PV to Thermal Store

[SeanK]

I've been pondering an off-grid standalone PV setup for dumping DC straight into a thermal store, to be supplemented with a wood stove in winter months... the simplicity of the system appeals and I wondered if anyone here has put together anything similar?

 

Seems some folk try and match the panels to the heater resistance - but as this figure varies there are a few products implementing MPPT to increase efficiency. One Austrian made and a few in South Africa:

 

https://zerohomebills.com/product/mypv-dc-elwa-solar-pv-water-heater/

 

https://www.sustainable.co.za/solarhotty-pv-water-heater-controller.html

 

https://www.sustainable.co.za/geyserwise-offgrid-pv-solar-water-heating-kit.html

 

The "Geyserwise" products use PTC elements which are very interesting, and seemingly inherently safe...

 

I've a distributor for SunAmp who is enquiring about their "dPV" units that might be a worth a look at for this purpose... (if any still exist).

[ProDave]

My only thought is why limit PV to just water heating?

 

Mine powers all sorts of things like the washing machine etc that we only run in the daytime close to mid day.  When nothing else is using the electricity it goes to water heating but why not use it for other things as well?

[SeanK]

1 hour ago, ProDave said:

My only thought is why limit PV to just water heating?

 

Mine powers all sorts of things like the washing machine etc that we only run in the daytime close to mid day.  When nothing else is using the electricity it goes to water heating but why not use it for other things as well?

 

This will no doubt raise more questions that it answers but - been looking into DC micro-grids, inspired by the unconventional use of PV, Nife batteries and "Daylight Drive" employed by these guys - http://livingenergyfarm.org/energy/

 

In line with their approach of being "multi linear" - I've been pondering standalone PV/hot water and PV/cooking systems for the summer so we're not relying on firewood or peat.

[SteamyTea]

DC requires larger switches.

And if low voltage, larger cabling.

AC is really the way to go as the kit is easily available.

And what happens if, for some reason, you get a current flowing through water between the heating element and the cylinder?

[SeanK]

21 minutes ago, SteamyTea said:

DC requires larger switches.

And if low voltage, larger cabling.

AC is really the way to go as the kit is easily available.

And what happens if, for some reason, you get a current flowing through water between the heating element and the cylinder?

 

Good points, thanks SteamyTea.

 

I'm coming at it from the ethos/approach taken at http://livingenergyfarm.org/energy/ - seeing if it's feasible to do away with inverters and such like. Is the problem you highlight exclusive to DC (or only able to be addressed in an AC system)?

 

Living Energy Farm actually use Solar Thermal with a little pump run by PV. Not sure how effective this would be in the Outer Hebrides and the idea of (perhaps) simplifying matters by eliminating the plumbing involved with Solar Thermal appeals. Don't know a great deal about Solar Thermal but I wouldn't have thought they would be able to get a thermal store up to temperature...

[SteamyTea]

14 minutes ago, SeanK said:

Is the problem you highlight exclusive to DC (or only able to be addressed in an AC system)?

It is really just an electrical problem.  The higher the current, the larger the switching needed.

 

Solar Thermal is an interesting problem.  It does, on the face of it, seem simple and reliable, and the efficiencies seem higher.

This is a bit of simplistic view of it as it does not take into account the times the system cannot function i.e. not enough light, system already up to temperature, not enough temperature difference to transfer energy.

This is where PV starts winning.  As long as the modules are producing enough voltage, the inverter can convert that to AC at at known voltage and resistance.  That can then be put into the store of water, or anything else that needs current.

 

Ideally you need to be able to store energy, water and stone i.e. storage heaters, are pretty good at this as they don't take a lot of looking after, resistance heating is very reliable, not pumps, antifreeze, sludge removal, leaks etc.

Storing a small amount in batteries is also useful as it is unlikely that a PV system will always be producing enough kW (power) to run a kettle (about 2.8 kW), but it can keep a 1 or 2 kWh (energy) battery charged that can power a kettle (as it only needs a fraction of a kWh to boil a litre of water (~1.1 kWh).

 

The real problem is that in the summer you need a small system, in the winter a large one.  Why small windturbines can be useful.  A 1 kW windturbine will probably consistently produce about 200 W, but for 24 hours a day (this is all weather dependant of course), but that is 4.8 kWh, which is about the energy my house uses in total on a non washing day (clothes not my body).

 

[ProDave]

@SeanK What are you trying to achieve?

 

Do you have a mains connection?  Or are you trying to power an off grid house?  And presumably in any case you still need electricity for normal stuff?

 

I like the concept of NiFe batteries and if I ever implement battery storage that is probably what I will use but so far I am just about able to self use all I generate without batteries.

[SeanK]

41 minutes ago, SteamyTea said:

It is really just an electrical problem.  The higher the current, the larger the switching needed.

 

Solar Thermal is an interesting problem.  It does, on the face of it, seem simple and reliable, and the efficiencies seem higher.

This is a bit of simplistic view of it as it does not take into account the times the system cannot function i.e. not enough light, system already up to temperature, not enough temperature difference to transfer energy.

This is where PV starts winning.  As long as the modules are producing enough voltage, the inverter can convert that to AC at at known voltage and resistance.  That can then be put into the store of water, or anything else that needs current.

 

Ideally you need to be able to store energy, water and stone i.e. storage heaters, are pretty good at this as they don't take a lot of looking after, resistance heating is very reliable, not pumps, antifreeze, sludge removal, leaks etc.

Storing a small amount in batteries is also useful as it is unlikely that a PV system will always be producing enough kW (power) to run a kettle (about 2.8 kW), but it can keep a 1 or 2 kWh (energy) battery charged that can power a kettle (as it only needs a fraction of a kWh to boil a litre of water (~1.1 kWh).

 

The real problem is that in the summer you need a small system, in the winter a large one.  Why small windturbines can be useful.  A 1 kW windturbine will probably consistently produce about 200 W, but for 24 hours a day (this is all weather dependant of course), but that is 4.8 kWh, which is about the energy my house uses in total on a non washing day (clothes not my body).

 

 

Thanks SteamyTea, plenty to mull over!

 

A small turbine ticking over straight into a thermal store sounds sensible. I'm put off slightly by the high outlay of a turbine and inevitable mechanical problems posed by fast spinny things! A D400 seems a good contender though.

 

We could meet our winter hot water/cooking/space heating needs with wood and peat (we'll have a peat bank). I've plans for a wood fired cook stove and water boiler and a HETAS installer to build them. I have been wondering whether PV could be dumped into the thermal mass of these heaters in summer...

 

13 minutes ago, ProDave said:

@SeanK What are you trying to achieve?

 

Do you have a mains connection?  Or are you trying to power an off grid house?  And presumably in any case you still need electricity for normal stuff?

 

I like the concept of NiFe batteries and if I ever implement battery storage that is probably what I will use but so far I am just about able to self use all I generate without batteries.

 

Sorry, should have given a bit more context! No mains connection, looking for a simple domestic system for off grid. The folk at Living Energy Farm have lived with their setup for over ten years and to me, it makes a lot of sense.

Interestingly, as laid out by Scottish Building Control - a conventional supply doesn't seem to be required where no mains is available.

 

Quote

 

Standard 4.6
Every building must be designed and constructed in such a way that electric lighting points and socket outlets are provided to ensure the health, safety and convenience of occupants and visitors.

Limitation:
This standard applies only to domestic buildings where a supply of electricity is available.

 

 

Obviously there are risks, good practices and it is worthwhile having an electrician okay it - but a 120V DC is considered extra low voltage and doesn't require a building warrant. Forgoing AC and a lot of standard appliances is something we would be fine with. 

[ProDave]

1 minute ago, SeanK said:

Interestingly, as laid out by Scottish Building Control - a conventional supply doesn't seem to be required where no mains is available.
 

But they DO still insist on an EIC whatever the installation.

 

One of my more unusual jobs was to wire a remote public toilet.  All it had was a 12V battery, a solar panel and a small (caravan) wind turbine to charge it.  The battery powered a fan to ventilate the composting toilet, and power for a 12V LED light.

 

And building control still insisted I produce an EIC before they would sign it off.

[ProDave]

But are you wanting mains for televisions etc? 

 

Another good read is this https://lifeattheendoftheroad.wordpress.com/

 

He lives at the north end of Raasay and there is quite a bit in his blog about his off grid energy system.

[SeanK]

Thanks for the heads up and the link @ProDave! 

 

We survive without a telly just now. You never know, might get a wee inverter to have on hand just in case...

 

Just having a quick look through the blog, great stuff there.

[SeanK]

Another couple of products from Poland that do this job, any folk experience with these?

 

https://actii.pl/en/ladowarki-prostowniki/979-mpm-pwm-heater-charging-controller-for-solar-solar-panels-actii-ac7391-6931240073919.html?SubmitCurrency=1&id_currency=4

 

https://ekopulsar.pl/news/sterownik-fotowoltaiczny-PulsarSimple

 

On a separate note, keen to hear any thoughts on DIY thermal stores. These tanks are suitable for hot water and a fraction of a price, is there anything in the regs that doesn't permit this approach? https://www.draytontank.co.uk/product_details_388.htm

[PeterW]

3 minutes ago, SeanK said:

On a separate note, keen to hear any thoughts on DIY thermal stores. These tanks are suitable for hot water and a fraction of a price, is there anything in the regs that doesn't permit this approach? https://www.draytontank.co.uk/product_details_388.htm

 

That is expensive for a glassfibre box - by the time you add in coils etc you will add £3-400 to the price and you'd get limited stratification in a tank that size.

 

 

[SeanK]

4 hours ago, PeterW said:

 

That is expensive for a glassfibre box - by the time you add in coils etc you will add £3-400 to the price and you'd get limited stratification in a tank that size.

 

 

 

I was imagining coils like these might do the job but I'm likely being naive... but you're right, upon further googling it seems there are pre-plumbed thermal stores available for around a grand.

[SteamyTea]

7 hours ago, SeanK said:

I was imagining coils like these might do the job but I'm likely being naive

Always worries me when I see things like this

Heat Coil 30 kw

And then a second time

Output:

30 kw

[SeanK]

5 hours ago, SteamyTea said:

Always worries me when I see things like this

Heat Coil 30 kw

And then a second time

Output:

30 kw

 

Seems like they plucked the number out of the air right?

[SteamyTea]

2 hours ago, SeanK said:

 

Seems like they plucked the number out of the air right?

Not the numbers, it is the units.

[SeanK]

1 hour ago, SteamyTea said:

Not the numbers, it is the units.

 

...kw instead of kW?

[SteamyTea]

kW

k for thousand.

W for watt.