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Old 02-23-2015, 02:08 PM   #71
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Re: Dual Voltage (12v dc / 120v ac) Small Electric Water Hea

My head hurts...

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Old 02-23-2015, 02:40 PM   #72
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Re: Dual Voltage (12v dc / 120v ac) Small Electric Water Hea

eddy: Yeah, I get that. But since he is showing his homework I can break his calculations into digestible segments between aspirin doses. Other times, its like leaning back on a New Wave Sleek from downstream current into the upstream current behind a rock and turkeyturning (i.e., I am underwater...)
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Old 02-23-2015, 02:47 PM   #73
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Re: Dual Voltage (12v dc / 120v ac) Small Electric Water Hea

boywonder - nice calcs!

So of the 37 amps used => ~22 amps went to 40 deg. rise heating the water and tank (plus the samples taken) and 15 amps went to heat loss? So finding an effective insulation solution for the tank is really important. This is why the Isotherm and other high end tanks are encasing in closed-cell-foam. Well-insulated would imply low maintenance of desired temp once achieved. So there is a light at the end of the tunnel with really good insulation. An engine coolant heat exchanger would be primarily effective but only for the first tank of water used after engine shut off, but it would also have the tank and 2.5 gals of water pretty hot to start with. By day two, though, we are back to the base amp requirements to heat water using this method.

Very interesting!
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Old 02-23-2015, 06:17 PM   #74
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Re: Dual Voltage (12v dc / 120v ac) Small Electric Water Hea

Quote:
Originally Posted by 1der
boywonder - nice calcs!

So of the 37 amps used => ~22 amps went to 40 deg. rise heating the water and tank (plus the samples taken) and 15 amps went to heat loss? So finding an effective insulation solution for the tank is really important. This is why the Isotherm and other high end tanks are encasing in closed-cell-foam. Well-insulated would imply low maintenance of desired temp once achieved. So there is a light at the end of the tunnel with really good insulation. An engine coolant heat exchanger would be primarily effective but only for the first tank of water used after engine shut off, but it would also have the tank and 2.5 gals of water pretty hot to start with. By day two, though, we are back to the base amp requirements to heat water using this method.

Very interesting!

Yes, roughly........if we assume 25 amps through the element for an hour, and if it takes an hour to heat the water and tank based on the rough calc above, then 25 amps went to raising the temperature and the other 12 amps for heat loss.....keep in mind that as the temperature rises the energy dissipated through the insulation goes up since the delta T is higher.

So if we used 12 amps of the 37 to replace the heat lost when warming up, it'll take more than 12 amps over a 90 minute period to keep the temperature at the set point on a continuous basis.


If someone made water heaters like a thermos, that would work very well, but it's cost prohibitive...the heat loss through the thermos vaccuum would be exceedingly low....we have to rely on insulation instead.
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Old 02-23-2015, 10:11 PM   #75
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Re: Dual Voltage (12v dc / 120v ac) Small Electric Water Hea

Hey Boywounder.
I applaud the effort and data driven info but it seems like a lot of power to warm up a few gals and an apparent slow time to recover. Makes me think more seriously about going with a propane heater. I know much more $$$
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Old 02-24-2015, 05:03 AM   #76
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Re: Dual Voltage (12v dc / 120v ac) Small Electric Water Hea

Not wanting to divert this thread (we already have one guy here for that! ) I'm curious how the efficiency could be increased if a "super insulated" containing vessel were used for this?

While we know the water temp rise and eventual steady state or while in holding the set temp we didn't see temp readings of the outside metal casing of the stock water heater. Noticing the relatively thin fiberglass insulation I wonder how much heat loss that creates?

Its far easier and makes perfect sense using off-the-shelf parts so designing and building another type water heater might exceed practicality.
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Old 02-24-2015, 08:45 AM   #77
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Re: Dual Voltage (12v dc / 120v ac) Small Electric Water Hea

Quote:
Originally Posted by JWA
Not wanting to divert this thread (we already have one guy here for that! ) I'm curious how the efficiency could be increased if a "super insulated" containing vessel were used for this?

While we know the water temp rise and eventual steady state or while in holding the set temp we didn't see temp readings of the outside metal casing of the stock water heater. Noticing the relatively thin fiberglass insulation I wonder how much heat loss that creates?

Its far easier and makes perfect sense using off-the-shelf parts so designing and building another type water heater might exceed practicality.
Well, it it were perfectly insulated (or a thermos, almost perfect...) once it was heated up is stays hot forever.....so heat it up when driving or plugged in then use it when you need it.....kind of like house batteries.

I can take readings of the metal casing temp and we can dive into the heat transfer aspects of this project (conduction, convection, radiation) but that would be significantly beyond the scope of the thermo calcs posted above......

Building another type of water heater is potentially dangerous as well...........I think the best solution here is what Ray is contemplating, lots of solar and a controller that diverts excess power to a dummy load, in this case 20 lbs of water........

As Nick just mentioned, propane has lots of energy in it, and heating water with propane does not drain the batts.....same for diesel and gasoline......gasoline has around 115,000 BTU's/gallon for example. We only needed about 900 BTU's to heat up 20 lbs of water 40 deg F.
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Old 02-24-2015, 09:33 AM   #78
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Re: Dual Voltage (12v dc / 120v ac) Small Electric Water Hea

....thinking about the BTU content of a gallon of gas on the train ride this morning.....let's compare 200 amp-hrs of battery capacity to a gallon of gasoline....just for fun.

Applying ohm's law P=VI

so 200 amp-hrs * 12V=2400 watt-hours

2400 watt-hours *(1 btu/.29 watt-hours)=8275 btu

the BTU content of a gallon of gas is roughly 115,000 BTU.......so the usable power from a couple of 4D batteries from fully charged to 50% discharged is roughly equivalent to a little over a cup of gasoline.....this is why electric cars have such a limited range compared to gasoline.....of course, lithium ion battery chemistry helps this quite a bit.
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Old 05-02-2016, 02:01 PM   #79
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In an effort to assist member 1der with his solar and water heating sizing, I did a simple (and extremely boring) experiment with my water heater yesterday....and also because I was just curious....

First, I turned the water heater on and allowed it to get to temperature....as mentioned earlier in this thread, that took about 90 minutes.

Then I observed my solar controller display....when the water heater element is not energized, my batteries are at around 12.8V.....with the element consuming 25A of current, the voltage drops to around 11.9-12V.

So I simply recorded the time that the element was energized to keep the water at temp (105F IIRC) and the time that the element was switched off.

It was very consistent......The element was on for 2 minutes about every 20 minutes, so roughly a 10% duty cycle.

This means that it's drawing 10% of full current 25A times 10%=2.5 amps per hour on average....better than I expected.
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Old 05-28-2016, 09:49 AM   #80
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More water heater related stuff....

I recently picked up a Xantrex Linkpro battery monitor from another member here (thanks SteveW!!).

I have been experimenting with using it to heat water in my 12V electric water heater with excess solar when the house batteries are charged by using the highly configurable low SOC/Low voltage alarm output on the Linkpro.

Member 1der and I have been playing around with this idea for awhile....at least since E350 posted some info from a guy named TechnoDave on the Midnite solar forum earlier in this thread.

1der has the Midnite Solar Kid controller which apparently does everything except slice bread, including using excess solar to power diversion loads like heat water....the kid has a handful of very configurable modes (well.....we'll see...) to do diversion loads and should provide hours of entertainment once 1der (Ray) gets his set up.

Since my controller is a bit more pedestrian (Samlex SCC-30 PWM unit), the Linkpro seemed like a nice addition to my setup...it does lots of things around monitoring battery health. For those wondering why I didn't just ditch the Samlex and buy a Kid.......I don't have the width to mount the kid in my panel and I like the Samlex...and it's paid for.....also for warm climate small systems the difference between PWM and MPPT is not much...

After mounting and wiring the shunt close to the house batteries, I had to fabricate another control panel to squeeze in the Linkpro.

...old panel on the left, newly fabricated panel on the right....those with a keen eye will notice a big round hole in the new panel...





...and the new panel bead blasted and installed......



To start, I configured the Linkpro low voltage alarm to be normally closed and to open when the battery voltage crops below 11.8V. This alarm output is wired in series with the switch the powers the water heater relay.

So, the use case is: If I want to turn on the water heater with the van parked, or keep hot water created while driving hot, I can turn on the water heater and leave the van without stressing about killing the house batteries, since now if they go below 11.8V the Linkpro will open shutting off the water heater relay.

When the sun is shining the solar system can pump amps into the batteries while the water heater is on.....that's the hope anyway.

The Linkpro has a few other settings for the low voltage alarm like also configuring it to activate at a low state-of-charge, and programmable deadband/hysterisis, on/off delays and seconds/minutes etc etc.

So we'll see what happens.....
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