Simple Solar For A Simpleton

[Dsanduril]

In the interest of simplifying (as the title of the thread requests) here is a graph of maximum possible output from a typical 100W, 36-cell solar panel vs. operating voltage. As previously stated, without a controller or with a PWM controller the panel operates at the battery voltage, whatever that may be.

[noelex 77]

This is a good discussion of MPPT technology.
I was pleased Dsanduril mentioned the voltage drop that solar panels experience when operating in the real world where cell temperatures are well above 25C. This is often forgotten when mathematically determining the gain from MPPT.

Factors that have not been mentioned are the inefficiencies MPPT has in converting the voltages and the inherent self consumption of the MPPT tracking circuitry.

It has been suggested in this thread the gain for MPPT on a boat is in the range of 10-20% on average. I think this is a reasonable figure although personally I think the average gain is a bit lower at 5-15%.

It is also important to note this is only with the very best MPPT controlers. Cheap controlers with poor tracking are much worse and they can loose out to simpler non MPPT controler.

Talk of a 50% gain is misleading. This sort of advantage will not occur unless the panel is incorrect for the instalation (Say using a 60 cell panel on a 12v system without MPPT for example.)

[goboatingnow]

The Vmp for a typical 36 panel PV is around 17v. your battery is heavily discharged. As was previously shown in the math, your looses can approach 45% without an mppt. Sure it's only for a while as the battery terminal voltage rises , but it is at a time when you need the output most.

Dave

[Jimbo485]

Originally Posted by goboatingnow

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

Dave

Agreed! After sitting down with an electronics engineer and going through it all, that is exactly what we did.

Now, how about some comparative costs? Once again, assume the same situation as described in your quote above. And then cost out less panels with an MPPT.

[Ex-Calif]

Very useful discussion. So far it appears the discussion is centered on parallel wired panels.

Clearly a boat is not the ideal place to mount panels as there are often things that will partially shade panels.

What are the trade offs using MPPT advantages/disadvantage with series wired panels?

[Dsanduril]

Originally Posted by noelex 77

This is a good discussion of MPPT technology.
I was pleased Dsanduril mentioned the voltage drop that solar panels experience when operating in the real world where cell temperatures are well above 25C. This is often forgotten when mathematically determining the gain from MPPT.
....
Talk of a 50% gain is misleading. This sort of advantage will not occur unless the panel is incorrect for the instalation (Say using a 60 cell panel on a 12v system without MPPT for example.)

One other item that hasn't been mentioned is irradiance values. All panels are rated at 25C (as previously mentioned) and 1,000 W/m2 irradiance. Except under some rare conditions that is pretty much the maximum sun brightness anywhere on Earth.

On our boats we will almost never achieve 1,000 W/m2 on the panels, because to achieve it the panel has to be perpendicular to the sun's rays. If the panel angle is off by 45 degrees (which would be pretty typical for a horizontal panel throughout most of the day) we only receive 71% of the actual irradiance. If you look at the I-V curve I posted in #254 you will see that VMpp also drops as irradiance gets lower, which reduces the voltage differential that drives the power improvement from MPPT. So, not only do you have the temperature effect dropping voltage, you also have effective solar intensity dropping it as well.

I use MPPT. I want to get the maximum from my solar, because I hate to listen to running engines (of any kind). I agree that in theory there are times when MPPT could gain you 50% in output. I also agree with Noelex that a real world, empirical, typical overall gain of 10% is about what you should expect on a boat installation. YMMV.

----------------

On a somewhat separate subject, we should bring up panel ratings in general. They are an advertising gimmick in the solar industry that tells you panel output under ideal conditions. Since they are so far out of whack with reality the industry came up with an additional NOCT (normal operating cell temperature) rating. This is the rating of the panel at ~45C and at 800 W/m2, both of which are typical conditions in a good installation, which is something we don't have.

I've included a datasheet graphic below for a 255W panel (I just happened to have this datasheet handy) that includes both standard and NOCT ratings. You can see that the panel is rated 255W under normal conditions, but just 184W under NOCT conditions. You can also see that Mpp voltage drops from 31.4 to 28.3V under NOCT conditions. If this panel was charging a 24-volt system under NOCT (which means fairly typical) conditions the MPPT gain would be zero, 28.3V is pretty close to the nominal charging voltage of a 24V system. You will see similar conditions if you look at the NOCT ratings for 36-cell panels used to charge 12V systems (sorry, I didn't have a handy datasheet that covered that).

If you want to be even close to realistic in estimating panel output on the boat you need to be looking at NOCT ratings. Some manufacturer's include them in datasheets, some don't. Almost all publish the NOCT temperature, using that and the published coefficients you can calculate it, or just assume something in the range of 75% of the panel's rating for the NOCT value.

[goboatingnow]

I use MPPT. I want to get the maximum from my solar, because I hate to listen to running engines (of any kind). I agree that in theory there are times when MPPT could gain you 50% in output. I also agree with Noelex that a real world, empirical, typical overall gain of 10% is about what you should expect on a boat installation. YMMV.

This isn't quite right both the average and maximum gains from mppt are in theory. Since it is a continuous process you would need to AH or WH count over conditions ranging from very discharged battery to poor Vmp matching to actually derive an improvement figure. It would primarily depend on your specific panels, your specific ambient conditions, your battery etc.

Dave

[Dsanduril]

both the average and maximum gains from mppt are in theory

Actually, I very specifically said that the 10% typical gain was empirical. Theory would be higher. The 10% average is my opinion and nothing more, value it however you would like.

Yes, it depends a great deal on battery state, charging voltages, panel installation, weather conditions, and a host of other factors. The 10% is a value I have observed over a number of installations. If someone asked me for a single number to use to estimate in charging a 12V boat installation that is what I would suggest. Actually measuring that is difficult. You either have to have two identical systems in parallel, or you have to switch back and forth between MPPT and direct throughout the day and measure input to the battery.

[VirtualVagabond]

Originally Posted by Ex-Calif

Very useful discussion. So far it appears the discussion is centered on parallel wired panels.

Clearly a boat is not the ideal place to mount panels as there are often things that will partially shade panels.

What are the trade offs using MPPT advantages/disadvantage with series wired panels?

I'm glad you brought up the subject of parallel or series connecting.
I've been so involved in working our the best way to attach the panels I havent given this issue any thought at all.

The big 320 watt panel goes on the davits and has it's own MPPT controller because of it's high Vmp voltage. (controller arrived yesterday and haven't even opened it yet). This is in effect a stand alone system.

The two 140 watt panels are going on the dog house and being wired directly into the battery bank, with an isolation switch.
Should they be wired to each other, and then to the battery bank?

What are the arguments for these 2 being in parallel or series?
At anchor I can pull the boom to one side and try to reduce shading on the panels, but under sail I assume one will frequently have more shade than the other.

If I run the wires directly into the batteries from each panel, what have I effectively got?

I've read the postings of the last few days and the 'Simpleton' tag is still firmly in place. I guess I know why my subjects were english and history

[noelex 77]

Originally Posted by Ex-Calif

What are the trade offs using MPPT advantages/disadvantage with series wired panels?

There has been a lot of discussion about the benefits of series, or parallel connection.
This is a very basic question, but there is no clear consensus as to which is better.

Originally Posted by VirtualVagabond

The two 140 watt panels are going on the dog house and being wired directly into the battery bank, with an isolation switch.
Should they be wired to each other, and then to the battery bank?

What are the arguments for these 2 being in parallel or series?
At anchor I can pull the boom to one side and try to reduce shading on the panels, but under sail I assume one will frequently have more shade than the other.

However series connection of (nominally) 12v panels is only possible with a MPPT controler. So you have to wire the 140w panels in parallel. They are normally joined at the panel, with just a single + and - wire leading to the controller (or direct to the batteries), but you can join them up anywhere if this is more convenient.
Make sure the wire size is appropriate.

[Ex-Calif]

I have read most of the solar posts - this is an area of my continuing education.

Series wiring seems to be a benefit when some of the cells on a panel will be shaded. Once some of the cells from the "nominal" 12V panel are shaded, voltage is less than the battery and charging stops.

The conundrum for Virtual Vagabond is in this case he would need a second expensive controller.

[atoll]

vic might want to read up here,some very good info on connecting up multiple systems,ie unregulated,semi regulated,and mppt regulated systems with solar and windgen
Coleman Air

[Nicholson58]

Originally Posted by tartansail

Many thanks to both of you for the responses. I agree with the need to take everything on the Internet with a grain (or pound) of salt, but getting different perspectives helps me move from simplistic confusion to a path forward. I appreciate the expertise reflected in both your answers. It was scary to see that 6.24 X 10^18. Something about Faraday or Coulombs? Physics was a looonnnng time ago, but I understand what you were saying. Despite that quote, I think Bob's main point about absorption voltage was that it was good to push in amps to the battery's tolerance. Stopping short was giving away useful capacity and, potentially, impacting battery life. The advice to follow the manufacturer's recommendation makes sense. If/when my batteries are toast, I'm planning to replace them with Trojans so the ability to adjust the absorption seems worthwhile. Since we are not liveaboards, the boat often has several days with good sunshine and trivial draw so it seems like we'll have a shot at getting close to 100% SOC. I like the idea of being able to start full when the fridge goes on.

Given that MPPT will have some benefit, it seems worthwhile when we're spending time on the boat as long as I can find one that's not too pricey. It sounds like the Rogue would be a good one to consider given its price and adjustable absorption voltage. It seems like it may be penny-wise pound-foolish to spend the $$ on the panels and then not be able to fully use the output.

It sounds like there is no disagreement that the controller should have temperature compensation, especially at higher voltages.

Consider that Handy Bob is an Engineer and that his advice is based on both technology and real life experience. His world is making RV systems work which is a bit different than boats. Its not as rigorous in the RV world and they can control the shading on panels. There is good advice there though.

In the RV world they frequently do not need MPPT because they do not buy high voltage panels and also, they buy by cost point, not on amps per square foot at any price. The typical RV user can simply add more panels and the space is notan issue. For boaters, the total amps/square foot is more important than the cost per amp. If you follow our needs, you will have high voltage panels which require MPPT to avoid frying batteries and which will help you net the best harvest/foot squared.

I swapped a few E-Mails with Handy Bob and it was clear that the RV bunch do not generally use MPPT. The physics of battery charging is right straight up Handy Bob's experience. I use AGM but if I was using traditional lead acid wet cells, I would follow Bob's advice. His point is that the makers of most charge eqipment do not pay any attention to the charge voltage numbers recommended by the battery makers. The charger makers are all too low so that the batteries all start out after a charge cycle significantly below full charge and partly sulphonated. I have made sure that my charge settings correspond with the battery maker's numbers, not the pre-programmed auto charge that came with the charger. ----- And as Handy Bob has noted, the charger voltage musty be read at the batteries and the cables need to be oversized. If you read the battery voltage a few feet away the droop is sufficient to screw up your charger's set points.

Its fundamental, big wires save money.

[Jimbo485]

Originally Posted by VirtualVagabond

I'm glad you brought up the subject of parallel or series connecting.
I've been so involved in working our the best way to attach the panels I havent given this issue any thought at all.

The two 140 watt panels are going on the dog house and being wired directly into the battery bank, with an isolation switch.
Should they be wired to each other, and then to the battery bank?

Parallel, Vic. With an individual breaker (not a switch) for each. It makes it simpler to use medium diam cable from each panel to an intersection point with the other panel. From here to the batteries you need heavy cable. We use 4 AWG, but you have to size yours for your situation.

I can't do a diagram, but the positive from each panel needs to have the breaker. The two positive cables join to each other after their respective breakers. We use a battery post in the cockpit and the breakers are immediately before this point. You might install it on the ceiling of your doghouse? The two negatives go straight to the negative post. From these two posts, you run the heavy cable directly to your batteries.

Most importantly, you need to have some way of measuring the voltage FROM THE COCKPIT, so you can turn off one or both panels when necessary. If your nav instruments cannot provide this info in the cockpit, you will need to install a voltmeter across the two posts. Or simply buy a cheap multimeter and "store" it in between these two posts where you can see the voltage.

If you leave the boat, turn the panels off and let the MPPT and your other panels handle everything. You will get in the habit, when in the cockpit, of naturally looking around at the wind, waves, boats dragging down on you, sail trim etc and part of that will be a glance at the voltage during the day. It is not hard, but needs to be part of your routine so that you don't overcharge your batteries.

[Dsanduril]

Originally Posted by goboatingnow

PV panels are current rather then voltage sources. Battery's are voltage sources

Dave

As pointed out above, solar panels are really devices to provide current. When you connect them in series the string will provide the current available from the lowest current panel. For that reason, when wiring in series it is critical that the panels be matched as to current. If you connect a 2A panel in series with a 6A panel the whole string will operate at 2A.

If you then refer to the I-V curves you see that current is a function of amount of light hitting the panels. If you have matched panels in series, but they are in different light conditions then the whole string will operate at the current provided by the least lit panel in the string.

Bottom line, panels in series need to be matched in terms of current output under all conditions, otherwise you run the risk of reducing the output to that of the lowest performing panel. If your installation can make sure that all panels are both matched in terms of specifications and matched in terms of operating conditions then series can make sense. If not then parallel is probably better.

Series wiring increases the voltage, so reduces the required wire size. Unless you are going into a higher voltage battery it almost of necessity requires that you install a controller that can convert the voltage, and that puts you squarely in the MPPT market.