PV Mounting Systems 1

[peebee]

I understand that the optimal angle for PV system mounting is roughly equal to angle of latitude. For the northern US, that angle is about 40 degrees, implying a fixed PV panel mounting system of 40 degrees. Moreover, for max output on a fixed mount, you'd manually adjust that by +/- 15 degrees each equinox, implying a mounting system adjustable from 25 to 55 degrees.

Why then, do most all commercially available roof mounting systems appear to be sold with a tilt angle of 5 to 15 degrees?

Even taking roof pitch into account, there seems to be an extremely limited selection of PV roof mounting systems out there. Am I just not finding the 30-tilt systems on the Internet? Or are there good practical reasons (wind load?) that I'm overlooking for the dearth of 25-55 degree adjustable PV mounting systems out there?

 

[BigInch]

While the sun's variation from the ecliptic is +/- 15 deg, the maximum deviation of 15 deg only occurs for about 1 week during the summer and 1 week during the winter, so for optimizing a maximum tilt angle for yearly power production it is not necessary to include those extreme values. Even if you could go to the max, you would still have daily variations in the E-W direction which would make hitting the maximum angle on the flat actually possible for only about 1 hour per day during those 2 weeks.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[peebee]

Thanks, but I think you missed the point of my question:

Why do most all commercially available roof mounting systems appear to be sold with a tilt angle of about 10 degrees, rather than with a tilt angle of up to 45 degrees or so?

 

[dpc]

It may have to do with weight distribution across the roof, and/or keeping the support frame rigid and stable?






"Theory is when you know all and nothing works. Practice is when all works and nobody knows why. In this case we have put together theory and practice: nothing works... and nobody knows why! (Albert Einstein)

 

[GregLocock]

The actual reason? because energy is still very cheap and there is no reason to aim for an optimal solution.

You should also bear in mind that solar intensity is very low near dawn and dusk, it is much more important to get the energy in the middle of the day. The energy gain from a system that tracks the sun exactly is around 15% over a horizontal panel, in Australia.

Cheers

Greg Locock

I rarely exceed 1.79 x 10^12 furlongs per fortnight

 

[IRstuff]

Most roofs already have a tilt, so there's only a small additional tilt needed.

TTFN

FAQ731-376

 

[peebee]

Most of the roofs I deal with are flat.

Merry Christmas

 

[IRstuff]

OK, so there's:

http://www.unirac.com/mounting-solutions/solar-mount.php?solution=ground-mount

TTFN

FAQ731-376

 

[jfpe]

peebee,
There are two basic reasons for flat or low tilt angle PV systems on commercial roof tops:
1. Lower tilt angles reduce the wind loading so the attacahment or ballasting requirements are reduced.
2. Lower tilt angles allow closer module spacing. The rows of module need to be spaaced out so they don't shade each other. The lower the tilt angle, the less separation is required. The less separation required, the more modules you can fit on a roof. With lower tilt angles, you get less energy over the course of a year per module, but you can get more modules on the roof, so you you get more energy for the roof space available.
Direct Power and Water and UniRac offer tiltable mounting solution.
Cheers,
John

 

[dalcazar]

The reason has to do with seasonal availability of light with the different seasons.

In winter the sun will be low, and your panels should point low towards the sun to capture the maximum amount of energy, but... during the winter you have a much shorter day and the energy captured is lower than the summer.

Now, take this low pointing setup to summertime and point it at the sky and your panels will no longer be at the optimum angle to absorb the most energy from the sun during the longest days of the year.

Especially in the case of grid-tied systems, the amount of power you can sell back to the utility during these longer days is much more than you can get during the shorter winter days and therefore it makes sense to produce your mounts to capture as much energy as possible during the longest days of the year. And that right there is the main reason.

Two-axis tracking systems will increase your total energy production over the year, however you can gain 80% of this increase for a much lower cost by employing a single axis tracking system, and these systems are still made to track the sun across the sky and not the angle of the sun as it changes over the seasons. Most of these systems have panels that are mounted flat or at low angles, if you angle the individual panels you lose panel density very quickly.

I have even seen a one-axis tracking system where the beams that held the panels were angled AWAY from the sun slightly (don't ask me why, I thought it was stupid too)

 

[peebee]

I think you've missed my point.

I understand why panels should be adjusted.

What I don't understand is why the available mounting systems allow for tilt angles of 5-15 degrees, when the required angle for the nothern US might be perhaps 25-55 degrees.

 

[IRstuff]

Not sure what you're referring to; This site shows all sorts of different mounting angles:

http://www.unirac.com/mounting-solutions/solar-mount.php?solution=ground-mount

TTFN

FAQ731-376

 

[cranky108]

5 to 15 degrees should be enough for a sloped roof. For a flat roof a custom unit might be needed, but maybe you just need to look harder.

 

[fcuneo]

I agree with jfpe, the tilt angle in a PV design is often optimized to produce minimum aerodynamic loads.

The success of a PV design is greatly determined by its suitability for a wide variety of installations/applications.

In this way, a PV array with low lift off forces can be installed in a greater variety of roofs for instance, were scenarios that are either limited by weight requirements, limiting ballast requirements (too avoid costly roof reinforcements); or do not allow roof penetrating mountings (as this are not preferred from a roof-maintenance perspective)


Fernando Cuneo

-------------------------------------------------------
Newmerical Technologies International
Montreal, QC, Canada
Web:

http://www.cwe.newmerical.com

 

[EnergyProfessional]

- because they are mostly sold in the south (no snow, low latitude). Maybe that is 95% of the market for them. Even some people here in WI are stupid enough to fall for that.
- because there is less wind load and they get away with ballasted systems (not sure about hurricanes, though)
- the spacing argument is not so true. Sure you get a little more panels on the roof. but the panels are the most expensive part. you'd be better off optimizing the slop and have fewer panels (for less money)

Dust will settle, they won't wash themselves clean. I'm not too excited about almost flat panels.