The Air Force Academy solar house

It's little known, but coming to light now as historical government documents are digitized: the United States Air Force Academy developed a solar house experiment in 1975.  It was not a purpose-built work of architecture, but rather a retrofit of an ordinary military housing unit.  They called it the Solar Test House.

The existing house was called a "Capehart unit," and it was among more than 1200 such units built by the Academy in 1958-59 in Pine Valley and Douglass Valley (on the USAFA campus).  Though there were several variations, the typical Capehart unit consisted of 3 bedrooms and 2 baths on about 1200 square feet on ground level, with a 700 square-foot basement.  The units had uninsulated brick walls (R-1) and "extensive air infiltration."  On average, each Capehart unit was estimated to need 30,000 Btu/Hr. each year.  With these factors, plus the excellent solar resource in Colorado Springs, it is easy to see why the Air Force Academy saw solar heating as an opportunity to manage energy costs.

The Solar Test House was also motivated by another set of historical contingencies.  Work on this project began in 1973, shortly after a crisis.  In the winter of 1972, Colorado experienced a shortage of natural gas, and the Air Force Academy was cut off for nearly six months.  (They used fuel oil, at great expense.)  Additionally, President Nixon announced "Project Independence" in June 1973, which aimed for national energy self-sufficiency by the mid 1980's, and the Air Force project was meant to contribute to this larger effort.

The solar heating system was rather conventional, consisting principally of 28 water-type flat-plate collectors manufactured by Revere.  Air Force researchers considered the design "unique" because, while half of the collectors were placed in a fixed array on the roof (at a 52˚ tilt angle), the other half were mounted on a ground array with adjustable tilt angles.  Apart from the ground array, the schematic design shown below strongly resembled earlier systems, especially MIT solar house IV (1957).  Extensive plumbing was required.  Also of note is that the 2500-gallon concrete storage tank was placed underground (outside the house) "for aesthetic considerations."  It was not insulated.

Aesthetically, there is not much of interest here, and architectural design is not mentioned in the official project reports.  Presumably the design was executed by the Academy's Department of Civil Engineering.  The major feature of the design is the addition of a solar roof form added to the existing standard roof of the Capehart unit.  Because these roofs follow different geometric logics and produce asymmetry, the result is somewhat incongruous, if rational.  In the larger history of solar houses, the eccentrically-shaped roof is a recurring theme; here that theme is not interestingly explored or well-resolved.

The researchers considered the Test House "a working solar energy laboratory" and modified it over a period of years.  In February 1977 they focused on efficiency, adding vestibules on the doors and urea formaldehyde (UF) foam insulation in the walls and ceilings.  (The safety of UF was the subject of much discussion at this time; the Air Force project found no harmful off-gassing.)

In late 1978 they installed evacuated tube collectors on the ground array.  These were found to be "not as effective as flat plate collectors," and more expensive.

How did the house perform?  The insulation reduced the heating load by 27%.  After that reduction, the solar heating system could provide 49% of the house's heating needs.  These figures were considered very reliable because the researchers also instrumented an identical Capehart unit as a Control House.

Then in February 1979 the researchers "decided to operate the house as though it had been completely cut off from natural gas."  (The solar heating system did depend on electricity.)  They found that the indoor temperature only fell below 60˚F twice, for short intervals. They wrote: "Therefore, a solar home occupant can survive relatively comfortably during winter weather until the supply of auxiliary energy is restored."  Clearly the themes of crisis and independence underscored the discourse about the project.

More broadly, Air Force Academy researchers concluded that the "new and growing" commercial solar industry could supply all the necessary hardware for such a system.  But the equipment was expensive: "In a competitive economic environment with conventional fossil fuels, solar energy presently falls somewhat short."

Some Air Force reports available on the web are linked on the Resources page.