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

Larry Weingarten

Hi and yes. Boxed and glazed collectors can get up to 400F. PEX may not like that much. If you used lower temperature collectors, like I mentioned in a blog post a while back in this thread, you could use PEX because it will never see temps over around 175F.

If it's a closed loop, with no little to oxygen getting in, cast iron could be okay. That likely would only apply if it were a glycol or other antifreeze loop.

Yours, Larry

desert_sasquatch

@Larry Weingarten

You're saying that Oxygen concentration impacts galvanic corrosion of metals in pumps, separately from corrosion caused by oxidation? Or are oxidation and galvanic corrosion connected in a way that I, with my very minimal understanding of chemistry, don't understand? Sorry if I'm garbling this.

I googled around a little looking for information about oxygen and galvanic corrosion and found this document but other than understanding that galvanic corrosion can be an issue with spinning copper or iron disks when the oxygen concentration gradient across the disks caused by the spinning motion creates a charge gradient across the disk I'm mostly left with the impression that it's complicated and I have no idea how much difference various oxygen concentrations would make on galvanic corrosion of an iron pump housing.

I'm also curious what you think about oxygen in drainback systems (since that's what I'd be using here). Does the small amount of oxygen in the drainback tank cause a problem or is it removed fairly quickly, effectively becoming a non-issue?

hot_rod

If it is a closed loop system, ferrous metals are fine. If it is an open unpressurized , you want stainless or composite

No Pex in the solar thermal side of the system

desert_sasquatch

Ah, ok, thanks @Larry Weingarten and @hot_rod. So a glazed collector--which I'd need in climate zone 6--will potentially get too hot for PEX. Though if I understand it correctly, that's really only if the pump fails or I stop pumping because it's summer and I've got all the heat I want. So it'll happen, but I guess I hadn't been thinking of it because it wouldn't happen when the system is active. (That's not saying this is unimportant, just explaining to myself why I was confused).

So that means that if I wanted to use a thermal sandbed to store the heat I would need some kind of tank in the middle that could accept hot water from the collectors via metal pipes so I could use the more temperate water (lets call it 90-150 F) to heat the thermal sandbed. IE panels --> mixing tank --> sandbed storage. (And yeah, the radiant floor would also connect to the mixing tank).

Which...is there even a controller that would be able to direct that?

Perhaps this is another reason why people generally use water tanks for thermal storage?

Larry Weingarten

Hi @desert_sasquatch , Just to touch on one point, water has about five times the thermal capacity of sand. Wet sand works better than dry sand for heat transfer. I like to think of water as thermal mass that you can move around for getting or giving BTUs. Yes there are pipes and pipes can leak, but the thermal system using water allows for more precise control than sand has been able to do. Maybe I just like water cuz I'm a plumber

Yours, Larry

psb75

You're last thought is 'spot on.' NO PEX in the solar thermal loop! Temperatures can approach 300°F (well above steam) in a solar thermal system, especially during stagnation episodes.

hot_rod

When the system is flowing I doubt you will ever see over 160 or so even in summer. The 300° plus is under stagnation condition. And you would only see a slug of a gallon or so as the pump will flow colder water when it starts again. That short hot slug however will soften Pex and cause it to rupture. From first hand experience
Some installers run 10' of copper from the collector and transition to pex. Not worth the $$ risk in my mind?

The tube in a tank or in the sand bed could be Pex.
Tom offered Pex coils in his unpressurized solar tanks. As I recall it takes 3 times as much Pex to get the heat transfer of copper coils in a tank. 300' of copper coil or 900' of pex.

Here is the potential stagnation temperature for a typical glazed plate type collector. 85° ambient, full sun condition.

desert_sasquatch

@Larry Weingarten oh yeah, I've been playing around with the numbers on this. Interesting to me that gravel has a higher thermal conductivity than sand does, seems like we should be talking about gravel beds for thermal storage rather than sand beds... But yes absolutely it does have a lower heat capacity. When I took the heat capacity and mass into account I found that, per unit volume, I needed about three times more gravel than I would water.

The big thing that has me still wondering about the thermal gravel bed instead of a huge water tank is that I *think* I can do an equivalent amount of gravel for about the same price as I'd pay for storing water. And unlike the propane tank, the PEX--and by extension the gravel pit--should last longer than the panels. (Assuming I protect the PEX from super-heated water, I guess.)

I know I'm being vague; I'll make a post and ask questions about the whole thing in more detail in a new thread once I've actually gotten to the part of Ramlow and Nusz's book where they describe thermal sandbeds. Because obviously right now I'm just guessing about how it should be done...

desert_sasquatch

@hot_rod OK, thanks. So it seems like this probably doesn't kill the gravel bed heat storage (I know it's not the favorite option on here but I'm going to doggedly keep investigating it until I find a reason that it's not the best option for me ). I would simply need for hot water run from the panels to be copper, and that run would have to pass through some kind of intermediate heat bank designed to absorb the excess heat from a high-temperature slug of water. If the heat bank absorbed enough heat from that very hot gallon then the pipe could transition to PEX. Which is what the gravel bed heat bank would use.
For instance, the intermediate heat bank could be made by placing one of those coiled copper heat exchangers in a lightly-insulated barrel filled with gravel. Or water, but gravel seems lower-maintenance, even if it takes up more space. It seems like that would solve this problem? Obviously there's some math that would have to be done but I figure that oversizing the component by a bit wouldn't be too spendy or difficult since we're only dealing with a gallon or so of very hot water...
Or is there another solution that folks prefer? Other than using a water tank for the main heat bank, which I am considering but just not in this post

hot_rod

Keep crunching numbers, you need to think in terms of multi thousand gallon water tanks to store days worth of heat. A 55 gallon barrel may only store 15- 30 minutes worth of heat. 500 gallons would run my small shop 20k load through the night. I could dump my wood boiler and solar into it to recover quickly. Solar alone might need several days to bring it back to useable temperature.

The main concern I have with sandbed is early fall t your slab could be in excess of 90 degrees, no way to shut it off. So the space could be uncomfortably warm for some of the year.

I have an article somewhere where a builder completely insulated around the sand bed to be able to better regulate it. Basically adding 2” over the top to separate from the slab 

desert_sasquatch

@hot_rod sorry if I didn't explain that well, the 55 gallon barrel thing was just to absorb the excess heat from the gallon or so of 300-400 F water on startup. So that it doesn't melt the PEX. But I do see now that, filled with gravel, a 55 gallon barrel would be unable to absorb sufficient heat unless the water was moving more slowly through the pipe than would be practical or the copper pipe was very very long (which would become expensive).
Sorry about that, I do try to do my research before I ask questions but sometimes in trying to explain where one weird question comes from I wind up creating a whole other tangent on a topic I've only thought about in vague terms...
Speaking of which...
Obviously one solution is the one yall have been suggesting, which is to use a water tank as the main heat bank. But for the sake of investigating this gravel bed heat bank thing to the end (which still looks to my untrained eye to be cheaper and more durable than a propane tank -based heat bank): It seems like a 50-100 gallon metal water tank should be enough to absorb the heat from a gallon or so of 400 F water. In other words if I put a tank between the solar panels and the heat bank, would that reduce the temperature shock on startup and protect the PEX in the heat bank?
I'd need some way to be confident that the hot water would mix properly before coming out of the tank...
Or maybe it'll be fine as long as the hot water inlet is at the top and the less-hot water outlet is on the bottom of the tank. And as long as the hot water is deflected via a T junction (as shown in...some picture I saw in some idronics, the idea being that the direction of the jet as the water leaves the pipe would be perpendicular to the vertical). Could I then hook up a PEX tube to the tank (or to a pipe coming directly off of the tank)?
I hope that made sense.
Also I hear what you're saying about the issues with the typical sandbed storage where there's no insulation between the sandbed and the heated concrete floor. I'm definitely planning on insulating the gravel on all sides.

hot_rod

desert_sasquatch said:

@hot_rod sorry if I didn't explain that well, the 55 gallon barrel thing was just to absorb the excess heat from the gallon or so of 300-400 F water on startup. So that it doesn't melt the PEX. But I do see now that, filled with gravel, a 55 gallon barrel would be unable to absorb sufficient heat unless the water was moving more slowly through the pipe than would be practical or the copper pipe was very very long (which would become expensive).
Sorry about that, I do try to do my research before I ask questions but sometimes in trying to explain where one weird question comes from I wind up creating a whole other tangent on a topic I've only thought about in vague terms...
Speaking of which...
Obviously one solution is the one yall have been suggesting, which is to use a water tank as the main heat bank. But for the sake of investigating this gravel bed heat bank thing to the end (which still looks to my untrained eye to be cheaper and more durable than a propane tank -based heat bank): It seems like a 50-100 gallon metal water tank should be enough to absorb the heat from a gallon or so of 400 F water. In other words if I put a tank between the solar panels and the heat bank, would that reduce the temperature shock on startup and protect the PEX in the heat bank?
I'd need some way to be confident that the hot water would mix properly before coming out of the tank...
Or maybe it'll be fine as long as the hot water inlet is at the top and the less-hot water outlet is on the bottom of the tank. And as long as the hot water is deflected via a T junction (as shown in...some picture I saw in some idronics, the idea being that the direction of the jet as the water leaves the pipe would be perpendicular to the vertical). Could I then hook up a PEX tube to the tank (or to a pipe coming directly off of the tank)?
I hope that made sense.
Also I hear what you're saying about the issues with the typical sandbed storage where there's no insulation between the sandbed and the heated concrete floor. I'm definitely planning on insulating the gravel on all sides.

I guess I'm not clear on why you will have that high temperature 300°slug?

Pretty much all solar controllers have over-heat protection built in now a days.
Unless you have frequent power outages? If so add a power backup, like you have on your computer. A rare pump failure would be another reason to expect stagnation.

Over heat protection works like this.
Suppose the pump turns off if it sees the tank or storage is up to temperature.

The control watches the collector temperature always. If it sees the temperature getting up around 250°F (adjustable) it kicks the pump back on for a short period to keep the collector from getting too hot, usually a few minutes until the collector sensors drops.

Now with a small tank it could run that tank up to 160F or higher if there is no other load, as it keeps shoving temperature into the tank whether it needs it or not. Summertime, no load, for example.

The control also has a tank cooling function. If it sees the tank has gone beyond the setpoint, it will run the pump again at night to cool the tank down, down to maybe 100F. So now the next sun up, you can do a harvest again.

The key always with ST is use it or lose it. You like a daily consistent load, hence the large storage beds.

If you have a large bed storage obviously you just run the pump whenever there is gain to be had. You would not enable night time cooling. The key to sand bed, and also the challenge, is they grab heat whenever the sun shines. So come fall you could have a excessively warm bed of sand or gravel under your building.

Think of hot beach sand, or a pool deck in Phoenix. At some point it becomes to hot to walk on in bare feet.

There is another option for collector over-heating called a steam back system. When the pump turns off, the fluid in the collector turns to steam and pushes the fluid in the collector piping back to a generously sized solar expansion tank. It requires a special ultra high temperature fluid for that system.

Jamie Hall

Frist, your 400 gallon tank will absorb about 3,000 BTU per degree F temperature rise. Do your math based on that.

I have encountered the occasional sand bed (saturated sand; if it isn't, it will be) storage system for hot water based solar heating (I've also encountered gravel storage bins for hot air systems -- they work moderately well). I have yet to see one which had solved the problem of reasonably rapid heat input and extraction. Since there is negligible convection, even in a saturated bed (there is none in a dry bed), you are entirely dependent on conduction, which is painfully slow and even with a very closely spaced piping grid results in warmer and cooler areas, as well as stunningly slow recovery.

This is not to say that mass solid storage of heat, if it is tightly integrated with the collector area (perferably part of it -- such as mass floors, or Trombe walls or the like) and is INSIDE the conditioned envelope doesn't work well -- it does, and indeed is a major part of the whole concept of passive solar heating. Where it doesn't work well is in systems where the heat is collected in one place, transported to another place to be stored, and then transported again to a third place to be used. The losses in transferring in and out of storage, never mind the complexity (and waste or space) of such systems just doesn't sit well with me -- it's so unnecessary.

desert_sasquatch

hot_rod said:

I guess I'm not clear on why you will have that high temperature 300°slug?

Oh gosh, did I misunderstand what you said previously? I was referring to what you said here

hot_rod said:

When the system is flowing I doubt you will ever see over 160 or so even in summer. The 300° plus is under stagnation condition. And you would only see a slug of a gallon or so as the pump will flow colder water when it starts again. That short hot slug however will soften Pex and cause it to rupture. From first hand experience

In other words, I was trying to figure out how to prevent the problem you described where the gallon or so of rather hot water coming from a stagnant panel melted PEX tubing. A gravel bed heat bank would need, as far as I can tell, PEX tubing. Because the gravel is not a fantastic conductor of thermal energy if I were to use copper pipe spaced pretty far apart it wouldn't be able to absorb enough of the heat. Whereas if I use PEX I can afford to space it much more closely, which I believe mostly negates the issue with mediocre thermal resistance because whenever I double the thickness of a material I double the absolute thermal resistance.

hot_rod

The solution is all copper tube, steel, stainless really any metallic tube.
The Watts Onix was rated for 300° for a period of time when they promoted it for solar thermal. It can handle intermittent slugs at that temperature. I suspect the attorneys saw that rating and flipped out It's back to 200°F.

What you will find is if that hot slug needs to travel 10, 20 feet or more, it blends with the cooler fluid still in the piping, probably fluid at ambient temperature. And the 300° slug of a gallon or so is diluted rather quickly.
I've never measured 300° back in the mechanical room if the fluid has traveled any distance. So copper from the collector down into the space, pex in the tank or sandbed will work.
Or if that frightens you use all copper.

hot_rod

Zilmet has this cool down tank for their solar thermal expansion tank protection. I don't know if any made it to the US. I think it is just a straight through tank, nothing inside. Easy enough to build from a piece of pipe.
It is just enough cool down "space" to protect the solar expansion tank diaphragms.
I'd use it on a steam back system.

hot_rod

From a Green Builders magazine article 2011. Many responses to the comments, as you might imagine. It may still be in their archive?

desert_sasquatch

Thanks @hot_rod ! A 20' copper pipe leading off the array is very doable. and if I was nervous I suppose I could increase the size of the pipe for some of that length. Anyhow, if you say that's enough to protect the PEX from melting then I believe you.

I've said it before but... I really appreciate this website and all the expertise folks are willing to share.

A bit of a tangent but... you mentioned onyx tubes. They're made with EPDM, yet there's one maker of solar water tanks who swear that EPDM breakdown products will prevent copper or maybe even stainless steel from creating the protective oxide layer. I'm curious if this is an issue you (or other folks here) have come across. http://www.americansolartechnics.com/helpful-heating-information/why-we-don-t-use-epdm/

desert_sasquatch

And thanks @hot_rod for sharing the paper from Fine HomeBuilding.

It's actually really helpful to get a bit more perspective on where folks are coming from when they say they prefer solar PV + heat pumps rather than solar thermal. Especially Martin Holladay, I've read some of his articles and subscribed to his website, even.

I will say though that I feel like the case they're making seems like it applies most strongly to the northeast USA, which I think is where most of these guys live. Especially the part where he poo poos a heat bank by explaining how it won't store enough heat for the whole winter. I was surprised to see that. Perhaps that really was what some people were expecting, a whole winter's worth of heat stored in the bank? If so it seems like that's not a common strategy these days, though, right?

But I certainly can see that in more polar latitudes grid-tied solar PV would be great because they could take advantage of the long summer days. Whereas closer to the equator that's not as big of an issue. And maybe winters are quite cloudy in Maine, that would impact this kind of thing too.

desert_sasquatch

OK, so to return to Ramlow and Nusz:

They describe a solar relay controller that controls two relays, one of which circulates water into a shunt load so that the system won't stagnate in the heat of the summer. They say this will extend the life of the solar fluid...but is this only for systems that use antifreeze? I'm thinking I'll probably use a drainback system and I thought that those didn't require antifreeze.

desert_sasquatch

Also, Ramlow and Nusz prefer aquastats to snap discs as temperature-triggered switches. They say that snap disks are not as reliable. Do folks here find that to be true? And how would folks characterize digital aquastats? Seems like when they were talking about the reliability of aquastats they were describing mechanical ones--the ones with some fluid that expands as it's heated. So what do folks think of the digital ones? Are they as reliable?

Jamie Hall

One might wonder why my father-in-law and I, and others such as Bill Shurcliff, kind of got discouraged and pretty well dropped out of the whole solar heating thing. Passive solar is so simple and so completely reliable and so easy to execute and works so well (though I will admit that the most northern building we did was in northern Maine, near Fort Kent)... but everyone wanted the fancy and complicated and whizzy.

There are a few fully passive buildings -- mostly residences -- being done every year, but we just couldn't compete with the pumps and tanks and controllers and all that which never worked as well... so we pretty well quit.

JakeCK

Jamie Hall said:

One might wonder why my father-in-law and I, and others such as Bill Shurcliff, kind of got discouraged and pretty well dropped out of the whole solar heating thing. Passive solar is so simple and so completely reliable and so easy to execute and works so well (though I will admit that the most northern building we did was in northern Maine, near Fort Kent)... but everyone wanted the fancy and complicated and whizzy. There are a few fully passive buildings -- mostly residences -- being done every year, but we just couldn't compete with the pumps and tanks and controllers and all that which never worked as well... so we pretty well quit.

Is it possible it isn't so much that people liked the fancy/complicated/expensive and more a consequence of not every lot and not every aesthetic being a good fit for passive solar heating? For example my tiny lot will all the houses packed in close with mature trees surrounding everyone is not the best by a long shot. I get a good amount of solar heating in the morning when it comes in the front window but by noon in the dead of winter my neighbors house is shading all but the gable of the house. My neighbor on the other side of me gets nothing because he has a bungalow and its considerably lower then my house. Come summer tho you bet your butt that my house is getting baked. And the master bedroom gets the most of it too.

Jamie Hall

Bottom line was -- and is -- if a lot (such as it might be yours) gets enough sun at some point during the day to even consider any form of solar heat or any usable PV, it can be done with passive. In fact, since the capture and storage efficiency of a well designed passive installation is greater than can be achieved with any remote collector/storage/distribution system, it's easier.

That said, however, retrofits are difficult -- and often impossible. Which is a limitation.

JakeCK

Jamie Hall said:

Bottom line was -- and is -- if a lot (such as it might be yours) gets enough sun at some point during the day to even consider any form of solar heat or any usable PV, it can be done with passive. In fact, since the capture and storage efficiency of a well designed passive installation is greater than can be achieved with any remote collector/storage/distribution system, it's easier. That said, however, retrofits are difficult -- and often impossible. Which is a limitation.

Except in my case the roof gets plenty of sun, Just not the sides of the house. So unless someone in a situation like mine wanted a bunch of sky lights pv or thermal solar might be a better alternative. 

Another example would be townhouses or row houses where the sides are attached. Unless the front or back is facing directly south passive solar would not work. But PV or thermal on the roof would.

And that still doesn't address the aesthetics. Maybe someone just doesn't like the look of a cave like box with large windows facing one direction. 

hot_rod

Here are some other opinions on sand bed systems. From Green Builder magazine 2011

desert_sasquatch said:

And thanks @hot_rod for sharing the paper from Fine HomeBuilding.

It's actually really helpful to get a bit more perspective on where folks are coming from when they say they prefer solar PV + heat pumps rather than solar thermal. Especially Martin Holladay, I've read some of his articles and subscribed to his website, even.

I will say though that I feel like the case they're making seems like it applies most strongly to the northeast USA, which I think is where most of these guys live. Especially the part where he poo poos a heat bank by explaining how it won't store enough heat for the whole winter. I was surprised to see that. Perhaps that really was what some people were expecting, a whole winter's worth of heat stored in the bank? If so it seems like that's not a common strategy these days, though, right?

But I certainly can see that in more polar latitudes grid-tied solar PV would be great because they could take advantage of the long summer days. Whereas closer to the equator that's not as big of an issue. And maybe winters are quite cloudy in Maine, that would impact this kind of thing too.

There are answers to all your questions. You are by no means charting new ground with your concept. Ramlow and Ben are in upper Wisconsin, many of their designs are around the WI,MN area. Not the sunniest winter climate🤓

Really what you need to do is start with a load to shoot for. It could be a calculated load based on a home not yet built,or a current home. Only with an accurate load can you design the most optimized system. The collectors, the storage, the back up needs all revolve around the load number.

To fine tune the system design even more, download the days of occurrence data, see home many days you are at design, above, below, and by how much. Most areas only see design days for a small portion of the heating season.
So really no need to design storage and collectors based on design day expectations all heating season. Perhaps some cold days you need to run the wood stove all day.
You would end up with a ma$$ive system if you try to design to the design condition.

As I may have mentioned, the solar thermal contractors I know in the sunniest SW areas, rarely design for more than 30% of the load. And they have customers with blank checks🤭

No need to deal with frequent stagnation conditions that an oversized system will experience.

hot_rod

I grew up in western NY I was very surprised when I first saw this data for Syracuse, typically a 0 to -5 degree design area.
Two take always, how well a modulating boiler suits this type of climate, and how few days at or below design.

Jamie Hall

I give up.

Here is a passive solar house I built in northwest Connecticut. Looks really weird, doesn't it?

JakeCK

Jamie Hall said:

I give up. Here is a passive solar house I built in northwest Connecticut. Looks really weird, doesn't it?

No offense was intended. That is a fine looking house. But it is not a house everyone would want. Some like Tudors, some like Greek revivals, some like contemporary ranches... 

And to elaborate on your point about complexity, that house I bet has a lot of precise engineering that must be closely followed and maintained. For example high performance houses need to very carefully manage moisture right? Proper location of vapor barriers, ect are critical. Where as a standard construction house is much more robust when it comes to mistakes during construction and the effects of age. 

I also see a lot of roof space for Pv and thermal solar. Lol

desert_sasquatch

hot_rod said:

As I may have mentioned, the solar thermal contractors I know in the sunniest SW areas, rarely design for more than 30% of the load. And they have customers with blank checks🤭

No need to deal with frequent stagnation conditions that an oversized system will experience.

Can you say a little more or recommend some reading? I'm not sure I understand the whole "stagnation" issue. Is it just what happens when your heat bank is so large that a lot of the time it doesn't get heated to a temperature where there's usable heat in the heat bank?

desert_sasquatch

@hot_rod I do see that there are diminishing returns as the design load for solar thermal approaches 100% of the heating requirement.
And also for what it's worth, I think I may have been overestimating the amount of heat I'd get on a truly cloudy day in winter. At least, I did some experimenting with a light meter and rather than getting 50% less light on a cloudy day, I would get...seemed like maybe 90% less light. Whereas the "cloudy" column on the SRCC chart assumes 50% less light. So that may explain my misplaced enthusiasm for a 90% solar fraction using lots and lots of panels.
Still, I haven't actually looked into what happens if I assume I can capture 3-4 days of heat in the heat bank (assuming there's a few days of nearly full sun, which is not uncommon here in the winter).
Aaanyhow, a detailed exploration of the cost benefit situation is a rabbit hole I'm not quite ready to go hop down. As you say, Ramlow and Nusz (and folks here) have thought about a lot of this already, so I'll work at finding out what their solutions were/are before I try to come up with my own...

Jamie Hall

Quite right, @JakeCK -- saltboxes aren't to everyone's taste! My father-in-law's house -- also passive solar -- was a very contemporary house with an almost flat roof. You are also right that attention to detail, particularly in construction, is mandatory -- things like vapour shields and the like have to be done right. You can't just slap it up and hope.

There would be ample room for a lot of PV. At the time we built that house (1985) PV just wasn't a thing. Could be retrofitted, though. It doesn't need any more thermal, though, although it doesn't have domestic hot water collection (if I were to do it today, it would have a heat pump hot water heater and enough PV to drive that).

With regard to one of @desert_sasquatch 's comments, the key to successful solar design -- passive or complicated -- is not so much the collectors -- there's ample sunshine, even here in the cloudy northeast -- but storage, and the name of the game is mass. Ideally, the storage will be built into the structure of the house, inside the envelope, and also ideally you want enough of it so that it can be at very close to normal temperatures for the use of the house. There is a problem with that, however, which I freely acknowledge: allowable temperature swing. If one wants a very tight temperature control -- say within a degree or two -- as many people assume is "normal", then clearly it can't be truly integrated into the envelope. If one can tolerate a wider swing, then it becomes easier -- and the wider a swing that can be tolerated, the easier it gets. I think this is why the default approach is active, and heat is stored (often in big superinsulated tanks) at a relatively high temperature, and metered out with active circulation to the living spaces. This increases the complexity (and cost) considerably, and also decreases overall collection efficiency. The house in the picture, for example, has a total of 100 tons of concrete which is the primary thermal storage. That stores about 70,000 BTU for every degree of temperature change. Since we (and the subsequent owners) could tolerate a 10 degree swing (75 to 65), this is enough for 2 days with no sun at all at design conditions.

Also to I think Jake's comment -- passive is very difficult to retrofit.

psb75

A timber frame within the house envelope is a whole lot of thermal mass that stick-frame houses don't have available. Large masonry mass in the form of a chimney and/or masonry heater in the core of the house is even more.

hot_rod

desert_sasquatch said:

hot_rod said:

As I may have mentioned, the solar thermal contractors I know in the sunniest SW areas, rarely design for more than 30% of the load. And they have customers with blank checks🤭

No need to deal with frequent stagnation conditions that an oversized system will experience.

Can you say a little more or recommend some reading? I'm not sure I understand the whole "stagnation" issue. Is it just what happens when your heat bank is so large that a lot of the time it doesn't get heated to a temperature where there's usable heat in the heat bank?

Stagnation is the condition where there is no flow thru collectors on a glycoled system.
It can happen in summer when there is no load.
It can happen anytime the loads is heated and satisfied.
It can happen is a component fails, control pump, sensors.

You want to limit this condition as much as possible if you have a glycoled system.
If you go with drain back or steam back, stagnation is not a concern, as the collectors empty out in any of the above noted conditions.

hot_rod

desert_sasquatch said:

@hot_rod I do see that there are diminishing returns as the design load for solar thermal approaches 100% of the heating requirement.
And also for what it's worth, I think I may have been overestimating the amount of heat I'd get on a truly cloudy day in winter. At least, I did some experimenting with a light meter and rather than getting 50% less light on a cloudy day, I would get...seemed like maybe 90% less light. Whereas the "cloudy" column on the SRCC chart assumes 50% less light. So that may explain my misplaced enthusiasm for a 90% solar fraction using lots and lots of panels.
Still, I haven't actually looked into what happens if I assume I can capture 3-4 days of heat in the heat bank (assuming there's a few days of nearly full sun, which is not uncommon here in the winter).
Aaanyhow, a detailed exploration of the cost benefit situation is a rabbit hole I'm not quite ready to go hop down. As you say, Ramlow and Nusz (and folks here) have thought about a lot of this already, so I'll work at finding out what their solutions were/are before I try to come up with my own...

Solar fraction is the % of the load that the solar is able to cover.

If your heating load is 60,000 BTU/hr, cover 30% or 18,000 worth of solar collection.

There is no rule about SF, the numbers I have come from solar installers that learn what % works best in their area. This is for lessening stagnation, roof space available as well as cost vs benefit.
Cedar Mt Solar did a lot of solar work around the NM area both heating and DHSoW, that was their rule of thumb number.

Remember also that solar energy is long wave radiation, beyond the light spectrum. It's not visible to the human eye. It doesn't need to be a bright clear day to still have some available energy.


Typically measured with a solar irradiance meter. Some of the name brand meter companies have solar meters. Your nearest weather station capturers that data also. Websites like NORA and NASA have that data. And it is all wrapped up in the solar simulation software packages.

Some favorite installation. The East/ West is a system I put on my neighbors home.

The 8 unit apartment building is in Switzerland. 100% year around ST for heat and DHW. They had extra energy and piped it to neighbors.

To accomplish that took a 54,000 gallon tank! Built into the ground up to the roof of the building. Actually delivered and installed by hand by the residences of the village.

Some other mount options. Reverse pitch on a north facing roof, Clarence Beaver a longtime solar guru in NC, RIP:(

The high tilt is a factory in Germany that made our SolarFlex. The steep pitch is best for heating, catches the low angle of the sun, and helps prevent overheating in the summer when the sun is high in the sky. Vertical mounts work well form heat only loads, a radiant floor day care building in NM.

A cabin in WI where the ground mount also held and shaded the AC.

The ballasted sawtooth mount was a way around the neighborhood covenants where nothing on the roof could be visible from the street in front of the home. 2X10' collectors

I used to teach an 8 hr solar thermal class, these are some of the "glamor" slides.

Jamie Hall

We used to call my father-in-law's house Experimental Manor, @hot_rod -- it had a flat roof (well, very slight pitch to drain to the north) and, since there was ample collector area from the south wall and windows to keep the family happy we could -- and did -- play with all sorts of collector designs on the roof. Lots of fun!

JakeCK

Reading all of this really has me curious about how much I could heat this house using solar thermal...

My neighbors already think I'm a bit eccentric, boarder line crazy. Whats one more project/experiment. lol

desert_sasquatch

hot_rod said:

Solar fraction is the % of the load that the solar is able to cover.

Yeah, that's what I meant. Er...justifying my 90% solar fraction dream will probably take a lot of space and involve a lot of assumptions that folks here will want to question I figure I'll do it on another thread. And tag you, if that's alright.

hot_rod said:

Remember also that solar energy is long wave radiation, beyond the light spectrum. It's not visible to the human eye. It doesn't need to be a bright clear day to still have some available energy.
Typically measured with a solar irradiance meter. Some of the name brand meter companies have solar meters. Your nearest weather station capturers that data also. Websites like NORA and NASA have that data. And it is all wrapped up in the solar simulation software packages.

Oh. OK. So the lux meter I got was the wrong tool for the job because it only measures visible light? And would you say that on a fully cloudy day there's about 50% of the solar radiation available as on a fully sunny day? (This might seem like a dumb question but the lux meter was showing that on a cloudy day there was about 10% of the visible light that I would have had on a sunny day when facing the sun directly).
The switzerland tank is amazing!
The vertical mount panel design makes sense, I was thinking I'd do something like that. Well, I was thinking 63 degrees at my latitude. But closeish...

hot_rod

Really no need to measure your self, NASA has been doing that for you, probably with more suitable meters, and over 20 years or more.
The SRCC shows collector performance under various conditions.

JUGHNE

This is our version of a passive solar heated house, we wanted something to look in place in the area but still take advantage of solar heat.

This picture was taken June 21, the summer solstice, you can see all the major south windows are shaded from sun. On Dec 21, the winter solstice, all glass is exposed to the sun. The sun reached about 17' into the 20' sun room.

That room is a 6" pour with ceramic tile installed, also about 1000' of tubing in the concrete. There is about 4' of sand under that. The outside foundation walls are insulated down to the footings.

The only difference in the structure is that the trusses are designed so that the inside 8' ceiling and the outside soffit ceiling are at the same level. There is a 30" overhang that provides the shade in the summer but allows wintertime penetration.

This 30" overhang was calculated and drawn out on graph paper based on our latitude, 42-43 degrees, and also the bottom of the windows. The first floor windows and patio Frenchwood door were placed under the standard 12" headers as usual.

If you look closely there is about 12-14" of wall above the windows, the result of truss design.
Most ranch houses here have only 2-3" of wall above the windows on the outside.

We started planning this in the early 90's and constructed in 1995.

We studied "The Passive Solar Energy Book" by Edward Mazria, published by Rodale Press in 1979. (this really dates me, doesn't it),
687 pages, packed with info.......old book but the sun is still working as it was then.

For passive cooling all rooms have small north windows that allow the summer breeze to flow thru the house. We did not get the AC until 2000. Something about aging changes things for the fairer sex.