Powering cylinder from solar panels in an off-grid situation

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Powering cylinder from solar panels in an off-grid situation

Postby kingblert on Fri Nov 10, 2017 2:20 am

At our off-grid cabin, I want to power a 240V 3 kW residential hot water cylinder more or less directly from a 5 kW solar array (when the sun is shining). The panels are not required to do anything other than heat the water. If the sun is not shining then we have no hot water - that's OK.

I want to use as few as possible parts, except for the panels themselves and the hot water cylinder.

Looking for design recommendations.

Here are some solutions I have rejected.

Solution 1 (the "chuck the kitchen sink at it" solution):

Connect the panels to a charge controller such as an Outback FLEXmax 60/80 (http://www.outbackpower.com/outback-pro ... exmax-6080)
Connect the charge controller to 4 x series connected 12V car batteries (no real storage effect - just enough to keep the inverter charger working)
To the "battery bank", connect an inverter/charger like the FREESCAL SX1250 (datasheet: http://www.kynix.com/uploadfiles/pdf9675/SX1250.pdf)
Connect the AC out from the inverter to the hot water cylinder
Add to the above some kind of circuitry to turn off the inverter when the sun is not shining (we never want to actually dip into the ultra light-weight battery bank).

I believe this would work, but would be hideously expensive. Also when the power from the sun drops below the 3 kW drawn by the water heater, the inverter needs to be shut down - whatever power is available is wasted.

Solution 2 (the "AC? who needs stinking AC?" solution):

Connect the panels in a series/parallel configuration, with a blocking diode in series with each string, such that the peak voltage from the array is < 240V (the rating of the water heater).
Connect the resulting DC directly into the heater element of the water heater.
I like this solution from the point of simplicity, and virtually zero parts cost. Also it makes best use of the available solar energy, and will work even when the sun is low, by just pumping a lower DC voltage into the heater.

However I am concerned about damage to the heater's element due to electrolysis, from running it on DC instead of the AC that it is designed for.

Solution 3 (the "dunno quite how" solution):

Some kind of DC to AC chopper that can turn the DC from the panels into AC, at <= 240V.
I don't believe the heater will suffer if it is driven from a lower voltage than it is designed for. This solution also makes maximum use of the available solar energy.

However I'm not familiar with building this kind of circuitry, or whether this is in fact a good idea or not.
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Re: Powering cylinder from solar panels in an off-grid situa

Postby Mike Curran on Fri Nov 10, 2017 6:31 am

You don't need a charge controller, inverter or batteries. Just wire enough panels in series to get 240 volts and connect it directly to the heating element of your "cylinder". The heating element doesn't know AC from DC, and there's no electrolysis since the element is insulated from the heated fluid (water, in this case). Water heating elements are commonly used as "dump loads" from DC systems.

The only downside of this is figuring a way to disconnect the panels when your water heater reaches the temp you want. With no other power sources you don't have ability to insert a relay in there somewhere. So maybe you do need a small battery setup, just for that purpose. Wouldn't require a complicated or expensive charge controller, but if you used OB equipment you'd have to tap into your panels someplace where the voltage would never exceed 150V max.

Two cents' worth.
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Re: Powering cylinder from solar panels in an off-grid situa

Postby blackswan555 on Fri Nov 10, 2017 9:05 pm

As Mike says but to add, I would wire panels for an approx VMP of 220v and add a use a thermostat (as it would do normally anyway) one thing to watch is the amp rating of the stat, It must be rated for the DC amps it is going to use, Not just a normal household AC type,

My comments are based on my experience and research, They are not endorsed or checked by Outback.I am an independent British electrician living in Spain, So please take this into account when reading /acting on my post`s.
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