on Fixed vs. Adjustable Mounting Systems
By Russ Lanoie
“Winchell Factor,” as it is often called here in New Hampshire’s White Mountains, is a phenomenon that anyone out on a cold, windy day has come to know, often intimately. It is the perceived decrease in air temperature felt by the body on exposed skin due to the flow of air, or the effect on anything that produces heat by cooling it off more quickly as wind increases. I recently had a chance to see how wind seems to have a chilling effect on photovoltaic (PV) panels that, although they do not produce heat themselves, do warm up when sun hits them.
While working on the roof of our local nature center where a section of glazing on a solar thermal system was broken during an attempt to clear recent stubborn snow and ice, I saw something going on that brought this phenomenon to mind. Although the air temperature at noontime was zero degrees F and the pitch of the roof is moderate to facilitate year round heat production, because the sun was out, the absorber plate of the section where the glazing was missing was heating up nicely and melting any snow we knocked onto it by mistake. This solar thermal system actually forms the roof of this portion of the center so is built into the roof.
While taking a break from the roof, I trekked over to the ground-mounted portion of the Center’s PV system to see if I could break off some of its accumulated ice. No chance, as the ice held the PV’s firmly in its grip. Even though there was a little section of the panels that was bare at the bottom of the array, the ice and snow above was firmly locked on and not about to yield to my gentle punching with my knuckles. The freestanding PV’s were at about the same angle to the sun as the solar thermal system on the main building, about 35 degrees.
Given the chance to reflect on this later, I came to realize that freestanding ground or roof panels mounted away from the roof on rails are much more subject to the outside air temperature, and particularly to a chilling wind, than systems built into a roof. Ironically, freestanding panels generally stay cooler which actually enhances electricity production, as heat decreases PV performance. But it leaves freestanding panels more likely to be affected by snow and, particularly, ice. Recent freezing rain has demonstrated this point.
A follow-up visit to Tin Mountain a day or two later confirmed my suspicion about built in versus freestanding systems. The PV’s built into the roof of the main building were clearing themselves of ice and snow, while the ground mount was still mostly covered with ice, and the PV’s on rails on the roof of the nearby intern cabin were completely socked in like my own system on the roof of my barn.
My PV system consists of 40 panels on the roof of my barn at about an optimum year round 38 degree pitch, with wind somewhat blocked by the roof behind it. However, they are mounted several inches away from the roof on rails that are, in turn, mounted onto four-by-four timbers running up and down the roof and screwed through the roof onto the purlins that run horizontally between the beams on this post and beam structure instead of the rails being screwed directly through the roof onto rafters. Because of this spacing, they are subject to chilling winds. And because the roof is way too high for me to reach with a broom that would only clear any loose snow over the ice anyway, I’m pretty much at the mercy of the sun and the air temperature to clear them. This is additionally frustrating when the sun is out as bright as it can be just after the winter solstice, but air temperature remains well below freezing both day and night, keeping ice and snow locked onto the array. I’ve noticed that most other local roof mounted PV systems also had trouble shedding the latest snow and ice, in great part because a light snow was quickly encased in ice when rain fell the very next day before the sun had a chance to clear the snow. In fact, it took over a week for my system and several others to finally shed their snow on a sunny day with temperatures that crept up towards 32 degrees F.
All of this brings another factor to light, the subject of tilt angle for PV systems. Built-in roof mounted systems, either PV or solar thermal, benefit from being insulated from wind chill while roof mounted PV systems on rails, though more efficient in cold weather, are more susceptible to nasty freezing rain events. Neither one, however, can be repositioned from summer to winter to take advantage of the sun’s declining elevation angle.
Pole ground mounted systems consist of a freestanding pole with a rack attached only to it and not to any other post or pole. The arrays are generally on a fixed position of about 35 degrees from horizontal at our 44 north latitude in Conway, NH. They can, however, be manually repositioned from summer to winter. This can not only provide benefit by keeping the PV’s more perpendicular to the sun’s rays to enhance performance, though admittedly not substantially, but the change from flatter to steeper also helps shed snow. In fact, a steeper angle helps keep snow from collecting on the PV surfaces, because the steep incline helps the snow bounce off and because the horizontal “footprint” of the PV’s is less.
I recently talked with an engineer and local weatherman friend, Ed Bergeron, whose freestanding two-pole system I helped install a couple of years ago, a school friend of my daughter’s also with a two-pole system, and also Nancy Rae Mallery, the publisher of Green Energy Times, who has a three pole systems in Vermont. They all have been changing the angle of their systems from summer to winter. Nancy sent me photos of her PV’s that are clear of ice and snow after our snow and rain event, and Ed tells me that he cleared his quite easily after the same event. I can clearly see the advantage of changing the inclination, both to collect less snow and ice and to put the PVs at an angle where the sun can do its job of melting, even though affected by chilling winds like we’d recently seen.
Considering all this, if a proposed installation calls for ground mounts rather that roof mounts, it may be more favorable to use multiple pole-mounted racks rather than in-line post type installations, as several owners have taken advantage of the simplicity of changing their pole-mounted system’s angle from summer to winter. For built in roof systems, Elon Musk’s new PV shingles will likely take advantage of being a part of the roof rather than hanging out where Winchell could keep them encased in ice for frustrating long times.
By the way, I did have to break away some of the ice and snow from the same event discussed here from my 45-degree-mounted solar thermal collectors by reaching the very bottom of them with a roof rake and then following up by accessing them through my two roof windows strategically placed between and beside the three collectors (coincidentally!) A strip of wood poked through the snow and ice got them partially cleared, and the warming sun did the rest. This is one of the few times in the 40 years that these collectors have provided my family with hot water that this has been necessary but may be a hint of what is to come with a warming climate and more freezing rains.
Russ Lanoie is a long time solar proponent in New Hampshire’s White Mountains and operated his Alternative Systems business in the 70’s and 80’s selling solar hot water systems, composting toilets and window quilts. He lives in a passive solar home which has had Daystar solar hot water for forty years and 11kW of PVs on his barn since 2015. www.RuralHomeTech.com