Electric panel resistive element water heating

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Re: Electric panel resistive element water heating

Postby davidg » Sun Jul 31, 2016 11:31 pm

APR wrote:I may then use a a 900v 30a DC-DC SSR to automatically disconnect one of the paralleled strings when insolation is high and the combined amperage of the paralleled strings would exceed the DC input rating of the MPPT.

Note thats a peak rating the proper rating is 420VDC further down the page, also typically SSR's should not have more than 60% max of the current rating continuous through it, otherwise things tend go very wrong, and they need a proper heat sink for the same reason.
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Re: Electric panel resistive element water heating

Postby APR » Mon Aug 01, 2016 9:16 am

davidg wrote:Note thats a peak rating the proper rating is 420VDC .... SSR's should not have more than 60% max of the current rating continuous through it .... a proper heat sink for the same reason.


Thanks for that Dave. I have ordered a heatsinks that is specific to the SSR, and do have a bit of heatsink lying around that would be more than adequate (overkill) for the expected dissipation. I am still to order this SSR, and am still deciding whether I will connect these latest panels to the inverter, or whether I will connect them straight to the HWS element. I have already ordered a 25 amp 250v DC-DC SSR to switch power to the HWS element if I go that way. In either instance the amps through the SSR would be a max of 10 amps.

If I connect these latest panels to the inverter I will be paralleling my existing strings and using the SSR to switch one of the paralleled strings in and out. So, this SSR would be connecting the OC voltage of one string to the loaded voltage of a second string and the reverse, and the voltage across the SSR would be no more than the 150 volts difference in panel voltages at the time of switching.
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Re: Electric panel resistive element water heating

Postby Quokka2 » Sun Oct 16, 2016 10:42 pm

Sorry to be a late starter on this thread but I have been very glad to find it after thinking about hooking up some spare panels to a HWS and wondering if it was a whacky idea.

With due regard for davidg's reminder about Mr Ohm's Law, I'm thinking it should be possible to closely match a solar PV array to TWO HWS elements in parallel (~8 ohms). With one of Gordon's virtual trackers you could have reasonably constant output all day, in which case would the $383 controller from Techluck be really necessary? You could use that sort of money to buy a small electric HWS to increase your hot water capacity, or you could use both elements of a larger HWS.

That begs the question of using mains backup with the same element; I guess some reasonably heavy duty 6 pole relay would need to be used (presumably triggered by the solar panel output). Has anyone out there tried this?
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Re: Electric panel resistive element water heating

Postby APR » Mon Oct 17, 2016 1:00 pm

Quokka2 wrote:I'm thinking it should be possible to closely match a solar PV array to TWO HWS elements in parallel (~8 ohms). With one of Gordon's virtual trackers you could have reasonably constant output all day...


I did have the same idea, but gave it away. I am using a power diverter that diverts excess solar generation, that would otherwise go to the grid, to the HWS element. The issue I had in winter was that I had days of low insolation when there would be no, or very little, excess solar generation to heat water. My solar generation first goes to the house, then to charge batteries, and any left over then goes to the HWS.

I considered installing a second tank that was vented, connecting panel DC to an immersion element in that tank, and running the inlet water flow to the main tank through a heat exchanger in this second tank to preheat the water going to the main tank. With the second tank being vented, if the water in the second tank boiled there was no chance of pressure buildup and a tank explosion. I was also proposing a temperature control system, but considered the need to vent the second tank a safety requirement. The existing HWS and excess solar generation power diverter was to be left in place.

There is an Australian Standard detailing how the electrical supply to the element in a pressurised HWS is to be switched, and my understanding is that the thermostat on a pressurised tank is to directly switch power to the element. The thermostat contacts cannot be used to control a second set of contacts switching the supply, and when fed DC, the HWS thermostat contacts may be unreliable because of arcing on DC. I understand the MPPT controller you mentioned is designed to directly use the thermostat to switch the element supply. In the end I decided I was better off installing a second inverter and keeping the HWS as it is. However...

With an element resistance of 8 ohms and 2 x 30.5v MPP panels in series, an 8 ohm element will draw around 7.6 amp at the panel MPP voltage for a wattage of 465 watt. But, if the panels are only capable of putting out 6 amp because of low insolation than the panel voltage (2 panels in series) will be pulled down to around 48 volt, and the wattage through the element will be 48 x 6, or around 290 watt.

If we go up to 3 panels in series we are looking at a string MPP voltage of around 91.5 volts, and with an 8 ohm element, that equates to 11.4 amp current draw which one 3 panel string of 250w panels has no hope of achieving. If we parallel another 3 panel string we are requiring less than 6 amp per string, but even that is an ask for a 250w panel in winter in my area.

I was considering an element of 19 ohm and feeding it with 2 strings of 5 panels. The panel MPP voltage for a string would be 152.5v, and the current draw of around 7.9 amps meant I would need 2 strings of 5 panels for around 1,200 watt through the element. So each string was to produce just under 4 amp which history demonstrates is achievable for adequate time on most winter days in my area to heat enough water to meet the demands of the house.

That begs the question of using mains backup with the same element; I guess some reasonably heavy duty 6 pole relay would need to be used (presumably triggered by the solar panel output).


This is another reason why I gave up the idea, as it gets too messy in my situation. If I need to heat water using AC outside solar generation hours the energy will come from the battery pack which I don't want to happen. While I could get around this, it was easier for me to throw more panels on the roof with another inverter so I have more than adequate power through winter to meet all my needs, and in summer I earn more $ from feeding back to the grid.
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Re: Electric panel resistive element water heating

Postby Quokka2 » Mon Oct 17, 2016 9:05 pm

Thanks APR for your considered and detailed reply. Although we get a nice FIT at the moment it is due to expire in about 3 years and I need to devise ways of using all my 5kW presently on the grid to stop the mercenaries at the utility getting even one kW-hr from us for 8 cents :-) So I am experimenting with another 2kW which we have off grid and a spare 1.5kW or so of panels still in the shed. They are all 190W panels, so voltages are a bit higher than your examples (37V pmax).

Your idea of a preheat tank with a heat exchange coil is actually what we did years ago using a big solar thermal array; we also had it connected to a house heating system for Winter and a pool heating system for Summer, but the latter two heat exchangers (water to air and water to salt water) eventually succumbed to corrosion and leaks. We tilted the array up to 57 degrees to even out the seasonal gain; this simple expedient seems to be overlooked by many.

Still looking for suggestions if anyone else is interested.
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Re: Electric panel resistive element water heating

Postby SandyP » Mon Oct 17, 2016 11:19 pm

APR wrote:I considered installing a second tank that was vented, connecting panel DC to an immersion element in that tank, and running the inlet water flow to the main tank through a heat exchanger in this second tank to preheat the water going to the main tank. With the second tank being vented, if the water in the second tank boiled there was no chance of pressure buildup and a tank explosion. I was also proposing a temperature control system, but considered the need to vent the second tank a safety requirement. The existing HWS and excess solar generation power diverter was to be left in place.


Another way of achieving a mains pressure vented hot water system you may be able to use, if you can add a pipe and/or re route your plumbing, is to have the upper hot water outlet of the second hot water tank open (pipe with no tap on it) going directly to your shower or basin/sink outlet where you want the hot water to go to. The "hot water" tap for that outlet is re routed to supply cold water to the base inlet of the second hot water tank.
Result : If the second tank boils the steam exits via the pipe to the outlet where generally it will condense before it reaches the shower head / basin outlet.
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Re: Electric panel resistive element water heating

Postby offgridQLD » Tue Oct 18, 2016 1:24 pm

Rather than start a new topic I thought this was somewhat on topic.

I have a friend who has developed a MPPT solar/ inverter unit that outputs AC to a standard resistive element hot water system. At this stage he is going through the process of all the approvals to have it listed for rebates. The unit can also power other (resistive loads or pumps though nothing with any smarts...electronics) . I have talked about testing one and giving feedback as I am in the need for a solar hot water system but not 100% on it.

The rebates REC/STC are the main draw card of the system as from what I can see. Say 2500w of PV is a larger rebate than the typical collector panel/tube hot water system would get. The price of PV makes it competitive . The systems are very simple to install and a lot of people have a tank already. Though the space required for 2500w of additional PV would be a issue for a lot of people.

Though I'm not a big fan myself of rebates and the time frame for him to get the thing approved might be a issue for me. So I haven't made any commitments to that system yet and I am looking at all my options.

At this stage I have been looking at components.

Tanks. No way do I want tanks on my roof. So I have been looking at 315lt ground mount systems. I also don't want anodes so the tank must be stainless steel.

Collectors. Seems to be 30 vac tube collectors are a good match for a 315lt tank and 3-4 person house.

Recirculation pumps and controller. lots of options though typically a 3 temp sensor unit with a good brand name brass 3 speed pump and most of the controllers are generic units used on lots of systems.

I need to get something soon (before the end of the year) as I will be living full time at my offgrid house and the instantaneous gas that's there now will be very expensive full time. (probably $750 - $1000 pa in gas and bottle rental)

Gordon, I had a look on the energy matters webpage for SHW systems ...pricing but I found it hard to navigate and looked like it wasn't up to date listing old hills systems. What's the best way to get a supply only quote of a split system 315lt SS tank, 30 vac tube manifold , pump /controller and miss valves ect. as a package.?

I have had one quote for $3500 on a system from the gold coast.

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Re: Electric panel resistive element water heating

Postby Gordon-Loomberah » Tue Oct 18, 2016 2:15 pm

Hi Kurt, try here for a quote, using the comments box for any features/specs etc that you want: http://www.energymatters.com.au/solar-h ... shw-quote/
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Re: Electric panel resistive element water heating

Postby offgridQLD » Tue Oct 18, 2016 2:41 pm

Thanks Gordon gave that a go. :)

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Re: Electric panel resistive element water heating

Postby offgridQLD » Tue Oct 25, 2016 6:10 pm

well no luck on a price or response from EM (perhaps a webpage malfunction). I ended up with 315lt SS tank and 30 vac tube split system should have it later in the week.

I still plan to help trial the Pv/electric inverter unit for my friend. At first just using the booster element in the tank. Though in the future if I stubble across a smaller tank 100lt+ I will use it full time for our 2nd bathroom 9that doesn't get used that often) as a electric only HWS.

Will report back when I have the test unit up and running.
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