We have solar PV panels, what Hot Water cylinder and methods to maximise use

Forums › Home Workshop DIY › We have solar PV panels, what Hot Water cylinder and methods to maximise use

jaybeedee

64 posts

Master Geek

#288370 24-Jun-2021 13:00

We are nearly completed a big reno. We were going to re-use the existing 180l HW cylinder (reduce, reuse,...) but the plumber has got us thinking it might make sense to replace now.

We have 4.9Kw panels and export excess back to the grid. Auckland location, panels almost directly north. For the next few years mostly just 2 of us living there. No EV yet but on the cards. We are at home during the day, but not always at home.

We are looking at Rheem 250 or 300l enamel cylinders and trying to get my head around the options and how best to configure our set up simply.

Should we get the 250l non simultaneous twin elements and just have the bottom element on a timer so it has power available only during times when our roof is likely to be generating ?

Or 300l and only have power to the cylinder during daylight hours ? (and try to remember to override the timer when the son comes to stay and runs deep baths)

Or something completely different ?

Simplicity is a key design requirement.

TIA

John

1 | 2

8274 posts

Uber Geek

#2733783 24-Jun-2021 13:59

What exactly are you trying to achieve?

Yes you could have a bigger cylinder to soak your PV surplus, but to what end,

With only 2 people in the household you will not often use 250 or 300 litres of hot water in a day... so you are just heating( and keeping warm) excess water rather than sending it to the grid (granted for little $)

You premised you question with a reference to (reduce, reuse) ... nearly doubling you hot water capacity is certainly not "reducing" even if you are powering it from your own PV...

tdgeek

29671 posts

Uber Geek

Trusted

Lifetime subscriber

#2733814 24-Jun-2021 14:17

I have Solar HW but not PV but I think I can add some value

Our 300L cylinder has two elements, top has a timer (via the Solar HW wall panel) and I had a timer put in the bottom as well. as was only mains on or off.

just have the bottom element on a timer so it has power available only during times when our roof is likely to be generating ?

Make sure you use the timer with your PV just for the top. You use the top, not the bottom. The top stays hot, the bottom has cold water added when you turn on a tap. Ideally Solar PV heats the top first, the grid stays away from the top to maximise savings. If you need to boost, (hot water use, no sun) you need to manage that somehow, not sure how you would fall back to grid when Sun is not out. Maybe use the timers to keep the cylinder off the grid, then turn the cylinder on later on vis the timer(s) (as if Sun was out, thermostat keeps the grid away, if no Sun the grid will boost it)

jaybeedee

64 posts

Master Geek

#2733819 24-Jun-2021 14:25

wellygary:
What exactly are you trying to achieve?

Yes you could have a bigger cylinder to soak your PV surplus, but to what end,

With only 2 people in the household you will not often use 250 or 300 litres of hot water in a day... so you are just heating( and keeping warm) excess water rather than sending it to the grid (granted for little $)

You premised you question with a reference to (reduce, reuse) ... nearly doubling you hot water capacity is certainly not "reducing" even if you are powering it from your own PV...

What I am hoping to achieve is to have sufficient storage capacity to have a shower in the evening, Mrs Jaybeedee to have a shower in the morning, do some dishes etc and minimise the amount of electricity needed from the grid, especially if we have overcast weather for a day or two.

I guess, as you identify, the heat retention of the system is something to take into account.

FineWine

2979 posts

Uber Geek

Trusted

Nurse (R)

Lifetime subscriber

#2733865 24-Jun-2021 15:48

Can not answer the hot water cylinder questions but the rest have a read at this Topic of mine: Solar Power Energy Solutions

We are very happy with the eddi EcoSmart Energy Diverter system. It lets us control our whole system fantastically. You also add on for EV charging later on or/and connect into a Tesla Power Wall. We got our system from Harrisons energy solutions

Whilst the difficult we can do immediately, the impossible takes a bit longer. However, miracles you will have to wait for.

compound

88 posts

Master Geek

#2733996 24-Jun-2021 20:10

I have a Paladin solar water controller fitted to our system which entirely controls the power to the HWC. Our cylinder is 300L and the controller measures by CT when there is export and then dumps to the cylinder, boosting temps past the preset 40, 50 or 60 boost options. Legionnaires is sorted by off peak boost if there has been too many hours since solar boost has pushed the temp past the critical minimum. It does a great job with minimum fuss. The cylinder has just the single bottom element and we transferred 11kWh today to the cylinder in preference to export. To keep context, the system is grid tied, no battery and total production today was 41kWh.

barclayg

16 posts

Geek

#2734015 24-Jun-2021 21:04

We also have a Paladin and would recommend it for most situations.

Having said that, we changed power companies to be with Flick. Rather than a paltry 8c/kWh like others, they pay whatever is the current spot price for exported power. Lately this has been anywhere between 18c and 40c/kWh. I ended up largely by-passing the Paladin and created a DIY solution which monitors spot prices and turns off the pool pump, HW, etc. if it's higher than our off-peak rates (~20c).

jaybeedee

64 posts

Master Geek

#2734020 24-Jun-2021 21:53

barclayg:
We also have a Paladin and would recommend it for most situations.

So how does the paladin measure the temperature? Does the hot water cylinder have an interface or just its own internal thermostats?

Hatch

Trade NZ and US shares and funds with Hatch (affiliate link).

colinuu

246 posts

Master Geek

#2734026 24-Jun-2021 22:06

If you are really considering a cylinder replacement then get a heat pump water heating system. Separate units (not integrated). Save real energy that way.

compound

88 posts

Master Geek

#2734036 24-Jun-2021 22:46

Paladin measures the temperature with its own temperature probe. The original thermostat is the over-temp cutout for the maximum cylinder capability. Controller current flow direction sensing at 3kHz? and switching at 50Hz through a solid state relay controls the element. The display shows current tank temp, throttle graphics, daily hours of transfer, grid activity in/out, daily transfer and daily top-up from grid.

SumnerBoy

2072 posts

Uber Geek

ID Verified

Lifetime subscriber

#2734080 25-Jun-2021 08:53

colinuu:

If you are really considering a cylinder replacement then get a heat pump water heating system. Separate units (not integrated). Save real energy that way.

I spoke with a guy who installs HWHPs for a living, and his view was they don't stack up financially particularly if you have PV. The cost to install and maintain, and the lifetime and high servicing costs, pretty much eliminate any savings in electricity you are likely to make. And with PV and a sensible system of maximising any excess (via a diverter if possible) you end up with a far more cost effective system.

YMMV.

SumnerBoy

2072 posts

Uber Geek

ID Verified

Lifetime subscriber

#2734083 25-Jun-2021 08:59

FineWine:

Can not answer the hot water cylinder questions but the rest have a read at this Topic of mine: Solar Power Energy Solutions

We are very happy with the eddi EcoSmart Energy Diverter system. It lets us control our whole system fantastically. You also add on for EV charging later on or/and connect into a Tesla Power Wall. We got our system from Harrisons energy solutions

I looked at the Eddie but it was going to cost $1700 (installed), and that was by Harrisons who are doing my PV + PW2 install in the new house - it was just far too expensive for the savings it would provide.

My current thinking is to use the battery as a kind of buffer, so the HWC element is turned on a 11am'ish and left to run for a few hours. If there is excess solar that will be used to heat the HWC. If there is limited or no solar, the battery will have been charged up over night at cheap rates, so I will be load shifting that cheap power effectively.

Then to get really clever I intend to build a little controller with 2 relays, one for the middle/bottom HWC element. Then each morning I can check the PV forecast (check out solcast.com.au) and automatically decide whether to enable the middle or bottom element. If there is high solar forecast I will heat from the bottom, if low then heat from the middle.

All theory at the moment however!!

jaybeedee

64 posts

Master Geek

#2734138 25-Jun-2021 10:38

Thanks for all the replies. It has helped clarify my thinking.

It seems that the diverter idea is what makes sense, as putting a timer in probably only marginally increases the proportion of self generated power, because if the PVs are not generating a spare 3kW when it kicks in, the rest is supplied by the grid. Being able to go as low as 100watts for the diverter makes a lot of sense.

So if we go that way, is there any real advantage in a dual element system ?

And how easy is it to retrofit the Paladin after we have lived here for a season or two and got a better understanding of our usage and our production ?

SumnerBoy

2072 posts

Uber Geek

ID Verified

Lifetime subscriber

#2734139 25-Jun-2021 10:40

I would hook up the middle element to an overnight timer, so you always have a hot top-half of the cylinder in the morning.

Then hook the diverter up to the bottom element. This ensures you convert as much excess solar as possible into stored hot water.

If you get a good solar day, the whole cylinder will be heated up and your overnight top up (middle element) won't kick in.

Kickinbac

417 posts

Ultimate Geek

#2735334 27-Jun-2021 12:19

Your hot water cylinder can be considered a battery for your solar PV.
I follow a couple of Aussie Facebook groups where they have a lot of PV. They tend to use hot water heat pumps (HWHP) and set them on a timer to run during the middle of the day. The heat pump reduces electricity use and is more efficient in the warmer period of the day. A 300 litre cylinder will store plenty of hot water until the next day. The best seem to be Sanden or Reclaim CO2 HWHP. You get what you pay for and they did have a bad reputation due to some poorly designed units on the market a few years ago. Also the Aussies seem to get some great rebates for HWHP and solar which makes it more affordable for them.

eonsim

391 posts

Ultimate Geek

Trusted

#2735369 27-Jun-2021 15:15

So we have a 5kw PV system with a Green-catch solar diverter (diverter + install was $980), hooked up to a standard 300L 3kw Rheem cylinder. The green catch is a fairly simple system that monitors the amount of power being generated and as soon as it sees any being exported it starts to redirect the power to the hot water cylinder, for it's use. Once that's heated to it's standard temp it turns off and allows the power to be exported, unless you've got something else setup to use the excess (our SPA heats up when there's spare power). The device has 4 settings heat now, allow up to 2.5hrs overnight grid usage, allow up 5hrs overnight grid-usage (does some tricks reducing the amount allowed each night if the solar power can fully heat the cylinder during the day) or solar only. It seems to work well, PV consumption pretty much perfectly tracks the amount generated for some hours each morning. Then in the afternoon we'll see a brief spike of exported power for a couple of minutes as it shuts down, before home assistant spots the wasted power and starts up the spa to prep it for the evening.

1 | 2