Very simple pv for dhw

[cwr]

Hi all

 

I am considering a very simple pv set up to help with dhw, and starting from scratch both in terms of hardware and pv knowledge (so lots of basic questions coming up...).

 

I'm just about to start collecting data to determine the power requirements,  but I guess a few hundred Watts will be in the ball park.

 

Is it as simple a buying an inverter and a few panels?

 

Should I keep it all extra low voltage and buy a new heating element, or go with inverter and stick with the 230v element in the hot water tank which is currently unused?

 

Can anyone recommend makes/suppilers to go to (or indeed avoid)?

 

Thanks 

[SteamyTea]

If you go the small inverter route, that make sure that the the load exceeds the inverter start up requirements.

 

If you are going to the bother of installing modules and an inverter, why not go grid tied and then use a diverter to the DHW?

[JohnMo]

PV and immersion heaters isn't as straight forward as DC to the immersion as the impedance (think that's the right term) needs to be balanced 

 

You need to use something like the attached spreadsheet to calculate the match between panels and immersion.

 

Or as @SteamyTea say, go AC and then diverter.

 

Solar PV Calculator (1).xlsx

[SteamyTea]

1 hour ago, JohnMo said:

DC to the immersion as the impedance (think that's the right term)

Impedance when AC, resistance when DC (I think).

All to do with root mean squared values.

Edited August 29, 2023 by SteamyTea

[cwr]

So simpler to stick to 230v with inverter?

 

Apart from wires fuses etc, is the main hardware just the panels and inverter?

I want to dump all of the energy into the dhw so avoid batteries and keep it off grid. 

 

[SteamyTea]

56 minutes ago, cwr said:

I want to dump all of the energy into the dhw so avoid batteries and keep it off grid. 

That does rather make it a one trick pony though.

If you have it grid tied, you can use excess production elsewhere in your house, and pay be paid a few quid for any exports (all rather depends how Octopus get on).

 

 

[cwr]

Yes, true. But to be honest I want to keep it simple and cheap, just to dip my toe in the pv world, can't afford anything else right now. 

Not sure if there's any meaningful payments fir exports in NI at present.

[SteamyTea]

7 minutes ago, cwr said:

Not sure if there's any meaningful payments fir exports in NI at present.

Not going to be any meaningful sun till may either.

 

Have you added extra insulation to your DHW cylinder?

Have you reduced the flow for showers?

Have you reduced the cylinder temperature to as low as you can get away with?

 

Those 3 could save you a bit.

[Crofter]

3 hours ago, cwr said:

Yes, true. But to be honest I want to keep it simple and cheap, just to dip my toe in the pv world, can't afford anything else right now. 

Not sure if there's any meaningful payments fir exports in NI at present.

I've been looking in to this too.

My background is off grid low voltage systems so my first thought was to go that route.

But I think grid tie actually makes more sense. 

AFAIK your options are:

- panels, battery, and LV heating element

- panels, battery, inverter, AC heating element

- panels, grid tie inverter, solar diverter, AC heating element.

 

It seems that you can't skip the battery if you want to go off grid, you need it as a sort of buffer to keep the system running.

 

The last option is the one I'm going to go with. Grid tie inverters aren't much more expensive than off grid ones, and it simplifies things hugely to be able to use the existing AC heating element in the tank.

[JohnMo]

10 minutes ago, Crofter said:

simplifies things hugely to be able to use the existing AC heating element in the tank.

It does but I have a diverter when I am transferring a few hundred Watts to the immersion, it's questionable how much heat it's actually transfering to the water. Because unless the element surface temperature is higher than the water temp, there is no heat transfer.

 

Think if I was doing it just for water heating I would buy a Willis heater casing, add a matched heating element (per the spreadsheet attached a few comments above) then hook up to the top and bottom of the cylinder. It would always be drawing through cold water and the hot water would go direct to top of the cylinder.

[SteamyTea]

36 minutes ago, JohnMo said:

Because unless the element surface temperature is higher than the water temp

I worked this out once when our old mate Jeremy Harris was building his own diverter.

Your point is the very point I raised.

While it may be possible to not export a Wh, that does not heat up much titanium or Inconel.

 

Titanium 520 kJ/kg.K and 22 W/m.K

Inconel 460 kJ/kg.K and 6.5 W/m.K

 

A Wh is 3.6 kJ

 

So

 

Titanium 0.007 K/kg

Inconel 0.008 K/kg

 

Now I have no idea how much metal is covering an element, 0.05 kg (50 g) maybe.

So a rise in temperature of about 0.15 K.

And that does not include the actual element or the insulation.

 

(I think, I have a headache, one of those ones that is so bad I somehow jumped a red light earlier)

 

 

[JohnMo]

I had an interesting conversation with the battery installer the other week, he was saying, one of the inverter manufacturers has a dedicated immersion output. But it looks at the state of battery charge and solar generation, and if it feels it is right, puts the whole 3kW in to the immersion for a set period, then assesses battery charge state and PV output, repeat...

 

Good concept if you have a battery and don't want to or can't export due to you G99 restrictions 

[cwr]

"Because unless the element surface temperature is higher than the water temp, there is no heat transfer."

 

Yes, i gree with that, but surely unless the temperatures are for some reason changing rapidly in the cylinder then the element will have stabilised to the same temp as the water. So any temp increase inside the element will result in the element surface temperature increasing and thus transfer to the water. The energy has to go somewhere. 

[cwr]

Thinking about this again, and looking at the 'panels, battery, inverter, AC heating element' option.

 

If I get four panels around 400w each, would this inverter be suitable?

 

https://www.cityplumbing.co.uk/p/solax-power-x-1-1-0-7kw-single-phase-inverter-1-x-mppt-inc-dc-switch-x-1-0-7/p/628024

 

And what size of battery would be recommended? Bearing in mind the battery is only to smooth out, not be used as a storage device.

[JohnMo]

Title says very simple?

 

Why do you need a battery, it brings nothing to the party. Your grid connected the grid will smooth stuff out.

 

That inverter is actually AC coupled so would be no use connected to a battery.

 

[Nickfromwales]

48 minutes ago, cwr said:

Thinking about this again, and looking at the 'panels, battery, inverter, AC heating element' option.

 

If I get four panels around 400w each, would this inverter be suitable?

 

https://www.cityplumbing.co.uk/p/solax-power-x-1-1-0-7kw-single-phase-inverter-1-x-mppt-inc-dc-switch-x-1-0-7/p/628024

 

And what size of battery would be recommended? Bearing in mind the battery is only to smooth out, not be used as a storage device.

4 panels @ 400w? You'd be seeing less than a kW net on the best of days. Categorically forget a battery!!! 

 

This 1kW would chew a fair bit out of your vampire & base loads, over the year, but for diversion to DHW?................"Behave!"   Please remember that for AC coupled equipment you have the inverter fed into the CU (fuse-board) so you don't get to decide where it (excess generation, should you ever have any with such a small array) goes.

 

Time to get the thinking cap back on, sorry. 

[JohnMo]

On 29/08/2023 at 13:36, JohnMo said:

PV and immersion heaters isn't as straight forward as DC to the immersion as the impedance (think that's the right term) needs to be balanced 

 

You need to use something like the attached spreadsheet to calculate the match between panels and immersion.

 

Or as @SteamyTea say, go AC and then diverter.

 

Solar PV Calculator (1).xlsx 14 kB · 2 downloads

This is as simple as it gets for just DHW, select the right combo of labels and immersion and a DC isolation switch switch between the two.

 

Semi clear day today nearly 7kW installed PV and at 11am I am generating between 2 and 4kW. So about half installed capacity.

 

So you need to rethink number of panels and how you hook up. Either water on its own in DC or AC connected and leave to cover house loads. Any less than about 3kW is a waste of time really. 3kW on a good summer day will also heat your water and cover all house loads.