Water heater as dump load

[vtpeaknik]

I have a 360W (peak) PV system that is mostly for back-up, so the batteries are usually full.  Seems like I should make use of the excess power for something?  I also have an on-demand water heater (burns propane) and the water temp fluctuates so I installed a small (4-gallon) electric hot water tank into the output pipe of the water heater.  That tank is not plugged in to electric power, it only serves to mix up the hot water to as to even out the temperature.  But it occured to me that the heating element in that tank is almost ideally sized as a dump load for my PV panels.  It's rated 115VAC and about 1200 watts (perhaps 1400), or about 9 or 10 ohms.  The PV panels' max power point is about 50 volts, 7 amps.  (These are two Evergreen ES180 panels in SERIES.)  Fed into 10 ohms that'll accept 5 amps or 250 watts.  Although the propane thing heats the water, the PV power would keep it from cooling off during the day when not used, offering instant very hot water in the evening for handwashing etc, with less waiting around with the water flowing for the temp to come up.

But, the contacts in the thermostat of the water tank are designed for AC - would they be quickly damaged by making them switch DC?  (And is it bad for the PV panels to have the current switched on and off?)  The DC power switched is only about 1/5 of the rated AC power, but DC is different from AC.  I put the thermostat on its hottest setting, which minimzes such switching, but it must still happen on sunny summer days.

Note: in dump mode, the PV is connected directly to the water heater, NOT through the charge controller that is only used for the other, non-dump, mode of charging the batteries.  That charge controller is a BZ MPPT500, and the batteries are 12V, in case you wonder.

[esc]

Is the propane heater an "on demand" heater?

If it is an "on demand" heater, I would consider placing the tank BEFORE the propane heater, that way, by feeding it warmer water, it would not have to burn so much propane.  The propane heater would become a backup heater for the electric water heater.

[spinningmagnets]

I am not qualified to comment on the heater elements, but I recall many stories where homeowners expressed great satisfaction with a solar water pre-heater.

A recent story has the primary-heater gas flame exhaust routed past the incoming water pipe for additional pre-heat. If done right, it passed safety building codes.

A salvaged electric water heater could be used as a solar pre-heater, and the proper elements installed to allow it to be used as a DC dump-load.

I'll bet Gary over at "BuildItSolar.com" has many links about this, with the warnings concerning experiments that failed and why (saving you time and money).

Best of luck!

[ghurd]

Curious.  How do you have the dump load not going through the controller?

The thermostat contact problem is fairly simple.

The contacts would be used to switch a circuit, and the circuit would control the heating element.

The dump load control would control the circuit.

The PV operating voltage with a 5A load may be a bit higher than 50V.

BTW, Neat idea.

G-

[scoraigwind]

Maybe use a second controller.  You already have a controller in series with the pv.  Use a second controller to dump off the battery at a slightly lower set point.  Use the thermostat to cut the sensing circuit on the the controller (assuming it is a Morningstar Tristar and not one of those rubbish Xantrex ones) by inserting  some volt-dropping diodes in series with the sensing circuit.

Hmm.  Problem is that then you will be dumping at 12 volts.

I wonder what effect it will have on your series controller when you disconnect the dump load from the input side.  Could cause some horrible voltage spikes.

another option is to use a Morningstar 'relay driver' to operate a dump load on the inverter.  That can be wired very conventionally in terms of its thermostat and so forth.

[vtpeaknik]

Some responses to the comments above:

(1) the dump load is connected directly to the PV panels, with no controller (other than the thermostat in the water tank).  Connecting it in any way to the 12V battery system (rather than the 50V PV voltage) would result in about 16x lower wattage, making it fairly useless.

(2) this is not an automatic dump load system, rather, on days when the batteries are full, I can use a manual switch to connect the PV panels to the dump load.  PV is a stupid way to heat water, but when the energy is going to waste anyway (would be blocked by the charge controller), might as well divert it to another use.

(3) a pre-heater tank with thermal solar input would be a totally different approach.  In my case there is not much point in pre-heat since the on-demand heater can only modulate the flame down so much.  Also I need the tank on the other end of the on-demand heater anyway, for the purpose of smoothing out the temperature fluctuations (unpleasant while taking a shower).  So the tank was already there.  And the PV panels already there.  Adding the wiring connection between the two was pretty much free, although of limited benefit.

So the question still remains, will the thermostat contacts (rated for something like 15A AC) survive occasionally breaking a 5A DC current?

[richhagen]

Most of the cheaper on-demand hot water heaters I have seen are 'dumb' in that they do not adjust their energy consumption based upon the incoming waters temperature, just the flow rate.  Preheating the water before feeding it into one of these would likely cause it to trip the overtemperature sensor and kick off.  There are some, specially made for the purpose that do adjust the heating energy based upon the incoming water temperature in which case preheating would work well.  Rich

[richhagen]

Great idea to use the ac thermostat to control a dc circuit for the element.  Rich

[richhagen]

I like Ghurds idea, run a tiny current through the thermostat and some resistance to control a DC relay or even build a circuit to switch on some FETs or something.  Rich

[ghurd]

I am guessing the panel will be at about 55V at closer to 6A with good sun.

I don't know the contact's ratings, but I wouldn't do it direct without finding out.

The circuit to control the heating element from the thermostat contacts is fairly simple.

Just throwing out an idea,

"mostly for back-up" means the batteries don't get used once a month?

How critical is it to have full batteries?  Life support?  Or "CSI" on Thursday?

I am thinking the unused battery bank can be fully charged in 1 day, once a week.  Maybe once every 2 weeks.  As long as it stays over 12.45V or so it should be fine.

The batteries will fail from age even if they are never used and always kept full.

The controller won't show they are "full" until they reach the regulation voltage, even if they were full when charging started.

I do not see any reason why the PVs couldn't be connected to the water heater most of the time.

Could add a 5 or 10W 12V PV just to help keep them topped off.  Sounds silly, but 100% of the power from small and large PVs would be being used.  That has to be better than only using 5% of the large PVs power.

It seems like a much better way to use what you already have.

G-

[vtpeaknik]

Interesting idea, ghurd, although in my case the small (4 gallon) water tank, in the output of the on-demand heater, can only take that much energy before the thermostat says "enough", thus I can't use 100% of the larger-PV-panels' power.  (In the summer that is.  This time of year I need the large PVs just to top off the batteries in the very short hours when it is sunny!)

A possible future project would be to add a larger water tank on the input side of the water heater and connect the large PV panels to its element most of the time.  If I were to use a standard 230V tank with two elements of about 20 amps each (is that correct?) that's 11.5 ohms for each element so one element would take about 220 watts from my panels in full sun.  That's not a lot of power, but over a sunny day it would be roughly 1 KWH, equivalent to about $50/year (but winter input would be much lower).  Is that worth doing?  (Note that around here propane and grid-electricity prices are similar, on a per-BTU basis.)

[vtpeaknik]

I should also add that I am trying to make use of the "excess" PV power for something better than resistive heat.  I'm now running the most commonly used lamp in the house, a 15W CFL, off a mini-inverter that is using the 12VDC battery bank.  (See my posting on that inverter.)  Also have one 12VDC CFL that is sometimes used.  Still looking for other such uses, perhaps replace some small AC fans that I use a lot with DC fans.

[scoraigwind]

I don't like switching high voltage DC.  I fyou ahve an inverter then why not connect the water heater to that and use AC as the heater and stat are designed to run on AC?

Using the stat to operate a relay is a solution that I often use when I want to change over to another heater.  But it does require a relay that can switch DC, and as the voltage gets higher this relay gets hard to find.