Off-Grid Solar Hot Water with Gas Booster?

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Sventja

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#299048 6-Aug-2022 11:17

We are planning the build of an off-grid home (~150m2) for a family of four. Solar panels and battery storage are being installed and when the sun is out we could use the energy created by the solar panels to heat water in a hot-water cylinder. Is it possible to use gas as a booster when the sun is absent? I found a discontinues system by Rinnai (Rinnai INFINITY® Solar Gas Boost 20 & 24), but nothing else. Our current alternative is to go with an infinity gas system, but I really would love to harvest the energy of the sun when available.

Thank you very much for sharing your knowledge.

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#2951258 6-Aug-2022 11:35

not sure on what they typically use, but you can use gas water heater with a circulation pump to heat up a hot water cylinder.

also look at PV hot water ie solar panel connected direct to hot water cylinder element.

with the multi connection tanks you can use the gas to bulk heat the water to a minimum temp and the solar to heat the rest to a higher temp.

CYaBro

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#2951272 6-Aug-2022 12:09

Who are you using for the solar system @Sventja?

tdgeek

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#2951296 6-Aug-2022 12:48

I would look at solar tubes to supplement your off grid solution. Our house was built 2011, the solar tubes (40 for larger family) was $8k, including 300L twin element cylinder. Given the impact of hot water on the power bill/kWH consumed, you can devote the solar PV for other uses.

Sventja

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Wannabe Geek

#2951300 6-Aug-2022 12:54

tweake:

not sure on what they typically use, but you can use gas water heater with a circulation pump to heat up a hot water cylinder.

also look at PV hot water ie solar panel connected direct to hot water cylinder element.

with the multi connection tanks you can use the gas to bulk heat the water to a minimum temp and the solar to heat the rest to a higher temp.

Would you know if that will reduce gas use (cost) over the course of a year compared with installing an infinity system only without a hot water cylinder?

Sventja

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Wannabe Geek

#2951303 6-Aug-2022 12:57

tdgeek:

I would look at solar tubes to supplement your off grid solution. Our house was built 2011, the solar tubes (40 for larger family) was $8k, including 300L twin element cylinder. Given the impact of hot water on the power bill/kWH consumed, you can devote the solar PV for other uses.

Do you have an additional system that kicks in when the sun is not shining?

tdgeek

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#2951304 6-Aug-2022 13:05

Sventja:

tdgeek:

I would look at solar tubes to supplement your off grid solution. Our house was built 2011, the solar tubes (40 for larger family) was $8k, including 300L twin element cylinder. Given the impact of hot water on the power bill/kWH consumed, you can devote the solar PV for other uses.

Do you have an additional system that kicks in when the sun is not shining?

It has two timers, I have both elements one for the Electric Kiwi free hour of power at 10pm. Winter I boost the top element manually for an hour some/most days. 8 months of the year I just forget it, as the evening boost doesnt kick in as the entire cylinder will be at 65-80. If we run low, just the top element is enough to cover the top 100+ litres, no need to heat the bottom. Its last third of Winter now, the sunny days we have had have almost taken care of the entire cylinder.

Mainly my point was adding solar tubes is effectively the same as you adding more PV panels

tweake

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#2951305 6-Aug-2022 13:07

Sventja:

tweake:

not sure on what they typically use, but you can use gas water heater with a circulation pump to heat up a hot water cylinder.

also look at PV hot water ie solar panel connected direct to hot water cylinder element.

with the multi connection tanks you can use the gas to bulk heat the water to a minimum temp and the solar to heat the rest to a higher temp.

Would you know if that will reduce gas use (cost) over the course of a year compared with installing an infinity system only without a hot water cylinder?

Yes.

There is some good video's and sites on the subject. Tho most i have seen has mains power as the back up.

Basically mains power/gas does the bare minimum temp and the solar does the max temp the hot water cylinder can handle. Then the tempering valve reduces that temp when you use the water. That way you get a decent amount of solar storage and have mains/gas back up for whenever the solar drops below minimum temp.

tweake

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#2951306 6-Aug-2022 13:11

Mates had solar hot water (non PV) and one of the issues is that its sized for summer. If you size it for winter you boil the tank in summer. But they had wetback for winter use and rarely ever needed the mains back up.

Thats why i like the PV hot water as its much more controllable. I understand there is systems that will use your main panels for the hot water, then switch to charging the batteries when the hot water is up to temp. But i do not know details on it.

tdgeek

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#2951308 6-Aug-2022 13:33

tweake:

Mates had solar hot water (non PV) and one of the issues is that its sized for summer. If you size it for winter you boil the tank in summer. But they had wetback for winter use and rarely ever needed the mains back up.

Thats why i like the PV hot water as its much more controllable. I understand there is systems that will use your main panels for the hot water, then switch to charging the batteries when the hot water is up to temp. But i do not know details on it.

Our system has peaked at 83 in the collector and the cylinder. Thats the highest Ive noticed. Its able to accept 116C

With either system you will have issues with bad weather and Winter, so freeing up the PV from HW heating helps the PV

PolicyGuy

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#2951370 6-Aug-2022 18:59

An Infinity-type 'instantaneous' hot water heater is ludicrously inefficient at heating - almost all the heat goes out the vent - they only work because they burn a lot of gas to heat up the heat transfer surface.
I reckon you need a storage-cylinder type system, this would enable to use your PV electricity to heat the water with something else as a backup. A heat pump electric hot water system is expensive but would give you the most hot water for each kWh of electricity input.

A wet-back of some type is probably the cheapest 'alternative' heat source, particularly as most off-grid homes have easy access to firewood

Jase2985

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#2951431 6-Aug-2022 20:35

why would you consider solar tubes if your investing in solar panels? for that price you could add an extra couple of Kw of panels and a large heat pump HWC.

Scott3

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#2951464 6-Aug-2022 22:40

PolicyGuy:

An Infinity-type 'instantaneous' hot water heater is ludicrously inefficient at heating - almost all the heat goes out the vent - they only work because they burn a lot of gas to heat up the heat transfer surface.
I reckon you need a storage-cylinder type system, this would enable to use your PV electricity to heat the water with something else as a backup. A heat pump electric hot water system is expensive but would give you the most hot water for each kWh of electricity input.

A wet-back of some type is probably the cheapest 'alternative' heat source, particularly as most off-grid homes have easy access to firewood

Typically they are north of 80% efficient, with some much better than that.

i.e. the Rinnai EF26 is a condensing unit, with a quoted efficiency of 92%, and the Rinnai N series have a quoted efficiency of 97% (they have a secondary heat exchange to scavenge heat from exhaust gasses.)

Scott3

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#2951469 7-Aug-2022 00:03

Sventja:

We are planning the build of an off-grid home (~150m2) for a family of four. Solar panels and battery storage are being installed and when the sun is out we could use the energy created by the solar panels to heat water in a hot-water cylinder. Is it possible to use gas as a booster when the sun is absent? I found a discontinues system by Rinnai (Rinnai INFINITY® Solar Gas Boost 20 & 24), but nothing else. Our current alternative is to go with an infinity gas system, but I really would love to harvest the energy of the sun when available.

Thank you very much for sharing your knowledge.

Short answer is, yes you can use gas to boost your solar hot water setup.

I pulled the schematic for that "Rinnai INFINITY® Solar Gas Boost 20 & 24" systems.

https://rinnai.co.nz/media/id3eom4n/solar-boost-oim.pdf

Read the not on Legionella on page 3. Very important not to end up with a system that can make you sick.

In case anybody finds the schematic confusing, here is what is going on in words.

A solar collector (in this case a remote one, with a pump to circulate the water) is used to heat a storage cylinder.
Hot water from the cylinder goes to a flow diversion valve. If more than 60c, water goes straight to the tempering valve, if less than 60c, it goes to the hot water heater to be boosted to 75c (ensuring any legionella is killed), before going to the tempering valve.
Tempering valve mixes the hot water above with cold water, such that the output temp is 55c, hence avoiding scalds.
Water is then used in the house.

Really any Califont (tankless water heater) that has the the abilty to have the output set hot enough can be used in this duty (buying the diversion valve individually)

Rinnai's current selection guide States the "Rinnai INFINITY HD and A28i models", noting that the N series is also works, but you don't get the higher efficiency by feeding it pre-heated water.

See page 47 here:

https://rinnai.co.nz/media/l3bgqzza/2021-cfwh-spec-guide-a-hd49-n-series-v3-web.pdf

I'm sure the other brands have similar products.

Main issue I can see with the above setup is that the gas heater will do lots of starts, will fire up for the cold slug of water in the pipe, before the diverted switches away from it when the hot solar water arrives.

Note that the common gas water heater's require require power to run their control system's.

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Consider using an evacuated tube setup as your primary means of water heating (rather than PV excess). Like the below graphic:

Evacuated Tube Collector - an overview | ScienceDirect Topics

They run at about 30 - 45 % efficiency. (with solar PV, you will do well to beat 20%, and that is before inverter losses (93-96% efficient is typical for the inverter).

And they are relatively cheaper to buy than more PV capacity. A 30 tube thermal collector (cira 1.8kW) runs at $2000 + installation hardware (cira $1000?), could even get two of them as per the above picture...

For comparison a 2kW solar PV setup (everything included) is going to be north of $10k...

Couple of things to note with solar thermal:

The style pictured above, with the storage tank separate from the cylinder requires a circulation pump, which uses a very small amount of power. also the pipe run (while insulated), will result in some heat loss. The style with the cylinder being mounted horizontally on the rooftop doesn't require a pump, and is very popular in asia, but dosn't seem available here.
System is uncontrolled. It can't switch off the heat. If the water gets near boiling the system will start dumping water (to allow more cold to enter the system), to protect itself. Obviously an issue if water is in short supply in summer. My grandparents are on rain water (Just 2 people in a house sized for a family), their solar panel is in a fairly accessible place, so every summer they cover 1/2 the panel with a mat to decrease the heat output from the panel. Works for them, they remove it in winter to get the full heat output then. Alternatively you could have a system that dumps heat to a radiator automatically if it gets too hot, but I have never seen such a setup.

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Re the solar PV off grid setup.

Not sure how much money you are going to put into this, but to give you some idea, family members have a quite expensive grid tie (but island capable) solar setup.

5kW inverter (5kW max feed into the grid in their area, apparently you can have more inverter power by setting the system up to monitor the load of the house and changing output to keep grid feed back below 5kW, but that option wasn't pitched to them at the time)
Substantially more than 5kW in panels (not sure, guess 7 or 8kW)
1x tesla powerwall 2
Timer to control hot water cylinder.

It's a big house, with a lot of load's (pool pump, multiple fridges / freezers, electric fence machine), but only 2 people & setup to be fairly efficient (led lighting etc).

On nice days in summer there is often lots of extra power, Hot water cylinder get's heated in the middle of the day, and the battery is full, and feeding back to the grid by late afternoon.

In winter they use wood for both heat and hot water (cylinder switched off), but it is not unusual for the battery to not be charged by evening despite this. (not a massive issue given they are on grid)

But for you being off grid, would suggest that there would be minimal to no power left over to heat hot water for much of winter, which could make a dedicate hot water solar system attractive.

A big decision you will have to make is if you are going to have a backup generator or not.

Jase2985

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#2951474 7-Aug-2022 06:36

Scott3:

They run at about 30 - 45 % efficiency. (with solar PV, you will do well to beat 20%, and that is before inverter losses (93-96% efficient is typical for the inverter).

until you connect it to a hot water heat pump

tdgeek

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#2951508 7-Aug-2022 08:47

Jase2985:

why would you consider solar tubes if your investing in solar panels? for that price you could add an extra couple of Kw of panels and a large heat pump HWC.

OP wants an off grid home, HW uses a lot of kW, by relieving the PV with tubes that makes sense to me. An extra 2kW would put you in the negative if the HW needs that and more, and a heat pump cylinder uses power that the PV also needs to provide. Even in the solar off season, you have two solutions accessing the Sun or whats available in non prime weather

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