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Heating water with excess power
#31
Just leave the main tank on all the time. It should not use much energy, getting refilled with lukewarm~hot water from the preheat tank.
Will save you a lot of hate from the 3 other members who have to shower cold.
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#32
Another option would be to just connect the tanks in parallel with a temperature regulated valve. Which ever side is the hottest it would allow through. You might would need to use two of them, tho.

So, to start with, the valve would allow both tanks to flow equally. When one side got hotter, it would switch more to that side.

This would allow you to have two tanks that would always provide hot water, regardless of which tank is hotter
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#33
Ok so this is the plan (subject to a lot of change). I have power distribution block for the positive and negative pv panel wires from each array that I use as a combiner. I’ll post a pic of it. Positive on top, negative on bottom. There’s openings-extra spots. I’m going to wire a positive and negative wire from this distribution block to the bottom element of my hotwater heater. I’m getting the dc for the element before the batteries-charge controller. This will be less stressful on the charge controller simply because the charge controller doesn’t have to power the element at all. It will also insure that the batteries are not drawn from at all to heat the element.
Question about that. Since the element is resistive will it be ok to NOT HAVE the charge controller mppt the power that goes to the element?

I said above that I could not have one of the elements powered by dc. This is because there was a possibility that the hot water heater did not receive enough excess pv power through the day to fully heat the tank. That I would have to heat the water with ac sometimes and with only one ac element at the top of the tank, the tank wouldn’t fully heat up.
I have figured out that since I have a pump that circulates the water in the tank to a copper coil on my roof . I can use that to circulate the heated water at the top of the tank that was heated by the top element to thoroughly heat the entire tank if needed. In the winter the water in the hotwater heater circulates through a copper coil in my wood boiler.

By the way, The cooper coil on the roof is only 3ft x 3ft, works well when suns really shining and the outside temps over 90 degrees. When it’s hot and suns shinning, then hotwater heater (when water in it is tap cold to start the day) only has to run for 20 minutes instead of an hour
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#34
Batteries are technically resistive load. So that doesn't matter

You should probably not run power from the solar panels in parallel, especially with one of the loads "not" being a controller. This could throw off the MPPT algorithms and you may end up with worse performance in the end.

Batteries won't dissipate "unless" there isn't enough power produced by other sources. So it'd be safe to connect on the battery side.
The battery will only discharge if there isn't enough current on the line to support the connected devices.
Since you'd be disconnecting the heating element when the batteries need to be charged, this isn't an issue.
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#35
For now I was thinking about continuing to use the 240v 3000w element that is in the hotwater heater. There’s 100v coming from the pv panels. I’m thinking that element would be able to use roughly 1000w max from the pv array. Is that correct?

Also what about the thermostat connected to the bottom element. I seen the contacts burning up when people were using those thermostats for dc? Should I get a new dc thermostat?

(09-01-2020, 03:08 AM)Korishan Wrote: Batteries are technically resistive load. So that doesn't matter

You should probably not run power from the solar panels in parallel, especially with one of the loads "not" being a controller. This could throw off the MPPT algorithms and you may end up with worse performance in the end.

Batteries won't dissipate "unless" there isn't enough power produced by other sources. So it'd be safe to connect on the battery side.
The battery will only discharge if there isn't enough current on the line to support the connected devices.
Since you'd be disconnecting the heating element when the batteries need to be charged, this isn't an issue.
Since the water heater element is a resistive load, shouldn’t the charge controller just basically see less power from the array therefore not affecting the mppt. Example total array power could be 3660w, element is using 1000w so array would just be a 2660w array that the charge controller could use till battery volts drop and element shuts off then array is again 3660max as far as the charge controller can tell.

When battery voltage drops from the array not giving enough power the aux of charge controller will open the relay allowing the rest of the power from the array to travel to the battery instead of the element
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#36
You won't be able to use standard contacts for DC that was designed for AC. That's why those thermostats fail. They aren't rated for DC loads. AC because it's alternating will quench the cutoff arc. DC will sustain the arc.
You need to use a contactor or SSD that is rated for high DC voltage and high DC amps. It would be good to go double the rating of what you need.

As far as the MPPT seeing less power. I wouldn't think so. It's the way that MPPT has to work. It tracks the power. In other words, it will actually change the voltage/amp output of the panels as it works. So even tho you have 3660W panels, the voltage will fluctuate as they are used by the MPPT. So at one point, the voltage may be 100V, another it might 90V, another 120V, etc. It all depends on what the MPPT detects as being the proper max "power" output of the panels.
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#37
Use the existing thermostat to control a DC rated contactor or maybe SS switch.
Be sure to consider what happens if eg you go on holidays, systems heats water to hot, but contacts weld or SS switch shorts & can't disconnect element.
Pressure relief valves must be installed & operational like "normal".
There's a Mythbusters episode about this.....
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#38
Preheating is just another way to store energy efficient and not having as much losses in terms of having the main system do all the heavy lifting. Of course you need to insulate. A tank that is insulated properly should keep decent showering heat for many days...

When i heat up my tanks i have showering water for 1.5 week atleast..

See it as you spend 5kwh of preheating power from the battery. Then you only need 15 more from the regular to get it up to full. What this does is just to make it simpler having 2 systems doing it and it can be the way to go or instead do it properly and divert the enrgy to the main directly.

I preheat at home but i do it with 2 sources. Excess solar vs firewood. Soon to have a 3rd that is direct solar. Still my main is the firewood since that can produce 50kW of heat compare to electric that is 9kW at most...
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#39
I want to heat my water to use up all of my available solar. My tank is insulated. They come insulated. I’m going to add more insulation. The water temp in the tank drops roughly 10 degrees overnight. I suppose this happens because the inside of the house is 72 degrees. This drops the water temp low enough so that it’s no longer at the temperature the wife and teenage girls want to shower at. I do not use preheating power from the battery. Right now I use the available excess solar, when the battery is full, I use the excess pv to heat the tank to 110 degrees. I have to do this manually by turning on the water heater. My tank takes 4Kwh to fully heat from 75 degrees tap cold to 110 degrees. In winter I use a wood boiler that will heat the water mostly on its own. During the summer I use the copper coil on the roof and I start the water heater when battery is full and there’s excess pv power. The copper coil drastically reduces the run time of the hotwater heater. I was thinking about making the copper coil bigger to fully heat the tank in the summer, but I have enough excess solar most days so I figured id go this route instead. So pv power wasn’t just sitting there not being used.
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#40
@LithiumSolar did a youtube using a pre-heat 6 gal water tank w/Sonoff Switch as control Smile
Heating Water with Excess Solar using a Reliance 6 Gallon Tank and a Sonoff Switch   https://youtu.be/jVEH0eAinBs
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