Passive Solar Design for New Mexico

Class VI

• Review of previous material
• Comfort: Mean Radiant Temperature - average temperature of surfaces:

• Overhangs:

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:

q_sun-axis = acos( sin(q_tilt)cos(q_season) )  (This yields the angle between the direction of the sun from the center of the earth and earth's axis).

q_{elevation, noon} = asin( sin(q_lattitude)cos(q_sun-axis) + cos(q_lattitude)sin(q_sun-axis)  ) (This yields the elevation of the sun above the horizon at noon).

Use: q_tilt= 23.47^o, q_lattitude= 36^o, q_season= (days from solstice)/365 x 360^o

Examples:

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(q_{summer elevation})/tan(q_{winter elevation})

L_height = L_window / (b - 1),  L_overhang = L_height / tan(q_{winter elevation})

• 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 phototropic growth.
  • 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 of vents.
  • 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 massive walls.
  • 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.