using excess power to heat water (For Dummies)

[ClassicCrazy]

He had a paper today with equivalent ohm ratings of various AC hot water heating elements. I was not able to stay for his whole talk.

OTOH if a higher DC voltage direct from PV array can rerouted to the element then lower cost AV rated elements can be used and respectable watts power used to heat water.
In my home system I use 90volt mpv PV strings (either 5 by 18v 140W or 3 by 250W 30v PVs).
To keep things simple when float is reached two 750w PV strings are diverted to the element using AUX1 float high to enable an SSR that connects the element to the PVs outputs.  A diode commoned 3phase rectifier between the SSR and Classic prevents the Classic trying to feed power to the element.

dgd

dgd  -  Could you elaborate on that wiring from two of your PV strings to the water heater element ?
  Is it like this
PV strings to rectifier to SSR to water heater element ?
You tie all the input on 3 phase rectifier together and use it only on the positive wire ?
Could you  sketch up a schematic for this ?
Thanks
Larry

[kitestrings]

He showed all the numbers he crunched and PV was less and do it  yourself easier.  The big benefits for him were just being able to run a wire in to the house instead of insulated pipes full of glycol , pumps, etc...
So without the cost of batteries or inverter it does seem to make sense for water heating.

Larry, I expected it would only be a matter of time before this started to make sense.  The price of the panels has really dropped a lot over the past few years, and as you say it is a much simpler install.  The cost and dead weight (and they are) is still the batteries in a system, so if you can avoid having to add more batteries as in this application, I can see where that would have appeal.  Hope to read this soon.  Thanks for sharing this article.

[kauaisolarman]

He had a paper today with equivalent ohm ratings of various AC hot water heating elements. I was not able to stay for his whole talk.

OTOH if a higher DC voltage direct from PV array can rerouted to the element then lower cost AV rated elements can be used and respectable watts power used to heat water.
In my home system I use 90volt mpv PV strings (either 5 by 18v 140W or 3 by 250W 30v PVs).
To keep things simple when float is reached two 750w PV strings are diverted to the element using AUX1 float high to enable an SSR that connects the element to the PVs outputs.  A diode commoned 3phase rectifier between the SSR and Classic prevents the Classic trying to feed power to the element.

dgd

dgd  -  Could you elaborate on that wiring from two of your PV strings to the water heater element ?
  Is it like this
PV strings to rectifier to SSR to water heater element ?
You tie all the input on 3 phase rectifier together and use it only on the positive wire ?
Could you  sketch up a schematic for this ?
Thanks
Larry

yes please elaborate on this if possible.

this link shows solar panels running direct to water heater element, but no 3 phase rectifier in the schematic.
http://waterheatertimer.org/Convert-AC-water-heater-to-DC-water-heater.html

[kauaisolarman]

You've just reinforced for me why DC makes more sense.  It's worked well for us since ~2007.  If you only need 1,000 watts then your only talking 40A when it is fully engaged.  Much of the time it will be something less than that.  They are readily available in the 900-1200 w range IIRC.  With DC elements you're not affecting the inverter at all, and you can use excess power from both of your arrays - well, assuming they are both tied to the one bank - you really are just taking would be lost energy once the bank is satisfied.  I can imagine buzzing LED lights, and potential noise on audio & electronics with AC, not to mention what toll it might take on the inverter long-term.

For us, on a sunny weekday when the kids are off to school and my wife and I are working, the batteries might be well into absorb cycle by late morning.  If they're calling for only ~500 watts, there is another 2.5 kW available for diversion.  There's something to be said for the KIS approach IMO.

The direct array approach sounds promising, and should be interesting to follow, but in my mind it's more of an alternative approach, say like considering a solar (thermal) water heating system.  Here you already have the investment in panels based on the electrical needs, and you're just looking to leverage more of the available energy.

~ks

thanks for the input KS.  are you running the DC off your batteries with AUX 2 PWM connected to SSR for dumping during absorb and float, or are you just setup for voltage connect and disconnect?

can you specify how you have your system setup?

does AUX 1 have the same PWM and wast not HI as AUX 2?

[kitestrings]

are you running the DC off your batteries with AUX 2 PWM connected to SSR for dumping

Yes, that's right.  We're using Waste-Not Hi on Aux2 on one of our two Classic's.  Aux1 does not PWM.  It is just a relay action.


Minor correction, but diversion, which is what we're collectively trying to do with hot water, is not really dumping.  Not to split hairs, but folks sometimes consider them to be the same.  Dump loading is usually a method of charge control, very often used in wind applications, but also solar with certain types of controllers.  Here we are letting the Classic do it's normal three-stage charging, but setting a voltage hysteresis relative to the set-points where we can divert energy when available.

~ks

[TomW]

sorry tom i dont understand exactly what you are referring to.  are you recommending not running a SSR with either AC or DC power?  i though the only way for AC is through the inverter and only way for DC was directly from the battery bank?

what exactly do you mean by "A.C. or the D.C. side of the inverter"?

Just saying that pulse width modulating the DC into an inverter is not going to work like you might think. Same with the AC out.  An inverter is much different than a heating element in how it works.

This does not apply to a simple on / off switching of an SSR or mechanical relay.

Maybe you should research a bit into how a PWM and SSR work.

It may not matter but I just would not do either. I can't even imagine what pulsing an AC source with an SSR would actually do.


I just have to comment when I see statements that don't seem to make sense.

Its your gear so best of luck with it.

Tom

[kitestrings]

Tom,

I agree that there may be some unintended consequences of switching AC elements out of an inverter.  I'm not sure that I agree that we're switching DC "into an inverter", but maybe I missed the reference.  At least in our case, we're switching/pulsing the heating (DC) elements in our water pre-heat tank. This is no different than PWM on the bank, or circuits in any number of applications, such as PWM lighting, charging, dumping/clipping for wind...I don't see where it involves the inverter.

~ks

[WillEert]

I have located the SSR between the inverter AC output and the HWT element. All the instrumentation shows is the amps to the tank element gradually changing as the SSR pulse on / off width alters. The voltage remains constant. I am just a learner but it seems OK to me.

Will

[kitestrings]

Will,

It sounds like you've work thru the obstacles, but I guess every approach has some.  With your set-up you have were not able to use the MS PWM.  I'm not sure as you mentioned what you went to - was it something homemade, or another diversion controller?  You also used a dedicated inverter and custom, electronic thermostats.  Sounds like no small challenge.

We originally kept the stock, inter-locked control sequence of the original tank, but eventually switch it to where both elements are pulsed (as I think you've done) so when more power is available it can be used.

[australsolarier]

as to my experience with pwm- ing  after the inverter:

my solar controller is speed controlling a 100w pump for the solar hot water circulation between tank and the collector. this regulates between 100% and 30%, which probably alludes to the speed. on the lower sides of the percentage it will definitively flicker an incandescent  lightbulb hooked to the inverter. but does not seem to create problems for the computer, monitors, etc.
the inverter is a small 2000w victron inverter. and i agree, it is probably not a good thing to do it. having said that the inverter has been running it for the last two years or so. i do not know the frequency of the pwm of the solar controller.

[dgd]

DGD, how or why does a 3 phase rectifier work on a single phase setup?

The 3 phase bridge rectifier is used to prevent current flowing out of the Classic into the element.
The 3 AC inputs are commoned together and the PVs +ve output connected to them.
The rectifier's DC +ve output goes to Classic PV+input
This means three power diodes  are used to protect the Classic's +ve pv input.
Usually the rectifier's body has a bolt hole, making it easy to mount in a  box or on heatsink if necessary.
dgd

[kauaisolarman]

again thanks for the input everyone.

next question.

i have my absorb stage set to end with ending amps from the WBjr shunt connected to aux 2. but,  I want to use aux 2 to run a pwm so i can connect the SSR and have "opportunity" load available to be during absorb stage.

should i disconnect the WBjr and just run a timed absorb stage and use AUX 2 with SSR and "waste not HI" to allow for harvesting of opportunity load,

or,

should i leave aux 2 with WBjr so my battery bank can end absorbtion stage on a set amps and run a seperate PWM controller (morningstar tristar ts-60)  off the battery bank to control the SSR for my needs? IS THIS EVEN POSSIBLE?

will both these options provide the same results and allow me to start harvesting opportunity load during the absorb stages?

of course assuming we are using water heating elements to use the load

thanks again

[WillEert]

Hi Kitestrings,

I started a topic in the Arduino section of the forum in order to build my own controller. Dgd and Paul Alting were very helpful to me as I learned Arduino C coding and put the hardware togeather. I do not want to highjack this topic but you can read about my learning and progress in the topic in that section of the forum.

Will

[kauaisolarman]

again thanks for the input everyone.

next question.

i have my absorb stage set to end with ending amps from the WBjr shunt connected to aux 2. but,  I want to use aux 2 to run a pwm so i can connect the SSR and have "opportunity" load available to be during absorb stage.

should i disconnect the WBjr and just run a timed absorb stage and use AUX 2 with SSR and "waste not HI" to allow for harvesting of opportunity load,

or,

should i leave aux 2 with WBjr so my battery bank can end absorbtion stage on a set amps and run a seperate PWM controller (morningstar tristar ts-60)  off the battery bank to control the SSR for my needs? IS THIS EVEN POSSIBLE?

will both these options provide the same results and allow me to start harvesting opportunity load during the absorb stages?

of course assuming we are using water heating elements to use the load

thanks again

[ClassicCrazy]

Did you ever find out if you can use Whizbang on one Classic and use the AUX 2 from the other Classic - assuming you are going to be using two Classics in Follow Me mode ?

If it was me I would use ending amps with the Whizbang to make sure my batteries were full so any power diversion didn't affect that.

Larry