Diversion Load Techinical question

trbarto · October 2013

I just finished installing a 3kW solar system and 2.8kW windmill, radian hybrid inverter (grid tied); Ouback FM 80 ch controller, flex net DC, Hub 5, 48V 370aH batt storage, and Mate3...the windmill came from MW&S - its their 48V dual PMA and I have it going into a Xantrex C40 Ch controller...when I bought the system, I understood I needed a dump load for the windmill and figured that the way it would work - would be, if the grid went down, and batteries were full and the no excess loads existed within the house, then I might dump...but I thought I'd only be dumping excess turbine power - NOT dumping everything coming into the battery pack - including the solar panel output...that IS allot of dump!

Anyhow the dump load is two 1000W 48V hot water heating elements in 40gallons of hot water...

My question - would it be possible to put a large diode between the xantrex C40 charge controller and the battery bank to prevent dumping the batteries and solar power energy into the water heater elements?...

Why am I concerned? Because the water heater elements I purchased have only 12ga wires coming out of them...and I've made them as short as possible and then have 10gaug coming off them, short run, to #6's which come off the ch controller...If a diode would work (like a 40A 600V blocking diode) then - in theory - if I go into dump, only the excess wind energy should divert thru the ch controller...which would protect the batteries from overcharge, but not heat much water...

But of course...no one seems to be able to answer this question...Anyone ever thought of this?

Tom

Ralph Day · October 2013

Re: Diversion Load Techinical question

Not to mention it's only 40 gallons. Is the wind turbine also grid tied through the Radian? If you had an extended outage 40 gallons of water might heat up pretty quick. (Maybe an ice storm or wind storm knocks out utility for days or more...the wind keeps blowing, you can only eat so much toast)

Ralph

trbarto · October 2013

Re: Diversion Load Techinical question

Yes - the turbine is also grid tied thru the radian...but I did the calcs on the 40 gallons and that windmill is unlikely to make that water even 100 degrees over the course of day...assuming I'm not using any hot water - so if we have an extended outtage - the 40gal tank is inline w the hot water tank and we're in northern MO...so the water coming out of the ground is a frosty 40 degrees or so...that'll cool out those DC elements in a hurry. The manufacturer told me its rated at 2.8kW - but is the point at which it melts - (industry standard -according to him)...he thinks nominal max is around 1kW and my flexnet DC shows it makes around 7-9amps at (55Vish) in a 30mph wind.

BB. · October 2013

Re: Diversion Load Techinical question

Normally, I would suggest setting the dump voltage at ~60.0 volts and the other charge controllers to 58 to 59 volts. And the Radian, once it decides the battery is charged, should keep the battery bank at ~52 volts floating (or whatever the float voltage is set at).

I don't know anything about the Radian... But I would guess that you will have to read through the manual closely and play with the sell voltage settings to ensure that the Radian is your normal "dump load" and that the C40 only dumps when the Grid is down.

If 60 volts is not high enough, according to the manual, you can clip a wire and get +8 volts added to the adjustment range (for use with alkaline batteries)--Perhaps that would work.

Setting Voltage Parameters for Alkaline Batteries

If using NiCad or NiFe batteries, the required charging voltages may be higher than the designed settings of the C-Series controller. Charging voltages can be augmented a little, if required. This can be accomplished by clipping the wire connecting the R46 Resistor to the circuit board. This augmentation will raise the designed charge parameters by 2 volts for 12-volt systems, 4 volts for 24-volt systems and 8 volts for 48-volt systems.

See Figure 2-14 for the augmented voltage settings

-Bill

clockmanfran · October 2013

Re: Diversion Load Techinical question

trbarto's question.

"My question - would it be possible to put a large diode between the xantrex C40 charge controller and the battery bank to prevent dumping the batteries and solar power energy into the water heater elements?..."

I have been thinking of some thing very similar using a blocking diode.

I have 3 Tristar 45's PWM controllers, 48v, set as diversion load with each matched to 2 kw heater loads for my 3 12 footers wind turbines. I also have 5kw of PV Trackers. All feeding to a 48v 1300ah battery bank.

Every now and then when its windy and its sunny, a Tristar controller on the wind turbines will go to float mode before the PV controllers go to float. Then my Dump Loads are taking a full 2kw from the PV backfeeding and things get mighty hot.

Diodes as a solution?

BB. · October 2013

Re: Diversion Load Techinical question

Don't know.. Diodes will give you a drop of 0.2 to 1.0+ volt forward current drop (depending on type of diode and amount of forward current).

Diodes > 8 amps or so are large and usually need good sized heat sinks (and heat sink needs to be protected against short circuits).

You may have an issue in that many charge controllers need to "see" battery voltage before they will "start up". A diode will block that voltage--But placing a small power resistor across the diode (to give you a few hundred millamps of current may take care of that problem).

The C40 controller had a resistor you could cut that would increase the voltage set points by ~1 volt... I believe the Tri-Stars are programmable with a PC (and adapter) so you can set their voltage > the Solar charge controller set points (perhaps set your multiple dump controllers at different values--14.9/15.1/15.3 volts (12 vref)-- That way you can stage the dump loads so they only go into maximum dump if you have maximum wind power).

In any case--Your dump loads should be wired/configured that they can run "forever" at rated power/loads. If you have a few days straight of high winds, you don't want something to fail 3 hours into the wind storm.

-Bill

NorthGuy · October 2013

Re: Diversion Load Techinical question

clockmanfran wrote: »

Every now and then when its windy and its sunny, a Tristar controller on the wind turbines will go to float mode before the PV controllers go to float. Then my Dump Loads are taking a full 2kw from the PV backfeeding and things get mighty hot.

You can try to adjust the absrbtion termination settings and make them different, so that your PV controler always goes to float before wind controller.

clockmanfran · October 2013

Re: Diversion Load Techinical question

Thanks BB, That's what I thought also,
"You may have an issue in that many charge controllers need to "see" battery voltage before they will "start up". A diode will block that voltage--But placing a small power resistor across the diode (to give you a few hundred millamps of current may take care of that problem)." I am not that keen on messing to much with the controllers.
Just a pain that only one diversion controller will go to float when it think the batteries are full and its not allways the same diversion controller.
My Dump loads are matched to each turbine, so they are okay, but squeezing all that PV into one dump load can be hot hot hot.

Northguy,
"You can try to adjust the absrbtion termination settings and make them different, so that your PV controler always goes to float before wind controller".
Yes, I have already done that, and yes it works most of the time but every now and then.....

I think its the nature of Wind Turbines that you need to use what they are giving no matter what. PV can be just shut down or throttled back.

Recently, this year, I have been dumping on the AC 220v side of my inverter when the battery charging voltage drops to float, ie, the batts are fully charged and therefore maximizing my PV.
Those wind turbines of mine are so idiosyncratic, they behave like 3 wild horses, never sure what they are doing.

Diversion Load Techinical question

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