Solar Hot Water ...

hbpowerwall

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Solar Direct or DC that is my next question... or challenge maybe even.


Really just a first look at how all this will work, now I've kinda got the jist of it I'll need to find somewhere useful to install it. If your keen Patron's already know :)

Kudom 100amp DC relay - https://ebay.to/2KIgNGG
DC Element 600w 24v https://ebay.to/2K5aVIk
Clamp Meter -- https://goo.gl/z8b4uJ
 
With HWS it's a really good idea to make sure the thermostat's contacts don't weld with the DC. They're only intended for AC.
Fortunately with that low wattage element, the tank pressure valve will cope easily if there's an event where the SSR fails shorted (on) & the thermostat's contacts are stuck.
You've seen the Myth Busters Hot Water Tank episode right?
 
Redpacket said:
With HWS it's a really good idea to make sure the thermostat's contacts don't weld with the DC. They're only intended for AC.
Fortunately with that low wattage element, the tank pressure valve will cope easily if there's an event where the SSR fails shorted (on) & the thermostat's contacts are stuck.
You've seen the Myth Busters Hot Water Tank episode right?

Pete's sensibly just using the tank stat to make/break the SSR control signal, it'llbe fine.
 
Sean said:
Pete, that SSR is junk, it's a RIP off Crydom/Sensata

There's an RS down under ....

https://uk.rs-online.com/mobile/c/r...5,4294522809,4294522952,4294523425,4294530390
This one is listed on rs but a few bucks cheaper and much cheaper post (and i need cheap lol) how you go with those trips any luck yet, i'll need one of this project :) . I'll pm you a private vid
Redpacket said:
With HWS it's a really good idea to make sure the thermostat's contacts don't weld with the DC. They're only intended for AC.
Fortunately with that low wattage element, the tank pressure valve will cope easily if there's an event where the SSR fails shorted (on) & the thermostat's contacts are stuck.
You've seen the Myth Busters Hot Water Tank episode right?

I am worried about that! was thinking of undersizing the cable or just put an inline fuse so if it does weld it'll blow (in theory) Yes, seen MB hws ep they love a good explosion lol
 
Sean said:
Redpacket said:
With HWS it's a really good idea to make sure the thermostat's contacts don't weld with the DC. They're only intended for AC.
Fortunately with that low wattage element, the tank pressure valve will cope easily if there's an event where the SSR fails shorted (on) & the thermostat's contacts are stuck.
You've seen the Myth Busters Hot Water Tank episode right?

Pete's sensibly just using the tank stat to make/break the SSR control signal, it'llbe fine.

That'll help with the tank stat's contacts but if the SSR goes short...
I guess the batteries being hauled down by a shorted SSR would be noticed before much danger with a 600W element.
If you left the original element in there, things could get interesting! Could become the owner of QLD's biggest steam whistle that isn't a steam train!
 
After a few conversations, I think I'll need to look elsewhere for another solution - I know there are complete elements/thermostats you can buy but they are quite expensive.
 
I've been researching this for some time now and about to put one inline. Your SSR setup is fine as long as you have a good SSR. This setup is not any more safe than the original thermostat getting stuck. Your hot water heater should have come with a external relief valve that would release once the pressure build up. If you want added safety, then have another thermostat in series so if one fails open then the other will be the backup.

More importantly, you need to size your panels to the heating element. If you run 120V to a 240V element you will only get about 1/4 the power. So a 3000W element will be no more than 750W.

 
hbpowerwall said:
I am worried about that! was thinking of undersizing the cable or just put an inline fuse so if it does weld it'll blow (in theory)
Under sizing the cable (bad idea) &/or fusing it (proper fuse or breaker good idea anyway) doesn't help the over temp issue but like the guys are saying, it should be fine with the regular stat > SSR & yes the over pressure valve is there.

not2bme said:
If you run 120V to a 240V element you will only get about 1/4 the power.
HBPW's down here in Australia so the mains is 230/240VAC but it looks like he might be running it direct off his panels.
Yeah the power matching video is good :)
 
If your hot water system is a header tank the relief is for the tank to blow back the water into the header tank, which if plastic, then melts...

Pressurised systems without a header tank would have an external vent pipe. From the video I'm not sure if the tanks is 3bar rated (typical header) or 5-10bar.

SSR and DC is the only way to go, just use the thermostat at 12V into the trigger for the SSR. The thermostat will work ok at 30-40V DC for a long while at small load, but not as long as the element. The thermostats are typically current rated for HV, so only a few amps, not 20-50A with LV DC... You could use just a few FET's on an aluminium bar as that is pretty much all that SSR's are internally...

Hot water should really be direct thermal, going DC is way less efficient and then becomes a spill battery in a way.
 
completelycharged said:
Hot water should really be direct thermal, going DC is way less efficient and then becomes a spill battery in a way.

Agreed, however, if the batteries are topped off and solar is still generating lots of power, then even if it's only 10% efficient, you're still using more than 100% more power than what would of been wasted anyways ;)
 
I'm hoping it'll work well from the batteries, seems it's useless off one 36v 250w panel (but pannel might have issues lol one cell on the solar panel is WAY hotter than the rest

image_peptsr.jpgimage_bcgbiu.jpgimage_ybjkxd.jpg


 
You'll need to get the load (ie the element) & the number of panels matched so that the panels can be operating near their max power point.
Eg you'll probably have to add multiple panels in parallel so that with the element connected, in full sun, the voltage on the panels ends up near 30V (ie approx Vmp for typical 36V VoC 60 cells panels). Otherwise you won't end up getting the expected amount of energy collected into the HWS.
 
Would it not be simpler and more efficient to use AC power from the inverter? The MPPT tracker optimizes power output of the PV panels, and no need to modify the boiler, which can safely be switched with a standard zero cross SSR or a relay.
 
Redpacket said:
You'll need to get the load (ie the element) & the number of panels matched so that the panels can be operating near their max power point.
Eg you'll probably have to add multiple panels in parallel so that with the element connected, in full sun, the voltage on the panels ends up near 30V (ie approx Vmp for typical 36V VoC 60 cells panels). Otherwise you won't end up getting the expected amount of energy collected into the HWS.
Yep working on this - the key is understanding it, and I learn by doing ... well sometimes :)
ajw22 said:
Would it not be simpler and more efficient to use AC power from the inverter? The MPPT tracker optimizes power output of the PV panels, and no need to modify the boiler, which can safely be switched with a standard zero cross SSR or a relay.
IT sure would but this isn't going to be used anywhere near a power pole and budget is limited & we need an effective dumpload for the lithium cells, Patreon members have already had a sneak peek at where this will be heading :)
 
One panel was dumb! lolz but we knew that right ?

 
Some discussion for you re the videos:
- the element is 600W at 24V so it's normal running current is is I = P/V = 600/24 = 25A
The element's resistance is R = V/I = 24/25 = 0.96 Ohms
- With one panel you measured approx 7.7V & 7.8A, so P-= V x I = 7.7 x 7.8 = approx 60W
- 60W is going to take quite a while to raise the water temp much!
- with three panels in parallel at approx 7A each = 21A that's be about 20V on the element & so about 20 x 21 = 420W going in.
at 420W, the 50L tank would take about 83 mins to rise 10 degC
- with 4 panels, it'd be approx 28A & 26.9V so approx 752W going in (you'd be pushing the element hard!)
at 752W, the 50L tank would take about 46.5 mins to rise 10 degC

If you got 600W going in, it would take about 4.85 hrs to raise 50L of 10 degC water to 60 degC

This calculator helps with the temp calcs:
http://processheatingservices.com/water-heating-time-calculator/

Basically, for your 50L tank & 60W input, it would take almost 1 hour to get just 1 degC rise!
With 600W going in, you'd see a 10 degC rise in approx an hour.
Yes there's some assumptions there...
 
Redpacket said:
Some discussion for you re the videos:
- the element is 600W at 24V so it's normal running current is is I = P/V = 600/24 = 25A
The element's resistance is R = V/I = 24/25 = 0.96 Ohms
- With one panel you measured approx 7.7V & 7.8A, so P-= V x I = 7.7 x 7.8 = approx 60W
- 60W is going to take quite a while to raise the water temp much!
- with three panels in parallel at approx 7A each = 21A that's be about 20V on the element & so about 20 x 21 = 420W going in.
at 420W, the 50L tank would take about 83 mins to rise 10 degC
- with 4 panels, it'd be approx 28A & 26.9V so approx 752W going in (you'd be pushing the element hard!)
at 752W, the 50L tank would take about 46.5 mins to rise 10 degC

If you got 600W going in, it would take about 4.85 hrs to raise 50L of 10 degC water to 60 degC

This calculator helps with the temp calcs:
http://processheatingservices.com/water-heating-time-calculator/

Basically, for your 50L tank & 60W input, it would take almost 1 hour to get just 1 degC rise!
With 600W going in, you'd see a 10 degC rise in approx an hour.
Yes there's some assumptions there...

Assumptions & math!! but I'm grateful for the effort - I can't wait to dotomorows test with more panels then the big final install running from batteries but that video will take a while to compete. And time isn't on my side.
 
Last one I'll be doing direct from solar but chuffed that with community input it's all working quite well and it's not even that expensive

 
Looking better there Pete :)
Getting the panels to right angles to the sun will help.
Re the control voltage for the SSR, if the thermostat opens, the panels will go to Voc, which is likely to be > than the SSR's max voltage.
Maybe try using a Zener diode + a resistor eg 27V 1W type across the control terminals with a 1k ohm 1W resistor going to the panels. That way, the 36V Voc will get safely limited to about 27V & the SSR won't get damaged.
So it'd be wired like this:

image_cfswkl.jpg


You could do this with a MOSFET too...

image_jewoad.jpg
 
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