It's not mysterious, it's that thermal convection is much stronger when the panels are vertical, and production is strongly correlated with lower temperature.
Angled mounts tend to have obnoxious cross-members that block the airflow that should otherwise be sliding up the back of the panel, particularly on roofs where there's basically a bed of hot air trapped underneath with no good way to escape. That boosts the panel temperatures even further than you'd assume given simply lower convection based on their angle alone. Vertical mounts cannot have framing in these places, so they don't.
> It's not mysterious, it's that thermal convection is much stronger when the panels are vertical
I agree that, when you factor in semiconductor physics, it's not a mystery but it isn't necessarily an intuitive result for most. I've been working in aerospace for 5 years and one of the things that has been very clear to me is that peoples' intuition about things breaks down very quickly when there's non-linear factors involved in an analysis. In aero it's primarily square-law/cube-law tradeoffs; in semiconductor physics it'll be more exponential.
For this particular problem you've got an exponential (semiconductor behaviour as a function of temperature) multiplied by a trig function/dot product (cosine of the angle of the sun relative to the normal of the solar panel), with a bit of natural thermal convection thrown in for good measure. Modelling this (digital twin, as they call it) is feasible but it's not something most people are going to have a good intuition on with respect to where the sweet spot is going to be.
The cited "digital twin" software[0] doesn't model convection (it just uses wind speed and an empirical factor), which is why it gives a higher predicted temperature than the physical model.
If you run PV/battery systems you pretty soon notice that in extreme cold events the controller can shut down charging because panel output becomes so high that the batteries will be overcharged.
The air near the top is 100% trapped, there is no way for it to escape and it's the hottest air under the panel. Overlap with the cells is anywhere from 1/2" to 1.5" so that's a sizeable fraction of the cells. Probably close to 25 to 30% or so.
The average person's intuition and 'mysterious' are two different things. One is someone with no knowledge assuming the wrong thing, the other is experts not able to figure something out.
The panels themselves are usually made from aluminum U profile and close to the edge there is a substantial amount of trapped air if the panel is at a bad angle. Given that most panels are at a bad angle this will cause the top edge cells to all be over temp and since they're all in series that drops the efficiency of the whole panel. So the cross members certainly don't help but the panel construction itself could do with some more ventilation near the top. I wonder if cutting some slots in the top members would drop the cell temperature in a way that it would show up on a measurement, this is pretty easy to test.
Indeed - I saw the title and thought “because convection” - I actually reinstalled my PV array last year with exactly this in mind, as while it was at an optimal angle for insolation, I was finding that yield was being hampered by them getting devilishly hot - the summer before last, when we hit 47C air, really underlined the issue, as the panels were getting up to over 85C.
The increase in yield from going near vertical (80 degrees was the best I could achieve using existing mounting gear), has been about 20% - I say about as I haven’t done a scientific study of it, just looking at year on year comparisons for cloudless days, and the panels are 60 C cooler, which is far better than I had hoped for.
So, a lot of the recent attention is on bifacial east-west arrays because they produce a complementary duck curve throughout the day. In that case, pure vertical makes sense, and production just takes a dip at local solar noon and recovers soon after.
But for traditional south-facing panels, I'd argue that straight vertical is still optimal, at least for higher latitudes. Vertical panels are extremely good at shedding snow. They produce more in winter when you need every watt you can get, and less in summer when the sun is high in the sky and you don't need all that extra power anyway. As soon as you tilt the panels to minimize cosine loss, you open yourself up to snow buildup which can dwarf any cosine gains.
I'm also a fan of vertical panels in snowy regions, but at the same time it kinda dodges the question.
In a non-snow region (which is most of the country, a fraction which is only increasing decade by decade), what is the optimal tilt angle? 70°? 80°? More? This is quite important for large solar panel farms in the hot and sunny South/Southwest.
The cosine loss is easy to model, so to answer the question what we need is a curve that describes how the convection cooling effect varies with angle.
I'm hoping to get solar on my next house, which we're designing now. Is there a way to install roof panels that improves convection / reduces temperatures, like using a different mounting system, if that's a thing?
Buy 2x extra panels. Honeslty the gains you might get by optimising panel placement (beyond matching to your hemisphere and latitude) will be outweighed by the additional cost. Domestic applications is not big enough to really give you significant gains in this department.
Make sure they face the right way (south for Northern hemisphere), and match the angle with your latitude. If you have a pitched roof thats +-10 deg in the correct angle, just lay them flat on the roof.
Edit: forgot to add, a while back there was an article here about a company that proved it was viable to lay the panels flat on the ground for massive solar farm installations. The savings from less installation labour and materials went to installing more panels. And they still came out ahead. Solar is getting cheap enough that the math gets weird. Your answer is almost always "just add more panels" unless you are seriously space constrained.
I don't know anything about rooftop solar mounting systems, I will show my ignorance here: I was imagining that perhaps you could attach the panels to rails running vertically rather than horizontally, to allow for convection airflow below the panels.
Or if the rails are horizontal perhaps they have holes in them to allow some airflow.
I'm sure this has been thought of and doesn't work some obvious reason I just don't know about.
Make sure you design a roof with the rigt heading, and slope. The rest will be done by the installer.
My rails run horizontally, but thats only because it required less rail and mounting hardware compared to vertically. The panels are mounted portrait, in a 5x2 square. Each panel has 2x rails under it, so 4 rails horizonfally on my roof. If you sketch it out then it'll make sense.
Had I gone with vertical rails, each column would've required 2 rails, so 10x rails vertically mounted on my roof.
I wonder how it would pencil out if you just pumped groundwater through them and returned it to an adjacent well, without even trying to harvest the heat. It cools the panels in summer, and warms them in winter when snow coverage might otherwise be an issue. Melting the snow off can turn an otherwise-nearly-zero period into a productive one.
But this is way simpler than domestic water heating. You don't care about thermostats and storage tanks, you don't worry about overheating, you don't even necessarily have to care about leaks since it's all going the same place anyway. There's only one pump and no valves. The panels themselves could be made to a lower standard since the whole thing could run at nearly-zero pressure.
The term of art there is "concentrated PV solar". Big fresnel lenses concentrating light onto a postage stamp of photovoltaic material. Requires sun tracking to keep the light on the PV, so it's useless for rooftop solar.
If you look it up you'll notice all the citations are decades old. Designed for a world where PV was terribly expensive, so you'd trade a lot of mechanical and packaging complexity to use as little of it as possible.
Then silicon got 100x cheaper. Now every solar install uses fixed-angle racks or just plopping the panels flat on the ground. You lose some efficiency, but land is cheap, so who cares?
Wow, I was just talking to my dad about this. He runs a startup that builds PV-embedded vertical masonry[1], and they have been getting significantly higher-than-expected yield on their installations. Though in their case they were mostly theorizing ground reflections
That's a cool project. One advantage solar masonry might have is the high thermal mass of masonry. The bricks average out day and night temperatures, and probably keep the panels a lot colder than they would be on their own.
If anything I would think that the poor thermal conductance of the masonry would make this inferior to simply mounting panels over the exterior with an air gap.
I have so many questions! While digging into the Applications section these look like conventionally-sized CMUs, the image on the front page looks like panels/blocks that are large enough to require a crane to lift? If that's the case, how fragile are they?
CMUs and other pre-cast concrete "lego blocks" have intrigued me for a long time. These look like they've got more robust interlocking features than conventional cinderblocks too? The idea of being able to order (even without the solar) say 8'x4' pre-cast "CMU-style" walls, have them show up on a flat deck, stuff rebar and mortar into ready-made holes, and grout between the blocks seems like it could dramatically speed up a lot of exterior construction. Being able to get them ready to wire for solar is delicious icing on the cake!
They don't need a crane. Maybe a confusing rendering for a future idea.
They make tiles and cinderblocks, and yes, they have some interlocking features. And yes, the idea is that installation will be way easier and cheaper than first building a regular wall and then attaching vertical solar to that after the fact.
Please ask your dad or whoever does the work on that website to update the home page so that the content does not do the reverse of its "intro animation" just because you scrolled back in the direction of the top of the page.
I was more distracted by that behavior and totally failed to get any value out of visiting your site.
They're embedded in the masonry. When you buy a CMU, you get one unit. Yes, this has better thermal properties, and this also makes them more durable and theft/vandal resistant.
Interesting, definitely a surprising result. I did wonder when I saw a thumbnail of a video discussing this on YouTube (but I didn’t have time to watch it) - I thought “Surely that only works for bifacial panels” so makes sense that the article confirms that that’s what they were testing.
The other factor is that (compared to where I live in Australia at least, but also all of the US too) the Netherlands is quite far from the equator, so I expect there would be a crossover point a bit closer to the equator where you start to get less efficiency than standard angled horizontal panels?
Although perhaps with some reflectors on either side it might still work with a vertical bifacial panel (in areas closer to the equator), maintaining the cooling advantage?
I'm curious about how well this performs in winter when the sun is low in the sky and you have a pretty big deficiency. Usually you use what ever angle the roof is at (typically 30 - 45 degrees) and leave it at that. By adding a vertical component you may be able to substantially offset the summer/winter difference. Vertical panels won't do much in the summer but that's fine, you'll have a surplus anyway. But in winter you need every little bit. But from a ROI point of view those would be pretty expensive KWh, because the total produced versus the capital expense won't be very high. And in plenty of places the local authorities might have something to say about covering the outside of the building with panels. I'm going to play around with this here to see what it does.
I live in the edge of a row of townhouses and have a huge wall doing nothing but cooling the house. I’ve been thinking about having vertical panels on the wall and my fear is exactly what you described, that all RoI calculations are based on producing a lot of power during the summer half of the year and I that I will not recoup the money but the problem is that when it’s sunny outside everybody is producing electricity and the prices are low so I think the RoI don’t really take this in to account. My wall is on the sout side and gets sunlight all day, is much larger than my roof, doesn’t get covered with snow. The prices electricity are usually 10-20x higher during the winter and our consumption is also 10x higher. I don’t know it just seems to make more sense to put them on the wall.
I think you should factor the savings on AC in the summer and heating in winter as well into your calculation. For that to be most efficient the air would need to be trapped behind the panel, I don't think you need to worry about overheating so much because they are going to be running at a low fraction of their theoretical capacity. There is a fair chance that including savings on heating and cooling it will actually work out but I haven't run the numbers in detail. But it certainly is an intriguing proposition, even if it would work only on South facing walls.
We don’t have AC installed so our only major energy cost during the summer is hot water . Yeah it’s definitely worth doing som maths. One major pain point though is that we need a permit for vertical panels because it affects the look of the house.
> we need a permit for vertical panels because it affects the look of the house
That's fairly common, but usually only on the front of the house. And on extra buildings like a garage or a garden shed such restrictions may not apply. This is pretty trick and it varies enough from one place to another (even within the same country, province or state) that it is worth researching before embarking on such a project.
Yeah, I'm quite curious about the overall tradeoff here. I'm at about 51 deg N and the solar elevation angle gets very very low during the winter (only 16 degrees elevation at noon) but high in the summer (63 degrees elevation at noon). Because of how low the sun is we end up with very long shadows even mid-day in the winter. Would love to spend the time breaking down:
- land area required for tilted vs. vertical
- net production over the year
- equipment cost for bifacial panels vs single-face panels
Anything that helps solar cost/performance is a huge win around here because we have max energy consumption in the winter when solar doesn't produce a whole lot.
If, like me, you don’t know what PV stands for: A photovoltaic (PV) cell, commonly called a solar cell, is a nonmechanical device that converts sunlight directly into electricity.
Summary (roughly, I skipped some things): while tilted panels produce more voltage, their efficiency drops due to heat absorption. Two sided vertical panels allow for better cooling, and on average produce more power, roughly 2%ish, albeit more expensive to produce.
I imagine installation costs are also substantially lower since the panels rest on a much simpler structure. Cleaning the panels should also be easier in this configuration. I wouldn't be surprised if these findings lead to vertical solar systems becoming a popular option, especially in places as far from the equator as Denmark.
The output can also be timed for better value (for example, increasing production in winter vs. summer, or peaking early and late for N/S aligned modules.)
Combinations of different orientations can smooth output over a day or year, which could reduce the mismatch between module output and inverter capacity.
Here in Australia, News Corp (owned by the same Murdoch family who is also behind Fox News) is currently running a campaign against agricultural solar. A key “argument” being that installing solar panels on fields makes it impossible to grow crops there.
I live in Aus as well, and I don’t expose myself to News Corp at all. Can you explain to me what the heck the issue is? We have so much space and we don’t need that much more solar, how is anyone’s crops at risk? Is this just a complete non issue? Like i agree that solar panels shouldn’t be installed over the top of crops but is that even a thing that is happening? Don’t the farmers have almost complete control of what goes on their property anyway? So confused
This seems like something that should have been verified in lab/field decades ago, before billions in mass solar roll out. I'm assuming it is and large scale solar infra are optimized according to site conditions?
> “For a standard system, we observed that under high irradiance conditions, the increase due to the light is offset by the decrease due to the higher operating temperature,” Van Aken stressed. “However, for the vertical system, we observed that the operating temperature is not increasing so much and the voltage increase and decrease are more or less balancing.”
Facing the bright sun increases temperature enough to offset the gains in voltage (since temperature increases presumably increase resistance if my EE101 classes hold in this era). Not facing the sun? Less heat -> more total power throughput.
Wow and wow! I've been wanting to jump into a home solar system, I just think there's to many inefficiencies. Hopefully this is the way solar will evolve in the near future.
Spoiler: vertical panels can rid themselves of excess heat more efficiently, thus operating temperatures are lower and, hence, output better. Vertical panels will also be more susceptible to damage from high winds but less susceptible to precipitation of water and dust, so my guess would be that the fixed slant orientation for PV panels in temperate latitudes will stay with us irrespective. I'm starting to think about having water pipes running along the backsides of the panels to cool them and obtain warm or hot water...
I’ve seen people cooling solar panels by using coreflute/Fluteboard at the back of a panel and circulating pool water through the channels. I think I saw this on Linus Tech Tips weirdly enough, not sure though
This was exactly my question, have people looked at the various thermal management techniques used in computing? Heat pipes on the backplane and fan cooled heat sink blocks could go a long way.
Angled mounts tend to have obnoxious cross-members that block the airflow that should otherwise be sliding up the back of the panel, particularly on roofs where there's basically a bed of hot air trapped underneath with no good way to escape. That boosts the panel temperatures even further than you'd assume given simply lower convection based on their angle alone. Vertical mounts cannot have framing in these places, so they don't.