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Forum Index : Solar : Solar hot water, elements in series?

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Gizmo

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Joined: 05/06/2004
Location: Australia
Posts: 4802
Posted: 02:57am 02 Mar 2020
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I currently use a 330 litre hot water tank and a couple of thermal solar panels for my hot water. Its got a controller and a 12v circulation pump. The plumbing is leaking, and in winter I need to cover the panels to stop them freezing, its a bit of a crude setup.

I was thinking of just heating the water with my existing off grid power, using 230vac and a timer so its only heating during the day, plus the tanks own thermostat.

Problem is my inverter is only good for 3kw to 4kW continuous, so if I'm running the air conditioner, and flick on the kettle, I really only have a 1000 watts to spare before the inverter starts beeping and shuts down.

I cant find a source of 230vac hot water elements under 1000watts. This hot water tank does have two elements, one near the top and another near the bottom. If I fit 2 1800 watt elements, wired in series, that should be 900 watts.

I'm guessing this would take 4 to 5 hours hours to heat up the tank, which should be OK. Plus the hottest water will be near the top outlet, and I'm living alone so dont need that much hot water.

Thoughts? Are there any issues with running elements in series?
People say 2020 is a terrible year, with the bush fires, COVID 19, and riots. But I see it as the year we woke up to ourselves.

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Warpspeed
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Joined: 09/08/2007
Location: Australia
Posts: 3263
Posted: 04:05am 02 Mar 2020
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Elements in series should be fine.
Cheers, Tony.
 
Solar Mike
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Joined: 08/02/2015
Location: New Zealand
Posts: 639
Posted: 04:22am 02 Mar 2020
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Electrically ok, see this Time To Heat Water link, if you only have 900w available and heating 330l by say 30c rise, then will take approx 12 hours.

I'm doing a similar scheme at a rental, except the mains power heats the top element only and its on a timer (not on during the day), lower element connects to 3.6KW PV array and acts as a pre-heater or if enough sun, the top element does nothing when it switches on.

At home I have DIY water heating panels on the roof, if its frosty, a valve on the panel opens and dumps the about to freeze water down the drain, replaced by warmer city supply water; approx 3L lost each occurrence, may do this 10x in a night.

Cheers
Mike
 
Davo99
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Joined: 03/06/2019
Location: Australia
Posts: 494
Posted: 08:09am 02 Mar 2020
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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
 
Gizmo

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Joined: 05/06/2004
Location: Australia
Posts: 4802
Posted: 11:21pm 02 Mar 2020
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Thanks guys.

The tank is well insulated, so it wont need to heat up from cold every day, which will reduce the time to reheat the used hot water. Of the 300 litres I'm probably using 50 or so litres daily, washing dishes and a shower or two.

I do have a gas on demand heater, so when its overcast and the tank is cold, I switch over to the gas heater. There have been cold mornings when I have been very very thankful for my gas water heater  

Yeah been very interested in using solar directly with a low voltage element. Project for next year.

Glenn
People say 2020 is a terrible year, with the bush fires, COVID 19, and riots. But I see it as the year we woke up to ourselves.

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Gizmo

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Joined: 05/06/2004
Location: Australia
Posts: 4802
Posted: 12:33am 05 Mar 2020
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I've changed plans. Decided to go the low voltage element path. I've ordered a 48v 1000w element, and sourced some 230 watt 60 cell solar panels locally for $55 each.  


I'll be using a different tank to. My 300 litre tank is getting rusty, and I have a 120 litre tank in good condition, plus it has a thicker layer of insulation. Using that formula Solar Mike linked to, and assuming I get 800 watts power from the panels, if the tank drops to 30c over night, 3.5 hours of sun will bring it back up to 50c.

Even in winter, say I only get 500 watts from the panels, it will take just under 6 hours.

Glenn
People say 2020 is a terrible year, with the bush fires, COVID 19, and riots. But I see it as the year we woke up to ourselves.

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Davo99
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Joined: 03/06/2019
Location: Australia
Posts: 494
Posted: 04:06am 05 Mar 2020
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You could always turn the thermostat on the tank up to 70 or 80 to bank some extra heat if you get a cloudy day.  If the tank doesen't get that hot, it will get as hot as it was going to anyway.

It's been lightly raining here all day today.
Just checked my main inverter and it's produced a whopping 2.65 KWh for the day.
On a clear day this time of year I'd expect to see a tad over 30 Kwh.
That's what you have to look out for as I'm sure you well know.

Some time back a mates brother worked for Dux. They were looking to take the water up to about 90oC. The reason was having the water hotter meant the tanks effectively were larger while actually being smaller to fit in apartments and the like without taking as much space. When used with the mixing valves there was effectively more " Useable" hot water.

I don't know if they ever went with that or it got dropped.
 
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