough - about normal for me. I was thinking of 2 tubes per row of panels as I hope is clarified by the (roughish) diagram below. The dotted lines suggest a panel layout although it will depend on the dimensions of the panels I buy. I think that is what you were suggesting with the addition of cross-pieces to connect each pair together as I show in the top row of panels. Would the panels themselves serve the same purpose as cross-pieces? The welding is no problem and the reason for asking these questions now. I have a welder here making a new steel frame to support a corrugated metal roof.

I'm thinking of doing it this way to avoid anything having to go through the roof with the inevitable potential for leaks. The only attachment points would be on the sides of the structure - welded to roof beams. Am I worrying too much about leaks?

The other big issue here is hurricanes with 100mph+ winds. That can create significant pressure differentials between the two sides of an object and I figured bigger gaps between panels would help equalize the pressure and reduce the chances of panels getting blown off relative to the situation if the panels created a single flat surface.

I'm planning on another 3 panels on a flat roof adjoining this roof for a total of 14 panels. I figure that should give me max about a kW with some power for around 10 hours of the day. Not really enough to keep AC going constantly but a fair return. I'm thinking a grid-tied system will be better than going off-grid in the middle of town.

Another issue with the hurricanes is power cuts which have sometimes lasted for days so I need a backup system. It seems to me that a 3 or 5kW 220/110 generator may be a better route than batteries and inverter. Any other thoughts?

 

Estragon

A typical steel roof I've seen is full of holes to begin with (to attach steel panels to substructure, and for vents etc.). I've tried to vent as much as possible out the side to avoid holes, but ended up with a couple of unavoidable ones. Because they open the low flat part of the profile, they're more prone to leaking than properly sealed lag screws or bolts through the higher ridge part IMHO.

I think there are off-the-shelf mounts made for most types of such roofing which will be engineered for a specific wind load. The rating could be important for insurance and/or building code purposes.

As a backup power system used for outages that happen for a few days to a couple of weeks per year, a generator will likely be a cheaper and possibly more reliable solution. Even if your array stays attached, storms like that often also deploy projectile weapons, like bits of your neighbour's less well engineered roof, etc. A 2x4 at 100mph vs solar panel may not end well

mcgivor

Tricksailing said:

Thank you very much for the response mcgivor.
It looks like I didn't explain well enough - about normal for me. I was thinking of 2 tubes per row of panels as I hope is clarified by the (roughish) diagram below. The dotted lines suggest a panel layout although it will depend on the dimensions of the panels I buy. I think that is what you were suggesting with the addition of cross-pieces to connect each pair together as I show in the top row of panels. Would the panels themselves serve the same purpose as cross-pieces? The welding is no problem and the reason for asking these questions now. I have a welder here making a new steel frame to support a corrugated metal roof.

I'm thinking of doing it this way to avoid anything having to go through the roof with the inevitable potential for leaks. The only attachment points would be on the sides of the structure - welded to roof beams. Am I worrying too much about leaks?

The other big issue here is hurricanes with 100mph+ winds. That can create significant pressure differentials between the two sides of an object and I figured bigger gaps between panels would help equalize the pressure and reduce the chances of panels getting blown off relative to the situation if the panels created a single flat surface.

I'm planning on another 3 panels on a flat roof adjoining this roof for a total of 14 panels. I figure that should give me max about a kW with some power for around 10 hours of the day. Not really enough to keep AC going constantly but a fair return. I'm thinking a grid-tied system will be better than going off-grid in the middle of town.

Another issue with the hurricanes is power cuts which have sometimes lasted for days so I need a backup system. It seems to me that a 3 or 5kW 220/110 generator may be a better route than batteries and inverter. Any other thoughts?

 

Was thinking along the lines of a lattice girder, 2 parallel 4 meter with triangulation between upper and lower chords as used in bridges.

https://www.scribd.com/document/262033156/Trusses-and-Lattice-Girders

Tricksailing

The system is for a house in Mexico which has grid power for all but a few days per year so grid-tie seems a logical option.  Outages tend to occur in the hot summers (hurricane season) when AC and refrigeration would be nice to have. As Estragon suggested, a  generator seems the way to go for back-up but it would be nice to use solar backup during the day. The Sunny Boy inverter seems to be the easy option there with a 2kW 'secure output' from solar for daytime use. Am I losing other benefits if I choose this over Solar Edge or Endphase?

Am I correct in noting that the grid-tie inverter connections are the same with the US split-phase and Mexican 3-phase 240V systems? And is there an issue with low grid voltage or high voltage spikes which seem to occur here?

mcgivor

Most hybrid inverter chargers will accept voltage above and below the nominal grid value , I believe my 230V unit will accept between 170-270V. The SMA gear would simplify the system as batteries, and a means of charging, are integrated with the whole setup communication taken care of. Check to make sure grid connection is possible and feed in, if desired, is allowed before hand. If I'm not mistaken the SMA gear has provision for both grid and generator via internal transfer switch, going on memory though. Mexico has the same standard for grid as North America.

Tricksailing

Thanks for the comments. I eventually went with two Ironridge XR 1000 rails mounted to 4" square steel tube welded to the roof support beams. No serious testing so far but the Ironridge Design Tool suggested it was  borderline cat 3/4 hurricane resistant - but I take the point about flying debris.
I used 18 Canadian Solar 275W panels (5kW total) into a grid-tied Sunny Boy inverter which produces as much as 4kW and 30 kWh per day in the summer. Since January (~9 mths) I've put almost 6 mWh back into the grid. Total cost was quite low - around $8,000 because in Mexico they allow allow DIY installations. You just have to make an application with design specs, they come and inspect the system and connect up if it looks OK. I got an overvoltage error when I turned the inverter on. Grid voltage was over 260V! I reported it and they fixed it within 24 hours but it makes me wonder how long I'd been cooking all the electrical devices in the house.