Simple Solar For A Simpleton

[tartansail]

Originally Posted by foggysail

OK, appears that you have some consideration to cost so I want to address Trojans.

This topic was discussed earlier in a different thread, the essence of the discussion was not about Trojan's quality but about cost. Further, we learned that the golf cart 105's are available in the Fort Lauderdale area in the $80 range. NOT BAD! Here in my Massachusetts area the least costly source I found wanted $220 a pop!! Further, Sams Club where I finally did purchase my 4 new golf cart batteries, Duracells for only $88 each which have a 230AH rating.

My only point is that there are many good lead acid batteries out there. The technology is over 100 years old. There are few surprises facing the manufacturers. I suggest that you keep an open opinion regarding new batteries until the time you need to say goodbye to your bucks.

Foggy

Thanks, Foggy. Good caution. My first effort will be to equalize the batteries when I get to shore power. If that doesn't work, I'll do some comparison shopping. It's sometimes hard to distinguish real vs. marketing differences. Yes, lead-acid is old technology, but there do seem to be significant differences between the way it is set up in deep cycle applications. I don't mind paying for meaningful improvement, but it can be hard to tell when the differences are real.

[DeepFrz]

David, check some of Mainsail's posts about batteries.

[GeoPowers]

Originally Posted by Jimbo485

Ok, thanks for the comments. I know from my own experience that each panel required 2 pieces of aluminium angle, 16 screws, Sikaflex and about 2 metres of cable. Nowhere near 4 x the cost of the panel. More like $50 more for each panel.

I still remain unconvinced and maintain that an MPPT is a consumer rip-off UNLESS you have run out of available space.

Using a generator is totally uneconomical while cruising in tropical and sub tropical regions UNLESS you run out of space for more panels.

So I return to the original question. Assume you have sufficient space. Why would you be silly enough to spend more on an MPPT than more panels???

FWIW, we have 970 W of solar panels, no generator, no alternator, 880 Ah of batteries. We live onboard permanently, run 3 computers, radar, autopilot, watermaker, fridge, etc etc. No problem. If we ever felt the need for more power, there is still no way we would install an MPPT. We would simply add another panel or two.

Nevertheless, your comments are appreciated, even though I do not agree with the logic!

Another point to consider is what constitutes suitable space to mount the panels. You, having a cat, have a lot more available space than a monohull, but even so I see monohull folks on here saying they have room for up to 900W of solar on a 40ish ft boat. While there may be room to physically mount such an array, especially if built into the bimini, how much will shading from the boom, mainsail, radar, standing rigging, etc. effect it, especially under sail? In your example if adding a 100W panel only gets you 40W for most of the day, then it may be better to go with the MPPT.

There seems to be two schools of thought-

A. Go for a fewer number of high-cost, high-efficiency panels, usually with a high Vmp, that are placed as to avoid all possible shading, coupled with an appropriately-sized MPPT that can handle higher Vmp levels and boost output to the max possible.

B. Go for a larger number of affordable, lower-efficiency panels placed in a quantity that compensates for any shading effects that may take place, coupled with an affordable regulator matched to the larger array size.

It seems like people have found success with their solar needs from both schools of thought!

In practice I agree with Ex-Calif that folks should decide to go MPPT or not in the planning stages. It also makes sense to me to buy a regulator you can "grow" into if necessary in case you find out you do need to add more panels in the future. I have 450W of solar on my arch, but can easily add at least one more 225W panel if I needed since I oversized all my wiring and my regulator. But then shading would start to become more of an issue...

Frank

[tartansail]

Originally Posted by DeepFrz

David, check some of Mainsail's posts about batteries.

Thanks, DeepFrz. I couldn't find anything here on CF, but I did find a great post from him on another site. Great validation for Sam's Club as a source.

[gchabs]

Thanks to all for a fabulous thread. In our case we have (2) 80W Shell solar panels and we are about to add more. I figure I can put on two more panels on the side of the boat. I don't want to pay the cost of an MPPT controller - too pricey, and too many other things to buy - so can anyone recommend a good 3-stage PWM controller that will do the trick. In 2004 -2005 we went cruising with a 350 amp hour battery bank and the two Shell panels. Ended up re-charging (via engine or Honda 2000) every other day. Two more panels say in the 120 W range each, should help. Thanks for the info. gchabs.

[goboatingnow]

I think people are mis understanding the difference between PWM and MPPT technologies. If you panel operating voltage is significantly away from its Vmp then you are not getting the design power from the panels. A PWM controller is just a variable duty cycle " relay" so for example if your batteries are at 11.8v and your panel Vmp is 14 v then you loose a reasonable amount of power. The I/V curve is not linear in a PV panel.

Hence depending on the Vmp match under the particular conditions you can get much lower actual outputs from a PWM system then you might otherwise expect.

A good MPPT controllers removes this by ensuring the panel operating point is maximised for the ambient conditions.

So of you panels have a significant Vmp mismatch or are subject to widely varying ambient , then mppt always is better. Not so much as better as in the fact that PWM can result in poor performance

Dave

[Jimbo485]

Originally Posted by goboatingnow

Hence depending on the Vmp match under the particular conditions you can get much lower actual outputs from a PWM system then you might otherwise expect.

A good MPPT controllers removes this by ensuring the panel operating point is maximised for the ambient conditions.

So of you panels have a significant Vmp mismatch or are subject to widely varying ambient , then mppt always is better. Not so much as better as in the fact that PWM can result in poor performance

Dave

Good points, Dave, but how about quantifying rather than using words such as better and poor? And then quantifying the cost and then making a conclusion?

As an engineer in a previous life, I still quantify almost everything and do not accept qualitative statements to arrive at a conclusion.

Assume unlimited space for panels which are fairly close to battery voltage. Say 17 - 20 V.

[goboatingnow]

Originally Posted by Jimbo485

Good points, Dave, but how about quantifying rather than using words such as better and poor? And then quantifying the cost and then making a conclusion?

As an engineer in a previous life, I still quantify almost everything and do not accept qualitative statements to arrive at a conclusion.

Assume unlimited space for panels which are fairly close to battery voltage. Say 17 - 20 V.

You'd want an examination of the particular panels under the actual ambient conditions. Quantifying PV output is notoriously difficult unless done with actual specific ambient conditions and particular panels.

PWM is a poor strategy anyway it only works because PVs are current sources. I'm an electronics engineer so like you I like numbers.

I have seen cases where nearly 50% of the power of the panel was being lost due to poor Vmp matching. A lot of the issues with poor performance from panels is often down to this, especially under varying ambient.

With unlimited space and close Vmp matching and constant ambient conditions, sure a switch will do , you don't even need a controller.

Dave

[Pete7]

Originally Posted by goboatingnow

I think people are mis understanding the difference between PWM and MPPT technologies. If you panel operating voltage is significantly away from its Vmp then you are not getting the design power from the panels. A PWM controller is just a variable duty cycle " relay" so for example if your batteries are at 11.8v and your panel Vmp is 14 v then you loose a reasonable amount of power. The I/V curve is not linear in a PV panel. Dave

Dave, I am trying to get my head around this, so is it not the case that if the battery bank is 11.8v and the panel produces 14v, then doesn't 14v go into the batteries which is what we want?

Yours, worried of Gosport with a couple of PWMs.

[cwyckham]

Hmmm... If I understand this right, a PWM does no voltage changing at all, it's just a smart on-off switch that is always on during bulk, then will modulate the current by pulsing on and off during absorption and float. If the battery voltage is higher than specified, it will increase the "off" time and vice versa. Is that right?

Example: PWM is in absorption mode and is set for 14.4V. It is pulsing 50% on and 50% off. It detects a voltage of 14.45, so it changes to pulsing 45% on and 65% off. The voltage should then decrease.

So if a particular panel in the particular conditions of the day has a maximum power point of 17V and 20 Amps, but is hooked up to a PWM controller in bulk phase at 11.8V, then it will be putting in roughly 20 Amps (the power curve is pretty flat, generally).

With an MPPT controller, however, it would be putting in 20*17/11.8= 29A, or a 45% improvement. Most batteries don't spend too much time down at 11.8, so it's usually a lot less dramatic than that, and at 14.4V it would be an 18% improvement using the same logic and math.

Is that right?

[Dsanduril]

Dave, I am trying to get my head around this, so is it not the case that if the battery bank is 11.8v and the panel produces 14v, then doesn't 14v go into the batteries which is what we want?

Panels don't work at a specific, single voltage, they work along what is called an I-V curve (I=current, V=Voltage). If you have a nominal 17V panels connected to a nominal 12V battery it will actually operate at whatever voltage the battery is sitting at. So, if the battery is at 11.8, the panel will be at 11.8 (not for long, as soon as it starts to charge the battery voltage will rise). As the battery state of charge increases so will the battery voltage, and thus so will the voltage at which the panel is operating. It may eventually get to 14V, and if the battery gets 100% it may actually get as high as the open-circuit voltage of the panel, whatever that is. You can use a directly connected panel to get your batteries charged, it just isn't the absolutely most efficient with the possible available energy, and you have to make sure you don't overcharge the battery. Bottom line, the battery determines the voltage at which the entire system is operating.

With an MPPT controller, however, it would be putting in 20*17/11.8= 29A, or a 45% improvement. Most batteries don't spend too much time down at 11.8, so it's usually a lot less dramatic than that, and at 14.4V it would be an 18% improvement using the same logic and math.

Nail on the head The actual real-world improvement seems to be in the range of 10-20% (on boats, with their less than ideal installations), which is in line with your 18%.

One other factor besides the battery spending little time at that voltage is that the panel will spend little time at that voltage as well. It will almost always be lower, as panel output (both voltage and current) is dependent on both panel temperature and solar irradiance. Changes in these values can greatly change the output point. Voltage tends to decrease ~0.3 to 0.4%/degree C, and most panels operate at 20-25 degrees C above ambient, but are rated at 25C. In the tropics it would not be unusual for a panel to be operating at 50-55C, which means a 17V nominal panel would actually be at about 15.5V. The MPPT keeps constant track of these changes and tries to make the optimum adjustments.

[goboatingnow]

Originally Posted by cwyckham

Hmmm... If I understand this right, a PWM does no voltage changing at all, it's just a smart on-off switch that is always on during bulk, then will modulate the current by pulsing on and off during absorption and float. If the battery voltage is higher than specified, it will increase the "off" time and vice versa. Is that right?

Example: PWM is in absorption mode and is set for 14.4V. It is pulsing 50% on and 50% off. It detects a voltage of 14.45, so it changes to pulsing 45% on and 65% off. The voltage should then decrease.

So if a particular panel in the particular conditions of the day has a maximum power point of 17V and 20 Amps, but is hooked up to a PWM controller in bulk phase at 11.8V, then it will be putting in roughly 20 Amps (the power curve is pretty flat, generally).

With an MPPT controller, however, it would be putting in 20*17/11.8= 29A, or a 45% improvement. Most batteries don't spend too much time down at 11.8, so it's usually a lot less dramatic than that, and at 14.4V it would be an 18% improvement using the same logic and math.

Is that right?

Exactly to use watts in the first case with a PWM controller your 340 watt panel has suddenly become a 236 watt panel, whereas with a ( perfect) mppt controller your panels is maintained at the maximum irrespective of battery voltage.

This is because the I/v curve of a PV panel isn't linear. But rather is flat. PV panels are current rather then voltage sources. Battery's are voltage sources

Dave

[goboatingnow]

Originally Posted by Pete7

Dave, I am trying to get my head around this, so is it not the case that if the battery bank is 11.8v and the panel produces 14v, then doesn't 14v go into the batteries which is what we want?

Yours, worried of Gosport with a couple of PWMs.

No the battery in general determines the voltage of the system. The input impedance of a discharged battery is way lower then the output impedance of a PV panel. Hence the battery determines the voltage and pulls the panel to it rather then the other way around.

Dave

[Dsanduril]

In furtherance of Dave's posts, sometimes a picture is worth a 1000 words:



This is the I-V curve and temperature dependence curve of a pretty typical 80W panel.

As we both have said, without a controller in the way (or with a PWM controller) the panel will operate at whatever voltage the battery is at. The current curve is pretty flat, so the total power you will get is:

Battery Voltage * Panel Current

If you look at the curve, on a typical 36-cell panel the maximum power point is just after the current starts to drop off, and is at a voltage of 16 to 17.5V. If you are charging into a battery at 14.4 you will come up a little on the current curve (about 0.3A), but not enough to offset the 1.6 to 3.1V that you are throwing away. The maximum output of this panel at 14.4V is 70W, and at 13.6V it is 67W. So, you end up giving up between 12 and 20% of the available power by not using a controller (MPPT) that matches the panel voltage to its ideal conditions while matching the battery to its ideal voltage.

[goboatingnow]

And even more if you have good sunlight and a heavily discharged battery. Just when you need the panel and you expect high output suddenly you have a panel nearly 50 % smaller then you thought you bought ! ! The average mppt benefit is 12-20% but the maximum benefit could be way more. To answer the mppt doubter, in that case you would have to nearly double your panel acreage to get the same output.

Dave