Steve Baer on Air Loop Rock Storage Systems — Nick Pine (1995)

Nick Pine — alt.solar.thermal / alt.energy.renewable / sci.engr.heat-vent-ac — July 26, 1995

Source URL: http://www.ibiblio.org/london/renewable-energy/solar/Nick.Pine/msg00008.html

Nick Pine (Villanova University, nick@vu-vlsi.ee.vill.edu) quotes extensively from Steve Baer’s Sunspots in a 1995 Usenet post.

Source identification

From his 1979 Cloudburst Press book, ISBN 0-88930-062-3, pp 62-66, Sunspots “An Exploration of Solar Energy Through Fact and Fiction.”

This confirms a 1979 Cloudburst Press edition of Sunspots — distinct from the 1975 Zomeworks Corporation first edition. The subtitle differs: “An Exploration of Solar Energy Through Fact and Fiction” vs. the 1975 “Collected Facts and Solar Fiction.” The archive holds only the 1975 edition.

Quoted content: Air Loop Rock Storage Systems (pp. 62–66)

Baer opens by noting institutional skepticism toward convective systems:

Much work still needs to be done on the behavior of convective air loop rock storage systems. The way they work flies in the face of the typical air conditioning engineer who can’t believe such systems could operate without fans. At a solar energy conference in 1968, Farrington Daniels mentioned the letters he had received from someone in New Mexico building solar chimneys that pushed air through rock storage bins. He was told by an engineer in the audience that the chimneys would have to be as tall as the Empire State Building. I was very pleased to report on the performance of the just-completed Drop City heater where the chimney was only 14 feet high.

Paul Davis house (Corrales, 1972): heated by a convective air loop, no fans. Los Alamos engineers who dismissed these systems “had never actually experimented with any rocks, but instead were using only computer simulations.”

Zomeworks 1969 experiments: 750 BTU/sq ft/day average storage in December Albuquerque weather, with collector and storage bin at the same elevation.

Solar chimney physics

A solar chimney is a sloping glass-covered channel. Air heated in the channel rises (chimney effect). The pressure difference driving the loop:

Example: 8-foot chimney, average rising-air temp = 130°F, average descending (storage side) temp = 90°F.

Pressure difference = (40/550) × 96 inches of air ≈ 7 inches of air ≈ 0.005 inches of water

This is extremely low — standard engineering tables rarely give flow data at such resistances.

Self-balancing: The chimney temperature rises until airflow increases enough to carry away collector heat. Poor design (high resistance) forces the system to overheat to get enough flow, increasing collector losses.

Collector mesh design

Five layers of expanded metal lath as heat exchanger:

Each sq ft of lath counts ~¾ sq ft transfer surface (both sides)
5 layers = ~5 sq ft transfer area per sq ft of glass
U factor (slow air to mesh): ~1.5
Delta T at 240 BTU/sq ft/hr solar flux: ~32°F
Mesh placed diagonally so rising air must pass through before exiting

15 design rules

Width of flow path ≥ L/15
Rock depth: 2 ft for 1” gravel; 4 ft for 6” rock
Collector slant ≥ 45 degrees
Insulate storage with ≥ 6” batt
Collector ≥ 6 feet long/tall
All flow channels ≥ 1/15 of collector area
Avoid corners in flow channels
Storage cross section ≥ 1/3 of collector area
Insulate divide between downflow and upflow with ≥ 1” duct board
Double-glaze collectors if climate ≥ 7,000 degree days
Hand-place rocks to avoid dirt layers in bin
Place all storage rocks above collector, or use damper
Build house above storage bin
Build vent flap at top of collector to open in summer (prevent overheating)
Heat house with trap door to rock bin; duct to cold underneath for return air

Baer notes further work in UN Solar Energy publications (vol. 5) and by Dunkel (Australia) and Scott Morris (Santa Fe, NM).

Significance

Confirms 1979 Cloudburst Press edition of Sunspots (not in archive)
Preserves air-loop solar chimney content from Sunspots — not covered in the archive’s 1975 PDF transcription
Paul Davis house confirmed as convective air loop (matches AIA slideshow description)
Drop City heater: 14-foot solar chimney, operational by ~1968

Source: Nick Pine, “Steve Baer on Air Loop Rock Storage Systems,” Usenet post to alt.solar.thermal, July 26, 1995. Archived at ibiblio.org. URL: http://www.ibiblio.org/london/renewable-energy/solar/Nick.Pine/msg00008.html