Page 11 — 26 Sep – 9 Oct 1973 The Tribal Messenger
Last week I visited a rock storage air loop collector that had some interesting features in its design. I was at first pessimistic about it, but after considering my own rules which were violated in the design I decided perhaps my rules should be modified. Last issue I went to great lengths to explain why in the storage bin the air should be introduced at the top rather than at the bottom of the rock bin. If it is introduced at the top it spreads the heat evenly throughout the bin; if it is introduced at the bottom (or so I claimed) it burns a hole through the bin and leaves without heating all the rocks. This must be qualified — the important matter is not where the air enters the bin so much as where the air leaves. If the air leaves at the top then it will warm all the rock, because although it seeks out the warmest rocks to rise up through it seeks the coldest to fall through; increased friction within the bin since the air is both rising and falling through the same cross section. In many bins this is not a problem.
[Design needs a door at the collector which is opened at night; tight fitting door; collector at top vs collector at bottom.]
The advantage of this design where the collector ends at the bottom of the bin: the rock bin where it releases the air to flow up inside the insulated wall and then back down through the rocks. Then rather than leave the south facing wall of the bin blank a second perpendicular collector can be built over the wall and fed directly into the house with small doors with which it can be shut off. It can heat outside air or be fed return air from the house or rock bin via ducts.
The advantage of this design is that you need no doors or control mechanisms. The system works by itself automatically adding heat to the storage whenever it is sunny.
How big a collector? How big a storage bin?
A large rock bin coupled to a small collector will warm only slightly during a day of collecting. A small rock bin coupled to a large house with great heat loss will cool considerably when supplying heat during the night.
[Box #1: 400 sq ft floor, 1120 sq ft surface, 2664 BTUs/day°F, 119,880 BTUs/day @ 20°F avg, 150 sq ft collector → size at 200 sq ft, ~34,251 lbs rock ≈ 340 cubic feet.]
[Box #2: 1600 sq ft, 3040 sq ft surface, 7008 BTUs/day°F, 315,360 BTUs/day, ~394 sq ft collector + fudge = 500 sq ft, ~90,102 lbs rock = 900 cubic feet, 6¾" under floor.]
Are these sensible assumptions to make about houses? Few houses are as well insulated as the two boxes I have described, but there is no reason they cannot be. There would be additional heat loss through the perimeter from the storage bin which would need extra thick insulation to protect the above 100 degree temperature. The house should be insulated from the storage bin by at least 6" of insulation below the floor. The ground directly below the storage bin should perhaps be insulated.
©1973 by Steve Baer
Source: The Tribal Messenger, 26 Sep – 9 Oct 1973. Text extracted by OCR from scanned document.
PDF: 1973-09-26-tribal-messenger.pdf