5000W/ 48V water heater dump load element [RE-wrenches]

Tue Dec 26 11:00:09 PST 2006

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Hiya Dan and All,
Since you are in the design stage here, I'd remind you that you need  
TWO controller load scenarios for shunt load regulation as per NEC  
690.72 (B)(1). I agree with David that dumping DC as the primary  
regulation is a much better scenario for a good charging regime  
because you can use a PWM controller like the Tristar, C-40, or  
Enermaxer. I've found the trick to not overcharging the battery is to  
set the regulation point fairly low. Around 55.2VDC for a 48V system  
where the batts are full or nearly full most of the time.
I also like David's DC water heating element. Too bad it comes with  
NPT instead of the threads that just screw into most water heater  
tanks. Unfortunately, that makes it unusable without plumbing  
contortions and even then, proper placement in the side of most tanks  
is hard. Pity there isn't a NPT/Whatever it is adapter bushing.
For the required secondary dump, I generally use the voltage  
controlled relay on the FX, VFX, or (SW in the old days...) to turn a  
SS relay such as Continental Industries makes to dump ac from the  
inverter to a cheap space heater or an ac water heating element. The  
ac elements are available in both 120 and 240V versions. I use a  
4500W/240V element in my home system. When fed 120V, it dumps about  
900W. Perfect for me. Ain't ohm's Law fun?
If you plan to make this regulation scheme your primary dump, the  
trick is to set the on voltage somewhat higher than you would in a  
PWM scheme because once that voltage is reached all charging stops.  
There is no absorption phase. For decent charging in a 48V system,  
I'd start with about 59.4 and dump 6-7 volts before turning it back  
off. You'll have to play around with those settings based on the  
charging sources to battery capacity ratio.
Best of Holidays to you all.
Amor y Paz,
bob-O

On Dec 26, 2006, at 8:42 AM, Dan Brown wrote:

>
>
> Thanks for all the input everyone.
>        At this point we're still pretty much in the design phase of  
> this
> project. First off -- yes, the 10K can spend over a third of it's  
> time regulating,
> (and they already use over 400kwh/mo.)
>         the  care taker / main brain of this project is an ex- 
> engineer now
> general contractor. I'm just a lowly system installer.  he's the  
> one pushing for
> a hot water heater thingy in line with the boiler. I agree, but  
> envision
> something more like a tarn heat storage system with multiple  
> inputs. the dump load
> from the XL-10 being just one of them. (I'd also like to see them  
> burning
> wood -- they're sitting on like 300 acres of woods, and are totally  
> reliant on
> propane.) or maybe even a dump load / heating element that fits  
> into their
> existing boiler. right now i have them set up to use two 1200w 110v  
> oil
> filled radiant heaters triggered (thru a relay) by an OB aux out.  
> not big
> enough.
>         anyway, they say they're considering all options,  and the  
> owner is
> receptive to efficiency, and environmental responsibility issues.  
> but cost does
> need to be considered.
>         the care taker dude was pushing for a conventional 240v hot  
> water
> heater running off the inverters. cheaper --yes. controllable? --  
> well, we do
> have two more OB aux outs to use. but running that much freight  
> thru that much
> circuitry makes me a little edgy. so I'm thinking -- at least keep  
> it on the DC
> side --yes, i could see setting up multiple hot water heaters, and  
> staging
> element start thresholds. but that conjures up even more control  
> issues i.e.;
> how is this mess disabled in the eq mode? i would like to keep battery
> maintenance as simple as possible. -- more likely to get done.
>        so, I'm pretty much fishing for  a consensus here -- what do  
> most
> folks do?  what works? what doesn't?  is there a reliable "off the  
> shelf"
> controller / element capable of handling this kind of logic -- at  
> that kind of
> freight? --( that doesn't require a Ph.D. to operate?)
>         i do value all your insight, and appreciate having this   
> forum in
> which to air such laundry -- thanks all.
>                                                   hope you all have  
> a safe
> and joyous holiday season. dan

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