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#2731 - 07/22/01 12:12 AM Powering Water heater circuit from Solar Unit?  
old Appy  Offline
Joined: Mar 2001
Posts: 118
Auckland, New Zealand
Hi there all, as they topic states this is something i have thought about for a while. With all the knowledge here-abouts, Whos got any ideas. Element is 3kW, supply is 240vac Do i use a inverter,step-up trans, alternator..?? looking as a part time alternative than a replacement.

Nice to see you all still here

[This message has been edited by old Appy (edited 07-22-2001).]

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#2732 - 07/22/01 04:26 AM Re: Powering Water heater circuit from Solar Unit?  
Scott35  Offline

Broom Pusher and
Joined: Oct 2000
Posts: 2,708
Anaheim, CA. USA
Hi Appy,

Well first off, the Elements of water heaters are typically just Resistors, so they are "Linear" loads as far as power goes [almost all True Power / Pure Resistive load].

What's all this baloney mean you ask?? Don't ask me [Linked Image] [just joking!].
Pretty sure that you will be able to use DC for the elements, as the RMS value of AC is equivalent to the heating effect of DC at the same levels [Voltage, Current and the resultant Power].
Problem is that you will need to have a voltage near 240 volts, in order to push enough Current through the element to equal the desired 3 Kilo Watts of power. If the voltage is too low, very little Current will be pushed through the Element - resulting in nothing more than a power consuming, meter spinning, water-warming [somewhat] deal!

Using Ohms law to figure the Element's Resistance [which might actually be the AC Impedance!], this is what comes up:
3000 Watts @ 240 Volts = 12.5 amps
12.5 Amps, pushed by 240 Volts = 19.2 Ohms Resistance.

Check the DC Resistance of the Element with an Ohmmeter, then figure what Voltage is needed to push a level of Current through it, which will equal 3000 watts.

The most simplest way to do this would be a DC to DC converter - which is usually an Inverter with a Rectifier for high power situations.
If you can connect your sources in series, that might work out...???

I'm not sure of what levels you have from your sources [Voltage, Current, Amp-Hours, etc.], or the possibilities available for connection schemes and/or storage vs generation, so fel free to throw some numbers in!

I'm sure that other members will be able to offer you much better answers or solutions than these!

Scott SET

Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

#2733 - 07/22/01 06:58 AM Re: Powering Water heater circuit from Solar Unit?  
old Appy  Offline
Joined: Mar 2001
Posts: 118
Auckland, New Zealand
well i had pretty much decided to use AC, DC hadnt even occurred to me.I was looking at installing a set of 12 volt storage batteries. Our hot water heating is controlled via a ripple control and is switched off in the evenings to reduce demand on the network, hence the part time option this is when i would like the aux to kick in.

#2734 - 07/23/01 04:01 PM Re: Powering Water heater circuit from Solar Unit?  

Scott is right that your best bet electrically is to have at least 20 battteries in series and avoid the losses from stepping up your voltage.

Frankly you are far better off directly heating the water in the sun and storing your hot water in an insulated tank. Just preheat your water over the roof before it goes into your electric water heater. Forty gallons of water on the roof costs much less than forty batteries and 4000 sq ft of photovoltaic cells and is a lot easier to install and maintain.

#2735 - 07/24/01 06:53 AM Re: Powering Water heater circuit from Solar Unit?  
old Appy  Offline
Joined: Mar 2001
Posts: 118
Auckland, New Zealand
Yup i think your are right there.
although the Lutec 1000 has really got me interested

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