Water heater as dump load

System · May 2006

I am considering using a 50gal water heater as a dump load for my PV and wind turbine. The wind turbine must have a load on it at all times. The combined PV and turbine could be 40A at 48V nominal. I was going to use a Tristar controller as the dump load controller. One of the elements in the water heater would be replaced with a 48V element. The wind turbine has a controller with dump load and will use that as a failsafe.

My question is, how do I keep the water heater tank from overheating if I end up dumping too much energy to it from the turbine and PV?

Frank · May 2006

Re: Water heater as dump load

The temperature/pressure valve will open allowing hot water to escape - cold water entering the tank drops the temps down. I plumbed mine outside as there was too much water ending up on the basement floor.

niel · May 2006

Re: Water heater as dump load

you could run a hose off of that relief valve. as it is under pressure watch it can't splash any surface you don't want to get wet.

System2 · May 2006

Re: Water heater as dump load

OR, you could use a thermostatically controlled solenoid valve to drain off water when it gets too hot. (similar to that used in wood fired hot water heating systems, except that these heating systems "dump" the heat into the house, instead of spilling hot water down the drain) The advantage is that YOU could set the max temp of your water, thus avoiding scalding yourself at the sink. The regular safety blow off is meant as a last resort safety device and the water normally is dangerously hot by the time it lets go, plus, many will practically drain all your hot water before shutting back off again. Some are even one time use type, with a metal pallet designed to melt and permanently open the blowoff, but I don't think there aren't many of those around.
Wayne

System2 · May 2006

Re: Water heater as dump load

Frank

As I remember, you actually powered two heating elements from your system. One was controlled using the Aux circuit on your MX60, and I think the other one came directly from your diversion controller on the batteries. Did one system work better than the other? What type of relay did you use for the Aux diversion load and what was its rating? Any thing else you could contribute would be helpful. You seem to have done a great job of working through the issues of coordinating the various setpoints on your controllers.

I have recently put in an MX60 (and love it), but would like to heat water in my root cellar during the winter time to help keep it from freezing during our long Alaskan winters.

Frank · May 2006

Re: Water heater as dump load

Nizina wrote:

Frank

As I remember, you actually powered two heating elements from your system. One was controlled using the Aux circuit on your MX60, and I think the other one came directly from your diversion controller on the batteries. Did one system work better than the other? What type of relay did you use for the Aux diversion load and what was its rating? Any thing else you could contribute would be helpful. You seem to have done a great job of working through the issues of coordinating the various setpoints on your controllers.

I have recently put in an MX60 (and love it), but would like to heat water in my root cellar during the winter time to help keep it from freezing during our long Alaskan winters.

Wow, you have a good memory! The MX60 drives a Crydom DC relay (Series 1?), which is connected to a 25A dump load. I think it's rated at 40 A but I never like to run stuff at full rated capacity. The TS60 still runs a 45A heater and they both work equally well. What I found was that with the TS only, my PV sometimes output more than the TS could divert and the MX would try to enter "Absorbing" mode. Both the MX and TS use PWM and I think they were fighting each other. The output amps would drop until output=input then the system would sweep again. Average output was down. Now the MX just generates amps i.e. it always stays in MPPT.

I generally set the MX diversion down about .5 volt and this means it will try to divert first; this eliminates excess heating in one tank and I don't think I've lost any hot water since I've done this. Once the 25A load is satisfied the 45A load takes care of the rest. Water flow is through the tank with the 25A heater then into the tank with the 45A heater then to the conventional system.

We are in the process though of doing a grid-tie arrangement. Out utility now does a 1:1 credit and I'd prefer to not cycle the batteries. We're in the process of installing a Tarm wood-gasification boiler with storage tank and have access to wood where we live. It won't take much wood to make up for lost preheat watts. I'm trying to get our house to be more carbon-neutral and will use PV and a small windmill to hopefully charge an EV someday. (Actually, I now use a homemade 36VDC m-cycle and would like to convert a car. Grid-tie allows us to recharge when we want.)

Sorry for the rambling reply. (I'll have to check this forum more often too.)

Do you have enough sun in Alaska during winter to do what you want to do?

halfcrazy · June 2006

Re: Water heater as dump load

Mnpower if you turbine has a controller and a dump load i would simply run the 48 volt element thru the thermostat or a aquastat so at a high setpoint it truns off then the batterys will start to climb the trick would be to set the tristar to dump to the heater a few tenths of a volt below the turbines voltage setpoint so basically at a certain voltage lest say 28.8 the tristar will trun on heater element when tank hits say 150 thermostat breaks the circuit so battery voltage rises to say 29.2 and the turbine controller will kick in. battery vlotages are just thrown in there they would be whatever you pick of course

System2 · June 2006

Re: Water heater as dump load

Frank

Thanks for the reply. As you can see, I don’t check this sight often enough either. But I really appreciate your input.

I only have a 580 watt system but will probably be adding 300 watts to fully compliment the MX60 and my 740 AH/12V battery capacity. In answer to your question, we can shut down our Sundanzer freezer in the deep winter time, but will still have a difficult time keeping the batteries charged while at our homestead with our present array capacity (A little bit of help from the Honda generator charges enough battery to get us through the night when necessary. We are about 100 miles form the grid). We use a root cellar to keep things cool year-round, but it begins to approach freezing levels during prolonged sever freezing situations. When we are at the homestead, we can route heat from the wood stove to the root cellar to keep it above freezing, but when we are away for sometimes a month, our batteries will fully charge even in December, and then I would like to use heat load diversion to a dummy load in the root cellar in the hope that it would be sufficient to forestall freezing. I could use an air heat exchanger, but feel that water heating may provide a more even scavenge heat source. I’m not as interested in hot water as I am in bleeding off that heat to keep the root cellar from freezing.

At any rate, I seem to remember that you have in excess of 2kw for your system and hence the need for the double diversion. It sounds like it is working great for you now. I would like to go with the TS60 but did recall that you were having difficulty getting it to sync with the MX60 prior to the second diversion. I am hoping that with my smaller array and the quick dissipation of heat in the cold root cellar that I won’t run into those problems. I should be able to adjust my set points to get MPPT all the time in the winter. Am I making a mistake here, or would I be better off using the Crydom relay? In the summer time we would shut down the dummy load and turn on the freezer (plenty of solar at 61 degrees latitude).

It’s great that you get one to one net metering. Alaska doesn’t have a net metering program yet, but I hope that we can do as well as your State (Maine?) when or time comes. I’ve got a friend with a small EV for the local area (McCarthy is not on the real road system) that he charges with his summer solar, and it seems to work well. That Tram wood gasifier looks like a good unit. I like the technology, but it’s heat rate and central heating component are a bit beyond our needs here in the bush. I like your concept of thinking globally and acting locally. Keep up the good work and keep us informed on how your projects are working out.

Thanks so very much for your help. I plan to plow forward with load diversion and really appreciate your thoughts

Nizina

Water heater as dump load

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