On 2022/08/02 at 7:56 PM, PeterW102 said:

Hi Guys

I have read just about every topic on Geysers, solar, etc.

I am in the same boat as many of you. The electricity costs are just too high these days.

This Geyserwise PTC DC element is actually very nice. Have a 150L Kwikot geyser and was wondering -

Will it be possible to drive this DC PTC element directly from a 48 Li-ion battery bank and then just charge the battery from 220V ? No solar involved. Surely the charge current will be less than the current directly being consumed by the element ?

 

An inverter battery combination should do what you are thinking with the option to add panels later. 

On 2022/08/02 at 7:56 PM, PeterW102 said:

Hi Guys

I have read just about every topic on Geysers, solar, etc.

I am in the same boat as many of you. The electricity costs are just too high these days.

This Geyserwise PTC DC element is actually very nice. Have a 150L Kwikot geyser and was wondering -

Will it be possible to drive this DC PTC element directly from a 48 Li-ion battery bank and then just charge the battery from 220V ? No solar involved. Surely the charge current will be less than the current directly being consumed by the element ?

 

I think this should be possible. But what would be the objective of this?
 

It seems like you want to save electricity cost by doing this (like all of us), but I dont see how this solution would achieve that?

On 2022/08/02 at 7:56 PM, PeterW102 said:

Hi Guys

I have read just about every topic on Geysers, solar, etc.

I am in the same boat as many of you. The electricity costs are just too high these days.

This Geyserwise PTC DC element is actually very nice. Have a 150L Kwikot geyser and was wondering -

Will it be possible to drive this DC PTC element directly from a 48 Li-ion battery bank and then just charge the battery from 220V ? No solar involved. Surely the charge current will be less than the current directly being consumed by the element ?

 

Not less used from battery and inverter. Loss from AC charging to battery about 10%. The only gain will be once you have panels to use the sun for free. DC element Watts to heat water = AC element. DC element cost a lot of money. Just a loss all the way.

On 2022/08/06 at 2:14 AM, Wanderer said:

Will it be possible to drive this DC PTC element directly from a 48 Li-ion battery bank and then just charge the battery from 220V ?

I also think it could be done but will not be a cost effective way of heating water. You will have losses to charge the battery, than losses again while heating the element. The only benefit of doing this  would be you have back up power to heat your geyser during load-shedding. I believe adding solar panels to charge the battery would at least give you a cost saving aswell.

 

To the best of my knowledge the water heating calculation still is the same. You need a certain amount of Kw to heat water. 

Kw=(2.4 × liter × ◇T)÷ 3600 

Where ◇T is the change in temperature. To install a 1Kw element in your geyser, you will basically use double the time to heat the same amount of water than if you would have used a 2 Kw element.  Except for the normal standing temperature loss. 

7 hours ago, Vaal said:

To the best of my knowledge the water heating calculation still is the same. You need a certain amount of Kw to heat water. 

Kw=(2.4 × liter × ◇T)÷ 3600 

Where ◇T is the change in temperature. To install a 1Kw element in your geyser, you will basically use double the time to heat the same amount of water than if you would have used a 2 Kw element.  Except for the normal standing temperature loss. 

I am not so sure this formule gives the right answer. Far too low.

1.1666xLitersxTemp change=Watts or 175xTemp change for a 150L=Watts

28 minutes ago, Scorp007 said:

I am not so sure this formule gives the right answer. Far too low.

1.1666xLitersxTemp change=Watts or 175xTemp change for a 150L=Watts

I get to 4 Kw to heat 150L by 40 °C. It appears to me that it might be close to the correct value. It would be nice if somebody with a "normal" geyser can please verify because I have a "solar assisted" one. Thanks in advance. 

2 hours ago, Vaal said:

I get to 4 Kw to heat 150L by 40 °C

I am going to throw a spanner in the works.

My heatpump does the same 40deg delta during summer for about 0.9 KWh and during winter about 1.8-2.0KWh (winter temps are around 2-6 degrees in gauteng).

The big plus for me is that I run it from my battery power so I remain off-grid. The downside is that the max temp of my heatpump is 55 degrees.

 

Heatpumps are great for saving money, if you use more than 100 liters a day. The more water you use the quicker the ROI. You can also heat 2-3 geysers with a single heatpump, it just takes longer.

If you use less than 100l your better option would be gas as there are no standing losses. People that claim to save money on gas with large amounts of water had geysers with poor insulation and/or poor quality geysers.

EV tubes also work great but do require a backup for those overcast days. If you tend to go away from home often you have the added problem of water overheating. 

If you already have excess PV then there are many ways to solve the same problem.

 

 

9 hours ago, Vaal said:

I get to 4 Kw to heat 150L by 40 °C. It appears to me that it might be close to the correct value. It would be nice if somebody with a "normal" geyser can please verify because I have a "solar assisted" one. Thanks in advance. 

You need 7kw to do what you indicated in your calculation. An easy one to find out. If a geyser was off for 2 days and you run hot water out to get it replaced by cold water it takes just over 2 hrs with a 3kw element. Never 1h20 to reach 60deg from 20deg. Your formula used a SHC of 2400 for water.

Calculate it from water SHC 4184 J⋅kg⁻¹⋅K⁻¹  

 

Edited September 13, 20223 yr by Scorp007

16 hours ago, Scorp007 said:

You need 7kw to do what you indicated in your calculation. An easy one to find out. If a geyser was off for 2 days and you run hot water out to get it replaced by cold water it takes just over 2 hrs with a 3kw element. Never 1h20 to reach 60deg from 20deg. Your formula used a SHC of 2400 for water.

Calculate it from water SHC 4184 J⋅kg⁻¹⋅K⁻¹  

 

I'm guessing he is taking a temperature measurement somewhere in the middle of the geyser and assuming all the water is at the same temperature.

1 hour ago, P1000 said:

I'm guessing he is taking a temperature measurement somewhere in the middle of the geyser and assuming all the water is at the same temperature.

Ok now things are getting interesting: Why would 

https://www.omnicalculator.com/physics/water-heating

show the calculations? If you google "water heating calculation formula" see what you get?

30 minutes ago, Vaal said:

Ok now things are getting interesting: Why would 

https://www.omnicalculator.com/physics/water-heating

show the calculations? If you google "water heating calculation formula" see what you get?

I feel there are some detail missing in your post. What are you trying to say?

From your link:

Edited September 14, 20223 yr by P1000

3 hours ago, P1000 said:

I'm guessing he is taking a temperature measurement somewhere in the middle of the geyser and assuming all the water is at the same temperature.

That is spot on. That's why I mentioned draining the hot water and get in cold water at 20 deg. Not easy as there might still be same water left that were at above 20 deg. My calculation is based on heating 150L of water by 40 deg.

Quite easy to prove with 1L in a kettle from say starting point to 100 and measure the power used.

Gr 12 Physics student should be able to calculate it as we did it at school decades ago.

 

Edited September 14, 20223 yr by Scorp007

 

Dear All

I thought it well to share my experience.

I converted my 150l geyser to PV about two years ago and recently did the same for my 2nd geyser. The cost is about 10G a geyser.

The geysers are off the grid in the summer and consumes about 2kw in the winter. However, I heat the geyser to about 60 degrees and via timer switches that off at night. In the morning after the evening's usage the water is about 25 degrees. Multiple baths in a day will have a different experience. We are 2 people per geyser mainly having one bath a day.

Now that I have set the scene, this is the set-up:

- a Geyserwise ac/dc ptc element with flange, gasket and thermo pocket replacement to kwikhot geyser. Ps some have 5 others 6 holes for flange:

- link 2x PV panels to dc section of element, adding the needed safety eg fuse of course; the solar panels Volts/Amps configuration must meet the PTC element constraints, choose the panels well;

- link the AC side to DB box with a thermostat/relay controller.

And that is it

Hi Pieter

I realise it is quite a while since you posted, but I am considering replacing my 2kW AC element with a PTC AC/DC element.  I currently run 3x geysers on 2kW via my inverter (staggered).  Has been working well for a couple of years, but when there are extended outages, I need to watch out for overloading the inverter (As there is no Eskom backup).  So dropping 2kW load from the inverter for a few hours a day will help.

 

So I was thinking to add 2 new PV panels to the DC side (keep the AC side connected to my inverter).  With PV prices as low as they are, I can get anything between 2x330W and 2x550W for a decent price.  I think this is similar to what you have done.  No MPPT, jsut straight to the DC side.  The Geyserwise MPPT is way more than a panel, wo it starts to destroy the ROI.

I understand that I need to match the resistance of the PV panels (e.g. 2 in series) to the DC element.  But the PTC resistance is stated strangely (as it is a variable resistor).  For normal elements, the calculation is quite straightforward.

If I decide to go with 2x550W panels at Vmp/Imp of 83V and 13A, that works out to about 6.2 Ohms on the panels' side.  I see the 900W element (which had a max of 1100W) has seemingly been replaced by an 1100W DC element now (with a max of about 1200W).  So 2x 550W panels is as high as I want/need to go.  If I get 900W for 3-4 hours a day out of it, I have covered most of my required output for one geyser.

The 1100W PTC element is rated at 72V DC.  That gives about 15A/4.7 Ohm.  But the spec states between 8-20 Ohm resistance at 25C.  (https://www.geyserwise.com/2kw-ac-1100-dc-72v-ptc-element/)

The variable Ohm I assume is as temperature rises.  So I am struggling to match the PV Ohms, and I know bad matching destroys the yield.  Since you have done this already, I wanted to know what you found the PTC resistance to be (or how you calculated the PV Vmp/Imp combination to match)?

Would you mind sharing?

Thanks

Gerhard
 

 

Hi Pieter

I realise it is quite a while since you posted, but I am considering replacing my 2kW AC element with a PTC AC/DC element.  I currently run 3x geysers on 2kW via my inverter (staggered).  Has been working well for a couple of years, but when there are extended outages, I need to watch out for overloading the inverter (As there is no Eskom backup).  So dropping 2kW load from the inverter for a few hours a day will help.

 

So I was thinking to add 2 new PV panels to the DC side (keep the AC side connected to my inverter).  With PV prices as low as they are, I can get anything between 2x330W and 2x550W for a decent price.  I think this is similar to what you have done.  No MPPT, jsut straight to the DC side.  The Geyserwise MPPT is way more than a panel, wo it starts to destroy the ROI.

I understand that I need to match the resistance of the PV panels (e.g. 2 in series) to the DC element.  But the PTC resistance is stated strangely (as it is a variable resistor).  For normal elements, the calculation is quite straightforward.

If I decide to go with 2x550W panels at Vmp/Imp of 83V and 13A, that works out to about 6.2 Ohms on the panels' side.  I see the 900W element (which had a max of 1100W) has seemingly been replaced by an 1100W DC element now (with a max of about 1200W).  So 2x 550W panels is as high as I want/need to go.  If I get 900W for 3-4 hours a day out of it, I have covered most of my required output for one geyser.

The 1100W PTC element is rated at 72V DC.  That gives about 15A/4.7 Ohm.  But the spec states between 8-20 Ohm resistance at 25C.  (https://www.geyserwise.com/2kw-ac-1100-dc-72v-ptc-element/)

The variable Ohm I assume is as temperature rises.  So I am struggling to match the PV Ohms, and I know bad matching destroys the yield.  Since you have done this already, I wanted to know what you found the PTC resistance to be (or how you calculated the PV Vmp/Imp combination to match)?

Would you mind sharing?

Thanks

Gerhard
 

One thing I found in a quick test of the DC PTC element is that the resistance seems to be very stable in say the 30-55 deg range but does increase fast at the higher temperature is reached. I have also tried over 150V to the element and it just drew a lot more power without damage. I only did it as a test for 10min outside of a geyser on the bench in a contained filled with water. They do seem to be relatively burn out proof as per the theory. 

 

One thing I found in a quick test of the DC PTC element is that the resistance seems to be very stable in say the 30-55 deg range but does increase fast at the higher temperature is reached. I have also tried over 150V to the element and it just drew a lot more power without damage. I only did it as a test for 10min outside of a geyser on the bench in a contained filled with water. They do seem to be relatively burn out proof as per the theory. 

Good to hear.  I have looked at a few combinations of AC/DC and AC elements.  For example, the spec sheet shows 8-20 Ohm resistance for the 1100W PTC element at 25C (rated at 72V).  But that cannot be the typical/stable resistance, as you would need about 15A at 72V to get to 1100W.  And that means the steady state resistance is at about 4.7 Ohm.  Same story for the AC elements.

I have two geysers in series (I heat up both in the day, staggered).  The reason is some shower in the evening, then the main geyser is still fed with hot water, and next morning others can shower again, still with hot water.  Works almost 100% of the time without needing any Eskom.   The second geyser was like a cheap battery for me.

To make a meaningful difference to the hours that I heat these two geysers off my inverter, I need about 10kWh (5 hours at 2kW currently).  In winter, I get about 3.5 full hours equivalent from my panels on average.  Maybe I can get 4.  So I would need 2.5-3kW PV to get to this target.  So I need a 3kW element.  I can then circulate the water between the two geysers right next to each other) to spread the warm water as if it is a large 300L geyser I am heating.   A cheap 10L/min circulation pump will mix them 50/50 in under 10 minutes a day, and fully cycle one geyser through the other one in 15 minutes.  So that is very easy.

 

Now the theory.  If I put 6x 470W (43V Vmp) panels in series (I will get less than 2.8kW from them, but let's assume I do), then I get to 260V/11A/2.8kW/24Ohm.  3kW element at rated 230V is 13A/18Ohm.  Do you think this will work without an MPPT/controller?  The PV string's resistance is higher than the element's, to avoid the element resistance blocking current (18<24), PV Volt is about at the max Volt of element (265V), PV power is within element Watt limit (3300W). 

I can get the PC for about R11k + delivery it seems.   R11k, say R13-14k with install to sort out two geysers winter and summer and give me back 2kW on my inverter capacity most of the day sounds like a good plan without any lengthy new pipework and associated risk.

I just want to test the theory with someone who has done it.

What do you think?

 

Enjoy the weekend.

 

 

Good to hear.  I have looked at a few combinations of AC/DC and AC elements.  For example, the spec sheet shows 8-20 Ohm resistance for the 1100W PTC element at 25C (rated at 72V).  But that cannot be the typical/stable resistance, as you would need about 15A at 72V to get to 1100W.  And that means the steady state resistance is at about 4.7 Ohm.  Same story for the AC elements.

I have two geysers in series (I heat up both in the day, staggered).  The reason is some shower in the evening, then the main geyser is still fed with hot water, and next morning others can shower again, still with hot water.  Works almost 100% of the time without needing any Eskom.   The second geyser was like a cheap battery for me.

To make a meaningful difference to the hours that I heat these two geysers off my inverter, I need about 10kWh (5 hours at 2kW currently).  In winter, I get about 3.5 full hours equivalent from my panels on average.  Maybe I can get 4.  So I would need 2.5-3kW PV to get to this target.  So I need a 3kW element.  I can then circulate the water between the two geysers right next to each other) to spread the warm water as if it is a large 300L geyser I am heating.   A cheap 10L/min circulation pump will mix them 50/50 in under 10 minutes a day, and fully cycle one geyser through the other one in 15 minutes.  So that is very easy.

 

Now the theory.  If I put 6x 470W (43V Vmp) panels in series (I will get less than 2.8kW from them, but let's assume I do), then I get to 260V/11A/2.8kW/24Ohm.  3kW element at rated 230V is 13A/18Ohm.  Do you think this will work without an MPPT/controller?  The PV string's resistance is higher than the element's, to avoid the element resistance blocking current (18<24), PV Volt is about at the max Volt of element (265V), PV power is within element Watt limit (3300W). 

I can get the PC for about R11k + delivery it seems.   R11k, say R13-14k with install to sort out two geysers winter and summer and give me back 2kW on my inverter capacity most of the day sounds like a good plan without any lengthy new pipework and associated risk.

I just want to test the theory with someone who has done it.

What do you think?

 

Enjoy the weekend.

 

Am I right that without the controller you feed the PV directly to the PTC that cannot burn out but no thermostat? 

If you use the normal thermostat which is in series with the PV it means it operates on DC and will weld due to DC drawing an arc or have you got another plan? 

Not anti PTC but a normal low Wattage element comes at a lower cost 

Going the heat pump route where you only use 33-50% in Watts to achieve the same end result is perhaps a more efficient system at a higher initial cost. 

Edited September 22, 20232 yr by Scorp007

 

Good to hear.  I have looked at a few combinations of AC/DC and AC elements.  For example, the spec sheet shows 8-20 Ohm resistance for the 1100W PTC element at 25C (rated at 72V).  But that cannot be the typical/stable resistance, as you would need about 15A at 72V to get to 1100W.  And that means the steady state resistance is at about 4.7 Ohm.  Same story for the AC elements.

I have two geysers in series (I heat up both in the day, staggered).  The reason is some shower in the evening, then the main geyser is still fed with hot water, and next morning others can shower again, still with hot water.  Works almost 100% of the time without needing any Eskom.   The second geyser was like a cheap battery for me.

To make a meaningful difference to the hours that I heat these two geysers off my inverter, I need about 10kWh (5 hours at 2kW currently).  In winter, I get about 3.5 full hours equivalent from my panels on average.  Maybe I can get 4.  So I would need 2.5-3kW PV to get to this target.  So I need a 3kW element.  I can then circulate the water between the two geysers right next to each other) to spread the warm water as if it is a large 300L geyser I am heating.   A cheap 10L/min circulation pump will mix them 50/50 in under 10 minutes a day, and fully cycle one geyser through the other one in 15 minutes.  So that is very easy.

 

Now the theory.  If I put 6x 470W (43V Vmp) panels in series (I will get less than 2.8kW from them, but let's assume I do), then I get to 260V/11A/2.8kW/24Ohm.  3kW element at rated 230V is 13A/18Ohm.  Do you think this will work without an MPPT/controller?  The PV string's resistance is higher than the element's, to avoid the element resistance blocking current (18<24), PV Volt is about at the max Volt of element (265V), PV power is within element Watt limit (3300W). 

I can get the PC for about R11k + delivery it seems.   R11k, say R13-14k with install to sort out two geysers winter and summer and give me back 2kW on my inverter capacity most of the day sounds like a good plan without any lengthy new pipework and associated risk.

I just want to test the theory with someone who has done it.

What do you think?

 

Enjoy the weekend.

 

Connect the PTC element directly to the battery bank and use a contactor to switch ON/OFF. I have connected mine that way and no issues so far. I power my geyser from batteries/DC every day from midnight till 07:00 for morning showers and then power the element from AC throughout the day. I use Geyserwise PTC element,  2kw Dual supply one. 

 

Hi Pieter

I realise it is quite a while since you posted, but I am considering replacing my 2kW AC element with a PTC AC/DC element.  I currently run 3x geysers on 2kW via my inverter (staggered).  Has been working well for a couple of years, but when there are extended outages, I need to watch out for overloading the inverter (As there is no Eskom backup).  So dropping 2kW load from the inverter for a few hours a day will help.

 

So I was thinking to add 2 new PV panels to the DC side (keep the AC side connected to my inverter).  With PV prices as low as they are, I can get anything between 2x330W and 2x550W for a decent price.  I think this is similar to what you have done.  No MPPT, jsut straight to the DC side.  The Geyserwise MPPT is way more than a panel, wo it starts to destroy the ROI.

I understand that I need to match the resistance of the PV panels (e.g. 2 in series) to the DC element.  But the PTC resistance is stated strangely (as it is a variable resistor).  For normal elements, the calculation is quite straightforward.

If I decide to go with 2x550W panels at Vmp/Imp of 83V and 13A, that works out to about 6.2 Ohms on the panels' side.  I see the 900W element (which had a max of 1100W) has seemingly been replaced by an 1100W DC element now (with a max of about 1200W).  So 2x 550W panels is as high as I want/need to go.  If I get 900W for 3-4 hours a day out of it, I have covered most of my required output for one geyser.

The 1100W PTC element is rated at 72V DC.  That gives about 15A/4.7 Ohm.  But the spec states between 8-20 Ohm resistance at 25C.  (https://www.geyserwise.com/2kw-ac-1100-dc-72v-ptc-element/)

The variable Ohm I assume is as temperature rises.  So I am struggling to match the PV Ohms, and I know bad matching destroys the yield.  Since you have done this already, I wanted to know what you found the PTC resistance to be (or how you calculated the PV Vmp/Imp combination to match)?

Would you mind sharing?

Thanks

Gerhard
 

Hi Gerhard

Yes,  I connected my panels directly with the Eco Controller improving RoI and I believe to be more durable.

You will be safe with the lower Ohms. My one Geyser is connected with only two 330W panels in parallel; being about 18A and 37V, and works well.

The other I connected 2x 390W in series and works just as well.