Solar Installation - PWM or MPPT ?

[Stormbringer]

Hi all

 

I've just picked up a couple of 100w solar panels to fit on the boat ....

 

Just wondering what people's experiences are regarding controllers.....

I have done a load of research on t'internet and watched far too many youtube videos - and now I am more confused than ever

 

the panels are 2 x 100w 20voc - I guess these are nominally what is known as 12v panels?

they will be feeding a 420Ah / 12v domestic bank

 

What is he best way to set these panels up ?

From my research it is suggested that PWM charge control is kinder to my bank and that MPPT gains are small on a 12v panel system.

Is this correct or have I missed the point somewhere?

 

So .... main questions are

 

Is it worth the extra for an MPPT controller - and if so would I wire the panels in parallel or series ?

If yes - what controller would you recommend ?

(I am aware of the fake Chinese controllers which proclaim to be MPPT but have no DC-DC converter so are obviously not !!)

 

Oh - and what will my Smartgauge make of it all ??

 

Thanks for your thoughts and experiences

 

Cheers

Andy

[Jess--]

Gut reaction is go with mppt even though it is a bit more expensive to start with, the few percent of extra efficiency adds up quite quickly.

Messing about with a damaged 140w (26v oc) panel and an mppt controller I was impressed with what it could get from the panel (so much so that I am fitting the system on my boat)

[Top cat]

I'm using a Victron Blue Solar MPPT controller with a single 130W panel and its working very well. The controller brings my batteries to full charge ( 14.4V) then holds them at float (13.8v) which seems pretty ideal. I paid the extra for the MPPT controller because of the claimed 30% improvement in power transfer, but I haven't done a side by side test. I have seen claims that PWM are just as good up to about 150W but no evidence was presented.

One feature of my set up is that should I want to add another panel I can do so easily, but so far the single panel has done all we require for the last two years.

 

I did learn the importance of mounting the controller near the batteries otherwise the temperature compensation can be incorrect.

 

There are quite a few threads on Smartguages getting confused by solar installations, but as I don't have one I can't comment.

 

Top Cat

[Chewbacka]

For my panels I wired them in series with an MPPT controller.

The OC Voltage for your panels will be about 40v which should be ok for an MPPT but check. My controller will go up to about 90V (if I remember correctly) so adding more identical panels is easy.

 

Advantages

1) In series you will get enough volts even when overcast to get some power, but if the sun is not very warm there will not be a lot, but better than nothing.

2) As the voltage between the panel and controller is high, the current is low (for a given power) so the losses are reduced if you have long cable runs.

 

Disadvantages

1) If one panel is shaded then the total power output is greatly reduced. With panels in parallel and each having a diode, then if one panel is shaded and the other in full sun, you will only lose half the power. But you do loose power in the diodes.

2) Series wired panels must be reasonably matched for full efficiency.

3) If you have a number of panels in series the voltage can become quite high (mine regularly run at 70V) which needs installing more like a mains system rather than a 12v system as 70v dc is going to hurt. From memory the regs require better insulation and protection above 32v, but I stand to be corrected on this. In your case it is only 40V oc, but who knows you may wish to add a third or forth panel.....

 

When I moor up I look for a place that is clear to the east to get the early morning sun, so normally shading is not an issue.

 

For me the advantages outweigh the disadvantages so I went for series.

 

Hopes this helps.

[Paringa]

I have 500W and a Steca PWM controller... I did a side by side un-scientific test with my friends barge and his new panels and MPPT and the difference was a 10% improvement with his controller. But these are big panels to 24V. I believe if you are using big panels and then wiring them in series a MPPT is the way to go to get maximum efficiency. I also believe that manufacturers test under bench conditions and in the real world things are a bit different, so get more Watts/panels up if you have the space.

 

Smartgauge just deals with charging and discharging as it did before you fitted the solar.

 

If you can afford it pay your money and do the job once properly. You also pays your money and takes your choice on the set-up...

 

And if you spend any time on a boat solar is just so the way to go. You wont regret it.

Edited September 3, 2015 by Paringa

[nicknorman]

Disadvantages

1) If one panel is shaded then the total power output is greatly reduced. With panels in parallel and each having a diode, then if one panel is shaded and the other in full sun, you will only lose half the power. But you do loose power in the diodes.

 

Isn't it normal practice to have a bypass diode for panels in series so that if one panel is shaded, the current can flow through the other one with just the diode's voltage drop? I think there is a difference in this respect (series vs parallel with one panel shaded) but not sure it's exactly as you say - with series, you are likely to lose 1/2 the power, with parallel you are likely to lose less than 1/2 the power unless the shaded panel is virtually in the dark.

[Traveller]

I'm using a Victron Blue Solar MPPT controller with a single 130W panel and its working very well. The controller brings my batteries to full charge ( 14.4V) then holds them at float (13.8v) which seems pretty ideal. I paid the extra for the MPPT controller because of the claimed 30% improvement in power transfer, but I haven't done a side by side test. I have seen claims that PWM are just as good up to about 150W but no evidence was presented.

One feature of my set up is that should I want to add another panel I can do so easily, but so far the single panel has done all we require for the last two years.

 

I did learn the importance of mounting the controller near the batteries otherwise the temperature compensation can be incorrect.

 

There are quite a few threads on Smartguages getting confused by solar installations, but as I don't have one I can't comment.

 

Top Cat

I have a single 150w panel and was told that if below 170w MPPT is not worth having. Apparently solar starts to hit the MPPT sweet spot at 170w. Seeing as I have a PWM I decided not to change to MPPT; not being a liveaboard it is very unlikely that I will add further panels

Edited September 3, 2015 by Traveller

[Jambo]

You may well find that 100W isn't enough so keep an eye to upgrading. It may well be that domestic panels might prove tempting in the future so investing in an MPPT with a decent power head room now to allow for expansion might be worth it.

[jenevers]

Isn't it normal practice to have a bypass diode for panels in series so that if one panel is shaded, the current can flow through the other one with just the diode's voltage drop? .

Where do you get these diodes from and is there a wiring diagram available anywhere?

Are they not already in the narrow, black box where the neg and pos wires are connected to the panel?

Edited September 3, 2015 by jenevers

[Phil.]

With that VoC and wattage MPPT is overkill so PWM. If however you will increase size of array then future proof with MPPT.

[nicknorman]

Where do you get these diodes from and is there a wiring diagram available anywhere?

Are they not already in the narrow, black box where the neg and pos wires are connected to the panel?

Yes they may well be. But probably panel-dependant. I'm not a solar expert, I was just pointing out the minor flaw in chewbaka's analysis of the different configurations.

 

You could test with a multimeter - panels totally in the dark, diode range, and expect to get 0.6v or so one way (current flowing from +ve terminal to -ve terminal of the panel) if there is a forward bypass diode.

[jenevers]

Yes they may well be. But probably panel-dependant. I'm not a solar expert, I was just pointing out the minor flaw in chewbaka's analysis of the different configurations.

You could test with a multimeter - panels totally in the dark, diode range, and expect to get 0.6v or so one way (current flowing from +ve terminal to -ve terminal of the panel) if there is a forward bypass diode.

Found this

http://www.electronics-tutorials.ws/diode/bypass-diodes.html

 

Starting to look complicated!

[nicknorman]

Starting to look complicated!

Not really! The blocking diodes stop reverse flow at night, and the bypass diodes allow duff or shaded cells, or areas of cells, to be bypassed so they don't block current from other good and well lit cells. As the article says, most panels come with these diodes already fitted.

 

I suppose if I had a couple of large panels in series, I would want to check that there is an overall bypass diode for each panel as well as bypass diodes integral in banks of cells, so that the maximum voltage drop for a shaded panel would be limited one diode's worth, each one being around 0.4 to 1v (depending on type and current). This diode would be located in the junction box as opposed to within the panel.

[trackman]

Not really! The blocking diodes stop reverse flow at night, and the bypass diodes allow duff or shaded cells, or areas of cells, to be bypassed so they don't block current from other good and well lit cells. As the article says, most panels come with these diodes already fitted.

 

I suppose if I had a couple of large panels in series, I would want to check that there is an overall bypass diode for each panel as well as bypass diodes integral in banks of cells, so that the maximum voltage drop for a shaded panel would be limited one diode's worth, each one being around 0.4 to 1v (depending on type and current). This diode would be located in the junction box as opposed to within the panel.

As Nick says in my experience.

 

Decent panels have bypass diodes to let power get past any defective or shaded cells in each panel. They also have doides to allow a whole panel to be bypassed when necessary too.

 

I can see the effects of this by looking at the voltages on the panels under differing conditions of shade.

 

Ours are in series. If a whole panel in our set of 3 is shaded it cuts the voltage by slightly more than a third. If I shade one or more cells in a panel the voltage drop is roughly proportionate to the proportion of the total cells I've covered.

 

The voltages can be quite high in strong sun. Each of our panels can kick out over 20V at best so 3 in series can approach 65 to 70V at times.

[Arthur Brown]

MPPT makes more of the available energy but costs more. If you hope or expect to increase the size of the solar array then a MPPT would make a lot of sense.

 

While a 200w array will do good things in the summer, a larger array will produce benefit for a longer season.

[Stormbringer]

Thanks everybody for taking the time to pass on your thoughts - it's very much appreciated.

 

I wouldn't really like to rule out another panel in the future (although we are not big users) so it looks like MPPT would be the way to go.

I suppose that this decision will force a re-think regarding my method of connection too - I was envisaging parallel connection but if using

an MPPT controller I guess series connection is preferable - have I got that right ?

 

Thanks for the interesting stuff regarding blocking / by-pass diodes too. What about the volt drop across the diodes ? Does that not 'cancel out' much

of the benefits of connecting in series? I'll be using 6mm cable from the panels and they are directly above my battery bank - so the runs are short?

I would have thought those 0.7v would add up to a fair power loss ?

 

So - is anyone really happy with their MPPT controller and, if so, would you like to share the make and model if possible so I can start to

look at some alternatives. I am keen to buy a decent bit of kit at a decent price, if that is possible these days !

 

Thanks again - the discussion has been very helpful indeed !

 

Cheers Andy

[Top cat]

I am really happy with my Victron blue solar MPPT 70/15, it has done all I have asked of it. It is fed by a single 130W panel but could take another one. It has 3 step charging for the batteries, so is kind to them. It does the job no fuss. Just follow the instructions and fit it adjacent to the batteries (I didn't at first).

 

Top Cat

[nicknorman]

I would have thought those 0.7v would add up to a fair power loss ?

Not really. The blocking diodes are only one per panel. In fact with a series arrangement you only need one diode total, and possibly not even that depending on the controller (it might have one itself). The bypass diodes do not come into play normally, they only come into play with a panel shaded, duff cell etc. If there is an overall bypass diode per panel then the most a panel will drop is one diode's worth for the bypass (if the panel is shaded) plus the blocking diode. If these are Schottky barrier types then they may only drop 0.5v each or so.

[Chewbacka]

I bought my set up from Bimble solar, they are good.

My MPPT unit is not listed anymore, all newer models, but here is a bit of interesting reading for you .

http://www.bimblesolar.com/offgrid/mppt

[Phil.]

Yes I am happy with my MPPT solar controller. It is an outback FM60. In addition to maximising output it also stores data and has a number of other functions including an equalise mode which I find useful.

[Stormbringer]

I am really happy with my Victron blue solar MPPT 70/15, it has done all I have asked of it. It is fed by a single 130W panel but could take another one. It has 3 step charging for the batteries, so is kind to them. It does the job no fuss. Just follow the instructions and fit it adjacent to the batteries (I didn't at first).

 

Top Cat

 

Thanks Top Cat - I've been looking at the Victron controllers and they do seem to be a good bit of kit - and not as pricey as I first feared. I would probably need a slightly bigger one to accept 300W (for future!). I like the 'no fuss' bit too !!

 

Not really. The blocking diodes are only one per panel. In fact with a series arrangement you only need one diode total, and possibly not even that depending on the controller (it might have one itself). The bypass diodes do not come into play normally, they only come into play with a panel shaded, duff cell etc. If there is an overall bypass diode per panel then the most a panel will drop is one diode's worth for the bypass (if the panel is shaded) plus the blocking diode. If these are Schottky barrier types then they may only drop 0.5v each or so.

 

Thanks for clearing that up for me Nick ! Obviously I need to do a bit more reading up !!! I didn't realise the bypass only came into play with the shade.

Back to skool for me !!

I bought my set up from Bimble solar, they are good.

My MPPT unit is not listed anymore, all newer models, but here is a bit of interesting reading for you .

http://www.bimblesolar.com/offgrid/mppt

 

Cheers for the link Chewacka - I've put that in my reading list ! Much appreciated

Yes I am happy with my MPPT solar controller. It is an outback FM60. In addition to maximising output it also stores data and has a number of other functions including an equalise mode which I find useful.

 

Thanks Phil

 

Whilst I would love an Outback controller, and salivate at data logging etc ( I was a chemical process operator for 20yrs and love a good graph - can't beat it for analysing what is going on !!!) I think the cost will be prohibitive - certainly at the moment

Maybe one day - or I might just build a datalogger with an Arduino !!! Hmmmmm