Off-Grid Solar Hot Water with Gas Booster?

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Jase2985

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#2951592 7-Aug-2022 10:36

tdgeek:

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

yes HW uses Kw but you just add more panels. and a hot water HP negates much of that using about 1/3 the power.

all im saying is if your already going PV why add another system when you can just add a slightly bigger system for not much more. Adding a HWHP reduces the energy draw by about 2/3. put a timer on that to run during peak solar generation and its a pretty bullet proof setup.

tube solar HW can only be used for hot water (maybe heating?) where as solar PV can be used for everything.

fyi, Yesterday we used 2h13mins heating our HWC (Family of 4, 180L cylinder) using 6.75kw of power. which was 17% of our total usage.

tweake

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#2951594 7-Aug-2022 10:56

what i was looking at was DC hot water element fed direct off the solar panel, so no inverter loss.

mains hot water element is only 1.5 kw to 3kw depending on tank. i think mines only a 2kw and that doesn't take long to heat up the whole tank, its on controlled supply. so you can do lower kw element and run it for a longer time on a smaller solar panel, which is what i've seen in the video's. (however that is with their sunlight conditions not ours)

what i don't know is if inverters take kindly to switching inputs. eg a relay on the hot water tank to switch the solar from tank and connect with the other solar array. so if hot water is at max the solar feeds the rest of the PV system. this also means the hot water is done separately so it doesn't drag down the main PV. of course you have the gas as back up for the hot water.

Scott3

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#2951721 7-Aug-2022 16:46

It is ultimately to come down to a balance of the following

- Amount OP is willing to spend on the system (noting that with a gas backup, the only purpose of the solar setup is to save gas costs & environment impact.

- Available space for solar panels

- Amount of Manual intervention OP is willing to do (i.e. covering solar panels seasonally

A few factors not mentioned:

- is if OP is going to have a wood burner, if so, getting one with a wetback would take care of most of the winter hot water needs, meaning the solar system just needs to be sized for summer water heating. (and there is likely to be spare PV capacity in summer)

- if OP is going to have a back up generator.

- How big the planned PV system is.

In terms of technology options (assuming a gas backup for everything)

-------

On heat pump hot water.

Yes, it is 3x the efficiency or of resistance heating. But it will add at least $2000 to to the cost. ($3500 Midea heat pump 280L from trade deport, vs $1500 250L mains pressure solar hot water cylinder from ice solar.

My thoughts is that on nice days in summer (based on my family members expensive grid tied PV / Battery system), there is going to be heaps of power spare on nice summer day's, and nothing spare on many winter days (so running 100% on gas regardless). Means you only get value from the $2k of extra hardware on moderate solar days. Perhaps 1/3rd of the year? My gut feeling is it wouldn't be worth the money.

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On 230v resistance hot water, fed by a diverted when the batteries on the solar setup are full.

Cheapest option for the hot water system there is, hence freeing up the most funds to poor into the PV solar / battery system, which will likely have the biggest impact on your quality of life (if no backup gen-set).

With a big solar PV setup this should give you free hot water for most of summer without issue, and the gas backup can be used (without user intervention) in winter.

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On solar thermal (evacuated tube).

Relatively cheap compared to up-sizing the PV system, and will contribute to hot water year round, (doing it fully in summer). Possibility of oversizing system so it will give decent amounts of hot water in winter too. (would require covering panels, or installing a radiator system to prevent loss of water from overheating in summer)

Most bang for your roofspace given the higher efficiency of evacuated tubes over PV panels

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On DC PV. Thinking a discrete system, basically a loop of pv panels to an element with a thermostat.

Basically the same deal as the above. Discrete system to the main PV / inverter / battery setup. Would contribute year round.

Fairly good value, No need for an inverter, and you can seek out clearance PV panels that don't match the rest of your setup:

https://www.trademe.co.nz/a/motors/caravans-motorhomes/parts-accessories/solar-panels-accessories/listing/3712883468

Means none of the thermal losses associated with piping, no circulation pump needed etc. Can be switched off with thermostat before the cylinder approaches boiling temp, so no risk of loosing water..

Could get a dual element cylinder to do both this and use any cast off power from your main PV setup.

But very geeky. Not much support for DC hot water PV setups around.

tdgeek

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#2951722 7-Aug-2022 16:58

Scott3:

It is ultimately to come down to a balance of the following

Compliments. As usual, extensive and detailed.

I felt my comments re tubes became a Ford Vs Holden, Apple vs Android debate.

Sventja

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#2951819 7-Aug-2022 21:09

I am still digesting all of the info you have provided so far. 🙂

The specs of our planned solar system are as follows:

6.3 kWp photovoltaic array

6 kW off-grid Inverter

Battery storage system (2 x 8.2 kWh)

Eventually: backup generator

We moved away from the wetback as our house is planned to be well insulated and hence a wood burner possibly too warm to run quite often.

We were hoping to use a generously sized PV system instead of an extra set up solar tubes. Space for PV panels is not an issue.

Scott3

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#2951834 8-Aug-2022 02:28

Sventja:

I am still digesting all of the info you have provided so far. 🙂

The specs of our planned solar system are as follows: 6.3 kWp photovoltaic array 6 kW off-grid Inverter Battery storage system (2 x 8.2 kWh) Eventually: backup generator We moved away from the wetback as our house is planned to be well insulated and hence a wood burner possibly too warm to run quite often. We were hoping to use a generously sized PV system instead of an extra set up solar tubes. Space for PV panels is not an issue.

That system sounds like it is a similar size to the one at my family members place. They have a bit more PV, but a little smaller inverter, and a little smaller battery.

Hard to make a determination on your energy consumption, knowing nothing about your family, but my gut feeling is you will end up with a full battery, and surplus power for most of the afternoon on nice summer day's. My gut feeling is that you will need to be pretty fugal with power on bad weather winter's days.

Makes sense to not have the surplus power go to waste, but you would need to be happy with a payback time in years.

Canstar blue says it costs approx $850 - 1250 to heat hot water with LPG for a household for a year. If you solar side of the setup could save $500 a year of this, and cost $2500, payback would be 5 years. Heaps better than bank interest rates if you have the capital. ($2500 based on a $1800 300L cylidner, and a $700 allowance for install, plumbing fitting's (including that diversion valve), wiring, solar controller. Could be way off).

Sizing the element of the cylidner is going to be interesting decision.

Reference Guide for Element Rating and Recovery

Recovery on a 50°C Rise

AMPS

2.0 kW

34 litres per/hr

8.7

2.4 kW

40 litres per/hr

10.4

3.0 kW

51 litres per/hr

13.1

3.6 kW

62 litres per/hr

15.7

4.8 kW

82 litres per/hr

21.0

5.0 kW

85 litres per/hr

21.8

6.0 kW

103 litres per/hr

26.1

2 x 3.0 kW

103 litres per/hr

26.2

2 x 5.0 kW

171 litres per/hr

43.6

3kW seems to be the default on single element 300L cylinders, so that is probiably what I would get. But the downsides are that it would take cira 6 hours to heat a 300L cylinder but hopefully you won't use that much every day, 3 hours would top the cylinder back off if it was only half used when the solar diverted kicked in. Possibly could set the system to come on before the battery is full in summer to make sure that the cylinder gets fully heated.

Other downside is it is using half of the power of your 6kW inverter. If somebody uses a hairdryer and vacuum cleaner at the same times as the hot water being heated, you will trip the inverter.

Might as well set the cylinder thermostat to it's max too (often 70c), given the power from the diverted is free (and output temp from the cylinder needs to be above 60c to avoid triggering the diversion valve.

If possible optimize the panel angle for winter generation (standard for off gird. Grid tie is often optimized for max generation).

With that size system and no gen set, it is going to be fairly important to move whatever heating loads you can to gas... Gas cooktop, gas oven, flued gas space heater, gas tumble dryer (if you have one at all).

Gas cooktops are common.

Gas oven are rare, but can be got from places like offgridcollective.co.nz

Flued LPG space heater are still fairly available, plus there often are a bunch of nice looking ones on trademe at a fraction of their new price, removed from (on grid) houses that have swapped to heatpumps:

www.trademe.co.nz/a/marketplace/home-living/heating-cooling/heaters/gas/listing/3716759012

Gas dryer are rare, only one I could find was the $3000 Speedqueen LDG3TR

elpenguino

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#2951915 8-Aug-2022 09:54

It's worth mentioning the inefficiency of PV solar panels at converting the sun into electricity - they're around 20%, and subtract further losses for inverters etc. The sun's insolation is around 1kW per square metre so to get 2kW to heat the HW cylinder you need 10 square metres of PV panels.

You can directly heat the water with much fewer thermal type of hot water panels which would leave a lot of roof space for PV panels.

Most of the posters in this thread are just like chimpanzees on MDMA, full of feelings of bonhomie, joy, and optimism. Fred99 8/4/21

Eva888

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#2951995 8-Aug-2022 11:23

@Scott3 You are truly impressive. All kudos and thanks to you for sharing your vast knowledge and always with such detailed responses.

eonsim

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#2952215 8-Aug-2022 17:38

Sventja:

I am still digesting all of the info you have provided so far. 🙂

The specs of our planned solar system are as follows: 6.3 kWp photovoltaic array 6 kW off-grid Inverter Battery storage system (2 x 8.2 kWh) Eventually: backup generator We moved away from the wetback as our house is planned to be well insulated and hence a wood burner possibly too warm to run quite often. We were hoping to use a generously sized PV system instead of an extra set up solar tubes. Space for PV panels is not an issue.

Looking at this I'd suggest just adding some more panels. With a 6kW inverter, you can add ~30% more panels which would give you 8kWp of panels. It will give you a fair bit more power without the cost of adding a bigger inverter. In winter the system will pretty much act as a 8kW system would and make better use of the inverter.

With a 6kW inverter with 6kW of panels most of the time the inverter is running well below it's capacity. Thus adding the extra 30% of panels increases the time the inverter runs at it's full power setting and will give you more power earlier and later in the day. It's cheaper to add the extra panels than increase both the inverter and panels to 8kw. In winter on reasonable days a system with a panel to inverter ratio of 1:1 will never get near it's peak generation. With 1.3:1 ratio the inverter will get closer to it's peak/optimal output. In summer the extra power may not be useful currently but if you get an EV in the future or add a spa or pool then the spare power will help. While in winter having 30% more power for a 5-15% increase in total cost is likely to be highly beneficial. Also you can potentially use the second MPPT from the inverter to run a string which has been optimised (angle) for winter generation while the first string is optimised for summer.

On really crappy winter days in the Waikato a 5kW system generates 2-4kW, so with 8kW you would likely get +60% more than that (SI likely a bit lower). Which is enough to warm up a reasonable bit of water or help the battery system keep house running.

If you a really worried about hotwater, have a very high electrical baseload/use and are in a part of NZ that gets torrential rain for weeks at a time it may pay to look at some sort of gas alternative. Or it may simply be cheaper and better to get the electrician to install a input-plug for a small generator and buy one of those as emergency power generation. With a 2kW Generator, 6-8kW of solar and 16kWh of batteries you would be unlikely to ever run out of power, and even if the generator isn't big enough to power the hotwater cylinder by it-self the batteries would be able to top up the power needed and then recharge off the generator once the cylinder was warm.

Sventja

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#2957513 22-Aug-2022 09:21

Scott3:

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.

Thanks for this info. The considerations pointed out are very useful. :)

Sventja

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Wannabe Geek

#2957531 22-Aug-2022 09:51

eonsim:

Sventja:

I am still digesting all of the info you have provided so far. 🙂

The specs of our planned solar system are as follows: 6.3 kWp photovoltaic array 6 kW off-grid Inverter Battery storage system (2 x 8.2 kWh) Eventually: backup generator We moved away from the wetback as our house is planned to be well insulated and hence a wood burner possibly too warm to run quite often. We were hoping to use a generously sized PV system instead of an extra set up solar tubes. Space for PV panels is not an issue.

Looking at this I'd suggest just adding some more panels. With a 6kW inverter, you can add ~30% more panels which would give you 8kWp of panels. It will give you a fair bit more power without the cost of adding a bigger inverter. In winter the system will pretty much act as a 8kW system would and make better use of the inverter.

With a 6kW inverter with 6kW of panels most of the time the inverter is running well below it's capacity. Thus adding the extra 30% of panels increases the time the inverter runs at it's full power setting and will give you more power earlier and later in the day. It's cheaper to add the extra panels than increase both the inverter and panels to 8kw. In winter on reasonable days a system with a panel to inverter ratio of 1:1 will never get near it's peak generation. With 1.3:1 ratio the inverter will get closer to it's peak/optimal output. In summer the extra power may not be useful currently but if you get an EV in the future or add a spa or pool then the spare power will help. While in winter having 30% more power for a 5-15% increase in total cost is likely to be highly beneficial. Also you can potentially use the second MPPT from the inverter to run a string which has been optimised (angle) for winter generation while the first string is optimised for summer.

On really crappy winter days in the Waikato a 5kW system generates 2-4kW, so with 8kW you would likely get +60% more than that (SI likely a bit lower). Which is enough to warm up a reasonable bit of water or help the battery system keep house running.

If you a really worried about hotwater, have a very high electrical baseload/use and are in a part of NZ that gets torrential rain for weeks at a time it may pay to look at some sort of gas alternative. Or it may simply be cheaper and better to get the electrician to install a input-plug for a small generator and buy one of those as emergency power generation. With a 2kW Generator, 6-8kW of solar and 16kWh of batteries you would be unlikely to ever run out of power, and even if the generator isn't big enough to power the hotwater cylinder by it-self the batteries would be able to top up the power needed and then recharge off the generator once the cylinder was warm.

We have have a little bit of extra space on the structure for the panels and are able to add two panels right away. Thanks for that tip.

eonsim

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#2957960 22-Aug-2022 21:41

Sventja:

...

We have have a little bit of extra space on the structure for the panels and are able to add two panels right away. Thanks for that tip.

Sweet, never heard anyone complaining about too much solar power (especially in winter). Adding a couple of extra panels is a very efficient use of cash (cheaper to get them on as part of the initial install). Assuming they're modern panels in the 390-410W range, it seems like that should get you to around 7kWp for an extra 10% energy, with 1.16:1 ratio for panels to inverter.

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