Opportunity Load - Heat Pump Water Heater - Anyone do it?

fowlerrudi

I know most off gridders use their AUX relay on their inverters/charge controllers to dump power to an electric element in a water heater - if their batteries are full. How would it be possible to throttle the load with this type of heater? Just thinking.

waynefromnscanada

Re: Opportunity Load - Heat Pump Water Heater - Anyone do it?

fowlerrudi wrote: »

I know most off gridders use their AUX relay on their inverters/charge controllers to dump power to an electric element in a water heater - if their batteries are full. How would it be possible to throttle the load with this type of heater? Just thinking.

I used to use the Aux signal from my MX60 to cause my freezer to run continuously until the voltage dropped at sundown, or whatever, but that used a whole lot less power than a water heater. On top of that, if it became partly cloudy and the voltage began to drop, the Aux would shut the freezer off, then with reduced load the voltage would go back up and the Aux would try to restart the freezer, which failed because back pressure on the compressor had not yet drained down. Cured this with a 10 minute delay added to the Aux turn on. Of course that wouldn't be a problem with a heating element, but does show the system will cycle under the right conditions. You might have to go with a reduced wattage element, or use a 220 volt element on 110 volts, which will then consume 1/4 it's rated power.

solarvic

Re: Opportunity Load - Heat Pump Water Heater - Anyone do it?

I have GE hybrid waterheater. It is wired for 240 AC so it wouldn,t operate on DC power. When full of cold water the high capacity heating element first comes on for a few minutes. After the water gets to a set temperature the heatpump system can take over. i have heard it takes about 700 watts 240 ac to run heat pump. So I don,t tjonk the heatpump idea will work. Solarvic

mtdoc

Re: Opportunity Load - Heat Pump Water Heater - Anyone do it?

Well I have a Nyle Geyser add on heat pump for my water heater and it works very well. It runs off of 120 - drawing about 600 watts. I am not off grid but have a hybrid system and sell extra power to the grid so I don't really need an opportunity load except possibly during extended power outages. But then again, those are usually in the winter when sunshine is at a premium.

This will be my first winter with my second array so if there's an extended outage and there is extra power then I'll run the heat pump to heat water. I've bought a coil to heat water in my woodstove but I haven't set this up yet.

I've thought about using the Aux out on one of my CC s to drive a 120 VAC relay to turn on the heat pump as an opportunity load in such cases but haven't had the need yet.

Eric L

Re: Opportunity Load - Heat Pump Water Heater - Anyone do it?

How would it be possible to throttle the load with this type of heater?

I don't think you can throttle the load with a heat pump water heater, since I think it needs full voltage. There may be compressor designs that could take a variable frequency drive that could be used as a heat pump, but it would be pretty complicated to get that to work smoothly with the aux controller on a charge controller.

You could try to have a mechanical relay turn to the heat pump on when the batteries had been in absorb charge for a while. Some charge controllers will let you specify that that after "x" minutes of absorb charging, a load is turned on by the Aux function. You'd probably have to experiment to see at what stage in absorption charging you had sufficient surplus power to carry the load without dropping out of absorb. I'm not sure how it would work when it got cloudy though.

Alternatively, many heat pump water heaters have an electrical element as well. If that's the way yours is set up, and you have the right kind of charge controller, like a Midnite Classic, you could power the electrical element using an SSR and the "Waste not hi" function on the Classic. Here's a discussion of it:

http://www.fieldlines.com/index.php/topic,147132.0.html

This is working pretty well for me.

Eric L

Re: Opportunity Load - Heat Pump Water Heater - Anyone do it?

I am not off grid but have a hybrid system and sell extra power to the grid so I don't really need an opportunity load except possibly during extended power outages.

mtdoc,

I expect you have a lot more favorable grid-tie interconnect agreements than I do in Alabama. But if, as in Alabama (specifically Alabama Power), you were only paid wholesale for all surplus watts sold, while being charged retail for all grid watts used, it would probably make more economic sense to heat your own water first using the SSR + Waste not Hi function.

Using this function, I get near 100% capacity utilization of my PV array, like a grid-tie system would, despite having a battery-based system. The one exception is that if we get multiple very sunny days, my hot water tank finally gets so hot that the thermostat shuts the elements off, and in those cases I lose a little production (the solution will be to eventually get a very large tank).

But otherwise, I'm very close to grid-tie levels of pv capacity utilization.

mtdoc

Re: Opportunity Load - Heat Pump Water Heater - Anyone do it?

Eric L wrote: »

mtdoc,

I expect you have a lot more favorable grid-tie interconnect agreements than I do in Alabama. But if, as in Alabama (specifically Alabama Power), you were only paid wholesale for all surplus watts sold, while being charged retail for all grid watts used, it would probably make more economic sense to heat your own water first using the SSR + Waste not Hi function.

Yep. I get paid 0.18 for every Kwh produced (whether consumed by me or sold to the grid) while I only pay 0.065 per Kwh bought AND I net meter..;)

As long as this incentive program lasts I get no benefit from using opportunity loads. My chargers stay in Bulk mode (at float voltage) and sell to the grid any power I'm not using. I meter all the power I produce and once a year get a check from the utility for the 0.18/Kwh.

Using this function, I get near 100% capacity utilization of my PV array, like a grid-tie system would, despite having a battery-based system. The one exception is that if we get multiple very sunny days, my hot water tank finally gets so hot that the thermostat shuts the elements off, and in those cases I lose a little production (the solution will be to eventually get a very large tank).

But otherwise, I'm very close to grid-tie levels of pv capacity utilization.

Yeah the way you and Chris are using the aux on the Midnite classics to heat water is excellent.