Is Wall-Mounted Solar Production More Efficient in Winter at High Latitudes?

[petterg]

I live at 60 degrees north. On december 22nd the sun is only 8 degrees above the horizon at noon. Here lots of people say they would go off grid if they just could have a reliable electricity production during the winter.
All the sellers of solar cells talks about maximizing the cells production over the year. They typically mount the arrays on rooftops, which typically has about 30 degree angle. At this angle snow sticks to the array half the day during winter.
What everyone experience is that their solar production is enormous in the summer and near 0 in the winter. To defend their investment they sell electricity cheaply to the grid in summertime and buys electricity at and high cost from the grid in wintertime. Hence they cannot go off grid. When looking at my electricity bills this winter 80% of the bill is the fee for being connected to the grid. If I should invest in solar cells without disconnecting from the grid, it would not do much to the monthly expenses.

What I don't understand is: Why don't people put their solar cells on the walls? Surely this reduces the electricity produced during summer, and thereby also reducing the total production over the year. But wouldn't it increase the production significantly during winter?
I've read that the sun has the potential to provide 1000w/m2. I'm assuming this applies on a clear day and a surface perpendicular to the sun rays. During winter the sun rays has to go through some more air before reaching us, but is the difference really that big as the solar cells generate near 0 electricity if they were angled for the winter? What would be the expected maximum energy from the sun at 60 degrees north on December 22nd on a surface perpendicular to the sun rays?

Some other factors I suppose comes into play is the reflection. Reflection from the ground (often covered with snow during winter) is probably helping out quite a lot when the array is mounted on the wall. While it doesn't contribute at all when the array is on the roof.

So why does the pros say you should mount the solar on the roof rather than the walls? (I suspect they make more money when the solar is connected to the grid as they'll get to sell the extra stuff needed to connect it to the grid, and the installation has to be done by someone certified (while arrays not connected can be done by the customer))

 

[jrmichler]

Your analysis is correct, although I disagree with your last sentence. I believe that the pros are trying to get the maximum kilowatt-hours per year from a system consisting of grid connected solar panels, while you are thinking of an off grid power system. The off grid power system must meet needs for the entire year, so needs to be designed for the worst week of the year. The worst week for solar panels is the week with the lowest sun angle and most clouds. The worst week for a wind generator is the week with the least wind.

If you search off grid power, you will find sites that discuss optimal power systems. An optimal power system typically has solar panels, wind generation, battery storage, and a backup generator. The optimal system balances the size of each component against the amount that the backup generator needs to run. A relatively low cost system might require the backup generator to run 100 hours per year, while a far more expensive system might have the backup generator running only 10 hours per year.

I live 46 degrees north. There is a solar farm only a few miles from here. I really should drive over there next winter to see if the panels are covered with snow.

 

[anorlunda]

It costs more money, but having a 2-axis movable mount for the solar maximizes the energy production. Not walls, not roofs, but free-standing and movable.

Sometimes they are nearly horizontal, sometimes nearly vertical. Most important, they rotate to follow the sun through the sky. At midsummer at high latitudes, the panels turn nearly 360 degrees. At midwinter in the northern hemisphere. , they point almost due south

The cost-benefit ratio for a 2-axis system depends on latitude. It is discussed in this other thread.
https://www.physicsforums.com/threads/sharing-solar-home-data.976496/

However, no matter what the mounting system, you can't escape much lower production at midwinter. Ask the Sami people living above 67 degrees latitude.

To live off-grid with solar, if your worst case day is 10% of rated production, you need to oversize your installation by a factor of 10. If you have battery storage, don't forget that battery capacity is lower in cold temperatures.

Don't forget to keep the panels free of snow and ice. Freezing rain would be troublesome for all kinds of mounts.

Have you looked into the possibility of micro-hydro for your home electric use?

p.s. I used to live at 60 degrees North, not far from you. Now, I'm retired at 29 degrees North.

 

[Rive]

Why don't people put their solar cells on the walls?

You can, but the main point is the length of the day. You can put the panels anywhere if you have only a few hours for production

 

[sophiecentaur]

You can, but the main point is the length of the day. You can put the panels anywhere if you have only a few hours for production

Any serious heating project needs to be looking at all possible aspects.
By the time you get to significant latitudes there may well be better ways of spending your money than a PV installation.
Cloud cover can have a massive effect on the yield from PV.
Extreme levels of insulation can get excellent value out of any money spent on Energy supplied.

 

[petterg]

In my case 2-axis movable panels has to have a lot less area than a fixed angle panel in order for the panels not to throw shade on the panel next to it. (The array has to be divided into several smaller arrays in order to have room to turn without hitting anything.)
I have not seen any side-by-side comparison of solar cells with and without solar tracking. Also I think tracking makes more sense the more horizontal the ideal angle of the panel is. At noon at 60degree north on December 22nd a 2m tall object will cause a 15.2m long shade. Before and after that time of the day, the shade will be even longer. If one account for panels should not make shades for neighboring panels between 10am and 2pm a free standing 2-axis array of 2x5m (h x w) will then need a ground area of about 550m2. Further south the shades aren't that long, hence moving arrays can be placed closer. An array mounted on a south facing wall, tilted 8 degrees back, will in the same 4 hours during the day never be more than 30 degrees off the ideal angle. Which means it will never catch less than 86% of an ideal angled panel. On total during the 4 hours it's probably just loose 5-7% of an moving panel. Which means if the cost of making the array move is higher than adding 5-7% more area, the increased area will give better economy as well as require way less mounting area.

I have not looked into how much extra a movable solution will cost, but I think it will be better use of the money to have panels on three sides of the house rather than small moveable ones. (As panels on three sides of the house will not reach peak power at the same time, all other components that scales with peak power does not have to be scaled up when adding panels to the sides.)

Solar cells are not for heating. Insulating the house and disconnecting the grid power, won't make any warm house or cold freezer. Some firewood, a battery and solar cell that produces enough power most days of the year will do both. Problem is to estimate what's enough solar. If the solar turns out to work very good, a heatpump powered by the solar with collector hose in the near by lake could be used to keep some heat in the house when nobody is home to handle the firewood.

What I was asking in this thread, that probably drowned was:
Does the sun rays provide about 1000w/m2 on a surface perpendicular to the rays even in winter at 60 degrees north? If not; What would be the expected maximum energy from the sun at 60 degrees north on December 22nd on a surface perpendicular to the sun rays? (assuming clear sky)

What is the most efficient? Installing the array more vertical to collect the rays reflected from snow on the ground or more horizontal to collect indirect rays from the sky?

 

[sophiecentaur]

I have not looked into how much extra a movable solution will cost, but I think it will be better use of the money to have panels on three sides of the house rather than small moveable ones.

Absolutely. If you are in high latitudes then you will get ice which can interfere with a mechanical solution. PV panels are 'understandable' and can be mounted on sturdy brackets. Repair would simply consist of replacing one panel at a time and wall mounted panels could even be moveable (manually) according to the seasons.
With a fixed array, you get the annoying fact that at least half the panels are doing nothing at anyone time (the 360° thing has been mentioned). Obstructions near the horizon would impose the biggest constraint in the Northerly directions in Summer and in Southerly directions in Winter.
In the end, the thing to do is to look around your district and see what's already in use. The installers are constantly trying to look good and will not instal systems where they won't impress the owners so whatever they put up is probably the best solution. People who use PV are 'enthusiasts' and there will be many owners who would be only too pleased to discuss how their setups work. Although, like gamblers, they will always reckon that they're winning in the long run.

 

[Rive]

Does the sun rays provide about 1000w/m2 on a surface perpendicular to the rays even in winter at 60 degrees north?

It does not. The 1000W/m2 refers to the overall incoming energy and contains the part what comes as heat too. It is a rule of thumb regarding solar based heating only (that's why the confusion). Regarding photovoltaics, it is something like 400W/m2 or so (I might be wrong: since this number usually has very little practical value I rarely use this).

Also a point regarding photovoltaics is that PV panels does not rely on the sun only, so the difference between a stationary panel and a movable one is usually far less than people think. That's why movable PV panels are not the mainstream.

One more thing is, that weather makes the raw value ~ useless. This is even more serious at north, since the effect of atmosphere is far higher. What you need for any practical use is the actual, measured average PV power generation based on a real project, and preferably not by just one season.

 

[petterg]

So you're saying that with panels having an efficiency of 17% they have a theoretical max output of 400w/m2 * 0.17 = 68w/m2 ? I thought the basis for this calculation was 1000w/m2. It doesn't really matter for the question of this thread though.

I'll try to rephrase my main question again: What I'm wondering is how the max input power to the panels (solar radiation) varies over the year assuming the panel is always perpendicular to the sun rays. There is no doubt that the suns energy to the ground is reduced when the sun rays angle to the ground goes from vertical towards horizontal. I think this is reduced by a factor of cos(angle) * [some other factor]. If this [other factor] is near 1, a solar panel that is always held perpendicular to the sun would produce the same power as long as the sun is above the horizon. What I'm trying to figure is how much the solar power drops beyond the cos(angle) factor

 

[hmmm27]

This calculator , given 60degN latitude and an 80deg tilt on the array, shows quite a variation, from $12.5kWh/m^2/d$ during the summer to $0.6kWh/m^2/d$ in the dead of winter.

 

[OmCheeto]

What I'm wondering is how the max input power to the panels (solar radiation) varies over the year assuming the panel is always perpendicular to the sun rays.

It's basically a function of 1/cos(angle), until the sun gets near the horizon, then there's a bit of a fudge factor thrown in.
On the horizon, the solar input is ≈38 times less than when the sun is directly overhead.
This is for sea level.
At only 1 mile altitude, the attenuation goes down to 31.
[ref]

According to my calculations, a 1000 watt panel will generate:
22 watts at sunrise/set,
380 watts when the sun is at your 8° winter elevation, and
940 watts when at your 52° summer elevation.

 

[petterg]

This calculator , given 60degN latitude and an 80deg tilt on the array, shows quite a variation, from $12.5kWh/m^2/d$ during the summer to $0.6kWh/m^2/d$ in the dead of winter.

That was a nice and simple calculator! Only thing is that it shows output total for the day, not peak when the sun is shining perpendicular on the panel. So it's including the factor for sunhours in the result.

If tilt angle is changed from 80 to 40 while looking at day 355 it's basically no change in the daily power input. Why is that? Is it because the less tilted panel catches some indirect rays earlier/later in the day, or that the indirect light from the sky is a major factor even at noon? (Or something completely different?)

It's basically a function of 1/cos(angle), until the sun gets near the horizon, then there's a bit of a fudge factor thrown in.
On the horizon, the solar input is ≈38 times less than when the sun is directly overhead.
This is for sea level.
At only 1 mile altitude, the attenuation goes down to 31.
[ref]

That's a interesting ref. When the sun elevation angle is 0 deg, you have nearly double the solar power, even though the change of airmass just goes from 38 to 31! Those last 7 air masses eats about as much rays as the 30 first air masses! (I guess that's because those last (lowest) 7 AMs are more dense than those above. The text states that the AM formula does not account for air being more dense near the Earth surface, so I guess the power data on 1mile elevation is not calculated from that formula.)

 

[Svein]

This calculator , given 60degN latitude and an 80deg tilt on the array, shows quite a variation, from $12.5kWh/m^2/d$ during the summer to $0.6kWh/m^2/d$ in the dead of winter.

I used a somewhat simpler calculation back in 1985 when wondering about solar panels on the cabin at 62° north. My conclusion was that in the wintertime it was more efficient to extract power from the heat of my own backside.

 

[sophiecentaur]

I used a somewhat simpler calculation back in 1985 when wondering about solar panels on the cabin at 62° north. My conclusion was that in the wintertime it was more efficient to extract power from the heat of my own backside.

So what's your conclusion about the 'value' you got from the installation? That's the bottom line. Would you do it all again or would you consider a different area of panels - or any other possible change?
I feel that we are likely to get a pretty unbiassed response from you, in the light of your comment about the winter performance.

My comment above about looking at other nearby installations (if there are actually any) could be the best source of unbiassed help with choice of installation. One big advantage is that the information would be more or less free!

 

[Svein]

Well - if left alone for 3 months, it had charged a 12V battery to full. That meant the possibility of using 12V lightning for a week if we were careful. It beat using paraffin lamps.

As I said, that was back in 1985. The panels may have gotten better, but the sun does not shine longer or stronger.

 

[sophiecentaur]

It beat using paraffin lamps.

In a subjective way but you could buy a lot of paraffin for the price of those panels, I think. And that glorious smell of burning paraffin!