Solar Heating - Packed (Pebble) Bed Storage?

I'm in the process of a solar thermal heating system, using vacuum tubes mostly.

The daytime system is pretty straightforward - run water through the collector to heat it, then through a radiator inside.

The more interesting part is the overnight system. It will collect and store heat during the day, for extraction and use at night. The basic idea is to have an insulated container of rocks that you blow hot air down into during the day. Air temps of over 80 F are best - not a problem with vacuum tubes, even in freezing weather. Air flow is reversed at night. Hot air comes out the top while (cooled) indoor air comes back in the bottom.

There have been lots of prototypes built and tested for large-scale use. It turns out that those prototypes are about right for residential use, usually around 5 cubic yards of gravel.

Any experience or thoughts about that?

I know a couple people that have electric heated gravel beds--they were built into the basement/crawlspace and super insulated on the floor and walls and open to the top. Heated with electric heaters when there was a surplus of power (for very little money) and then the heat just radiates up into the living space. One did have an air loop running through it to push the heat to the far end of the house, but not sure if they actually used it (they had a woodstove on that end of the house).

They used a lot more gravel than that, but not sure of the cu yd of gravel to sq ft of house--plus they were in Minnesota.

Paul has done a lot of the work with his pebble bench as far as the performance of the heat storage and transfer properties.

Thanks.

The bed sizing is controlled by the temperature it can be raised to. Some industrial applications can use 500 C, but that's probably an upper limit for rock integrity (thermal cycling) and starts to get into failsafe problems. If you lose control of heating circulation, you don't want to have passive convection run away with a 500 C bed of rocks.

The storage vessel can be made for horizontal air flow, but vertical is ideal. Hot air charging goes down, discharging goes up. The temperature of discharged air remains pretty constant as the cool "front" slowly advances up through the bed.

The usual problems encountered in basic residential systems are related to temperature mismatches - charging with air that is too cool, allowing the bed to condense water, etc. Not too hard to avoid those with basic electronics. I'm instrumenting my stuff for temperature and will be using PLCs and relays to control fans.

Air to rock is pretty terrible heat transfer. Air doesn't hold much heat, rocks hold a ton, so you need a LOT of flow, and the rocks will condense the water out of the air, which then promotes mold, which you have no way of cleaning out. For example, a cubic foot of rock requires 20 BTUs of heat to change 1°F, A cubic foot of air contains 0.02 BTUs of heat per °F. Air at 80°F and rock at 50°F means that 33 cubic feet of air must be moved to raise that rock 1°F. You are going to need tens of thousands of BTUs to heat the building over the night.

Water storage is my first choice, Large safe storage (i.e. no risk of large leaks), I would use water to rock (sand really). Preferably storage into the actual thermal mass of the structure.

That said, I don't think you should worry about overnight heat storage. Insulate the house properly, Reduce air leakage (and pre-heat incoming air), add the right amount of thermal mass, and the difference in heat overnight shouldn't be uncomfortable.

Topher Belknap wrote:You are going to need tens of thousands of BTUs to heat the building over the night. . . . .

Heat collection is available. It works out to two systems: one for day, and a second to charge the overnight system. Heat collection for the overnight system needs to be 3 or 4 times the day system, if memory serves (been a while since I've gone through the calculations). Getting through the night in cloudy weather in January is the controlling design problem.

The condensation is a central issue. Inadequacies in control can cause condensation, and I'm not aware of any commercially available control equipment that can do it. What I am doing is designing PLC-controlled custom one-off equipment where condensation will not be allowed. It's another reason for vacuum tubes too. The collector I'm using now will build up to 200 C in direct sunlight easily with air in the manifold. I'll need to do some testing to establish the air temperatures I can actually get, but I don't expect they will be higher than that. My mention of 500 C is just because that's what some industrial systems do. They use concentrated solar to reach those temperatures.

But yes, bad air quality would be a deal breaker. It's something I've not done elaborate design calculations on, but it could potentially cause a shift to a different storage material.

Water certainly holds a lot of heat, but not a stable temperature. Paraffin is theoretically nice that way; phase change is absolutely constant temperature. I don't like the fire hazard though.

I have used activated alumina as a thermal store using solar to dry the alumina during the day then using moist air to recover the heat overnight.

My parents built a house with rock thermal storage in the 1980's (in Wisconsin). Basically, there was 60 tons of 1-2" gravel that comprised most of the basement and warm air was drawn from inside the house near the peak (steeply pitched roof, 16:12 IIRC) in the summer. Heat was drawn off the rock for winter heating. There was a large sunspace on the south side to aid in the passive heat gain.

There were a few issues with the house that decreased efficiency. A fairly high amount of electricity was used to move air (into and out of the rock bed) for a relatively low amount of heat stored/recovered. The back-up heat source provided the majority of the winter heating because the rock never got that warm and the heat exchange was mediocre. Heat storage was maximized when the house was allowed to get hotter than comfortable in the summer (we usually opened sky lights in the summer to vent off the heat).

Your idea eliminates some of the problems. However, using the rock as storage will always have some inefficiencies due to the problems of getting the heat into and out of the rock using air.

IMO the thermal mass is better used within the living space to serve as a thermal flywheel of sorts, along with normal passive solar techniqiues to maximize heat gain during the heating season.

Someone did them math somewhere in a rocket stove thread on the thermal mass value of cob vs. gravel vs. water. Water wins as long as you want to stay under 160~180 F.

Ernest Smith wrote:

Topher Belknap wrote:You are going to need tens of thousands of BTUs to heat the building over the night. . . . .

Heat collection is available. It works out to two systems: one for day, and a second to charge the overnight system.

The collection was not what I was trying to point out. Ten of thousands of BTUs requires that ten of thousands of cubic feet of air moved. Among other things, that is a lot of energy used by fans.

Getting through the night in cloudy weather in January is the controlling design problem.

That says to me, 'more insulation and better air sealing'. My house is heated on a single burn of the wood stove per day. It coasts for the entire night, and most of the next day.

The condensation is a central issue. Inadequacies in control can cause condensation, and I'm not aware of any commercially available control equipment that can do it. What I am doing is designing PLC-controlled custom one-off equipment where condensation will not be allowed.

Could you be a little more specific on what you mean by 'not allowed'? If your storage rocks are say 50°F, your air from the solar is 80°F at 40% RH, what is the controller doing to prevent condensation?

It's another reason for vacuum tubes too. The collector I'm using now will build up to 200 C in direct sunlight easily with air in the manifold. I'll need to do some testing to establish the air temperatures I can actually get, but I don't expect they will be higher than that.

200°C (about 400°F) is awfully high. While it does reduce the air flow needed to move the BTUs you need, the losses through the insulation are going to be commensurately higher. Simple, reliable solar is correlated with small delta T. That, by the way, is the sort of heat that goes through my chimney, so I would want it to be as protected as my chimney (especially as you are fire-adverse). That means no flammables within at least 2".

But yes, bad air quality would be a deal breaker. It's something I've not done elaborate design calculations on, but it could potentially cause a shift to a different storage material.

I would love to see those elaborate design calculations when you get them done. As for storage material, rock and it relatives have a lot to be said for them. Phase change materials work best with very small delta T, as temperature variation mitigation, once you are outside that temperature, they work less well. I certainly wouldn't want to be sending 400°F air though wax! That is above its flash point. Hydrated salts have problems disassociating (or whatever the proper chemical term is for becoming non-hydrated) at high temps.

Water certainly holds a lot of heat, but not a stable temperature. Paraffin is theoretically nice that way; phase change is absolutely constant temperature. I don't like the fire hazard though.

Phase changers are only stable for a certain amount of heat storage. Why do you care about a stable temperature anyways? If you are designing a computer control system for input, do one for output as well.

Topher Belknap wrote:

The condensation is a central issue. Inadequacies in control can cause condensation, and I'm not aware of any commercially available control equipment that can do it. What I am doing is designing PLC-controlled custom one-off equipment where condensation will not be allowed.

Could you be a little more specific on what you mean by 'not allowed'? If your storage rocks are say 50°F, your air from the solar is 80°F at 40% RH, what is the controller doing to prevent condensation? . . .

It's not really a big deal to program a PLC to only turn fans on when condensation is not a problem. Temperature (and possibly other) sensors are used as input, and the logic can be programmed to suit. IF THEN, AND, stuff like that.

So, for example, warm air charging might require that incoming air is at least 80 F and above the bed temperature, or the fans shut off after some time delay (can't have a lot of on/off switching during morning startup). Discharging might stop when the top of the bed (last to cool) gets below a certain temperature. The coolest the bed would get at the bottom would be whatever temperature the cool air from the return ducting is, should be over 60 F.

What I usually do for heat transfer problems is write up some time-stepping C++ code and simulate it every second or something. I've not done that yet. I also need to go through a winter with some data logging on my recirculating water system. It is sort of disturbing how some of the physical properties are not well established. Evidently no one really had cause to give these things much consideration before. I'm expecting it will be a fairly iterative process, might take a couple of winters for prototyping.

. . .

Off topic - is there a way to increase magnification on the reply box? I can hardly see what I'm typing . . .

Ernest Smith wrote:

Topher Belknap wrote:

The condensation is a central issue. Inadequacies in control can cause condensation, and I'm not aware of any commercially available control equipment that can do it. What I am doing is designing PLC-controlled custom one-off equipment where condensation will not be allowed.

Could you be a little more specific on what you mean by 'not allowed'? If your storage rocks are say 50°F, your air from the solar is 80°F at 40% RH, what is the controller doing to prevent condensation? . . .

It's not really a big deal to program a PLC to only turn fans on when condensation is not a problem. Temperature (and possibly other) sensors are used as input, and the logic can be programmed to suit. IF THEN, AND, stuff like that.

So, just don't run the fans when under those circumstances. Next question, what are you doing to get rid of that condition? With the fans off, the collectors are going to get hotter, but they are never going to reduce the dew point of that air. 80°F at 40% relative humidity becomes 120°F at 12% RH, but still the water will condense at 53°F. If you have a Psychrometric Chart, this is just moving to the right, when what you need to do is move down.

What I usually do for heat transfer problems is write up some time-stepping C++ code and simulate it every second or something. I've not done that yet. I also need to go through a winter with some data logging on my recirculating water system. It is sort of disturbing how some of the physical properties are not well established. Evidently no one really had cause to give these things much consideration before. I'm expecting it will be a fairly iterative process, might take a couple of winters for prototyping.

Right there with you.

Off topic - is there a way to increase magnification on the reply box? I can hardly see what I'm typing . . .

I usually adjust the font size on my browser (firefox has a '+' at the top-ish far right.

Topher Belknap wrote: Next question, what are you doing to get rid of that condition? With the fans off, the collectors are going to get hotter, but they are never going to reduce the dew point of that air. 80°F at 40% relative humidity becomes 120°F at 12% RH, but still the water will condense at 53°F. . . .

I'm hoping to avoid bed temperatures too far into the 50s F. If I can't make that work it will be time to start looking at other materials.

The idea is to keep indoor air temperatures close to 70 F. The thing should be sized to keep temperatures somewhere around mid-to upper-60s F all through the night, so return air will not cause bed cooling below that. Then when recharge starts in the morning, it will only be gaining heat.

The vessel containing this probably will be an insulated concrete. I've made concrete that floats in water before. It's amazing what can be done with it. Anyway, if interior space gets cool enough to cause condensation problems, it's time to switch that off and run a different heating system.

I'm getting to the "rubber meets the road" point with this packed bed idea. I like it conceptually, but it needs a brutally honest evaluation.

Ernest Smith wrote:

Topher Belknap wrote: Next question, what are you doing to get rid of that condition? With the fans off, the collectors are going to get hotter, but they are never going to reduce the dew point of that air. 80°F at 40% relative humidity becomes 120°F at 12% RH, but still the water will condense at 53°F. . . .

I'm hoping to avoid bed temperatures too far into the 50s F.

I picked 50 out of my hat. It is the concept that I am trying to convey not the exact number.

How are you going to prevent condensation? I don't see what mechanism your proposed controller has to adjust the condensing point of the storage. I don't see any method for removing moisture from the air. I do know a number of people with blocked off rock heat storage systems, that developed mold. Near the end of summer, you will need to transition from not storing heat, to storing heat. The storage will be cooler than room temperature (otherwise it would baking the house in summer). If your summer/fall are similar to mine, you will be in a high humidity situation outside. What happens next?

Topher Belknap wrote:
How are you going to prevent condensation? I don't see what mechanism your proposed controller has to adjust the condensing point of the storage. I don't see any method for removing moisture from the air. I do know a number of people with blocked off rock heat storage systems, that developed mold. Near the end of summer, you will need to transition from not storing heat, to storing heat. The storage will be cooler than room temperature (otherwise it would baking the house in summer). If your summer/fall are similar to mine, you will be in a high humidity situation outside. What happens next?

The short answer is that operation where condensation forms will not be allowed. It will shut down, self-protectively, and some other system will be used.

In use the bed won't go below the lower 60s F, which would require very high humidity for condensation. If incoming air becomes unmanageably cool, it will use a self-protective shut-down and heating will transition to another system. This will all be handled automatically by PLCs programmed with the logic required to do that. I haven't found plug-and-play control equipment that is even remotely able to handle that. This will have to be an industrial process control type show. Pretty much all of the failures I have come across stem from lack of adequate control logic - preventable problems that came to be by oversimplification.

Heat loss during shutdown will be pretty slow due to insulation of the vessel. It will recirculate indoor air - outdoor air will be excluded.

Some condensation can be tolerated for short periods under some circumstances. It should be able to generate a killing heat, with discharge to waste, during transitional periods like spring and fall. Overheating interior spaces is super easy to avoid. If it has to operate it can dump the heat to waste by various means.

I'll have to instrument and log data from the water system to establish ground truth on a realistic energy flux from collectors. Then I'll do the detailed computer simulations to evaluate probable designs. Then I'll build it and see. If at any point the obstacles to making this work become less desirable than some other system, then something else will move ahead of it in the priority list.

. . .

P.S. I should add a little about the air flow.

Thermal charging (daytime) will be a closed loop. Air will be drawn from the bed bottom, run through heat collectors where it is heated, then put back into the bed top-down. Hot air temperature might need to be controlled by fan speed (slower movement means hotter air). So, air comes out the bottom, through heating, back in the top. At no time will it condense water.

Discharge at night will be a more or less closed loop too, with indoor air instead of running through the heat collector. Warm air goes out the top of the bed to the interior space. Interior air (mid to upper 60s F) comes in the bottom to replace it.

At no time during normal use does the bottom of the bed cool below room temperature. It is either recharging with heated air during the day or taking in room temperature air at night. There will be short lag time with no air flow (probably) but it will be too short for significant cooling.

. . .

P.P.S. The daytime system will be totally different - water recirculates between heat collectors and radiators inside. The thing about pebble bed heat storage is you cannot charge it and discharge it at the same time.

Ernest Smith wrote:

The short answer is that operation where condensation forms will not be allowed. It will shut down, self-protectively, and some other system will be used.

You mentioned this before; what I am wondering is how you get from this 'not allowed' condition back into an allowed condition. Once the rocks are cooler than the dewpoint of the air, how do you correct that condition?

In use the bed won't go below the lower 60s F, which would require very high humidity for condensation.

Be sure to tell it that.

15% RH at 120°F would be sufficient.

If incoming air becomes unmanageably cool, it will use a self-protective shut-down and heating will transition to another system. This will all be handled automatically by PLCs programmed with the logic required to do that. I haven't found plug-and-play control equipment that is even remotely able to handle that. This will have to be an industrial process control type show. Pretty much all of the failures I have come across stem from lack of adequate control logic - preventable problems that came to be by oversimplification.

I am familiar with needing to write my own control systems, I did so for my solar hot water, and heating system. That isn't really my concern here, I don't think you have the physical controls necessary to correct a problem condition. For example, in my design, if the water in the collector, approaches the freezing point of the glycol mix, I run the pump to move heat from the tank to the collectors (and sound an alarm). This is just an example of correct design in a control system. However, if the heat in the tank is exhausted, either the tank or the collectors are going to freeze. I have no further automated way of averting the problem. Good design needs to acknowledge and understand the limits of the system. Emergency manual intervention must occur at this point, but I have a way of doing that, adding hotter water to the tank. I want to know your plan, controlled or manual, for the condition where your storage is colder than the dewpoint of your warming air.

I'll have to instrument and log data from the water system to establish ground truth on a realistic energy flux from collectors. Then I'll do the detailed computer simulations to evaluate probable designs. Then I'll build it and see. If at any point the obstacles to making this work become less desirable than some other system, then something else will move ahead of it in the priority list.

Which brings me back to my original point. Why an air-to-rock system instead of an easier water-to-rock system? Especially when you already have a water system. Why add in another system using air?

Thermal charging (daytime) will be a closed loop. Air will be drawn from the bed bottom, run through heat collectors where it is heated, then put back into the bed top-down. Hot air temperature might need to be controlled by fan speed (slower movement means hotter air). So, air comes out the bottom, through heating, back in the top. At no time will it condense water.

Discharge at night will be a more or less closed loop too, with indoor air instead of running through the heat collector. Warm air goes out the top of the bed to the interior space. Interior air (mid to upper 60s F) comes in the bottom to replace it.

That sounds to me to be the description of an open system. A closed system, the fluid (air in this case) from collectors to storage never mixes with the fluid which goes to the end user.

At no time during normal use does the bottom of the bed cool below room temperature. It is either recharging with heated air during the day or taking in room temperature air at night. There will be short lag time with no air flow (probably) but it will be too short for significant cooling.

No cloudy days where you are?

P.P.S. The daytime system will be totally different - water recirculates between heat collectors and radiators inside. The thing about pebble bed heat storage is you cannot charge it and discharge it at the same time.

I have seen designs that did. Fortunately, I was able to talk those people out of the idea of air-to-rock storage altogether. You certainly can heat your house, and store surplus in rock at the same time, which would seem to me to be sufficient.

Thank You Kindly,
Topher

p.s. No engineer should let another engineer get away with 'that won't be allowed'. First one plans to avoid any problems than one assumes that they will happen anyways. Plan for a way to fix it, and then assume that it doesn't work.

Topher Belknap wrote:
You mentioned this before; what I am wondering is how you get from this 'not allowed' condition back into an allowed condition. Once the rocks are cooler than the dewpoint of the air, how do you correct that condition?

Control logic, using temperature sensors as input, possibly others.

If the sensors do not meet criteria programmed into the PLC, interior heating is not allowed. Period. An IF THEN ELSE is not satisfied, so it does the ELSE.

Actually, I don't know how the thing ever would get below 65 F or so, and the dampest, coldest air it will ever see is room air at that temperature on a rainy day. There might possibly need to be a dehumidification for interior space (for comfort), but that would be a separate system.

But just for the sake of discussion, lets say the rocks freeze somehow. Maybe I'm gone for a week and the neighbor broke my collector tubes, then sprayed a hose into the rock bed. When I replace the tubes, there would be a recovery procedure, part of that ELSE above.

Air from the bed is recirculated through the collector until the bed is sufficiently warm. Maybe that is 150 F or something to kill mold. Maybe that will require humidity or some equivalent other sensor. Maybe that would require some sort of moisture removal. But heat the bed until it meets recovery criteria. Then the bed would be placed back in service.

In normal service, I don't see how condensation would happen. There is no source of moisture but the air, and there is pretty much no way to get below about 65 F.

Ernest Smith wrote:

Topher Belknap wrote:
You mentioned this before; what I am wondering is how you get from this 'not allowed' condition back into an allowed condition. Once the rocks are cooler than the dewpoint of the air, how do you correct that condition?

Control logic, using temperature sensors as input, possibly others.

Control logic doesn't DO anything. What are the OUTPUTS and what do they do.

In normal service, I don't see how condensation would happen. There is no source of moisture but the air, and there is pretty much no way to get below about 65 F.

Why do you think 65°F is a limit beyond which condensation won't occur?

You are making me more and more apprehensive about this project. Vague assurances about things that 'won't happen' isn't reassuring.

Well, you asked for experience and thoughts; my experience is that air-to-rock heat storage doesn't work well, sometimes it works so badly, that its owners block it up. My thoughts are that you need to do some calculations starting from all sorts of starting points, and bizarre conditions, and figure out how to return to a workable equilibrium from there. And don't believe your own propaganda about how it is going to work. Good luck. Let me how it is progressing.

I still think you will be MUCH better off just including your storage into your water system. 1 pump, some pipe, perhaps a solenoid valve.

Thank You Kindly,
Topher

Topher Belknap wrote:
Which brings me back to my original point. Why an air-to-rock system instead of an easier water-to-rock system? Especially when you already have a water system. Why add in another system using air?

P.P.S. The daytime system will be totally different - water recirculates between heat collectors and radiators inside. The thing about pebble bed heat storage is you cannot charge it and discharge it at the same time.

I have seen designs that did. Fortunately, I was able to talk those people out of the idea of air-to-rock storage altogether. You certainly can heat your house, and store surplus in rock at the same time, which would seem to me to be sufficient. . . .

I keep rereading this post, trying to figure out what mechanism you are thinking of for condensation.

It seems like you are envisioning some other kind of rock heat storage. Every time I have encountered it on a discussion forum, there are complaints of mold, and usually also talk of charging and discharging at the same time. But in the technical literature it is pretty clear there can be no simultaneous charging and discharging in what I'm talking about.

Here is a physical description of it:

There is an insulated concrete cylinder, something like 10 ft tall and 6 ft wide. It's mouse-proof, vapor-proof, and filled with round rocks 1" diameter or something. Charging it with heat involves blowing hot air in the top and out the bottom. Discharging is the reverse: in the bottom, out the top. The purpose of that is to keep the air from it at a constant temperature. The lower temperature of return air works its way up from bottom to top. Outgoing air suddenly drops in temperature when the bed is depleted.

The things you've mentioned, cold air, outside air, overheating a house, heating and storing at the same time, controls not being able to prohibit operation . . . I don't know what kind of a system you are thinking of, but it's not at all what I'm talking about.

Topher Belknap wrote:

Control logic doesn't DO anything. What are the OUTPUTS and what do they do.

In normal service, I don't see how condensation would happen. There is no source of moisture but the air, and there is pretty much no way to get below about 65 F.

Why do you think 65°F is some magical condensation prevention? . . .

Control equipment runs the fans, etc. If it is programmed to not turn on a fan under a certain circumstance, it won't.

For example, let's say air in the heat collector is cooler than the top of the bed. Charging is not allowed. The fan will not run until the collector warms up more. End of story.

The 65 F thing . . .

Right now my room air is 70 F. It is cooler on the floor, possibly 65 F early in the morning. Let's say I've been cooking a lot and humidity is 90%. That air goes in to the bottom of the rock bed, 65 F and 90% humidity. Then it works its way up and is warmed, its relative humidity drops, and it gets blown back into the room and its humidity comes back up. And so on. How is that air supposed to condense anything? That's the lowest temperature it ever gets to, 65 F or something. If it won't condense in the room, how is it supposed to condense in a rock pile at the same temperature?

Recharging would be the few cubic yards of air that was in the bed recirculated through a collector the opposite direction. There would be a tiny bit of heat loss from the lag (if there was one) between depletion of the bed early in the morning and the start of recharge in daylight, at most a degree or two. Vacuum collectors work in overcast, cold weather, That only adds heat, not moisture. The worst state the air is in then is whatever the return air was when heating stopped less heat lost in a (possible) delay before recharge. There would be a tiny pulse of cold air during startup, the stuff that was in the collector overnight.

Actually, control equipment can be programmed to keep heating from rocks until the water radiators are running. There does not have to be a lag unless the rock bed gets depleted.

The "magic" is that nothing happens to the air to increase its moisture content. The air moves through a pile of warm rocks in a great big heater every so often. As I type this that same room air is moving through a warm electric heater. No condensation there either. Magic.

Ernest Smith wrote:
Control equipment runs the fans, etc. If it is programmed to not turn on a fan under a certain circumstance, it won't.

What about thermosiphoning?

Right now my room air is 70 F. It is cooler on the floor, possibly 65 F early in the morning. Let's say I've been cooking a lot and humidity is 90%. That air goes in to the bottom of the rock bed, 65 F and 90% humidity. Then it works its way up and is warmed, its relative humidity drops, and it gets blown back into the room and its humidity comes back up. And so on. How is that air supposed to condense anything? That's the lowest temperature it ever gets to, 65 F or something. If it won't condense in the room, how is it supposed to condense in a rock pile at the same temperature?

Why would I be concerned about condensation when the heater is in a discharge mode?

90%RH air at 70°F most certainly WILL condense (both in the house and in the rock storage at 65°F! Please, please, please consult a Psychrometric Chart. If you don't know how to read one (and it is complicated), I would be happy to help you.


I advise caution when believing the brochures on those vacuum tubes as well. By the way, where are you getting vacuum tube collectors that work with air?

Topher Belknap wrote:

What about thermosiphoning?

90%RH air at 70°F most certainly WILL condense (both in the house and in the rock storage at 65°F! . . .

No . . .

65 F and 90% RH to start with, and no appreciable temperature drop. And that is just a fictitious way-past-worst-case scenario that in reality would be fixed with dehumidification and only lasts a matter of hours even without.

Typical indoor humidity is in the range of 50%. Controlling that is a matter of using related systems, as I prefer to not ask too much of any given system - heating is heating; cooling is cooling; humidity control is humidity control. They need to work together, but asking one system to do too many things can cause problems to come out of the woodwork. Just my own personal preference.

Thermosiphoning risks runaway heating. I think you have some other type of system in mind. I've often wondered why what I am pursuing has not been more widely used.

It would appear the answer lies in chronic oversimplification: some convenient change works "better" and won't change anything. Except it does change things, and causes an intractable problem. The kind of pebble bed storage I am talking about is something that has to be taken seriously in its finer details or it won't work correctly. All of the failures I have come across have been due to things that fundamentally were design flaws which could have been avoided with adequate control: trying to charge with cold air, uncontrolled convection, simultaneous charging/discharging, allowing attempted operation outside of acceptable parameters, mice (!), . . . it's a long list. There are a lot of things about these systems that can be screwed up. Like I said, they need to be taken seriously or not attempted.

I'll bow out now and descend into the "nerd cave" to do my studies and calculations.

As an aside, there are ways do do concentrated solar that works even in overcast weather. Put a vacuum tube with a head pipe at the focus of a parabolic, or even circular trough. Coating of the reflective surface can be even certain white coatings (titanium-based pigment is good), and the "focal" part is not that important in diffuse lighting. It requires a custom manifold for the tubes, but allows one tube per foot or something, instead of the usual couple of inches between tubes (saves lots of cost). Requires quite a bit of advanced design though, and you have to watch sunny weather performance - too much of a good thing.

A related project is a sunny day repurposing of an old TV dish antenna. Metal pot at the focus. It will char wastes. Charcoal is stable on the surface (doesn't rot), and provides huge ion exchange capacity (sort of - it's a biologically mediated ion exchange) for soils. In other words, it is effective carbon storage and boosts growing potential of the soil. But you do have to be a little careful of the raw materials, for metals and things.

Ernest Smith wrote:
I'll bow out now and descend into the "nerd cave" to do my studies and calculations.

I look forward to seeing your results.

For others thinking about this, I implore you to consider these words:

Ernest Smith wrote:The kind of pebble bed storage I am talking about is something that has to be taken seriously in its finer details or it won't work correctly

My experience is precisely that. Failed systems; too heavy to be removed.

Just my $0.02, but I've considered all kinds of thermal storage possibilities, and I keep coming back to water. I tend to agree with the concerns about the high energy required to move air through a packed pebble bed (not to mention ducting). A very low power mag drive pump sending hot water to a fan coil unit seems hard to beat.