Solar, A/C, DC Hot Water Heater Project

[1der]

Working on our Hot Water system right now. We are using an Isotherm SPA15 water heater which come stock with a 110v/750w heating element as well as a heat exchange coolant loop. We are adding 12VDC /200w element to this system. This is a very high quality/high efficiency 4 gal all stainless tank rated to 160 degrees and utilizes an adjustable mixing valve to bring the tap temps down.

In the current plan, we are not going to use the coolant heat exchanging capability due to the efforts of taping into and running long heater hoses. We are going to primarily use the A/C element to heat the water via the Samlex EVO 2212 2200 watt inverter when we are driving. Using the AC, an hour, or so, of driving will be enough to heat the hot water. This will require about 60 amps which will be supplied by the OEM dual alternator setup we have. This approach may not be viable for a single alternator system due to the higher draw rate in addition to the normal vehicle demands. If this does not prove to be a workable solution, then the coolant runs will be implemented.

The added custom 12VDC 200w element is used primarily as a solar dump load to heat the water using excess solar energy not required by other DC systems when the solar panels have output. The DC element can also be powered by the house battery bank if needed to "top" off the hot water. Or, it could be used as the sole source of water heating on a longer drive only using about 20 amps per hour. Lastly, the DC element can be run in addition to the A/C to heat the water even faster, likely only if connected to shore power.

In theory, the 200w element would use about 18 amps and raise the water temp by around 20 degrees in one hour. It takes approx 400 watts (not allowing for some inefficiency/losses) to raise four gals of water by 40 degrees in one hour. 200 watts would take about 2 hours to do the same. The time/watts/temperature/gals relationships are fairly linear. What is NOT linear is the watts fed into a heating element and resultant effective heating . 100 watts (9 amps) into a 200 watt element yields about 50 watts of heating because of resistance losses. (cue: boy wonder here) So, even though the 300 watts of solar could provide in excess of 240 watts with the batteries in float, I spec'd the DC element at the lower end of available solar to hopefully find a sweet spot. Too small and risk burning out the element, or too big and lose out to resistance loses.

I am finalizing the layouts for plumbing and electrical runs, both AC and DC. Hope to have this done by middle of January. Then we will see what this endeavor can really do. More pics to come.

BEFORE ADDITION OF DC ELEMENT

[86Scotty]

[Flux]

So you can put two elements on this to go AC or DC power?? Am I understanding this wrong??

I sure do like that unit though.

[Glider]

I like it!

[1der]

Originally Posted by Flux

So you can put two elements on this to go AC or DC power?? Am I understanding this wrong??

I sure do like that unit though.

Here are some of pics after modification to add the 12v element. This required drilling two new holes in the flange.

[boywonder]

Originally Posted by Flux

So you can put two elements on this to go AC or DC power?? Am I understanding this wrong??

I sure do like that unit though.

Correct......1der has procured a custom fabricated 200W 12V element for his water heater. He sent the element flange to the vendor and they added the second element.

1der and I have been discussing how to properly wire this up in a safe manner. As he mentioned, he wants to use the inverter or shore power to drive the 750W AC heating element and use the excess solar power (once the batteries are topped off) to power the 12V 200W element, and, ideally have the option of using both elements simultaneously. His solar controller is a Midnight Solar Kid which does everything except slice bread.

So far he's contemplating two options:

Procure and mount a second thermostat and temp limit switch and wire that to the 12V element, so there would be two completely separate systems on one tank, 12 VDC and 110VAC.

Procure a DPDT switch rated for the current and voltage of either element and connect the positive/hot leads of each source as shown in this incredibly crappy drawing:



The DPDT switch uses the existing thermostat and overtemp safety switch for either element; the downside of this approach is not being able to power both elements simultaneously. This approach would also require that the existing thermostat and safety switch have a current rating at least as high as the 12V element current requirements.

If anyone here has other brilliant ideas on how to accomplish this, please feel free to share ideas/designs.

[hogasm]

Where did he find the custom element?
I have a steamer sink in my kitchen that Kohler does not have the replacement element anymore so I am going to need one custom made

[boywonder]

Originally Posted by 1der

(cue: boy wonder here)

This will require about 60 amps which will be supplied by the OEM dual alternator setup we have.

750W/12V=62.5A (Ohm's law)

...neglecting inverter losses of course.......more like 68A including these

Originally Posted by 1der

In theory, the 200w element would use about 18 amps and raise the water temp by around 20 degrees in one hour.

200W/12V=16.67A (Ohm's law)

4 gallons=.53 cu ft

.53ft3*(62.5lbs/ft3)*(1 BTU/(lb*degF)*(20 DegF)=662 BTU's

662 BTU's=194 Watt-hours (lazy...online conversion )

(194 Watt-hours)/200 W=.97 hours

Originally Posted by 1der

It takes approx 400 watts (not allowing for some inefficiency/losses) to raise four gals of water by 40 degrees in one hour.

200W/12V=16.67A (Ohm's law)

4 gallons=.53 cu ft

.53ft3*(62.5lbs/ft3)*(1 BTU/(lb*degF)*(40 DegF)=1325 BTU's

1325 BTU's=388 Watt-hours (lazy...online conversion )

(388 Watt-hours)/(1 hour)=388 watts

Originally Posted by 1der

200 watts would take about 2 hours to do the same.

It sure would....

Originally Posted by 1der

The time/watts/temperature/gals relationships are fairly linear.

Completely linear (assuming adiabatic conditions...ie no heat loss)

Originally Posted by 1der

What is NOT linear is the watts fed into a heating element and resultant effective heating . 100 watts (9 amps) into a 200 watt element yields about 50 watts of heating because of resistance losses.

Well, not exactly......it's the amps (current) that makes things non-linear......due to "I squared R" losses....

P=I^2*R (Ohm's law again)

Calculate the resistance of a 200W element that's rated at 12V operation.

P=V*I (ohm's law again) so I=P/V=200/12=16.67A

rearranging the above equation, solve for R...

R=(P/(I^2))=200/(16.67*16.67)=. 72 ohms

Calculate the power out of 9 amps flowing through .72 ohms:

P=(9*9)*.72=58W

Calculate the voltage required for the above current and resistance:

P=V*I (ohm's law...again!!)

V=P/I=58W/9A=6.5V

...so using a 200W/12V rated element at 6.5V only gives 58W.....so in this case roughly half the voltage yields approx 1/4 the power...




Note: All of the above calcs are simple thermodynamic calcs, and do not account for any heat transfer through the water heater insulation, etc. (Adiabatic calcs) They also do not include the energy/power required to heat up the metal tank, insulation, etc.

Real life will require more power and or time......


...and 1der gets an A- for a grade......missed one!

[1der]

boywonder - Thxs! That made my afternoon!

Re: Heating element - there are two sources I narrowed it down to. TempCo was my choice in the end because of their attention to a custom application. The other would have been OEMheaters.com, also seems like a good company.

The big challenge was getting a loop 12v element made. There are many sources for a 12v 1" npt fitting, and lots of 120v loop elements but not 12v loop. The flange set up on the Isotemp SPA15 will not accommodate 1" npt fitting.

I hope to get some time soon to test and finish this off. Will post findings!

[arctictraveller]

Boywonder, If you redo those calcs at something like 13.2 or even 14vdc things will probably look even better.