In 1998 Marc Rosenbaum was working on a 22 unit cohousing development in Harland, VT. Amory Lovins told the client that she should build a passive solar house without any backup heat. Marc didn’t buy it, and they went back and forth discussing how it could be done. Then Marc published their correspondence at BuildingGreen.com. It’s definitely worth reading.
Posts regarding ‘Design’
A low mass sunspace is meant to serve as a heater, not a greenhouse for plants or a comfortable place for humans.
William A. Shurcliff:
It is hard to think of any other system that supplies so much heat at such low cost…
One could shorten the warm-up time of the enclosure and increase
the amount of heat delivered to the rooms by making the enclosure
virtually massless–by greatly reducing its dynamic thermal capacity.
This can be done by spreading a 2-inch-thick layer of lightweight
insulation on the floor and north wall of the enclosure and then
installing a thin black sheet over the insulation. Then, practically
no heat is delivered to the massive components of floor or wall;
practically all of the heat is promptly transferred to the air.
And since the thermal capacity of the 100 or 200 lb. of air in
the room is equal to that of one fourth as great a mass of water
(about 25 to 50 lb. of water), the air will heat up very rapidly.
I estimate that its temperature will rise about 40 F. degrees in
about two minutes, after the sun comes out from behind a heavy cloud cover.
At the end of the day, little heat will be “left on base” in the
collector floor or north wall and, accordingly, the enclosure will
cool off very rapidly.
New Inventions in Low Cost Solar Heating–
100 Daring Schemes Tried and Untried
Brick House Publishing, 1979
This works well with airflow between the sunspace and living space
during the day and no airflow at night.
- Max space heating and cooling energy < 1.4 kWh/sf/year (4777 btu/sf/year)
- Max primary energy usage < 18.6 kWh/sf/year (63,500 btu/sf/year)
- Air tightness < 0.6 air changes per hour @ 50 Pa
The energy calculation uses these values:
- Indoor temperature: 68 deg F
- Internal heat gains (from lights, people, cooking, etc.): 0.7 Btu/hr/sf
- Occupancy: 377 sf/person (other values between 215 and 538 sf/person may be used with an explanation)
- Domestic hot water use: 6.6 gal/person/day
- Domestic hot water temperature: 140 deg F (120 is standard in the US)
- Domesitic cold water temperature: 50 deg F
- Average air flow rate: 12-18 CFM/person
Passivhaus certified windows must meet these standards:
- U value < 0.14 (R-7)
- SHGC > 0.5
This presentation is a good overview:
For many homeowners, building an attached solar greenhouse is very appealing. They believe that they can extend their garden’s growing season while reducing their home heating bills. Unfortunately, there is a contradiction between the use of a greenhouse to grow plants and the use of it as a solar collector for heating the house.
• To provide heat for a home, a solar collector needs to be able to collect heat in excess of what plants can tolerate.
• Much of the heat that enters into a greenhouse is used for evaporating water from the soil and from plant leaves, resulting in little storage of heat for home use.
• A home heat collector should be sealed to minimize the amount of heat loss. Greenhouses, however, require some ventilation to maintain adequate levels of carbon dioxide for plant respiration and to prevent moisture build-up that favors plant diseases.
Greenhouse management practices also can affect heat storage. For example, a full greenhouse stores heat better than an empty one. However, almost half of the solar energy is used to evaporate water from leaf and soil surfaces and cannot be stored for future use. Solar heat can be complemented with heat from compost as described in the ATTRA publication Compost Heated Greenhouses. Besides adding some heat to the greenhouse, increased carbon dioxide in the greenhouse atmosphere, coming from the decomposition activities of the microorganisms in the compost, can increase the efficiency of plant production.
Because of the concentrated air use by plants, greenhouses require approximately two air exchanges per minute.
Shading provided by mature trees is not recommended. Older books on solar greenhouse design argue that deciduous trees can provide shade in the summer but allow for plenty of sunlight to enter through the glazing in the winter after the leaves are gone. However, more recent literature notes that a mature, well-formed deciduous tree will screen more than 40% of the winter sunlight passing through its branches, even when it has no leaves.