Thoughts On "Solar Water Heating: A Comprehensive Guide..." by Ramlow & Nusz

jumper

This thread got me thinking. OP is in desert. Question is does he/she wish to store energy for short term since she can usually count on sunny weather most of the time. Or does she want to go long term? Accumulate enough warmth in soil. Hopefully somebody has INDEECO design manual?

Toronto went without electricity for over a week several winters ago. Some fifties homes had floor heating in basements. I presume there was no insulation in those days. So ground was being warmed for sixty years. Those basements remained tolerably warm.

desert_sasquatch

OK, so after spending some time looking into PV hot water heating I see...I guess I'm not sure what I see. There are people who hook up solar panels directly--without a charge controller--to a heating element. I finally found a good paper on it, though they don't go into what safety precautions people take to prevent a dry tank and a hot heating element from starting a fire...

But I'm not sure I should give up (and I'm open to thoughts on this). The things that keep me thinking about solar PV direct hot water heating are:

1) Low-light winter efficiency.
2) Simpler storage using higher temperature water

desert_sasquatch

However as Jumper helpfully reminded me, it might make sense to store heat for more than one day--something that solar thermal does a lot better than the PV setup I mentioned above. I had been sizing the system by basically looking at a graph of the daily heating I'd require and trying to draw a line across it that would provide me with 90% of that. Like this:



And then I'd try to figure out how much low-light power I could expect from panels in the winter on a cloudy day and size the array for that. So with the solar thermal I'd look at the power output for a "cloudy day" (1000 BTU/ft^2/day) with the approximate temperature difference I'd expect during daylight in that month and go with that.

Solar thermal though, unlike the solar PV setup I mentioned earlier in this post, is much better at ramping up heat production in fair weather. And while it loses efficiency as the temperature difference grows...it may not matter too much. For instance, with the SunMaxx panels the SRCC results for a cloudy day with 90 F temperature difference is 3.9 kBTU, while the results for a sunny day with 144 degree temperature difference is 11.2 kBTU. So using the methodology I described above, that sunny day would be able to store enough heat for maybe three days or more, provided it had somewhere to store it at that temperature. Two or three sunny days would do even more, though I'm sure with diminishing returns whose magnitude would depend on the size and insulation of the heat bank...

Sorry, I feel like all my thoughts here are half-finished. In spite of my equivocating, the comments here really are helping, though.

desert_sasquatch

jumper said:

Question is does he/she wish to store energy for short term since she can usually count on sunny weather most of the time. Or does she want to go long term? Accumulate enough warmth in soil. Hopefully somebody has INDEECO design manual?

Interesting @jumper. I wouldn't have thought those basements would have kept warm for so long! Maybe it's just a matter of heat loss decreasing as the indoor temp gets cooler? 6 degrees C is "tolerable" with warm clothes, and at that temperature maybe they weren't dumping huge amounts of heat into the homes, and the ground under them was at a similar temperature so there wasn't heat loss that way? Just thinking out loud, let me know if you think it was something else like massive amounts of thermal mass.

To answer your pondering, my main concern is honestly economic. I'm in the boonies, so the usual cheap option for heating--natural gas--isn't possible. So it's either electricity from the grid, propane, or solar. And even though our electricity is a bit below average in price, it's still about the same price as propane per BTU. Which tends to be 2.5X or so as much as natural gas, iirc. There is wood, but this will be a tiny triplex. Tiny stoves are a bit annoying--you have to keep feeding them. And three big stoves would be way too much. And if a tenant went away I'd need electricity as a backup anyhow... And a wood boiler winds up costing less than propane but more than solar. So here I am

I suppose the silver lining is that it makes a higher solar fraction more reasonable from a purely financial point of view. And I admit that I do like the idea of having a home that at least probably won't freeze if the grid fails in the winter, and I do like the idea of doing relatively right by the environment. Plus, I like the idea of being insulated from future energy price increases. But at some point, were another option cheaper, I would go with it.

So if getting a large storage system and a smaller array will save me money on my energy bill, I'd be into that. But if getting a larger array and a smaller storage system is the way to go, I'd be into that too.

hot_rod

Sun Bandit was one of the first PV direct to DHW tank system. The appeal was it was a standalone DHW system that didn’t require all the permits and utility tie in paperwork, approval, etc. It doesn’t connect into your homes electrical system.
super simple install no piping, pumps, fluid involved. A wire from the PV module to the controller on the tank. If the tank runs dry, the small element pops, not unlike an old flash bulb, no safety or fire hazard inside the tank. Size the PV module to the amount of DHW you want. Typically for small family DHW systems or as a pre-heat for the second tank or tankless.

Also know those btu outputs for ST are per day, not per hour!A solar day is generally considered 6 hours. So 39,000 per day from 1 panel, 6 hours to get there.
So when you start talking about producing and storing a days worth of energy, be realistic. Depending on your heat load, you are talking a thousand gallons or more of insulated storage. They usually an open unpressurized tank to be remotely affordable in those capacities American solar technics, Tom up in Maine has been building those knock down tanks since the 1970s.

desert_sasquatch

solsean said:

I have been involved with lots of projects that went for the big solar heating fractions and very few of them are operational. Drainbacks help avoid fluid stagnation but the collectors are going to expand and contract significantly. Seen more than a few with the absorber plates loose from the riser tubes. The best results I have seen for large solar heating fractions have come from "seasonal storage" tanks with 10:1 storage:glazing or even bigger. In terms of text for solar thermal, Planning & Installing Solar Thermal Systems by Earthscan Publishing is the best I have found.

Interesting. A little discouraging but better to learn now than once the system is bought and paid for.

What does 10:1 storage:glazing mean? Is that measured in gallons of water to square feet of glazing? Or $ invested in the system?

On the question of fins getting separated from the risers, do you think that's an issue to do with build quality or is it more to do with use? It seems like some folks here say that their flat plate collectors have lasted a long time, so what do you think the deciding factor is?

Thanks for the book recommendation. Not being an engineer I'm a bit concerned that if I bought it I'd be wasting $300 better spent on a proper solar consultant, though. Nonetheless I wish I was near enough to a library that had it to check it out.

desert_sasquatch

Thanks @TonKa

I've heard about something like that but hadn't seen anyone doing it. Cool to see!

Probably not right for this application, though, as the increased efficiency occurs in hot weather, which isn't really what I'm worried about.

Jamie Hall

In some ways, at least, @desert_sasquatch , I fear you may have the cart before the horse here, and that just doesn't drive well.

When dealing with solar power -- whether it is direct thermal space heating, direct thermal hot water, PV, or whatever -- the first questions you need to answer are a, how much power do you need at any given moment? b. how much power do you need on average? c, how much energy do you need to store? d, how much solar radiation can you expect with regard to day/night cycles and longer term cloudy or stormy conditions?

The last question up there is location dependent, and it will determine everything else in connection with the others. To give you some idea of what I mean by location (and to a certain extent design) dependent, a tracking thermal solar collector in a southwest desert environment can confidently be expected to produce about 1 kilowatt per square meter of collector area for 12 hours per day, every day (12 KWh per day per square meter), with no long outages. On the other hand, a flat panel thermal solar collector in New England can be counted on only an average of 3 KWh per day, with an expected longest outage -- no significant power output-- of 3 days. High quality photovoltaic panels will give you about 20% of that.

So, for the sake of discussion, let's say that you have determined that you have a thermal power demand of -- for convenience -- 48 KW on average for 24 hours a day (1152 KWh). That tracking thermal direct thermal collector will need a total area of 96 square meters, and you will need to be able to store about 580 KWh (2,000,000 BTU) of heat somewhere. On the other hand, for the same load in New England with flat place collectors, you would need to store a good bit more energy::3,700 KWh or 12,700,000 BTU, and you would need 390 square meters of panel. If you are doing all that with electricity, the storage doesn't change -- but the area of panel required does: it would be about 5 times as much.

One must use the advertised peak power output of various types of collector or panel with a good deal of caution, as it really isn't of much use in determining what you actually need -- and it doesn't address storage at all.

desert_sasquatch

@hot_rod Thanks

OK, so the sunbandit system did use some kind of fuse that would break if the temp of the tank got too hot... Would that work without the controller?

Good to know about the solar days! I was just going by some online "daylight hours" calculator. I suppose the first and last hour don't produce much so they don't really count? Or was six solar hours a general rule, and it's actually worse than that in the winter? Comparing the SRCC and Keymark categories (SRCC is per day, Keymark is per hour) I can see that they do become about equal if you assume the solar day is six hours long. So....are winter solar days shorter?

That said I did check the average irradiance for my location with X tilt to the solar panels and as far as I can tell the average irradiance in January is about 1500 BTU/ft2/day, so right in the middle of the SRCC weather range in spite of the month...

desert_sasquatch

@Jamie Hall Sadly, I do feel a bit "all over the place" with this, as my inquiries keep swinging from one system to another as I try to build a better picture of how they operate and what the constraints--very much including environmental constraints) are.

I feel like I have a decent idea of what the power requirements for heating are, as per BEopt. Thermal storage I'm less clear on, as the only data I have regarding how many very cloudy days I can expect to have in a row is basically limited to my own experience. I'd say that usually clouds will last 1-2 days in the winter, though occasionally they'll last longer. But most of the data I've found is aggregated by month so I'm not sure how to expand upon that. Is there some website or software that will tell me?

I do use this solar insolation calculator. So unless it's wrong, that seems to me like a good estimate (again, aggregated by month) of the total solar insolation I can expect. The NOAA climate data on cloudy/partly cloudy/sunny days tells me that there's a few more sunny days than partly cloudy days, and a few more of them than cloudy days, but in the end there's a good mix of all of them.

So the only thing I'm unclear on there is how many of the cloudy days happen in a row.

desert_sasquatch

Regarding the PV panels and their rated output: I...tentatively (because how many times have I reversed myself here) think I'm seeing why folks here are gently suggesting that solar thermal may be better for my application.

For what it's worth, I was definitely not assuming that I'd get their full rated output all the time. I think my mistake was thinking that I'd get usable sunlight from dawn to dusk, which looked to me like it was 9 hours or so in the winter.

Anyhow, I did find this paper on direct heating of resistance elements with solar panels. And it has a good graph, and I was taking my panel efficiency estimates from that:



As yall probably know, (and as it took me a week to figure out) without a charge controller we wind up optimizing our panel's performance for a certain level of irradiance. Eyeballing the above graph it seems like at 400 W / m^2 irradiance the panels would give me about 35% of the rated output at MPP. Fairly reliably, as long as the irradiance doesn't drop below that. To get the power up to the "red line" in the image I posted above I estimate I'd need about 56 kWh of heat per day. With six hours of useable sunlight for the panels that's 56/(.35 *6) = 26.6 kW of panels. Which...admittedly is is definitely getting more expensive than solar thermal plus storage. Back when I thought I only needed 17 kW of panels to achieve the same thing it seemed more reasonable

So thank you everyone for all the gentle corrections (and I'm sure there are more to come).

Jamie Hall

And how many cloudy days in a row is very much dependent on where you are. So... some idea as to where this is going to be installed?

On the hours and so on... yes, with ordinary flat plate collectors -- which is 99 percent of them! -- a decent guess for clean ones is that the effective capture is equivalent to about 6 hours. Reason being partly that the solar constant is less in the evening and morning (more atmosphere), but more it's a factor of the angle at which the sunlight is hitting the collector. At noon, of course, with a south oriented and properly tilted collector, the effective area is the same as the actual area -- but as the sun moves (well, as the earth spins -- let's be at least 16th century here!) the effective area is reduced -- the actual reduction is the cosine of the angle off noon either way. Which works out to more or less half on the average over a day.

That calculator you mentioned looks OK -- at least it agrees closely enough with my experience around New England...the figures it quotes, though, are for total energy -- and do not take into account the relative inefficiency of PV panels. So the numbers are OK for direct thermal collection, but must be reduced by a factor of 5 for PV (electric) panels.

hot_rod

Well you will never figure out Mother Nature. So for simulation or projections the best you can do is use 20 or 30 year data. That is built into programs like T-sol.

The weather going forward, again a best guess.

Solar energy outputs is an instantaneous number. At some exact second, the radiation was at some specific number. Seconds later it could be completely gone. A gust of wind changes output.
It can take months for a lab to get test results for a collector if they are doing the test outdoors, like the FSEC lab in Florida. More testing is done under powerful lights now.

Thousands have gone through this exact same exercise, before you. No one will ever get you exact numbers. Often times the amount of solar a customer installed is based purely on their budget for the quest. Maybe that is a lot less stressful way to invest in RE😎. I have 20 grand budgeted for energy independence, what will that buy me.

Personally I cover 50-70 % of a DHW load with thermal, simple and affordable. Put the rest in PV since electricity is an everyday need. Solar thermal doesn’t do a lot for you in summer months, can’t easily run AC from it

desert_sasquatch

@Jamie Hall Thanks. Yeah, that's how I was using the insolation calculator--just using it to see if I need to adjust any estimate based on SSRC data, or seeing if a very cloudy day in the winter might have even less than the 200 W/m^2 I've seen mentioned. Seems like probably not, where I am.

@hot_rod Thanks. Yeah it seems like maybe I'm getting closer to the point where I can ask useful questions of the local solar guy (he mostly does solar thermal I think). I imagine he'll have the experience and tools to answer these questions of cost and weather in this particular place.

I think maybe the main thing left to do before I meet with him is to learn more about thermal storage options. But in any case I do see that even with the best data available there's still a certain amount of guesswork involved here.

Still though--my aim is to add solar capacity until the cost per BTU from solar over the life of the system becomes higher than the cost per BTU from electric. Obviously there's no need to bullseye that target but I figure I'll aim for the ballpark, especially as I'm building from scratch and I think that might afford me a few opportunities that retrofits don't have.

jumper

In south west desert most winter days one can count on five hours of productive sunshine.
So ten kilowatts of panel can produce one and a half therms per day. Length of payback depends on future prices of electricity or propane. Remember that solar heating does not need to supply all heating requirements. Still saving $$$ if solar supplies some of your heating requirements.

PV can also supply some A/C as well. Trick is to segregate one's PV from grid. Otherwise "they are going to get you" one way or another. Since OP is building from scratch she can have an ice bin to store cooling energy. The heat rejected is ample for DHW if she can escape double wall heat exchanger.

hot_rod

It’s An intriguing technology

https://www.viessmann.family/en/newsroom/solution-offering/viessmann-ice-energy-storage-20-now-in-plastic

There were some attempts at energy fence products being imported back in the last solar wave, 2007- 2010. I saw some clever, modular composite systems at the InterSolar show back in those years

Larry Weingarten

Hi, Here's a link to some copies of that book ... and not so expensive. https://www.addall.com/SuperRare/UsedRare.cgi?title=&author=&title=Planning+&+Installing+Solar+Thermal+Systems&keyword=&isbn=&exclude=&bookshop=&binding=Any+Binding&min=&max=&dispCurr=USD&order=PRICE&ordering=ASC&match=Y&timeout=15&store=ABAA&store=Alibris&store=Abebooks&store=AbebooksAU&store=AbebooksDE&store=AbebooksFR&store=AbebooksUK&store=Amazon&store=AmazonCA&store=AmazonUK&store=AmazonDE&store=AmazonFR&store=Antiqbook&store=Biblio&store=BiblioUK&store=Booksandcollectibles&store=Ebay&store=EbayUK&store=EbayFR&store=LRB&store=ZVAB&via=used

Something you could do is to design the system so it's easy to expand if needed. It isn't too hard to twin tanks together, or have a place for more collector racks. Start small and very, very efficient. See what you really use and need from there.

Yours, Larry

hot_rod

A clever thermal storage concept. The owner of the former SolarSkies buried one of these, put a ground mount drainback array above it.
A nice option if you don’t have solar facing roof.
with a drainback, the summer overheating issue goes away. Ground mounts can be Earth screwed down, glass cleaning is a simple task.

As for collectors, The AET brand out of Florida has the copper absorber forge welded onto the tube, impossible for that bond to come apart. Silver soldered headers, it is a robust collector for drainbacks.

The insulation was always the weak link for collectors, drainbacks especially. The rock wool would outgas and cloud the glass. Todays foam do a better job.

The Caleffi collectors were injection foamed. Turned the entire frame into a very solid assembly, air tight and waterproofed. Frames were welded corners, not pop rivets.

https://www.latitude51solar.ca/commercial-solar-water-heating/2-uncategorised/148-cocoon-large-solar-storage-tanks

desert_sasquatch

@Jumper I agree, I would do that if I was further south but where I'm at is a bit cooler than that--there's no need for cooling in a home with decent insulation and/or thermal mass.

As for solar PV for AC power... It's something to think about. If I was building a cabin just for me I might do that. But I'll be renting out portions of the home and I suspect that renters will be less enthused about monitoring their electricity use carefully and dealing with the consequences of going over their alotment. Plus what happens if someone has an electric vehicle that requires charging? And as much as prices are coming down in parts of the whole solar thing I'm not sure it's cheap enough to justify it. For now I think I'll go with a small battery bank charged by PV (which dumps the rest into DHW) to power essentials if the power goes out and keep it at that.

desert_sasquatch

@hot_rod Interesting stuff, I had not heard of an energy fence. I guess the main benefit is that it frees up space on the roof for PV, and frees up space in the yard for...tomatoes or whatnot? And having a big cistern in the yard is definitely an intriguing way to do ground source heat pumps, especially in northern climates where air source heat pumps are trickier to do...

desert_sasquatch

@Larry Weingarten that is a better price! I may get the book, then. Though one of the reviewers says it doesn't help with sizing a system / storage. But eh, I can always sell my used copy again, I guess.

About starting small: Hmm, good to know that's an option. I might do that.

That said, I'm a bit concerned that low-temp thermal storage may be trickier to expand after the building is built. I prefer to put that within the building envelope, but naturally it requires a good bit of space to do so. Without a big basement that's not being used (I won't have that) it seems like some forethought is warranted on that front...

Larry Weingarten

Hi @desert_sasquatch , Forethought is always good! Just to throw out another crazy thought, I burn 1/4 cord of wood a year for an 1800 square foot house. It is in a mild climate, but temps have ranged from 18F to 110F. I built with SIPS, using 8" thick walls, 10" thick floor and 12" thick roof. I basically live in a big ice chest, and was criticized for using too much insulation. Not any more however. Doing this made finding any needed heat a simple task.

Similarly, structural waste in hot water delivery systems can account for as much as 80% of the hot water use. Designing the system to hold as few gallons as possible between heat source and end use will give you faster hot water delivery, waste less water and of course reduce heating needs.

Efficiency is a powerful tool that is far easier to use in the planning stage of building. Anything you can find by Bill Shurcliff on efficiency: https://en.wikipedia.org/wiki/William_Shurcliff is worth reading.

Yours, Larry

Jamie Hall

I think the thing which drives me nuts fastest when working with solar power enthusiasts for new construction is that fundamentally it is so simple -- and people insist on making it so complicated. (The underlying problem is not confined to solar power -- I see it all over the place. Medicine is a very bad example...).

First, you have to define what you are trying to do. Heat a house? Get a hot shower? Power a toaster?

Then you need to consider just how much power and energy you need (the two are NOT the same). And this is one of the places that things go bad fast. There are two halves to this. First, what is your energy input? Obviously, if you are looking at solar, the sun. OK then, what are the power characteristics over time, and what is the maximum instantaneous power. Second, what is your energy use? Again, look at the power use over time, and the maximum instantaneous power required. It's usually better to start with the second. Plot the total energy as a function of time. Now take your power input. Plot that as a function of time. Run both out for a week or two -- long enough to even out any variations. Look at the graphs. If over that duration you have used more energy than you have put in, you need to adjust what you put in -- more collector. That's easy. Now look at the two graphs as they go along. The distance between them will vary. Slide the input graph up or down so that it is just tangent to the highest points of the demand graph. Good. Now the distance between them is the amount of storage you will need...

Easier to do than to describe. Several points appear, though. First, you have defined the amount of effective collector you need. Second, you have defined the amount of storage. You're well over half done with the design at this point. Therie is another point to note, though -- if you provide storage at all, the peak output of your collector system does not have to match the peak demand of your facility. Useful to know.

The next step is a little more complicated, but not much. The question you are asking is in what do you need from your output power? Space heating? OK, nice to be between let's say 65 and 80 in terms of temperature. Domestic hot water? OK, that's 140 or so. Want to power that toaster? OK -- now you need electrons. You may find that you need some of each -- and it might be smart to go back and redo your storage question so that you know how much of each you need to store -- and what form (heat or electricity or both) you need to collect.

Now, and only now, can you begin to contemplate what kind or kinds of collectors you want and what kind or kinds of storage you want, and how big they need to be -- and, if you are converting from one kind of collector or storage (say electricity to heat -- heat to electricity is a lot harder) what is associated with that.

There is a prime directive:: Keep it Simple, Stupid. Corollary: if you need a widget to fix a problem resulting from another widget, you probably don't need either widget.

end of rant...

Jamie Hall

Practical example. Well, sort of. Let's suppose I want to power Cedric's home with solar energy. Just the heating and domestic hot water, since that's what I have handy. Over one heating season it uses about ,, 420,000,000 BTU, or 123,000 KWh. What colllector area do I need? Well, I know from another source that I can get 3 KWh per day, on the average, from 1 square meter of direct thermal collector. There are, roughly, 180 days in a heating season around here, so... mumble mumble... i need 230 square meters or 2300 square feet of collector. Assuming I have storage, anything more than that is pure gravy. Very encouraging. As it happens, that's only about three quarters of the total area of the roof, so I'm good to go -- direct thermal collectors on the roof.

Goody! Now some fiddling will get me the needed storage...

Not that I'm going to do it, mind you.

jumper

hot_rod said:

It’s An intriguing technology

https://www.viessmann.family/en/newsroom/solution-offering/viessmann-ice-energy-storage-20-now-in-plastic

There were some attempts at energy fence products being imported back in the last solar wave, 2007- 2010. I saw some clever, modular composite systems at the InterSolar show back in those years

I think that last solar wave featured concentrated solar collectors. Less efficient (first law pov) but more useful temperature wise (perhaps more efficient second law pov). Also there were less expensive less efficient PV panels that may be more practical.

desert_sasquatch

Thanks @hot_rod

If you (or other folks) are willing...what are three or four flat plate solar thermal collector companies that folks like? I realize that I don't even know what brands I should be comparing, for example, the AET collectors to. For instance the SunEarth collectors seem to be about the same price, but with lower shipping because I'm closer to California than Florida. But if there's a probable maintenance issue with SunEarth when used in a drainback system then paying a bit extra for AET could be worth it...

desert_sasquatch

And I'll have to come back to that storage idea. I'm a bit concerned that in this area if I bury a bunch of styrofoam without protecting it from mice--there is a metal exterior to it but they call it an "embossing" so I assume it's basically thick aluminum foil--the mice will just use the styrofoam as a mouse apartment complex . But I'm not sure how to protect it without spending a bunch more money...

Well, other than by putting it in the home. I'd have to think about that. I can always put this or something like it in an insulated and heated crawl space. I just need to figure out how maintenance would work in that situation. And whether I'm comfortable with the chemicals that would be required to prevent bacterial and fungal growth.

On the other hand I did have a thought about the thermal sandbox thing. If gravel generally has higher thermal conductivity than sand, and if high quartz sands have higher thermal conductivity than sands without quartz, then quartzite gravel should have higher thermal conductivity than either. If the conductivities of different gravels scale linearly with the conductivities of similarly-formed solid rocks (quartzite is supposedly 2.5X more thermally conductive than sandstone) then this might be enough of a boost to the thermal conductivity of the storage system such that it becomes possible to cram 20 hours of heat into it during the 4 hours that the panels are cranking. I've found some folks to ask about this; we'll see what they say. And we'll see if it's available locally and at what price.

desert_sasquatch

@Larry Weingarten

Oh yeah, I'm definitely a fan of more insulation. Obviously there's a point where you save more on heating expenses by adding another solar panel and/or thermal storage than you do by adding another inch of insulation but I'm definitely aware of the role that insulation plays in this.

That's interesting about hot water heat loss. Where are you getting that figure from? I checked the EPA (not that I trust them in all things) and they put the figure at around 10-15% of waste in the DHW system through the pipes.

Also how do you bring the DHW source closer to the fixtures? I figure I could put an electric point of use on demand heater on all of the faucets. But it seems like that wouldn't prevent the loss of heat between the DHW tank and the faucet so much as it would make sure that any heat lost that way was immediately made up by electric heating from the grid. And it would do that even if the DHW tank had sufficient hot water for the day, so it would wind up wasting energy on any day that the solar system provided sufficient DHW.

The other question I have is how much heat loss matters if the heat is lost into the home. Obviously during the cooling season that's an additional cooling load, and that's an issue. But there is no cooling season here. So all the heat lost into the home is just space heating by another name. I'm not saying I plan on leaving all the DHW pipes uninsulated--far from it--but I'm just saying that if the pipe going to the shower isn't used for 24 hours and the 0.5 gallons of water in it go from 120 F to 70 F I won't cry over the misallocation of those 208 BTU. I suppose I'd be more concerned with the 125 BTU that gets flushed down the drain (assuming the water was heated from 40 F). And if that happens a few times a day at each sink and once a day at each shower, and there are three sinks and three showers then I guess that does become about 1 kBTU that's flushed down the drain. But that's like...0.3% of a bad day's heating requirement. Maybe 3% of the DHW requirement. I guess I'm not too concerned. Should I be more concerned?

I did actually make it about halfway through Shurcliff's "Super-Insulated Houses". I didn't know the author had such a pedigree though! And that must be where I read about @Jamie Hall's Shrewsbury House. It was fascinating to read about. And very creative!

Personally I have some concerns about condensation and mold when pushing hot, moist air with probably at least a tiny amount of dust into a cool, underground space. I don't know whether a problematic mold would grow or a more benign one but as I'm rather sensitive to some molds I'm kind of conservative with those things. Which is why I have thus far preferred to use a "liquid-in-pipes" transport system for heat.

desert_sasquatch

@Jamie Hall Good to hear someone say it's fairly simple. I definitely agree that with this kind of thing just getting into the ballpark is good enough. Admittedly I am having fun doing my best to drill down into the math of it, though

desert_sasquatch

@jumper are there any residential concentrated solar collectors you're aware of? Especially for thermal hot water? I've wondered about that technology but so far all I've run across is some guy in...Poland or something...with some DIY concentrated solar hot water heating collectors.

hot_rod

desert_sasquatch said:

Thanks @hot_rod

If you (or other folks) are willing...what are three or four flat plate solar thermal collector companies that folks like? I realize that I don't even know what brands I should be comparing, for example, the AET collectors to. For instance the SunEarth collectors seem to be about the same price, but with lower shipping because I'm closer to California than Florida. But if there's a probable maintenance issue with SunEarth when used in a drainback system then paying a bit extra for AET could be worth it...

Sun Earth and Heliodyne are California based. Look like Heliodyne also used a welded on absorber
Really not much maintenance to collectors. If you have a lot of dust or pollen, rinsing them off keep efficiencies up. Performance differences is mostly in the special coatings or the crimping of the absorber to increase surface area. Insulation r value would have some effect on heat loss.

Some brands had plastic backs instead of aluminum to lower that conductive loss. Roth built an entire composite collector.

Low iron solar glass helps also, but it is getting hard to find, if you order a semi load it can be had. Pebbled glass surface helps, less reflection.

hot_rod

desert_sasquatch said:

@Larry Weingarten

Oh yeah, I'm definitely a fan of more insulation. Obviously there's a point where you save more on heating expenses by adding another solar panel and/or thermal storage than you do by adding another inch of insulation but I'm definitely aware of the role that insulation plays in this.

That's interesting about hot water heat loss. Where are you getting that figure from? I checked the EPA (not that I trust them in all things) and they put the figure at around 10-15% of waste in the DHW system through the pipes.

Also how do you bring the DHW source closer to the fixtures? I figure I could put an electric point of use on demand heater on all of the faucets. But it seems like that wouldn't prevent the loss of heat between the DHW tank and the faucet so much as it would make sure that any heat lost that way was immediately made up by electric heating from the grid. And it would do that even if the DHW tank had sufficient hot water for the day, so it would wind up wasting energy on any day that the solar system provided sufficient DHW.

The other question I have is how much heat loss matters if the heat is lost into the home. Obviously during the cooling season that's an additional cooling load, and that's an issue. But there is no cooling season here. So all the heat lost into the home is just space heating by another name. I'm not saying I plan on leaving all the DHW pipes uninsulated--far from it--but I'm just saying that if the pipe going to the shower isn't used for 24 hours and the 0.5 gallons of water in it go from 120 F to 70 F I won't cry over the misallocation of those 208 BTU. I suppose I'd be more concerned with the 125 BTU that gets flushed down the drain (assuming the water was heated from 40 F). And if that happens a few times a day at each sink and once a day at each shower, and there are three sinks and three showers then I guess that does become about 1 kBTU that's flushed down the drain. But that's like...0.3% of a bad day's heating requirement. Maybe 3% of the DHW requirement. I guess I'm not too concerned. Should I be more concerned?

I did actually make it about halfway through Shurcliff's "Super-Insulated Houses". I didn't know the author had such a pedigree though! And that must be where I read about @Jamie Hall's Shrewsbury House. It was fascinating to read about. And very creative!

Personally I have some concerns about condensation and mold when pushing hot, moist air with probably at least a tiny amount of dust into a cool, underground space. I don't know whether a problematic mold would grow or a more benign one but as I'm rather sensitive to some molds I'm kind of conservative with those things. Which is why I have thus far preferred to use a "liquid-in-pipes" transport system for heat.

A DHW recirc loop provides instant hot at the fixtures. I think California energy code requires 1” wall insulation, that along with a smart pump all but eliminates the heat loss from that 120 degree loop. No need to add point of use heaters which also consume electricity and add more heat loss surface area.

When you look at the numbers, those copper tube drain water HX reclaimers are the best ROI of any energy saving devise.

hot_rod

desert_sasquatch said:

And I'll have to come back to that storage idea. I'm a bit concerned that in this area if I bury a bunch of styrofoam without protecting it from mice--there is a metal exterior to it but they call it an "embossing" so I assume it's basically thick aluminum foil--the mice will just use the styrofoam as a mouse apartment complex . But I'm not sure how to protect it without spending a bunch more money...

Well, other than by putting it in the home. I'd have to think about that. I can always put this or something like it in an insulated and heated crawl space. I just need to figure out how maintenance would work in that situation. And whether I'm comfortable with the chemicals that would be required to prevent bacterial and fungal growth.

On the other hand I did have a thought about the thermal sandbox thing. If gravel generally has higher thermal conductivity than sand, and if high quartz sands have higher thermal conductivity than sands without quartz, then quartzite gravel should have higher thermal conductivity than either. If the conductivities of different gravels scale linearly with the conductivities of similarly-formed solid rocks (quartzite is supposedly 2.5X more thermally conductive than sandstone) then this might be enough of a boost to the thermal conductivity of the storage system such that it becomes possible to cram 20 hours of heat into it during the 4 hours that the panels are cranking. I've found some folks to ask about this; we'll see what they say. And we'll see if it's available locally and at what price.

Build an underground storage tank out of ICFs. Get mass and insulation. Foam is attached more by bugs than mice, there are coatings and wraps to prevent that. Usually it happens when the ground contactof foam is moist. Bugs need moisture to tunnel into foam, according to bug experts. I have seen foam board turn into Swiss cheese around foundations that had poor drainage. My own😳

desert_sasquatch

hot_rod said:

When you look at the numbers, those copper tube drain water HX reclaimers are the best ROI of any energy saving devise.

Yeah, I'm definitely planning on getting a wastewater heat reclamation tube (or whatever it's called). Brilliant devices.

desert_sasquatch

hot_rod said:

Build an underground storage tank out of ICFs. Get mass and insulation. Foam is attached more by bugs than mice, there are coatings and wraps to prevent that. Usually it happens when the ground contactof foam is moist. Bugs need moisture to tunnel into foam, according to bug experts. I have seen foam board turn into Swiss cheese around foundations that had poor drainage. My own😳

Definitely something to consider. I'll have to see how the cost of that stacks up with the thermal gravel pit though ( or whatever we're calling it). The gravel pit benefits from using the existing under-slab insulation, the existing roof insulation, and much of the existing foundation wall. Which is why I keep coming back to it. Much of that stuff has to be done again if I put the thermal storage tank outdoors.

My sympathies on your foundation insulation! So many pieces to the puzzle of home construction...

hot_rod

desert_sasquatch said:

hot_rod said:

Build an underground storage tank out of ICFs. Get mass and insulation. Foam is attached more by bugs than mice, there are coatings and wraps to prevent that. Usually it happens when the ground contactof foam is moist. Bugs need moisture to tunnel into foam, according to bug experts. I have seen foam board turn into Swiss cheese around foundations that had poor drainage. My own😳

Definitely something to consider. I'll have to see how the cost of that stacks up with the thermal gravel pit though ( or whatever we're calling it). The gravel pit benefits from using the existing under-slab insulation, the existing roof insulation, and much of the existing foundation wall. Which is why I keep coming back to it. Much of that stuff has to be done again if I put the thermal storage tank outdoors.

My sympathies on your foundation insulation! So many pieces to the puzzle of home construction...

You will triple the conductivity of sand or gravel by adding a few bags of Portland and some water😚 lots of air space between the gravel or grains of sand.

Search Ramlow sand bed, dozens of real life experiences pop up from Fine Homebuilding site to multiple solar chat rooms.

I agree that the entire bed should be insulated, all four sides. Then Pex in the slab. This way with a simple small circ you can manage how the heat comes out of the bed. Where you need it when you need it. The south facing cave dweller lifestyle is so BC.

One owner claimed his bed got to 160f. That could make for an uncomfortable floor temperature in August or September. The plan was to open windows at night to make it livable. Seems to defeat the purpose of storing thermal energy in my mind. Running a 100w solar pump all summer to collect and store, then let it go out an open window!

So the only selling feature of a non Pex slab over non insulated sand bed sand, becomes initial cost, but an unwieldy heat system for the entirety of the building.

Manage, store, and maximize that ‘free” energy.

jumper

desert_sasquatch said:

@jumper are there any residential concentrated solar collectors you're aware of? Especially for thermal hot water? I've wondered about that technology but so far all I've run across is some guy in...Poland or something...with some DIY concentrated solar hot water heating collectors.

Too long ago, maybe ten years, I was at a trade show in Long Beach,California. I remember trough type concentrated solar collector exhibit. Also somebody was offering 9% PVs as well as 18%.

hot_rod

Arctic Solar in Jacksonville makes concentrated ST for industrial use. But I think you are getting way out over your skis thinking you need 400 degree collectors?

Even evac tubes are more than most need or can handle.

you would need special high temperature fluids to work with those temperatures or very high pressures to prevent water or glycol from flashing. Probably not for the novice solar enthusiast 😗

desert_sasquatch

hot_rod said:

Arctic Solar in Jacksonville makes concentrated ST for industrial use. But I think you are getting way out over your skis thinking you need 400 degree collectors?

Oh, are they all for high temperature applications? Duh, I guess that makes sense. I was just thinking that since panels become a lot less efficient in the winter due to the temperature differential that concentrated solar would avoid a lot of that due to greatly reduced surface area for the heat to escape though.

But yes, I don't want to complicate things (and potentially increase maintenance issues) by going with really high temperature collectors....

desert_sasquatch

To bring the conversation back to the original topic: Ramlow and Nusz say that they don't like using pumps with cast iron housing on solar thermal loops because they cause galvanic corrosion. I assume this is when they are in contact with copper pipes?

But what happens if the pipe that the pump is mounted on is PEX? Or is there a reason not to use PEX in a solar thermal loop? I know it doesn't do well with temps over 180 or so...