Small scale Borehole Thermal Heat Storage

Hello all
after learning about this idea I’ve been wondering if it could be done on a single home scale. my thought was drilling boreholes in about a 50ft radius at a depth sized to storage load and insulate the top. charge the system during the spring summer and fall with evacuated tube collectors. but here in the pacific northwest we can go months with little sun so my thought was store heat all summer and use it in the winter. or does this technology only work at large scale.





http://www.dlsc.ca/how.htm

I think it works better in large scale, since the heat loss depends on the surface of the storage, while the capacity is dependent on volume (mass). The bigger the storage the better the ratio.
But i have good news as well. You don't need expensive evacuated tube collectors usually to charge this kind of system. Even low performance collectors are suitable, since you dont need high heat, or insulation, but big collector surface. This is because it will be charged mostly in summer, so the insulation is not critical. Also lower grade heat can be used if a smart charging system can be used. (if the inner boreholes are warmer than the current imput, we can charge the outer, colder boreholes.)

Imagine a poly tunnel type greenhouse, with a really shallow pond inside, like 1 cm deep, lined with black material. And on top of the pond swims a transparent insulating material, like clear bubble wrap. From my experience with similar low tech stuff, i predict on my location the water can reach up to 90C temperature. This is with a collector that has material cost in the 20$ /m2 range. This collector could charge the mass ideally in summer months, and do a fine job late spring and early fall. The problem with this nice set up is it wont work winter time.

I know of a single dwelling, small lot application. They put 3 heat wells below the driveway in a triangle, 11ft apart, and 60ft deep, top 12ft insulated, if I recall. They used evacuated solar tubes and heat from the AC condenser/heat pump unit to heat the system. They told me it is going to take 3 years to fully season the well, and they calculated 11 million BTU total max storage with the materials in their sub soil. In the winter they run the heat back through the heat pump coils that are zoned systems to only heat the rooms being used. They claim less than $7 a month in heating costs. This was retrofit into a 1960 era brick home. However they did extensive insulation, and special windows (double pain glass, double pain plastic seasonal inserts) to mitigate heat loss.

We love the idea of storing heat from one season to use in the next. For our use, warmer attic air warms a mass in the basement and it works ok spring and fall, but not enough heat is in the attic over the coldest part of winter.

The cost was less and one day, a solar panel can push the air. The thermostat wasn't an issue to set it in the attic to cool so it draws air, the other thing the fan could do is blow out hot air over sunny summer days.

And, to save costs, as some thermal storage systems can be expensive, we are adding a wood mass heating system into the basement mass to finish off the design.

I can see spending $30k for a system that will heat for 100 years, but we didn't have the upfront cash.

I know it runs against the grain of a lot of folks on this forum, but I have successfully used the exhaust of both my generator and wood chipper as sources of high grade heat to charge the earth under my insulated shop floor. There is a LOT of waste heat coming out of even very small internal combustion units and it can be stored effectively under ground for use at a later date. I figure I'm using these units anyway so its hard not to consider them as primary sources of heat. Someday I may switch to solar, but for now, I'm sticking with waste combustion heat.

Those purists who plan to use solar heat might still consider using generator waste heat as a source to "quick charge" their soil heat storage field during the first season in order to jump start their system.