pv dump load to water heater on midnite solar classic?

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  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    Re: pv dump load to water heater on midnite solar classic?
    gridloose wrote: »
    I doubt it. 10 volts out of 200 is 1 volt across each of 10 panels strung in series, where each panel has 20 volts across it.
    A quick look at the curves for a typical PV panel meant for a 12v lead acid battery show a very little loss when running 1 Volt away from a Vmp near 20 Volts.
    And 10 volts should be more than plenty, the PWM ripple could be brought down to under a volt as shown in a previous post.

    Edit: corrected to read 10 panels, not 20

    I have 2 arrays with Vmp about 95V. When it goes over a 100V, the production drops dramatically. At 105V, there's almost nothing.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: pv dump load to water heater on midnite solar classic?
    NorthGuy wrote: »
    I have 2 arrays with Vmp about 95V. When it goes over a 100V, the production drops dramatically. At 105V, there's almost nothing.

    Which is exactly what should be expected from a current-based power source.

    Vmp is a term which really only applies to panel specs, not to the Voltage point chosen by an MPPT controller for maximum power. It means "Voltage at maximum power" and relates to the panel meeting its Wattage rating at Vmp * Imp. As far as a PV is concerned Imp comes first, then Voltage. It is able to maintain Imp up to the Vmp rating, after which V point the current will drop off.
  • Vic
    Vic Solar Expert Posts: 3,208 ✭✭✭✭
    Re: pv dump load to water heater on midnite solar classic?

    grid ..,

    Sometimes links have a limited lifetime on some Forums, but here is one that raises many of the issues. If you follow links to other discussions on other sites, this is a good background on some of the issues:

    http://forum.solar-electric.com/showthread.php?19126-Classic-water-heater-diversion&highlight=waste+not

    As you can see, this Thread is in off-grid on this site, and is called Classic water heater diversion and began on March 13, 2013, in case the above link expires. Believe that most link probs are caused by links to sites from a Search Motor.

    FWIW, Enjoy, Vic
    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH X2@48V, 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
  • gridloose
    gridloose Solar Expert Posts: 40
    Re: pv dump load to water heater on midnite solar classic?

    NorthGuy wrote: I have 2 arrays with Vmp about 95V. When it goes over a 100V, the production drops dramatically. At 105V, there's almost nothing.

    Hmm
    Here's a panel picked at random from our hosts:
    http://www.solar-electric.com/solarworld-sunmodule-sw250-monocrystalline-solar-panel.html
    Click "Details" and download the spec sheet pdf, turn to page two, see graph in middle of left column .
    The sheet says Vmp at STC is 31.1 Volts, and Imp is 8.05 Amps.
    Find that point in the graph for the blue line at 1000 W/m2, then move 1.6 volts to the right, or 1/6 the distance between 30 and 40 Volts.
    I see a current of around 7.5 Amps, suggesting we lose less than 10% from maximum power.
    (I'm moving 1.6 volts to the right, cuz 31.1 volts is about 1.6 times the 20 volts of the original discussion.)

    If what you say is accurate, I'm left puzzled as to where the missing power might be going.
    As the battery becomes charged, the CC is no longer sucking current, and the panel voltage should be jumping up toward Voc.

    Perhaps there is a panel overvoltage protection scheme in your CC kicking in that shunts away some current if it doesn't need it?

    Perhaps the power curves on your panels are drastically different?
    At 5% over Vmp, what percentage of Imp do those curves show the expected current to be at?
  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    Re: pv dump load to water heater on midnite solar classic?
    gridloose wrote: »
    Perhaps the power curves on your panels are drastically different?
    At 5% over Vmp, what percentage of Imp do those curves show the expected current to be at?

    No. I have exact these panels. Except I have 255, not 250, but I don't think it's any difference.

    The curve isn't stable. It changes with temperature, so Vmp may be higher or lower. This has an effect of shifting curves to the left or to the right. You can aso see that when insolation drops, the curves move to the left.

    See how these curves move down quickly to the right. If you operate in this area, very little increase of voltage causes dramatic drops of current. If you want to pick up a voltage that is guaranteed to be higher than the MPPT voltage chosen by the controller, you'll be in this area. If the curve moves left a bit, you loose most of the production.
  • gridloose
    gridloose Solar Expert Posts: 40
    Re: pv dump load to water heater on midnite solar classic?

    Vic suggested this thread:
    http://forum.solar-electric.com/showthread.php?19126-Classic-water-heater-diversion&highlight=waste+not

    I had browsed that thread, just did a thorough read.
    The original post is talking about DC off the battery to the water heater, most of the thread then becomes a discussion of running a water heater off an AC inverter.
    There is some mention of using DC from the panels, mostly saying nobody has figured out how to make it work.
    They run into plenty of trouble just running off the battery or through an inverter, and DC from the panels will be significantly more difficult than either.
    But I think it can be done, and is worth doing.

    boB hops in briefly with this notable tidbit with regard to Aux2 PWM mode:
    "Also, I would tend to pick 1V width. that is around 500 Hz PWM. 5V width is around 100 Hz PWM frequency."

    As stated before, I think the switching freq from Aux2 PWM is too low to use in front of the CC,
    I'd be happiest up around 30 kHz.

    With natural gas coming on so cheap, a craigslist electric water heater in good condition is almost free.
    Would be nice to find a way to use them offgrid effectively with $50 worth of extra electronics.
  • gridloose
    gridloose Solar Expert Posts: 40
    Re: pv dump load to water heater on midnite solar classic?

    NorthGuy,

    You're probably right about the curves not being stable enough.
    I did mention compensating our Vmp for PWM with measured panel temp, I believe in the first post.
    But perhaps some other approach is needed to determine when to turn on PWM, and what duty cycle it should run at.

    On the Classic, perhaps this scheme:
    The separate PWM controller electronics package monitors the waste not 10-500 Hz PWM signal to determine the duty cycle
    that the Classic wants us to run at, and re-creates a 30 kHz PWM signal of the same duty cycle to drive the FET with.

    I like that. Would like it even better if the Classic were to have a 30 kHz PWM option for Aux2.
    I think it would work. And I'd be happy to buy a Classic.

    But am still interested in a generic method that would work with other MPPT controllers such as Outback.
    Perhaps monitoring current from the CC toward the battery/inverter is part of the solution?
    The PWM duty cycle to the heater is some adaptive linear function of PV voltage and CC current?
    The word linear in there suggests to me that it could play well with an unaware generic MPPT algorithm in the generic CC.

    There's almost certainly a way.
  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    Re: pv dump load to water heater on midnite solar classic?
    gridloose wrote: »
    But am still interested in a generic method that would work with other MPPT controllers such as Outback.
    Perhaps monitoring current from the CC toward the battery/inverter is part of the solution?
    The PWM duty cycle to the heater is some adaptive linear function of PV voltage and CC current?
    The word linear in there suggests to me that it could play well with an unaware generic MPPT algorithm in the generic CC.

    There's almost certainly a way.

    You can measure battery voltage. Turn your heater on if it is above threshold, and turn it off if it is below your threshold. Set your threshold 0.3V (or as close as you reasonably can) below CC's absorb/float voltage. You need something to create some hysteresis. You will also need a big low pass filter to remove all the inverter ripple. With big enough filter you can get away with very low switching frequency and eliminate nearly all switching losses.

    There must be diversion load controllers that already do that.
  • gridloose
    gridloose Solar Expert Posts: 40
    Re: pv dump load to water heater on midnite solar classic?

    NorthGuy:

    Your scheme is simple, should work, and is a lot better than throwing away all power generated while the CC is in float.

    But you're wasting a lot of power from the panels as the current tapers off from C/10 at the start of the absorb cycle to close to zero Amps at the end of the absorb cycle. And in my case, where I plan to have an oversize array of panels for cloudy winter weeks, I won't even be using all the available current during the bulk charge cycle either when in full sun. I'd be much more worried about those losses than the 5% or so lost to 30 kHz transistor switching. At 5% of 4KW that's 200 Watts, and I have no intention of working with transistor heat sinks any bigger than that. Maybe I should just bolt those transistors to the water tank? ;-)
  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    Re: pv dump load to water heater on midnite solar classic?
    gridloose wrote: »
    Your scheme is simple, should work, and is a lot better than throwing away all power generated while the CC is in float.

    But you're wasting a lot of power from the panels as the current tapers off from C/10 at the start of the absorb cycle to close to zero Amps at the end of the absorb cycle. And in my case, where I plan to have an oversize array of panels for cloudy winter weeks, I won't even be using all the available current during the bulk charge cycle either when in full sun. I'd be much more worried about those losses than the 5% or so lost to 30 kHz transistor switching. At 5% of 4KW that's 200 Watts, and I have no intention of working with transistor heat sinks any bigger than that. Maybe I should just bolt those transistors to the water tank? ;-)

    No. It'll do exactly what you wanted to do, but is not charger specific.

    If it is not enough energy to reach the threshold voltage, the heater will be off.

    If it is enough energy to hold the threshold voltage with your heater, the heater will be on (be careful what you wish for).

    If it is something in between, it'll be switching. The frequency depends on your hysteresis and the magnitude of low pass filter. You cannot switch faster than 120 Hz, because it is a strong ripple from the inverter, so your feedback is inherently slow. But something like 20-30 Hz should work fne.

    If you do not do the filter, you will be switching at 120 Hz as the battery voltage changes with the inverter ripple. This may even be a good thing.
  • gridloose
    gridloose Solar Expert Posts: 40
    Re: pv dump load to water heater on midnite solar classic?

    NorthGuy:

    > If it is not enough energy to reach the threshold voltage, the heater will be off.

    So, exactly what threshold are we talking about? The threshold between absorb and float? Even with a small panel array, much of the power available during absorb gets thrown away since the current into the battery will be tapering off toward zero as it reaches the threshold. And in my case my battery bank may be way down in absorb, several volts below the threshold, the CC dumping C/10 into the battery, and my oversized bank still generating plenty of unused power
  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    Re: pv dump load to water heater on midnite solar classic?
    gridloose wrote: »
    NorthGuy:

    > If it is not enough energy to reach the threshold voltage, the heater will be off.

    So, exactly what threshold are we talking about? The threshold between absorb and float? Even with a small panel array, much of the power available during absorb gets thrown away since the current into the battery will be tapering off toward zero as it reaches the threshold. And in my case my battery bank may be way down in absorb, several volts below the threshold, the CC dumping C/10 into the battery, and my oversized bank still generating plenty of unused power

    The controller regulates battery voltage during absorption. Say, 59V. You take this task out of controller's hand and start regulating with your heater. You choose a voltage close to controller, but not quite. Say, 58.7V. If voltage goes above 58.7V you turn on the heater. This sinks the voltage, it starts go down. Once it goes down enough, say to 58.6V, you turn your heater off. The voltage will start rising. As soon as it hits 58.7V, you turn your heater back on, and battery voltage goes down. And so on ... With 10-30 Hz frequency.

    This will keep the heater off until absorbtion is reached (aka 58.7V), then it starts switching, effectively directing all the excess energy to the heater. When controller manages to overcome your heater it'll rise the voltage to 59.0V and continue with the absorption. At this point, since the voltage is above 58.7V, your heater will be on.

    Makes sense?
  • gridloose
    gridloose Solar Expert Posts: 40
    Re: pv dump load to water heater on midnite solar classic?
    NorthGuy wrote: »
    The controller regulates battery voltage during absorption. Say, 59V. You take this task out of controller's hand and start regulating with your heater. You choose a voltage close to controller, but not quite. Say, 58.7V. If voltage goes above 58.7V you turn on the heater. This sinks the voltage, it starts go down. Once it goes down enough, say to 58.6V, you turn your heater off. The voltage will start rising. As soon as it hits 58.7V, you turn your heater back on, and battery voltage goes down. And so on ... With 10-30 Hz frequency.

    This will keep the heater off until absorbtion is reached (aka 58.7V), then it starts switching, effectively directing all the excess energy to the heater. When controller manages to overcome your heater it'll rise the voltage to 59.0V and continue with the absorption. At this point, since the voltage is above 58.7V, your heater will be on.

    Makes sense?

    Makes sense.
    Probably works well enough, better than the battery charger in my pickup for example.
    Sounds like the CC would always think it is in the absorption phase.

    But with big bucks invested in batteries, most will want all the high end bells and whistles
    of the CC's charging algorithm available, That would be difficult to duplicate.
    Take a look at the charging curves here:
    http://batterytender.com/resources/battery-basics.htm/
    Note that the "float" voltage is less than the voltage near the end of "bulk".

    And I'd still be wasting power from my oversized panel array in sunny weather even down at the start of the bulk charge,
    since it is sized to well over my battery's C/10 so I can get some power for lights during cloudy winter weeks.

    I think waste-not from the Classic's Aux port somehow controlling a 30 kHz PWM of power direct from the PV panels into the heater is the way to go.
    Might need a PID (proportional-integral-differential) algorithm inside the PWM control electronics to stabilize the control loop, given all the time delays involved.
    Solving the case for generic MPPT CC's would be cool, but perhaps should wait till the above scheme is working.
  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    Re: pv dump load to water heater on midnite solar classic?
    gridloose wrote: »
    Sounds like the CC would always think it is in the absorption phase.

    No. If it was thinking it's in absorption, it would cut off the power and wouldn't follow the MPPT point. However, you do not let it reach the absorption voltage. Every time it gets close to what it think is absorption, you throw your heater on it. Therefore, it continues to think it is in bulk mode and try to follow the MPPT point. So, you get all the benefits of MPPT.

    It will continue doing so until it overcomes your heater and gets to its absorption voltages, but at this point, even with your heater, you don't have enough loads to take all the energy.

    When the absorption ends and it goes to fload, you simply turn your heater on. But be careful not to overheat your heater.
    gridloose wrote: »
    And I'd still be wasting power from my oversized panel array in sunny weather even down at the start of the bulk charge,
    since it is sized to well over my battery's C/10 so I can get some power for lights during cloudy winter weeks.

    What's your max charge rate with your array. You say it's over C/10, but what exactly is it?
    gridloose wrote: »
    I think waste-not from the Classic's Aux port somehow controlling a 30 kHz PWM of power direct from the PV panels into the heater is the way to go.
    Might need a PID (proportional-integral-differential) algorithm inside the PWM control electronics to stabilize the control loop, given all the time delays involved.
    Solving the case for generic MPPT CC's would be cool, but perhaps should wait till the above scheme is working.

    What I'm telling you will work with any controller. Of course, it does involve voltage measurement and requires some circuicity, so it is not that simple as wiring SSR to the PWM output.
  • gridloose
    gridloose Solar Expert Posts: 40
    Re: pv dump load to water heater on midnite solar classic?

    >> Sounds like the CC would always think it is in the absorption phase.

    > No. If it was thinking it's in absorption, it would cut off the power and wouldn't follow the MPPT point.

    Yes. I meant bulk. Really. These terms aren't quite second nature to me yet.

    I've got 240 AH at 48 V of battery now.
    Bought used, and getting badly manhandled through the house building process.
    Would like to eventually invest in 500 AH of good new batteries, once I've practiced on
    and destroyed these eight old T-125's.

    I bought a pallet of those Sunelec Evergreen ES-E's, so have 4400 Watts.
    Currently have 6 of them up on a temporary rack, more than plenty for several saws and a compressor
    in this clear fall weather. If I put them all up, that's up around 4400/48 = 91 Amps.

    Water heater elements go to 6KW or more, would take close to my 4.4KW at 200 Volts.
    So one water heater element could deal with the entire panel farm.
    I could run your scheme and not leave bulk for weeks, if I ever get around to setting up
    that large insulated cistern for hydronic floor heat. But for domestic hot water and a reasonable
    amount of sun, you're right, the heater would eventually reach temp and shut down to allow
    the CC to continue on to absorption. If the CC makes it into float, your heater trip point best be set
    below the float voltage, which is well below the voltage seen at the end of bulk stage.
  • gridloose
    gridloose Solar Expert Posts: 40
    Re: pv dump load to water heater on midnite solar classic?

    Post #13 of this thread reports success driving a hot water heater direct from PV using Aux2:
    http://midniteforum.com/index.php?topic=727.0
    But he's just turning on a solid state relay with Aux2 to drive the water heater.
    No attempt at matching the water heater load to the power currently available from the panels.
    And no real insight into exactly what Aux2 is doing or any analysis of how it might effect the CC's MPPT.

    Would be nice if an Aux could be programmed to simply tell us that the CC is sending all the Amps it wants
    off to the battery, so there is surplus power available from the panels. This signal would be true when
    the CC hits a battery current limit setpoint, or when the battery is near full charge. We could suck power
    from the panels to the heater whenever this signal is true, gradually ramping up the current by changing
    that 30 KHz PWM duty cycle until we see this Aux signal go false. At that point we ramp the current to
    the heater back down till it goes true again. And then go back to ramping up, ad infinitum.
    The CC's manual should spell out the maximum rate at which the diversion current from the panels
    can change in Amps/sec.

    Keep it simple.
  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    Re: pv dump load to water heater on midnite solar classic?
    gridloose wrote: »
    I've got 240 AH at 48 V of battery now.
    Bought used, and getting badly manhandled through the house building process.
    Would like to eventually invest in 500 AH of good new batteries, once I've practiced on
    and destroyed these eight old T-125's.

    I bought a pallet of those Sunelec Evergreen ES-E's, so have 4400 Watts.
    Currently have 6 of them up on a temporary rack, more than plenty for several saws and a compressor
    in this clear fall weather. If I put them all up, that's up around 4400/48 = 91 Amps.

    That's really a lot of panels compared to batteries. You probably do need to limit current, but it's much better to simply use the controller function instead of building a separate diversion controller. You'll have huge amount of power when it's sunny. It'll be enough to keep a lot of hot water in the storage. When it's very cloudy you still won't have enough.

    If your batteries are sized to your loads (except water heater, which you do not run at night) then you do not need a bigger bank. However, if they're too small for your loads, it'll be a lot of troubles living with such small batteries. If you go to 500AH or so to match your loads, you will no longer be over-paneled.

    How big are you loads?
    gridloose wrote: »
    Water heater elements go to 6KW or more, would take close to my 4.4KW at 200 Volts.
    So one water heater element could deal with the entire panel farm.

    4.4 kW can boil 50 gallons in an hour (or heat 100 gallons to good hot water temperatures). How much hot water do you need anyway?[/QUOTE]
  • gridloose
    gridloose Solar Expert Posts: 40
    Re: pv dump load to water heater on midnite solar classic?

    Primary electrical loads are a fridge and a couple freezers.
    That could grow, we're not yet living in the house.
    If the hot water went into hydronic floor heat, we can use pretty much all we can get Nov-Feb.
    As the other threads show, there are many ways to pursue hot water.
    I may just hang a 1500 Watt element off our inverter like normal people, shut it down when the
    battery is not fully charged. But I've now got my teeth in this direct-from-pv project,
    it's a fun challenge, and could be a much better solution.

    I think direct-from-pv can be done well if the Classic would somehow tell us when it is tracking
    and when it is just skimming some Amps off the top. Waste-not by name suggests it should
    tell us this, but the documentation in the manual is not convincing. Will poke further through the
    other threads to see if anybody really knows what's going on under waste-not.
  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    Re: pv dump load to water heater on midnite solar classic?

    To heat a house you need much more energy than you have.
    gridloose wrote: »
    I think direct-from-pv can be done well if the Classic would somehow tell us when it is tracking
    and when it is just skimming some Amps off the top. Waste-not by name suggests it should
    tell us this, but the documentation in the manual is not convincing. Will poke further through the
    other threads to see if anybody really knows what's going on under waste-not.

    The people who developed Classic frequently post here, They also have their own Classis forum. You can create a new thread ask the direct question about how their "Waste Not" works. This should attract their attention.
  • gridloose
    gridloose Solar Expert Posts: 40
    Re: pv dump load to water heater on midnite solar classic?

    NorthGuy:

    > To heat a house you need much more energy than you have.

    Time will tell.
    We're straw bale, tight, wood stove, lots of southerly sun coming in.
    Occasional hydronic heat in just two or three relatively small areas would be nice.
    I once did heat loss calculations, found that a 12x12x8 foot cistern with 1' of foamboard insulation
    in the basement that gets heated to 140F in Oct could heat the house through a cloudy Nov
    of the typical ~30F'ish weather. (Not that I have any immediate plans of doing that.)
    And when it gets to 10 below, we typically have snow on the ground and bright sun,
    and of course some very efficient PV panels.

    Could add an outside wood boiler as backup.
    And/or at $0.50/Watt, add bunches more panels, in which case direct from PV is mandatory.
    And power the new fangled electric car if all that new battery technology ever arrives.

    I'm not necessarily being rational here.
    Rational would be a grid tied house in suburbia.
    But in my book, this is far more interesting than sitting back watching football with a beer.

    Will dig around for additional nuggets of real information on waste-not.
    Thanks for all the advice, it's been truly helpful.
  • gridloose
    gridloose Solar Expert Posts: 40
    Re: pv dump load to water heater on midnite solar classic?

    Post #5 of this thread has some of the missing details with regard to the hows and whys and why nots of waste-not on the Classic
    http://www.midniteforum.com/index.php?topic=890.0
  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    Re: pv dump load to water heater on midnite solar classic?
    gridloose wrote: »
    Post #5 of this thread has some of the missing details with regard to the hows and whys and why nots of waste-not on the Classic
    http://www.midniteforum.com/index.php?topic=890.0

    Looks like what you need, but it still won't work on limit your current during bulk.
  • gridloose
    gridloose Solar Expert Posts: 40
    Re: pv dump load to water heater on midnite solar classic?
    NorthGuy wrote: »
    Looks like what you need, but it still won't work on limit your current during bulk.

    Yup.

    There were a few sharp curves, but I think I figured out all he was trying to say.
    Except just what the heck's going on regarding Aux2's PWM duty cycle and frequency.
    But I'm no longer planning to use Aux2 PWM anyways.
    I'll post a response to the Midnite thread cited, see if there's any interest in
    finding a better way to do waste-not. In particular, see if there's any interest
    in having the Classic emit a signal to show when it is using MPPT.
    Will probably have it up within a day or so, once my registration gets through.

    Did you notice at the start where he said Bulk but probably meant Absorb?
    Makes me feel much better.



    Here's a waste-not algorithm that might work with any generic MPPT controller:

    Add external electronics to monitor current and voltage from the panels,
    use a microcontroller to calculate how much power is coming from the panels.
    The external electronics slowly ramps up the duty cycle of the 30 kHz PWM
    to the heater until it detects that total power from the panels is dropping,
    then ramps down from that point to perhaps 5% less power, then repeats.
    The ramps up and down should be slow enough to have this inspection rate be
    slower than the MPPT inspection rate happening inside the charge controller.

    So we're doing our own simple minded MPPT at the same time
    that the charge controller might be actively running it's MPPT.
    If the charge controller has it's MPPT active, our algorithm soon finds
    there's no extra power to be had from the panels and pretty much gives up.
    If the charge controller is getting all the power that the battery wants,
    then our MPPT will grab any excess power for the waste-not load.
    If the battery then needs additional power, the charge controller will
    have to wait till our waste-not PWM backs off the 5% before the
    charge controller can take it.

    There are potential issues. Could be local maxima in the panel power curve
    due to shading or mismatched panels. Or unexpected trouble due to user loads
    switching in and out. But I suspect it could be made to work as well as
    most of the other waste-not schemes I've looked at.
    .
    Primary parts cost would be the caps and FET and FET gate driver as previously discussed.
    So under $50 in parts, but figure $500+ if anybody ever builds a low volume product
    like this that jumps through all the hoops to meet UL and FCC approval.
    An appropriate MSP430 microcontroller with A2D conversion and programmable flash
    can be had for under a dollar these days. Enough math and logic going on here that it
    wants to be a microcontroller, not op amps and comparators and such.
  • gridloose
    gridloose Solar Expert Posts: 40
    Re: pv dump load to water heater on midnite solar classic?

    Northguy:

    You once said:
    > No. If it was thinking it's in absorption, it would cut off the power and wouldn't follow the MPPT point.
    > However, you do not let it reach the absorption voltage. Every time it gets close to what it think
    > is absorption, you throw your heater on it. Therefore, it continues to think it is in bulk mode and try to
    > follow the MPPT point. So, you get all the benefits of MPPT.

    That's starting to make a lot of sense.
    My objection at the time was that you were hanging the CC up in bulk, and those expensive batteries
    might suffer for never seeing absorption, float, and equalize.

    But you could raise the CC's programmed float voltage to get it out of the way and out of mind.
    And set your waste-not battery voltage trip point to a voltage equal to what should be the float voltage.
    We now miss the top of bulk and all of absorb, which I believe only means that the last 20% of charge
    is happening at the float voltage, and thus will take much longer. So might even be easier on
    the batteries. If that last 20% of charge needs to be fast, could just get a slightly bigger battery.

    All that's missing is equalize. Might be important if the battery never sees the absorption voltage.
    Could manually turn off the waste-not load once every few weeks for equalize to happen.
    Or even automate that, should be relatively easy.

    The waste-not load could be tripped by a relay if it's on the battery (or inverter),
    since the battery can smooth things out for us.
    Should have some hysterisis in the waste-not trip voltage to avoid relay chatter,
    so one voltage setpoint to turn on the load and one setpoint a bit lower to turn it off.

    If the waste-not load is at the panels, I think we need high speed PWM with a duty cycle that slowly
    ramps up until the trip point is reached, then slowly back down till the trip goes away.
    No need for hysterisis in this case, since we're always chattering at 30 kHz anyway.
    But may need lots of high surge caps across the panels, enough to smooth out the PWM from
    the panel voltage to a low enough voltage ripple that it does not disturb the CC's MPPT.
    Would be nice if the CC designers could spec a maximum value for that ripple.
    A good chance that the CC has a low pass filter in the voltage sense (with a corner at ~1 Hz?),
    so this may be mostly moot.

    If the Midnite guys were to add the "MPPT-Active" Aux output feature (with the configurable on/off delays
    of waste-not to avoid relay chatter?), everything works the same except we could use that for the trip point.
    And we now get the top of Bulk, plus Absorb, Float and Equalize happening as god intended.
  • gridloose
    gridloose Solar Expert Posts: 40
    Re: pv dump load to water heater on midnite solar classic?

    Oh, right.
    The other objection was that this does not divert any of my excess panel capacity
    when the CC's bulk mode current limit is reached. Still doesn't, and for me that's
    a significant issue. An "MPPT-Active" Aux output from the CC would be very nice.

    But your solution would be an easy way to implement waste-not on most any MPPT CC.
  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    Re: pv dump load to water heater on midnite solar classic?
    gridloose wrote: »
    We now miss the top of bulk and all of absorb, which I believe only means that the last 20% of charge is happening at the float voltage, and thus will take much longer. So might even be easier on
    the batteries. If that last 20% of charge needs to be fast, could just get a slightly bigger battery.

    Float is the mode that you use when batteries are already charged. You need to somehow pass the energy to the loads, and your CC is connected to batteries. CC tries to regulate it so that all current goes to the inverter, and none to batteries.

    Absorption is the mode where current into batteries is higher than they can take. This causes bubbling, which mixes up the electrolyte, which would otherwise stratify. You must do absorptions at least once a week or so, or your batteries die.

    Bulk is when batteries are discharged and therefore can take lots of energy, so the voltage simply cannot reach absorption level.

    If you want so much from the controller, you can build your own, which you can control as you wish. It'll probably be more expensive than a stock unit, but not by much. Won't be UL approved neither.
  • gridloose
    gridloose Solar Expert Posts: 40
    Re: pv dump load to water heater on midnite solar classic?
    NorthGuy wrote: »
    Float is the mode that you use when batteries are already charged. You need to somehow pass the energy to the loads, and your CC is connected to batteries. CC tries to regulate it so that all current goes to the inverter, and none to batteries.

    Absorption is the mode where current into batteries is higher than they can take. This causes bubbling, which mixes up the electrolyte, which would otherwise stratify. You must do absorptions at least once a week or so, or your batteries die.

    Bulk is when batteries are discharged and therefore can take lots of energy, so the voltage simply cannot reach absorption level.

    If you want so much from the controller, you can build your own, which you can control as you wish. It'll probably be more expensive than a stock unit, but not by much. Won't be UL approved neither.


    I think occasionally equalizing might create enough bubbles.
    Lots of people go away, leaving their systems sit for months on float.
    We could take this to equalize once a week or so.

    I've got plenty to learn about batteries.
    But I'm not yet convinced that this would not work pretty well.
    Except for wasting power when current limited in bulk.

    Yes, I've looked at building a controller from scratch.
    Designing a 5kW full bridge switching power supply is non trivial, lots to learn first
    about magnetics and FET switching losses and snubbers and such. And batteries.
    Even people who've done it for a career will spend months getting it to work well,
    with occasional 5kW craters in the board where a FET once was.
    That PWM circuit might be plenty for me to mess with.

    All I want from the controller is an "MPPT-Active" signal coming out.
  • gridloose
    gridloose Solar Expert Posts: 40
    Re: pv dump load to water heater on midnite solar classic?

    Could put the CC"s Absorb and Float voltage setpoints both a bit above the target Absorb level.
    Build our PWM circuit for the two different setpoints. First it goes to Absorb, then shifts down to float
    when the Absorb current gets to some minimum value. Just like the big boys. Easy enough.

    Might add Equalize in there too if the CC's unused Absorb and Float setpoints can be raised high enough,
    though many choose to go to Equalize with manual intervention anyways. Make it part of regular
    battery maintenance, along with checking the electrolyte.

    Gives us a generic waste-not that treats the batteries well, can be used with most any MPPT CC.
    Unfortunately, this does not send excess power to waste-not when the CC is current limited in Bulk,
    an issue for me but not for most.
  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    Re: pv dump load to water heater on midnite solar classic?
    gridloose wrote: »
    the CC is current limited in Bulk, an issue for me but not for most.

    I think the issue is that your bank may be too small. You may have troubles living through the night without discharging batteries too much. That is just my quess.

    If you get to 500AH, you won't need to limit current.
  • gridloose
    gridloose Solar Expert Posts: 40
    Re: pv dump load to water heater on midnite solar classic?

    Good point.
    We might be good through the night given the expected loads.
    Through a bunch of cloudy days, certainly not.