MPPT V's PWM regulators

Things 12V and 24V
sea_bee
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Re: MPPT V's PWM regulators

Post by sea_bee » Fri Aug 16, 2013 1:19 pm

I have 12v solar at home and have excess power for 9 months of the year.

Seeing all the excess power being wasted, I have just built a prototype Arduino device to operate relays based on battery voltage.

The Arduino is a programmable IC that is really simple to build. There is a whole industry out there catering to just this platform.

Anyway, I have the Arduino now displaying the battery voltage on a LCD display, and opening and closing a relay as the voltage goes up and down. I intend to connect that relay to an invertor to run a load when there is excess power. I've been planning on connecting to a pool pump. I'm not sure I would want to heat water off an invertor, but if you can find a hot water tank that has a 12v element, that might be the way to go.

Off topic, but I plan on using solar hot water.

Chris

puremajek
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Re: MPPT V's PWM regulators

Post by puremajek » Fri Aug 16, 2013 2:58 pm

we are pretty much in full agreement then, just wasnt sure if a shaded panel did enough damage to overall output on a marginal day where you are only just exceeding 12v to drag it all below 12v in a parallel wired system
The information I have shown are just numbers. While numbers explain the theory, it is not what actually happens in real life. We have found that shading is more of an issue on a cat than a mono.

For the same length, a mono-sailboat will sit into the tide in all but the strongest winds. Due to the windage and the light weight of cat-sailboat (for the same length), the challenges are different and there is not much written about it.

We have found that even in light winds, the cat swings constantly up to ninty degrees to the current, sometimes doing a full circle.

Together with the swell (or any movement), partial shading from the mast, rigging, sheets and boom does affect the cells/panels, constantly changing the voltage (in very small amounts) as the cat swings. While not much voltage is lost, it will stymie those using pure theory to calculate exact values for their solar panel charging.

Theoretically, we reduced panel stated voltage by 2-Volts and hours of daylight to 5-hours and this seems to more accurately account for most varriables including temperature, cloud effect, shading, swing and system efficiency.
Great Whitsundays report btw, only one problem with it....wasnt long enough, as soon as I had read it I wished there was more. Great reading, thanks
Thank you for the Whitsunday Blog feedback. A far more detailed log of provisioning, sail plots and anchorages is in draft form for a new ‘Cruising Volume’. On the drawing board are Lady Musgrave and Cato Islands that I want to include. But I diverge from this important topic.
James
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http://www.diycatamaran.com
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kjay
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Location: brisbane

Re: MPPT V's PWM regulators

Post by kjay » Fri Aug 16, 2013 3:43 pm

Yes there is a 12 volt heating element available, $140us I think, I found the link for it on the Cruisers forum website however the saved link was on my other laptop that died. If you do a search you should find it. I thought as Groper said it would be ideal to channel any excess solar power into you hot water storage tank thus saving gas when showers are wanted.

John

mahnamahna
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Re: MPPT V's PWM regulators

Post by mahnamahna » Sat Aug 17, 2013 12:51 am

In Europe and Asia single fawcett electric instant water heaters are very popular.

http://www.ebay.com.au/itm/271013462443 ... 1423.l2649

I am leaning toward fitting one in each hull or even 3 (they certainly are cheap enough)one in each bathroom and one in the galley. My reasoning being, there is really only 2 places where 2 fawcets could ever be on simultaneously, either the kitchen sink, or in one of the bathrooms, so if each area had their own that would work from a water flow point of view, would not like to have more than one eating into my battery bank at the same time, but again, how often would someone need hot water for a sustained period in the galley whilst someone else was having a shower?

In each of our bathroom there are 2 outlets, shower and sink but never would both run at the same time so in this case it would be ok to connect both outlets to the one heater and rather than trying to send water 12 meters from the aft of one hull where one bathroom is to the bow of the other hull where the other bathroom is, I can more easily send power and these units are quite small (so too is their heating output but I also figure that life in the tropics will mean that we wont need that much hot water coming out of a shower rose). So it would not be over the top to imagine having 3 of these fitted. Just have to make sure that unless we are in a marina on shore power (and water) that only one is ever used at any point in time.

How they work is, they rely on low flow, the more you turn the tap on, the less time the unit has to heat the water so it comes out colder. But they are capable of raising the water temp to about 50 degrees, but I more realistically expect about 35-40 degrees at reasonable flow. They burn through power, but again I figure the water will run out before the power, so showers will need to be short anyway and no tank to keep heated so they ought to be more economical than keeping 20 litres warm whether used or not. Actually I am told that in reality people with elec tank water heaters turn them on half hour before showers and then turn them off again. Not sure how I would adjust to having to wait half hour for hot water? but I think these low flow units might work for me. Anyone ever tried them?

There are heavier use ones, but at 6kw or 40amps, but I think that might be too much power, not even sure shore power is at 40amps is it?

http://www.ebay.com.au/itm/Electric-Hot ... 3462443%26

Thoughs?

groper
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Re: MPPT V's PWM regulators

Post by groper » Sat Aug 17, 2013 8:21 am

This is off topic... but the problem i see is that each heater is 3kw. So if you turn 2 of them on simultaneously then your inverter needs to supply 6kw - will your inverter handle this?

I also dont like that the temperature is controlled via the water flow rate. If the unit works really well, then you will need the water blasting through to keep it from being too hot, youll waste heaps more water than you need to. On the other hand if the flow rate needs to be low for the correct temp, then you might not get a very good shower stream, although this is less likely and at least your water useage will be minimal.

If people like to have 10min showers, then power consumption will be quite high.
3000w for 10mins = 500w/h of energy but youll loose 10% from the inverter losses so thats 550w/h from your batteries.
550w/h @ 12v = 45amp.hours gone in a 10 min shower.
2 people aboard and both have showers = 90amp.hours gone.
4 people aboard = you can see where this is going... it will effect your energy generating and storage budget significantly.

Im thinking of one of these personally - http://www.countryoutdoorsupplies.com/
Not bad for $300. As far as i can tell, it meets the relevant standards. Same goes for cooking with gas.

Back to the solar panels and regulation;

It seems the problem arises when the max power point of the panels falls low to near 13v and then the MPPT charger wont deliver the charge. This happens when the panels get hot - which is a very real situation. Thus if multiple panels are used in series and the voltage can be raised well above this, the MPPT charger will still deliver an efficient charge current when the panels Max power point drops to low voltages.

With many panels, say 4 or more, a combination of series and parallel can be used so that the voltage is raised above that of 1 panel, but not too high as to cause a hazardous voltage which many dont feel safe with. A single MPPT can still be used like this.

But it still seems best to divide large arrays into smaller groups with separate MPPT`s for the case of partial shading. The shading will only reduce the output of the shaded group, rather than the entire array, and it also depends on the panels your using and how they are setup with bypass diodes. Seems the more expensive panels have multiple diodes which can bypass a small group of cells, whilst cheaper panels generally only have 1 bypass diode which would bypass the entire panel. On solar forums, people that have done alot of testing show that shading really does drastically reduce your output, so its worth considering carefully on a sailboat which could be / will be shading the panels via the rig and rigging often - and the effects on your energy budget.

Careful shopping can still find MPPT`s for around $100. Their algorithm or method of these cheaper units may not be as efficient as the more expensive ones - wiki lists 4 main methods for determining the MPP used by these controllers along with their pros and cons. But the difference would be small, and splitting the array up into smaller groups would no doubt pay back far more energy with regard to partial shading.

It all seems a pretty in depth topic really...

puremajek
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Re: MPPT V's PWM regulators

Post by puremajek » Sat Aug 17, 2013 10:07 am

people that have done alot of testing show that shading really does drastically reduce your output
Sorry Groper, if the inference here is 'total shading of a panel', yes - it will definitely affect any power production (series or parallel). If the inference is 'total shading of a cell or a few cells', then no, I do not agree, given our first hand experience over the last four years. The comment does not talk of 'partial shading' which is the most common issue on a yacht.

We often have partial shading on our panels from components I've mentioned in previous posts above and this does not drastically reduce our power production by any means.

Additionally, all our 4 panels are now all in parallel at 12vDC with one MPPT charger???? We originally had them in two pairs at 24vDC.
James
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http://www.diycatamaran.com
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groper
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Re: MPPT V's PWM regulators

Post by groper » Sat Aug 17, 2013 11:13 am

Yes, i meant partial or full shading of the array, not just a few cells etc... I figured on a sailboat under sail, there would be alot of time spent with the entire array shaded by the sails, boom / lazy jacks etc.

Why did you change from 2x2 @ 24v down to 4X1 @ 12v? What changes did you notice?

puremajek
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Re: MPPT V's PWM regulators

Post by puremajek » Sat Aug 17, 2013 3:34 pm

Please excuse the crude drawing. It is supposed to be:
> The sun rays (blue arrows),
> Solar panels (1,2,3&4- dark lines),
> Over the aft section.
Image

We tried pairing panels (1 & 3) with (2 & 4), (1 & 2) and (3 & 4), even (1 & 4) and (2 & 3)
and found that due the:
> Panels angles (or 'panel-arc') on the aft surface, and
> Swell...
Voltage and amps were down for a few hours each morning and afternoon.

1/3 of the time, by noon to mid-afternoon, cloud-effect was also in play. So not only did we lose out from panel-arc and swell, but cloud-effect too.

In parallel, we think we get the maximum performance from each panel, including the yacht swinging around while moored.

Just one other important point that no one has raised yet. Many of the larger MPPT (and some PWM) controllers draw power to operate (up to 10W with one of the larger popular brands). Whats is not told is that power draw comes from your solar panels. That means that the first bit of power you make goes to running the Solar Charger. You can normally locate this in the finer print.
James
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http://www.diycatamaran.com
______________________________________________

groper
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Re: MPPT V's PWM regulators

Post by groper » Sat Aug 17, 2013 7:50 pm

Ah i see...

You are correct in running strings (or modules/panels) which have different angles of incidence, in parallel. This is no different to matching cells within a panel, as a strings output is determined by the cell with the minimum output. Same goes for a string of cells, and one of them gets shaded - the entire string is reduced output. This is the reason for the bypass diodes, to prevent them from reducing the entire panels output, restricting it to the effected string by dropping it out to save it ruining the output of the unshaded cells.

I have 1 as yet unasnwered question, which relates to the MPPT and all of these setups. The voltage across everything upstream, is determined by the MPPT - thats how they work, by adjusting the voltage to find the MPP. It tests different voltages and settles on the one which produces the highest wattage. How can it differentiate between 1 panel to the next when they are series or parrallel? Its an irrelevant point when all panels have the same angle and no shading, as all panels should be very close in optimal MPP voltage.

If a string is dropped out due to partial shading of a panel, and the whole array is in series, then it simply adjusts to the new, lower sum of voltage and finds the MPP of the group minus the dropped string. Simple enough. But when there are multiple panels in parralel, the MPPT sets a voltage across all of them... how can it know which panels have a dropped string and optimally require a lower voltage? It cant because the voltage is equal to all panels in parrallel.

Thus i cant figure how running panels in parallel can be achieved optimally, without a seperate MPPT channel for each group in parallel. So if you have 2 parallel groups (which may be grouped by multiple panels in series) then you need 2 MPPT`s or a MPPT with 2 channels - which do exist.

This whole reasoning is probably why in many circumstances, its probably just as effective to use PWM rather than MPPT, because if the MPPT cant be implemented properly, then youd most likely end up with worse performance than the PWM.

Most of these problems do no exist in house/ fixed installations, as all the panels generally have the same angle of incidence and little shading considerations, so either method works just fine. We on boats, have a much more complicated set of considerations....

Also, with regard to parallel wiring, i was under the impression that if circumstances arise, whereby the MPP voltage can potentially drop to below battery voltage after getting very hot in midday summer, the MPPT will stop charging as there is not enough voltage to allow current to flow into the battery? This will never happen with series wiring because the voltage is summed with multiple panels and thus the MPP voltage is always going to be much higher than the battery voltage and so the current will still flow?

groper
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Re: MPPT V's PWM regulators

Post by groper » Sun Aug 18, 2013 8:05 am

Upon waking this morning it instantly dawned on me... The parallel strings that are bypassed are still at the mpp voltage set by the controller, it's just that no current flows from that bypassed string.

So either configuration works pretty much the same with regard to partial shading.

So it seems the main consideration when choosing which way to wire things up is what type of panels your using... If using low nominal voltage panels, you could often find no benefit to using an mppt when the panels heat up and the mpp voltage becomes close to the battery voltage. You'd only get a small gain early morning and late afternoon.

Also worth noting, is that you'd almost certainly get more total energy if you spent the money on an extra panel and just used a pwm controller as panels are so cheap now. The only caveat is that space is often at a premium on our boats so simply adding more panels isn't an option...

Another thing I've noticed, is that most mppt controllers are limited to 150v DC max open circuit voltage. So if series wiring and the panels are 48v Voc then your limited to series wiring no more than groups of 3 etc. And if you look at the efficiency curves from the manufacturer, these higher voltages results in greater step down losses. Morning star publishes this in the mppt booster manual. Thus I think it better to limit the voltage to a moderate level whereby the wire gauge is kept reasonable and always slightly above the mpp of the array, and the step down losses are not too great.

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