I have a small 30 watt solar panel (about 1.5 feet by 3 feet) that's in a frame.  My flat roof is actually positioned ideally for the sun's angle.  Can I just lay the panel on the roof, or should I put it up an inch on "legs" or a frame that allows air to circulate underneath?  The roof gets pretty hot in the summer...it's painted with reflective paint, but still gets hot. 

Posted By stuart.wyss on 02 Apr 2010 10:09 AM 
My flat roof is actually positioned ideally for the sun's angle. 

Do you live on the equator?

Silcon solar loses a significant amount of output with elevated temp due to leakage of thermal electrons. It's 30W rating is only good at 25C/77F and 1kw/m^2 insolation. Flat up against 150F/65C roof deck you're giving up about 20% of the available power- it'll only put out ~24-25W at standard insolation levels.

Giving it some space for convective cooling on the back side will increase it's net output. If you give it enough pitch that it stays below 115F due to convection cooling you'd only be losing at most ~10%, plus a modest amount for solar geometry reasons. How much you retain in practice kinda depends on factors like roof pitch air temp, the thermal R of the panel itself, etc. At a pitch of less than ~2:12 you won't be getting much in the way of natural convection. Putting it on a stand with sufficient pitch to convection cool will often more than compensate for loss relative to variance from ideal solar geometry. Even if the rule of thumb calls for a latitude minus 15 degrees angle for the PV panel, going with latitude minus zero is only giving up ~5% of theoretical annual production that assumes an operating temp of 25C, but if it increases your summertime gain by 5% due to cooler operating temps you're actually ahead of the game, since the geometry is now better optimized for the shoulder-seasons, taking on more than it would have at latitude minus 15.

At least one company (Sundrum), thermally marries PV panels to high flow/low temp water heating thermal panels, which water-cools the PV using the heat drawn from the PV panel as pre-heat for domestic hot water.

Equator? No, Philly, basically, just north of.

But the sun, at its peak in summer, is directly down on my flat roof.

Posted By stuart.wyss on 02 Apr 2010 05:52 PM
Equator? No, Philly, basically, just north of.

But the sun, at its peak in summer, is directly down on my flat roof.

No, it isn't.  

Philly is at about 39.5 degrees, so "just north of" is ~ 40 degrees north latitude. At solar-noon on the summer solstice the sun is no more than ~74 degrees up from the horizon (never 90) and that's the highest it'll ever be for you.

Dead-level flat would be more than 15 degrees off of being perfectly pitched toward the sun at that time, and it would be even worse the rest of the hours of the day/year.  In winter you'd be severely compromised (even if it DIDN'T collect snow on the panel), based on solar-geometry alone.

The rule of thumb for maximum annual insolation (ignoring weather factors) for a non-tracking collector located above the tropics is to put it oriented due south, pitched about 15 degrees less than your latitude from the horizon. For you that means tipping it 25 degrees up from dead-flat.

But there are other factors that come into play (seasonal haze & average weather, etc.) Find yourself on the map on p.6 in this document to figure out your best orientation.   Looks to me like 32-34 degrees from dead-flat would be a better bet in Philly, based on their analysis, which would also be sufficient to provide reasonable convection cooling as well.

Ok, ok. LOL. I know every situation's different and I haven't been completely honest....my roof isn't actually perfect flat, it's probably about 12 degrees already. And it's facing south. So, if about 33 is the best, I'm about1/3 way there already. I would only tip it another20 degrees or so.

I'm trying to avoid drilling holes in the roof...it's a rubber membrane roof, and we just moved in...I'm nervous. How about a frame weighed with a couple bricks? Would that be low-profile enough to sit up there, or should I be trying to find a way to truly anchor it?

I might be able to attach it to the vent stack pipe somehow, just to stabilize it....would have to design a custom "rack" but not impossible. Some galvanized piping and aluminum stock, perhaps.

You could build a rack that straddles the ridge and add some weight, even if it crosses over for only a few feet it would help out a lot. But as you add the correct tilt to the panels they become sails in the wind, the last thing you would want is in a storm to hear a bunch of loud banging and then see the panel flip off the roof or be bouncing around up there punching holes in your rubber membrane. In that case your better of with the panel as close to the roof as possible and losing some output, but at least it would stay up there
The first panels I had on the roof I used strapping to go from under one eve, up across the ridge and back down to under the other eve. Then I attached the frame to the strapping, it was a wood frame with an additional 15* tilt on a 9 in 12 roof. It sat on the roof for 3 years with no issues before I went to mounts before the roofing was installed on this new house.

Could you avoid the roof? Maybe a pole mount or heck even ground mount if you have a good sun spot?

I have a pretty wooded yard, but for some reason the house is in full sun.

Some other options I might try are chimney mount, like a TV antenna system. I'm also going to watch today (really sunny) and see if there's a place next to the foundation of the house that gets sunlight most of the day. There are some trees near the house, but I think the angle of the sun gets through, and it will only get higher as we head into summer. Perhaps a low ground mount for now. Plus, I'll be off the roof (which my wife would prefer).

In my previous house, I used iron piping and a custom-made aluminum stock frame to mount a small panel on a shed. I figured it was a shed...it wouldn't matter if it wasn't the most weather-proof attachment. Worked really well, but now my shed is across the yard under several trees. That will never work.

If the chimney is on the north side of the building or has a significant amount of roof between it an the south edge of the building, a vertical rather than horizontal or angled mount can be surprisingly good on a reflective flat roof. The solar reflectivity of the roof boosts the insolation significantly over a broad range of angles, and while your summertime peak output levels go down signficantly the annualized return (and particularly the winter performance) can be far better than projected by solar-geometry alone. Without the reflective roof, the annualy uptake with a south-oriented vertical mount is only reduced by ~15-20% at your latitude, most of which will be returned by the scattered solar-mirror represented by the reflective roof.

Mounted it on a masonry chimney, give at least 1.5-2" of air between the brick and the panel for maximum convective cooling.

Hmmm...sounds like you are looking for a ballasted solar mount that uses sandbags or concrete blocks to hold the mount down.
Try either http://www.google.com/search?q=ballasted+solar+racking+systems or http://www.google.com/search?q=ballasted+solar+mount

In a high temperature situation, take a look at thin film amorphous panels - they have about 1/2 the loss of output caused by heat.

Yeah, I was thinking about making my own...it's just a small panel!

Instead, I mounted it to 2 square hollow aluminum stock rails with spacers between the rails and the panel to allow air in from the sides. Plus, the rails will keep it at least an inch off the roof. I'm going to attach it to a different part of the roof, where it's got a standard ridge vent. The ridge vent is nailed on and I discovered that 2 of the nails were easy to pry out. I'm going to tuck an L shaped galvanized piece of metal under the ridge vent flashing, put the nails back in (or replace with screws/rubber washers maybe). That will anchor the two L brackets to the roof by way of the ridge vent. Then I'm going to attach a cross piece of hollow square stock between the 2 rails on the panel, and attach the cross piece to the L brackets. The panel will "hang" from the brackets that are tucked and nailed under the ridge vent. The panel will be between 1 and 2 inches off the surface of the roof, and angled at about 40 degrees....should be ok for summer and winter.

That's about right- 40 degrees is well inside the 5% center oval of the bullseye in the optimization target for your location, and it'll boost the winter production slightly. Having it at 40 degrees means it'll slough snow fairly quickly too.

50+ degrees is better in that aspect for colder snowier climes than yours, but even that would still be at the edge of the 10% contour of the bullseye for your location.