Efficiency of different solar panels when not pointing directly at sun? - Page 1
Forum section: electronics / general

author: Freesurfer
author_rank: Regular Contributor
timestamp: October 31, 2020, 02:24:43 pm
content: I am going to install some panels on a roof which is not quite at the ideal angle for solar panels. That is, they are pointing where the sun is in the morning, and at the peak hours the sunlight will come in from an angle. I can accept that and just get some extra panels to make up for the inefficiencies.It is obvious that when the panel is pointed directly at the sun, you get the greatest efficiency and cover an area of 100% of the sunlight. Then if the angle is off, it is simple geometry to calculate how much less sunlight hits the panel. So with a panel tilted 45 degrees away from the sun, you only have "1 / sqrt(2)" (70.7%) the amount of sunlight hitting the surface and can expect to lose at least that much power.But other than the panel covering less surface area, I wonder if the cells are as efficient at catching sunlight from all angles, or if there is a derating, (similar to how light fades when you watch an old LCD screen from an angle)And if so, are there any differences with the different kinds of panels? E.g. are monocrystalline better than poly or vice versa?

author: Kleinstein
author_rank: Super Contributor
timestamp: October 31, 2020, 02:55:49 pm
content: Usually the loss due to reflection at the surface also gets a little higher when the light comes at an angle.There is an additional loss in efficiency when the intensity is low, independent from the reason. The current generated is to a good approximation proportional to the intensity, but the voltage available goes up with more light. With twice the light this may be another 20 mV more and thus some 3% extra voltage. So the efficiency would go up by these 3% (of the efficiency, not absolute number). On the other side there is a temperature effect - in the full sun the cells get hotter and from this reason less efficient. This may compensate some of the intensity effect, especially at the very bright end.The thin film cells often have a more loss due to series resistance, so there the efficiency does no increase as much or in the extremes may even go down with very high intensity. So the thin films cells are a little more competitive in low intensity than they are in full sun. They are still usually lower efficiency under all intensities - more like they may not like full sun as much.

author: Freesurfer
author_rank: Regular Contributor
timestamp: October 31, 2020, 05:16:29 pm
content: By "thin film cells", are you thinking of flexible/amorphous silicon cells?I think my options will be mono or poly panels with an alu frame. I have nearly decided on some 300W monocrystalline panels, which have a rated efficiency of 20.8% from a reputable supplier. But since I live in a nordic climate where we have many overcast days, and because they are going to be installed on the mentioned roof, I just wonder if they are the best option under non-ideal conditions, or if poly of similar power may be a better option. I think amorphous will take up too much space to be a viable option. Derating from heat won't be a huge problem as our summers aren't that intense, and I am oversizing the array a bit to account for longer periods of poor weather. So even if the panels become less efficient on some of the hottest days, they are already supplying a decent amount of power.

author: fourtytwo42
author_rank: Super Contributor
timestamp: November 01, 2020, 11:19:11 am
content: I once read an article that claimed poly was better in diffuse light situations but then I have also seen material claiming the opposite because mono is typically 2% more efficient than poly. Other articles point to the economics in that you get considerably more watts per dollar using poly. In your case where you want to makeup the lack of a good angle by increasing surface area poly may be your most economic solution. There are plenty of free resources on the web for estimating energy production based on where you are and the angleshttps://ec.europa.eu/jrc/en/pvgisfor example.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 01, 2020, 02:28:59 pm
content: Thin film panels are amorphous silicon and some CdTe or similar cells. AFAIK they are not very popular anymore with the prices for the normal silicon cells going down. The effect of heat and low light should be comparable for poly and mono cells - not much difference. The poly cells are a little cheaper, but lower efficiency. I have not look at up to date number, but the difference was larger than 2% points (and thus some 10% of the output).With relatively low intensity and often some wind the heat effect would not be so much.There is some advantage in having the same panels as already there. In case of a partial damage it makes swapping easier and the fastening system is also known.

author: Freesurfer
author_rank: Regular Contributor
timestamp: November 01, 2020, 09:37:36 pm
content: I went ahead and ordered 6 of the 300W mono panels. I hope I won't regret :-)Now I just need to find a suitable way to install them on a trapezoidal sheet metal roof. I want to install them directly on to the roof without any rails. The trapezoidal pattern should give enough back venting on its own. The summers here normally don't get hotter than perhaps 25C. I can live with some inefficiencies during the very hottest days, I am sizing the array to provide enough power on dull overcast days in the first place.

author: trobbins
author_rank: Frequent Contributor
timestamp: November 02, 2020, 12:51:43 am
content: As an aside comment, some suburban areas have a high penetration of roof-top PV, and it is now being recommended to face roof-top panels to east and west, rather than north-south (depending on hemisphere), so as to flatten the total PV generation curve within an AC feed distribution zone, and that profile typically aligns better with domestic daily load profiles, where peak demand can be in the early morning and late afternoon. There can also be a net $ benefit if your load profile is also more aligned to morning/afternoon, than noon, and your grid electricity import cost is high (or time of day scheduled), and your export revenue is low.

author: Freesurfer
author_rank: Regular Contributor
timestamp: November 02, 2020, 03:22:40 am
content: Yeah that's the problem that PV suffers from when everyone feeds back electricity at the same time and the peak doesn't match the consumption profile. If there aren't any good peaker alternatives like hydro then solar isn't as great as it may seem.This is for an off-grid setup in a cabin though, so I don't need to worry as much about when the peak is, as long as it can top up the batteries between each setup. I basically just need whatever can catch as much power as possible in a day.But one thing to consider is of course that if the panels can match the consumption in the cabin better, then it can put less stress on the batteries as the PV sort of feeds directly into the inverter, saving the need for charge/discharge and the inefficiencies of the batteries.

author: trobbins
author_rank: Frequent Contributor
timestamp: November 02, 2020, 04:42:29 am
content: That round trip efficiency can be lower than 80%, so it would depend on frequency of visits, and whether PV can top up batteries in between visits even if panels were say east/west, so maybe not a simple assessment to make.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 02, 2020, 11:16:36 am
content: With roof top panels, usually the roof sets the direction. If the roof is facing south it does not make sense to have then at a different angle. It is only the question whether it is worth mounting them in a east or west facing slope or not. If power is also needed in winter times a south-east or south-west facing wall may be an alternative.There are tools to estimate the result for different angles.

author: tautech
author_rank: Super Contributor
timestamp: November 13, 2020, 09:27:57 am
content: Multiple arrays with controllers that support such are the solution.Son in Perth has roof angles that aren't optimal so installed 2 arrays totalling 21 panels for 30+ KWH daily generation.A single 325W panel we have charging batteries on a remote data relay installation faces the morning sun and easily refreshes 2*195AH 12V SLA's in an hour or so.Screenshot that just missed a peak of its max 20A charge rate: