A Forum run by Enthusiasts of MidNite Solar

I'm sitting here, watching the sun pour down. The Midnite 150 is perilously close to dropping into float. I am currently running the fridge off the inverter, living room lights at night. But right now, peak of production time, there's potential power not being stored/used.

On days we're here, we've been doing things like plugging the washing machine into the inverter, stuff like that. But when there's no immediate load ... potential power is going nowhere.

Thoughts?

Quote from: Vern Faulkner on November 22, 2012, 12:21:21 PM
I'm sitting here, watching the sun pour down. The Midnite 150 is perilously close to dropping into float. I am currently running the fridge off the inverter, living room lights at night. But right now, peak of production time, there's potential power not being stored/used.

On days we're here, we've been doing things like plugging the washing machine into the inverter, stuff like that. But when there's no immediate load ... potential power is going nowhere.

Thoughts?

Vern;


Everyone uses hot water and that is my main "opportunity load". Our system is unusual but works. Not automated yet but that is in the plan.

We use 2 10 gallon electric water heaters one feeding into the other. The "main" heater (closest to the faucets) is grid connected and the first one in the line is powered from our inverter.  On low power days it is usually just "off" but on high power days it gets turned "on".  We decide what to do on the fly manually but the decision making is being automated.

We just added 1500 watts of solar so this is just me starting to figure out where to put the power.

Our original 820 watts of solar & the turbines kept a fridge, 2 freezers and our satellite TV and wireless internet going along with night lites and occasional opportunity vacuming, heating of water and running the TV. We haven't gotten a solid plan in place for using the new power we are getting. My next step is probably getting our submersible well pump on the RE but that will not keep ahead of the incoming power.

There is always the dump load to waste power option but I prefer to use it.

Hope this helps some.


Tom

This is exactly what "Waste Not" Aux modes are for.   Waste Not is mistakenly called "Opportunity" in
the manual.

boB

Bob,

Can you explain clearly what 'waste not' does? The manual just leaves me scratching my head.

Cheers

Quote from: zoneblue on November 22, 2012, 04:21:40 PM
Bob,

Can you explain clearly what 'waste not' does? The manual just leaves me scratching my head.

Cheers

Yeah, I went back and reread that section and I really don't get it, either.

Clarification & an example would be nice because I think I will be needing functions it may do.

Tom

I am looking at the manual now and trying to see if I can add some clarity to waste-not as well
as other things that may need some better explanation.

First thing to remember about Waste Not is that, when the charge controller is done with
its MPPT mode, whether it be Bulk MPPT, Float MPPT or EQ MPPT, it is regulating the battery
VOLTAGE and can not use all of the available power from the PV array or wind or whatever
the power source is.  In MPPT mode, the battery voltage is below the Absorb, Float or EQ
voltage set point and the system is trying hard as it can to get the voltage up to that
set point voltage.

So, while in Absorb, Float or EQ voltage regulation, the Classic raises the input voltage in
order to push it more towards open circuit voltage and use LESS power and energy to
keep the battery voltage from being overvoltaged.

Instead of raising the PV voltage to reduce the power and keep the battery voltage down,
why not load it down with your own loads ?  Might as well hold that voltage steady
by switching on and off a water heater or pump water etc rather than just leaving
the power source unloaded.

That is the gist of how it works.  Now I will explain a bit how to adjust it in the Aux
menu....

Since the Classic has 3 different possible voltages it can regulate... Absorb,
Float or EQualize, we need to make the diversion load relative to that
charge stage voltage.  This is why the waste-not is relative to the charge
voltage set point.  This charge stage voltage set-point will change slightly
with temperature if the battery temp comp sensor is connected and attached
to the betteries.  But that is taken care of automatically the Classics' waste not
logic.

Since the Classic will still raise the PV or turbine input voltage if the battery
goes above the temperature compensated set point voltage, the waste-not relative
voltage must be slightly lower than that set point voltage if it is going to turn
on your diversion load before that set point is reached.  If for some reason your
diversion load (water heater, etc.) goes open and disconnects, the actual set point
voltage will take over and increase the PV input voltage to keep the battery from
being over voltaged and over charged.  You can think of the raising of the input
voltage as a "fail safe" in this case.

Using Waste-Not will add just a slight compromise in voltage regulation tolerance
because of this relative difference.  This compromise can be reduced further by
slightly raising the Classics' Absorb, Float and EQ set point voltage.  This way,
when Waste-Not is regulating the battery charge voltage, it is closer to the
desired charge voltage of the battery and the actual set point voltage set up
in the Classic's voltage screen be the fail-safe voltage just slightly higher
than the waste-not regulation voltage.  This difference will only be a couple
to a few tenths of a volt so won't be harmful.

Another way this compromise can be reduced will be in a future update
in Classic code but that will not be coming for a while.

Aux 1, at the moment, is a fairly slow On-Off (bang-bang) method but the
delay time and hold time is adjustable in tenth of a second intervals (0.1 second)
The DELAY time is how long the Aux 1 output goes active to turn on the diversion load
after the battery voltage rises above the HIGH voltage set point.  The HOLD time
is  how long it takes for the Aux 1 output to go inactive after the battery voltage
falls below the LOW voltage set point.  The difference between HIGH and LOW
voltage set points is called the hysteresis voltage.  The hysteresis can be as small
as 0.1 volt.  The Delay and Hold times can be adjusted for a small amount of time
so that a Solid State Relay (SSR) can be used turn on and off the diversion load,
or can be set for a large time difference that is more suitable for a mechanical
switching method such as a relay contact.

Aux 2 is similar except that it does not have a Delay or Hold time.  Instead, Aux 2
uses a semi-high frequency Pulse Width modulation (PWM) method to smoothly
go from full off to full on.  The hysteresis in this case is called the voltage WIDTH.
The Width can be adjusted as small as 1.0 volt from full off to full on up to
5.0 volts full off to full on.  At a voltage width setting of 1.0 volt, the PWM frequency is
about 500 Hz appearing at the Aux 2 output.  At 5.0 volts width, the PWM frequency
is about 100 Hz.

I hope this explanation helps some.  Maybe some of it will go into the manual.

Happy T-Day !!

boB




boB

 Vern Faulkner,

I have the same issue. My battery's are on Float by 9:00AM most mornings. With 4kw of pv (set North east to catch the morning sun) and low overnight consumption of around 3kwh this is what happens. Its a good problem to have though :)

Some Idea's are...

* Heat water
(Personally i feel if you have this much PV to wast you most likely have lots of sun so why not heat the water from a dedicated vac tube solar hot water system, though there is the extra expense of the solar hot water system)

* Install a swimming pool where you need to  run a pump for a few hrs a day (This might be my personal choice.  A bit of the good life off grid is never hurt)

* Build or purchase a electric car or ebike and charge your battery's.

* Run a cooler or small Air conditioner unit. (usually its hot on a sunny day)

* Do some bulk cooking-baking (saves stress on your battery's at night)

* Pump rain water to a tank  higher than your home to gravity feed your house water when on battery's. (Even a pumped hydro system is possible if you have the space and slope, not that efficient but better than wasting the energy)

I haven't used the "waste not" feature through and Aux switching yet.  Basically all I do is let the classic reach float (usually by 9 - 10am) then its free power time until about around 3pm. I can usually sustain loads up to 2000w without dropping out of float on a nice day.So I run the dishwasher,  cloths washer,  charge my ebikes, vacuum, pump water. Any high load tasks.  After a while you learn to judge what kind of load vs the days weather conditions your system can handle and still maintain float using only excess power.

The classics remote app a very handy tool to motor this. We have a tiny laptop in the kitchen running the remote app 24/7 so all can see when we are on float and if the classic is able to hold the loads and float during our free power time. ;)

Kurt

Quote from: boB on November 22, 2012, 06:24:29 PM

I am looking at the manual now and trying to see if I can add some clarity to waste-not as well
as other things that may need some better explanation.

Hut how can those outputs  - which I assume are accessible off the motherboard somewhere -- be used to add loads? I mean, could I use the signal to trigger a relay to trigger a switch to turn an inverter on?

Thats certainly a simple way to do, connect Aux1 to a tiny relay, which is in turn connected to a dedicated invertor's remote on.  You cant pulse a remote on.

But apart from the expense of the inverter, the other downsides are that aux1 is either on or off. Depending on where you are in the charge cycle, how much pv output and load you have you can set up a not so nice oscillation, where the extra load drops the controller back into bulk, relay drops out, repeat etc. That's where the delay and hold times are important to slow the oscillation down to manageable levels. Because its a bit rough, this method may also add extra cycles to the battery but im not sure how much. Will depend on how big the diversion load is i suppose.

The upside is you minimise reported buzzing noise from PWM routes.

Quote from: Vern Faulkner on November 22, 2012, 09:38:18 PM

Hut how can those outputs  - which I assume are accessible off the motherboard somewhere -- be used to add loads? I mean, could I use the signal to trigger a relay to trigger a switch to turn an inverter on?

Yes, you can drive an external relay or contactor but that is mechanical and can click and buzz.
An SSR is a better way to go.  There are DC SSRs (transistors/IGBTs or FETs), and AC SSRs (typically thyristors or triacs).

AC SSrs can not be used on the battery side so is only used on the AC output of an inverter or the AC side of a turbines' rectifier.

AC SSRs are probably more preferable than DC SSRs because AC SSRs are typically thyristors that turn off at AC zero crossing and usually turn on at zero crossing as well.  This minimizes inductive spikes from inductance in the diversion loads.  AC diversion using SSRs can have drawbacks in that the typical drive to the SSR does not remember which AC phase/polarity was last turned on or off and so the magnetics can saturate.  This is not usually a problem but it should be remembered in case it does become a problem.  Typical problems with AC diversion due to saturation may be made apparent by either transformer buzzing and grunting or turbine "clanking" once in a while.

DC SSRs  are used for battery side diversion and for some alternators where the rectifiers are built into the turbine and only two wires are run from generator to the charger, batteries and inverter system.  Because the DC load does not have a zero crossing to turn off at, there may be dangerous voltage spikes that can harm the SSR or other circuitry.  So for DC SSRs, the frequency should be lower to reduce the energy dissipated in the SSRs or other circuitry.

The Classic's Aux 2 has an AC and a DC diversion mode selectable in the CLIPPER aux 2 menu.  This is not applicable for the Aux 1 output.

MidNite has the "Clipper" which come in AC and DC versions and are preferable for Classic PV/wind input side clipping.
The Clipper contains the resistive loads and SSRs and rectifiers for AC 3-phase clipping operation.
Both AC and DC Clippers have an Aux input that is driven from the Classic's Aux 2 output (normally) and also includes a power supply that runs from the input side that operate a "fail safe" circuit in case the Classic should stop commanding the Clipping.  The fail-safe clipping voltage is adjustable by a trimmer resistor on the Clipper circuit board.

boB

boB's description of waste-not in post #5 was a difficult read.  I tried to rewrite it, filling in what I thought were blank spots.  Comments/corrections welcome.  See post number 66 of http://www.wind-sun.com/ForumVB/showthread.php?21017-pv-dump-load-to-water-heater-on-midnite-solar-classic

Quote from: gridloose on September 21, 2013, 12:51:30 AM
boB's description of waste-not in post #5 was a difficult read.  I tried to rewrite it, filling in what I thought were blank spots.  Comments/corrections welcome.  See post number 66 of http://www.wind-sun.com/ForumVB/showthread.php?21017-pv-dump-load-to-water-heater-on-midnite-solar-classic

Good questions.
I would like to copy and paste what I wrote over in the NAWS forum just now.

Hi Gridloose (et al. Whoever "Al" is ?)....

The PWM on Aux 2 is normally set for a 1 volt width. The frequency of the PWM at that width is
about 500 Hz. The reason for that is that the Classic's software that runs this is at a fixed
timing of 200 microseconds. If you take 200 microseconds and multiply that by 10, you get
2 milliseconds which is 500 Hz.

OK, so having divided up a 1.0 volt width by 10, you get 0.1 volt divisions or increments.

When the Classic encounters the low end of that 1.0 volt, it starts to pulse the Aux 2 output
at one tenth of its 2 millisecond (1/500 Hz) time. That would be 10% duty cycle.
As the battery voltage rises every tenth of a volt, the Classic increases the duty cycle
by 10%. At the top of that 1.0 volt width, the duty cycle would be at full on or, 100%.

When the width is widened to, say, 5.0 volts, the 5 volts is divided up into 50 tenths
of a volt. The frequency of the PWM in this case is 5 times slower, or 100 Hz.

The tenth of a volt division is because that is the minimum voltage change the Classic
sees and has to work with.

boB

Ill try to summarize the choices:

1.  AC diversion

Up side: really simple to setup, the only extra hardware is the SSR. Existing heating elements, and thermostat both usuable as is. Aux 1 or aux2 take your pick.
Downside: takes up inverter capacity/cost of inverter capacity, less of an issue if the midday heating period is considered off-peak for your demand patttern.

2. DC diversion

Upside, highest overall efficiency, ie no inverter losses.
Downside, may have extra dc wire losses, depending on distance and wire. Less choice and range of DC elements. Extra components required to avoid dc arc issues on the thermostat.  No 'perfect solution' that mitigates completely the PWM ripple effects on both diversion and battery. Replacing element risks damage to cyllinder if seized.

3. PV diversion

Upside, high voltage means less losses and ability to match to available elements
Downside, High input voltage reduces overall controller efficiency. Interferes with controller (requiring diode at minimum), plus the same issues with thermostat as for dc diversion.

4. Stand alone (seperate PV for HWC)

Upside, reduces complexity
Downsides, doesnt use waste power. Need something to match mppt. Thermostat issues as above.


Conclusion: if you arent grid tied, at the end of the day you cant really ever use every last drop. Something that works, ie heats your water, is probably the objective. When there isnt enough hours in the day, ac diversion is clearly the winner.

I still want to give dc diversion a go, (because i try to keep our invertor off), even if it means building a small interface board to handle the thermostat. If you are going to build a board then a mosfet driver and few mosfets and an optoisolator, is not really extra trouble. Will it happen?, ask me again in another year.

BTW i was planning on using a pair of these 0.7 ohm units, at $100 ea. That gives me 2kw for 2kw array. http://ecoinnovation.co.nz/p-631-07-ohm-water-element.aspx

Many thanks to boB for the complete description of Aux2 PWM waste-not.
All is clear!

Zoneblue seems to be considering the same issues as I.  A fun puzzle, and a lot more going on here than you might think.

I could imagine using all the water heater power I can get in the winter months, heating an insulated basement cistern for hydronic floor heat if nothing else.

Power straight from the PV panels would be ideal, avoids heating up the Charge Controller unnecessarily and allows the use of high power 120 Volt (or 240 Volt) heater elements, much cheaper than the low voltage DC heater elements.

But I haven't seen anybody out there who has successfully captured a majority of waste power direct from the PV panels.

If you just use Waste-Not and a relay to connect the panels to the heater element  (panels simultaneously sending power to the charge controller), we must avoid sending too much current to the heater element or the voltage at the panels will nosedive and efficiency will suffer.  To say nothing of disruptions to the Charge Controller's MPPT.  Drawing DC from the battery does not have this issue since the battery serves as a huge capacitor, smoothing out any variation in battery voltage between heater on and heater off.  Aux2 PWM at 500 Hz would help, but it would take a 10,000 uF cap for every 10 Amps to the heater element to bring the panel voltage ripple down to 1 volt pk-to-pk (when operating at 500 Hz with a worst case 50% duty cycle).    Might be better to raise the PWM switching frequency, perhaps by monitoring Aux2's 500 Hz PWM and duplicating its duty cycle at 30 KHz.

Some people are going to oversized panel arrays to keep their batteries up during moderately overcast weather, now that PV panels can be had for well under $1 USD/watt.   So bulk mode might go into current limiting,and waste-not will waste that excess power.   Extra credit if you can monitor current to the battery from the charge controller and set up a threshold there as well, just before the charge controller gets into current limiting.  Of course, as has already been pointed out to me, could just find the bucks for a bigger battery to better match the worst the panels can put out, as that would also help us through cloudy weather.

Quote from: gridloose on September 22, 2013, 02:59:15 PM
Drawing DC from the battery does not have this issue since the battery serves as a huge capacitor, smoothing out any variation in battery voltage between heater on and heater off.

What is your view on the effect on the bank of that ripple? Worse case scenario i have a 1ah battery and i put a 0.5amp discharge pulse followed by a 0.5amp charge pulse, i am cycling that bank? Does the ripple interfere with battery chemistry or 'heat' the battery, ie the old addage that battery chargers should have minimal ripple.

QuoteAux2 PWM at 500 Hz would help, but it would take a 10,000 uF cap for every 10 Amps to the heater element to bring the panel voltage ripple down to 1 volt pk-to-pk (when operating at 500 Hz with a worst case 50% duty cycle).

Hang on, if you are still talking about dc diversion there is no ripple on the pv, the controller integrates that. If you are talking about battery ripple, then yes big caps have been reported to help and reduce the buzzing sound emitted by the elements as well.

QuoteMight be better to raise the PWM switching frequency, perhaps by monitoring Aux2's 500 Hz PWM and duplicating its duty cycle at 30 KHz.

I suspect my own pondering out loud whether the aux2 output accurately reflects spare power, is mostly a product of never having tried it. Most of the parts are laying around. mumble mumble.

QuoteSo bulk mode might go into current limiting,and waste-not will waste that excess power. 

Im confused, aux2 will  let you use spare bulk power when under current limit or it wont?
I am trying to follow your rash of writing on this but find a lot of it difficult to follow.

Quote from: gridloose on September 22, 2013, 02:59:15 PM
Many thanks to boB for the complete description of Aux2 PWM waste-not.
All is clear!

Zoneblue seems to be considering the same issues as I.  A fun puzzle, and a lot more going on here than you might think.

I could imagine using all the water heater power I can get in the winter months, heating an insulated basement cistern for hydronic floor heat if nothing else.

Power straight from the PV panels would be ideal, avoids heating up the Charge Controller unnecessarily and allows the use of high power 120 Volt (or 240 Volt) heater elements, much cheaper than the low voltage DC heater elements.

But I haven't seen anybody out there who has successfully captured a majority of waste power direct from the PV panels.

You could do that as long as you do not load the input to the charge controller directly.  This is because
it is bi-directional and creates an input voltage from the battery that matches what you see on the
PV terminals.  So, you could use the waste -not PWM control on say, the anode of a diode that
is connected to the PV positive and the diode's  cathode tied to the CC's positive input terminal.
The PWM'd load could connect to that diode's anode to load down the PV.  This might let you divert
from the PV directly while in current limit, as you pointed out below.

Quote from: gridloose on September 22, 2013, 02:59:15 PM
If you just use Waste-Not and a relay to connect the panels to the heater element  (panels simultaneously sending power to the charge controller), we must avoid sending too much current to the heater element or the voltage at the panels will nosedive and efficiency will suffer.  To say nothing of disruptions to the Charge Controller's MPPT.  Drawing DC from the battery does not have this issue since the battery serves as a huge capacitor, smoothing out any variation in battery voltage between heater on and heater off.  Aux2 PWM at 500 Hz would help, but it would take a 10,000 uF cap for every 10 Amps to the heater element to bring the panel voltage ripple down to 1 volt pk-to-pk (when operating at 500 Hz with a worst case 50% duty cycle).    Might be better to raise the PWM switching frequency, perhaps by monitoring Aux2's 500 Hz PWM and duplicating its duty cycle at 30 KHz.

Some people are going to oversized panel arrays to keep their batteries up during moderately overcast weather, now that PV panels can be had for well under $1 USD/watt.   So bulk mode might go into current limiting,and waste-not will waste that excess power.   

Extra credit if you can monitor current to the battery from the charge controller and set up a threshold there as well, just before the charge controller gets into current limiting.

I'm not exactly sure what you mean here but the WhizBang Junior battery shunt monitor may provide this limiting function.
That should be either in the first go of the WB Jr. Classic code or soonly thereafter.  ("soonly", my new word of the day)

The PWM frequency is low-ish for a couple of reasons.  First, it is done in a timer loop in software and not in
hardware.  Second is that high frequency (30 kHz) PWM tends to bring out the problems with resistors in
that they can be fairly inductive and at those high frequencies can cause grief in voltage spikes heating up
the Solid State Relays, snubbers or actually raising the voltage rather than lowering it.  This is why  in DC
clipper mode, the Classic operates at lower frequency like around 20 Hz.

Quote from: gridloose on September 22, 2013, 02:59:15 PM
  Of course, as has already been pointed out to me, could just find the bucks for a bigger battery to better match the worst the panels can put out, as that would also help us through cloudy weather.

ZoneBlue,

I've often wondered how much the battery might be getting used up when quickly cycling power in and out.  Good question, I have no idea.  But it's often done, and it's what makes waste-not while drawing power from the battery (perhaps via the inverter) a fairly easy thing to do.  A solved problem.

I'm wondering if it's worth trying to put a waste-not load directly across the panels.  If you use Aux2 PWM at 500 Hz to switch 10 Amps of that PV power directly from the panels to a water heater element, I figure that even with a 10,000 uF cap across the PV panels you could still see something like a volt of ripple across those panels.   That's just a quick calculation of how much a cap would discharge in a millisecond at 10 Amps, a full analysis would need to know the panel characteristics, how much light is available, and how much of a load the charge controller is presenting.   (The effect of all that ripple on the Classic's MPPT might not be so bad if the Classic's A2D has a low pass filter in front of it, and if we phase lock our 30 KHz PWM to the Classic's Aux2 500 Hz PWM and thus hopefully to when the Classic takes an A2D sample).

When in Bulk mode, the Classic cannot divert any waste-not power.  Waste-not only works in Absorb, Float and Equalize, where the Classic is trying to raise the battery voltage to a specific level.  But if we were especially ambitious, we might add our own electronics external to the Classic to implement a current based version of waste-not to make use of any excess panel power when the Classic gets near its battery current limit while in bulk mode.

Quote from: boB on September 22, 2013, 07:14:16 PM
You could do that as long as you do not load the input to the charge controller directly.  This is because
it is bi-directional and creates an input voltage from the battery that matches what you see on the
PV terminals.  So, you could use the waste -not PWM control on say, the anode of a diode that
is connected to the PV positive and the diode's  cathode tied to the CC's positive input terminal.
The PWM'd load could connect to that diode's anode to load down the PV.  This might let you divert
from the PV directly while in current limit, as you pointed out below.

Not quite sure what you mean by "creates an input voltage from the battery that matches what you see on the PV terminals".  But I had considered a diode at the charge controller PV input such that when the heater is switched on it does not discharge the input caps inside the charge controller. 

Quote from: boB on September 22, 2013, 07:14:16 PM

Quote from: gridloose on September 22, 2013, 02:59:15 PM
Extra credit if you can monitor current to the battery from the charge controller and set up a threshold there as well, just before the charge controller gets into current limiting.

I'm not exactly sure what you mean here but the WhizBang Junior battery shunt monitor may provide this limiting function.
That should be either in the first go of the WB Jr. Classic code or soonly thereafter.  ("soonly", my new word of the day)

The PWM frequency is low-ish for a couple of reasons.  First, it is done in a timer loop in software and not in
hardware.  Second is that high frequency (30 kHz) PWM tends to bring out the problems with resistors in
that they can be fairly inductive and at those high frequencies can cause grief in voltage spikes heating up
the Solid State Relays, snubbers or actually raising the voltage rather than lowering it.  This is why  in DC
clipper mode, the Classic operates at lower frequency like around 20 Hz.

I'll look into the battery shunt monitor. 
What I'm suggesting is that if somebody cares about wasted power if bulk mode hits the charge controller current limit, we could add external circuitry to monitor charge controller current to the battery.  This circuitry would turn on a waste not load when the current approaches the limit we have programmed into the charge controller, with hysterisis.  This is very much like the existing waste-not for Absorb, Float, and Equalize, except it is monitoring current rather than voltage.  However, this would only work for a waste-not load tied directly to the solar panels, not the battery, since the charge controller current limit would not allow the waste-not power to make it to the battery.  Also, not of much use to most people, since the battery is usually sized to be able to handle all the power that the panels can send.

I could be wrong, but I suspect that inductance in a waste-not load need not be much of a problem.  The circuit topology sorts out to something very like a buck mode switching power supply.   The buck switcher's  free wheeling diode across the FET switch allows the inductor current to keep flowing as long as it wishes.  But unless there is good reason to go fast, slower switching is certainly better.  Especially at power levels of several kilowatts, which is something I have no experience with.

Quote from: gridloose on September 23, 2013, 12:18:28 PM

Quote from: boB on September 22, 2013, 07:14:16 PM
You could do that as long as you do not load the input to the charge controller directly.  This is because
it is bi-directional and creates an input voltage from the battery that matches what you see on the
PV terminals.  So, you could use the waste -not PWM control on say, the anode of a diode that
is connected to the PV positive and the diode's  cathode tied to the CC's positive input terminal.
The PWM'd load could connect to that diode's anode to load down the PV.  This might let you divert
from the PV directly while in current limit, as you pointed out below.

Not quite sure what you mean by "creates an input voltage from the battery that matches what you see on the PV terminals".  But I had considered a diode at the charge controller PV input such that when the heater is switched on it does not discharge the input caps inside the charge controller. 

What I mean is that the Classic is Bi-Directional...  From PV  to battery terminals and battery to PV terminals.

Quote from: gridloose on September 23, 2013, 12:18:28 PM

Quote from: boB on September 22, 2013, 07:14:16 PM

Quote from: gridloose on September 22, 2013, 02:59:15 PM
Extra credit if you can monitor current to the battery from the charge controller and set up a threshold there as well, just before the charge controller gets into current limiting.

I'm not exactly sure what you mean here but the WhizBang Junior battery shunt monitor may provide this limiting function.
That should be either in the first go of the WB Jr. Classic code or soonly thereafter.  ("soonly", my new word of the day)

The PWM frequency is low-ish for a couple of reasons.  First, it is done in a timer loop in software and not in
hardware.  Second is that high frequency (30 kHz) PWM tends to bring out the problems with resistors in
that they can be fairly inductive and at those high frequencies can cause grief in voltage spikes heating up
the Solid State Relays, snubbers or actually raising the voltage rather than lowering it.  This is why  in DC
clipper mode, the Classic operates at lower frequency like around 20 Hz.

I'll look into the battery shunt monitor. 
What I'm suggesting is that if somebody cares about wasted power if bulk mode hits the charge controller current limit, we could add external circuitry to monitor charge controller current to the battery.  This circuitry would turn on a waste not load when the current approaches the limit we have programmed into the charge controller, with hysterisis.  This is very much like the existing waste-not for Absorb, Float, and Equalize, except it is monitoring current rather than voltage.  However, this would only work for a waste-not load tied directly to the solar panels, not the battery, since the charge controller current limit would not allow the waste-not power to make it to the battery.  Also, not of much use to most people, since the battery is usually sized to be able to handle all the power that the panels can send.

I could be wrong, but I suspect that inductance in a waste-not load need not be much of a problem.  The circuit topology sorts out to something very like a buck mode switching power supply.   The buck switcher's  free wheeling diode across the FET switch allows the inductor current to keep flowing as long as it wishes.  But unless there is good reason to go fast, slower switching is certainly better.  Especially at power levels of several kilowatts, which is something I have no experience with.

Yeah, the inductance can be a problem if the switching is happening real fast and the resistors are wire-wound and big.
Just gotta be careful.  Slow is good if possible.

Quote from: gridloose on September 23, 2013, 12:18:28 PM
 
What I'm suggesting is that if somebody cares about wasted power if bulk mode hits the charge controller current limit, we could add external circuitry to monitor charge controller current to the battery.  This circuitry would turn on a waste not load when the current approaches the limit we have programmed into the charge controller, with hysterisis.  This is very much like the existing waste-not for Absorb, Float, and Equalize, except it is monitoring current rather than voltage.  However, this would only work for a waste-not load tied directly to the solar panels, not the battery, since the charge controller current limit would not allow the waste-not power to make it to the battery.  Also, not of much use to most people, since the battery is usually sized to be able to handle all the power that the panels can send.

An interesting idea that will depend on having an excess of PV power, amps, beyond what the max the controller can deal with.
This may be a quite common scenario as low cost PVs makes it easier to install larger PV arrays to still provide some decent power in adverse weather conditions.
As you say its going from monitoring battery voltage to monitoring current to battery then using this as a control to an external controller for redirecting PV power

I eventually gave up on using AUX2 pwm wastenot as a control for switching in diversion of PV power to water heater. I noticed too often that in non ideal weather the switching out of part of my PV array would drop the Classic back into bulk mppt mode then a few minutes later wastenot would enable again, and this could repeat for some time, neither letting absorb complete or heat water.
Of course in good weather it worked much better.

Instead I changed to monitoring the input voltage to the Classic (output V from PV array) and calculated a water heater turn on voltage that permitted a chunk of my PV array to be diverted to the water heater without causing the Classic to drop out of Absorb or Float.
Early in this process I used AUX1 PV high as the control but eventually wanted to separate the Classic from the hot water control process.
Now I am  using 2.8Kw PV in four strings of 5 series 140 watt panels, each 18.16v mpv and 7.85a for about 91v mpv per string.
I divert one string then a second into a 120v 2Kw element.
With the Classic in Float that pv voltage can rise up over 112v. So some experimenting with this I found a suitablepoint to divert one string
and as expected there was a small down-blip in voltage but it starts ti rise again and at anoher higher point the second string gets diverted too.
On a bright day this leaves 1400watts of PV to the Classic. A good amount of this is wasted during Float so time to get some AC loads active.

Once PV strings are diverted to heating I use a 20 minute timer to hold them at water heating then the battery input v is measured again and if the Classic has dropped into bulk mppt then the diverted pv strings are released to the Classic.
I also had to include a clock to stop early morning high PV voltage from enabling diversion before the Classic starts taking power.

The controller for this PV input diversion management is an EEEPc with an HP voltage monitor intrface. My current project is to get this simple application moved to a Beagle board controller

dgd

Quote from: boB on October 09, 2013, 09:20:46 PM
What I mean is that the Classic is Bi-Directional...  From PV  to battery terminals and battery to PV terminals.

If it is truly bidirectional, does that mean it is sometimes taking power from the battery and pushing current back toward the PV panels to raise the PV voltage?  My guess is that it does not.

From your post to this thread in reply #5:
"So, while in Absorb, Float or EQ voltage regulation, the Classic raises the input voltage in
order to push it more towards open circuit voltage and use LESS power and energy to
keep the battery voltage from being overvoltaged."

My guess is that the Classic raises the input voltage by setting its current limit for power from the PV panels to a lower value.
So rather than call it "bidirectional", I would say that the Classic is capable of setting a current limit.
Probably comes down to how we choose to model this in our heads, but "bidirectional" seems a bit weird to me.

My understanding of how an MPPT controller might work is that it is a switch mode power supply that can operate with both a max current setpoint and a max voltage setpoint.  If the max voltage setpoint is reached (or if the controller is sending the maximum current to the battery during bulk that was programmed into it by the user) then the controller falls out of MPPT mode and the battery is getting all it could use.  Otherwise,  the controller is sending all the power to the battery that it possibly can.  It does this by scanning through the power curve for the PV panels given the conditions of sunlight and temperature and shading at that moment.  When scanning, it sets the current limit to a selection of possible values and measures the resulting PV voltage, computing power into the controller as current times voltage.  It then selects the current limit value that yields the maximum power and runs with it for a few minutes, at which time it performs another scan.

Quote from: dgd on October 17, 2013, 05:52:20 AM
Now I am  using 2.8Kw PV in four strings of 5 series 140 watt panels, each 18.16v mpv and 7.85a for about 91v mpv per string.
I divert one string then a second into a 120v 2Kw element.
With the Classic in Float that pv voltage can rise up over 112v. So some experimenting with this I found a suitablepoint to divert one string
and as expected there was a small down-blip in voltage but it starts ti rise again and at anoher higher point the second string gets diverted too.

Very cool, sounds like you've moved well beyond armchair engineering to actually getting something to work. 

I had considered splitting my array, using a system of relays to send all power to the charge controller when it needed it or sending some or all of the array to the water heater, also varying the power to the water heater by running 4kw and 2kw water heater elements in various series and parallel arrangements.  That might be the best approach if high speed PWM does not work out, though high speed PWM would provide more finely grained control.

Quote from: gridloose on October 23, 2013, 01:27:09 PM

Quote from: boB on October 09, 2013, 09:20:46 PM
What I mean is that the Classic is Bi-Directional...  From PV  to battery terminals and battery to PV terminals.

If it is truly bidirectional, does that mean it is sometimes taking power from the battery and pushing current back toward the PV panels to raise the PV voltage?  My guess is that it does not.

Yes, the hardware is basically bi-directional and ~could~ put current back into the PV by raising the PV voltage beyond
the last measured Voc of the array but it does not want or need to do that.   We try to avoid that from happening.

You should not have to think about it's bi-directionality in normal use though.  The only time you might want to
think about that is when thinking about connecting something other than the PV array to the Classic's input,
such as another Charge Controller PV input, a water pump  or some other kind of load or a short circuit to negative.
The Classic watches for  negative current and normally protects just fine.

How the Classic and other MPPT CC's regulate battery or PV voltage or limit output current is by setting the
duty cycle for the switching converter such that the input voltage to output voltage ratio is whatever
it needs to be to keep the output voltage (battery V) constant in the case of EQ, Absorb or Float,
keep the output current limited to some maximum  amperage or to keep the PV input voltage
sitting at the Maximum Power Point voltage.   That doesn't matter if the
CC is bi-directional or not.  The bi-directional nature is just an artifact of the way the switcher is
designed to be more electrically efficient.

The charge controller doesn't really need to or want to be bi-directional unless you want to actually push current
back to the PV array like you would if you were going to actively melt snow for instance or
charge a higher voltage batter from a lower voltage battery.   These bi-direactional modes
of operation are normally disabled at the moment in software.

Quote from: gridloose on October 23, 2013, 01:27:09 PM
From your post to this thread in reply #5:
"So, while in Absorb, Float or EQ voltage regulation, the Classic raises the input voltage in
order to push it more towards open circuit voltage and use LESS power and energy to
keep the battery voltage from being overvoltaged."

My guess is that the Classic raises the input voltage by setting its current limit for power from the PV panels to a lower value.
So rather than call it "bidirectional", I would say that the Classic is capable of setting a current limit.
Probably comes down to how we choose to model this in our heads, but "bidirectional" seems a bit weird to me.

Voltage regulation (EQ, Absorb, Float) could certainly be done by lowering the current limit. They are both
accomplished the exact same way by raising the PV input voltage, or if you  want to think about it
in non-bi-directional terms, by "letting" the PV voltage rise rather than the CC pulling the PV voltage down.

Another way to look at it is that the CC (Classic or other MPPT) is firstly not allowing ANY power from
the PV input and its voltage is then at Voc (an open circuit) and to get power to come up, the PV's
voltage must be "dragged down" towards the battery voltage.

Of course dragging that PV input voltage down below the Maximum Power point voltage will start the power
going down again. But dragging the PV voltage below its MPP Voltage is not NEARLY as effective as
letting the PV voltage rise towards Voc to regulate battery voltage or limit current.

Quote from: gridloose on October 23, 2013, 01:27:09 PM
My understanding of how an MPPT controller might work is that it is a switch mode power supply that can operate with both a max current setpoint and a max voltage setpoint.  If the max voltage setpoint is reached (or if the controller is sending the maximum current to the battery during bulk that was programmed into it by the user) then the controller falls out of MPPT mode and the battery is getting all it could use.  Otherwise,  the controller is sending all the power to the battery that it possibly can.  It does this by scanning through the power curve for the PV panels given the conditions of sunlight and temperature and shading at that moment.  When scanning, it sets the current limit to a selection of possible values and measures the resulting PV voltage, computing power into the controller as current times voltage.  It then selects the current limit value that yields the maximum power and runs with it for a few minutes, at which time it performs another scan.

Yes, pretty much.  The CC's will have both a current limit and voltage limits.  Sometimes they are trying to
regulate battery voltage and have a current limit, and other times they will regulate PV input voltage
when trying to put out maximum power/current into the battery.

Either operation is keeping an eye out for too high of voltage or too high of current.

boB

boB,

Thanks for the very well thought out reply to a guy with a whole bunch of inane
questions about internals that most customers have no reason to care about. 
You clearly have a passion for this, not just trying to get through another day at work.
And that's a deciding factor when I decide what to buy.

KE7ER

Quote from: gridloose on October 23, 2013, 05:47:37 PM
boB,

Thanks for the very well thought out reply to a guy with a whole bunch of inane
questions about internals that most customers have no reason to care about. 
You clearly have a passion for this, not just trying to get through another day at work.
And that's a deciding factor when I decide what to buy.

KE7ER

Thanks GridLoose !

Yes, this is a passion.  Well, electronics in general is a passion.

I wish there were more young ones these days with the interest.

I think that most of the ones that are that way are working here now !

Always on the lookout for some we may have missed though.

boB

Quote from: gridloose on October 23, 2013, 01:46:50 PM

Very cool, sounds like you've moved well beyond armchair engineering to actually getting something to work. 

I had considered splitting my array, using a system of relays to send all power to the charge controller when it needed it or sending some or all of the array to the water heater, also varying the power to the water heater by running 4kw and 2kw water heater elements in various series and parallel arrangements.  That might be the best approach if high speed PWM does not work out, though high speed PWM would provide more finely grained control.

Array splitting is one option and not too difficult to manage with a few SSRs and not gates.
In my case I just limit the DC load to no more than half of my array but don't actually disconnect them from the CC.
It works and water gets heater. But it is simple and not too intelligent. The amount of array to divert and the heater resistance/wattage
were arrived at by iterative trail and error.  Much excess power is still wasted. When bank is full and 200litre tank is full of 66deg C water then the wasted pv power is excessive.
As you say a PWM approach to managing excess power would or probably would make better use of power. Maybe all it would do is get the water tank hot sooner.
Also with PV panels sooooo cheap its tempting just to connect a Kw+ worth onto the water heater element.

The Black box project looks the way to go for more intelligent use of excess power.
I still think direct PV diversion is best option with some feedback via additional sensors to guage PV potential at any time and use a PWM technique to divert the excess to the DC load. I would like to see the efficiency of diversion going up from the 50-60% effectiveness of my current setup to perhaps in excess of 90% with a black box (embedded controller system).
However, I am aware that diverting the first 66% excess is simple but the complexity to get the rest is difficult and will need some pretty interesting software to allow for the many variables.
This design is the good fun part, time consuming, lots of research and endless thought experimenting

dgd

Quote from: dgd on October 24, 2013, 01:26:32 AM
Array splitting is one option and not too difficult to manage with a few SSRs and not gates.
In my case I just limit the DC load to no more than half of my array but don't actually disconnect them from the CC.
...
As you say a PWM approach to managing excess power would or probably would make better use of power. Maybe all it would do is get the water tank hot sooner.
...
Also with PV panels sooooo cheap its tempting just to connect a Kw+ worth onto the water heater element.

The Black box project looks the way to go for more intelligent use of excess power.
I still think direct PV diversion is best option with some feedback via additional sensors to guage PV potential at any time and use a PWM technique to divert the excess to the DC load.
...

If you "don't actually disconnect them from the CC", that sounds a lot like you are not splitting the array at all.  I'm not quite sure what you are doing here.

Yes, I've thought about just having another batch of panels set up only for the water heater.  But then I start thinking about how much less I would need to start up a generator if I somehow tied them to the CC occasionally in foul weather.  I don't think I could live with myself unless there was at least a manual switch to send both banks to the CC when things got tight.

Having both the CC and this blackbox independently making decisions about when and how to switch power around will be confusing.  Would be best if this were all somehow coordinated inside the CC.  Here's my best shot (as of this particular minute) at a solution to making full use of an oversized array of PV panels, sending excess power directly from the panels to a water heater so as not to burden the CC or the battery or the inverter with the extra load:

All PV panels are in one array with a DC voltage of around 100 volts (optionally 200 volts?) so we can use standard water heater elements.
Power from the PV panels is sent to the CC through a large diode to deal with the bidirectional issue that boB raised.
A bunch of caps are added across the PV panels (perhaps 1000 uF if PWM frequency is up around 30 KHz) to smooth out the voltage ripple when we PWM power from the panels into the water heater elements (caps capable of very high surge currents, placed very near the PWM switch, and PWM switch has a large freewheeling diode to allow current to continue to flow through any water heater inductance the instant after the PWM switch turns off).
The CC tells the water heater PWM controller box when there is more power available, and the PWM slowly ramps up power to the water heater (at an Amps/sec rate specified by the designer of the CC) until the CC tells the water heater PWM controller to slowly ramp back down.  That communication from the CC to the PWM controller could be a single wire, high to ramp up and low to ramp down.
When the CC is performing an MPPT scan, it could hold the PWM controller at a constant average current to the water heater by toggling the wire with a 50% duty cycle.  The CC might choose to ramp down the current to the water heater before starting the MPPT scan.

I looked briefly at the blackbox project thread, not obvious to me how having all those sensors would make a solution to this particular problem any easier or better.  But the sensors could be very useful to record current conditions for a better understanding of how the system is performing.

Quote from: gridloose on October 24, 2013, 11:47:34 AM
If you "don't actually disconnect them from the CC", that sounds a lot like you are not splitting the array at all.  I'm not quite sure what you are doing here.

If you have a diode from from each PV array partition into the CC as boB suggested, that might be sufficient.

"When the CC is performing an MPPT scan, it could hold the PWM controller at a constant average current to the water heater by toggling the wire with a 50% duty cycle.  The CC might choose to ramp down the current to the water heater before starting the MPPT scan."

A first pass at CC firmware to do this could be kept very simple.  Do away with MPPT, have the user program in a datasheet Vmp and provide a temperature sensor on the panels.  Then the CC increases power sent to the battery/inverter until the panel voltage falls to the temperature compensated Vmp value, and if the battery/inverter cannot provide enough of a load to do this the CC starts ramping up the PWM controller for the water heater.   MPPT could be added later to the CC firmware once it looks like this is working with no change to the PWM controller.  The "PWM controller" could use other methods to vary the power into the water heater, perhaps relay switching a bunch of heater elements into various serial/parallel arrangements (though that may not be fine grained enough).

Perhaps a combination of PWM and parallel elements.  For example, imagine a tank with four heater elements of 1, 1, 2 and 4 kW each, a PWM circuit driving the first 1 kW element and simple relays able to turn on the other three.  Such an arrangement could sink any amount of power from zero to eight kilowatts, but with a max of 1 kW being switched by the PWM controller.

But a single PWM circuit driving a single heater element would be the simplest from a system design standpoint, and easily used with any craigslist electric water heater.  The thermostatic control should be included in the PWM controller since water heater thermostats assume an AC source and our DC could arc long after the thermostat contacts open.  May include a circulation pump to send water to a large insulated cistern when the water heater gets up to temperature.

Quote from: gridloose on October 24, 2013, 11:47:34 AM

If you "don't actually disconnect them from the CC", that sounds a lot like you are not splitting the array at all.  I'm not quite sure what you are doing here.

Array is not split as such, just the heater is placed via ssr into output from part of array, this part has diode (actually3 in 3phase rectifier) before CC. Other part of array goes direct to CC. So only current from part of array goes to heater. No backfeeding from CC or rest of array.

Quote

Yes, I've thought about just having another batch of panels set up only for the water heater.  But then I start thinking about how much less I would need to start up a generator if I somehow tied them to the CC occasionally in foul weather.  I don't think I could live with myself unless there was at least a manual switch to send both banks to the CC when things got tight.

ok. But what about  just connectiing a large PV array to CC so in adverse weather there is still good PV output. Cheap PVs = add more to primary array then also consider separate bank for water heater/other DC loads.

Quote
Having both the CC and this blackbox independently making decisions about when and how to switch power around will be confusing.
Would be best if this were all somehow coordinated inside the CC.

Not sure about this. The CC's primary role is battery charging. I like the idea of separating water heating from the CC and battery bank/inverter by taking power directly from the source generator - the PV array. A Black box processor could easily minimum interface to the CC yet provide more effective measurement of available PV power and adjust the DC load to match.
The BB could also provide all the reporting ever needed and perhaps a web server with decent tcp stack for multiple web connections.

Quote
  Here's my best shot (as of this particular minute) at a solution to making full use of an oversized array of PV panels, sending excess power directly from the panels to a water heater so as not to burden the CC or the battery or the inverter with the extra load:

All PV panels are in one array with a DC voltage of around 100 volts (optionally 200 volts?) so we can use standard water heater elements.
Power from the PV panels is sent to the CC through a large diode to deal with the bidirectional issue that boB raised.
A bunch of caps are added across the PV panels (perhaps 1000 uF if PWM frequency is up around 30 KHz) to smooth out the voltage ripple when we PWM power from the panels into the water heater elements (caps capable of very high surge currents, placed very near the PWM switch, and PWM switch has a large freewheeling diode to allow current to continue to flow through any water heater inductance the instant after the PWM switch turns off).
The CC tells the water heater PWM controller box when there is more power available, and the PWM slowly ramps up power to the water heater (at an Amps/sec rate specified by the designer of the CC) until the CC tells the water heater PWM controller to slowly ramp back down.  That communication from the CC to the PWM controller could be a single wire, high to ramp up and low to ramp down.
When the CC is performing an MPPT scan, it could hold the PWM controller at a constant average current to the water heater by toggling the wire with a 50% duty cycle.  The CC might choose to ramp down the current to the water heater before starting the MPPT scan.

Would this not require some serious re-writing of the CC embedded software? Not sure. Can the CC actually be 'aware' of what the max power available from the pv array is, at any moment?
The CC is in current limiting mode during Absorb and Float so would it have to temporarily go back to bulk mppt to test max power availabililty?   I cant see how this could occur often enough to control PWMing the available excess power to the heater without messing up Absorb and Float modes - or have I misunderstood this process?
Perhaps the current limiting output interface of the CC connecting to the batteries can be controlled/monitored separately from the input stage connecting to the PV array? So therefore the input stage can somehow have knowledge of what the array capability is at any time?
If this is the case then an AUX output, single wire, as you suggest could use this PV power availabililty info to control an external PWM controller.

dgd

Quote from: dgd on October 24, 2013, 08:05:40 PM
... consider separate bank for water heater/other DC loads.

PV is cheap, but not yet free.  The electronics to store otherwise wasted power as hot water should be considerably cheaper than an extra 5 kW of panels.

Quote
A Black box processor could easily minimum interface to the CC yet provide more effective measurement of available PV power and adjust the DC load to match.

Perhaps you can glean enough information from the register interface of the CC through a com port to make this work.  But it would be easy to disturb the CC's MPPT scan if you are switching kilowatts of power from the PV panels to an opportunity load without carefully considering what is going on inside the CC.   Not obvious to me how the black box provides a more effective measurement than the CC's MPPT scan, or how you plan to adjust the DC load.

Quote
...  Can the CC actually be 'aware' of what the max power available from the pv array is, at any moment? ...

The CC does an MPPT scan every few minutes if it is not getting all the power it wants.  But you're right, there is a complication here if the PV array is larger than the CC can handle, or if the battery/inverter can't load the CC enough to bring the panels down to Vmp.   I was thinking the water heater could be sucking most of that current when the CC is doing the MPPT scan.  But it should do a full scan of the entire power curve for the panels to avoid getting stuck on a local maxima.  The CC could measure the average water heater current with an external shunt, and control that water heater current with the AUX signal to scan the entire power curve of the PV panels.  That's getting a bit messier than I had first imagined.

Perhaps you're right, and we should simply have a dedicated array for the water heater.  With perhaps a switch to allow us to manually send all panels into the CC when the weather gets gloomy for a few days, and a relatively small waste-not load on the CC for when the weather is sunny.  I think you still need the PWM controller to send a varying amount of power to the water heater, such that the dedicated array panel voltage stays near Vmp.  And eventually something monitoring time-of-day, insolation, and battery state to automate that switch, only allowed to flip it once every hour or so to avoid messing with the MPPT scans?  I guess that could be your blackbox.

Quote from: gridloose on October 25, 2013, 12:20:28 AM

Perhaps you can glean enough information from the register interface of the CC through a com port to make this work.  But it would be easy to disturb the CC's MPPT scan if you are switching kilowatts of power from the PV panels to an opportunity load without carefully considering what is going on inside the CC.   Not obvious to me how the black box provides a more effective measurement than the CC's MPPT scan, or how you plan to adjust the DC load.

Therefore to eliminate risk of disturbing CCs mppt scan it's best to only consider diversion to heater when CC is in float mode.
The BB can use various methods/sensors not available to the CC to measure insolation, at any time, which can then be used to evaluate pv power capability. The BB software then uses this data and loads the pv array appropriately - likely using pwm.

Quote
..  I was thinking the water heater could be sucking most of that current when the CC is doing the MPPT scan.  But it should do a full scan of the entire power curve for the panels to avoid getting stuck on a local maxima.  The CC could measure the average water heater current with an external shunt, and control that water heater current with the AUX signal to scan the entire power curve of the PV panels.  That's getting a bit messier than I had first imagined.

And with the Classic it would need significant firmware changes to achieve this. Since this is not open source we would depend on MN to implement such changes. I won't comment on the likelihood of this happening.

Thanks for your suggestions and ideas on this diversion issue. It's been interesting for me to read about other control options and possibilities.

Dgd

Quote from: dgd on October 25, 2013, 06:12:56 AM
...
Therefore to eliminate risk of disturbing CCs mppt scan it's best to only consider diversion to heater when CC is in float mode.
The BB can use various methods/sensors not available to the CC to measure insolation, at any time, which can then be used to evaluate pv power capability. The BB software then uses this data and loads the pv array appropriately - likely using pwm.
...
And with the Classic it would need significant firmware changes to achieve this.

Yup, we're pretty much on the same page here.  I think the CC firmware changes could be quite trivial if the CC uses a temperature compensated Vmp as the target panel voltage based on a user entered value from the panel data sheet and a temp sensor on the panels, so the CC does not use MPPT.   But that's not a plug and play solution, and probably not something midnite would want to put their name on.  Depending on user configured loads to implement MPPT at power points greater than that which can be handled by the CC is most likely not in the cards as well.

That calculated Vmp approach might work better for you than measuring insolation.

I think a common problem here is that of implementing PWM to a water heater, with power coming directly from PV panels.  You need big enough caps (with high enough surge current capabilities) near the PWM switch such that the panel voltage remains pretty much constant, otherwise the panels will be quite inefficient with the panel voltage a square wave popping far above and below Vmp.  So either a whole bunch of large high voltage caps, or fast PWM.  And with fast PWM you get FET switching losses.  I'm figuring on getting to ~30 kHz, perhaps with four FET's each operating at 7.5 kHz and each handling about a kilowatt.  Avoid long wires from the PWM FET's to the water heater to reduce the inductance of the load, perhaps mounting the FET's on the water heater.  Drive the FET's very hard with a gate driver capable of several Amps to reduce switching losses.  Have large schottky diodes across each FET to allow any inductive current to continue to flow as the FET turns off.  Snubbers at the FET's might be needed, but I'd guess the heater element inductance is low enough to get by without them.

Quote from: gridloose on October 25, 2013, 11:34:14 AM
I think the CC firmware changes could be quite trivial if the CC uses a temperature compensated Vmp as the target panel voltage based on a user entered value from the panel data sheet and a temp sensor on the panels, so the CC does not use MPPT.   But that's not a plug and play solution, and probably not something midnite would want to put their name on.

Does the CC register interface allow us to know when the CC is in MPPT mode, taking all available power from the panels?
Or perhaps midnight could provide a testpoint which is high when the CC is in MPPT mode (preferred, as that is a much simpler interface).

If so, then we can do everything else in this PWM controller for the water heater.

Whenever the CC indicates it is in MPPT mode, the PWM controller ramps power to the water heater down to zero, waits a few minutes for the CC to perform an MPPT scan, then if the CC is still in MPPT mode the PWM controller records the panel voltage for future use as the target Vmp.  When the CC leaves MPPT mode for more than say 30 minutes, the PWM controller ramps up power to the water heater until the panel voltage falls to the Vmp value previously recorded.  The PWM controller then regulates power to the water heater in such a way as to maintain the panel voltage at Vmp.  The PWM controller may or may not include a temp sensor at the panels to allow it to adjust the recorded Vmp value to meet current conditions.

(edit:)  The PWM controller could probably detect when the CC is doing an MPPT scan by monitoring panel voltage, checking for the characteristic pattern.  That assumes the PWM controller is slewing power to the water heater slowly enough.  But having that information come from the CC as previously suggested would be quicker and more robust.

Quote from: gridloose on October 25, 2013, 12:38:23 PM

Quote from: gridloose on October 25, 2013, 11:34:14 AM
I think the CC firmware changes could be quite trivial if the CC uses a temperature compensated Vmp as the target panel voltage based on a user entered value from the panel data sheet and a temp sensor on the panels, so the CC does not use MPPT.   But that's not a plug and play solution, and probably not something midnite would want to put their name on.

Does the CC register interface allow us to know when the CC is in MPPT mode, taking all available power from the panels?
Or perhaps midnight could provide a testpoint which is high when the CC is in MPPT mode (preferred, as that is a much simpler interface).

If so, then we can do everything else in this PWM controller for the water heater.

Whenever the CC indicates it is in MPPT mode, the PWM controller ramps power to the water heater down to zero, waits a few minutes for the CC to perform an MPPT scan, then if the CC is still in MPPT mode the PWM controller records the panel voltage for future use as the target Vmp.  When the CC leaves MPPT mode for more than say 30 minutes, the PWM controller ramps up power to the water heater until the panel voltage falls to the Vmp value previously recorded.  The PWM controller then regulates power to the water heater in such a way as to maintain the panel voltage at Vmp.  The PWM controller may or may not include a temp sensor at the panels to allow it to adjust the recorded Vmp value to meet current conditions.

(edit:)  The PWM controller could probably detect when the CC is doing an MPPT scan by monitoring panel voltage, checking for the characteristic pattern.  That assumes the PWM controller is slewing power to the water heater slowly enough.  But having that information come from the CC as previously suggested would be quicker and more robust.

What you are talking about is "Waste-Not" mode.   Waste not allows the water heater or other load to be the diversion load that
regulates the battery voltage at Absorb or Float or EQ so that the CC does not have to reduce its power output to keep
the voltage from going higher than the regulation set point.

Usually the relative voltage (relative to the Absorb or Float or EQ set point) is somewhat lower  and so those Absorb, Float and EQ voltages only come in to reduce the power when the Waste-Not load can't do the job.  That might happen if the water heater
turns off because the water is already hot.

When the Classic is in MPPT mode, the battery voltage is below the Waste-Not set point and the Aux output is off so the
diversion load is not drawing any power at that time.

boB

Quote from: boB on October 25, 2013, 02:07:27 PM
What you are talking about is "Waste-Not" mode.

Yup.  I dropped the discussion for a month, and now I'm circling back around to what I was thinking then.  I'm still a bit hung up on wanting an oversized array to be useful during bulk, though perhaps that's not really much of an issue.  Another possible issue is that if we have an oversized array, the CC's MPPT can only scan part of the power curve on a sunny day since much of the power is getting shipped off directly to that water heater from the panels.  Perhaps that's not an issue unless the panels get to 5x oversized or so.  A third is that even fast PWM direct from panels to water heater could cause enough voltage ripple to disrupt the MPPT algorithm.  So I was trying to figure out how to only send power to the water heater when the CC was not in MPPT.  I'm probably over thinking this, and could do well to just try using the existing waste-not AUX signal to ramp up that 30 kHz PWM of power from panels to water heater.

Quote from: gridloose on October 25, 2013, 03:48:17 PM

Quote from: boB on October 25, 2013, 02:07:27 PM
What you are talking about is "Waste-Not" mode.

Yup.  I dropped the discussion for a month, and now I'm circling back around to what I was thinking then.  I'm still a bit hung up on wanting an oversized array to be useful during bulk, though perhaps that's not really much of an issue.  Another possible issue is that if we have an oversized array, the CC's MPPT can only scan part of the power curve on a sunny day since much of the power is getting shipped off directly to that water heater from the panels.  Perhaps that's not an issue unless the panels get to 5x oversized or so.  A third is that even fast PWM direct from panels to water heater could cause enough voltage ripple to disrupt the MPPT algorithm.  So I was trying to figure out how to only send power to the water heater when the CC was not in MPPT.  I'm probably over thinking this, and could do well to just try using the existing waste-not AUX signal to ramp up that 30 kHz PWM of power from panels to water heater.

Remember that Waste-Not is intended to be diverted from battery and not PV input voltage.

I wasn't sure which side you were thinking of diverting from.

boB

I may be driving off into the weeds here, but there seem to be a few others with the same goal of putting up a 2x sized array to avoid foul weather generator use, and are then too cheap to just let that power go to waste when the sun's out.  I'd like to store that power in a large insulated basement cistern, use it for hydronic floor heat.  A few have figured out ways to more or less successfully send surplus power direct from the panels to an electric water heater, but all the schemes I've seen thus far look to be either inefficient or overly complicated to get going, often both.  The outline in posts #32 and #33 still seems reasonable the next morning, which for me at least is quite remarkable.  Hard to say where my head will be in a week.  Standard AUX Waste-Not is a potential plan B if the objections of post #35 don't get in the way (would need fast PWM with large caps, ramp the power very slowly to avoid disturbing the CC's MPPT).

Quote from: gridloose on October 26, 2013, 11:22:08 AM
I may be driving off into the weeds here, ...

Well, if you are off in the weeds, this is out of left field:

Maybe the easiest and simplest thing to do is grab an el cheapo Chinese inverter in an European configuration of 240? volts 50 Hz for your battery bank. Use a relay to toggle it on and off by whatever means and criteria you choose. Drive whatever loads you desire from AC like common everyday water heater elements  or AC resistance heaters for heating. A few losses but..

Personally, I would heat water and the rest would go to space heating in cooler weather. Or just let it go to waste.

Sadly, I don't have this problem of excess power much except in mid summer and long days.

Just thinking into the keyboard.

Tom

Quote from: TomW on October 26, 2013, 12:37:35 PM
Maybe the easiest and simplest thing to do is grab an el cheapo Chinese inverter in an European configuration of 240? volts 50 Hz for your battery bank. Use a relay to toggle it on and off by whatever means and criteria you choose. Drive whatever loads you desire from AC like common everyday water heater elements  or AC resistance heaters for heating. A few losses but..

That's hardly left field thinking.  Most successful attempts at heating water from PV I see posted here involve sending it through the CC and battery and inverter as you suggest.  But if I have a 4 kW water heater, that's probably twice as big as any other loads I plan to burden the system with, so everything would have to be sized much bigger.  If I can go direct from panels to water heater with $50 in electronics (and 6 months of screwing around, it's a hobby!) that's a big win.  If you do use the el cheapo inverters unattended, be sure to fuse them well and put them in a fireproof box.  I've seen them blow for no apparent reason in most spectacular fashion, amazing how quickly and thoroughly a bank of lead-acid batteries can destroy a few FET's.

Quote from: gridloose on October 26, 2013, 02:14:17 PM
  I've seen them blow for no apparent reason in most spectacular fashion, amazing how quickly and thoroughly a bank of lead-acid batteries can destroy a few FET's.

I hope they (inverters) were well protected by CB or fuse at the battery,  as you comment about using a fuse implies...??

Quote from: gridloose on October 26, 2013, 02:14:17 PM
amazing how quickly and thoroughly a bank of lead-acid batteries can destroy a few FET's.

I have witnessed a few spectacular failures involving Fire Emmitting Transistors over the years.

T

Quote from: gridloose on October 26, 2013, 02:14:17 PM

If I can go direct from panels to water heater with $50 in electronics (and 6 months of screwing around, it's a hobby!) that's a big win.

And just to say how easy that is...
Use dc SSR, snubber diode and decent blocking diode. SSR control to aux1 set to Float High. Use SSR to switch in heater to output from pv array - or just part of array that has block diode between array and CC. Leave enough of array direct to CC  to maintain float with normal loads.
Aux1 has a few seconds off delay that stops SSR chattering.

> I may be driving off into the weeds here, but there seem to be a few others with the
> same goal of putting up a 2x sized array to avoid foul weather generator use, and
> are then too cheap to just let that power go to waste when the sun's out.

Thats about an apt a summary of what we are trying to do as well, as youll ever get.

And I also am suprised at how hard it is to do. I refuse to tie up that much inverter capacity. But we have short distances and lots of fat wire laying around, so the plan here is to use DC elements, 100A DC SSR, and waste not.

But ill be interested on hearing how your pv diversion method works out.

...and using aux1 float high for heater controlmeans aux2 could be used as input for WBjr so ending amps will probably terminate Absorb sooner with over sized array. Hence reducing potential pv power waste during Absorb.  :)

Quote from: gridloose on October 26, 2013, 02:14:17 PM
That's hardly left field thinking.  Most successful attempts at heating water from PV I see posted here involve sending it through the CC and battery and inverter as you suggest.

I too live completely off-grid, and faced this issue.
I hate to say it, but for hot water, PV really isn't the way to go IMHO.
With typical panel conversion efficiencies of what... around 15%? Then a few more losses in the charging process, and a few more going back to AC, and a few more in the cables, I reckon you get perhaps 10% of the suns energy as degrees rise in your water.

I purchased some evacuated tubes (60), a manifold and frame. It heats the water directly. A small circulating pump and a $12 temperature controller complete the setup. Close to 90% of the available sun goes in to heating the water.

I know, comparing apples to oranges - you can't use the hot water to charge your batteries, and this discussion is probably more about "using surplus PV", but sometimes its easy to focus too hard on one solution and miss another.

Quote from: dgd on October 26, 2013, 03:17:08 PM

Quote from: gridloose on October 26, 2013, 02:14:17 PM

If I can go direct from panels to water heater with $50 in electronics (and 6 months of screwing around, it's a hobby!) that's a big win.

And just to say how easy that is...
Use dc SSR, snubber diode and decent blocking diode. SSR control to aux1 set to Float High. Use SSR to switch in heater to output from pv array - or just part of array that has block diode between array and CC. Leave enough of array direct to CC  to maintain float with normal loads.
Aux1 has a few seconds off delay that stops SSR chattering.

That might work well enough, but is not ideal.
Those panels probably won't be operating anywhere near Vmp most of the time unless you vary the power to the heater.
Also, the CC is trying to do MPPT tracking while you are switching in and out that power to the heater.

Quote from: RossW on October 26, 2013, 03:45:04 PM
I hate to say it, but for hot water, PV really isn't the way to go IMHO.
With typical panel conversion efficiencies of what... around 15%? Then a few more losses in the charging process, and a few more going back to AC, and a few more in the cables, I reckon you get perhaps 10% of the suns energy as degrees rise in your water.

I purchased some evacuated tubes (60), a manifold and frame. It heats the water directly. A small circulating pump and a $12 temperature controller complete the setup. Close to 90% of the available sun goes in to heating the water.

I know, comparing apples to oranges - you can't use the hot water to charge your batteries, and this discussion is probably more about "using surplus PV", but sometimes its easy to focus too hard on one solution and miss another.

That 90% efficiency on the evacuated tubes seems rather high, can you point to a data sheet?
I'd guess the Y intercept is likely below 50%, and with a 100 F delta T (ambient to water temp) it drops another 10 or 15 %.
And then you still have all the plumbing losses.

The 15% efficiency of PV is only an issue if you lack the real estate.
What I care about most is dollars per watt.

PV efficiency goes up as ambient temps fall, so works best when you need it most.
Solar hot water efficiency falls with ambient temps.

When I investigated solar hot water a couple years ago, I concluded that PV was about as good on dollars per watt if you wanted it to work at ambient temps of 30 F or so.  And that's dedicated PV panels into an electric hot water heater, never mind that electricity can be put to many other uses.

If we have enough PV for lights and fridge and washer here during the winter, we will have plenty of PV hot water most days in spring summer and fall.  Most solar hot water systems don't do much good in winter.  (Your evacuated tubes might be an exception though).

Quote from: gridloose on October 26, 2013, 11:40:32 PM

Quote from: dgd on October 26, 2013, 03:17:08 PM

Quote from: gridloose on October 26, 2013, 02:14:17 PM

If I can go direct from panels to water heater with $50 in electronics (and 6 months of screwing around, it's a hobby!) that's a big win.

And just to say how easy that is...
Use dc SSR, snubber diode and decent blocking diode. SSR control to aux1 set to Float High. Use SSR to switch in heater to output from pv array - or just part of array that has block diode between array and CC. Leave enough of array direct to CC  to maintain float...

That might work well enough, but is not ideal.
Those panels probably won't be operating anywhere near Vmp most of the time unless you vary the power to the heater.
Also, the CC is trying to do MPPT tracking while you are switching in and out that power to the heater.

i agree but you did say $50  ;)
Surprisingly it does appear to work quite well... its takes some matching of heater wattage to available diverted strings wattage and CC seems to not get confused at it sees strings not diverted as better optimal V and A for continued battery charging.
dgd

Quote from: gridloose on October 27, 2013, 12:50:41 AM
That 90% efficiency on the evacuated tubes seems rather high, can you point to a data sheet?

Don't have the sheets of mine handy, but I spent a fair while comparing various tubes.

Here's the first one I found on the 'net.
http://endless-solar.com/evacuated-tube-efficiency.htm (http://endless-solar.com/evacuated-tube-efficiency.htm)

Mine are wet tubes, which when I was doing all the numbers, were somewhat more efficient than the heat-pipe type, but had other downsides. Fortunately, the downsides that were mentioned were not an issue for my system because I was able to design around them.

My actual measured efficiency wasn't far off the mark either. I didn't have a flow-meter on my circuit, but the temperature difference between water in and out and the flow rate of the pump with the measured head came to about 86% conversion of the "available" power measured by my pyranometer at the time. Plenty of scope for errors though, because I was measuring the power available in the cosine-corrected pyro, comparing to the calculated sun angle, re-correcting that for the actual alignment of the evac tubes and then comparing to the temperature-delta*flow-rate method of determining input power.

Quote from: RossW on October 27, 2013, 04:40:27 AM

Quote from: gridloose on October 27, 2013, 12:50:41 AM
That 90% efficiency on the evacuated tubes seems rather high, can you point to a data sheet?

Don't have the sheets of mine handy, but I spent a fair while comparing various tubes.

Here's the first one I found on the 'net.
http://endless-solar.com/evacuated-tube-efficiency.htm (http://endless-solar.com/evacuated-tube-efficiency.htm)

Mine are wet tubes, which when I was doing all the numbers, were somewhat more efficient than the heat-pipe type, but had other downsides. Fortunately, the downsides that were mentioned were not an issue for my system because I was able to design around them.

My actual measured efficiency wasn't far off the mark either. I didn't have a flow-meter on my circuit, but the temperature difference between water in and out and the flow rate of the pump with the measured head came to about 86% conversion of the "available" power measured by my pyranometer at the time. Plenty of scope for errors though, because I was measuring the power available in the cosine-corrected pyro, comparing to the calculated sun angle, re-correcting that for the actual alignment of the evac tubes and then comparing to the temperature-delta*flow-rate method of determining input power.

Choosing evacuated tubes is definitely better than flat plate collectors if you want solar hot water in winter.
Could well pencil out to be cheaper than PV for hot water in many cases.
But they pretty much have to be roof mounted on the house to avoid plumbing losses.  Worst case when offgrid is winter, I'd like collectors somewhere between 60 deg and vertical, and able to deal with high winds.  For me, it's a lot more convenient to park a whole bunch of PV panels out of sight 100 yards away, and run power back to the house over some 10 gauge wire at 200 volts or so. 

That datasheet you pointed to shows "normalized" efficiency with no explanation, which is to say it was probably put together by the marketing department.  An honest graph would not show the flat plate collector to have a lower efficiency than evacuated tube at small deltaT's, even if the curves are "normalized". 

The graph on the top right of page 4 here:
    http://www.homepower.com/view/?file=HP123_pg66_Marken
compares some flat plate with evacuated tube designs.  With collector water at ambient air temp the evacuated tube is shown to be around 50%, falling to about 25% when the difference in temperature gets to 100 F (say 20 F outside, and 120 F hot water).  There are plenty of yeah-buts here that could effectively raise those figures, such as the round tubes collecting heat for more hours in a day than a flat plate, or if you figure just the area of the tubes themselves rather than the entire assembly, but I suspect 86% is more than a bit high for a complete system including plumbing and such.  This webpage suggests around 80% transmission of the sun's heat energy through a single pane of uncoated glass:
    http://www.commercialwindows.org/shgc.php

Quote
i agree but you did say $50  ;)
Surprisingly it does appear to work quite well... its takes some matching of heater wattage to available diverted strings wattage and CC seems to not get confused at it sees strings not diverted as better optimal V and A for continued battery charging.
dgd

Those big cyllinders have two elements right? If you used two, and sized them at a bit less than half the array, you you bring them on line in staged manner.

This is the one i had my eye on. 0.7 ohms, 1000W: http://ecoinnovation.co.nz/p-631-07-ohm-water-element.aspx .Gasp at the price if you want *everything* is expensive here.

Quote from: zoneblue on October 27, 2013, 05:30:33 PM

Those big cyllinders have two elements right? If you used two, and sized them at a bit less than half the array, you you bring them on line in staged manner.

Yes they do - some even have 3 element  positions. One is usually at bottom of cyclinder and other nearer top. The top is to provide hot water in smaller volume for immediate use instead of waiting for lower element to heat whole tank of water.
I had considered enabling a top 1kw element first then the lower if excess power continued. I just use lower element position as 200 litre tank gets hot in a few hours.

Quote
This is the one i had my eye on. 0.7 ohms, 1000W: http://ecoinnovation.co.nz/p-631-07-ohm-water-element.aspx .Gasp at the price if you want *everything* is expensive here.

..too expensive for me. I bought one similar to this one  from Ebay

http://www.ebay.com/itm/Camco-2000-Watt-120V-High-Watt-Density-Water-Heater-Screw-In-Element-/400581805927?pt=LH_DefaultDomain_0&hash=item5d44894767
Note element only no thermostat.

and used this for water temp  control
http://www.jaycar.co.nz/productView.asp?ID=ST3821&form=CAT2&SUBCATID=969#1

Placed it direct to top of tank with heat sink compund and used plastic wire ties to hold in place. It is in wire path from Aux1 to SSR.

and these as blocking diode  :D
http://www.ebay.com/itm/261260112924?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649

The Crydom 20A ssr i use also from ebay. I seem to remember the lot was less than USD50 no including postage..

I didn't have an HWC so trademe was were I found good price mains pressure one and valve set. My HWC now feeds Rinnai LPG water heater. I checked that Rinnai monitored incoming 'cold' water feed so it did not activate gas heater if incoming water temp was already high enough. It did. Now I use less than a 13kg bottle of LPG in 12 months instead of two 45Kg bottles worth every 8 months.  :) :)

dgd

Quote from: dgd on October 27, 2013, 06:58:33 PM
Now I use less than a 13kg bottle of LPG in 12 months instead of two 45Kg bottles worth every 8 months.  :) :)

that's quite a reduction!  looks like somewhere between 15% to 20% of previous consumption...

Quote from: Westbranch on October 27, 2013, 07:13:39 PM

that's quite a reduction!  looks like somewhere between 15% to 20% of previous consumption...

It was also saving $ because the LPG supplier would not fill 'user owned' 45Kg bottles so I had to pay rent each month for use of their bottles. I terminated that deal  a year ago (they didn't even bother to pick up the bottles - yet!)  and bought a 13Kg that I could get filled almost anywhere.

Quote from: dgd on October 27, 2013, 06:58:33 PM
...  used this for water temp  control
http://www.jaycar.co.nz/productView.asp?ID=ST3821&form=CAT2&SUBCATID=969#1

One thing to keep in mind.  The contacts on that thermostat are rated for 240 VAC, nothing said about DC.
With AC, the voltage goes to zero once each half cycle, so with 60 Hz line freq that's 120 times per second.
The voltage is at zero long enough for any arcing to break and the ozone disperse a bit to where when it comes back
there is no more spark.  With DC, the contacts might continue arcing indefinitely.  Circuit breakers such as the SquareD QO series that are rated for both AC and DC generally are considered good at DC for only a fraction of the AC voltage.
    http://static.schneider-electric.us/docs/Circuit%20Protection/Miniature%20Circuit%20Breakers/QO-QOB%20Circuit%20Breakers/0730CT9801R108.pdf

So maybe best to wire the thermostat to the control side of that SSR?
One would hope that they would fail burned open, but I could imagine them welding closed.

Quote
Now I use less than a 13kg bottle of LPG in 12 months instead of two 45Kg bottles worth every 8 months.  :) :)

Very cool.  That does indeed work well enough.  I'm tempted.

But I think the scheme I outlined a few posts back with PWM to the heater and monitoring the PV voltage to check for a pattern indicating the CC is performing an MPPT scan and recording the Vmp, that could be built into a nice cute box that mounts at the water heater, wired to the PV panels with blocking diode into the CC, and just work with no tuning of any sort (assuming the panel voltage is up high enough to get enough power into the heater element).  And operate at close to optimum efficiency in bulk, absorb, float and equalize.
So a turn-key solution with no head scratching, possibly work with other CC's as well with a firmware change to match the MPPT pattern.

Quote from: gridloose on October 27, 2013, 07:32:49 PM

Quote from: dgd on October 27, 2013, 06:58:33 PM
...  used this for water temp  control
http://www.jaycar.co.nz/productView.asp?ID=ST3821&form=CAT2&SUBCATID=969#1

One thing to keep in mind.  The contacts on that thermostat are rated for 240 VAC, nothing said about DC.
With AC, the voltage goes to zero once each half cycle, so with 60 Hz line freq that's 120 times per second.
...
So maybe best to wire the thermostat to the control side of that SSR?
One would hope that they would fail burned open, but I could imagine them welding closed.

Yes that is what I do, this switch interrupts the control from the Classic AUX1 to the SSR. So its only dealing with 12v about 10ma max.
I checked with the supplier tech dept to see if this was ok for their AC switch and they assured me it was and if it ever failed to bring it back for replacement. 8)

Quote
But I think the scheme I outlined a few posts back with PWM to the heater and monitoring the PV voltage to check for a pattern indicating the CC is performing an MPPT scan and recording the Vmp, that could be built into a nice cute box that mounts at the water heater, wired to the PV panels with blocking diode into the CC, and just work with no tuning of any sort (assuming the panel voltage is up high enough to get enough power into the heater element).  And operate at close to optimum efficiency in bulk, absorb, float and equalize.
So a turn-key solution with no head scratching, possibly work with other CC's as well with a firmware change to match the MPPT pattern.

I had thought about your scheme and maybe all it would take is the Classic to signal via an AUX1 output or perhaps setting a modbus register when it is doing an mppt scan. The register may not be the way as it would need to be continually polled as no interrupt processing would be possible, BUT the aux1 could prompt a BB to read the voltage over next 20 or so seconds to get the high Pv reading then the BB could go from there.. :P

dgd

Quote from: dgd on October 27, 2013, 08:07:30 PM
I had thought about your scheme and maybe all it would take is the Classic to signal via an AUX1 output or perhaps setting a modbus register when it is doing an mppt scan. The register may not be the way as it would need to be continually polled as no interrupt processing would be possible, BUT the aux1 could prompt a BB to read the voltage over next 20 or so seconds to get the high Pv reading then the BB could go from there.. :P

I'm warming to the thought of inspecting the PV voltage for the MPPT scan pattern.  Then the only connections needed into this PWM box are power from the panels, power to the heater element, heater tank temp sensor, and perhaps a circulation pump SSR if heating a larger cistern.  This box needs to watch the PV voltage anyway, as it is trying to send only enough power to the heater to hold the panel voltage at Vmp.  A TI MSP430 Launchpad at $12 would give all the processing power and the ADC as well.  I distrust solutions involving more than a billion or so transistors, often having another billion little flash cells of electrostatic charge slowly leaking off toward senility.

Edit: That billion transistor thing does not necessarily apply to the BB, and the BB could well do a fine job of this and perform other tasks as well.  But an MSP430 in a DIP20 costing around $1US (single unit, from Mouser or Digikey!)  should have all we need here for the water heater PWM.  At $12, the Launchpad gives a complete development system including the USB cable to your laptop.  Unfortunately TI dropped the ball a bit on packaging the software tools, it's non-trivial to get that first blinky light going even after you get through the 50 megabyte download and install.  Silly.

Quote from: dgd on October 27, 2013, 06:58:33 PM
..too expensive for me. I bought one similar to this one  from Ebay
http://www.ebay.com/itm/Camco-2000-Watt-120V-High-Watt-Density-Water-Heater-Screw-In-Element-/400581805927?pt=LH_DefaultDomain_0&hash=item5d44894767
Note element only no thermostat.

Are you sure that one for 27v dgd? 2000W at 120V is  7 ohms. At 27v thatll be about 100W.

Quoteand used this for water temp  control
http://www.jaycar.co.nz/productView.asp?ID=ST3821&form=CAT2&SUBCATID=969#1

Placed it direct to top of tank with heat sink compund and used plastic wire ties to hold in place. It is in wire path from Aux1 to SSR.

Quite the diode, do you have a wire diagram of all this? Sounds interesting.

QuoteI didn't have an HWC so trademe was were I found good price mains pressure one and valve set. My HWC now feeds Rinnai LPG water heater. I checked that Rinnai monitored incoming 'cold' water feed so it did not activate gas heater if incoming water temp was already high enough. It did.

Ive asked several califont reps this question and all i get is blank stares or fudged answers. It seesm clear that the cheaper ones dont regulate gas pressure, are either on or off, so not good for this.  One guy told me that the bosch or better grade do regulate gas pressure, but only in a band. if you put in 30C water it boosts it to temp nicely. But if you put in 45C water it gets confused and heats it to 80. Gas cant go low enough. SO then you have to put in a temper valve to cut the top off. What model rinnai is yours?

Quote
Now I use less than a 13kg bottle of LPG in 12 months instead of two 45Kg bottles worth every 8 months.  :) :)

Nice job. Our gas usage is the last bastion.

Quote from: zoneblue on October 28, 2013, 11:08:49 PM

Are you sure that one for 27v dgd? 2000W at 120V is  7 ohms. At 27v thatll be about 100W.

Its no good for 24v but then I'm connecting it to the output of PV's, 5 to a string = about 88V mpv.  That is about 1Kw with this element

Quote
Quite the diode, do you have a wire diagram of all this? Sounds interesting.

I will draw up a diagram and get it posted here...

Quote
Ive asked several califont reps this question and all i get is blank stares or fudged answers. It seesm clear that the cheaper ones dont regulate gas pressure, are either on or off, so not good for this.  One guy told me that the bosch or better grade do regulate gas pressure, but only in a band. if you put in 30C water it boosts it to temp nicely. But if you put in 45C water it gets confused and heats it to 80. Gas cant go low enough. SO then you have to put in a temper valve to cut the top off. What model rinnai is yours?

I'm using an old Infinity 22. Seems to be ok with taking water from a tank that is already hot enough although it does start up the gas burner but turns it off almost immediately. I still use a tempering valve out of the HWC which keeps the water to 55c even though the HWC can get to 60c.
The Rinnai pdf online manual recommends in a solar system where a tank feeds the continuous flow heater that the heater is set to 75c
and tempering valve then drops it to 50c. Its for bacteria control of water from the tank (eg legionaires). I might revisit this sometime with my setup but it would probably mean more LPG use. Also I think an HWC at 60c may be sufficient for basteria control.

Quote
Nice job. Our gas usage is the last bastion.

Same here.

Quote from: TomW on October 26, 2013, 12:37:35 PM
Maybe the easiest and simplest thing to do is grab an el cheapo Chinese inverter in an European configuration of 240? volts 50 Hz for your battery bank.

I tend to go for lowest price, figuring everybody else is just ripping me off.
But that's not always the case.  Here's an inverter story that should scare you good:
  http://ludens.cl/Electron/chinverter/chinverter.html
Lots of other interesting stuff on that website if you are into electronics.

I've used a bunch of PowerBright and Wagan modified square wave inverters (I refuse to call them modified sine wave) with reasonably good luck.   Keep all such gear away from any moisture (such as a puff of powdered snow).  Expect fireworks when you eventually hook them up backwards to you car battery some dark night.  Mostly work well enough, but there are reasons for building inspectors to insist on UL 1741 certified gear.