Solar Choices

[CarinaPDX]

Can you confirm Tootsie's suspicion about the voltages? What is the Vmp (voltage at maximum power) for each panel?

Vmp in the 17-19VDC range benefit very little if at all by MPPT controllers. Last week I went through the computations yet again for a local boat wanting to install solar, and found the benefit to be about 5%, depending on installation details. The point of MPPT controllers is to allow running panels in series to obtain higher voltages, which in turn require smaller cables and less loss. With an 18Vmp the extra power obtained by operating at the slightly higher voltage is small, and somewhat offset by the efficiency of the controller. I encourage you to work the numbers out for yourself.

Greg

[Dsanduril]

One of the problems with Vmp is that by industry consensus panels are rated at a panel temperature of 25C. Panels normally operate at ~20C above ambient temperature, so normal operating cell temperature (NOCT) is around 45C for anyone using them in the tropics. For the two Kycocera panels mentioned, the decrease in Vmp is 0.52%/C.

So, with a nominal 16.6V panel operating in 25C ambient conditions, the actual Vmp is:

16.6 - 20*0.0052*16.6 = 14.9V

at typical tropical operating conditions. That significantly cuts into any increase you could see from a DC-DC conversion. For instance, if the DC-DC conversion is 95% efficient then you drop to 14.1V output, which is not sufficient to completely charge LA batteries (most good MPPT controllers would switch to PWM at this point, but this depends on the software and how well it is written). For higher voltage panels you need the MPPT DC-DC conversion to effectively utilize the available power from the panel, but with 32-cell and 36-cell panels you may not see any increase in warmer climates.

[GILow]

I was planning to wire the 215s in series if I went down that path, on the logic that once I was forced to go to MPPT, I might as well reduce the current in the system for the benefits that implies, particularly as I am moving the battery bank to midships, so the panels will be 20 feet away from the batteries horizontally, and a good 10 feet or more vertically. So there's at least a 30 foot cable run, more like 40 by the time all the corners are factored in.

Of course this works against the idea of multiple MPPT controllers, so that's one negative against the postive of redundancy.

Then there is all the comprehensive electrical considerations documented above, which I am still absorbing...

Will get back to you when I reach float.... still in bulk absorbtion mode.

Matt

[GILow]

Oh, I should probably add to all of this, that our engine needs 24 volts, and one thing I would like to be able to do is point the MPPT controller at the engine bank in an emergency to recharge it from the solar. This is a secondary consideration, but on the wish list.

Matt

[TacomaSailor]

Could someone please explain to me what I am missing in the new Solar Panel / Controller world?

I put 4x125 watt Kyoceras up on our radar arch in June 2000 and hooked them up to a Trace C-40 controller. I used 2/0 battery cable to attach them in parallel to a 625 amp hour battery bank.

That system has worked perfectly for 14 years. In the tropics I could run the watermaker, run refrigerator, and trickle charge the batteries (about 25 amps at 14 V).

That install seemed stone-simple with no complications. There are seemingly endless complications being discussed here about what I thought was the simplest electrical system on our boat.

What in the world am I missing with all the new MPPT / series-vs-parallel, stepping down voltages and other very confusing things.

[GILow]

I think it's a bit like anchoring. I used to set the anchor without a care in the world then I started reading more on CF and became surprised I am still alive.

Sounds Ike you have a good system with plenty of capacity and wired in well. Therefore any small sub optimal setup issues have not mattered.

Just my guess.

Starting with a clean slate I am keen to know all the issues so if I do have to compromise for whatever reason at least I fully understand the impact of those choices

Matt

[CarinaPDX]

Originally Posted by Dsanduril

For instance, if the DC-DC conversion is 95% efficient then you drop to 14.1V output, which is not sufficient to completely charge LA batteries (most good MPPT controllers would switch to PWM at this point, but this depends on the software and how well it is written).

I don't think this is quite right. The 95% efficiency will reduce the power (watts) output by 5%. Given a decent voltage control the voltage could be maintained at the desired level, but at reduced current. It might even be able to raise the voltage as needed, depending on the circuit design. The voltage could be reduced by the diode drops of the transistors in the circuit, which is a typical concern of PWM controllers. It is something that would vary based on the specific design.

The high temp derating is very real. OTOH I got the most out of my panels on a cold, clear day here in Portland, even with the sun being low in the winter. Never saw that much power since then.

In addition to considering the available voltage at realistic temperatures, the voltage drop at maximum current in the wiring must also be factored in. Also, if any diodes are used (to isolate the panels, or the batteries) then that must be used to reduce the available voltage (as well as the voltage drop through the controller). By the time all of these voltage drops are factored in there is precious little voltage difference for the MPPT circuit to work with. Edit: unless you have a MPPT of 20V or more.

Greg

[CarinaPDX]

TacomaSailor- You're not really missing anything. If you were to be starting today to install a solar system like yours you might use the same panels, with perhaps a modest output increase due to technology improvements. You might well choose to use the large battery cable and simple controller again. Or, you could wire the panels in series for 48V nominal or in series/parallel for 24V and then reduce the wire size to reflect the lower current, using an MPPT controller to drive the batteries at the correct voltage. This change might result in a 5-10% increase in output, depending on a bunch of things. Given the higher price of the MPPT controller, you may or may not have a lower cost due to the smaller cables (battery cables are very expensive these days). Also, you will no longer be able to bypass the controller to manually regulate in the event of failure, unless you rewire the panels back to parallel (in emergency) and upgrade the cables. Your choice, but the new way is not that big of a win.

Greg

[Dsanduril]

Originally Posted by TacomaSailor

Could someone please explain to me what I am missing in the new Solar Panel / Controller world?

You're not missing anything. If you have enough room on the boat to put up the panels to do what you've done you come up with the simplest, cheapest solution. But not all boats have the room, and as cruisers put more and more electrical equipment on the boat sometimes you need to see if you can eke a little bit more efficiency out of the system.

Originally Posted by CarinaPDX

I don't think this is quite right. The 95% efficiency will reduce the power (watts) output by 5%. Given a decent voltage control the voltage could be maintained at the desired level, but at reduced current. It might even be able to raise the voltage as needed, depending on the circuit design.

Yes and no, my example was very much overly simplified. MPPT controllers can be buck, boost, or buck and boost. The latter is more expensive, and thus pretty rare. Most are built with simple buck converters, and those will need some voltage overhead in order to operate. In reality what happens, if they don't switch to PWM is they move the solar panel off (and above) Vmp in order to maintain their required overhead. And the power curve as you come off Vmp is very steep, resulting in lost power production.

[CarinaPDX]

I think we're saying pretty much the same thing.

Good to meet someone who is also not blinded by the latest and greatest hype.

Greg

[onestepcsy37]

tacoma sailor - i used to be even simpler. apparently its ok to go directly from the solar panel (12volt) to the battery bank, as long as you are not handing the batteries more than 1.5% of their capacity.

so with my one 135 watt kyocera (7.5 amp output) and a 550 amp battery bank i just hooked it up directly. ran that way for maybe four years. no problems. but now i want to add a second kyocera 135 and the output will exceed the 1.5% limit so i just installed a morningstar 20 amp PWM controller. things are getting complicated....

[noelex 77]

Originally Posted by TacomaSailor

What in the world am I missing with all the new MPPT / series-vs-parallel, stepping down voltages and other very confusing things.

You are not missing out on much, but compared to a more expensive top of the line MPPT controller you have the following disadvantages.

A small loss of power. This is in the order of 5-15%

The inability to use the latest high efficiency panels (these are high voltage). This is of no consequence with your current panels, but if you did replace them (unlikely given their long life) it would be frustrating.

Some lack of adjustability in the battery regulating. The Trace C-40 is actually quite good and this is not the issue it can be with many cheaper controllers, but it is still less adaptable than the best controllers.
Reduced adjustability can shorten battery life and make it difficult to use the controller with lithium batteries for example.

These issues are not great and I would not suggest upgrading, but if starting from scratch some of the high effieciency panels with MPPT controller could be appealing, especially for boats which don't have the room for large solar arrays.

[Tootsie]

Originally Posted by shipofools

I thought most of the benefit of MPPT was when used with more than one panel? Anyone confirm.

Dear all,
For a non- electrician like me, I wish to try and agree on what the basic benefit of an MPPT controller is:
My understanding is like this;
The required voltage to charge a 12 V battery bank is 13,6 Volts (not sure about the exact decimal here)

Assume you have a solar panel with an output of 19,6 Volts.
Without an MPPT controller, 19,6 minus 13,6= 6 volts, which will be "lost" i.e. not utilized. That is significant.

With an MPPT controller, the extra voltage will be transformed into more amps of charging.
19,6 volts is 44 % more than 13,6.

I need to check why we end up with only 5-15 % gain as some in this group are claiming...
I know I have seen this calculation somewhere.

[RainDog]

Originally Posted by Tootsie

For a non- electrician like me, I wish to try and agree on what the basic benefit of an MPPT controller is

To me the basic benefit is it allows you to use the larger panels. For example, a 215 Watt Kyocera panel is $218. A 140 Watt Kyocera panel is $285. That means for a bit over 400 Watts of panels, it would cost $436 for two of the larger panels, but $855 for the smaller panels.

The larger panels require MPPT as I understand it.

Prices are from Solar Electric Power Systems For On & Off Grid

[TacomaSailor]

"Assume you have a solar panel with an output of 19,6 Volts.
Without an MPPT controller, 19,6 minus 13,6= 6 volts, which will be "lost" i.e. not utilized. That is significant.

With an MPPT controller, the extra voltage will be transformed into more amps of charging. 19,6 volts is 44 % more than 13,6."

I do not understand all this about converting volts into amps.

- Solar panels convert solar energy into electrical energy
- maximum energy produced by the solar panel is constant in a given solar situation i.e the maximum number of electrons knocked free by the solar radiation is a constant in any given situation
- electrical energy is measured by watts
- Watts=Volts*Amps
- Amps = Watts/Volts
- lowering the voltage increases the amperage (current) because the solar panels are still producing the same number of electrons

How does an MPPT controller make more current available by changing the voltage if there is a constant amount of energy (electrons) delivered from the solar panel?

What do I have wrong about energy and voltage?

Is it that the solar panel only flows the maximum possible number of electrons (current or amps) at the maximum open circuit voltage? I=VR and the R stays the same (wires and internal battery) in a given solar situation.

Is that why my 125 watt panels produce only 8.2 amps at 14.4 V (118 watts)?

"The required voltage to charge a 12 V battery bank is 13,6 Volts"

I think that statement is incorrect. My lead-acid batteries require 14.4 V for some length of time to be fully charged. They float at 13.6V which is an indicator of a full charge but the charging source must raise the BATTERY voltage to 14.4 to completely convert all the sulphate back to electrolyte.

Thanks to all for bringing this old solar dinosaur into the modern error. I have paid little attention to solar panels and controllers since 1999.