48v 1000 Watt Heating Element with Adjustable Thermostat

[SolarMusher]

Hi all, I've seen this 48Vdc heating element with thermostat on Missouri wind&Solar website (http://www.mwands.com/index.php?main_page=product_info&cPath=40&products_id=493). It could be interesting for those who would want to divert solar excess power and do not want to use AC power from inverter to heat/preheat a water heater.
I would like to use two of these elements to power a used electric tank ahead of my propane water heater and control them with a DC SSR on Classic Aux2 waste not PWM. Now, with a DC thermostat, I think it could work fine but from what I've read in other threads, I'm in doubt. Is there a problem to PWM a DC SSR?
What do you think of that? This could be an inexpensive option to heat water.
I have also an old Tristar C60 that I could use to divert directly from battery without SSR but would prefer doing it using Classing Waste Not.
As usual, your inputs are very appreciated.
Thanks,
Erik

[dgd]

Never had any problems with aux2 pwm into a DC ssr.  The ssr will get warm so needs a heat sink and of course a breaker to the battery. I would try with one element first and ssr like Crydom d1d100
100v @ 100A, I would probably give each element it's own ssr. The classic aux can drive two Ssrs.

Dgd

[SolarMusher]

Posted by: dgd
« on: Mars 13, 2013, 02:51:30 pm »Insert Quote

I would probably give each element it's own ssr. The classic aux can drive two Ssrs.

Dgd

Hi David,
I think that I will follow your SSRs setup. I would like to begin with 2x 700/800W elements and keep it at around 1500 max.
If I use 2x 1500W/120V elements, it would give me 1500W total on 48Vdc battery (59,2V absorb), is this correct?.
Or do you think it could be more efficient to connect only the bottom element with a 3000W/120V?
Could you explain how you can set the Classic to drive two SSRs? I do not follow.
Erik

[dgd]

Eriik,

If you half the voltage then you roughly get a quarter the power rating, so a 120v 1.5 kw element would be about 375watts with 60volts.
(1.5kw at120v=12.5amps=9.5ohms element, 60v puts 6.25amps into 9.5ohms giving 375watts)
So it may be better to use a 120v element rated at 3kw to get 750watts at 60v
You just connect the DC inputs to the Ssrs in parallel to the aux2 output on the Classic.
I would start with one ssr and element and see how it goes then if power still available maybe put in second element.
When power scarce I would just open breaker to one of them, or else insert a miniature switch in the aux to ssr wire and open it so ssr not enabled by classic.

Dgd

[SolarMusher]

If you half the voltage then you roughly get a quarter the power rating, so a 120v 1.5 kw element would be about 375watts with 60volts.

Oops sorry, bad math! I could have a used tank for...free  . The plumber told me that it has already 2x 3000W/120V elements, so it would be perfect!

You just connect the DC inputs to the Ssrs in parallel to the aux2 output on the Classic.

If I read the Crydom correctly, I should use 4 and 3 for pos/neg from Aux2, 1 for DC+ from bat and 2 for DC+ to WH. Is that right?
Erik

[dgd]

Erik,
Yes the existing two 3Kw elements would be perfect       that wiring for the ssr looks right.  The crydom web pages for D1Dxx series have a wiring diagram,  note it shows a diode across the load to stop voltage spikes, include this in your wiring.
A schottky diode is good, ebay..
The D1D20 would be ok for your 3kw element, cheap on eBay 

dgd

[SolarMusher]

A schottky diode is good, ebay..

Hi David,
The only diode I've found which could match amp/voltage is this one: http://www.ebay.com/itm/LOT-OF-10-WIND-GENERATOR-SOLAR-40-AMP-BLOCKING-DIODE-/250742187967?pt=LH_DefaultDomain_0&hash=item3a616627bf
It could be ok for D1D20 or D1D40 SSRs. Where do you put it, in battery/SSR or in SSR/WH?
Would you have a picture on how is your SSR wiring and diode?
Erik

[jp_le_gros]

Hi, that diode do not have to "take" all the power, it's there to control high voltage create by rapid discharge of a inductive load, like a large coil.
JP

[SolarMusher]

Hi JP,
So what do you think a good size would be (volt/amp) for this schottky diode?
Erik

[jp_le_gros]

Erik, as i understand, your load will be two 3kW (at 120V) heater (about 25 amps at 48 V by each) and you plan to drive those with some D1 series Crydom SSR.  If it's true, the diode in the circuit is there for protecting the SSR output from overvoltage create by turning off inductive load.  So, you could use, as indicated by  dgd a schotkky diode, but where i'm worked till last december, we generally use zener or transorb to do the same job.  With those, the protection is in both side... my recommandation for you is something like a 1.5KE100.
JP

[SolarMusher]

Thanks a lot JP, but now that I 've read Chris post on NAWS, I'm really confused about using DC to power these elements. Problem is that nobody really wants to explain why and where AC would better or more efficient if batteries are close to WH. From what I've read here and there, it seems that adding a cheap inverter to power it on AC is the way to go. I don't understand why people here at Midnite are not more helpful (no offense here, please) on aux2 waste not uses. It would be a major reason (one more) for people to buy a Classic though. I know the crew is busy but...
I also believe that Aux2 diversion should be by far more important than a local app from an off-grider point of view...
Just me,
Erik

[dgd]

Erik,
With DC elements the distance from the battery to the element needs to be considered.
Unless fairly close to each other, a few metres, then the DC cabling will need to be large enough size to reduce voltage loss. If the elements are  30 metres away then the cable cost would be significant,  30m x2 of #2 is not cheap. So it is then much easier to use AC to keep cabling costs low.
The use of a cheap inverter just stops loading main inverter which already my be quite busy.  A modified sine type is great on resistive loads like elements.

If your battery and elements are reasonably close then DC seems least costly.
The is no difference in efficiency and the elements don't care if fed DC or AC
The Classic aux will  drive an AC  or DC relay so long as the control inputs are DC.

Your idea of perhaps some info on aux applications, maybe with examples is good.
Not sure MN should do it, liability and all that stuff..

Dgd

[SolarMusher]

Erik,
With DC elements the distance from the battery to the element needs to be considered.
Unless fairly close to each other, a few metres, then the DC cabling will need to be large enough size to reduce voltage loss. If the elements are  30 metres away then the cable cost would be significant,  30m x2 of #2 is not cheap. So it is then much easier to use AC to keep cabling costs low.
The use of a cheap inverter just stops loading main inverter which already my be quite busy.  A modified sine type is great on resistive loads like elements.

If your battery and elements are reasonably close then DC seems least costly.
The is no difference in efficiency and the elements don't care if fed DC or AC
The Classic aux will  drive an AC  or DC relay so long as the control inputs are DC.

Your idea of perhaps some info on aux applications, maybe with examples is good.
Not sure MN should do it, liability and all that stuff..

Dgd

Hi David, I could even put this secondary WH close by 36 Inches to the battery box, so DC voltage drop would not be an issue.
What would be your schottky diode choice to protect the Classic? JP thinks a 1.5KE100 could do the job, what do you think about it?
I would like to be sure to get it right.
Was thinking about AC SSR using a dedicated inverter but the only one that could match 1500 element would be a Cotek/Samlex 1500/48 inverter no charger and I'm afraid that it would be too short to power 1500W for 4/5 hours regularly, I have not Seen/found serious 2000W/48V unit even in MSW model (Ebay, Amazon).
There were comments on NAWS that recommend against DC SRR for safety/contact (arc) reasons and it seems to be a consensus, I don't understand why and where using a quality Crydom DC SSR which was built especially for this use would be dangerous  .
Why contacts on WH could be a problem at 60Vdc??? It's over my head!
Could this problem occur when the Classic PWM the diversion? I'm pretty sure that you never have experimented this kind of problem on your DC set-up.
Not a drop of liability here, we're just talking  .
Erik

[TomW]

Erik;

I wonder if the issue they are concerned with is the contacts welding shut or arcing over @ the higher DC voltages?

Seems the switches for that kind of use are "special" with protection from arcing and contact welding? A fused limit switch could be very catastrophic in nature.

Perhaps there are limit switches available for this application?

Just an idea.

Tom

[SolarMusher]

Erik;

I wonder if the issue they are concerned with is the contacts welding shut or arcing over @ the higher DC voltages?

Seems the switches for that kind of use are "special" with protection from arcing and contact welding? A fused limit switch could be very catastrophic in nature.

Perhaps there are limit switches available for this application?

Just an idea.

Tom

Tom, yes you're right, that's the main problem but I've never seen a safe switch for that specific use. I wonder if using Float on Aux1 (on/off switch) wouldn't be a better option for DC direct use fom batteries to supply a full steady 1500W on a voltage range. I'm not sure that Aux1 Float would worth the cost of an efficient WH/elements/SSR when only 2 or 3 hours on float are available.
Maybe waste not on Aux1? My concern would be to not overcharge the battery with that option which would result in extending the bulk charge time and therefore absorb time.
So to resume, water heating wouln't be possible/safe on PV/batt DC and very costly on AC as it needs a dedicated inverter to work  .
I Think a last option would be to go with Nyle Geyser pump 600/800W through an AC/SSR on Aux1 opportunity or Float only as Mtdoc
had proposed.
Erik