The tree and me. MPPT or chain saw?

Bob Wallace

Off the grid. 1.2kW of panels on two ground mount racks. (8-75 each rack). Wired 24vdc.

The two months of the year when the Sun is lowest a Doug Fir casts a mid-day shadow on my lower/more southern rack. I could cut the tree, but I'd like to avoid that if possible.

I was considering separating my racks by running them to separate controllers. (I'm using a C60 and have a C30 as a backup.) That way when the lower rack voltage drops I'd still get full flow from the un-shaded rack.

I tried that idea out on the tech guy at another online store and he suggested I switch to a MPPT controller. I've read the page on this site, I've got a basic idea how a MPPT controller works, but I don't understand why this might solve my shade problem. (Or perhaps it won't and the other person didn't quite understand my situation.)

Would it work to turn the two racks into one big 48vdc array and feed them through a MPPT? Or is there some other trick I might use?

mike95490

Re: The tree and me. MPPT or chain saw?

as magical as MPPT is, it does not fix shadows. Splitting the racks to 2 controllers, PWM or MPPT would work best, as shadow happens, you loose the lower rack. The unshaded upper one, on it's controller, continue as normal.

Bob Wallace

Re: The tree and me. MPPT or chain saw?

Thanks.

Now, could I put the unshaded rack on a MPPT and the shaded rack on a PWM?

I could set it up to do that temporarily for the couple of shady months. Put both racks on the MPPT for the other ten.

Or would it work to put the lower on a MPPT so that it got cranked up to match the system voltage?

(Or am I badly confused?)

BB.

Re: The tree and me. MPPT or chain saw?

With PWM controllers--Your Array has Vmp set to just a few volts over the battery charging voltage--So, covering one panel only kills the current from that one panel (or string of panels if 24 or 48 volt battery bank).

No matter what you do with the PWM/MPPT/Array wiring--It is just a matter of another year or three before the tree covers the next higher row of panels (I assume).

Something is going to have to give--Accept loss in power, move array, cut tree...

-Bill

waynefromnscanada

Re: The tree and me. MPPT or chain saw?

Agree with Mike and BB. They know their stuff and are telling it like it is.

solar_dave

Re: The tree and me. MPPT or chain saw?

I vote chain saw, if it is shaded now it can only get worse.

Alternative is to move the array further North, maybe much further.

SolaRevolution

Re: The tree and me. MPPT or chain saw?

A few thoughts-

The chain saw does not have to cut at ground level. Many arborists may frown at topping trees but if the alternative is felling? It will eventually grow back. Is it worth your while to have it trimmed again every 5-10 years or so?

Also, how is your power production? Do you have enough PV to meet your needs in the winter even if you have full sun? Most areas where Douglas Firs grow have their cloudiest weather in the winter. If you have a costal, light sort of overcast for much of the time (no hard shadows) your tree shadow may not have as much impact as you may suspect.

The suggestion of using an MPPT controller may have been so you could get better efficiency from what you have. Cool winters and low batteries are the conditions where MPPT has the greatest benefit.

Just guessing here but judging by your 75 watt panels and a c-30, your system is probably >15 years old so the tree may not originally have been a problem. If your battery bank is larger than 4, L16s they may want more charging current than your array can produce so you may be regularly supplementing from a generator anyway. The daily PV production you would be loosing from 100% shading of 8-75 watt modules could probably be made up with less than an hour of generator run time. (depending on your system capabilities, of course)

Alex

niel

Re: The tree and me. MPPT or chain saw?

i have to clarify something here.

an mppt controller will not recoup power lost due to shading. mppt will only work with pv voltage outputs high enough to rework the voltage lower and will also only pass reworked currents being presented to the controller. basically if you have xxxx watts coming into the controller then it's impossible to get that much or more out of it.

this is a 2 fold question and not an either/or for i say address both aspects as both will benefit you. trimming or felling the tree will up your production by taking away the shade. an mppt cc will also up the available current. if the power presented by the array is upped back to normal by taking away the shading your available power to be worked upon by the mppt controller is now larger too.

the alternative to cutting into that tree is to move the array where it won't be shaded if such another place exists for you.

westbranch

Re: The tree and me. MPPT or chain saw?

IIWM I would use the chainsaw.

Remember the midday sun is the strongest.
In low insolation period's (winter) I prefer MPPT so you get the max out of what you get vs PWM

To cover off what SolaRevolution said, I would up the PV voltage to 24 or 48v and add MPPT. Covers off a multitude of issues.
Consider moving midday shaded panels to where they get sun, even the slightest (leafless branch) shadow has a dramatic effect...

HTH

SolaRevolution

Re: The tree and me. MPPT or chain saw?

niel wrote: »

the alternative to cutting into that tree is to move the array where it won't be shaded if such another place exists for you.

Well heck, If you are going to buy a new MPPT controller and if moving the array is a possibility you can now go much farther from your power room than you could just a few years ago.

If you were to move all 16 of your 75 watt panels you could arrange them in two parallel strings.
This should give you a PV output circuit with:
Voc +/- 175 volts (check your site's lowest historic temp for proper correction factor. +40 volts for a worst case scenerio)
Vmp +/- 136 volts
Imp < 9 amps

A Midnite Classic 200 can handle 224 max Voc on a 24 volt battery system.

You can then go
+/- 350 feet on a # 6 wire run.
+/- 500 feet on # 4.

Alex