[RE-wrenches] Off Grid, Cloudy Days, Solar Tilt Question

[Eric Smiley]

It is difficult to quantify but this was pointed out to me years ago by a
long-time off-grid installer, who also lives off grid on Vancouver Island
where weeks of solid cloud isn't unusual. We never tilt to latitude + 10 to
15 for off-grid, as many textbooks recommend.
Latitude works quite well. When I've tried modelling it, January is the
only month that seems to outperform when tilted at less than latitude,
which is often our cloudiest month. We also have a lot of conifers here so
tilting to 60 degrees often means the modules are facing the trees anyway.
Fortunately we don't get much snow so a shallow tilt is OK.


*Eric Smiley,* Design Manager
eric at vecoop.ca
T: 250.703.6004 <+12507036004>
T: 888.386.0116 <+18883860116>
*VIRIDIANENERGY.CA <https://viridianenergy.ca/>*

* <https://viridianenergy.ca/>*


On Wed, 25 Oct 2023 at 17:42, integrityenergy101 via RE-wrenches <
re-wrenches at lists.re-wrenches.org> wrote:

> This is too funny.  I just stumbled upon this phenomenon last week and
> have tested it several times with my irradiance meter over the last week or
> so.  I'm seeing consistently higher irradiance/panel output with panels at
> a very shallow to horizontal angle than those set to latitude on cloudy
> days in VT.  And we have plenty of them this time of year!
>
> Amos
>
>
> Sent from my Galaxy
>
>
> -------- Original message --------
> From: Bradley Bassett via RE-wrenches <re-wrenches at lists.re-wrenches.org>
> Date: 10/25/23 7:03 PM (GMT-05:00)
> To: RE-wrenches <re-wrenches at lists.re-wrenches.org>
> Cc: Bradley Bassett <bbassett2 at gmail.com>
> Subject: Re: [RE-wrenches] Off Grid, Cloudy Days, Solar Tilt Question
>
> I used to have both an array at a 60° tilt and one at 14° tilt. In the
> winter the high tilt array did better on sunny days, and on cloudy days the
> shallow array did better. They were different modules and different sized
> arrays, so without more analysis than I did, it would be hard to tell which
> is better through the winter, but it looked like they were similar in
> overall output. However, once March rolled around the steeper array started
> to do much better (more sun), and by summer the shallow array did much
> better. I'm at 47° N and I think if I had a choice of any tilt, but without
> adjustability, I'd probably set it at latitude or therabouts. If you're
> west of the mountains you're probably going to need another source of power
> anyway. I say that, but judging from the modelling I did of the output of
> my micro-inverter system it might just be possible to get through the
> winter with an array 500% overisized. And that with only 2 or 3 days of
> battery autonomy. This was surprising to me. Do keep in mind that an array
> with snow on it has 0 output.
>
> Brad Bassett
>
> On Wed, Oct 25, 2023 at 3:47 PM Kirk Bailey via RE-wrenches <
> re-wrenches at lists.re-wrenches.org> wrote:
>
>> Hi All!
>>
>>
>> We don't do a lot of pure off-grid systems and I recently ran across
>> something I wanted to run by folks with more experience in this area:
>> Optimal array tilt for our very cloudy PNW winters!
>>
>> My understanding has always been that latitude plus 10-15 degrees was the
>> best tilt to address our winter energy shortage. However a paper I recently
>> read (1), makes a compelling case for a much shallower tilt in situations
>> where the cloud cover is so heavy that "diffuse" solar radiation is all
>> that makes it through. They indicate that under those conditions a
>> horizontal array will produce significantly more energy than even a
>> two-axis tracker!
>>
>> Given that the challenge in our off-grid setups always seems to be making
>> it through the really cloudy stretches, and that there is usually enough
>> energy the rest of the time, should we be installing off-grid arrays at a
>> shallower angle?
>>
>> Anyone tried this?
>>
>>
>> Kirk Bailey
>>
>> kirk at abundantsolar.com
>>
>> www.abundantsolar.com
>>
>>
>> (1) Kelly, N.A., Gibson, T.L, 2011, Increasing the solar photovoltaic
>> energy capture on sunny and cloudy days. Solar Energy 85, 111-125.
>>
>>
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