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You're essentially looking at making a thermal mass that simply swamps out the level of "losses" to the outside world. Thermally 100% correct I'd say . . if it's truly big enough, and can exchange it's heat with the surroundings efficiently enough. No matter HOW efficient the place could be; there will be losses . . . and at a time of year when you want them the least . . winter . . when they are also least available to replace.

I'd say s'can the flat roof . . . put enough pitch on it to shed rain . . and . . perhaps more importantly; snow . . like metal roof. The weight can get incredible. Insulate the space between inside roof and the "attic" . . . will help cool in the summer, and avoid icicles in the winter.

You are barking up the right tree with regards to surface area vs volume; but I believe that bees have it right: Believe that a hexagon, or septagon, or whatever they have; actually gives the most volume per lineal outside material. May not lend itself to livable space terribly well . . although bees seem to do fine with it . .

Running tubing throughout the concrete mass I think would be a good idea . . . just don't pour all at once lest ye collapse forms as the weight / pressures can get quite high as the mass gets larger. Pressurize the tubing during the pour !! lest ye collapse it. As far as pumps / controls etc for circulating fluid; why not use passive thermo-syphon ? ? Only ONE moving part . . . a check valve. No motors / pumps / controls / sensors etc. Heat during the winter; and oodles of heated water during the summer months. Follow the KISS principle: Keep It Simple Stupid . . less crap to go wrong . . . easy to protect against freezing etc . . . Circulate well water through the mass in the summer . . get free cooling ( and likely some condensation ) while pre-heating your domestic hot water. Perhaps some cavities / "tunnels" through the mass . . which could have air blown through them to help exchange along better by virtue of more surface area ? ? ? Make the mass be part of the "architectural" design . . not just put there to serve the main purpose. Incorporate masonry stove etc into it . . as PART of the living space . . . shelving . . cubbyholes . . a small sitting area . . "couch" . . all as part of it ? ?

While thinking of cheap AND efficient . . ever look into straw bale construction ? ? ? I've read some about it . . and it is certainly cheap . . and if PROPERLY done ( to exclude water infiltration ) will last a VERY long time. Low tech, cheap, works well. How's 'bout earth-bermed or "underground" construction . . . a few feet down and the temperature doesn't vary nearly so much . . . a much more constant "outside" temperature to your structure ? ? ?

Don't forget that building very tight . . . . also means lack of fresh air . . . you'd have to have ERV or such to get that . . . and there are some losses with it.

An interesting idea . . . curious what others may say . . and what you end up doing with it . . . .

Bob

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I'll make two additional points . . .

1) Thermosyphon does NOT require the collectors to be below what you're heating . . not at all . . perfect example is the Copper Cricket . . . intended to use collectors on the roof . . and storage etc in the house / basement / whatever. NO moving parts but for a single check valve . . driven ONLY by the heat collected . . . thus being self-regulating . . also freeze-proof . . no controls / sensors / pumps . . elegantly simple and effective . . a basic but very clever design. I'll supply more info on it / the prinicples & details if you'd like.

Regardless of the physics; there's always the "human" or "comfort" factor. Heat transfer is Q=mc * delta T. You probably know that . . . but my point is that for the concrete "blob" to be effective in warming a room; the higher the temp differential between it and the surrounding air; the faster the heat transfer. In other words; having the blob at 70 deg F and room air at 65 that you're trying to warm for example . . would suggest moving air over the blob to increase heat transfer . . . HOWEVER . . the moving air will "feel" colder due to the increased evaporation off your skin. Air, to "feel" warm; is much warmer than you'd think . . . measure the air coming out of a register for example . . you'll be surprised how hot it must be to "feel" warm. Lower air speed would make it feel "less" cool; but would also result in lower heat transfer. Bottom line: If the room needs some heat; waiting for it to come off the blob will happen . . but it will not be fast, and will not "feel" warm.

Not trying to rain on your parade by any means . . . but no matter the physics . . there is still unfortunately the "human" factors that we are all affected by; no matter WHAT our wives may say . . . .

Bob

Your idea is flawed in many fundamental ways I'm afraid. How much heat do you think a house requires? What is the heat capacity of a large mass of concrete? Same for cooling. The "ton of cooling" figure for airconditioners comes from the heat absorption capacity of a ton of ice. If you run a 3 ton airconditioner for 10 hours, that's equivalent to melting thirty tons of ice!

A quick websearch reveals that concrete has a heat capacity of about 0.22 - 0.25 BTUs per pound per degree F (sorry for the lack of metric units). So, if your house requires 20000 BTU/h of heating in winter (and this is quite a low figure!) this would need a block of concrete weighing 96 tons - and this is assuming you could have warmed it by 10 degrees F. Assuming a desnsity of around 110lbs per cubic foot, that would be a block 12 feet per side! The cost of that concrete would also be of the order of $5000.

You'd be much better off spending that $5000 and drilling a well for a ground-source heat pump which can provide all the cooling and heating you'll need. If you want solar, you could install enough cells to provide the approximately 3kw (peak) you'd need to run the system - but it would be doubtful if you'd have enough sunlight to run the system when you need the heat. Perhaps with a small wind turbine to supplement this (with sufficient storage) and you'd be OK.

Another flaw in passive solar designs for climates which are both hot in summer and cold in winter is that you pretty much need to use low-E coated windows to prevent heat gain in the summer. These, however, don't let any heat from the sun into the house in winter, though they do reflect the long wavelenght infrared that's already in the house back inside.

Good luck!

Paul.

Gary,

thanks for the interesting reply. My figure of 20kBTU/h is based on real requirements here in Quebec where the mean January temperature is -8.9C. If you do an ACCA Manual J load calculation you'll find that this figure is quite conservative for an indoor temperature of 20C. If your goal is 100% passive, you have to ensure enough heat/cool capacity for worst case conditions - even average conditions here in January are pretty severe with a temperature differential of around 50F.

You're right about passive heat collection being more cost effective than photovoltaic but the problem is often that the heat is low grade when you need it to be high grade - i.e. cloudy days in winter. You're right about the thermal mass of the building being important - the house I live in in Montreal has 20" thick brick and stone walls and these really help smooth out changes in external temperature. Fortunately here 98% of our electricity is generated by hydro schemes so it is pretty efficient to use for heating (or running a heatpump). For northern lattitudes, I think a ground-source heatpump is a good compromise system in that it does utilize solar energy but transforms it into a more useable state and, of course, it should be combined with design techniques which minimize energy requirements to begin with.

By the way, I do have two houses which use ground-source heat pumps. One is new construction (~4200 sqaure feet heated space, R29 walls, low-E windows with large overhang, R40 roof etc.) and one is a retrofit to a 107-year old house with the 20" walls and around 1950 square feet. The running cost is about the same for both at around 90kwh per day (this includes lighting and hot water too) and is much less than the $400 per month oil bill for the old house prior to installing the heatpump. As for cost of installation, with a new house, the differential cost of the geothermal is not too much (mainly the drilling) since a well designed HVAC and active ventilation system is a code requirement here due to the low infiltration required. We designed the new house to last at least 100 years and so wanted a proven system with the lowest life cycle costs. I thought about adding passive solar but the snowfall reality here is such that it would be difficult to make it effective, even though there is often quite a lot of sunshine in winter.

Anyway, this is an interesting discussion!

Paul.

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Solar Cricket . . being of the engineering trade ( electronics in my case ) it still amazes me with it's utter simplicity . . the true hallmark of a GOOD engineering design in my eyes . . . enough of that . . . .

Only two key things in the entire system:

Firstly; using a mix of alcohol and water ( forgot the details; and my high school chemistry ) causes two things of crucial importance. The mix will not freeze at any "normal" temperature ( varies with mix and vacuum ). The system operates under a slight vacuum. Just as pressure in your car radiator RAISES the boiling point of the contents; so too will pulling a vacuum REDUCE the boiling point of the contents. So, alcohol / water mix, and the vacuum on the system; move the freezing AND boiling points to appropriate values for operating such a system. More on this in a bit . .

Secondly, the collector. Vertical risers exposed to the sun . . . tie into a manifold at the top. The manifold is several times the diameter of the colleting tubes. The collecting tubes go INTO the manifold; such that fluid can boil out INTO the collecting manifold; but CANNOT run back down into it. Now tie it all together . . . .

A check valve is located BELOW the collector risers. Fluid IN the risers gets warmed . . and by choosing the mix and the vacuum on the system; will boil at a much lower than normal temperature. When this happens; hot fluid and vapor is expelled into the collecting manifold; and cannot run back down it since the risers stick INTO the collecting manifold. This fluid in the manifold drains by gravity down into whatever heat exchanger etc you have; down in the basement etc. When the fluid runs DOWN; an equal amount of fluid is pushed back UP into the collecting risers . . fluid seeks it's own level. The check valve ensure that boiling energy goes into pushing the fluid into the collecting manifold.

It self regulates; as when the sun goes away, there is no heat to drive the boiling . . hence no liquid movement. The very vacuum that reduces the boiling point and makes it work; ALSO reduces the freezing point.

I have long since forgotten the way to figure out concentration vs change in freezing / boiling points; but it's really basic chemistry.

There it is in a nutshell; if it's not clear I can email you a functional diagram that will make it more clear. Don't know why Copper Cricket is no longer made . . the solar collectors I see now mostly have tanks AT the collectors; where it's cold . . . and insulated or not; just seems silly to me . . .

Bob

Lots of interesting ideas and critical points, certainly more than I expected, and all of which are very welcome as they help improve the design.

Having read them all, I'm starting to see an issue with the quanity of thermal mass and the heat stored/released. From what I understand more mass means more heat stored, but to some extent the heat can become lost inside it and take a long time to radiate back out again. I hope to use the ability of concrete to store heat and only allow it out slowly, like a time delay for the energy stored during the day from the solar water heater, but I need to find an ideal range where the mass asorbs enough heat *and* allows it out at about the right time.

When I came up with the idea I thought heat passed through concrete much faster, so I pictured a very big block. However, it's excellent news that I was wrong as I can use far less expensive, heavy and bulky concrete. Now I'm seeing it as a much thinner block, more like a thick wall of perhaps a foot or two thick. I could run this wall the length of the house, right through the middle, giving it a much more reasonable surface area to volume ratio. This would also maximise the surface area exposed to sunlight.

During the day it would collect heat in two ways, first the heat directly from the sun, some of which would be stored in it. It would also collect heat inside from the solar heater, which won't radiate out until the evening/night. If the thermal mass is sized well and the solar panels big enough, then it should result in enough heat to keep the house warm overnight.

I agree with the comments that it won't be really hot, but I do think that a relatively low surface temperature would be acceptable due to the large surface area. Underfloor heating relys on a large surface being warm and it seems to heat houses very well. The quantity of heat delivered in to the house is the object's surface temperature multiplied by the surface area. Provided one of these numbers is large enough it should work well.

I am quite tollerant of the fact that such a house will vary in temperature, both during the day and the year. I don't mind it being warmer or cooler at times, provided it stays within a fairly wide range of comfortable temperatures. I can easily enough open windows or wear thicker clothing.

Anyway, thank you all for taking the time to respond, it has helped clarify the good and bad parts of the design. I'm feeling more confident of the idea with these new ajustments. There are a number of other areas I'm interested in, particuarly the solar heating systems that don't use any pumps/thermostats, but I'll have to explore that angle later when I have more time.

Here's one for you: Instead of a giant block of 'crete, build a vertical concrete TANK, with walls of 4" thick. Fill your tank up with water and heat the water using the thermosiphon. Now you have the mass of the concrete, with the added heat from the water.
Water too has great thermal capacity. In highschool chem we watched a flame held to an inflated rubber ballon with a small amount of water inside. It took a lot of heat to get the water up to the temp where the balloon would burst. I digress...

I think the best part about this idea is that you have a much easier way of moving the heat. You could directly tie your hot potable water and a hydronic (zoned too!) circulation system into this thermosiphon/giant tank. And if that's not enough, on a cloudy day you could use a ground source to water heat pump or a small (wood fired) boiler. Oh, and it just came to me: instead of a boiler, just put a fireplace at the bottom of the tank with the flue through the middle of the tank!

Obviously this is going to need a tad more engineering to make the 'crete watertight. What do you all think? I'm excited... Let's build it!

Sorry to be late to this forum, but you might pick up several interesting ideas from the book called Solviva.

I read this whole thread and right from the start my first thought was "concrete is expensive, it would be more eficient to make the whole block a cistern, not only do you get cheap heat storage, but you also get a supply of clean safe water that can last you and you nieghbors a long time in the event of a natural or political disaster" Something I didn't see mentioned were shutters. A south facing window in Anchorage Alaska with an R-9 shutter put on it at night will result in a net solar gain 12 months of the year, thats alot in a place with -20 F nights.

Flat roofing requires more lumber than say shed roofing, trusses really are good, they are always engineered. Also, firewood has no cost if you grow it yourself.

Finally Rcmjr if you put water into a vacume it will infact lower the boiling point, but it will also RAISE the freezing point. Ice takes up more volume than water, due to the hydrogen bonds forming together.