Project Journals > Users Projects

Off grid water heating project

(1/11) > >>

dochubert:

In planning my off grid existence, I quickly realized that water heating was going to be a large challenge.  The house came with two 50gal electric water heaters.  One seemed to be solely dedicated to the kitchen sink and dishwasher with the other doing everything else.
Running one was bad enough energy-wise, but two was unacceptable.
Redesigning and re-piping followed with the kitchen waterheater removed and the whole house worked off one unit.  I did add a hot water return line and a small circ pump so the kitchen had hot water quick.

For off grid water heating the initial plan was a 24v bank of four 200ah batteries running my oldest powerjack, my 24v/8kw.  These batteries are separate from my main bank for powering the house.  I put in a manual transfer switch so I could select inverter or grid to power the water heater.
I guess I didn't really grasp how much power an electric water heater draws.  Switched to inverter and when the water heater tried to run the pj went immediately into overload and shut down.  4500w elements were just too much for an 8000w pj.  Plan B was to hook up just 110v instead of 220v from the inverter.  Now it ran but of course only heated half as fast with about 2500w measured by the powerjack meter.  Also I only had half the solar hooked to it so far that I had planned, so after 2 days use I had to switch the waterheater back to grid to let the batteries come back up.  So solar water heating 2 days of each 3.  It was a start.

Next, I moved that extra water heater next to the main one and piped it in as a preheat tank for the main tank.  Planned to heat this wh with dump load controller from the main battery bank.  I installed 2 double 1000w 48v elements into the preheat tank.  Got one of these;



Wired it up to power one 1kw upper element and one 1kw element lower.  Planned to get a second controller and hook to the other two elements after trying this one out.  It seemed to work fine.  The tank heated up nicely, using excess solar the batteries didn't need.  Then ran the circ pump to heat the whole loop up and keep the main tank hot enough to not turn on its elements.  Eventually, the circ pump will be on a timer and temp switch control.

That all worked well enough for about a month.  Then that solenoid relay cooked.  I bought it because it was 48v and rated for 440 amps.  Figured that was exagerrated but should still be enough to handle the 30-45 amp load I needed handled.  Only about 10% of rated.  So much for optimism.
While messing with the solenoid while it was failing, I realized the 48v elements were frying too.  What the heck?  The upper element had opened and was doing nothing, and the lower was cooking.  One connection started glowing red hot!  More junk overrated parts apparently.

Back to the drawing board.

Time to rethink this whole deal. 
After much thought, I decided to put two 2000w 120v elements in place of the 48v elements.  Ordered a dc controlled ac relay.  I now plan to control the new relay with the same style voltage controller as was used with the bad solenoid.  The relay will switch regular ac 120v from my main powerjack as an ac dump load into the water heater elements.  Switching ac is much easier on contacts than switching dc.
As of today, I'm still waiting on delivery of the new relay.

Hopefully, this will solve the water heating issues but now I need to think about a real dump load.  I want protection for my batteries that I can depend on.  Eventually, I will have 3 times the solar that I'm using now.  Dump load becomes a must!  The water heater setup described above should usually use up the excess solar, but if the inverter isn't working for some reason or the tank is up to temp, ac dump load doesn't dump.  So in addition to ac dump, will add a dc dump set a trifle higher voltage using this guy;




Its a dc contactor rated 500 amps!  This one looks like its actually got a chance of meeting its specs.  60vdc contact rating and a 60vdc coil.  I won't be doing anywhere near that of course.  I like to use a space heater for my dump load.  The heater that looks like an old style steam radiator except on wheels.  I pick them up at flea markets and garage sales for $5.  They are oll filled and don't have a fan.  You need to bypass the thermostat because it can't handle the dc.  I just remove the slide connectors from the thermostat and use a large size 30a fuse.  The tabs on the fuse match the slide connectors.  Connect the slide connectors to the fuse instead of the thermostat.  This also gives some short ckt protection.  The heater's switches let me use one or both elements.  The switches don't much like the dc either, but work ok.(probably shortened lifetime)  With one element engaged I see about 5 amps at 55vdc.  Different brands of heater are slightly different.
So for cold weather I'll have a heater or two in the basement or back room.  In summer I'll probably put some regular dump resistors outside on the chimney out of hands reach.

Back to water heating; (What! Me ramble?)
The 120v 2000w elements are installed and wired and ready.  The controller is installed and ready.  Just waiting for the ac relay to arrive.  Supposed to hit 100 deg F today so hot water without the preheat tank won't be a problem.
That's where it stands.  I'll report progress when I have some

Pete:
Hi Doc, I am not clear on why you need dump loads. Surely your regulators will open circuit the solar panels when your batteries are full, or when the water heater thermostat opens.
Maybe with that much power you could investigate hydronic heating. Saves lots of cords and individual heaters running on DC.
Anyway it sounds like you are having fun
Cheers
Pete

dochubert:
Hi Pete,
Always having fun!

We mostly heat with wood by preference.  That said, I intend to do some amount of warming inside by electric heat thru solar.  Then burn less wood to reach comfort level.
The dump load is not truly needed by a solar power system, true.  ! like Xantrex C40 controllers.  They work very well.  I have found that despite settings, if there is lots of sun and no load, that voltage tends to creep up.  Batteries are probably the most expensive part of my system so I like to be sure they are protected.  The water heating dump will be set to voltage low enough that it comes on and heats the tank well before the charge controllers go to float.  The dc dump in winter will be set higher than the water heater dump  but not so high that it doesn't come on and move some solar energy into the house for heat.  And it will work more and put in more heat after the tank is up to temp.
 
I do plan to have 3 times the solar wattage I have now when the panels are installed on the roof (soon hopefully).  In winter that will hopefully be enough to run the house fairly normally.  In summer it will be way way plenty (with the exception of the water heater setup which will be hopefully adequate), so some protection makes me feel better.  Two dc dump loads are in my plans, one set to put heat in the house daily in winter, the other set just to protect batteries.

Another point which I have shelved for the present is the use of a true diversion control for banks of solar panels.  With diversion, any time your solar panels aren't pumping power into your batteries, that power is instead pumped into a heater or water heater or something that actually makes use of the power.  This without pulling on the battery, leaving it able to concentrate on running the inverter to power the house.

And finally, my long range plans include possibly adding a wind turbine or 4 to the setup.  I have a vertical 300w stuck up above the shop roof now, but so far receiving negligible output in decent wind.  It came with one of those cute electonic controls with leds that blink encouragingly, but no output to speak of.   Just took that out and wired in a regular 3 phase bridge rectifier instead.  Now I'm waiting on a windy day.

I guess I should answer your question after all the above (More rambling).  Why do I need dump loads?  I don't really need them, but prefer to have at least one.  When we 'Go to Town' we're usually gone most of the day.  I want to be confident (as much as possible) that things are safe and under control while I'm not home.  Redundant safeties make it easier to believe nothing serious will go wrong while we're gone.  Bottom line, I feel better.

eidolon:
I heat water with PV diverting power from the panels DC buss with whatever isn't used to charge battery.  My power point is about 60V and use the same 2,000W heating elements. That gives me up to about 500W.  If you half the rated voltage, you get 1/4 of the rated power. Not a fan at all of heating water with batteries.  Everyone thinks I will just divert when the battery is full. That is a very bad assumption.  I sure can't tell when a battery is full.  And you need a bigger inverter, bigger battery, and bigger charge controller wasting more money. Shame there aren't products to do this on the market. Dump load? I don't have any surplus of panels, just what I need to run the system.  Still I find my water heater shutting off about 10am. I'm going to add another 30 gallon tank just for the excess.  It is no wonder you are cooking elements, you can only dump so much into a tank.  I just did some data collection over 11 days and it surprised me. Lowest temp was 107 over night. Tank shuts off at 127F.

I've done the oil finned heaters too. It is amazing how hot they get with just 300W.  Unless the space is really cold, I would avoid putting more than that in them.

MadScientist267:
Hate to put it so bluntly, but if you can't tell when a battery is full, "you're doing it wrong". In a battery based system, that's in the top 3 list of things you best know how to determine unless you like throwing away good money after bad (or worse).

Dump loads by virtue of correct implementation do not "use battery" to DO anything. It's always surplus power. Just like when you "leave the lights on" in your car - with the engine running. The battery doesn't even know they're on; the alternator is doing all the work. You do not need a bigger battery to use a dump load. In fact, you're doing the same thing (just more directly) by diverting PV to the elements. Dump load, diversion load, shunt regulation... Synonyms.

Dump loading is just a "special" form of regulation, "usefully wasting" power that is otherwise available rather than letting the potential of which go completely to waste by not being tapped. A very useful and practical strategy when it's done right. Domestic hot water is an EXCELLENT place to put surplus electric power for multiple reasons. Like anything, yes, a water heater can reach capacity (and they do serving as dump loads) - If you're not using the heated water enough to naturally keep the heat down (which would appear to be the case the graph there indicates, most of the time displayed lol), the tank needs to still have a way to stay below a safe temp. So safety mechanisms need to be kept in place. Simplest method for this, aside from something like the standard switching thermostat, is to have the tank simply "blow off the steam" by discharging some of the water (usually somewhere protected from human or other access, outside, or down a drain that can handle it). Auxiliary tanks, yes, are another way.

Inverters are used to get the elements to operate at capacity, provide maximum dump ability, or both.

And yes, there are commercially available products to do just this... They're called... (Wait for it...)

Dump controllers. Diversion controllers. Shunt regulators.

Decent MPPT controllers either support a dedicated dump mode directly, have a feature to drive a set of contacts (mechanical or solid state, whatever floats your boat), or can even be "tricked" into running as one where neither of the other 2 methods apply.

I'm in agreement however on the oil heaters. That's playing with fire and hoping to not get burned. Better off with plain resistance in open air.

I've used wax personally with some success, but only with a purpose built supply to control it... The physics of the phase change can take a lot more energy input before overheating than the simple liquid oil in heaters.

No matter the route, a means of handling dangerous conditions needs to always be in play. Just like a battery, you can only put so much energy into any energy storage system before that cup overflows too.

Steve

Navigation

[0] Message Index

[#] Next page