DC Water Heating

System · March 2008

Hello, I am using the xantrex c40 to divert surplus 24 volt DC power into two AC heating elements. The heating elements get hot, but I noticed that they are not drawing much power, only about 1 or 2 amps. I would like to increase the amount of power that they draw and make them get hotter.
What type heating element would work best? 220vac, 110vac, or 24vdc?
Is there a way to increase the amount of current that is fed into the heating elements?
Thanks alot.

Brock · March 2008

Re: DC Water Heating

Depends on how much load you want it to pull. A 24vdc element would pull the most, although I used to use dual 2000w 120v elements and they took quite a bit of current, it also let me run them from 120 in a pinch.

BB. · March 2008

Re: DC Water Heating

From basic electricity, for a first approximation:

V=I*R
I=V/R
R=V/I
P=V*I=I^2*R=V^2/R

So, if you have a 24 volt heating element, and apply 24 volts, it will draw rated current (for example, lets say 10 amps)

P=24v*10a=240 watts...

If you have a 240 volt element and it is rated at 10 amps (P=240v*10a=2,400 watts), but run it at 24 volts:

R=V/I=240v/10a=24 ohms (element resistance)
I=V/R=24v/24 ohms= 1 amp (current thru element at 24 volts)
P=24v*1a=24 watts (power dissipated by element at 24 volts)

So, your choices are to find heaters that are rated at the voltage you want, find very large heaters and run them at lower voltage (and wattage), or find adjustable heaters that you can change taps on to run at lower voltages.

Or, run the heater on the output of an inverter... If you are trying to heat air, the losses in an inverter are not an issue--just heats the air too. If you are trying to heat water, then the inverter will lose approximately 15% of your diversion power.

A resistance heater should run fine on AC or DC so 24VDC=24VAC in terms of heating... However, the switches and fuses/breakers have to be rated for DC (AC is "easier" to switch).

Also, as a second level concern--many materials change resistance at they heat up (frequently resistance rises with temperature)... So, if you run a heating element at a cooler temperature--measure the current--it may be higher than the simple V=I*R equation would indicate.

Low voltage heating elements are not common--but they can be found... Here is one that came up with google (has information on how to use with diversion controllers--I don't know anything about this store or product)...

-Bill

mike95490 · March 2008

Re: DC Water Heating

Most common water elements are 120 or 240 V Less voltage would still work, but at reduced wattage. A speciality shop may have 24V elements, expressly for water heating.

What do you have that needs a dump load? Solar PV does NOT need a dump load. Wind or Hydro use dump loads.

Truck Stops often have 12 or 24V electric window defrosters. That'd make a good dump load. I'd be tempeted to use a "Koolatron" fridge or some other "thermoelectric" style cooler, and have a frosty one.

System2 · March 2008

Re: DC Water Heating

Thanks I am trying to heat water through two 240 elements with 24 vdc but the heaters do not specify rated amps. It says 4500 on the tank specs for each heater. Dont know what that means. Sounds like if I step down to two 120's I should be in better shape.

System2 · March 2008

Re: DC Water Heating

Mike, I have 400 watts solar and 400 watts wind and my batteries are always full. I want to use the surplus power to heat water to pump though my radiant heat floor.

BB. · March 2008

Re: DC Water Heating

P=V^2/R
R=V^2/P=(240v^2)/4,500w= 12.8 Ohms

V=I*R
I=V/R=24v/12.8 Ohms= 1.875 amps
P=V*I=24v*1.875a=45 watts

45 watts per 240V / 4,500 watt heating element at 24 volts...

Obviously, not much heat--and 400 watts of sun and 400 watts of wind is not that much extra energy. It may not even be enough to worry about trying to get this type of system working.

Instead, you might be better off building/installing a solar thermal collector panel / system to heat water/air for your home... A 40"x40" thermal collector (~1meter square) will catch ~1,000 watts (per square meter) x the number of hours of sun per day x panel efficiency (some 50-80%)...

-Bill

System2 · March 2008

Re: DC Water Heating

Thanks Bill, actually it is for my barn which is only 15x30 feet. It does not take much to heat and I already heat the radiant water with wood so I was looking for another heat source to intrigrate. The only thing that runs 90% of the time is a circulation pump at .2 amps. So the rest of my power is dumped. I want to get these things learned and I plan to expand. Thanks alot.

System2 · March 2008

Re: DC Water Heating

How well do those thermal collectors work in cold climates like northern michigan?

BB. · March 2008

Re: DC Water Heating

JW,

Basically, there are several classes of thermal collectors... The simplist is just an extruded vinyl sheet with interior channels. No insulation, cheap, and you see these used for heating swimming pools. Since a swimming pool does not heat much above 82F, it is very efficient (water temperature is very near air ambient--so not much heat loss).

The next type are the glass (or plastic covered) with insulation on back and sides. These are good for cooler areas and/or for domestic hot water heating (or where you want to heat a water tank for storage). Sort of the standard that is used everywhere (except swimming pools).

The last type is built like a glass thermos bottle. Outer tube, vacuum, inner tube of metal to collect heat and transfer to working fluid. Very efficient for low temperatures and at least one style I saw, had the ability to "stop heat transfer" if the the temperatures got too high (prevent boiling in the working fluid). Here is one vendor I found while looking around a couple years ago...

So--first you have to choose you working "fluid"... Air, water, anti-freeze, oil... And typically, the type "2" is used for hot air...

Regarding heating your barn--there have been a few threads around here about space heating and home made space heating (some as simple as building a box on the side of the barn, painting it black, and covering with corrugated fiberglass. Cut an opening at the top and bottom to allow air to circulate--use flap valves or shutters of some sort to block heat loss).

Googling the web and looking for home made solar heaters brings up a lot of examples... I would think that any of these would be better for space heating than trying to make/buy dump heaters (I assume that you already have good insulation and double pane/storm windows, etc.).

If you are looking for daytime space heating on sunny days--A hot air collector with a small fan (use 24vdc if you want) would probably do more than trying to use a dump heater.

As for particular brands/styles/etc., I don't have a clue since I have not built/installed any solar thermal systems yet for my home. My big issue with solar thermal is long term reliability and maintenance... There have been quite a few solar thermal systems (mostly for domestic hot water and pools) around are area since the 1970's oil shock--and most of them seemed to only have lasted a few years--then slowly decay.

My two cents would be to make a thermal air collector with fan--not much to go wrong, relatively cheap to build/buy. Not much to go wrong. Easy and cheap to experiment with.

-Bill

Telco · March 2008

Re: DC Water Heating

This guy's my hero, might check out what he did for whole house heating with water. Sez in there that on a 25 degree day he is seeing water temps to almost 200 degrees 10 minutes after the sun comes up. He also kept updating the page after the unit was in operation, pointing out the flaws in his system he discovered during operation. When I go offgrid I can only sell my wife on it by making it appear seamless to her, and this page is what cleared the whole heating issue up for me. No way the 'ol lady would have supported a 100 percent wood heat system, since she'd have to mess about with moving wood some of the time.

So far as the 70s oil shock system deterioration, I'd have to guess that this was more due to a bunch of people running to the next great thing rather than people dedicated to getting a system and keeping it working. I've seen these failures too, first hail storm comes along they disconnect rather than repair, and call it a day. Energy got really cheap after the 70s oil crunch, and when power is cheap nobody cares how much they use. Why maintain an expensive piece of hardware to save a few kilowatts when kilowatts are a few cents each?

System2 · March 2008

Re: DC Water Heating

Bill, While running 24 volts into two 120 volt heating elements it looks like it is drawing around 5 amps and it does heat the water, but slowly, but it does help. Can you work out the equation for 24 volts into 120 volt heating element? I dont know the amps of the heating elements, it does not say. And I dont know where you get the ohms, other wise I could do it. But I would like to know how many watts I get. Thanks alot.

BB. · March 2008

Re: DC Water Heating

JW,

Actually, a few posts up I did the math based on your installation... I kind of took the long way around and used simple equations instead of doing algebra to get the exact equation... Either way works fine.

Assuming 4,500 watt element at 240 Volts and you are using two of them:

For review, here are a few of the basic equations for AC/DC resistive devices (don't even ask about capacitor, inductors and AC)...

V=I*R (voltage = current time resistance)
I=V/R (and you can through simple algebra, isolate any particular variable on the left)
R=V/I (and the last variant

For power, some of the basic equations:

P=V*I (the basic definition)
P=I^2*R (you can substitute V with I*R and get this)
P=V^2/R (and so on...)

From your installation, we know two variables V=240 Volts (AC or DC), and P=4,500 watts of power, So we pick this equation that has P and V):

P=V^2/R

Solve for resistance R and substitute the numbers we know and solve for the answer (^2 means raised to the power of 2 or "squared"--V^2=V*V)

R=V^2/P=(240volts^2)/4,500watts= 12.8 Ohms

Now, we can figure how much current you use at any particular voltage... First solve for "24 volts"...

V=I*R (basic equation, solve for "I" on the right)
I=V/R=24v/12.8 Ohms= 1.875 amps

Now, in reality, you are probably pumping ~28-29 volts to your dump heater, using the above equation:

I=V/R=29v/12.8 Ohms= 1.875 amps 2.267 amps

If you have two heaters, then:

2*I=2*2.267 amps = 4.531 amps for two heaters in parallel...

To figure out the amount of power per heater:

P=V*I=24v*1.875a=45 watts
P=V*I=29v*2.267a=65 watts

Of course, you double the total wattage to 130 watts of heat from two heaters in parallel.

-Bill

System2 · March 2008

Re: DC Water Heating

Thanks Bill but I requested for two 120 volt heaters, not 240. I switched them out and that is why I asked. Still way over my head. Thanks anyway.

System2 · March 2008

Re: DC Water Heating

There is nothing to indicate polarity on the heaters so I am to assume that no matter which way they will be in parallel?

System2 · March 2008

Re: DC Water Heating

I got it:

120^2/1500 =9.6ohms 24/9.6=2.5 amps X 2 equals 5 amps total.
24 X 2.5 = 60 watts each.
Cool stuff. Thanks alot.

Still curious about polarity. Thanks

BB. · March 2008

Re: DC Water Heating

JW,

I am sorry--I missed that you asked about a different element. Glad the equations helped.

For resistance heaters, there is no polarity issue. You can wire them either way.

-Bill

DC Water Heating

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