Solar heat storage in sand bed under house slab?
Can anyone comment on the following technique? This is excerpted from a Breaktime post at http://forums.finehomebuilding.com/breaktime/energy-heating-insulation/pex-layed-sand-between-concrete-insulation
What you are thinking of doing is what we are already doing in the solar side of heating. I am building a solar heated house in st paul Mn using FOUR FEET of sand below the slab. This sand has it’s own pex tubing for injecting the heat from the hot july and august sun, which will provide heat for the house through the new year!
In my house, the sand bed is fully insulated on all sides, top, and bottom, so it is a little different than what you are doing. Additionally, the slab has it’s own pex heating loop since it does not get heat directly from the sand.
What you are doing is very similar to the common design in central wisconsin advocated by Bob Ramlow. In his design, the sand below the slab is not insulated from the slab and it acts like a big ‘flywheel’ to hold heat energy for a few days and sometinmes months, if the sun doesn’t shine. This is a well proven design with many examples up and running for many years.
Sand is not an optimal material for moving or storing heat. We use it because it is cheap, easy to work with and it won’t leak all over the place causing your house to collapse. My gut feeling is that you want a bit more than two inches to make it through the day. That depends on the heat loss of your building, how hot you get the sand, and whetherit is a house or a utility building. The farther down the PEX is, the slower the response to a heat call will be. I think the Ramlow method uses pex in the sand and pex in the slab, but the slab PEXmay be optional. His followers tend to use two feet of sand, but thay are banking weeks worth of heat, not just the 8 or 10 hours you need.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
seasonal thermal storage. the euros have been using similar technology for a while, though they tend to use superinsulated tanks filled with water or water and rocks.
i've been especially intrigued with this technology in places that get little sun in winter, but oodles in sumer (like seattle or alaska)
http://bruteforcecollaborative.wordpress.com/2010/03/16/seasonal-thermal-storage/
there is also a passivhaus project in galway (IE) that is utilizing a superinsulated water tank to provide all space heating. sexy.
http://www.scanhome.ie/research/solarseasonal.php
Mike,
Intriguing idea, but the data don't back up the theory. In short, when winter comes around, the sand isn't warm enough to provide useful heat.
Building an insulated sand bed costs money -- and the yield on the investment is nil or very low. Storing useful amounts of heat for more than three or four days is hard.
sand, no...
but the seasonal tank storage has been used fairly successfully over the last 2 decades.
I’ve heard this strategy discussed many times among architects I know. While I think in principle low-tech passive strategies represent the best solutions, I think the sand bed storage route is too complicated. There are too many variables that can’t be modeled and analyzed as a system (as far as I know) – daily weather patterns, heat storage capacity of the bed, efficiency of capturing solar heat, the rate of thermal transfer, the heating design load, heat distribution, etc. This strikes me as a ‘shooting from the hip’ approach. In addition, when designing a sand bed for heat storage and redistribution it is probably recommended to have a back-up heat source. The back-up heat source will likely be sized to meet the design heat load and that becomes second redundant heating system and additional expense.
I’m becoming more and more convinced by the PassivHaus strategy for cold climates which is a heat retention model vs. a heat storage and redistribute model. There is software to model the heat loss of a designed thermal envelope and the modeled heat loss is based on air tightness measured in the field. Passive solar gains when wanted are collected directly through high SHGC glass to reduce demands on the heating system. A well designed thermal envelope can go far reducing the design heat load and therefore radically reducing the size of the heating system required allowing for much simpler solutions.
While the PassivHaus method is not yet a time tested approach in US cold climates there are multiple buildings meeting this standard completed or near completion. Because of PassivHaus’ tract record in Europe and all the engineering behind the approach I have more confidence in it then employing a sand bed for heat storage.
Long term thermal storage has never yet panned out in practice.
You want to save all that extra solar energy from the summer and use it in the winter? Get a PV system and put it on the grid. Then pull it off the grid. The grid stores it for you, not in Btus or even kwhs, but in Dollars. Problem solved.
Kevin,
You're exactly right. Although solar thermal systems collect more BTUs per dollar invested than PV systems, they yield less useful energy on a year-round basis -- much less in winter, when thermal heat is needed.
The simple fact of the matter is that solar thermal systems collect too much heat during the summer, when no one wants it -- and that solar thermal heat is almost impossible to store.
Grid-connected PV systems, on the other hand, give homeowners credit for 100% of the energy produced -- so they come out ahead.
We built a number of homes in the mid 90s that used high mass solar thermal systems and have lived with these systems for 15 or more years. The basic deal is that large scale (320 to 400 ft2 flat plate) solar thermal systems can pretty easily provide about ½ the space heating load primarily shaving the shoulders of the heating season. These systems which are optimized for space heating also easily provide 75 to 95 % of the domestic H2O.
What we observed with these systems is that they were not as nimble and responsive as large tank based systems and in this century we have used fairly large tanks (100-1200 gal) integrated with modulating boilers with good results. In the end high mass systems seem to be better suited to owner builders with sweat equity to invest. If you are interested in seasonal heat storage the Drake Landing Project has a district heating system that is exploring that approach but as Martin points out it is very difficult to store heat seasonally especially in individual detached homes.
Martin’s point about the advantages of investing in grid tied PV is one I would echo although with the note that there are some trade offs in making a complete commitment to your utility. Both from the point of grid dependability and future access costs. Martin and I have both lived in off grid homes for the past 30 + yrs and speaking for myself I would appreciate access to the gird and it’s boundless appetite for energy, especially in the current climate of feed in tariff opportunities and time of use rates. But in off grid homes and for clients who are skeptical of putting all of their space heating eggs in the electrical grid’s basket space heating scale solar thermal systems can I think be a useful piece of a very low energy house. The current Federal tax incentives make both investments much more affordable although here in Wisconsin the addition of State incentives on PV make solar electric a better deal.
I have followed with great interest the conversations about the Passive standard and I believe there are some real lessons in that path although I have found Robert Riversongs critiques thoughtful in expressing some of the limitations of the Passive standard in its embrace of a house that is hermetically isolated from our natural environment and in it’s tendency to depend on foam based insulation. We know similar levels of performance can be (and sometimes are) achieved using environmentally friendlier cellulose Insulation. A product which is far superior from the point of view of embodied energy and global warming potential.
Mark,
Nothing in the Passivehouse standard mandates the use of foam insulation or hermetic isolation, Robert Riversong's claims to the contrary. It's just an energy use standard and quality building program, it does not mandate materials or techniques to achieve the energy use targets. Yes, walls and building components need to meet specified air leakage rates, but it's still a house with windows that can stay open all summer long, just like any house.
Many US builders trying to meet the standard have used spray foams, but that's rare in European practice. Breathable, vapor open wall systems are common practice over there, just like Robert encourages.
Jesse Thompson
Kaplan Thompson Architects
Mark & Jesse,
Riversong's Vapor Open, allow it to dry strategy makes sense to me.
I DO NOT understand his aversion to Air Control.
Robert ,Mark(or anyone),
Why don't you tell us exactly where you think our Enclosures should be LEAKING?
So that we can be sure that we have enough leakage everywhere that we need it.
Jesse Thanks for your thoughts, I guess that I wasn't as clear in acknowledging that homes can be built to the Passive Standard of insulation and air sealing using cellulose walls and ceiling insulation as I intended to be to be. I am skeptical about the increasingly common practice of air sealing new homes with spray foam. I am more of an air sealing believer than Robert seems to be and we routinely seal our homes to less than1 AC/H @ 50 Pa and have done so for the last 20+ yrs.
The idea of a home that breathes to me is one with skin similar to our own. No air leaks, punctures, gaps but still has variable permeance as needed.
I think this point is very misunderstood.
With proper permeance and low ACH and windows that open when desired along with Superinsulation a low E home is achieved less reliant on an ERV. Not saying it has to eliminated.
Build with zero VOC materials and you have a pretty good habitat.
On topic.... I like water as a heat storage medium.
We did this about 25 years ago. It works good for carrying a superinsulated structure for a week or so in the winter here in Maine.
Seasonal storage is not realistic, but weekly storage is. This can even out sunny and cloudy days and is a decent tool with that realistic expectation in mind.
IN1983 & 84, PRIOR TO BUILDING A NEW HOME I RESEARCHED THE SOLAR HEATING FIELD. MY COMPANY HAD BUILT OR WAS IN THE PROCESS OF BUILDING SEVERAL SOLAR-FRIENDLY PROJECTS. I DECIDED TO INCORPORATE SEVERAL INTO OUR NEW HOME. ON DEC. 1st 1983 WE EXCAVATED 4 FEET BELOW GRADE & INSTALLED FOOTINGS & 4' BASEMENT WALLS. DURING THE NEXT MONTH, IN FREEZING WEATHER, MY SON, MY SON IN LAW & I INSTALLED:
1. 2" OF FOAM INSULATION AT THE BOTTOM & UP THE WALLS;
2. ONE FOOT OF COMPACTED SAND;
3. PVC TUBING IN PATTERN TO COVER THE ENTIRE 2400 SQ. FT. BASEMENT;
4. 3 ADDITIONAL FEET OF SAND;
5. A SECOND LAYER OF PVC TUBING;
6. A 6" CONCRETE FLOOR.
PROPER SOLAR ORIENTATION, DOUBLE GLASS WINDOW WALL & SEVERAL OTHER SOLAR APPLICATIONS WERE UTILIZED. OUR 5,000 SQ FT EXPOSED BASEMENT HOME HAS TEN (10) 22,000 BTU REMOTE LOCATED SOLAR COLLECTORS. OUR HOME HEATS FOR 1/4 OF THE ORIGINAL CALCULATED HEATING COSTS. LAST YEAR PROPANE HEAT COST WAS $1049.00 & THERE WERE 7500 DEGREE HEATING DAYS HERE IN SOUTHEASTERN WISCONSIN. I WOULD BE HAPPY TO DESIGN, SITE PROPER, HOME AND SOLAR SYSTEM FOR INTERESTED PARTIES.
YES - SAND IS A FINE MEDIUM FOR STORING HEAT & WE HAVE 25 YEARS OF SUPPORTING DATA.. WE CAN GAIN 3 DEGREES, WITH SUN & ZERO DEGREES OUTSIDE & USE 1 1/2 DEGREES OVER NIGHT TO ASSIST IN HOME HEATING..
Rules to Live By:
#13: Anyone who makes outrageous claims and writes in ALL CAPS is not to be trusted.
Martin, can you remove my email address from this thread? I'd rather not get emails about new posts to it. Not sure if that's possible, but appreciate anything you can do.