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Old 16-05-2021, 02:12   #1
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Bifacial Solar Panels

Apologies if this has been covered.. I couldn't find a lot of discussion on it and thought it an interesting idea.

On mono-hulls where everyone tends to mount solar on an arch, how come nobody mounts a thin/lightweight downward facing panel to the underside of the top one (double-sided if you like) ?

I realise the idea of a solar panel facing the floor seems pointless, but when on passage I would imagine there is enough diffuse/reflected light to provide benefit. Since it wouldn't really take up any additional space, the only downside I can see would be cost and weight.

Has anyone ever tried this ? Did it work at all ? Is there a reason that you never see it ? Has it ever been discussed ?

Interested..
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Old 16-05-2021, 17:06   #2
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Re: Bifacial Solar Panels

Bifacial solar panels exist -- the back is specially prepared so that reflected sunlight hitting the back of the panel is absorbed, increasing the output of the panels. For the existing ones the back efficiency is less than the front efficiency, but they do add.


Example: https://sunwatts.com/bifacial-solar-panels/
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Old 16-05-2021, 17:23   #3
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Re: Bifacial Solar Panels

Bifacial panels are going to give you only a couple percent boost on a low albedo surface like water (5% albedo vs potentially over 50% albedo for something white like ice or snow). Even optimally a bifacial panel won't give you more than a 15% boost. If most folks cleaned their panels regularly they would get a bigger boost than from a bifacial panel over water.
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Old 16-05-2021, 17:24   #4
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Re: Bifacial Solar Panels

The reason they aren't popular is that the extra power you gain is very slight, relative to the extra cost and weight.
Glare off the sea may look like a lot of light, but on average, only about 6% of sunlight that hits the water is reflected.
So, you take a 200 watt panel, and coat the back side of it with more photovoltaics, and the most you're ever going to get out of that is about 12 watts. Less, in practice, because the back side needs to be a polycrystalline or thin-film type that'll work in partial shade, and is less efficient overall than the nice monocrystalline cells on the top.
In exchange, you've significantly increased the cost, made it heavier, and made it harder to mount.
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Old 17-05-2021, 12:37   #5
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Re: Bifacial Solar Panels

Thanks all..

I wasn't expecting a lot, but I figured it would be more than just 5%.

I see why no one bothers. Darn. Lol.
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Old 17-05-2021, 13:42   #6
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Re: Bifacial Solar Panels

Additionally, many panels are mounted on an arch with a dinghy stored on davids underneath, essentially shading the lower side.
Makes bifacials even less efficient.
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Old 26-05-2021, 13:44   #7
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Re: Bifacial Solar Panels

It's maybe worth mentioning that the bifacial solar panels are usually glass - glass panels. That means that the backside is also covered by glass instead of a foil like most "normal" panels.
This adds extra weight but also improves the protection for the solar panels (or so they claim).
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Old 14-12-2022, 06:39   #8
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Re: Bifacial Solar Panels

WOW, I am glad that I found this because Socks and I have been talking about bifacail panels for Faelan. I would have thought that there there be more light reflection off the water making it worth while.
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Old 14-12-2022, 12:37   #9
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Re: Bifacial Solar Panels

Quote:
Originally Posted by marshmat View Post
The reason they aren't popular is that the extra power you gain is very slight, relative to the extra cost and weight.
Glare off the sea may look like a lot of light, but on average, only about 6% of sunlight that hits the water is reflected.
So, you take a 200 watt panel, and coat the back side of it with more photovoltaics, and the most you're ever going to get out of that is about 12 watts. Less, in practice, because the back side needs to be a polycrystalline or thin-film type that'll work in partial shade, and is less efficient overall than the nice monocrystalline cells on the top.
In exchange, you've significantly increased the cost, made it heavier, and made it harder to mount.
Bifacials are an absolute game changer.
bifacials solar panels use diffusion light so they are in general less prone to shadings then conventional panels. in solar Parc they are installed vertical.
they produce up to 25% additional of the backside.
I switched from very good standard LG panels 345W to Longli 365W bifacials of the same size which actually deliver 450W, additional 90W from the backside into my Victron MPPT. on 3 panels that a gain in real watts of 3x105W=315W an additional panel virtually added plus it starts much earlier and stops later as the reflection from water the panels start slowly after sunrise with 3x 80W and then when sun is high enough the front side kicks in too. At passage I went from 40% output to 70-80% of the on anchor production. as they are less prone to shading from the sails as enough diffusion light is always around.
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Old 14-12-2022, 17:02   #10
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Re: Bifacial Solar Panels

https://prairiesunsolar.com/product/longi-365-watt/

Thats the one I am having. 3 of them are installed flat parallel to water and below with 20cm distance is the dingy hanging, each has its own 50/100 MPPT.
I see regularly real 430-450W on my Victron MPPT, the 365W rating is always just the front side. What I discovered is that the bifacials each panel profits from its own MPPT, you get 10-15% more out of a panel then you paralleled. Must have to to with tracking of both sides of the panel.
Bought 4, the 4th panel is installed on my buddy boat delivering the same output.
I will only buy bifacial in the future. The big difference is that they work with diffusion light and on water you always have plenty. On high quality bifacials you see additional gains just from backside 20-25%, which kick in shortly after sunrise as light reflects in water to the backside. So another hour after sunrise and one before sundown gives you 2 hours additional power input just from the backside, with my 3 panels only that is measured another 250Wh input additionally which you only get with bifacials. As long as sun is out it delivers, if it gets shaded by main sail during passage I see 70% of what I get at anchor or sun going fully on it, the conventional LG panel delivered 40% (same size and 10W less wp 355W).
Longli has a new 550W bifacial, 230cmx105cm that a friend recently installed on his boat, he regularly sees a whooping 690W and the Victron 50/100 MPPT goes into the 690W limit, the panel can even do a bit more.
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Old 14-12-2022, 17:31   #11
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Re: Bifacial Solar Panels

Quote:
Originally Posted by CaptainRivet View Post
https://prairiesunsolar.com/product/longi-365-watt/

Thats the one I am having. 3 of them are installed flat parallel to water and below with 20cm distance is the dingy hanging, each has its own 50/100 MPPT.
These panels only have an 18.5% efficiency. The better solar panels are around 22%. As most yachts have limited realestate to install panels the best option (if cost is not a consideration) is to select high efficiency panels. These will have a higher output per surface area than these bifacial panels even after taking into account the contribution from the second surface.

I think some high efficiency bifacial solar panels may be available in future. These may be the best option if you can harvest the output of the second surface. Even Longi promise better panels with up to 19% efficiency albeit with a pessimistic +/- 3% rating on output.
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Old 14-12-2022, 17:38   #12
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Re: Bifacial Solar Panels

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Originally Posted by libspero View Post
Thanks all..

I wasn't expecting a lot, but I figured it would be more than just 5%.

I see why no one bothers. Darn. Lol.
I bother, see above. Real world gains with the right panels is +20-25% plus.
I went from same size 3x355W LG Neon to 3x365W Longli bifacials and it was a gamechanger especially on passage. It is virtually a 4th panel added for free plus less shading losses. Real world Experience after a full year 100% live aboard with them. LG are top notch, if you had cheap panels that gain is more like 40% which my buddy boat experiences where a longli bifacial 365W replaced 3 older 120W panels from victron.
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