Solar geyser converter kit - why a controller?

[frankvw]

To me a solar geyser is a no-brainer here in SA with all the hot sun we're having. So I'm looking to convert my existing geyser into a solar one. To me that makes sense: why stick a new tank on top of the roof if the existing geyser tank is there already? I'm at the south coast where frost is unheard of, so a direct system will be fine.

Now in my mind this should be simple: a small PV panel with a 12V pump, and a solar collector. When the sun shines the PV panel will run the pump; when there's no sun it won't because you don't need water to run through the collector, and that's the job done, right?

Well, apparently not; a proper conversion kit (like this one) also in includes a battery and a fancy programmable timer/controller. Which I'm sure would not have been included if they weren't necessary; that would have lopped a big chunk off the price and make it sell a lot quicker.

So why is a backup battery and controller needed and why will my simple idea of a directly PV powered 12V pump alone not work?

 

[DrJohnZoidberg]

The point of the controller is to regulate the temperature. The controller has a temperature probe and it tells the pump (and the element) when to turn on and off to maintain the correct water temp in the geyser.

 

[konfab]

The point of the controller is to regulate the temperature. The controller has a temperature probe and it tells the pump (and the element) when to turn on and off to maintain the correct water temp in the geyser.

To add onto what the good Doctor is saying: you want your geyser to operate as normal. Which means that you probably want the hot water from your taps to be the correct temperature such that it doesn't flay your hand if you accidentally touch it. Having a system that just dumps heat into the geyser means that there is nothing stopping your pump from raising the water temperature in your geyser to 95% degrees.

The other aspect is that when the sun is shining, you do not want your geyser to be using electricity to warm up, you want all the energy to come from the sun. Your geyser thermostat doesn't know about the sun or your 12V pump, so it will just go on in its merry business.

This is why there is a controller there, which knows to turn off the electricity to the geyser, when the sun is up, and to turn it on if you need the geyser to be a certain temp by a certain time.

What you are describing is ok for a pool, where there is a very large amount of water to heat up, and that water isn't sitting in an insulted vessel designed to keep heat. This means that you can have a more DIY system in place with relative safety.

 

[frankvw]

Aha. So if I understand you both correctly, such a conversion kit is designed based on the assumption that the electrical heating element in the geyser will still be active as a back-up for when there's not sun and you have no hot water. That makes a lot more sense then. What I had in mind (and why I didn't even consider the electrical element) is to use a gas geyser (which I already have) as a backup with a thermostatic diverter valve that will switch over to the gas geyser when the water from the solar geyser drops below a certain temperature. So I won't be needing the electric element. In fact I don't want the geyser to use any electricity.

The point about limiting the water temperature is well taken, but that could also be done with a simpler system, e.g. a bog standard thermostat or a thermostatic valve (like this one) . No need for a programmable Wundermachinen that can sing and dance.

Or am I talking out of my backside here?

Update: maybe not:

 

[leon.davibe]

With the flat panels you are unlikely to have a ridiculous temp like 95 imo and even if you do you would just use less hot water and more cold water

And just the solar panel on the pump should work fine

You could always run it as you intend and monitor and if you have circumstances that require intervention then get the controller

But i doubt it

The only thing i have heard is that sometimes you struggle to get dc pumps that have the same head stats vs ac pumps

I have a diy collector with a ac pump on a timer
The collector is a too small for the tank i have 200L and i don't care to have it perfect as it acts as a feeder tank for my actual geyser

I think the diy panel is about 1m² if i remember correct
(Was just a test we built many years ago with some copper pipes and plate tied to the pipes sprayed black with wooden frame and perspex cover)
When prices went up instead of making a propper size i just plonked this on the roof and felt any savings is better than nothing

I leave my geyser element to function as normal

The feeder tank could lift the 15c supply to 35-40c weather dependant i can't recall when i did that check i think it was somewhere between aug -nov

Lately i have a 1kw element that can act as a dump load when my batteries are charging too quick
I inject about 5-6kwh a day

Since mixed supply lately no idea the contribution of this diy collector

Calculations would suggest between 4.5 to 5.5kwh on a sunny day

 

[Tomtomtom]

Do tubes on the roof still make sense with solar PV prices what they are?

From a reliability PoV it seems a no brainer anyway. Solar PV with a DC element means no new piping and no moving parts.

 

[Stokstert]

If OP is living in a frost area and use a flat panel, the controller and standby battery is necessary to keep the panel from freeze bursting. In the event of frost, the controller pumps warm water of the geyser through the panel with short intervals, which will keep it from freeze bursting.

Happened with my panels when the installer didn't set the controller correctly.

 

[Segg]

I've disconnected the heating elements in 2x 200L geysers and attached a 220v circulation pump to an evacuated tube array for each geyser.

There is a day/night switch wired to a NC contactor that activates the pumps when the sun comes up and turns them off at night.

The house in question has an inverter & solar system to run the pumps so no 12v nonsense required

The geysers are then joined together and plumbed in series with a 16L Dewhot gas geyser that "trims" the water to 55 degrees if necessary. If the water coming from the geysers is 50 or above, the geyser doesn't even turn on, and allows the hot water to bypass

Saving electricity & Gas By the end of the day, the mixer taps are in a noticeably colder position than the start of the day

 

[frankvw]

Do tubes on the roof still make sense with solar PV prices what they are?

From a reliability PoV it seems a no brainer anyway. Solar PV with a DC element means no new piping and no moving parts.

In the process of digging deeper as part of this conversation I've begun to ask myself the same question. And I'm more and more leaning towards "no".

The last time I looked at solar collectors was about 10 years ago. Then a conventional geyser was a no-brainer. Now, however, the price of PV conversion kit is only fractionally higher than a pumped direct heated conversion kit, but the latter doesn't include the cost of the associated plumbing which is considerably more expensive than running a length of wire.

Which changes the picture considerably.

Unfortunately the options for DC heated geyser elements on the SA market still seem to be rather limited; the Geyserwise products are pretty much it as far as I can see. Running the DC through an inverter into a 220V element makes no sense to me. More investigating to do, then...

Onward!

 

[Tomtomtom]

Unfortunately the options for DC heated geyser elements on the SA market still seem to be rather limited

That's the thing. Not sure we have enough players in this market yet.

Thinking it should practically be a SABS requirement to accept dual element / DC input on new geysers.

 

[leon.davibe]

Yea must say if you have nothing i would not , invest money in solar geyser

I would rather plough that money into an inverter, plus panels

If you don't care for battery backup some inverters can actually run batteryless

Bringing down initial layout

So buy the panels you would have for solar geyser (pv electrical type) and instead of spending money on dc element and controller (which is just a mppt anyway) connect it to either the 450v voc mppt axpert type or growatt inverter models that can run without battery that way you have some backup power while sun is shining and be able to use the power for geyser too maybe just with a lower KW element

And then later maybe buy a battery since that helps to get more out of panels