Saturday, August 26, 2017

Solar panel connection reseach. Interesting findings on YouTube

Well now, there's a thing! Helpful comments from Frank and Dave (thanks chaps) on my last post caused me to doubt my decision to connect solar panels in parallel rather than in series. Their arguments were erudite and persuasive. So wanting to delve further, I called upon my secret weapons, Rick, the smartest Engineer (with a capital E) I know, and I know a fair few, and Peter my brilliant Cambridge PhD scientist son (well I have to play the proud Dad sometimes) who has taught me so much inluding most of what I know about electronics.

Rick has never looked into Solar energy in any detail, but his knowledge of electrics and his sense of logic led him to side with me in so far as recognising that serial linking two panels with similar voltages but potentially different currents might not be advantageous. I left him to ponder.

Peter had never looked into solar power either, but he is nothing if not a quick researcher (with a good knowledge of electronics) and went off to look at a lot of graphs and interview the internet. One of his many mantras is that "Theory tells you which experiments to run" so he set out searching for people who had actually compared series and parallel solar panel connections and taken proper measurements under different conditions. What he found was very interesting.

There is a series of YouTube videos presented by a smart lady called Amy from the eltstore which I believe is in Canada. I wont go into all the detail here, but she connects up panels one way and then the other and takes readings of panel volts and amps and the amps delivered by a connected MPPT controller. If you go to Youtube and search for solar mismatch, you'll soon find her.

I was particularly interested in the mismatch topic because my intention is to add a new panel with an old one, and to some extent they will have different characteristics.

What Amy's tests clearly demonstrate is that when adding a second panel with a similar voltage, but materially different current from the first one, it is much better to connect them in parallel. All the specs and graphs i have looked at show that panel voltages are nearly always remarkably similar and stable but amps generated vary a lot with panel size and solar energy input. Furthermore, and most interestingly she demonstrates that under partial shading, series connections suffer a much larger drop in ouput than parallel. Go see for youself if you don't believe me. The videos are very good. She also does a good demo of the effects of tilting the panel in low sun, and another on the effects of temperature on panel performance.

This, as I see it, is the difference between ideal conditions and the reality of solar panels on a boat. Frank and Dave are quite correct and in ideal conditions I would follow thier advice to the letter. The ideal would indeed be for me to have two identical panels, each with their own controller, and each receiving the same amount of sunlight. The next best thing would be two identical panels connected in series to one controller and getting the same sunlight.

But that ain't gonna happen. I will have two different panels, generating different current, but closely matched voltages sharing one controller (because of expense and installation challenges), and on many occasions when we have to moor under trees or next to a wall or building, or the morco chimney casts a shadow over part of one panel, we will encounter partial shading. Unless anyone can prove Amy wrong, I'm going parallel. Having seen her videos, I might even consider getting a 150W panel rather than a 100W to add to the existing 95W.

Now I need to remeasure the roof space.


stevefree said...

We had a 2x165w solar system fitted by Tim at in June. He has done over a thousand boats so knows his stuff. His website has a wealth of information and this is what he had to say about series and parallel
Our controllers support serial or parallel connection. This clever feature means we can connect the panels in serial to achieve a high voltage input (which means better output in lower light levels) and with 495 or 660 W systems, we can then parallel banks of two serial connected panels together to maximise output in conditions where panels might be shaded.
One of his biggest points is the actual panels. Most panels may be high wattage but are also high voltage (often 48v) as they are for domestic usage. So a 165w panel would give only about 3.5 amps. He uses 18v panels to give 9.5 amps. Add on the improvement from an MPPT controller and the gap widens.
We are light electric users and could go 48 hours before we needed to run the engine. Since having his panels fitted we have never even had to think about running the engine. We moored up Friday lunchtime with a full charge and at 9am on Sunday morning I am showing 94% on the SmartGauge. It will be 100% again in a few hours. Of course in the Winter with much less usable light I'm not going to get this but for 9 months of the year it looks like I can forget about battery charging.
Because the batteries remain at a high level of charge we have not been able to see what could come in. However the day he fitted them when I was at a low charge level, I saw 21 amps going in, which is more than my battery charger.
It did cost us £1000 but he does the full job anywhere in the country and it only took about two hours.
Steve - Tumbleweed no5

Vallypee said...

I'm going to come back here when we get solar panels. There's so much interesting information here, Neil!

Paul said...

Am I missing something here folks? The max charge current delivered by the panel (eg a 150w unit) is apprx 150/12 = 12.5amps, ignoring all losses and in perfect sun. The higher voltage is stepped down by the MPPT controller to 12v. If anything higher voltage is better as it means less amps to the controller input and so lower cable losses and thinner cables. Same principle as 40,000v power lines.

Neil Corbett said...

You are not wwrong Paul, but you seem to have missed the point. Higher voltage will indeed reduce the current for a given power and allow thinner wiring, but the problems of matching dissimilar panels negate that benefit in my case. BTW so called 12v panels actually put out 17.5 v at max power and of course 12v batteries can be anything from 12v or less if they are flat to 14.4 or so depending on their state of charge. The mppt controller cleverly juggles volts and amps in real time to optmise amps to the battery at any given state.