Heating water from PV system

[Tony Swash]

Hi All,

I was always keen on heating the water in my 150l geyser so I installed a 2kw element 220 vac and had it switch on via a Geyserwise controller from 6am - 8am.

Result- Disaster. The current drain on the batteries was so high that after running the house all night, once the geyser came on the inverter tripped due to low voltage. I spoke to the guys at Geyserwise and they sent me a 400 watt PTC based 48 vdc  element. This gets turned on at 9 am and merrily runs until 5 pm using the excess power from my panels. 

At the end of the day the geyser (inside and lagged) reaches a temperature of 56 degrees C. I find that this is sitting at 50 C in the morning and easily provides a bath and a shower. I have forgotten it on a few occasions and the temp was 72 the next day. The PTC can run dry if the geyser has no water. I use the geyserwise set to 52 degrees so if the weather is cold it kicks in the 2000 watt PTC element (220 VAC ) and runs for about 15 mins.

[Guest]

16 hours ago, Tony Swash said:

Result- Disaster.

Welcome Tony.

Use batteries to heat water is seriously expensive in my opinion. Eskom would be cheaper.

Best is to heat it daytime using spare solar power.

With us in CoCT limited to size of inverter, it is better to use EV tubes (they are better in winter) for heating water, than precious solar capacity.

[DeepBass9]

So how does the 48V element work, do you run it directly from your batteries? 

[SilverNodashi]

I use the same setup, Geyserwise + 2000W PTC but mine is set to run from 12:00 to 16:00 at which point it gets to 60 degrees (which is the limit of the geyserwise) and this works perfectly well for us.

[SilverNodashi]

45 minutes ago, DeepBass9 said:

So how does the 48V element work, do you run it directly from your batteries? 

It can run off some PV panels + MPPT

[DeepBass9]

400W will need to run for quite a long time to heat up any volume of water. To raise 100 litres by 20 degrees will take 8400kJ, so that element will have to run for about 6 hours. 

It is an interesting option though, I will be replacing one of my solar geysers soon, so it may be an option to go that way. What size of cables do you need to run 48V to the geyser?

Edited March 11, 2019 by DeepBass9

[PhatWheZ]

Im busy working on installing a DIY mk2pvrouter system for myself.
I am grid tied but get charged to export back to the grid (Thanks Pre-Paid meter). So in attempts to not purchase a inverter and batteries just yet. This is my next logical step.

The system can run with a trigger to turn the geyser on and off over a specific period of time so you can ensure you have nice hot water

[Tony Swash]

Hi,

I run 4 mm cables to the geyser, about 6 meters.

The water after shower/bath is normally 34 37 degrees so its not coming from 20 degrees up to 52.

Normally we have used the dishwasher, iron etc by 13h00 then we have excess power any way. I run it directly from the batteries.

I turn it on at 9 am and off at 5pm

Been running for a year now. not used Eskom once

 

[SilverNodashi]

3 hours ago, DeepBass9 said:

400W will need to run for quite a long time to heat up any volume of water. To raise 100 litres by 20 degrees will take 8400kJ, so that element will have to run for about 6 hours. 

It is an interesting option though, I will be replacing one of my solar geysers soon, so it may be an option to go that way. What size of cables do you need to run 48V to the geyser?

but it will start warming up the element from as early as about 7am. If you bath / shower at night, it will be warm enough by then

[DeepBass9]

2 minutes ago, SilverNodashi said:

but it will start warming up the element from as early as about 7am. If you bath / shower at night, it will be warm enough by then

400W spare at 7am? Not on my system!

[SilverNodashi]

3 minutes ago, DeepBass9 said:

400W spare at 7am? Not on my system!

 It's a heating element so it should use whatever is available at that time. My system produced 423W at 7:23 am yesterday.

So, let's say you get 400W by 08:00, by 14:00 it would have heated your geyser by +20 degrees  

[Gabriel_2018]

I think it woud be better to use a SSR when the system is floating. Or, if the charger has divert signal, as FM80, directly use the ssr via FM80 signal. The advantage is that it is possible to feed geyser in absortion stage. 

[SilverNodashi]

10 minutes ago, Javi Martínez said:

I think it woud be better to use a SSR when the system is floating. Or, if the charger has divert signal, as FM80, directly use the ssr via FM80 signal. The advantage is that it is possible to feed geyser in absortion stage. 

Yes, that would be a wise option. The 48VDC PTC element can be run off it's own solar panels + MPPT as well, if desired. 

[___]

1 hour ago, Javi Martínez said:

FM80 signal

Essentially a mirror signal to the PWM that drives the MPPT's buck converter? That is rather brilliant

(Probably a bit more complex than that, especially if the converter is interleaved, but the basic idea is to switch the dead-time into a heating element).

[Tony Swash]

I did say 9h00

[Tony Swash]

1 hour ago, SilverNodashi said:

 It's a heating element so it should use whatever is available at that time. My system produced 423W at 7:23 am yesterday.

So, let's say you get 400W by 08:00, by 14:00 it would have heated your geyser by +20 degrees  

Exactly. My water is 30 degs after showering. At 14h00 its 50 at 17h00 its 56

[SilverNodashi]

7 minutes ago, Tony Swash said:

I did say 9h00

aah, I missed that. 

 

How do you turn it on? Seeing as it runs off battery, I am rather curious what you do. 

[DeepBass9]

1 hour ago, SilverNodashi said:

 It's a heating element so it should use whatever is available at that time. My system produced 423W at 7:23 am yesterday.

So, let's say you get 400W by 08:00, by 14:00 it would have heated your geyser by +20 degrees  

My first priority is charging up the batteries again, so once that is done, I would be able to use the excess for water heating.

[Tony Swash]

3 minutes ago, DeepBass9 said:

My first priority is charging up the batteries again, so once that is done, I would be able to use the excess for water heating.

True, I dont think my system misses out on 275 watts per hour .The water is now 52 degrees

 

 

 

 

[Gabriel_2018]

7 hours ago, plonkster said:

Essentially a mirror signal to the PWM that drives the MPPT's buck converter? That is rather brilliant

(Probably a bit more complex than that, especially if the converter is interleaved, but the basic idea is to switch the dead-time into a heating element).

Yes. That feature is included in some chargers. It is a 0 +12 PWM signal, and the best way to heat water without drawing batteries. What i´ve known can work with MOS(DC) PWM 500 Hz, or AC Triac with much longer period.

Edited March 11, 2019 by Javi Martínez

[Gabriel_2018]

9 hours ago, SilverNodashi said:

Yes, that would be a wise option. The 48VDC PTC element can be run off it's own solar panels + MPPT as well, if desired. 

Yes, with  only solar pannels and this kind of resistor, one can have three levels of Power:

2 Parallel: 1500 w

Only one resistor: 750 w

2 in series: 375 w

https://www.amazon.es/Calefacción-Inoxidable-Alimenticio-Resistencia-Calentador/dp/B01EHWIZ76/ref=sr_1_1?ie=UTF8&qid=1552320774&sr=8-1&keywords=resistencia+48v

Depending on radiation, one or the others can be adopted.

 

If you like spanish, jeje, you can read this:

 

https://adnsolar.eu/viewtopic.php?f=17&t=146

 

Edited March 11, 2019 by Javi Martínez

[KLEVA]

I have to say NOOOOOO!

Solar water heating is one of the cheapest and best ways to save power. It is also the only way to save money overall (Solar electricity is still far too expensive vs Municipal power).

You will save money, and your entire electricity power setup if you just do a proper solar water install.

You need to also remember all the losses you incur by converting from Solar Electricity -> Battery -> AC Power -> Geyser Element.. Every step in that process looses power/heat. Any element that uses heating will kill your batteries and need you to provide much higher invertor specifications that you need.

Get a decent solar geyser with some tube heating instead. You are wasting money otherwise.

My Water heating with a Geyserwise does 60 - 85 degrees per day with no electricity input. You would need at least 2-4 hours of electricity input at 2kW to provide similar.

[KLEVA]

Just to add - even on a bad day (like today/yesterday when it just rains and rains, with almost no sun) my water heating is still at 38 degrees - enough for at least 3 people to shower.

Running costs are minimal, maintenance minimal, I cannot emphasis enough that a proper solar water install is hugely better than trying to heat your geyser with solar electricity.

[Tony Swash]

1 hour ago, KLEVA said:

Just to add - even on a bad day (like today/yesterday when it just rains and rains, with almost no sun) my water heating is still at 38 degrees - enough for at least 3 people to shower.

Running costs are minimal, maintenance minimal, I cannot emphasis enough that a proper solar water install is hugely better than trying to heat your geyser with solar electricity.

15 hours ago, DeepBass9 said:

My first priority is charging up the batteries again, so once that is done, I would be able to use the excess for water heating.

How much does  a solar water heater cost plus installation?

My element cost R600 add two panels to that R5000. No thermostat needed.  Cabling etc . Total cost R6000

No need to turn it on and off.

 

 

 

 

Edited March 12, 2019 by Tony Swash
forgot something

[SilverNodashi]

2 hours ago, KLEVA said:

I have to say NOOOOOO!

Solar water heating is one of the cheapest and best ways to save power. It is also the only way to save money overall (Solar electricity is still far too expensive vs Municipal power).

You will save money, and your entire electricity power setup if you just do a proper solar water install.

You need to also remember all the losses you incur by converting from Solar Electricity -> Battery -> AC Power -> Geyser Element.. Every step in that process looses power/heat. Any element that uses heating will kill your batteries and need you to provide much higher invertor specifications that you need.

Get a decent solar geyser with some tube heating instead. You are wasting money otherwise.

My Water heating with a Geyserwise does 60 - 85 degrees per day with no electricity input. You would need at least 2-4 hours of electricity input at 2kW to provide similar.

Unless you have free excess PV power to re-purpose for water heating. A cheap solar geyser will cost say R17,000+ A decent one with long lifespan will set you back about R30k. 

I already had the inverter solar panels and batteries in place for electrical usage. And now I am use excess, unused, wasted solar energy. My geyser doesn't run off batteries though, it's straight from PV - > 220V through the inverter. Even with the losses, how is that more expensive than thermal water heating?