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Need some element advice


Arzy

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So our house currently has 2 200l (could be 150's) geysers with 3kW elements that are coupled to evacuated tubes on the roof and managed with the basic GeyserWise controllers. The temperature on both is set to 55c with timing done so that they don't run at the same time. Generally, they don't need to switch on in the late afternoon/early evening as they have reached the desired temperature which covers bathing the children etc. I have set the main bathroom geyser to run in the early morning to get ready for work etc. while the second geyser "tops up" between 8 and 9 so that there is hot water for cleaning.

 

As of last week we now finally (also) have our own solar system installed consisting of a 12kW Deye 3-phase hybrid inverter, 10.4 kWh in batteries, and 6ish kW in panels.

 

With that bit of background done, I am looking for advice on what to do about the geysers.

 

As I am not around during the day I'm slightly concerned about drawing too much power at any given stage but I believe I've mitigated this by staggering the operating times of the geysers and setting a reasonable target temperature. On the flip side, I would also like to utilize my PV production more efficiently. For instance, my household consumption is currently only 364W all fed from the PV so there is surplus generating capacity available. This is low though as I switched the pool pump off this morning due to my concern above and the gardener being at the house today and I can't anticipate when he mows the lawn (electric lawnmower).

 

I'm thus planning on replacing the 3kW geyser elements so that I can, safely, run them for longer periods during the day but have no idea what I should be stepping down to. Would really appreciate some input and wisdom on what to do considering the overall setup. The key times when hot water is an absolute requirement are around 19:00 at night and from 6:00 in the morning.

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The way to save energy on a geyser is to only heat the amount of water you need up to the temperature you need right before you need it. This idea is to a certain extent incompatible with heating with alternative sources/supplies throughout the day. The point is that you probably need the 3kW element if you want to take a bath in a hurry. When I had a grid-tie inverter and a pre-paid meter, I had a similar problem. I used a contactor and Sonoff switch to put the 3kW element in series with two 70 µF running capacitors during the day. This brought the power down to somewhere between 500W and 1kW. If the Geyserwise switched on, the full voltage was applied to the element.

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58 minutes ago, Arzy said:

Would really appreciate some input and wisdom on what to do considering the overall setup. The key times when hot water is an absolute requirement are around 19:00 at night and from 6:00 in the morning.

The morning run is going to have to be powered by battery. For the evening run you can probably heat during the day if your geyser is well insulated. I run my heat pump starting between midday and 1 pm, and the water is still hot at 8pm.

So that morning run is the problem. PV won't help you  then, and a 3kW element is going to take a chunk out of your battery. Even if you downgraded, the element will take less power for longer and probably take just as much out of the battery.

So how much battery do you have? And what is typical SOC at about 5:00?

Back to my heatpump: I can run that early in the morning because we use very little during the night and so usually have about 60% at 6:00.

Heatpumps are helpful because they are so much more efficient than an element. But that's more money to spend.

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I can agree with @Bobster.

My heatpump is now about 10yrs in service. I heat an hour before we wake up and in the afternoon. The other day the timer missed the afternoon, next morning and afternoon switch on due to LS. So we only discovered it at 19h30 when we found the water was not very hot but at 40 degrees C. Inside a roof the heat during the day adds to keep the water hot. Our day temps are still high. 

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I have a similar system - two geysers.

2 hours ago, Arzy said:

So our house currently has 2 200l (could be 150's) geysers with 3kW elements that are coupled to evacuated tubes on the roof and managed with the basic GeyserWise controllers. The temperature on both is set to 55c with timing done so that they don't run at the same time. Generally, they don't need to switch on in the late afternoon/early evening as they have reached the desired temperature which covers bathing the children etc. I have set the main bathroom geyser to run in the early morning to get ready for work etc. while the second geyser "tops up" between 8 and 9 so that there is hot water for cleaning.

As of last week we now finally (also) have our own solar system installed consisting of a 12kW Deye 3-phase hybrid inverter, 10.4 kWh in batteries, and 6ish kW in panels.

With that bit of background done, I am looking for advice on what to do about the geysers.

As I am not around during the day I'm slightly concerned about drawing too much power at any given stage but I believe I've mitigated this by staggering the operating times of the geysers and setting a reasonable target temperature. On the flip side, I would also like to utilize my PV production more efficiently. For instance, my household consumption is currently only 364W all fed from the PV so there is surplus generating capacity available. This is low though as I switched the pool pump off this morning due to my concern above and the gardener being at the house today and I can't anticipate when he mows the lawn (electric lawnmower).

I'm thus planning on replacing the 3kW geyser elements so that I can, safely, run them for longer periods during the day but have no idea what I should be stepping down to. Would really appreciate some input and wisdom on what to do considering the overall setup. The key times when hot water is an absolute requirement are around 19:00 at night and from 6:00 in the morning.

TIP: Treat your geyser as an energy store (aka battery for excess Solar PV).

I have a similar evacuated geyser system (installed in 2010).
Since getting Solar PV in 2021, I change the timer programs to use the geyser as a store.
I previously had the Eskom recommended geyser timings of 4h00-6h00 and 16h00-18h00
The theory is that you allow the sun to heat the geyser during the day 6am to 16pm.

The standard GeyserWise is limited to 65C and has 4 time zones.
I set the daylight time zones as high as possible to 65C.
Note that the GeyserWise TSE / Tuya upgrade allows for 75C.

My average background house load is ~700W and my batteries are normally full by 13h00.
On most days (i.e. sunny days), my PV is generating 4kW by 10h00, which us enough to start heating water and also run the house and not draw from batteries.
So I changed the 16h00-18h00 timer to 10h30 to 18h00.
I use excess Solar PV electricity to boost the geyser temperature at 10h30 to 65C. The boosting takes 30m-1h.
The evacuated tubes then takes the water to 70C or 80C.

Make sure you have an automatic mixer on the hot water outlet of the geyser to prevent overly hot water feeding your home.
AFAIK, the mixer is a code safety requirement.

image.thumb.png.937a18f23394252baa9a6b2f0a09a4b3.png

My second geyser is a normal 100L for a single person guest house, and I have a basic timer set for 13h00 to 16h00 only to 65C - this uses excess Solar PV electricity.

Edited by system32
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Energy-Jason
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Modina was awarded the badge 'Helpful' and 25 points.

There have been various suggestions of what you could do.  I will concentrate on reducing your heating element radiated power.

You could replace your 3KW element with a lower power element.  Reading this forum, it seems 2KW elements are common.  The disadvantage with a new element is that the geyser(s) need to be drained to fit the new element.

Although I have spoken a lot of using capacitors to reduce the brightness of LEDs, I never thought of using a capacitor for high power applications.  I have not checked @frivan  suggestion of using 2x 70uF capacitors, meaning I have not made any calculations to see how much power your element would see.  The calculation is not difficult, we know that a 3KW element has a resistance of about 17.75ohms and it is simple to calculate the impedance of a 140uF capacitor @ 50Hz.  Then it is a simple voltage divider calculation.

Another way to reduce the power going into a 3KW element is to reduce the input voltage with a mains transformer.  You would connect the primary as usual, across the 230VAC and the secondary winding would be connected in SERIES with the element, in such a way that the transformer secondary is 180deg out of phase with the mains.  A 40V transformer would reduce 230VAC to 190VAC and with this lower supply voltage the 3KW element would dissipate 2KW.  The current would be reduced from 13A to about 10.7A.  This would mean your transformer should be rated at about 450VA.   By varying the output voltage of the transformer, you could set your power dissipation in your 3KW element to whatever value you choose. Now, a 450VA transformer would be a large lump of iron and it would be pretty pricey.  Most likely over R1500.  The alternative (for a DIYer) would be to get an old microwave transformer and remove the high voltage secondary, and then rewind with much thicker wire, but only a few turns.  (They often use these transformers to make spot-welding machines).

Power could also be reduced with phase control using a triac, but this would produce a lot of electric noise and a very bad power factor that your inverter would handle, but be a bit unhappy about.   Another way to reduce the power would be a simple electronic circuit that removes a certain number of full sinewave periods, switched on/off at the zero-crossing point.  The average power would go down, no electric noise and the power factor would be maintained at 1.  However, you still have 3KW impulse loads.

For you @Arzy there is one other solution but it also has certain disadvantages...  Because you have TWO geysers, with (I presume) two identical 3KW elements, you could connect both in series.  Each element would then dissipate 750W and your inverter would see a resistive load of 1.5KW.  This would be an elegant solution, but it would treat both geysers the same.  One would be able to refine this idea by adding relays that could switch, and thereby revert the one, or other, or even both geysers back to full 3KW operation.

Then there is another cheap and easy way of reducing your power by 50%.  Simply connect a suitable 20+ Amps 400V diode (mounted on a heatsink) in series with your element.  This would result in half wave rectification and would reduce your 3KW element to dissipate 1.5KW.  One could then also connect a relay across the diode, as @frivan did with his capacitor, to revert to full 3KW dissipation.  Again, a diode would not cause electric (switching) noise and the load would remain resistive with a PF=1.  I am not sure if pulsed DC through your element has negative consequences.  I am not sure if galvanic corrosion could occur with current only flowing in one direction.

Edited by Modina
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First of thanks for all the feedback and suggestions, given me way more to think about than I had anticipated.

Short term option seems to be to push the geysers a bit during the day potentially upping the temp to 60 and letting the tubes carry on from there. Will have to see if this possibly caters for the morning.

Connecting the geysers to each other may prove a tad more difficult as they are at opposite ends of the house in separate roof spaces.

And then possibly look at either a technical solution with capacitors or just drop the elements down to 2kW

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20 hours ago, frivan said:

I used a contactor and Sonoff switch to put the 3kW element in series with two 70 µF running capacitors during the day. This brought the power down to somewhere between 500W and 1kW. If the Geyserwise switched on, the full voltage was applied to the element.

Please pardon my rudeness, but is this legal as in an electrician would sign a COC? Capacitors have a way of going pop eventually. Do the capactitors have to be in some kind of enclosure? 

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1 hour ago, Bobster. said:

Please pardon my rudeness, but is this legal as in an electrician would sign a COC? Capacitors have a way of going pop eventually. Do the capactitors have to be in some kind of enclosure? 

I put my capacitors and control gear in a polycarbonate box. A steel box will have earthing benefits. Because the capacitors are in series with the resistive element, they don't work very hard. I can't see why such an installation won't get a CoC. 

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