Solar Diverter for water heating in NZ? Would appreciate your experiences.

Forums › Home Workshop DIY › Solar Diverter for water heating in NZ? Would appreciate your experiences.

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2391 posts

Uber Geek

#3153684 30-Oct-2023 12:57

Terciops:

Paladin (paladin.nz) is still here, manufacturing in NZ and selling Paladins as required.

We no longer bulk manufacture units though. I was rather underwhelmed with the quality control of our units out of the factory and post Covid I moved the manufacturing to my workshop. This better suited the ongoing volume of sales and has subsequently made a huge improvement to the build quality. The other upside is that the latest Paladin firmware now incorporates a rigorous test and tuning sequence that allows each individual unit to be 'tweeked' to provide much more accurate readings of grid, solar and transfer activity. Paladin has always been accurate enough to do the diversion portion of its' task extremely well, but the daily totals could drift outside the nominal +/-5% I use as an acceptable limit.

For the standard Paladin user this is never an issue, but as Paladin expanded to do more sophisticated tasks via a LoRa broadcast for 2nd tank, remote temperature probe, Air Con DRED control etc for specialised applications, I wanted to tighten this up. Until the V69x firmware any tuning adjustments from standard had to be via a recompile and effectively a dedicated firmware load to match each main board and CPU combination. Now this can be done on the standard firmware and each unit that leaves the test rig has its own tuning values stored internally. This is total overkill for the standard unit, but as it is now part of the testing - there is no extra work and it allows a high confidence that each unit is as good as it can be when it leaves the 'shed'.

I use the Paladin FB page as a sort of ongoing tech blog (https://www.facebook.com/groups/1108024512629017) and for those who like following this sort of thing - it is worth a read.

T

do you do direct setups? ie pv just for the hot water tank.

Terciops

26 posts

Geek

#3153782 30-Oct-2023 19:00

Direct PV to HW - I assume you are thinking about DC->DC. No this is not a Paladin thing at all. DC->DC is doable with the correct element to match the panel voltage(s) but DC switching to provide a thermostat function can be problematic.

tweake

2391 posts

Uber Geek

#3153787 30-Oct-2023 19:25

Terciops:

Direct PV to HW - I assume you are thinking about DC->DC. No this is not a Paladin thing at all. DC->DC is doable with the correct element to match the panel voltage(s) but DC switching to provide a thermostat function can be problematic.

looking at DC-AC setups at the moment due to that thermostat issue. the ones i've seen sounds like a combo mppt controller and inverter.

Terciops

26 posts

Geek

#3154195 31-Oct-2023 19:28

There is a simple way to control a couple of panels, say 300W each using a DD (DC) SSR. Below is a rough and ready diagram. Here are some rough and ready numbers.

To keep the voltage down use panels in parallel. A 300W panel is roughly 24V under full load and sun. So 600W @ 24V = 25A. This is well inside the limits of a cheap DC SSR - 60V @ 40A.

You can drive the control side of the SSR directly from the panels via a thermal switch that is NC (Normally Closed) that opens at 70C (or as required) water temperature to protect the tank. THE SSR will switch the DC when active, and that will be whenever the panels produce more than 3V or so under load.

The DC element wants to be 24V and a minimum of 600W for this example.

A standard 180L tank uses 3100Wh to heat the water by 15C, so your 600W of panels would heat the water by 2C (or so) per hour in full sun. In a summer's day you will get some 3600Wh from those 600W of panels so that is not enough for such a large tank. A smaller tank would give better results. If you can find 350W or larger panels, use them, but watch the top amps.

The SSR will need to be kept cool. Bolt it to a square foot of aluminium plate using heat transfer paste please. Place the plate vertical in the shade and nature will do the rest. Paint the plate black for better cooling if needed.

SSRs are here : https://vi.aliexpress.com/item/1005005423995957.html

24V heater elements : https://vi.aliexpress.com/item/1005002292547788.html

Thermal switch : https://vi.aliexpress.com/item/4001292393887.html (10A-Ceramic hole-NC)

This may help

Use the ceramic. They are 10A but you only need 200ma to drive an SSR.

Notes.

I did this a few years back for a friend as an experiment using a 80L tank and it is still working fine. Cheap, simple and effective. You don't need MPPT or any other control, just let the panels generate the angry pixies and the element absorb them.

Keep cable runs as short as possible.

Use silicon on / over the terminals to avoid corrosion.

tweake

2391 posts

Uber Geek

#3154211 31-Oct-2023 20:43

wow thank you for all that.

i havn't had a look at the 24v setups yet so that helps. i was looking at the 1500w into mppt and inverter then into stock thermostat and a 1000w 240v element.

keep in mind we are only lifting the temp 10-20 degrees, so 2c lift per hour is not bad. (once temps drop to much normal mains element kicks in so its never doing from cold).

tdgeek

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#3154214 31-Oct-2023 21:09

Throwing a spanner in the works. We have solar HW. Its cheap for what you get. Is it feasible to install Solar HW tubes, thereby getting the benefit of the Sun but not tapping into the PV panels?

SomeoneSomewhere

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#3154216 31-Oct-2023 21:27

Yeah, direct solar thermal is *probably* still cheaper than PV, even with the general crash in PV prices. Downside is the extra plumbing and possible new cylinder - but for a DC solar system, you're probably still going to want an AC backup element and most fitted tanks won't have two elements anyway.

On DC PV heating... do you need to even switch it? Thermal solar usually just lets the cylinder boil and operate the relief valve. Allowing the cylinder temp to rise above 70C allows you to store more energy in the tank and for it to stay hot longer - you'll lose more heat, but you're losing even more by turning the panels off.

SSRs tend to fail closed when they fail.

tdgeek

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#3154222 31-Oct-2023 21:41

SomeoneSomewhere:

Yeah, direct solar thermal is *probably* still cheaper than PV, even with the general crash in PV prices. Downside is the extra plumbing and possible new cylinder - but for a DC solar system, you're probably still going to want an AC backup element and most fitted tanks won't have two elements anyway.

On DC PV heating... do you need to even switch it? Thermal solar usually just lets the cylinder boil and operate the relief valve. Allowing the cylinder temp to rise above 70C allows you to store more energy in the tank and for it to stay hot longer - you'll lose more heat, but you're losing even more by turning the panels off.

SSRs tend to fail closed when they fail.

When our house was built, 2011, the solar tubes were $8k. Then less EECA discount. So around 5.7k or so. That includes a 300L cylinder, tubes (40 or 50) and the install and plumbing/wiring. Its awesome. Whether the roof space is better utilised by more PV panels, unsure. Re cylinder temp, ours is rated to a max of 116C. Most I've seen in high Summer is 83 in the collector and top and bottom cylinder sensors.

Terciops

26 posts

Geek

#3154224 31-Oct-2023 21:45

I have solar water heating as well, but it was far from cheap to install given the new tank, pump and plumbing. But 11 years ago it was cheaper than PV in terms of $ per joule / Watt. It's horses for courses really. If you just want a simple system to boost water temps and have a dual element tank, a couple of panels direct connected to a DC element is all you need. If you are only putting in a max of 600W or 3600Wh / day, with a normal tank then forget the SSR / thermal switch etc, you will never get into a top temp problem.

SomeoneSomewhere

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#3154273 31-Oct-2023 22:49

tdgeek:

SomeoneSomewhere:

Yeah, direct solar thermal is *probably* still cheaper than PV, even with the general crash in PV prices. Downside is the extra plumbing and possible new cylinder - but for a DC solar system, you're probably still going to want an AC backup element and most fitted tanks won't have two elements anyway.

On DC PV heating... do you need to even switch it? Thermal solar usually just lets the cylinder boil and operate the relief valve. Allowing the cylinder temp to rise above 70C allows you to store more energy in the tank and for it to stay hot longer - you'll lose more heat, but you're losing even more by turning the panels off.

SSRs tend to fail closed when they fail.

When our house was built, 2011, the solar tubes were $8k. Then less EECA discount. So around 5.7k or so. That includes a 300L cylinder, tubes (40 or 50) and the install and plumbing/wiring. Its awesome. Whether the roof space is better utilised by more PV panels, unsure. Re cylinder temp, ours is rated to a max of 116C. Most I've seen in high Summer is 83 in the collector and top and bottom cylinder sensors.

I hadn't considered that the water pressurisation delays boiling... you're looking at about 150C for 500kPa.

WellWhat

12 posts

Geek

#3154342 1-Nov-2023 10:15

tweake:

wow thank you for all that.

i havn't had a look at the 24v setups yet so that helps. i was looking at the 1500w into mppt and inverter then into stock thermostat and a 1000w 240v element.

keep in mind we are only lifting the temp 10-20 degrees, so 2c lift per hour is not bad. (once temps drop to much normal mains element kicks in so its never doing from cold).

This guy has a pretty simple setup, dump the solar panels energy straight into the hot water cylinders dc coils with ac coils for backup for the cloudy/busy days.

Worth a watch none the less, does a decent job of showing whats involved with important things like strengthening the roof

https://www.youtube.com/watch?v=r7g-27AXFEg

cthombor

81 posts

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#3154394 1-Nov-2023 10:39

The economics of rooftop PV in NZ are greatly improved by adding a solar diverter to the system, but I think they're still pretty sketchy -- unless you're off-grid or you find it convenient to charge an EV at home during mid-day.

Some folks are happy to pay $10000 or even $20000 for a household storage battery but (unless they're offgrid) I think it's hard to make an economic case for this. (From an ecological perspective, I think it makes much more sense to use lithium batteries in utility-scale installations, and in neighbourhood-scale ones, where they can be thermally managed and run on really gentle charge-discharge cycles -- so that their cost per kWh of timeshifted energy is far below what's possible in a household-scale BESS). Case in point: if you spent "only" $10k on a Tesla PowerWall 2 (they now cost more like $20k installed), and if (somehow!) you manage to get 3.8MWh throughput per year for ten years without voiding its 10-year warranty, hmm... $10000/38MWh = $0.26/kWh is the added cost of timeshifting your household's "excess" PV energy to times when the sun isn't shining. Fine if you're offgrid, but why would you do this if you're ongrid?

All to say that I think solar HW diverters, when used in conjunction with charging EVs at mid-day, are NZ's best hope for decarbonising its electricity.

And yes the good-old solar hot-water heaters are still a viable option, especially if you're offgrid, but also in ongrid households without an EV and with enough roofspace to hold one of those... ideally in a location where their late-afternoon noise on sunny summer days won't disturb the sleep of anyone who works nightshifts! Back to the future: "In Pasadena, California by 1897, 30% of the homes had solar water heaters." https://www.osti.gov/biblio/6772173

tweake

2391 posts

Uber Geek

#3154605 1-Nov-2023 18:00

SomeoneSomewhere:

Yeah, direct solar thermal is *probably* still cheaper than PV, even with the general crash in PV prices. Downside is the extra plumbing and possible new cylinder - but for a DC solar system, you're probably still going to want an AC backup element and most fitted tanks won't have two elements anyway.

On DC PV heating... do you need to even switch it? Thermal solar usually just lets the cylinder boil and operate the relief valve. Allowing the cylinder temp to rise above 70C allows you to store more energy in the tank and for it to stay hot longer - you'll lose more heat, but you're losing even more by turning the panels off.

SSRs tend to fail closed when they fail.

i actually have a dual element cylinder, but one downside is they are only rated for 70c. you need to go to the solar/wetback ones to get higher temp rated ones which costs $$

one of the attractions to PV hot water is easier to control. mates had thermal solar system but they are sized for summer to avoid excessive temps. so it doesn't work in winter but they use their wetback in winter. so overall a good system. however i don't have a fireplace and frankly plumbing is far more difficult and risky then electrical.

i spoke to a crowd many years ago that did thermal solar systems that worked in winter but they had a heat sink to dump the excess heat into during summer. eg that might be a water tank or swimming pool.

tdgeek

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#3154628 1-Nov-2023 19:18

tweake:

i actually have a dual element cylinder, but one downside is they are only rated for 70c. you need to go to the solar/wetback ones to get higher temp rated ones which costs $$

one of the attractions to PV hot water is easier to control. mates had thermal solar system but they are sized for summer to avoid excessive temps. so it doesn't work in winter but they use their wetback in winter. so overall a good system. however i don't have a fireplace and frankly plumbing is far more difficult and risky then electrical.

i spoke to a crowd many years ago that did thermal solar systems that worked in winter but they had a heat sink to dump the excess heat into during summer. eg that might be a water tank or swimming pool.

Can you expand on that?

Ive got solar HW, and being nerdy/geeky I have an intimate knowledge how my system works, and I manage it accordingly. Not disparaging your post at all, just keen to hear more

tweake

2391 posts

Uber Geek

#3154732 1-Nov-2023 20:44

tdgeek:

tweake:

i actually have a dual element cylinder, but one downside is they are only rated for 70c. you need to go to the solar/wetback ones to get higher temp rated ones which costs $$

one of the attractions to PV hot water is easier to control. mates had thermal solar system but they are sized for summer to avoid excessive temps. so it doesn't work in winter but they use their wetback in winter. so overall a good system. however i don't have a fireplace and frankly plumbing is far more difficult and risky then electrical.

i spoke to a crowd many years ago that did thermal solar systems that worked in winter but they had a heat sink to dump the excess heat into during summer. eg that might be a water tank or swimming pool.

Can you expand on that?

Ive got solar HW, and being nerdy/geeky I have an intimate knowledge how my system works, and I manage it accordingly. Not disparaging your post at all, just keen to hear more

sorry not a whole lot. i never went into details on the systems. even mates one was a long time ago. afaik the problem they mentioned was boiling the water in summer due to the solar. so its kept small to avoid that and they used wetback in the winter. most likely it would have been low pressure system. we used to boil our wetback so i know how much fun that is. the company i spoke to said they used things like outdoor swimming pool or even house water tank, to dump the hot water into to stop the hot water cylinder from overheating.

the idea of the PV is you can use a simple thermostat to turn it on/off. (except dc is problematic). if you want to work well in winter put bigger panels in as in summer the thermostat just turns it off. a waste of generation it may be.

cost wise i have no idea.

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