Passive Solar Design for New Mexico
- Review of previous material
- Comfort: Mean Radiant Temperature - average temperature of surfaces:
|Mean Radiant Temp.
Geographical method: Use SunCharts to make a scale drawing. Overhang is
determined by intersection of a summer ray hitting the bottom of the window on
the date that you want full shade to begin or end, and a winter sun ray hitting
the top on the date you want full sun to begin or end.
Semianalytical method: To get full shade for 3 weeks in summer and full sun
for 3 weeks in winter, use a summer ray that is about 2.5 degrees less than at
summer solstice and a winter ray that is about 2.5 more than at winter solstice.
For 5-6 weeks of effect, use 5 degrees instead of 2.5 degrees. This is the
default rule used in the Energy-10 program for overhang sizing.
Exact Analytical method: First calculate elevations of the sun at noon at solstices
or nearby times:
qsun-axis = acos( sin(qtilt)cos(qseason)
) (This yields the angle between the direction of the sun from the center
of the earth and earth's axis).
qelevation, noon = asin( sin(qlattitude)cos(qsun-axis)
) (This yields the elevation of the sun above the horizon at noon).
Use: qtilt= 23.47o, qlattitude=
36o, qseason= (days from
solstice)/365 x 360o
|Time of beginning and end of full shading effect
|Only on summer solstice
|Only on winter solstice
|+/- 3 weeks from summer solstice
||20.7o (= 21/365x360o)
|+/- 3 weeks from winter solstice
||159.3o (180o - 20.7o)
|+/- 6 weeks from summer solstice
||41.4o (= 42/365x360o)
|+/- 6 weeks from winter solstice
||138.6o (180o - 41.4o)
Note that for the 6 week periods, the change in overhang angles is slightly
larger than 5 degrees, in agreement with the semi-analytical rule (the Energy-10
rule) described above.
Now calculate overhang ratios:
b = tan(qsummer
Lheight = Lwindow / (b
- 1), Loverhang = Lheight / tan(qwinter
|Only on solstices
|+/- 3 weeks from solstices
- Guidelines for Attached Greenhouses
- Food Chart: page 53 of the book, "The Food and Heat Producing
Solar Greenhouses. Design, Construction and Operation", 1996, by
Rick Fisher and Bill Yanda, John Muir Publications.
- Temperature is allowed to swing in greenhouses.
- Thermal mass: Use three times area of glazing. Or 2-4 gallons of water
per square foot.
- Need mixture of colors on back wall to provide reflection to avoid
- Use 6 foot vertical spacing between interior vents. These should have
a combined area of about 15% of the glazing.
- Exterior vents should be located to allow summer venting by prevailing
winds (low vent in the west, high vent in the east).
- Exterior vents should have total area about 1/6 of the floor area.
Door counts as a vent. High vent should be about 1/3 larger than lower
vent. Example: 160 ft2 greenhouse: 4 ft2 low vent,
6 ft2 high vent, 18 (3 x 6) ft2 door = 280 ft2
- All vents (both exterior and interior) should be operable manually.
- Exterior door away from winter winds (east end).
- Need good drainage away from the greenhouse.
- Dimensions: length 1.5 - 2 times width.
- Width less than 10 feet is convenient.
- Winter sun should hit well up on back wall (6-7 ft), while summer sun
should not hit thermal mass materials. Example: Roof: 4-5' overhang and
3-4' clear with slight tilt from horizontal, the rest glazing tilted at
60 degrees from vertical.
- Most flowers, shade-loving plants, and cool-weather veggies can go
into the back area. Full light loving plants (tomatoes, cucumbers,
peppers) can go in the front.
- Tilt glazing to be perpendicular to sun during coldest months (60
degrees from vertical in NM). More importantly, avoid perpendicular
during the summer.
- Outer walls should be insulated walls, or even better, insulated
- Foundation (perimeter) should be insulated. Trench should be at least
16 inches deep.
- Floors can be insulated with sand, pumice, or other porous materials.
- The Yanda greenhouse's used flat fiberglass (acrylic) for exterior
vertical (or near vertical) clear walls, corrugated plastic for clear
roof areas, and 6 mm polyethylene with an ultraviolet inhibitor for the
interior clear wall and roof areas.