PV-> DHW

[Crofter]

I'm kicking about some ideas for how to make a dent in my electricity bills without having to spend thousands on a big MCS approved PV/battery project.

 

Currently have a direct UVC.

 

Plan A: swap the existing bottom immersion element for a dual voltage AC+DC version. Wire up the AC side as usual, then DC side goes to a couple of panels, small battery, charge controller, possibly some sort of low voltage disconnect relay. Should be able to put something together for £250-£500 depending on how powerful I want it.

 

Thoughts?

 

Plan B would involve using grid tie equipment instead, but I don't see that coming in anywhere near the cost for my basic off grid approach. It would obviously have advantages though.

[JohnMo]

1 hour ago, Crofter said:

DC side goes to a couple of panels

Trouble with DC you need to get a balance system, if the impedance (think that's the correct term) isn't the same you get rubbish output.

 

Also to get the best out of PV you could do with mppt.

 

Not cheap but does what you want out the box

 

https://www.windandsun.co.uk/products/my-pv-elwa-dc-immersion-heater

[SteamyTea]

2 hours ago, JohnMo said:

impedance

That is for AC, impedance is sometimes called AC Resistance.

Don't make much difference to the calculations for a simple resistance heater really.

 

You are right though that to get maximum power, resistances need to match.

I don't know how well better charge controllers, or cheap sine wave inverters do this.

 

My main concert with low voltage DC is the switching current, may cause reliability issues quite quickly.

 

3 hours ago, Crofter said:

depending on how powerful I want it.

I think the trick is to not be greedy. A couple of 330 W modules in series, via a regulator (for want of a better term) into a 400 or 500 W element will still give a useful saving, and if it did 'stick on' the cylinder overpressure/over temperature mechanisms cal safety devises should be able to cope with it safely.

[Crofter]

2 hours ago, JohnMo said:

Trouble with DC you need to get a balance system, if the impedance (think that's the correct term) isn't the same you get rubbish output.

 

Also to get the best out of PV you could do with mppt.

 

Not cheap but does what you want out the box

 

https://www.windandsun.co.uk/products/my-pv-elwa-dc-immersion-heater

That's neat, but very expensive, and doesn't appear to take an AC input?

 

Not sure what the balancing is all about for a DC immersion... I thought you'd just need to make sure it had enough volts and amps?

 

I'm probably looking at a 600w DC immersion at 12v, so 50A draw. Use a Victron Battery Protect to switch it on and off according to voltage. Undecided on the battery, in theory any old lead acid would do but I imagine the system will work better with e.g. 100Ah LFP instead. But there goes another £200...

[JohnMo]

9 minutes ago, Crofter said:

doesn't appear to take an AC input?

It does you need to read the manual.

 

I have attached a spreadsheet that allows to match different mixes of series and parallel solar panels to different immersion heaters to get the correct response from both the PV and immersion.

Solar PV Calculator (1).xlsx

[JohnMo]

The video about the spreadsheet

 

https://youtu.be/kVaxAfXQPDU?si=BXgkkLaUM7II-OOb