Heating water from PV system

[Guest]

1 hour ago, Tony Swash said:

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.

Tough to beat that with EV tubes.

So if it works, then why not.

Only system that I am aware of that will beat R6k are low pressure flat panel water heating system, if you shop right.

 

[SilverNodashi]

10 minutes ago, The Terrible Triplett said:

Tough to beat that with EV tubes.

So if it works, then why not.

Only system that I am aware of that will beat R6k are low pressure flat panel water heating system, if you shop right.

 

R6K would exclude the pipes, probably the circulation pump and installation + COC. Installation would be about R3k - R5k, depending on area and what needs to be done. These are the prices I've seen around.

[Fuenkli]

1 hour ago, Tony Swash said:

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

We have to compare apples with apples here. The above gives you +- 3Kw/h per day. An average EV Tube system will give you at least 12kW/h per day. I do however agree that it makes sense to use excess PV production to heat water. To upgrade a PV system to cover the hot water requirement will be more expensive than to install a solar water heater.

[SilverNodashi]

11 minutes ago, Fuenkli said:

We have to compare apples with apples here. The above gives you +- 3Kw/h per day. An average EV Tube system will give you at least 12kW/h per day. I do however agree that it makes sense to use excess PV production to heat water. To upgrade a PV system to cover the hot water requirement will be more expensive than to install a solar water heater.

No always. If your existing PV system use say 300W panels, and you can add 3 more 300W panels, it would cost about R4k to upgrade. If that extra 600W won't be used for anything other than warming the element, you still have a cheaper (read: more cost effective) way of heating water. Add another 300w if you like. For a 150L geyser this is sufficient. 

The picture would change a bit if you need 300L water heated, then EV tubes might become more cost effective. 

[Fuenkli]

3 minutes ago, SilverNodashi said:

If your existing PV system use say 300W panels, and you can add 3 more 300W panels

I agree. But I guess that the average system does not have sufficient extra free capacity to add the required PV. In addition the 3x300W panels will not cover the average household hot water requirement.

[Tony Swash]

My sentiments exactly. For some guest houses it works great as it is so simple. Even if the geyser runs dry the PTC will not burn out. I have a 220v PTC element as back up just in case but I have not switched it on in a year. This picture shows the 400 watt element next to a standard element.

 

[Tony Swash]

As with any power saving idea, you need to lag the geyser and the pipes. Mine has 2 jackets on it. 

[Youda]

EV tubes for heating the water are great, but the PV panels are great too.
I would say that both solutions have it's own benefits and depending on actual situation one or the other might be the better way to go. To me, it's similar like comparing the diesel cars with gasoline cars. When looking strictly at power/consumption/range the diesel is the winner. But for a lot of people, depending on their actual situation, a small gasoline car is a better choice.

Therefore, you can't just say - "diesel is everytime better". And I believe that the same logic applies to EV tubes vs PV.

Personally, I don't mind to work with any electrical stuff. Being it AC, DC or digital. But I'm not a friend with plumbing, pipes, pressure, temperature calculations etc. And since I already had a roof full of PV panels, in my case it was a logical decision to use PV for heating the geyser:

https://powerforum.co.za/topic/2322-youdas-off-grid-lab/?do=findComment&comment=46645

Long story short, you can't just put a geyser on the battery and let it work. That's really killing the batteries. So as a first version, I used a simple contactor, that was turning geyser on/off depending on the battery SoC. Then I moved to SSR and finegrain tuning of power that's flowing into geyser's heating element.

Edited March 12, 2019 by Youda

[Guest]

1 hour ago, Youda said:

EV tubes for heating the water are great, but the PV panels are great too.
... like comparing the diesel cars with gasoline cars. 

This is so true.

My 2 cents: 

My experiences are that EV tubes are better over a one year period seeing as the Western Cape is in a winter rainfall region with EV tubes outperforming PV by a long stretch under clouds.

On top of that seeing as we in CoCT are limited to inverter size based on our DB breaker ito max inverter size, that complicates things a wee bit more.

However, if you have a PV system and you have spare panels, then using the spare PV capacity is a no brainer - like I'm doing with one geyser.

The other geyser has been on EV tubes since <2012 - made my money back years ago. Pump only now showing wear and tear some of the tubes may need replacement on of these years. Eskom usage, I don't know, maybe 5 times a year I think.

 

4 hours ago, SilverNodashi said:

These are the prices I've seen around.

Jhb install by my brother, 200l flat panels system self installed with pipes was under R4k. He shopped hard.  

 

3 hours ago, Tony Swash said:

My sentiments exactly. For some guest houses it works great as it is so simple. Even if the geyser runs dry the PTC will not burn out. I have a 220v PTC element as back up just in case but I have not switched it on in a year. This picture shows the 400 watt element next to a standard element.

Seems to me Tony's system works.

 

[Gabriel_2018]

No inverter is needed for a PV Geyser, only solar pannels and a resistor acording them. Nowadays, comparing costs of both technologies, i only woud install pipes if I hadn´t enough space for PV. 

[SilverNodashi]

4 hours ago, Fuenkli said:

I agree. But I guess that the average system does not have sufficient extra free capacity to add the required PV. In addition the 3x300W panels will not cover the average household hot water requirement.

What is average?

Most people's home PV systems have excess energy from time to time, unless you have a small enough system to cover just the basics. 

[SilverNodashi]

4 hours ago, Tony Swash said:

As with any power saving idea, you need to lag the geyser and the pipes. Mine has 2 jackets on it. 

That also helps a bit

[DeepBass9]

6 hours ago, Tony Swash said:

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.

 

 

 

 

A 150l tube geyser costs half that price and is much more efficient. It will heat water from 20 to 80 deg in a few hours of good sun, so it doesn't really compare.  

[___]

6 hours ago, Tony Swash said:

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.

A good solar geyser is around 30k, as @SilverNodashi pointed out. Well you can make some plans that I will get to next, if 30k is too much.

What you should be doing is calculating your cost per day (assuming you use more or less the same amount of hot water per day, which is probably an acceptable assumption). Now right off the bat, you can see that the solar geyser costs about 5 times more, so to be victorious in the race it merely has to make 5 times more hot water per day. And that it does, it generates easily the equivalent of 8kwh (how much you need approximately to heat 150 liters from 15 to 55 °C), and quite often it will do this by lunchtime. So in terms of sheer heating efficiency, there is no question: The solar geyser wins.

But what if I don't need 150 liters of hot water? It's rather silly to buy a beef sharebox at MacDonalds (meant for a family of 4) if it's just me and the wife, right? So how much hot water does two panels and a PTC element make? Well let's assume you have two large panels up there, which nowadays means you have 4m^2 covered in PV panel. With strong sunshine, that means 4kwh every hour, times roughly 5 for our climate, so you have 20kwh beaming down onto that area, of which the average good panel can turn 17% into electricity, so you have 3.5kwh of electricity. Assuming no other losses, and assuming the same 40 °C elevation (from 15 to 55), and given that it takes 1.16Wh to heat one liter of water by 1°C, 3500/(1.16*40) = 75 liters.

So it can work if it's just you and the dishes. It will be less than 75 liters though. And you have to subtract the standing loss of around 2kwh, which is a bit complex (cause it depends on the average tank temperature), but I would be surprised if you have much more than 50 liters to work with.

Also, the moment you have a bit of cloud cover, you basically have no hot water.

So what I would do, if I didn't have 30k, is install a heatbox. They cost around 10k. This is a pre-heating solution, feeding a normal electrical geyser. Then you can use PV-panels (and a PTC element if you really want to) to heat that tank. Best of both worlds.

[Fuenkli]

18 minutes ago, plonkster said:

A good solar geyser is around 30k

You can get decent systems for less than that.

http://ubersolar.co.za/prices-and-systems/

[DeepBass9]

5 hours ago, Fuenkli said:

You can get decent systems for less than that.

http://ubersolar.co.za/prices-and-systems/

Much much less.

https://www.pricecheck.co.za/search?search=105l+low+pressure+solar+geyser

R30k for a solar geyser is just robbery.

Edited March 12, 2019 by DeepBass9

[Guest]

10 minutes ago, DeepBass9 said:

Much much less.

That's what I said.

My Brother did it all in for under R3k - self install.

[DeepBass9]

I have a 300l high pressure geyser that is about 12 years old now (early adopter, expensive exercise), but when it finally collapses in on itself, I will replace it with a low pressure geyser.

[SilverNodashi]

2 hours ago, DeepBass9 said:

Much much less.

https://www.pricecheck.co.za/search?search=105l+low+pressure+solar+geyser

R30k for a solar geyser is just robbery.

Well, you get the cheap stuff and you get the good stuff  Same with inverters and batteries. 

[SilverNodashi]

7 hours ago, plonkster said:

A good solar geyser is around 30k, as @SilverNodashi pointed out. Well you can make some plans that I will get to next, if 30k is too much.

What you should be doing is calculating your cost per day (assuming you use more or less the same amount of hot water per day, which is probably an acceptable assumption). Now right off the bat, you can see that the solar geyser costs about 5 times more, so to be victorious in the race it merely has to make 5 times more hot water per day. And that it does, it generates easily the equivalent of 8kwh (how much you need approximately to heat 150 liters from 15 to 55 °C), and quite often it will do this by lunchtime. So in terms of sheer heating efficiency, there is no question: The solar geyser wins.

But what if I don't need 150 liters of hot water? It's rather silly to buy a beef sharebox at MacDonalds (meant for a family of 4) if it's just me and the wife, right? So how much hot water does two panels and a PTC element make? Well let's assume you have two large panels up there, which nowadays means you have 4m^2 covered in PV panel. With strong sunshine, that means 4kwh every hour, times roughly 5 for our climate, so you have 20kwh beaming down onto that area, of which the average good panel can turn 17% into electricity, so you have 3.5kwh of electricity. Assuming no other losses, and assuming the same 40 °C elevation (from 15 to 55), and given that it takes 1.16Wh to heat one liter of water by 1°C, 3500/(1.16*40) = 75 liters.

So it can work if it's just you and the dishes. It will be less than 75 liters though. And you have to subtract the standing loss of around 2kwh, which is a bit complex (cause it depends on the average tank temperature), but I would be surprised if you have much more than 50 liters to work with.

Also, the moment you have a bit of cloud cover, you basically have no hot water.

So what I would do, if I didn't have 30k, is install a heatbox. They cost around 10k. This is a pre-heating solution, feeding a normal electrical geyser. Then you can use PV-panels (and a PTC element if you really want to) to heat that tank. Best of both worlds.

As matter of interest, our geyser hardly ever start off at 15 degrees. It's in a tiled roof with a geyser blanket and it's on average 35 - 39 degrees before the geyserwise kicks in. 

[Tony Swash]

The math that you propose is correct. However in my opening statement I did not say that the water is starting at 15 degrees. Its anywhere between 34-39. This is the whole point of the exercise. Prior to installing, the geyser would be in the low 20's and the rise from that to 50C was taking my batteries out in the morning. now they have all day to get to 52 which is my set point. If I want more hot water we just leave it on. I have forgotten to turn it off and have seen the water in the morning at 72C.

This morning without any showering done the temperature is 50 degrees. As it is every day.

One aspect to consider is this. My wife does not like ugly water heaters on the  roof. We also have a thatch.  I have to grow trees to hide the neighbour's ugly water heaters that litter the roofs of Robertson.

Its also one more thing to worry about, with the PV its install and forget. and the panels are out of sight. 

[DeepBass9]

11 hours ago, SilverNodashi said:

Well, you get the cheap stuff and you get the good stuff  Same with inverters and batteries. 

Not 10x better. A 30k geyser wont make water 10x hotter than a R3k geyser, neither will it last 10x longer.

[Gabriël]

this is not in reaction to any previous post on this matter, just to mention how great our solar geyser now works since the 3kw element was replaced with a 1kw.

the pv system has no problem with the load and the element is on from 10:00 to 15:00. the thermostat temp for the water is set to 60c, in other words the load will switch off there although the sun does most of the heating anyhow.

that is the up side.

the down side [isn't there always one ] is that we have a 200l geyser and are only two people, so as soon as we use warm water a lot of cold water gets in and lowers the temp - but then, most geysers have that problem...

[___]

1 hour ago, Tony Swash said:

did not say that the water is starting at 15 degrees

It's a worst case scenario. It should be around the average soil temperature or a bit above, wherever the pipe is running, which is supposedly around 18°C.

I'm not against a PV solution per se. I have a house with a very stupid low roof pitch which means I'm constrained for space. There is no way I'm sacrificing PV-module space to heat water. In Cape Town we have generation limits of 3.5KW on a 60A breaker. Again, I'm not wasting 20% of that to heat water. That doesn't mean it can't be a solution... but it is not "better"

25 minutes ago, DeepBass9 said:

Not 10x better. A 30k geyser wont make water 10x hotter than a R3k geyser, neither will it last 10x longer.

Again, I sort of agreed with Silver as a worst case scenario. I also linked a 10k heatbox, which proves that it can cost way less. I doubt you'll get a decent high-pressure solar geyser for just 3k. Mine was 14k back in 2011 (and I would rate it as "average"). So I really think you need to look at systems between 10k and 20k if you want something at least modestly decent.

So that's about twice the cost of the PV solution, but I promise you it will make way more than twice the hot water using half the space.

Also: As far as possible use a Themosyphon system. You really don't need the stress of melting pumps and stuff.

Again: That does not mean that PV can't be a good solution. I just believe the conventional answer has way more going for it.

Edited March 13, 2019 by plonkster

[DeepBass9]

30 minutes ago, gabriel said:

this is not in reaction to any previous post on this matter, just to mention how great our solar geyser now works since the 3kw element was replaced with a 1kw.

the pv system has no problem with the load and the element is on from 10:00 to 15:00. the thermostat temp for the water is set to 60c, in other words the load will switch off there although the sun does most of the heating anyhow.

that is the up side.

the down side [isn't there always one ] is that we have a 200l geyser and are only two people, so as soon as we use warm water a lot of cold water gets in and lowers the temp - but then, most geysers have that problem...

What I am planning to do is to get two 150l tube geysers and put them in series so the first one fills the second. In the second geyser I'm going to put a heat exchanger on my generator exhaust and circulate the water through there when the generator is running. Generally our solar geysers are fine if it is sunny to partly cloudy, but if it s overcast I run the generator a bit in any case to change the batteries, so I can heat the geyser as well using the exhaust waste heat. Hopefully I'll have this system in place by winter. As I mentioned above I have an old flat plate indirect solar geyser which is less efficient that EV tubes, and is also about to fall apart as it is 12 years old.