Diversion ...

[Vern Faulkner]

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?

[TomW]

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

[boB]

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

boB

[zoneblue]

Bob,

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

Cheers

[TomW]

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

[boB]

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

[offgridQLD]

 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

[Vern Faulkner]

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?

[zoneblue]

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.

[boB]

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

[gridloose]

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

[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

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

[zoneblue]

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

[gridloose]

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.

[zoneblue]

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.

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).

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.

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.

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.

So 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.