Has anyone tried using 55-gallon barrels filled with water as "thermal storage"? The exhaust vent would be run back and forth underneath the barrels. Brick would be used on both sides of the vent for support. Gravel and Cob would be used as fill up to the top of the bricks. The water would not be used for any other purpose of than thermal storage. This arrangement would be set up in a greenhouse.
Are there any issues with this approach?
I have purchased and read your (this) book and found it very informative.
Various forms of water containers have been used as thermal storage in greenhouses. What you describe sounds quite practical. I would keep the fill over the ducts to a minimum, with the barrels as close to direct contact as possible. I presume you are talking about steel barrels; plastic has obvious issues in this setup. If plastic, do keep some separation to avoid possible overheating next to the ducts.