annualized solar - heating well above ambient?

So I was thinking again the other day...

Most of my knowledge of PAHS/AGS stuff doesn't come from here/i'm not in a position to even build an earth bermed house, I just remember the original book.  Blow air thru thermal mass and you have whats probably tons of earth or rock or something I seem to remember talk of like 5 million btu thermal storage.

One thing I was wondering is why not just heat things alot hotter?  Like instead of 10 tons at 20 degrees warmer, having 1 ton at 200 degrees warmer type of thinking here...

But instead of having a coupled thermal mass to 100% of the floor, have it so you can control the radiation of heat into the room.  An example would be insulated floor panels and you remove a few insulation panels so that more heat is radiating upwards like into a corner of the room.

Or maybe the thermal mass is connected to something like a Trombe wall - think one heated alot hotter than normal - and you have sliding insulation panels to decide whether it's re-radiating out 10% or 30% or 100% of it's heat.  Letting you adjust the interior heat gain of the building for comfort instead of being stuck to one year round temperature.

You could also heat it with extra heat to be sure that a cooler than average summer wouldn't leave you SOL with a much colder than normal winter near the end of it cuz it was drawing down less heat than normal/you'd be encouraged to overheat the wall really, and be stuck with either some waste heat or leave the panels covered all summer as the norm (in terms of thermocoupling to the house itself).


You could even add the ability to pre-heat/pre-charge or late season rapidly charge the thermal mass with heat if say this is a summer project starting too late to get a full summer's gain.  Solar concentrators/mirrors might let you gain heat alot faster than normal.  Or you might even use a rocket stove.

Just like a rocket stove burns fast then stores heat to reradiate away, since youre above ambient anyways, just rocket stove some extra heat into the wall (separate from the solar gain) esp if there's maybe some waste biomass at the end of farming season or something...  annualized heat is all about the insulation level and at what speed the heat creeps through, dry earth or whatever.  But maybe instead of 20 feet of dirt you dig three feet of trench filling it with pumice or perlite or vermiculite around the sides of the under-house thermal mass (connected to trombe walls) or even under it - like the ground layer of a trench filled with the insulative rock, then putting in a large mass of thermal mass stones however many feet thick the length/width of much of the house probably - and then atop that a combination of permanent and moveable insulation panels so that the too-hot-rocks don't overheat the house inside.

Probably because it is easier to have a lot of "cheap" storage than a little bit of high temperature storage.
The energy flow though the insulation is proportional to the temperature difference, so you need a lot more insulation to keep the energy loss the same.

The energy stored is temperature difference * volume.
The energy loss is temperature difference * surface area / insulation thickness
The insulation required is surface area * insulation thickness.

Assuming 10m³ of material in a cube shape at 10°C difference and 1cm of insulation you need 0.27m³ of insulation.
1m³ at 100°C difference needs 10cm of insulation to keep the energy loss the same and that is 0.6m³ of insulation.
And pipes that last a long time at those temperatures will be much more expensive.

And if it is sufficiently large, the material itself also acts as an insulator.

Some thermal conductivity values for reference in W(m · K)
dry sand: 0.15 - 0.25
urethane foam 0.021
perlite: 0.031

So a meter of dry sand insulates as good as 13 - 20cm of perlite.

Collecting heat at low temperatures is also much easier than at high temperatures.
Water pipes in an insulated roof for example will cool the roof and give you heat to store at the same time, where as high temperatures require vacuum insulated tube collectors which are much more expensive, but don't give you more energy.

Brian Shaw wrote:So I was thinking again the other day...

I've had those kinds of thoughts too, mostly because my land is topography-deficient and we have a high water table. I'm drawn to the concept of earth-bermed homes but the logistics and expense of bringing in enough material to envelope a home is daunting. I'm glad that you've brought up this topic.

Is it viable,  most certainly.  But it has some serious flaws too.  

The biggest is while there are lots of ways to get a 20 degree rise naturally out of the environment there are very few ways to get a 200 degree rise.  Basically you are talking about some sort of fired heater which is sort of defeating the purpose.  Or some sort of fancy optical concentrator probably with tracking or some sort of pressured evacuated tube type solar collector.

Now one version of the idea that isn't, is a large photovoltaic array running an electric resistance heater in sand heating it to 600 or 800 degree F.  Turns out the insulation to do that effectively long term is the problem cost wise.  Great battery.  Never needs repair and mostly really low maintenance.  But needs photovoltaics with their expense and limited life expectancy.

Now another version is using hot water in a large tank.  It probably comes the closest to what you are talking about.  Start roughly 1 hour and 7 minutes in.  Watch the next 40 or 50 minutes.
 


 Combine it with a drain back collector design and it can be even more simplified.  Add ceiling based radiant heat tubes and it should get better yet by getting rid of radiators and fans.  Combine it with a ground based cooling and should be able to heat and cool for less than 100 watts with very limited complexity.

Sebastian Köln wrote:And if it is sufficiently large, the material itself also acts as an insulator.

So a meter of dry sand insulates as good as 13 - 20cm of perlite.

Collecting heat at low temperatures is also much easier than at high temperatures.

Yes that was one of the most interesting things about the PAHS book is that idea that there is a thermal lag through DRY soil/earth and such, it's just that "20 feet out and 20 feet down" may not always be viable in all situations and it made me wonder about ways to adapt the principle, like if you were limited to 8 feet on one side could you put a trench of perlite to insulate?

Heating the thermal mass well above ambient isn't a requirement - going to 200 instead of 20 - it's just more "well whats standing in the way of taking things to an extreme?" or "why does it have to be the other way, going for the lowest grade heat dispersed over the largest heaviest area?"  It's the kind of thing that makes me want to test "do these things work how we think they do" so that they become engineering problems you can design around by trying to come up with systems that work at both extremes.

I think my biggest desire is still to have a PAHS style home with VARIABLE radiant heat indoors - if i'm sick I might want that room at almost 90 degrees to help burn out the fever.  If i'm overheated from exercising really hard more like 68 might be ideal for me.  I want PAHS with a thermostat!  And the only way to make it happen would be some kind of insulation panels over the wall so you sort of open or close your radiant heat area.

The other reason for wanting to go above ambient is to see if it's possible to more fully charge the earth if you had some kind of seasonal variation.  Some people are concerned about climate variability in the future, well if we had some extremely cool summer, having the ability to dial up heat gathering 50% into the thermal mass would get the same consistent total BTU input every summer.

Or concerned about abnormally cold winters - so overbuild how much storage you have, so that you have some heat to literally waste if needed, this will also give you warmth right through a late winter or into a chilly spring.  I don't want to live right at the bleeding edge of something and find that my year round 70 degree average house is now a 60 degree average house if just a few extra meters of planning or an insulation trench or some other minor modification could more reliably guarantee that comfort into the future.

My final reason for being curious about above ambient is things like a heated greenhouse, you might want it 90 degrees in there year round.

Brian Shaw wrote:
I think my biggest desire is still to have a PAHS style home with VARIABLE radiant heat indoors - if i'm sick I might want that room at almost 90 degrees to help burn out the fever.  If i'm overheated from exercising really hard more like 68 might be ideal for me.  I want PAHS with a thermostat!  And the only way to make it happen would be some kind of insulation panels over the wall so you sort of open or close your radiant heat area.

What about having different temperature zones in the house instead?
The warmest room in the middle and the coldest on the outside.

I reckon just wearing a good jumper or coat would be almost as effective.