Save on Heating Bills with Do-It-Yourself Passive Solar Upgrades
One of the most famous passive solar devices is known as the Trombe wall. Named for French solar scientist Felix Trombe, the primary force behind the world famous French solar facility at Odeillo in the French Pyrenees mountains, the concept of the Trombe wall is in fact much older. Indeed, as far back as the Greek and Roman civilizations of antiquity (not to mention ancient civilizations in the American southwest), whole cities were designed to take advantage of passive solar energy.
The basic idea behind using passive solar to heat is to allow solar heat in and insulate like crazy against allowing radiant heat to escape. In the northern hemisphere, of course, the source of solar energy is always to our south. Therefore, the Trombe wall needs to be built on a southern or at least southeast or southwest facing wall. Conversely, insulation should be added, windows should be covered with storm windows or plastic and heat leaks caulked or covered on north facing walls intensively and to a lesser extent on all walls not involved with the Trombe wall.
The National Renewable Energy Laboratory (NREL) has put out a paper that goes into a fair amount of detail about one Trombe wall design that has been used for buildings in Zion National Park as well as at the NREL site itself. The link is http://www.nrel.gov/docs/fy04osti/36277.pdf . Mother Earth News has also featured what are essentially Trombe wall designs over the years although they are not always labeled as such.
The “Build it Solar” website shows a very simple Trombe wall built over an uninsulated south-facing masonry wall. The link there is http://www.builditsolar.com/Projects/SpaceHeating/nicksthrombe.htm. The “Build it Solar” design does not include holes in the walls, which is usually incorporated in most Trombe wall designs. Instead the increased heat is absorbed by the wall and slowly radiates to within the structure. The disadvantage of that particular design is that it takes longer for the solar heat to work its way into the living space. An advantage is that even on cloudy or stormy days there is no heat loss as might occur with the hole design if the homeowner does not properly plug the holes on unsunny days. Wikipedia also provides a very good reference article on the Trombe Wall. The link there is http://en.wikipedia.org/wiki/Trombe_wall .
Yet the basic concept behind the Trombe wall is relatively simple. The idea is to increase the thermal mass of your south facing wall and allow that heat to radiate into your living space. Thermal mass can be composed of rocks, bricks or blocks. The best thermal mass would be black either naturally (in the case of usually the stone basalt in a rock wall) or painted black. The thermal mass is covered by a glazing, which is basically just a covering of glass or plastic. You can get a glazing that is specifically for solar uses, or as has been suggested, although not yet tested to my knowledge, one way mirrors which would allow light and heat in but not allow them to radiate out. The glazing should be as airtight as possible and cover the entire thermal plus an area just above and just below the mass to allow for vents.
To me the vents are the most ingenious part of the Trombe wall. Since hot air rises and hot air off of the thermal mass will rise to the top of the enclosure the idea is to have a vent that will allow hot air to flow into the house, but not back out. Therefore a vent needs to be cut into the wall that contains a flap on the inside that will move out of the way for hot air to enter and close close if hot air tries to escape. Conversely a vent or vents cut below the Trombe wall ought to allow cold air to leave the living space but not re-enter. Therefore a flap needs to be placed on the outside of the living space within the Trombe wall enclosure for that purpose.
When cutting the vents for the Trombe wall, save the cut-out areas and insulate them to the extent possible, creating vent blocks. That way during long periods of stormy or cloudy weather the vent blocks can be reinserted into the vents to prevent heat loss. A Boulder, Colorado resident who built a Trombe wall swears that it has practically eliminated his winter heating bill. Of course, Colorado has an unusual number of sunny, albeit often cold winter days. Also the Trombe wall needs to be under an overhanging roof so that while it is affected by the low sun of the winter, it is not so affected by the high sun of summer. When summer comes the vent blocks also ought to be inserted and, if necessary the glazing opened up for venting or one way mirrors turned around.
If you are even more ambitious and want to see a full range of passive solar techniques that are currently being used on a house at the 6000 foot elevation level in Utah, check out http://www.allanstime.com/SolarHome/index.html . Besides a Trombe Wall, Allan uses a solarium, solar hot water panels, a eutectic salt chamber, berm insulation and, in the summer, passive solar air conditioning, using black painted chimneys that, pull hot air out of the house to be replaced by cooler air from a 50 foot tube in the lower northwest portion of his house that uses the principle of evaporative cooling.
