Need 'Skimming" MPPT CC

Vic

Have been waiting for the new round of MPPT Charge Controllers to emerge. Would like to wait to see what the Midnite Classic feature-set is when shipping, but there may be someting out now which would do the following:

For my Off-Grid system, would love to find a CC, or CC accessory which would skim excess PV power (when in Asorb or Float) directly into an electric water heater. For my application, the Vmp from the array is 106 VDC nominal. Would be nice if the main CC would stay in its current charge mode, and just the excess fed to the water heater DC bus.

I do realize that there may be some code issues, and that the thermostatic switches have the potential of welding from large DC currents and so on, but perhaps the DC to the heatetr could be PWMed, or DC relay-controlled.

An external box might be able to infer the present state of the main CC, and try to take just enough of the available PV to keep the main CC in its current mode, but really, at the very least there should be some communitation twix the main CC and any auxillary skimmer.

Some of the newer CC's seem to have a communication buss, but this may be propritary/SECRET information, available only for that company's use.

Does anyone here know of such a CC shipping now ? Any news on the internals of the Classic and ship date target for it ? Am I just lost in space ?

TIA Vic

boB

Re: Need 'Skimming" MPPT CC

Hi Vic. Yes, the Classic will do just what you are asking for. I call it "use it or lose it" mode. (UioLi, sort of like Louie-Louie", the song)

The only problem I can see is that if you are depending on the energy to be there, you will have to just take what is available of course.

Sorry, it's still vapour-ware, but we should be seeing beta units this summer at the latest.

Also, the Classic communications will be pretty much open and documented for the user to interface to. There might be a couple proprietary commands, but is mostly
a Modbus communications system. This is also what Xantrex uses in their "Xanbus", the specs of which are freely available for download from their web site. There will certainly be differences, but it looks like Modbus is being used by a few renewable energy companies now and it's time we sort of got on the same page, so to speak.

boB

Vic

Re: Need 'Skimming" MPPT CC

boB ! Great news on a number of fronts ... yes, just hate to waste the energy. Havvan LP WH now, but for ten-or-so months per year the PV/Electric approach should do just fine -- typicallly use 6KWH per day ... will have to work harder to waste more energy.

Will just have to wait for the MidNite Classic. Sounds as if the wait will be worthwhile.

73 TU again. Get back to work ! (please). Vic

mike95490

Re: Need 'Skimming" MPPT CC

Vic wrote: »

I do realize that there may be some code issues, and that the thermostatic switches have the potential of welding from large DC currents and so on, but perhaps the DC to the heatetr could be PWMed, or DC relay-controlled.

A FET switch would be perfect, they have very low ON resistance - almost as good as a relay.
Rds On (Max) @ Id, Vgs @ 25° C 165 mOhms @ 14A, 10V

translated = 165 mOhms @ 14A with 10V control voltage (at the Gate)
in/out differential voltage = Drain to Source Voltage (Vdss) 250V abs max.



http://search.digikey.com/scripts/DkSearch/dksus.dll?PName?Name=SUM18N25-165-E3CT-ND a $5 part ! Stick it on a $10 heatsink, and you are all set
http://www.vishay.com/docs/72849/72849.pdf

You will need snubbing caps, to keep little spikes from blowing it out, but if you wired a couple in parallel, and in series on feed & return to the heater (or other load) it should be pretty reliable.

nigtomdaw

Re: Need 'Skimming" MPPT CC

On a similar vien to what Vic has asked I would like the charge controller to be able to notify me when it drops in to float. My system works very well on a sunny day I can hit float around 2 oclock this is only going to get sooner as the solar day extends as we approach summer. If the unit could allow a siren of klaxton to sound on tripping to float that would allow non technical members of the family to be aware we are wasting energy and address this by putting a wash on of plugging in the clothes iron.8). Providing shes home and not shopping for shoes or something else important

Vic

Re: Need 'Skimming" MPPT CC

Hi Nigel (I hope that is your name !),

Notice that you have C-40's, but the MX-60's Aux has this ability, as I recall, via the Aux output.

The XW CC has an Aux output, bur dunno if one of the options is for it to signal FLOAT .

Think that the MidNite Classis is going to have two Aux outputs. I would guess that this ability must one of its many tricks.

Think that the Morningstar Realy Driver board could be configured to do this, perhaps by detecting a rise in PV voltage, altho it might give a false signal early in the morning.

Bet others have a better solution for now.

Nice to see you here. Enjoy the summer in Spain or the UK. Vic

n3qik

Re: Need 'Skimming" MPPT CC

Was thinking of the same thing as others here. Right now have a C60 controller set for a PMW controller, when voltage hits a set point, I have a PLC to turn on a fan on. On nice bright days this works find, on cloudy days, the PLC is switching on/off many times.

Was thinking of getting another C60 and setting it as a shunt/Diversion Controller. This C60 would regulate the batteries and dump the extra into the water heater. By leaving the other C60 set as a PMW controller, I can use it to record total HA's and as a fail save if the shunt/Diversion Controller or the heater would fail.

Thoughts???

Vic

Re: Need 'Skimming" MPPT CC

Just to clarify what I am looking for:

Would like to find an MPPT CC with similar general capabilities of an MX-60, which would place a priority in charging off-grid batteries, but would divert excess power during Asorb/Float to an electirc water heater.

To me, this is best done inside the main MPPT CC. Would guess that perhaps there would need to be an external PWM diversion driver under the control of the main CC, such that ONLY excess power is diverted -- the main CC wold always stay in its current Aosortion or Float modes.

A completely independant PWM box could kinna do this job, perhaps adequately. But the master control really belongs inside the main MPPT CC. It knows what should happen, and what it is currently doing. Attempts to infer what the main CC is doing seems to be loaded with exceptions that would need to be handled.

Although, in my situation on most sunny days there are many KWH of excess power available, so absolute perfection is not required.

Just my thoughts. Thanks, Vic

nigtomdaw

Re: Need 'Skimming" MPPT CC

Vic Im with you on this but it might be easier if we looked at it from an AC view.

If we use the aux output of MX or XW cc to control an SSR DC-AC type then it could power an ordinary AC heater element. and adjust the set points on the CC accordingly , so we use the inverter to use the excess power to skimm the batteries.

Vic

Re: Need 'Skimming" MPPT CC

Hi Nigel,

Yes you are correct, using AC as it was intended is the "correct" way to do it.
But I am compelled to be a bit different. Thinking about PWMing the PV to the heater element seems like a very direct method of allowing the CC to just stay in its Asorb or Float mode without oscillating in and out of a charge mode.


I do realize that once the CC is about half way through Asorb, I could just enable the water heater to take all it needs from 240 VAC and forget about the CC mode. It might need to do a Rebuld or the like to replenish the charge taken from the batteries and so on, but am just intrigued with the thought of skimming.

And while this is not such a novel concept, had thrown it out a couple of times just to try to make certain that those designing new CC's might consider it during the design process -- altho, if it is a valid concept, then these sharp folks have already thought about it.

I do plan to run my Air Conditioning loads (on 240 VAC) in the way you described -- they would be enabled late in Asorb and for a number of hours in Float. Will need to Air Condition the power room, and my small cabin during the warmest months.

Thanks, Nigel, and others here for our thoughts. All the Best. Vic

Solar Guppy

Re: Need 'Skimming" MPPT CC

I don't see this ever happening, regards to boB. What the OP is requesting in essance is a second buck converter, one for the primary input and a second for the heater.

When there is excess, the second converter is brought online to regulate the array voltage as the first is now limiting the current to the primary load.

In the dog eat dog market, doubling the cost of the unit, that being the converter parts are the marjority of cost, FETS, caps, inductors makes little sense.

This could be done today with a pair of controllers that would have a master/slave on the mppt tracking sharing a common array with seperate outputs or the load side a second unit that is programed as diversion load below the mppt units absorb threshold, in either case it requires a second unit and doesn't make sense to package into a single unit cost/sales wise

Vic

Re: Need 'Skimming" MPPT CC

Hi SG,

Well, what I thought I was asking for was the ability to add a PWM driver for the second DC load. This second load -- the water heater in my example would have second priority. It would be driven from the master such that only excess power would be diverted.

The intelligence should be in the master, otherwise, I could see the demands from the water heater causing the PV MPPT CC to oscillate between modes, and other hiccups.

This thought would require some code, and two or three terminals to interface to the external PWM driver (probably some FETs on a heat sink etc).

Personally, I see no need to do another converter etc -- I just wanna PWM the DC output of the PV array into the WH. This is untraditional.

AND, thanks for the comments. Probably enuf bandwith on this subject.
Thanks again Vic

boB

Re: Need 'Skimming" MPPT CC

Vic wrote: »

Hi SG,

Well, what I thought I was asking for was the ability to add a PWM driver for the second DC load. This second load -- the water heater in my example would have second priority. It would be driven from the master such that only excess power would be diverted.

The intelligence should be in the master, otherwise, I could see the demands from the water heater causing the PV MPPT CC to oscillate between modes, and other hiccups.

This thought would require some code, and two or three terminals to interface to the external PWM driver (probably some FETs on a heat sink etc).

Personally, I see no need to do another converter etc -- I just wanna PWM the DC output of the PV array into the WH. This is untraditional.

AND, thanks for the comments. Probably enuf bandwith on this subject.
Thanks again Vic

Hi Vic...
I understood what you meant.
boB

Solar Guppy

Re: Need 'Skimming" MPPT CC

OK, guess I didn't understand what you were asking for ...

The Xantrex XW-60 should do what you want as is.

The Aux output can be triggered by a bunch of user selectable values and the output voltage is user adjustable, the time is adjustable down to a second so it could direct drive external switches ( FETS ) and produce a low duty cycle drive as your looking for. Your heater element ( one leg ) would be connected the battery and your N-Channel FETS on the other leg of the heater, with and the FET to Ground. ( or a solidstate relay might work better for the do it yourself crowd ).

Check out the XW-60's manual:

http://www.xantrex.com/web/id/1876/docserve.aspx

Page 3-12 is where the document has the AUX control parameters