Off-Grid in Australia - Yes it does get kinda cold.

coldAussie

Hi from Australia,

Down here we don't have nearly the heating challenge of our northern hemisphere friends, but in my zone we run from -8degC to 43degC over the year, have a genuine four seasons and heat for about 4-6 months depending on the year, typically wood heaters for rural situations. I'm working on a rural house design for our farm and considering an integrated heating system which would primarily use wood for heat in winter and solar for DHW in summer. The backup would be LPG (propane). I've attached the P&ID concept and would appreciate a critique or recommendations from others experience.

Thanks in advance.

STEVEusaPA

How does the hot water from the boiler get into the storage tank? Unless the mixing valve you are showing is an actual Caleffi thermobloc.
I also don't see why you would want to bring your solar hot down so low in the storage tank.

coldAussie

STEVEusaPA said:

How does the hot water from the boiler get into the storage tank? Unless the mixing valve you are showing is an actual Caleffi thermobloc. I also don't see why you would want to bring your solar hot down so low in the storage tank.

Thanks, the boiler loading valve has a circulator, the laddomat 21-60 was what I was thinking,

http://www.termoventiler.com/en/products/laddomat/laddomat-21-60

 Very similar to the Caleffi thermobloc.

I guess with the low solar I was expecting to mostly be dealing with excess heat over summer since we have plenty of sun, so leaving it low might give some chance of avoiding boiling or running the dump excessively.

Alan (California Radiant) Forbes

I haven’t had much luck with Baxi boilers although I have no experience with their new generation models. They stopped supporting their older ones quite awhile ago which didn’t impress me. 

jumper

If you're off grid how do you drive your pumps?
I'd go photovoltaic with high temperature storage.
Year round. Gleaning and stacking wood only works when there's nothing better to do. Is the LPG delivered?
BTW LPG differs from propane around here.

Zman

The design looks solid. How do you plan to control it? I believe the controls will make or break the system.

hot_rod

My system is similar 500 gallon, about 1900 L steel pressurized tank.

At the top is about 160 gallons 605 L of "space" that acts as my expansion bubble and drainback solar air bubble

Wood boiler input, 160 sq ft, 16M2 solar thermal, Lochinvar LP combi boiler on outdoor reset for back up boiler. Entire system is at 20 psi 1.3 bar.

Hot water its via a plate HX and flow switch which starts pump. I can shower with tank temperature as low as 110F, 43C.

A Resol solar control runs the entire system. My EKO 40KW wood boiler does have a Caleffi 281 boiler protection pump.

Solar is my first option, wood input whenever that is fired, LP if neither wood or solar.

Note the LP boiler only heats the very top of the tank, 35 gal just enough to buffer the multiple micro zones.

So basically a plain steel tank, former LP vessel, one HX, inputs and outputs via pumps controlled by Resol MX control.
Heat emitters are radiant slab and some panel rads. I do have a 3 way motorized valve for the radiant, (not shown) tekmar control, although the Resol could also run that if I had a proper motor, mine is floating 24VAC.

Apricus evacuees tube by chance? Those could be run as a drainback, no glycol my system is plain water with Rhomar conditioner.

Tank, plate HX, 5 circulators, controller, boiler with 281 pump block.

The pumps could be 12VDC nB&G Laing Solar S5 if you want to run via PV and batteries.

I have a bunch of 12 and 24VDC Resol controllers you are welcome to, cover the shipping.

coldAussie

jumper said:

If you're off grid how do you drive your pumps? I'd go photovoltaic with high temperature storage. Year round. Gleaning and stacking wood only works when there's nothing better to do. Is the LPG delivered? BTW LPG differs from propane around here.

For this house there is also a grid-ready solar PV electric power system with batteries with diesel gen backup, it would be sized to suit the heating system loads (as well as A/C and house loads). On $ these are competitive with running the power poles in on many farms now in Australia, even allowing for 2x battery replacements and a set of panels for a target 40year life. Self contained PV systems can often be more reliable than the rural grid in some areas.
I hadn’t thought of oversizing the PV system and using electric resistance for the heat source, I’ll look into it, it would simplify the system. Flickering flames and a warm kitchen cookstove has other appeals for us though.

With LPG, our gas vendors use a mix of propane and butane and deliver by tanker or bottle swap, available Bottle form factors are twin 45kg cylinders, 90kg or 210kg with a 45kg swapover/reorder bottle. The LPG would also be used for summer cooking.

Thanks for the reply.

Zman said:

The design look solid. How do you plan to control it? I believe the controls will make or break the system.

My first thought was just to put it all in a PLC for flexibility, but the more I look at it, the boiler circulation, heat dump and EV solar controls are simple and may suit relay control, standard parts.
Exception would be if I want some intelligence to the heat dumping.

On distribution, the dP control and outdoor reset is standard, but selecting the priority of heat source is a bit unusual, I’m not sure if any standard controls exist for this, would love any product suggestions.
fallback is a PLC, but I would prefer to simplify.

Thanks for the reply.

My main concern is sizing the coils to achieve the required heat transfer, unless I get this right, three possible problems:

- a long shower may drop the delivered water temp, if the DHW coil can’t pick up enough heat from the storage tank at that flow rate. Worst case is this happens even with the LPG Boiler firing.
- a large heat demand from the house may drop the delivered temp and cause the boiler to fire if the central coil can’t pick up enough heat from the tank to reach setpoint.
- if the Central coil is too small the LPG boiler may short cycle if it can’t reject enough heat to the tank on minimum firing rate in indirect mode.

hot_rod

Page 35 on are the basis for my system, also some control logic.

https://www.caleffi.com/sites/default/files/coll_attach_file/idronics_6_0.pdf

hot_rod

The controls I have to offer.
https://www.resol.de/en/produktdetail/88

coldAussie

hot_rod said:

Page 35 on are the basis for my system, also some control logic. https://www.caleffi.com/sites/default/files/coll_attach_file/idronics_6_0.pdf

Thanks for the great feedback @hot_rod.

Your system looks neat and simple, would love to use a similar design. In aus our code requires all wood heated water systems to be open vented, no exceptions. The code considers wood an uncontrolled heat source, even if you use an electronically controlled wood furnace. The proposed wood heater is a hand fired kitchen stove so I need to stay open vented.

My first rev of this system had just a single coil for DHW and all other systems adding or removing heat from the open system, with drainback solar. I changed for corrosion reasons in the radiator circuit. I think keeping the radiators in a closed loop is good, but could switch to an open drain back on the solar and a two coil tank.

I also switched to glycol in the solar and a non drain back design when I started considering multi coil tanks. I was concerned about overheating and thermal shock in the EV tube array impacting longevity. The stagnation temp for the tubes would be >300degC in our summers. Keeping it flowing and dumping the heat may make the array last longer, at the expense of a more complicated system? I’m interested in your experience of this?


Many thanks for the offer on the solar controls. I’ll have a read into them.

hot_rod

Over the years I have tried many designs, including two open systems with copper coils. It is not so easy to pull heat out of a tank of non moving water with a coil, it takes a lot of surface area. Pic shows what we call reverse indirect tanks, a steel tank with a LOT of copper coil area to product hot water. The tank needs to be kept fairly hot also to get performance, not unlike boilers with a tankless coil.

I avoid the open systems due to corrosion, it takes a constant chemical romance to keep ferrous metals safe Potential pump cavitation also.

If you use an external double walled, vented flat plate HX for DHW is that acceptable? It would be outside the pressurized tank and double wall are vented, open to atmosphere if a leak develops. You can size flat plat HX to the exact requirements of the system. define the rpm you want and the tank side temperature. I use a 40 plate 5X10 and can shower with tank temperature down around 110f, 43c. it would take a very large coil in a tank to get that close approach temperature. Unless the tank is stirring the coil ends up in a pocket of cold water. But stirring the tank busts up the stratification, so either way it is a tough go.

I do everything I can to avoid glycol in solar, as you mentioned the high stagnation temperatures in EV collectors when a pump or controller fails.
To me a heat dump for solar or wood is counter productive, with enough tank capacity I wonder that you need a dump for either. The math would be figuring the maximum wood capacity of your stove, convert to BTU/hr, and see if the tank could absorb that. With an open tank you could allow it to boil I suppose, that is how many open wood boilers in the US run, not always on purpose
Attached is pic of a a typical OWF unpressurized wood furnace.
Heating or cooking with wood requires that delicate balance of fire size to load size, it takes a few toys to get the hang of firing to the load requirement, probably more so with a cook stove? i doubt the hot water coil is very large? It would run the wood stove too cold and be a creosote monster!

Are the vac tubes heat pipe, or direct flow? The Resol controls have several protection logic for evac tube arrays, they can lock out at extreme conditions, or pulse the pump as the tank exceeds set point to prevent those high stagnation temperatures. The never ending challenge with solar, too much sun when you least need it.

For sure use the special high temperature glycol for EV collectors. Hard to find the high temperature glycol here as it doesn't always meet the FDA approval due to all the inhibitor required to run high temperature applications.

What about flat plate style collectors? There is really not a huge performance difference, EV do perform better in extreme cold, or if you need 160F in cold months. The graph shows the performance curves of typical collectors. Collector operating temperature vs ambient is the key.

coldAussie

@hot_rod Thanks again for the feedback.

I did another revision incorporating your style system but retaining the open venting on the storage tank. Being able to use a repurposed pressure vessel would be great. Something like an industrial air receiver or propane tank would have plenty of corrosion allowance and easy to weld on the fittings needed.

The external plate HX's would be much better than the coils both on performance and serviceability. Being a ceiling mounted tank this would be very important 20 years down the road. Never want to pull that tank out of the roof.

With the code requirements, I think they're less concerned with the possibility of DHW contamination from a leak than they are about blowing up storage tanks thru uncontrolled heating and over pressure with a wood heat source. Running DHW thru a plate HX should be OK.

With the wood heat source, there are a couple of options, cook stoves here are available up to 20kW output (68,243BTU). For this house it would likely also have a wood fire in the living areas, I can also at a 20Kw exchanger in that. as you mention they need a loading valve (Laddomat or Caleffi 821 Thermobloc etc) to avoid condensation and creosote.

For the EV tubes I was considering the heat pipe style vac tubes since we get the cold winters but not settled yet, thanks for the references. On the rev 2 I switched to a drainback system to simplify.

What I couldn't settle in this rev2 was the auxiliary boiler integration. I've shown it in the open system as it was the most simple. For operating pressure, most manuals call for about 0.5Bar, to do that could mount the boiler low under a bench and use static head. The other issue is the boiler now has to swallow the open vented water with corrosion issues. Upside is, I don't need a modulating boiler and the controls are simple to achieve a tank top-up approach to DHW priority or heating use.

I started out trying to put a modulating boiler in the closed heating circuit then using three port valves & reversing the HX to achieve the tank top up function for DHW priority. The controls looked doable but complex, and more components.

Perhaps rev 3 will settle on another circulator and HX to give a non-modulating boiler its own closed circuit.

hot_rod

Correct you are in that finding a large pressure vessel is the key. Old or brand new LP tanks are rated vessels, air compressor tanks, ammonia fertilizer tanks, similar to LP tanks. I'm not sure what else you may find down there? Buying new insulated pressure vessels around here could run 10- 12 bucks a gallon for ASME tanks!

If you do go with un-pressurized or just "open" static pressure you can add O2 scavengers yearly, that is very common with open type wood boilers here. Probably a simple as a sodium sulfide or more expensive blended hydronic conditioner.

Why would you need that open tank up high if you use an open tank? That open space at the top of the vented tank acts as your expansion.

Does your boiler have a low pressure switch? Does it need one? Typically that act as a low water protection device, you could add a probe style low water safety instead.

What temperature would you run the LP backup, if below 60C I doubt you would ever flash to steam? As long as all the circulators are mounted low in un-pressurized type systems you should be ok as far as potential cavitation. If you start running 85- 90C I'd worry more:) NPSHR goes up with temperature below 60C I think Grundfos shows 1.7 psi, .117bar. At 200F I think they want around 5- 7 psi, .5 bar should be adequate.
It's the vapor pressure formula basically. We talk about that in Idronics 10 with un-pressurized boilers.

Do you have enough wood or solar HP to ever overheat the tank? I doubt that 20KW cookstove would. Depending on what tank size you end up with. How large of an array? The solar could stop when tank approached 80C or whatever, that is why I like drain back.
Yes, the heatpipe style are better for draining back. I think the Apricus Wombat was designed to be a drainback-able EV tube system?
I've been tinkering with systems like this since the 1970, have some scars to prove it, burn and mental

Some additional design info here, storage sizing, control stratigeries, etc.

https://www.caleffi.com/sites/default/files/coll_attach_file/idronics_10_0.pdf

hot_rod

Tom from American Solartechnics hangs out here. He is the master of those open style tanks and HX sizing, an example of one of his coils. Coils could be pex also, I think it takes 3 times much pex to get the same transfer as copper.

coldAussie

So rev 3 went in a different direction for the backup supply and also incorporated a living room wood stove with 20kW heat exchanger to allow flexibility and heat capture for whichever wood appliance has been fired.

I recognised that the house is an off-grid installation and will have a PV system with batteries and diesel generator backup for electricity supply with no option to feed excess PV generation back into the grid. An abundance of free energy on Sunny days.

Since the PV array will be oversized on sunny days to provide useful battery charging on less than ideal days I tried to see if I could incorporate that into the heating system. There is a bit to like with this since it eliminates the LPG boiler, as well as the evacuated tube solar thermal system. Along with that disappears:
- The extra capital and maintenance costs of LPG boiler and EV tube system.
- The LPG boiler corrosion issues I was wrestling with.
- The summer overheating issue and possibility of a heat dump
- Several components eliminated.

The DHW priority is still achieved with the full available use of the electric system (PV, batteries or Diesel Gen) to keep the top of the tank hot enough for DHW.

The tradeoff is losing the option to run the heating system on LPG if we were ill or too lazy to burn wood and it was persistently cloudy. Though realistically such LPG use would send us broke quickly anyway. The DHW priority element needs to be sized for the flat battery case, i.e. diesel generator capacity less house loads, say 3kW to DHW leaving 4kW available for house loads and battery charging using a 7kW backup generator. No real heating use would be possible there, but this would be a rare scenario.

I think if the storage tank is a repurposed pressure vessel, I could also set it up either open vented to meet the code requirements with the option to switch to closed with PRV if the storage tank is designed large enough to prevent boiling. If a genuine muppet were to leave the stove doors open and overfire both stoves for many hours with a starting tank temp of 95degC and no heat demand then the PRV could be sized to ensure the vessel is not pressurized excessively, would not be code approved but possible.

As far as the ceiling mounted tank goes, its partly a packaging thing to fit it into the house. I was also considering the potential to connect two stoves to one tank where the stoves are some distance apart, not sharing a common wall with a mechanical room. In order to have continuously rising & falling thermosiphon piping the piping & tank would need to be in the roof space to achieve two heat sources into one tank. If the tank was at grade the piping would traverse the living space to get to the tank.
The elevated tank also helps for NPSH considerations on the circulators in a open vented system. I could redesign the layout to incorporate a shared wall plant room, but I like this layout.

hot_rod

How old are you? That has a lot to do with wood burning for heat and cooking
Resol builds a clever device that shed excess PV directly to ST, pump, control, etc. https://www.resol.de/en/produktdetail/208

With 270 days of sun in my area I get a good 4 months of DHW from just the sun. That 430 gallons of thermal storage allows me 3 no sun days of DHW. I never burn wood in the summer, I gather then.

You could size down the ST and just tap into the top of the tank?
ST is low hanging fruit to me, so easy to harvest and almost a daily load, just don't over do it.

Are you building hot water coils for those stoves? We call those homemade devices closet bombs over here I wonder that small KW like those are even worth trying to squeeze hot water from? Seems like a lot of technology for little energy conversion. Is the juice worth the squeeze?
Let the 16 KW do its job of heating the space and cooking. Any hot water you try to pull subtracts from it's intended design. Most wood burner output is fully loaded burning full out, hard, dry wood, that rarely happens. Over-heat protection is critical as they can flash to steam very easily. And return protection to protect against creasote formation.

I think sourcing a tank should be the first step, if a pressurized vessel is doable, lose most of thoise HX and open expansion tank. or attach one until the inspector leaves

Any idea what the heat load of the home is, and the size of the thermal tank and array? with that info and the city near you I could run a T-SOL solar simulation, give you some ideas if and how much you might spend chashing "free" energy. With just a location I could show a SF solar fraction, what % of your load is actually realistic in your area.

The holy grail for ST is storage, period. Same as batteries are for PV offgriders, how much can you fit in or afford really.

If you do go with a fossil fueled boiler, go with a combi,soi you can quickly, easily and efficiently get DHW, pre-heated by solar maybe.

There will be days where you want to flip a switch for heat and or dhw, I promise. so many good options, hard to focus on the best for you and the family.

hot_rod

Try this calculator

https://www.lgenergy.com.au/calculator/suburb/melbourne-vic/3000

kW to BTU 3412 multiplier
1 kW= 3412 BTU/hr

jumper

Curious how you end up using both LPG and Diesel?
For intermittent use an engine using LPG is less troublesome.

coldAussie

@hot_rod We're a 30 something growing family, sourcing and burning wood is standard fare for a farm in our area, comes with the territory. Cooking on the woodstove would be optional since a summer LPG stove would be installed also.
The labour free options for us are Air-Air heat pump A/C, which would be the button on the wall solution. A zoned system that does the main living area ~100m2 and provides A/C in summer would be doable off-grid, but a full house solution pushes the PV-battery feasibility on heating, which is typically an evening/night time load, so the diesel generator would auto for heating if we wanted to let the fires go out and do it with A/C.

Most places with grid power run ducted A/C for heating & cooling and supplement the heating with a dry wood heater. A/C is doable off-grid in summer, if you don't need to run it thru the night which is typical of our climate. Nearest city, Orange NSW.

On sizing;
My very rough calc of steady state heat loss is between 18-23kW (61,416-78,476BTU/hr) on a -7degC day for the full house 245m2. dependent on chosen glazing & insulation spec. Still need to confirm that heating load. The bulk of the loss is in the main living area, which would also receive the substantial radiant heat from the wood appliances when they're running. The typical use for the hydronics would be to keep the bedrooms & bathrooms warm, which are remote from the wood heaters, by rads and towel rails. The approx load for those areas is about 9kW (30,708 BTU/hr) steady state heat loss.

For vessels, thin wall insulated tanks are readily available in 315, 400, and 500L sizes. Thick wall vessels like anhydrous bullets can be had in 800L. Air receivers and LPG tanks can be found in 980, 1000, 1200L sizes, bigger than that starts to push the limit on cost and weight bearing. The tank would be the only modified component if I went with a thick wall vessel, otherwise would all be commercially available equipment. Mods to the vessel could be done to the vessel code by a coded welder (I do a bit of this in another life)

Hopefully no closet bombs here, the stoves are commercially available, supplied with heating jackets by the vendor. www.wiseliving.com.au is the locally made products, there are also imported options (Esse, Rayburn etc.). They are all available dry or wet.

I did have a look at using an LPG combi prior to rev1, there is limited options in Aus since hydronic is uncommon, so the sizing is often bumped up to cover the most number of applications with one or two locally certified products. Either a 24 or 37kW (81,888 or 126,244BTU/hr) mod-con model would be available from reputable vendors, quite expensive, and probably oversized though they turndown say 5:1.
The other downside is the DHW firing cycle is triggered by a flow switch, so unless I had some control logic interrupting the call for DHW, the combi would short cycle every time a tap is opened even with preheated DHW available from the storage system. That interrupting control may need to modify the combi since the DHW controls are onboard the combi. Some models may be able to receive a digital input.

There are a few more simple & cheaper capital options in the mix too, involving either higher running cost and/or a bigger PV-battery system or not heating the bedrooms & bathrooms which is normal around here. The wood appliances are going to stay in the design even if they were dry, just 'cos we like a wood fire and a warm stove ticking over in the kitchen. So this exercise is to stretch the brain and see if we can make a useful integrated system. Cost/benefit analysis comes next which may also torpedo the idea, but lets see.

coldAussie

jumper said:

Curious how you end up using both LPG and Diesel?
For intermittent use an engine using LPG is less troublesome.

Thanks @jumper this triggered me to review the available LPG generator options, as it turns out there are many available locally at reasonable cost. I was running on the assumption that diesel was the standard option, and diesel fuel is readily available on the farm. It would be a neat integration to consolidate the storage and distribution for the house to one fuel, will definitely throw this into the mix.

hot_rod

Yet another option would be A2WHP. They are becoming more popular in the US, allows for high efficiency heating, AC, and DHW from electricity. If -7C, 20F is design day that is plenty workable with new A2WHP. With a buffer tank, the wood fired could integrate.

Sounds like wood and solar are you cheapest energy, if you can harvest affordably. What about LP vs electricity cost? Do you get FIT credits?

Here is some more light reading on HP, fresh off the press.

https://www.caleffi.com/sites/default/files/file/idronics_27_na.pdf

coldAussie

hot_rod said:

Yet another option would be A2WHP. They are becoming more popular in the US, allows for high efficiency heating, AC, and DHW from electricity. If -7C, 20F is design day that is plenty workable with new A2WHP. With a buffer tank, the wood fired could integrate.

Sounds like wood and solar are you cheapest energy, if you can harvest affordably. What about LP vs electricity cost? Do you get FIT credits?

Here is some more light reading on HP, fresh off the press.

https://www.caleffi.com/sites/default/files/file/idronics_27_na.pdf

Great article @hot_rod I'll do a rev4 with an A2WHP as the main hydronic heat source for heating and cooling along the lines of figs 8-5 & 8-6 integrating a wood heat source as well and see where that lands. Probably with a dry fire in the living room and the kitchen stove sized down to 5 or 8kw as predominantly a DHW and high temp heat source. This would make the wood even more 'optional' on most days.

With sufficient buffer tank size, it may well be possible have controls to load up the PV array when batteries are full to capture enough surplus solar during the day into heat and/or chill tanks for use in the evening most of the time. On really cold or hot nights light the fires or open natural cross ventilation.

We do get FIT for surplus solar about 8-16c/kWh usually set up with the higher FIT for say the first 10kWh/day then dropping lower. In the future this is likely to go to lower FIT since the volume of rooftop solar in AUS is now causing negative wholesale prices during sunny windy days. Power companies currently have to pay to keep fossil generators connected on these days, yet homeowner rooftop PV is still paid FIT for their generation. I expect this will need to be reconciled in future as rooftop PV gets even bigger.

In the case of this house, running in the grid power poles and buying a transformer is $55-60KAUD. On current budget numbers the off-grid electric system would be a little over half that, even if substantially oversized. Going off-grid should allow enough budget for maintenance and component replacement, and enough leftover for a neat HVAC system, plus no utility bills.

hot_rod

I think you are on a better path, load up on the PV not thermal to avoid all that over-heating nonsense. The off grid friends I have basically learn to live within the parameters of what their arrays and battery budget can provide. Use smart efficient , bulbs, appliances etc.
New heat pump technology with inverters and vapor injection, I suspect you will get close to a 2.5 COP even on those design days IF you can keep required SWT 130F or lower. Think large radiant surfaces, generously sized panel rads, for example.

Depending on how much data crunching you want to do, see if you can find hours of occurrence data for your location. Look back 20 years to see what average temperatures are throughout the year. ASHRAE has that data maybe a database in your country? Here is an example for upstate NY, certainly a severe winter climate area.

With that you could predict closely what type of COP or EEF you could expect, then of course how your predicted array size, storage and additional loads match up.

An accurate heat and cooling load number helps get the numbers dialed in, yours seemed high, but it is based on the building design and construction. Insulation is the gift that keeps giving
A substantial rise in net zero buildings in the US, up 240% in Canada last year. All the new technology makes this very doable without sacrificing lifestyle.

coldAussie

Here are rev4 and rev5, showing an A2WHP as the main heat source with thermal storage to make maximum use of the surplus daytime PV generation. Rev4 looked expensive with the two buffer tanks. For the most part our shoulder seasons of autumn and spring are mild and no heating or cooling is needed, as such a seasonal changeover of the buffer tank from heated to chilled would be no inconvenience.

I included functionality to be able to export space heating from the open vented system, though this may be added complexity for little gain if the heating jacket is sized down to predominantly DHW needs. It could likely allow wood to keep the chill off bedrooms and towel rails if there were a particularly poor stretch of PV generation weather, but in such a case there is also the backup generator.

With these smaller heating jackets the stove vendors supply them in stainless steel, and the open vented ceiling tanks are available up to 500L in either S/S or copper. As such the whole open vented system could eliminate ferrous metals for corrosion control. Some farms in Australia run the DHW gravity fed directly from the ceiling tank and learn to live with the lower supply pressure, or use a reverse indirect smooth coil for mains pressure DHW (typically 1" 45m long copper.)

I chose to include a plate HX to export DHW for three reasons;
- to allow mains pressure DHW,
- to improve performance with low temp DHW should it be sourced only from the heat pump (~55degC tank)
- to mitigate legionella risk and the need for disinfection heating cycles using the backup element.

rev4 has the heat pump supplying the DHW tank via a reversible import/export HX. Rev5 uses an indirect coil straight off the heat pump. I much prefer rev5 for simplicity, but would need to be careful on sizing the coil to ensure I can get enough heat transfer into the tank with only 55degC max available out of the heat pump.

I think this about completes the possible options for integrated systems that suit the application. From here I'll work on the design heat loads, PV & battery sizing and sizing the major HVAC components. Then I'll pull together a few costs and see how scared I get.

@hot_rod Thanks for the guidance on hours of occurrence data, I can get hourly weather data for my local station for reasonable cost from our Bureau of Meteorology and do the histogram analytics myself. Typical Meteorological Year data is also available pre-crunched for reasonable cost. It will be good to link this up with expected PV generation data and confirm feasibility & sizing.

Rev 4



Rev 5

jumper

Photovoltaic produces higher temperatures than solar thermal unless you use concentrating collectors. Higher temperature storage is more useful and smaller. Photovoltaic can also be used for A/C.

hot_rod

Yeah, that is the next big question a single or hot and cold buffer. It comes down on when to make the switch over. With two tanks you can quickly go from a heating to cooling condition, with a single tank, you would be starting out with the wrong temperature, so to speak.
You are also correct in that it comes down to how much $$ you are willing to spend for the perfect, or best system.

The brand new Idronics 27 shows both simple and more complex piping options. it available in digital form now also at the www.caleffi.com site.

https://www.caleffi.com/sites/default/files/file/idronics_27_na.pdf

Sounds like you have the ability to crunch the numbers for solar feasibility. All that data resides in solar sim programs. I use the T-sol thermal program, and it has a worldwide data base.

I'm not sure how much they allow you access to on the 30 day free trial?https://valentin-software.com/en/downloads/
Give it a try before you pay for the data.

coldAussie

jumper said:

Photovoltaic produces higher temperatures than solar thermal unless you use concentrating collectors. Higher temperature storage is more useful and smaller. Photovoltaic can also be used for A/C.

Thanks @jumper are you suggesting to consider more electric resistance heating using PV power rather than heat pump?

coldAussie

Thanks again @hot_rod I will access the T-sol database and see what’s possible. I went with four pipe for the buffer tank but I see idronics 27 has good discussion of the options so will study that.

hot_rod

The reality is that solar thermal is much more efficient than PV. Thermal can approch 90% range under the right conditions, like pool heating. The problem with ST is having a consistent load. There are some chillers, AC running off vac tube ST, but the technology if a bit too pricey.
So PV, since it is an everyday use will make more sense.

The SRCC www.solar-rating.org has a database of solar thermal collector performance, you can predict fairly closely what you will get from the various collectors at your location.

The heat pump under your conditions should get you a 2.5 COP or higher, your mild weather and the ability to use low SWT in heating mode. Really no reason to use resistance heat, especially with your wood backup systems.

jumper

coldAussie said:

jumper said:

Photovoltaic produces higher temperatures than solar thermal unless you use concentrating collectors. Higher temperature storage is more useful and smaller. Photovoltaic can also be used for A/C.

Thanks @jumper are you suggesting to consider more electric resistance heating using PV power rather than heat pump?

Do you want to store energy at 120° or at 200°? Latter require less tanks. Plural emphasized. A depleted large tank requires many more hours of sunshine.

hot_rod

I would think you would be further ahead by turning electricity, grid or self generated into 2- 3 units of energy for every one you pay for? Judging by your plan and climate, running 130F or lower SWT on design that is possible.
As far as storing energy for extended periods, thermal would probably be the most cost effective, compared to batteries or low $$ FITS.

But as you can see thermal energy storage for a few days worth of heat energy takes a lot of well insulated gallons, literes.
Energy, all energy starts slipping away as you start pumping, heat exchanging, storing, etc.

A mix of both would be nice if you are headed to net zero or completely off grid, thermal storage and a battery bank.
It can be addictive, I started out with 160 gallons, went to 220, now I have 500 gallons and would like 1000 or 1200 gallons

Yet another Idronics showing the advantages to lowering the required temperatures.
https://www.caleffi.com/sites/default/files/file/idronics_25_na.pdf

jumper

>>think you would be further ahead by turning electricity, grid or self generated into 2- 3 units of energy for every one you pay for?<<

In the days before compulsory double wall I could generate quite hot water with desuperheater segregated from condensing preheater. 120° is okay for DMW but for heating with that temperature you may want blower terminals? I treat PV as marginally free energy. So high temperature energy storage is my initial choice.

Cooling storage is a different issue.