PV->hot water

[Aphers]

I'd like to add PV to my house. I think I've got a lot to learn, although I do know some of the basics as I built a 1.2kw system on my boat already.

At it's most basic, can I install some panels, a grid tie inverter, and a diverter, to use solar to heat a tank of water?
Are there any other components required?

I'm probably going to go with a more complex system eventually, involving export and eventually an EV, but I need a simple basic plan to get started with first.

Thanks!

 

[Tomthumb62]

Why a grid tie inverter for an off grid location? Why not the usual PV>SCC>battery >inverter >hot water tank?

 

[Solar Guppy]

Solar thermal is typically lower cost and smaller footprint for the btu's to heat energy than solar electric.

 

[Tomthumb62]

Solar thermal is typically lower cost and smaller footprint for the btu's to heat energy than solar electric.

True but what do you do in winter? Wouldnt a solar thermal hot water heater on the roof freeze? Then you’d have to drain it come winter and do solar electric anyways. Our wood fired hot water.

 

[GSXR1000]

your electric company will let you grid tie without an electrician?

 

[Tomthumb62]

your electric company will let you grid tie without an electrician?

But there’s no grid…he said this an off grid location.

 

[Aphers]

But there’s no grid…he said this an off grid location.

No sorry it's not off grid, I didn't word it very clearly!

I *currently* live off grid, on a boat. Just mentioning that to show that I do have some familiarity with PV.
But I'm moving back to my house, which is on grid, and want to add PV there.
I currently heat the water using direct electric on an immersion (no mains gas and I don't want to use oil) so looking at PV->DHW.

 

[Aphers]

your electric company will let you grid tie without an electrician?

Not sure. I'm in the UK. I'm aware that anything above 4kw needs DNO approval if you wish to export.
Obviously I'll get an electrician in if that's required.

 

[efficientPV]

There are a variety of water heater controls like this in Europe that use voltage directly from array https://www.ebay.co.uk/itm/263190867548 where this is more of the norm.

Heating from batteries is a rather poor way to do it. Even from a 48V system the currents are enormous with serious losses. I heat all my water with PV using a similar diversion method that allows me to use a charge controller in parallel. Easy and cheap to build, but nobody here builds anything.

 

[jasonojordan]

your electric company will let you grid tie without an electrician?

Mine had no problem with me doing my own install with an inspection. Passed inspection and has been running for over a year.

 

[SeaGal]

Not sure. I'm in the UK. I'm aware that anything above 4kw needs DNO approval if you wish to export.
Obviously I'll get an electrician in if that's required.

If under 16A (=3680W) you just need G98 compliant inverter and inform the DNO within 30 days of comissioning.
If over 16A, you will need to get DNO approval before installing a G99 compliant inverter.
All electrical work must be IET BS7671 and building reg's part-P compliant. Whether you need a sparky for that will depend on your own competence and the topology of your existing wiring. In most cases you will need one to at least sign-off the work.

Regarding diverting Solar to Immersion - yes there are a few systems out there to do that - Marlec Solar iBoost being a popular one, albeit not that cheap. I build my own based on designs from Robin Emley & Martin Roberts. Here's a good starting point if you're going to DIY it...

Choosing an Energy Diverter — OpenEnergyMonitor 0.0.1 documentation

 

[J.P.]

True but what do you do in winter? Wouldnt a solar thermal hot water heater on the roof freeze? Then you’d have to drain it come winter and do solar electric anyways. Our wood fired hot water.

There are different types of systems. Look into a Drain Back System. Like this design. https://solarconsultants.com/drainbackrheemstecaman/
If you have enough solar to produce PV to heat your water in winter, there should be enough for this system to work even better.

 

[rhino]

I also don't understand why there isn't more off the shelf options in the USA like in other countries to go directly from PV to hot water tank.

I have zero info about this but thought it was interesting to find IMO supposedly has relays that can switch high voltage and high amp PV lines so I would think it would then be possible to have a solar charge controller and water heater connected in parallel and use one of these relays to switch over when necessary.. This avoids this "waste" of having a PV array that is only capable of heating your water.

 

[SeaGal]

The issue with switching with a relay is it is 'all-or-nothing'. With a triac-based a.c. diverter, I am able to divert as little or as much power to the immersion heater depending on amount of PV, battery SOC and current house usage.

 

[Aphers]

There are different types of systems. Look into a Drain Back System. Like this design. https://solarconsultants.com/drainbackrheemstecaman/
If you have enough solar to produce PV to heat your water in winter, there should be enough for this system to work even better.

I'm not really interested in solar thermal because AFAIK I'd need a new tank- my (relatively new) UVC is twin immersion, no coil.

 

[Aphers]

If under 16A (=3680W) you just need G98 compliant inverter and inform the DNO within 30 days of comissioning.
If over 16A, you will need to get DNO approval before installing a G99 compliant inverter.
All electrical work must be IET BS7671 and building reg's part-P compliant. Whether you need a sparky for that will depend on your own competence and the topology of your existing wiring. In most cases you will need one to at least sign-off the work.

Regarding diverting Solar to Immersion - yes there are a few systems out there to do that - Marlec Solar iBoost being a popular one, albeit not that cheap. I build my own based on designs from Robin Emley & Martin Roberts. Here's a good starting point if you're going to DIY it...

Choosing an Energy Diverter — OpenEnergyMonitor 0.0.1 documentation

Thanks, that's really helpful.
A related question: using basic geometry I *think* my proposed 6kw array would only be able to output 25A max (at 240v) because it would be on the E/W slopes of a 42⁰ pitched roof, at a latitude of 58⁰N. Or am I oversimplifying things?

The diverter I'm considering is a Cool Energy DiverTech: https://coolenergyshop.com/products/cool-energy-solar-power-diverter

 

[J.P.]

There are a variety of water heater controls like this in Europe that use voltage directly from array https://www.ebay.co.uk/itm/263190867548 where this is more of the norm.

Heating from batteries is a rather poor way to do it. Even from a 48V system the currents are enormous with serious losses. I heat all my water with PV using a similar diversion method that allows me to use a charge controller in parallel. Easy and cheap to build, but nobody here builds anything.

I have that exact unit and it has been working well for a few years. It is even mounted in my hot attic (125F+) in the summers and so far it's on it 3rd or 4th Summer and still working.

Regarding building things, I love to build things but do not understand electrical/electronics as well as you do. I have read (most if not all) of what you have written about PV diversion to hot water but still can't quite figure it all out but if I had a good design plan to build from I certainly could and would build it.

 

[Tomthumb62]

There are different types of systems. Look into a Drain Back System. Like this design. https://solarconsultants.com/drainbackrheemstecaman/
If you have enough solar to produce PV to heat your water in winter, there should be enough for this system to work even better.

Ah ok, these are newer designs than the ones I’ve seen. That’s smart.

 

[Delmar]

I don’t know why the market hasn’t offered a dual PV-thermal panel. Can preheat the water before the storage tank, and will cool the panels adding to their efficiency. Reduces the footprint and install cost over separate PV and thermal panels.

 

[SeaGal]

A related question: using basic geometry I *think* my proposed 6kw array would only be able to output 25A max (at 240v) because it would be on the E/W slopes of a 42⁰ pitched roof, at a latitude of 58⁰N. Or am I oversimplifying things?

You can put the figures in here and see what your panels can produce with that configuration. Then factor in inverter efficiency.

JRC Photovoltaic Geographical Information System (PVGIS) - European Commission

Photovoltaic Geographical Information System (PVGIS)

re.jrc.ec.europa.eu