Solar hot water, elements in series?


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Davo99
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Joined: 03/06/2019
Location: Australia
Posts: 519
Posted: 08:09am 02 Mar 2020      

Finally, something I know a bit about!  :0)

First suggestion to over come the load problem is a voltage sensitive relay.
You connect this between the water heater ( through a larger normal relay or contactor) and the inverter. It monitors the voltage in the system and you set it up so when the voltage sags due to load, the thing kicks out. When there is plenty of power, it will kick in.

I have this set up on my water heater.  When the solar pushes up the voltage in the wiring, the relay kicks in and sends the excess power to the heater. When there is no excess or there are other loads that pull the voltage down ( thing is sensative to a volt) it dumps the load.

voltage relay

These are available in voltages down to 48V and run on DC so you could set one up potentially to monitor your actual battery pack voltage rather than the inverter voltage and load control from that.

Another option is to get some used panels and set them up in a direct heating application to take the load off the inverter and your batteries all together.
Having a dual element tank is ideal as you could set the dedicated array to one element and use the other on the inverter which would reduce the over all power consumption.

Next Suggestion and also one I use ( several of) with the water heater is a PWM Controller.  The beauty of these is you can vary the effective wattage of your element from a few watts to full tilt.  I use mine with a power meter with an inductive pickup and can see how many watts I'm driving the element to.

4Kw PWM

Rather than have a fixed amount of power as in using 2 elements, This would allow you to use whatever power you have to spare. Seems to me it would be annoying to have 3 KW to spare because your batteries are charged and maybe you are even going out and won't be using anything but the heater is crawling along at 900W when you could be using that power to full advantage. Using one of these controllers lets you regulate the power to what ever you want.  Could  be 300W or could be 3 KW.
It's also a lot cheaper than buying 2 new elements.


With the thermal water heaters, a lot of them are drain back now.  The water is pumped through them but once the pump stops, they automatically drain back to the storage tank and empty themselves so there is nothing in the collectors or tank to freeze. Don't know if it's possible to re engineer older systems but just throwing food for though out there.

Another setup I have used VERY successfully but probably too out there for most sane people is to modify a gas heater by replacing the burner with an oil burner.
You just fire the thing up and have VERY hot water in very little time. It's way harder to keep the output of a burner down to 20Kw than it is to make one that cranks out 100Kw.

I used this in real world practice when my water heater bust its seams at the old place.  Pluumbed the oil burning heater in to the hose with hose and sat the heater out on the back verandah.  Fired it up and got it up to speed then everyone took the longest, hottest showers we ever did have.  Thing kept up with demand very easily, in fact the blow off valve let go at one stage due to the water being too hot.

The only problem with this is that gas heaters are hard to find in anything over 125L.  It's good to only have to fire the burner up every 3 days so what I did was get a big 400L electric tank and use that just for storage and got a solar circulation pump and ran the water round so I had 500L of hot water.
Lasted the 4 of us for about 4-5 days.

You can burn any old oil you like, I prefer used veg oil but you can burn engine oil, Lard/ fat, transmission oil or mix them all together.   You could also pretty easily make up a little fire grate and use wood to heat the tank if you have it.  Circulate the water in your main tank and you'll have more hot water than you ever had before in winter and plenty of power to spare as well.

I am in the process atm of building  a heater for the house out of a little 100Kw spa heater. Removed the gas burner and built a little oil burner and will run that at about 10 Kw but also think I can stage the thing pretty easy so it will be able to do maybe 5Kw  or flick up to 50kw for faster heat ups.  Also looking at using a 400L water tank so I can store excess heat and have that in the morning and don't have to go out and light the thing till it warms up to a cosy 5 oC or so before I venture out.  :0) $00L would store about 30Kwh of heat energy so definately a worthwhile amount.

I was thinking of using a little 20 plate Flat heat exchanger and using a circ pump to heat the hot water tank but I think with the house heating load removed or greatly reduced, I'll have enough solar to get me through the winter with the extra 5Kw or so of panels I'm putting up.


This is the energy/ time / power calculator I use when doing the mental arithmetic on water heating:

https://bloglocation.com/art/water-heating-calculator-for-time-energy-power


Anyway, a few potential solution suggestions to your problem, hope something is workable for you.
Edited 2020-03-02 18:57 by Davo99