Closed Loop Water Storage Tank Lifespan

desert_sasquatch

Newbie here.

I've been looking into the different thermal storage options for a solar hydronic system--and it seems it's not an insignificant portion of the cost!

I don't want to do an unpressurized system--I have concerns about legionella or mold or whatnot growing in one of those tanks, it's just not a risk I want to take. So I'm looking at pressurized systems.

It seems like a lot of people use the insulated 119 gallon water tanks. At 120 gallons I gather there are added requirements regarding pressure testing and as far as I can tell that seems to mean that it's hard to find pressurized tanks over that size--at least online.

However it gets cold here in climate zone 6, and if I'd like to store enough heat from the solar panels to make it through a cold winter night, and if I'd like those solar panels to operate with a decent efficiency, then I would need a large water storage tank to do so. Like 1500-2000 gallons (this assumes the panels put out 90 degree F water on a...maybe a 20 degree F day, I think. Something like that.

So this is sort of a two part question. The first is: Is there a solution for finding a pressurized tank of this size? I've looked at prices (rough, on the internet) for large propane tanks. They are pressurized and I have a vague idea that they might work for water storage too (if I insulated them)....but is that actually the case? If it is then it seems like I could probably get a 1500-2000 gallon tank for ... $4,000ish aboveground [https://home.costhelper.com/propane-tank.html]? Plus a bit more money to surround it with mineral wool, stainless steel mesh (so the mice don't burrow into the mineral wool) and corrugated metal to make it look like a low shed and protect it from the elements).

The second question is: How long would we expect a tank like this to last? Energy.gov says that in a closed loop hydronic system corrosion and scale aren't really issues--at least if you don't do something stupid like connecting the tank to the hose with a fitting that has dissimilar metals. https://www.energy.gov/energysaver/solar-water-heating-system-maintenance-and-repair .

Would it require a sacrificial anode to prolong the lifespan?

Basically, if I get an expensive propane tank--or if folks say that's not the way to go, then if I get another large pressurized water storage tank--and use it for warm (ie 90 F in winter) water storage, and take care of it in every way that one could...how long might it last?

Hot_water_fan

Solar thermal is tough road. One option is to use ice + water source heat pump. That way you can store much more energy per gallon and the thermal panels operate at highest efficiency. 

Or consider solar PV, which is what the market has mostly decided on. 

Caleffi’s Idronics journals cover solar thermal well 

Larry Weingarten

Hi, An approach I've used successfully is to use a large plastic tank, essentially for heat storage, and then use copper coils high and low to put heat into the water or remove it. This way you can have a long-lived tank, but not under pressure.

Yours, Larry

hot_rod

Lp tanks in a closed loop system will last a very long time, I’d guess in excess of 20 years. Insulating them to r-10 or more gets tricky. They are generally a heavier gauge metal, and are ASME rated.

What size is your thermal array? You want somewhere between 1-2 gallons per square foot of collector.

2 gallons for warmer sunnier climates, 1 gallon for winter solar harvest would be adequate.

The colder you run the collectors, the higher the efficiency. 110- 120 in winter from a flat plate collector is about as good as it gets. Evac tubes can go a bit higher.

Trying to drive them to 140 in cold winter temperatures would put you down around 10% efficiency.

95 degree supply from the collector with 20 ambient could get you 35% or more efficiency

Read through this Idronics to learn how to use the SRCC test data to predict performance.

https://www.caleffi.com/sites/default/files/file/idronics_3_0920.pdf

EBEBRATT-Ed

We have used air compressor tanks also ASME rated

hot_rod

Fuel storage tanks can be sealed, but not pressurized, maybe 2 psi according to a manufacture that I bought waste oil storage tanks from.

I tried some plastic HDPE tanks, around 140F they start to get really soft and flex. I had a pallet tank and the plastic started pushing through the mesh around it at 140!

It will cost you another circulator, but by far a flat plate HX will be the most efficient way to transfer heat from a tank,as you have two moving flows. The water around coils in a tank tends to stagnate, unless you churn it up from time to time🤔

desert_sasquatch

@Hot_water_fan I will look into the heat pumps some more, but I think I would only do them in combination with solar hydronic. [Edit: I see that may be what you're suggesting? I'll have to look up what an "ice source heat pump" is].

In any case the calculus for what the best system is may be different for me than for some people. That's for a few reasons:

1) My health is not great. I have environmental sensitivities--mostly to mold--but I'm aware that many other people in my position become sensitive to "EMFs". Solar panels contribute a great deal of "dirty electricity" to the grid when they are tied in without a filter. The filter I know of that is up to EMF-sensitive people standards will cost $6,600 (https://defiltersllc.com/product/the-new-ind100-inductor/?v=93b46a3fc67d). This filter--which would probably need to be replaced every 20 years--makes a small solar electric array that's plugged into the grid a poor investment.

2) Heat pumps are either ground/water sourced or air sourced. The ground/water sourced ones require expensive excavation. The air sourced ones have a similar potential to attract mold as AC units that use condenser coils. For many people that wouldn't be a concern, but for me it will be. That forces me to use the more expensive ground sourced heat pump if I want to use a heat pump. Which further changes the calculus for me.

3) It seems like air source heat pumps are set to dominate the market. I wonder how easily I will be able to find ground-source heat pumps (or people to service them) once my current one breaks--which it sounds like it will in perhaps 12 years or so (longer winters in climate zone 6 make me think that I'll be using the heat pumps a bit harder than "average," thus the shorter lifespan estimate). So that makes me even more nervous.

I'm not saying that I definitely won't do the heat pump thing. But in order to understand the financial pros and cons, I need to price out all the options--very much including solar hydronic. And in order to do that I need to price out the options in terms of thermal mass storage.

hot_rod

Directional boring is another way to get a loop field installed. Less tearing up of the property. I’m not sure how it prices compared to trenches or bore holes.

Hot_water_fan

The air sourced ones have a similar potential to attract mold as AC units that use condenser coils. 

You mean outdoor mold?

I know nothing about EMFs so can’t give good advice on that. 

It seems like air source heat pumps are set to dominate the market. I wonder how easily I will be able to find ground-source heat pumps (or people to service them) once my current one breaks--which it sounds like it will in perhaps 12 years or so (longer winters in climate zone 6 make me think that I'll be using the heat pumps a bit harder than "average," thus the shorter lifespan estimate). So that makes me even more nervous. 

No doubt about it, ground source heat pumps are rare because the excavation is pricey and therefore is often disadvantaged compared to air source. They should last longer than 12 years though. That said, solar thermal is even rarer. Solar thermal + ice storage + water-to-water (air and ground source are not the only options) is probably the rarest of all, but your situation is particularly unique. 

Larry Weingarten

Hi @desert_sasquatch , You said "I don't want to do an unpressurized system--I have concerns about legionella or mold or whatnot growing in one of those tanks, it's just not a risk I want to take. So I'm looking at pressurized systems." The bugs really don't care about pressure or lack of pressure. They can grow anywhere. I think a fairly non-toxic way to manage an unpressurized tank is to periodically add some hydrogen peroxide to it to burn off the bugs. Also, if the water is used solely for thermal storage, but otherwise not communicating with the indoor air, it seems a pretty safe approach. This saves you from pressurized metal and the corrosion concerns. My 1000 gallon tank is made from PEX, so has no problem with the temps flat plate solar thermal panels produce.

Yours, Larry

desert_sasquatch

You mean outdoor mold?

@Hot_water_fan Honestly, I don't really know what kind of mold I expect to grow on the coils of the coils or fan. It could be an outdoor-type mold--the kind that munches of leaves and compost--and I'd not be concerned. My concern is that it would be the kind that you sometimes find in ducts when there's enough dust and condensation from the AC. That kind of mold might be a problem for folks like me, even if it's coming from an outdoor source near the house rather than a box with air being pushed directly into the house. Or it might not be an issue. However if it is an issue, remediation can be very expensive and sometimes even then it doesn't work. Plus there would be the expense of replacing the heating unit with something else.

Basically, I'd consider air-source heat pumps if the environmentally sensitive community had had some time to see what's what. So if I'm building a home in 10-20 years I may revisit this. But for now, I do not want to experiment with an air source heat pump.

desert_sasquatch

Directional boring is another way to get a loop field installed. Less tearing up of the property. I’m not sure how it prices compared to trenches or bore holes.

Thanks @hot_rod. Unfortunately everything I've seen is that the bore hole heat pumps are a little bit spendier. See the chart here, for instance: https://climatebiz.com/cost-of-a-ground-source-heat-pump/ For ground to water (they call it water to water) it's about $4,000-$5,000 more for the vertical loop vs a horizontal loop. And in any case I'm out in the boonies and I think that I'll have space for a horizontal heat pump, should I go that way.

desert_sasquatch

No doubt about it, ground source heat pumps are rare because the excavation is pricey and therefore is often disadvantaged compared to air source. They should last longer than 12 years though. That said, solar thermal is even rarer. Solar thermal + ice storage + water-to-water (air and ground source are not the only options) is probably the rarest of all, but your situation is particularly unique.

It's true, solar thermal plus storage is often pretty rare. I do live in a bit of a hive of alternative building though, so in this particular place it's not too rare to see solar thermal panels on roofs--though I suspect that usually those panels are either just heating the domestic hot water or heating DHW plus a simple radiant floor (a design which might work fine on a cold winter day but runs into issues if you expect to keep a place warm at night with stored heat).

I guess my hope regarding maintenance and professional knowhow was just that the system would run very similarly to and rely on essentially the same hardware as any other hydronic system; as far as I'm aware (and folks here please let me know if I'm wrong) there aren't special pumps or temperature control valves that are only used for solar thermal systems that aren't also used for, say, radiant floors or wood-fired boilers. Radiant floors, at least, are quite popular over here. So I kind of figure that if someone knows how to fix a radiant floor loop they'll know how to fix the loop going through the solar panels.

I should have been more clear regarding my 12 year estimate and fear of being able to replace it (or repair it): That was just for the heat pump itself, based on what I had read, which was that the newer heat pumps probably will last 15 years on average, but that the actual lifespan will obviously depend on how much and how hard it's used. And being in a cold climate means you'll use it both more and harder. I would of course expect the underground tubing to last longer. I just...I read Hot_Rod in a recent post talk about the difficulty of replacing parts for condensing on demand water heaters. Maybe I'm being paranoid but I feel like gosh, if those are becoming obsolete that quickly what can I expect for ground-source heat pumps when air source heat pumps are making such strides?

Hot_water_fan

I think I understand @desert_sasquatch. Air source means using heat from outdoor air. It sounds like you’re worried about mold inside the house, particularly in the ducts. Therefore, air source or ground source isn’t relevant here. You seemed concerned about using forced air vs forced water. Air source can do either.

desert_sasquatch

Hi @desert_sasquatch , You said "I don't want to do an unpressurized system--I have concerns about legionella or mold or whatnot growing in one of those tanks, it's just not a risk I want to take. So I'm looking at pressurized systems." The bugs really don't care about pressure or lack of pressure. They can grow anywhere. I think a fairly non-toxic way to manage an unpressurized tank is to periodically add some hydrogen peroxide to it to burn off the bugs. Also, if the water is used solely for thermal storage, but otherwise not communicating with the indoor air, it seems a pretty safe approach. This saves you from pressurized metal and the corrosion concerns. My 1000 gallon tank is made from PEX, so has no problem with the temps flat plate solar thermal panels produce.

Thanks @Larry Weingarten. Hmm...I've read other people say otherwise but I'm new to this I'm obviously just starting to get a handle on all these things, and on what the arguments are. So it's helpful for me to hear that you disagree.

Is there a document somewhere on thermal mass storage? Does one of the Idronics cover it, @hot_rod ? (I've read Idronics 17--which like all the Idronics I've looked at was great--but while it covers a lot of thermal storage stuff I don't recall anything about large unpressurized tanks used for thermal storage).

I feel like I'm kind of flailing around here reinventing the wheel when someone has probably done the math on it already. I can see how, if the heat was added to and removed from an outdoor unpressurized water tank through some kind of heat exchangers then the risk of Legionella exposure might be minimal. Similarly, if the tank were vented to the exterior then my concern about the tank contributing moisture to a home which could result in mold would be moot. Er...is that how it works? Is there even a vent?

So then why do people buy these 119 gallon pressurized thermal buffer tanks if unpressurized tanks can do the job more cheaply? Do the plastic tanks not like the hotter temperatures?

Thanks for helping me get some kind of grasp on this stuff, guys.

desert_sasquatch

I think I understand @desert_sasquatch. Air source means using heat from outdoor air. It sounds like you’re worried about mold inside the house, particularly in the ducts. Therefore, air source or ground source isn’t relevant here. You seemed concerned about using forced air vs forced water. Air source can do either.

I appreciate your patience, but actually, I'm a bit more paranoid than you infer: I'm concerned that mold growing on, say, the fins of the outdoor fan of an air-to-water heat exchanger might become problematic for me whenever I walked outside. My concern is that, being a dark, moist environment that dust gets blown through, it might grow inside the heat exchanger and then blow on me when I walked by it.

As I said, I'm unsure whether the mold that would grow in this situation would actually be all that problematic. And if it were sufficiently far from the home...well that would also reduce any risk (though obviously to insulate the pipes coming from the unit would cost extra money).

I don't know. I'm kind of going in five different directions at once here trying to price out options.

Ultimately I hope to be able to sit down and say "ok, here's the price of the heat pump with ground source, (and now you mention it) here's the price of an air source heat pump placed far from the home, here's the price of a wood fired boiler, here's the price of solar without heat storage, here's the price of solar with heat storage" et. Unfortunately since my health puts some weird constraints on my plans, the typical calculations don't always apply. So I wind up asking questions like this.

desert_sasquatch

And by the way I'm still curios if anyone has an answer: Will a 2000 gallon propane tank last twice as long as a 2000 gallon polyethylene tank because it'll be pressurized and will be spared damage from oxygen and biocides? Or does that not matter?

Larry Weingarten

Hi @desert_sasquatch , there are lots of factors at play here. One is what delivery temperature of the heated water is needed. The lower the better! I can keep my house at 70F with 80F water, but then it seldom goes below freezing here. If you design to use lower temp water, you probably will want more gallons of storage per collector area. I use three gallons per square foot. This makes the collectors perform better and I get a lot of low grade heat to use. I don't know of a document that spells all of this out, but I'd bet that @hot_rod does! But, essentially, you want to store enough heat to meet your needs for some number of days. Doing a heal loss calculation to get the daily number for BTUs times number of days gives you the amount of heat to store. Then you look at haw much heat you can store based on how warm the solar can make it and how low a temperature you can use. That translates into how many gallons the tank should be. Hope that helps!

Yours, Larry

hot_rod

Idronics 6 is on solar combi systems and has some info on open tanks

Issue 9 is geo hydronics, 10 is on Wood Fired, 17 is thermal storage options, 25, 27, 29 all apply to what you are doing.
Thermal storage comes into play a lot with most any hydronic system, so various issues mention options.

May as well download and read them all, a massive amount of knowledge and actual working systems documented. Can’t beat the price for a lifetime of information🤓

https://idronics.caleffi.com/

gsk3

Builditsolar.com has designs for collectors and very large unpressurized tanks. Worth looking at for ideas.

desert_sasquatch

@Larry Weingarten Thanks Larry. I did actually catch on to that dynamic just a few days ago--I was kind of excited to learn about it, actually --and I've been calculating my storage needs based on that. Though admittedly I was aiming for more like Ti + 25 degrees F (Ti being the indoor temp so...95 degrees coming from the collectors). 80 degrees would obviously make the panels even more efficient but would require I want to say 4000 gallons of water to store one night's worth of heat. And of course there's just no getting around the issue of paying for an insulated space, wherever it is.

And thanks to everyone else for responding! I'm working my way through a lot of reading and pricing of stuff and I'm not responding more simply because when I open my mouth I want to have something useful to say. But my silence doesn't mean I don't appreciate the responses.

desert_sasquatch

hot_rod said:

Read through this Idronics to learn how to use the SRCC test data to predict performance.

https://www.caleffi.com/sites/default/files/file/idronics_3_0920.pdf

Thanks @hot_rod. I have a question regarding the solar thermal collector efficiency dynamics described in Idronics 3: Where does the target water temperature come into it? In other words, the inlet fluid parameter takes into account the inlet fluid temperature and the ambient temperature outside the solar panels as well as the irradiance available at that time. And I assume it takes into account the ambient vs inlet temperatures because the greater the difference between the temperature of the water being heated and the air outside, the more energy will be lost to the air outside.

Just to make things clear, Idronics 3 says that the inlet fluid prameter is (Ti-Ta)/I

Where
Ti: Temperature of the Inlet Fluid to the Collector
Ta: Temperature of the Ambient air around the Collector
I: Irradiance (BTU/hr/ft^2)

However if that's what's happening then I would also imagine that, as you said, the warmer you heat your water the less efficient the collector will be. In other words lets say it's a 50 F day with 200 BTU/square foot irradiance. If I take 40 F water and heat it to 95 F that should be a more efficient use of the collector than taking 40 F water and heating it to 160 F. However as far as I can tell, that's not reflected in the inlet fluid parameter equation, since in both cases the inlet temperature, ambient temperature, and irradiance would be the same.

What am I getting wrong here?

Hot_water_fan

@desert_sasquatch the issue is that the water will not remain 40 degrees inlet - maybe only 10 degrees are added per hour but eventually you’re putting 150 degree water into the panel. 

desert_sasquatch

Hot_water_fan said:

@desert_sasquatch the issue is that the water will not remain 40 degrees inlet - maybe only 10 degrees are added per hour but eventually you’re putting 150 degree water into the panel. 

I understand why the temperature of the inlet water would impact the time it takes to heat that water to a set point--say 150 F.

However what confuses me is this: I thought that the issue here is a decrease in efficiency due to increasing heat loss as hotter and hotter water loses more and more heat to the exterior. If that's the case then the inlet temperature is only part of that story. It seems to me that in addition to the exterior temperature, irradiance, and insulation value for the panel we'd also need to know the temperature of the outlet water. All else being equal, if the outlet water is 190 degrees, it'll have lost more heat along the way than if the outlet water is 150 degrees. Right?

As far as I can tell if one knew the speed at which the water was circulating through the panel then it would be possible to figure out the approximate temperature of the outlet water. But we don't know that, do we?

Or maybe it's just that while we don't know the exact speed of the water passing through the panel, practically speaking there's a limited range of speeds for the water in a good system and within that range we know that the heat gain will be minimal in one pass-through so we can still estimate efficiency using just Ti, Ta, and I?

Thanks for bearing with me on this

hot_rod

you have it correct. You would vary the flow rate through the collector to get that delta T. Typically you want a 3-5 delta across the panel 

Its the load you apply to the collector or the temperature you drive them to.

obviously you want useful energy, a tank of 80f water for a load requiring 90 isn’t much good 

that is why the relationship between the array size and the tank is important to the operating condition/ efficiency. You want to find the sweet spot efficiency while still covering the load 

scrub the heat out as fast as reasonably possibly.  Most of the digital solar controls have a variable speed function, as the collector temperature starts to rise, the pump speeds up, or slows down as a cloud comes in😎

as for efficiency it is just a single point in time. A full sun is considered 317 btu/ sq ft hitting the collector. That can change second by second, as can the fluid entering the collector, wind speed, ambient temperature 

If you look at how that formula applies to a bare rubber tube swimming pool collector, 90% efficiency!  How is that possible?

well the collector inlet is close to the ambient, so very little loss. 80 degree pool water to the collector on an 80 ambient day, 82- 85 coming out of the collector toy the pool 

desert_sasquatch

Thanks @hot_rod I think that clears it up.

So what I was missing was that the delta T between inlet water and outlet water is fairly constant, allowing the inlet temperature to stand in for the average of the inlet and outlet temperatures... Always satisfying to see the pieces fall into place

desert_sasquatch

@hot_rod just to be sure before I redo all my solar thermal calculations:

If the chart for solar thermal collector efficiency indicates efficiency at a particular moment in time, then if I want to get the average efficiency with which a collector will heat water from, say, 40 F to 130 F, then I need to 1) Check to see that 130 F is doable (ie it's not so hot on a cold day that it passes into the realm of "impossible to do") and 2) if #1 is true then take the average of 40 and 130-5=125 (if I understood you correctly if the outlet water is 130 then the inlet is probably 125). Average of 40 and 125 is 82.5 F, so then I'd look up the efficiency (or power output) associated with that value of Ti-Ta.

I hope that's understandable. Is that correct? Because if so then I've been underestimating the output of solar thermal panels by calculating water heating efficiency using the least efficient part of the cycle, when the water was heated to within 5 degrees of the target temperature.

hot_rod

Head over to this site and try the free simulator
It provides a lot of the info, specific to your area and the system requirements. It’s the best way to look at solar, and may also show the ROI

desert_sasquatch

hot_rod said:

Head over to this site and try the free simulator

Thanks @hot_rod , that's interesting software.

Unfortunately I'm hitting a snag using T*Sol. I have asked folks on their forum about it but thus far I've not gotten a response. The snag is: I'm getting very different output figures from T*Sol than my back-of-the-napkin calculations. And while it's entirely possible I screwed something up, I've checked my math a few times and found no issue. And the math doesn't seem that difficult.

Whereas if I assume that the T*Sol software is forcing the panels to operate at a high temperature (and thus a lower efficiency) then that explains it. And the visualizations appear to back me up.

I wont' get into more details here since the Valentin folks have their own forum but for anyone interested here's a link to the thread I started there.

So I guess this leads me back to sort of a basic question: It seems like the best way to use solar with some kind of backup on-demand heat source would be to prioritize the higher efficiency / lower temperature outputs first, and only once that has been satisfied move on to higher temperature / lower efficiency outputs. It doesn't *seem* like that should be all that hard. Yet so far as I can tell there's not a good way to model it in the premier hydronic modeling software. Is it impractical for some reason? I'm also open to having missed some option in T*Sol that would work but, as described in the thread I linked above, I did try messing around with what seemed like the pertinent options, to no avail.

hot_rod

Yeah it is a powerful, but complex SIM program I took two of their online classes when we bought the program, and I just scratched the surface. They had an office in So Cal for awhile and a gal there was knowledgeable and helpful. I think it all goes through Germany now?

F Chart TRNSYS another program that the old solar gurus use for simulations, comes out of UW Madison. Also the Canadian program RETScreen may still be a freebie. You’ll want a tutor for any of them.

But you already know the best way to leverage ST😉

The Caleffi/ Resol controllers had some clever loading options. You could use two or 3 storage tanks and load them in sequence. Load the first tank to maybe 120, then go to the second cold tank, load it to a temperature, then on to the 3rd

Then come back to tank 1, take it up another 20 degrees. So basically the control picks the best load to keep efficiency of the collectors at the highest possible operating condition

You may drive yourself nuts chasing small %s of free energy, however

desert_sasquatch

OK, thanks @hot_rod. Perhaps I'm getting close to the place where I need to hire an expert.

Still, I'm hoping I'll be able to get to the bottom of this. As far as I could tell this wasn't just a few % of the solar output, it was maybe a third, and most of that came from loss of efficiency in the coldest part of the year, meaning that adding 33% more panels wouldn't bring me back up to where I was before.

I guess either I'm missing something or it's rare for people to use solar thermal as a primary home heating source. As I think about it more...maybe that is it. Maybe there are reasons that what makes sense for me rarely makes sense for other people:

In northern climates in the northern hemisphere, winter days are shorter than in more southern climates (in the northern hemisphere). If you're looking for a high % of heating to come from solar thermal in a non-tropical climate then winter efficiency is more important that summer efficiency, since the increased home heating need and lower panel efficiency will necessarily make that season the place where most of your heating shortfall comes from.

So northern climates are less likely to find solar thermal panels efficient in the winter. Which isn't to say they're no good, but depending how far north one is there will be a decent chunk of winter where you rely almost exclusively on whatever your other heating source is. And since that's the time when you need the most heating...a lot of one's home heating would be coming from another source, even if you design a system like I suggest that's designed to heat the home with 90 F water. At some point this makes even hooking up the solar thermal panels to the home heating side of things not very cost effective, since the heat bank is sized for the heating load and costs essentially the same no matter how much you use it. One could, of course, just pump heat into a concrete slab but if you want to heat a home at night that means a hot home during the day, and I gather slabs are less popular in northern climates where the foundation goes so low you might as well have a basement.

So instead people up north are (I assume?) just using solar thermal as a thing they can plug into their existing DHW system. It gives them hot water or at least pre-heated water much of the year and because it doesn't require the whole heat bank infrastructure that using solar thermal for home heating would require it's fairly cost effective in spite of the latitude.

In southern climates in the northern hemisphere folks don't usually have basements. And that means that there might not be a convenient place to put a couple thousand gallons of water with which to store heat from 90 F water. Plus most southern places have, by definition, more mild winters, which means that hot water, which is required all year round, becomes a larger % of the heating load, and when thinking of saving money on heating and cooling, cooling becomes the more important of the two. And obviously solar thermal panels don't help with that, while heat pumps do.

Also just generally the only people who would invest in a system with a 20-25 year payback are homeowners. Builders won't do it, as it doesn't add enough (or sometimes anything) to the sales price. So there's this issue where if there's no spacious basement then you need a home that's been designed from the start to use solar thermal. And unless you're building it yourself, that's not what you'll get, because no builder wants to "build themselves into a corner" like that.

Anyhow I guess I'm just thinking out loud. But if these reasons stand up, it might explain why what seems like the most reasonable approach to me is not usually the one that's taken. Which obviously I'd like because the alternative is that people aren't doing it because the approach isn't as good or cheap as I think. Though obviously if folks do think that's true, I'd appreciate hearing it so I don't make a big mistake!

Hot_water_fan

You’re coming around haha. Solar thermal isn’t popular for all of the reasons you’re discovering.

hot_rod

The solar contractors I know in NM would shoot for a 30% SF for heating as I remember. Beyond that you tend to run out of roof square footage, or customers $$ budget

The other challenge us what to do with large arrays in summer months with just a small DHW load?

So the financial part is always part of the picture. T-sol crunches those numbers also, as do the other programs

actually Wisconsin and NJ have been strong solar thermal markets, over the years, NC also driven by the courses offered down there for 50 years or more now.

Bob Ramlow taught sand bed storage for years in WI, I believe his son in law Ben took over that position at the community colleague.

Unquestionably the best, simplest use is passive solar for heating. Worked for the cavemen and SW Cliff dwellers, still works the same today.

Concrete is an easier medium to store that energy in, since it can be used as the structure also you get double duty But has a mind of its own when trying to control it.

The ST motto has always been “use it or lose it”
storage the holy grail yet to be discovered. Plenty before you have traveled this pursuit😎

Last fall we did a webinar A2w slab radiant. 80-90 required SWT is an ideal match for HPs
Maybe a PV array driving a device that can heat, cool, provide DHW pencils out better?

Plus you can run the lights and plug in your EV

Windy, in the 20, bright blue sky, here is my 60 gallon, one 4x6.5 collector system
HP is the hours the pump has run since I installed it, fall of 2022

desert_sasquatch

hot_rod said:

Bob Ramlow taught sand bed storage for years in WI, I believe his son in law Ben took over that position at the community colleague.

Thanks! I see he wrote a book. I've bought it and started reading.

I already have a question (for everyone): He claims that for every $100 in yearly energy savings from a solar hydronic system you'll add $2,000 to the value of your home. I assume that's for a new system and I assume it depreciates over time, but basically it sounds like if you get a system with a 20 year payback and had to sell the home after five or ten years that you should expect, roughly, to recoup the expense of installing the system. Does that match with people's experience?

Because I've also watched a video from some Florida real estate agent who was complaining about how solar PV doesn't add value to a home when you sell it, that the only way it'll pay out is if you hold onto the home long enough to recoup the expense through energy savings. I would have assumed that to most prospective buyers solar PV and solar hydronic would be essentially the same--they'd be asking the same questions, which would be "how much money will I spend" and maybe "how long will it last before I have to replace or service it". But maybe solar thermal does add value where PV doesn't? Or maybe that guy was being too pessimistic about solar PV?

hot_rod

pros and cons about sandbed radiant.

Sand isn’t a great conductor, blending some Portland in the mix would help conductivity.
Id put a layer of 2” foam between the sand and slab to better control the output
Come fall that bed can be warm to the point of over heating some spaces.

You will find a wide variety of opinions on solar, benefits, drawbacks, liability or asset, financially viable or not. At days end, does it make sense for you is the only question to answer.

Im on my 6th ST system, starting in the 1980’s on four different homes and shops. Two PV systems.

I did all but the current 6kw PV installation. Got great deals on all the thermal equipment. Do all my own service. I’m far ahead of the game in my case.

But I also do it because I enjoy the technology, it’s a hobby as much as a utility.

I doubt I’ll see a payback on the $$ package I bought to get heated seats and steering wheel on my truck, it’s not always about the money for me.

Hot_water_fan

 He claims that for every $100 in yearly energy savings from a solar hydronic system you'll add $2,000 to the value of your home. I assume that's for a new system and I assume it depreciates over time, but basically it sounds like if you get a system with a 20 year payback and had to sell the home after five or ten years that you should expect, roughly, to recoup the expense of installing the system. Does that match with people's experience?

Not a chance - if a solar thermal system paid back at resale (or original sale) every new house would have one, as builders build what people pay for. Instead, almost no one installs solar thermal.. Solar PV might be different - people install those in much greater numbers, probably 1000x as many per year? Somewhere around there. 

hot_rod

Tom Lane, another old solar dog. Installer/ trainer from Florida had some fuzzy math on ST payback also.

Larry Weingarten

Hi, I'm usually the outlier, so will throw out some weird thoughts for fun. I use solar thermal for water and space heating. I'm off grid and burn roughly 1/4 cord of wood a year. Propane water heating. 200 gallons lasts about four years. Efficiency and solar do the rest. I have a 1000 gallon unpressurized water/heat storage tank with coils top and bottom. The bottom coil is for solar input and the two top coils are DHW preheat and space heating. I'm in a temperate climate, a bit away from the coast in California. Temps outside get from 18F to 110F. Solar works for me because I went nuts with air sealing and insulation. I use secondhand glazed collectors, with black painted copper absorber plates. Overheating was a concern so I made automatic vents for the collectors should they got too hot.

Solar thermal can work if you make the building shell really good and have storage for however many sunless days you typically have in a row. It's important to keep the system simple, even if you lose efficiency points. That way there is less to fail. Redundant systems are a plus. Not having to hook up to grid power and finding inexpensive parts to install myself made payback a moot point. I can keep the house at 70F with 80F water, but only because the shell is efficient. That's a snapshot of my crazy!

Yours, Larry

hot_rod

Water checks a lot of boxes for thermal storage, safe, inexpensive, safe, easy to move around. But the “box” the keep it in can get complicated if you want to drive high temperatures. Because of course the higher the storage the more useable it is.

I see Roth, which is a plastic and hydronic company makes black colored HDPE tanks for water and septic yes. Maybe their formula allows higher operating temperature.
I also tried one of the plastic direct burial septic tanks from the lumber yards I worried about the seams at above 120F.

And another attempt was a 1000 gallon concrete septic tank. I had to put an EPDM liner in it as they are not necessarily watertight😳. It ended up with some sort of bacteria film and smell. Maybe something in the rubber roofing material supported the growth. So that didn’t end well. Ended up with a used 500 gallon lp tank, sealed, 15 psi operating pressure.

In the concrete tank I had a single horizontal copper coil top to bottom and used a reverser valve. Pulled from the bottom to load it up. Reversed flow to pull heat out.
Tall skinny tanks stratify much better. The 500 lp tank was mounted on end. It would stack 30 degrees or more temperature delta. That drives up the exergy up

Some math on concrete vs water heat capacity, and a kitty dity about stratification and exergy.

desert_sasquatch

@Larry Weingarten Sounds great I am hoping I can do something similar, though about half the plots here actually have grid-tie electricity so if my plot has that I probably won't be able to justify the expense of being totally off-grid...

Larry Weingarten

Hi @desert_sasquatch , Just to stir the pot a bit, my off-grid house cost about $100 per square foot of living space to build when the going rate was $250. Efficiency and off-grid doesn't have to cost more. It does take careful planning, shopping, and some sweat equity.

Yours, Larry