Measuring and acting on solar export


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Davo99
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Joined: 03/06/2019
Location: Australia
Posts: 1507
Posted: 10:48pm 11 Jan 2022      

  Technophiliac said  [

I've been pondering the possibilities while trying to use one thermostat, for example running one element on night power, and doing green things with the other two elements, for example green power into one element or 2 elements or even 2 elements in series with switching between these options possible.


I have been mulling over the Exact same thing.  Saw it suggested somewhere, maybe here and got me thinking.

In theory it sounds good but trying to do the mental arithmetic for me seems to come up short.
I can run 2 elements off panels, no problem so we have hot water at night when we shower. Thing is if the 3rd element is then connected to mains, we shower at night, the thermo kicks in and we pay for the water to be heated and the solar powered elements do bugger all, just maintain the temp and make up for any of he minimal use through the day.

Ideally would want to be using the mains in offpeak times which is not when I want the heating.  We shower at say 10-11 PM, the off peak comes in at 11 and again does all the heating when we don't need it. I'd really want it to kick in about 8 PM so it makes up any shortfall in the tank temp and is right for when we want to have the main use.

Only thing would be to have the mains power switched manually so it only kicked in  during bad weather when the solar was lagging. Be handy to have a temp read out to know when it needed boosting or switch it to the mains.

If I have 2 elements on solar and with the element I bought being 6 KW ( although not fitting any standard tank here which is a problem) If I'm getting about 1.5KW out of them each or even half that in crappy weather, then the times I should need to heat from the mains will be minimal here.

If I put a controller in between the panels and element, Would be easy to overclock the element so there was good power in any but the worst weather and I should only need a single element if I have plenty of panels on it.  even 1500W will raise 400L of water from 50 to 70oC in 6 hours so that would add up.  I don't know what the tank drops to in the morning. Might be an idea to start taking temps and see.  

I thought I can do that with a Simple DPDT relay.
Have the thing wired on the resting side to the Solar and have the Mains Pull it in when powered. That way the 2 sources will never be connected at the same time.
The solar side will need to go through a DC relay from the thermostat but that's no issue either and that won't be closed when the mains is in so again the 2 would be isolated and no problem.

My 3rd idea is having a small generator for when the weather is bad. Use one of the little diesels I have just driving an IMAG.  Perfect job for one of these. Only thing they are good for is constant non switched loads so the thing could sit there for a few hours putting away and I wouldn't need a mains connection at all.

My hot water is 400L so we get about 3 days out of it before it gets too cold for a shower which makes me think the water temp isn't dropping a lot and I might get 2 days worth of heating, certainly 1 in every 3 days of bad weather which would give us a window of 4 days minimum. Being a grub, I don't always shower every day in winter anyway. If I have been inside and not out working, I don't stink so will pass that night. That would reduce the load a bit as well and might give a bit more breathing space before outside power is needed.  Thinking next heater I get will be a twin element but not really sure how advantageous that would be in reality. Make it easier to put 2 Solar feeds I guess.