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Solar rock storage

ajgoyal47 | Posted in Green Building Techniques on

I have a 800sft insulated cabin with a 16’×48’×2.5′ rock storage, with 4″-8″ rocks, and insulated (R-8) on the vertical sides and bottom with vapor/water barrier. The top surface of the rock storage is UNINSULATED and in direct contact with a 3″-4″ concrete slab above with vitreous tiles on top.

The 1000sft transparent polycarbonate solar roof / insulated attic below (with black metal sheet) gets very hot (> 120F in Dec) and a 1500CFM blower below it transfers this hot air (> 110F) to the rock storage below and is discharged from the four corners, at 50F, to the attic space again. The blower turns on in the morning and is shut off at sunset.

The design/concept called for the heat in the rocks to percolate up, after sunset, and heat up the slab, which is in direct contact with it, on top and thereby heat the cabin above it.

While it is clear the heat is going into the rocks….. the slab is still cool….. about 60-65F only….. not sufficient to heat the cabin.

I am looking for ideas, comments, suggestions.

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  1. Expert Member

    Ajay, Sounds like a variation on the '80s designs of William Shurcliffe. As I recall the Book was "Super Solar Houses". He later moved on to super insulated houses, seemingly believing they held more promise. I bet Martin remembers a lot more about these houses.

  2. charlie_sullivan | | #2

    I'm trying to understand a little better: "The top surface of the rock storage is UNINSULATED and in direct contact with a 3"-4" concrete slab above". I'm having trouble imagining how you build that--when you pour the slab, what prevents the concrete from running down between the rocks?

    Do you know what temperature the rocks are at? Are you saying they are still at 50 F?

    It might help to time the blower based on air temperature, such that it turns on only after the air space has warmed up.

    Based on the symptoms, it seems like overall you may simply have too much heat loss compared to heat gain, and may need to add insulation or do better air sealing, or add another heat source.

    But 1500 CFM at a delta-T of 60 F is 97 kBTU/hr, which seems like more than enough to heat an 800 sq ft cabin with decent insulation. One question would be how many hours a day or days a week do you get those conditions. And other is whether 1500 CFM is really the airflow or just the rating of the blower.

    Another possibility is that your "duct" system for that air exchange is leaky, and you are driving infiltration as much as the desired effect with big fan. A duct leakage test or blower door test on that system might be a good thing to try.

  3. ajgoyal47 | | #3

    Thanks for your comments...... let me respond

    1. There was no problem in installing/laying the concrete slab over the rocks.... as the surface of the rocks was rough, some concrete must have filled in the "gaps" but was thick enough to not flow down into the rocks.

    2. All that I have at present is the temperatures of the air going into the rocks (the discharge duct between the blower outlet and rock storage below is a 12" sturdy vertical PVC pipe used for water
    systems and eadily accessible. There are no leakages here as you can see with a (design) discharge of
    1500cfm.... through this 12"dia duct gives a good speed of 2000fpm.... or almost 30mph..... easy to detect with your hand.

    3. As far as the rock storage itself is concerned.... the sides are poured concrete with a water/vapor barrier (a plastic sheet taped at the joints) and 2" styrofoam rigid insulation (actually 2-1" with offset joints to prevent leaksge) inside. The base is rammed earth, clean of rocks/stones on the surface,
    levelled with plastic sheet and thermocol insulation above like the vertical walls. Most of this "insulated storage" is underground and leakage is highly unlikely. The concrete floor on top is flush with the top and "end to end" with no gaps at the edges. This should ensure a leaktight rock storage.

    3. As I did not have any flowmeter with me I could not verify the actual air flows but shall do so after I get one and return there in March/April. However, at the four corner exit ducts (~18"×18") of the rock storage I can easily "feel a draft" with my hands..... remember the flow is only 1/4 of 1500cfm ~400cfm or ~2- 3mph only over these ~2sft openings.

    4. This Dec I was there and the sun hit the solar roof at around 10am (sun is delayed due to a hill in the South East corner) and continues till 4:30pm even on Dec 22nd. The skies are absolutely clear, no fog, pollution, etc., and the sun is strong at this 6,000 ft altitude. So I get a minimal of 5-6 hours of strong
    sunlight. This gives about 5-600,000Btuhr/day (@100KBtu/hr)..... enough to heat the 200,000 lbs rock
    (Specific heat of 0.2) by 10-12°F/day. It is clear from the discharge temperature of 50°F that the rocks are not getting uniformally heated and there is a gradient/stratification between 100°F-50°F between the entry and exit of the rock storage..... and the slab surface temperature (which I have measured) is barely 5°F higher in the evening after the system has been operating the whole day. In addition, there is no
    "thermal lag" in the evening / nite (as I had expected) and the slab slowly cools down over the night, not
    enough to heat the cabin above. The cabin is provided with "standard" 4" (R-11)/8"(R-20) glasswool insulation in the walls and ceiling. Windows and Glass Doors are insulated tempered glass with Argon Gas which manufacturer "claims" will give me R-10 insulation (I dont belive them... but bought it on "good faith" for a nominal extra charge".

    From your commrnts that I may have a leaky system....I will also check for thermal leaks (with thermal
    scanner and flowmeter) and see if this solves the problems.

    I have a feeling, and there is some indication of this on web articles, that WITHOUT air circulation in the rocks.... the heat transfer through them to the concrete slab above is very poor..... what do you feel.... not sure myself..... (I had installed a similart, smaller system about 30 years back in Southern NJ and it worked fine..... but I was ALWAYS blowing air through the rocks to charge and discharge the thermal energy from the rock storage.

  4. ajgoyal47 | | #4


    I am not familiar with the book "Super Solar House"...... any idea how I can contact William Shurcliffe....????

  5. GBA Editor
    Martin Holladay | | #5

    There are lots of questions that arise from you situation. Rather than raise them all, I'll cut to the chase: a solar air system that stores heat in rocks does not gather enough heat to justify the cost of the equipment required to collect it. These experiments date back to the 1940s -- and superinsulation beat active solar some time in the late 1970s or early 1980s. That battle is over.

    Build a very tight, well-insulated thermal envelope.

    Don't puncture the envelope with ducts. Don't invest in blowers. Don't build an uninsulated slab. Don't think that R-8 on the exterior of your rock bin (which, after all, is inside your thermal envelope) is adequate.

    Keep heat in -- don't try to collect it with a solar air system. If the sun really shines every day, all year long, where you live, hook up a few PV modules to a ductless minisplit.

  6. GBA Editor
    Martin Holladay | | #6

    William Shurcliff died on June 20, 2006.

  7. ajgoyal47 | | #7

    I thank you for your quick response..... it is too late now..... cabin is already built.... heat is going in.... but NOT coming out.....

    FYI...... In 1980's I built a (25'x12x12ft) high solar green house as an add-on in Cherry Hill, NJ (near Philly) for $11,000.... including Four Seasons greenhouse kit, solar rock storage, blowers, etc.. I got
    $4,000 back under the Carter Renewable EnergyTax Credit plan.... as it was limited to this amount. I saved 60% on my heating bills which were about $1,200/yr at that time (interest rates were 10%+ in
    those days as against almost 0% now). In addition, I got 300sft of "premium sunspace" @ $25/sft for "free".... So while this covers "financial" issues there were others related to social and personal
    responsibility towards the environment and conservation of limited energy resources.

    In this case it is similar.... the "rock storage foundation" and concrete slab was free..... as I need this in any case. Extra for rocks (plenty in the Himalayas), rock insulation, blowers, ducts, etc.... $3000. Solar Collector consisting of Polycarbonate sheet/glazing ($2/sft) on roof is same cost as a traditional free once again.... In addition I am trying (not very successfully right now due to government red tape) to get Carbon Credits (335/yr) worth ~$5000/yr, (taking the Tax Credit in the US was so much easier and simpler).

    Now..... even with solar panels, batteries, inverter (I am NOT connected to the grid as it is not reliable at this remote location) at less than $1/W it would be quite difficult to "financially compete against the above.... I do have a 1.5KW PV system with charger, batteries, inverter, etc. that also powers the blower during the day time and does not take any power from the batteries, and so fits nicely in the "scheme of things".

    Since 1940's many things have changed...... most important is cost of PV panels (from $100/W (??) in the 1960's to less than $1/W nowadays)..... and Carbon Credits (not fully applicable in the US). While the former encourages the use of PV panels (10% efficiency) and heat pumps (COP of 3-5) the latter
    encourages direct solar thermal energy (75% default efficiency petmitted) if it replaces electic heating (the preferred, legal, way to heat in the Himalayas.... as oil, kersone, gas is not permitted by law). Gets interesting in this "dynamic energy market"..... does it not....????

    Anyways..... thanks for your valuable comments / insight...

  8. Expert Member

    Ajay, William Shurrcliff managed to build a couple of well performing houses using similar methods to yours, but as Martin and I have intimated, even he abandoned the approach in favour of extra insulation.
    Since, as you say, you are already committed to solar air collection, there are a couple of differences between what he did and your set up that might allow you to get better results. He didn't try and heat the house from the slab over the rock storage, but used it as a buffer between the ground and house above. He used the attic collection area in much the same way and ducted the air from the rock storage into a south facing greenhouse.
    Essentially what his houses did was surround the building envelope as far as was possible with the warm air collected from the attic, not try and bring it into the envelope either as radiant heat, as you do or as air warm enough to be comfortable in the home.
    For a much better description of his projects you might want to buy his book:

  9. ajgoyal47 | | #9

    Thanks.... have already ordered the book..... looks like I will be busy.... soon....

    Once I have the flow in/out data..... I will know for sure how much heat is being transferred.... right now

    only my hand tells me..... yes I have flow.... but this is the best I have for the moment.... The flowmeter
    showed up last nite..... so just have to wait till I get back to take actual measurements.... and confirm solar energy storage values.

    Still surprised at the "constant low temperature" of the uninsulated floor slab above the rock storage.

    Thanks.... anyways...

  10. GBA Editor
    Martin Holladay | | #10

    Where are you in the Himalayas? I spent several months in Nepal in 1978.

    My first recommended step would be to control the fan with a thermostat so that it only operates when the attic air is hot. That makes more sense than a timer.

  11. ajgoyal47 | | #11


    The cabin is near Pauri, Uttarakhand in India..... west of Nepal..... cabin is about 5km
    West of Pauri in a village Khandu San.... right near the top of the hill..... may see it on Google Earth...the water storage pool is quite clear.....

    Right now I operate the blower manually and have not yet connected the Diff. Temp Controller with energy recorder yet..... will do this once I can get the heat out...

    Hope you had an enjoyable trip to Nepal.... maybe you have to "pay your homage" to the Himalayas.... again......

  12. GBA Editor
    Martin Holladay | | #12

    I imagine that any trip to the Himalayas is unforgettable. Mine certainly was. My brother and I spent a month trekking round trip from Lamusangu to Kala Patar, above the Everest base camp. (We couldn't afford to fly to Lukla; I'm glad.)

    I hope that you can work out some of the kinks with your project so that it performs better than it does now.

    Where does your electricity come from?

  13. ajgoyal47 | | #13

    Here are some green features.....

    1. Electricity is from 6-230W PV Panels, Battery Charger, 4-12V 150Ahr Exide Sealed Gel Battey, Inverter.This is an Off Grid system.

    2. Hot Water - 6x2m2 Hot Water Solar Copper Panels with 2x300L insulated HX type solar hot water tanks, circulating pumps, etc

    3. Solar Heating - You know that already

    4. Water Supply - 125, 000L. Concrete/tiled open (for now) rainwater harvesting tank. Domestic water recycled in garden, Septic treated on site and gray water recyled in garden. Zero discharge.

    5. Cooking - LPG Gas for now (may use PV electricity in future).

    6. Automobile - Petrol powered ( may use electricity in the future).

    So there you are.....

    Now.... plan your next trip to the Himalays in India, Bhutan..... and remember..... Pauri cabin...

  14. GBA Editor
    Martin Holladay | | #14

    My brother and I never made it to Bhutan, but we spent many happy months in India.

    If you have an off-grid system, I don't think that using electricity to operate a fan is a good use of your limited power supply. But it's a little late to redesign your house.

    For off-grid homeowners (I'm one of them), the best way to gather solar energy for space heating is using south-facing windows and a little bit of extra thermal mass. (You don't really need a basement full of rocks.) If your building envelope is close to airtight, and well insulated, you shouldn't need any supplemental heat on sunny days. Just the sun shining through your south-facing windows should provide all the heat you need.

  15. ajgoyal47 | | #15

    I think you and your bro need to "finish off what you did not..... go to Bhutan, and the parts of Himalayas you "missed out" last time..... age will soon overtake you and there will just be regrets..... By the way I am impressed by your "trekking prowess".... going to Mt Everest base camp (20,000ft +...???) Is high enough for me. I was brought up in the Himalayas over 50 years back under the "protection and shadows" of Kanchengunga (28,000ft+)....if you have not seen this.... you missed the "Real Mccoy".....

    much prettier than the Everest group.....

    Pauri is similar.... 20 peaks between 22-25,000ft..... now you know why I built this cabin there.... I love seeing the year round snow peaks from the "comfort of my solar cabin"... specially during sunrise and sunset when nature puts up it almost unreal "show.

    Regarding passive solar heating..... the picture (from the Southside) in the beggining shows you the large windows and the center is actually a large 16ft wide rolling shutter with insulated glass doors
    behind. (Similar arrangment on the other, North, side..... open up to the Himalayas... in your face view...
    what you see is barely 10% as peaks are further to the East. The 1000sft roof has enough incident solar radiation for this small 800sft (48'×16') cabin with temperatures rarely falling below 25°F.... only for maybe an hour or so at night (not every nite... maybe 2-3 times a year). Now I just have to get the heat out to the slab/house. I will be tightening up the house too also next time am there as suggested by you.

    Now regarding the blowers.... it works very nicely..... when the sun shines and I have maximum PV power.... these blowers operate directly from the charge withoutvdrawing from the battery.... on cloudy
    days the blowers are not needed and the PV panels still have dnough to charge the batteries. Works quite well actually... and this was in December only.... when the sun was weakest..

    See attached pics of Kanchenjunga and Pauri Himalstan Peaks

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