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Comment by schiffern | Hacker News Reader

  >we avoid most capital expenditures... since no provision must be made for moisture management or geotechnical engineering

No summer rains in this (presumably agricultural) project area?

I see no math for heat transfer due to rainwater percolation through the pile. "Assuming all voids are filled with water" is great and all, but (with apologies to Jurassic Park) water... uh... finds a way. Meltwater will even tunnel its way through compacted glacial ice.

Plus the "dry" insulation layer won't stay dry for long.

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Good catch :)

I thought about this for awhile and the gap between harvest time for many crops and first frost isn’t enough to get more than a few inches of rainfall in most agricultural regions with favorable economics.

I think the wetness will wreck the insulation of the first meter or so, but won’t lead to much convective heat transfer if the outside never gets saturated. A big if, to be sure.

As an aside, it’s common practice to leave large piles of grain outside overwinter in the central USA and it’s not optimal, but they certainly don’t saturate the whole way through with water.

Beyond that gap time period, I was thinking more about longer-term storage and heat transfer. What happens over the second summer?

Grain piles have free drainage so we don't expect saturation, but if water freely drains through this system it seems problematic.

The spring-summer timeframe is the biggest issue to deal with here. I think that this design would have to have a sacrificial layer with decomposing, anoxic material which degrades during the warmer months. Perhaps it could be mitigated with various expedients to decrease the ambient temperature like flooding the surrounding landscape or putting a shade over the pile.

This scheme only makes sense if the amount of biomass added every year is much larger than the amount which is present in this external layer.

Anecdotally, my father has told me about soaked hay bales out in the pasture which still had a frozen interior core by late May.

With intentionally wet hay (or similar) I would start worrying about spontaneous combustion.

https://news.okstate.edu/articles/agriculture/2020/stotts_br...

https://www.beefmagazine.com/livestock-management/-what-are-...

Perhaps worthwhile to annually top with a cover membrane (possibly bioplastic) to control moisture? Maybe even two layers (bottom airtight and top vapor permeable), sandwiching the dry "insulation" to avoid moisture ingress from both sides. If the area-to-volume ratio is so favorable the cost might be managed.

Thank you, very interesting proposal and discussion. I'm all for it!

Any idea is enlivened by the possibility of criticality and runaway thermal energy production as the melting hay produces heat, thus melting more hay and so on.

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