Passive solar heating is one of the most efficient uses of the sun’s energy possible. Once the building is designed it requires little to no energy input and the heat it produces is basically free. In traditional passive solar homes the sun heats a thermal mass during the day and the radiates nice even heat throughout the night. In places like the American south and midwest this works great but in areas like the beautiful pacific northwest what do you on days when the sun doesn’t shine (and it frequently doesn’t)? In situations like this Annualized Geo-Solar (AGS) design is a good bet.
Basically heat from the abundant summer sunshine is stored in a large thermal mass. Original Passive Annual Heat Storage (PAHS) designs involved either an underground home or a home with a green (dirt) roof. Don Stephens developed the AGS technique to apply passive solar heating to areas with limited winter sun without the need for earth-sheltered houses.
AGS basically involves collecting heat from an isolated source such as a solar collector or greenhouse and transferring the heat from the air to the earth under the house via ducts. The system requires (at its most basic): 1) a well insulated structure, 2) dry earth under the structure for storage, 3) an isolated solar heat collector, 4) a method of controlling when the collected heat is dispersed into the structure, 5) a radiant floor in contact with the soil.
#1 can be accomplished by superinsulating your building envelope, such as is found in a straw bale house.
#2 is accomplished by surrounding the house with a insulated water repelling apron.
#3 can be accomplished in a number of ways. Hot air can be collected from a sunspace, attic, or even under metal roofing. Dedicated solar collectors can also be built and ideally situated downhill from the structure, then the hot air will naturally siphon through ducts under the house without the need for fans.
#4 is important because you don’t want the house to begin receiving heat until approximately 6 months after you begin collecting it. One way is to deposit the heat deep enough under the structure that it takes 6 months reach the level of the floor (about 9-10′ deep). Another way is to deposit the heat under insulated floor 9-10′ from uninsulated floor and use the horizontal distance to control the when the heat begins to be released.
#5 This design obviously requires a slab type floor. The floor must be in contact with the soil underneath in order to radiate the stored heat into the house.
How to heat? We want something environmentally friendly yet also inexpensive, low maintenance and actually works.
Our current plan is to design a house that will get most of its heating needs from a free source, the sun. Unfortunately it is not so simply as merely putting in a lot of windows all over the house. Then you will roast when the sun is out and freeze at night (during the summer you will just roast). So the answer is to concentrate your glazing on the south side where it will receive the most sun and provide plenty of mass to store the heat and radiate at an even temperature throughout the day. You still don’t want to do this on a summer day so the answer is to provide overhangs/shading in the summer. In the summer the sun is at a higher angle so a 2 ft or so overhang (depending on window height) with stop the sun from coming in in the summer but allow the sun to enter in the winter when it is at a lower angle. The best kind of shading, shades only when the weather is warm. You can either do this with manually adjustable shading or even better plants.
We don’t want to have to incorporate large amounts of mass into our living spaces so we are planning on using a sunspace. Basically a green house attached to the south side of the house. The green house will trap the heat of the sun, as green houses do so well, in an area insulated from the actual living space. When we want the house to warm up we can open doors and windows into the sunspace letting in heat. In the summer we can keep the doors closed and additionally provided the green house with a bit of shade and open some windows for ventilation. The green house will have a high mass floor (some sort of stone), dirt planting beds and possibly additional solar mass in the form of water barrels to help mediate the temperature.
This set up should provide us anywhere from 50-70% of our heat in the winter. The
remainder of heat will be provided by a wood burning stove. I realize that wood burning isn’t at the top of the list as an ecologically friendly material but it is at the top of our list for cost effectiveness. Our site is conveniently located for the sustainable harvesting of scrub oak. So we will have a free relatively clean burning wood to use. Also, because the house will be well insulated and partially heated by other means, we shouldn’t need more than 3 cords of wood each year, probably much less.
