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I just came across the concept of STES - Seasonal Thermal Energy Storage and more specifically BTES - Borehole Thermal Energy Storage.

(Example project)

I wondered if anyone had built a small scale version to store summer heat and release in their homes during winter.
The concept seems remarkably simple but I wondered whether it only works on larger scale projects. Specifically I can’t wrap my head around how the heat loss/disapation of the stored heat in the soil isn’t significant - everyone seems to claim it’s very low.

I also wondered whether storing the heat in a phase change material like fat wouldn’t be more effective but I can’t seem to find any documented small scale implementations.

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STES (Seasonal Thermal Energy Storage) is, in most cases, too expensive to really be practical because of the huge quantity of the storage medium needed to supply the seasonal demand. Typically, you loose a good portion of the heat through conduction or have to invest heavily in insulated containment. I find it difficult enough designing thermal stores for demand balancing over a 24 hour period!

BTES (Borehole Thermal Energy Storage) is an exception. GSHP (Ground Source Heat Pump) is arguably the most efficient method of providing domestic heating. Boreholes are commonly used as a source of heat for GSHP. A common problem with GSHP is that material surrounding the borehole can cool down over several years causing the heat pump to become less efficient and even unusable due to freezing. Using the borehole for passive cooling in the summer will put heat back into the borehole. Putting excess solar thermal heat into the borehole during the summer will also help. Even where ground conditions aren't particularly good for retaining the heat, you can still use the borehole for passive cooling and be assured that it will function well as a heat source. Combining GSHP with solar thermal also removes the problem of stagnation where the thermal store inside the house can't take any more heat. It's quite a good idea to size the solar thermal array to put roughly the same amount of heat into the ground in the summer as is taken out in the winter.

I wouldn't dig the borehole yourself, that's definitely a job for the professional!

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I have done this sort of, in reverse.

I'm a tree farmer. I get seedlings and to extend the time I can plant them I have made a cold room.

This is a 12x12 foot room with 4" fiberglass + 12" strawbale insulation. Inside are 10 45 gallon drums of water on sections of pallet and 4" of styrofoam chips on top. In the winter, the door is left open and the fan on. The barrels freeze. The chips slowdown ice formation on the top so that when middle of the barrel freezes it breaks the ice at the top instead of bulging the bottom.

The cold also sinks deep into the ground.

In late winter I augment this by putting a foot of packed snow on the ground.

Boxes of trees start arriving in late April, early May. I have ice until June.


This illustrates some of the problems at small scale:

  • It takes a great deal of insulation.
  • You don't have good economies of scale.

If I wanted to keep ice for another month, I would have to triple the number of barrels of ice. That would fill up the shed.

If I double the dimensions of the shed, it takes twice as much exterior insulaiton, but I get 4 times the inside area. (Ignoring the roof for now.)


If I were to do this again, I would build a 30x30 building, and inside create a 4 foot deep pit with posts. The posts would be space so tht spandard pallets would span them.

The pit would be lined with bentonite clay and plastic. I'd put a couple thousand feet of 1/2" drip line manifolded into 100-200 foot chunks. these would be attached to the posts at varying heights.

Another array of pipe would be on the north slope of the shed, held on spacers a few inches above the roof.

A small pump on a timer would circulate antifreeze solution between the two manifolds. This would be on a differential thermostat so that when the antifreeze in the roof section was colder than the pit section, the pump would run. Ideally the speed of the pump would depend ont he differential.

In summer mode, the pit would be covered with pallets, with a couple of open spaces to mount fans. Whenever the room temp was over the set point, fans would pull cold air out of the top of the pit and distribute through the room.

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