Passive Solar - Notes on Bachman
Robert Johnston was interested in obtaining material from last month's presentation by Leonard Bachman. I am providing a summary from my written notes, FYI.
Leonard Bachman goes back to the 70's and 80's in his association with the Houston Solar Energy Society. He is a professor at the University of Houston, in architecturue. He presents really well. (My notes will not do him justice.) See www.uh.edu/~lbachman.
Solar is highly site-specific, with micro-climate an important factor. Houston has a large potential for passive solar techniques (50% and more of heating and cooling requirements can be met passively). To characterize Houston's climate, 1/3 too cold, 1/3 just right, and 1/3 too hot for comfort. Notable, is the fact that 60 - 75% of the sun's clear-day energy passes through on cloudy days.
Aperture and Thermal Mass are important elements. Aperture is the access or direction of energy into the building [my definition]. Thermal mass utilizes the thermal capacitance of materials to moderate temperature extremes. There are four basic types of passive solar design:
Thermal Storage Wall
Sun Space works best for Houston, having an isolating quality, and working well for heating. I interpreted the concept as similar to a "sunroom".
Heat Storage Properties - concrete (4 inches are optimum) and water (good heat absorber). When using the floor as a thermal mass for heating, it is a good idea to have windows extend as close to the floor as possible. Use of reflectors outside is also a good idea. The Trombe Wall was mentioned, as it contains mass just behind the glass.
In Houston, sudden and large climatic temperature changes are common. Therein lies the difficulty, since the unexpected temperature swings can run counter to the intent of using thermal mass as a moderating medium.
In Houston, sloped glazing on windows (as on high ceilings) is not recommended, except perhaps for asthetic reasons.
The following list captures major elements of passive solar design:
Home Orientation (south facing +/- 15 deg)
Window Location (no/few East-West windows)
Radiant Barriers (in attic, not walls)
Insulation (R-30 ceiling/attic, R-19 walls)
Shading (as by trees, structures, overhangs, drapes)
Ventilation (sophet to ridge, etc.)
Attic Fans (for humidity control)
Exterior and Interior Walls (must "seal")
Moisture permeability is very important in walls.
Put thermal mass in interior walls (vs exterior).
Need thermal heat sinks for cooling. Some schemes:
Direct Loss (Ventilation, delta p - Natural or "Stack")
Roof Pond (heat sink during day)
Earth (constant temp, about 74 degrees 8 feet down)
Evaporative Coolers (good in Pheonix, not so in Houston)
An interesting phenomenon was discussed - a convective rain storm, with its accompanying quick cooling, common in Texas. Temperature shock and extremes are tough on roofs - oils and sealants seep, materials expand and contract; roofs experience shock, sometimes catastrophic. My personal solution is to cover roofs with Solar PV panels! Of course, tiles work well.
Leonard Bachman provided an excellent overview on passive solar concepts and design, focusing on the concerns in Houston. I left many interesting details, examples, and anecdotes out of these minutes, as well as references to good written materials. If anyone in attendance can recall the references, please forward them to the group. Thanks.
Jonathan A. Clemens