Solar Heating Pierre SD
Sioux Falls, SD
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Since at least as far back as ancient Greece, Western civilization has been reaping the benefits of free solar heating through south-facing windows. Unfortunately, night losses historically canceled daytime gains.
With the advent of insulated glass, selective coatings and window insulation, the heating equation has changed. Now south-facing windows can be incorporated specifically to reduce a home?s annual heating bill. But it is not simply a matter of more or bigger windows. To realize a net heating reduction, the homeowner must consider two factors:
. The optimum area of the south-facing glass, evaluating the availability of sunshine, the severity of the winter climate, the size of the building and its insulation level.
. The building?s thermal mass, which stores excess solar heat gain and prevents the building from overheating on sunny days.
Deducing these two factors requires incredibly complex calculations on a computer. Fortunately, research allows us to use simple rules of thumb to approximate these factors. The design procedure presented here is derived from the Passive Solar Design Handbook, vol. 3, published by the U.S. Department of Energy, and Adding Thermal Mass to Passive Designs: Rules of Thumb for Where and When, by the Northeast Solar Energy Center.
The procedure assumes first that the proposed south-facing windows have full access to the sun. Note that this is not often the case in existing homes. Neighboring structures and trees often shade south windows at least partially or during parts of the day. So the first step in the procedure is to determine whether you have adequate solar access.
Determining Solar Access
Illustration 1, Sky Chart for 40Ã?° North Latitude, is a map of the sky showing the path of the sun from sunrise to sunset on the 21st day of each month. The map extends vertically (altitude angle) from horizon to directly overhead and horizontally (azimuth angle) from east (sunrise) through south (noon) to west (sunset). The yellow shaded portion is the critical area of the sky (Sept. 21 through March 21, and 8 a.m. to 4 p.m.) that delivers 90 percent of the sun?s energy during the heating season. (Additional sky charts for latitudes 28Ã?° N through 48Ã?° N can be found in my Visual Handbook of Building and Remodeling.)
To use a sky chart, first photocopy the one closest to your latitude. Then, standing with your back against the proposed glazing at noon, locate a reference point indicating due south (0Ã?° azimuth) directly below the sun. Now you can measure and plot on the chart the outlines of obstructions in altitude (vertical angle) and azimuth (horizontal angle from south). Illustration 2 shows how you can use a simple plastic protractor, available at any stationery store, to determine both altitude and azimuth.
Illustration 3 shows the sky chart for a site that has excellent solar access. From the proposed window location, nothing will block the rays of the sun from 8 a.m. to 4 p.m. throughout the heating season. If a neighboring building, trees or other obstructions block more than half of the direct rays of the sun, you have poor solar access and should read no further.
Assuming you have good solar access, the remaining procedure results in the recommended area of south-facing glazing, the required amount and location of thermal storage mass and the estimated percentage reduction in annual heating bill. You need to supply only:
. Geographic location
. Building floor area
. Building insulation level
. Type of glazing.
Location and Building
Table 1 lists select cities throughout the country. (A complete list of 50 cities -- one city for each state -- is available on our website, http://www.SmartHomeOwnermag.com .) Interpretation of the table is simple:
The smaller of the two window-area and fuel-saving percentages corresponds to ordinary houses with the customary window areas, with the exception that all the windows are located on the south wall. The greater set of figures gives the maximum recommended target percentages for the geographic location.
The fuel savings depend on the type of window: double-glazed; triple-glazed or equivalent R-3 glazing such as low-E; quadruple-glazed or equivalent R-4 glazing such as Heat Mirror; or double-glazed with R-9 night insulation. Values for double-glazed and double-glazed with R-9 night insulation were derived from the computer simulations; triple-glazed and quadruple-glazed values were interpolated assuming the use of R-1 and R-2 night window insulation over double-glazed windows.
For example, a home in Chicago that has a south-facing window area of 17 to 35 percent of the floor area can be expected to reduce the winter fuel bill by 17 to 23 percent if it is double-glazed, and 27 to 39 percent if it is triple-glazed or low-E glass.
The table assumes a building heat loss of 6 BTU per degree-day per square foot of floor area. This corresponds roughly to the current energy standard for northern states: R-19 walls, R-38 ceilings, double glazing and an infiltration rate of three-quarters air change per hour. If your home?s insulation levels are different, the table values can be adjusted proportionally. For example, if your home has R-28 walls, R-57 ceilings, triple-glazing and half air change per hour (1 1/2 times as much insulation and two-thirds as much heat loss as the standard), you can reduce your window areas by one-third and still achieve fuel savings of the same percentage.
Sizing and Placement of Storage Mass
All buildings contain mass in their floors, walls, ceilings and furnishings. If they didn?t, they?d all overheat on sunny days. Table 2 shows the approximate maximum percentage of south-facing-window area in average-insulated and well-insulated wood-frame homes before overheating occurs.
By referring to this table, for example, you can determine the maximum allowable area of south-facing windows for a 1,800-square-foot home in Chicago (6,000 heating degree-days with a base of 65Ã?° F) constructed with R-25 walls, R-49 ceiling and R-3 windows, using only the mass of ordinary wood-frame construction. In this case, the energy efficiency of the home is close to the well-insulated house in the table, so the appropriate percentage is 7, and the maximum glazed area is 7 percent of 1,800 square feet, or 126 square feet.
This example is for a typical nonsolar home. For the higher fuel savings listed in Table 1, a much greater window area is suggested. For example, the suggested solar window percentage for Boston is 15 to 29 percent, or two to four times the percentage in the example. Such a home would require additional storage mass.
We?ll now explore four distinctly different patterns for adding thermal storage mass. For each pattern, the accompanying table specifies the material, thickness and surface area of mass required per square foot of glazing in excess of the norm. The patterns are not mutually exclusive. You can use multiple patterns, if necessary, to achieve the required mass.
For example, assuming you wish to achieve 25 percent solar savings for the home in Chicago having triple-glazed or double-glazed with low-E windows, you will need about 17 percent of the floor area in south glazing. You therefore need additional thermal mass to compensate for 17 percent -- 7 percent = 10 percent of the floor area, or 180 square feet of extra glazing. Using Pattern 1 and assuming a bare 6-inch concrete slab as the mass, you?ll find you need 3 x 180 square feet, or 540 square feet of slab in direct sunlight.
The Mass Patterns
Pattern 1: Floor or Wall
in Direct Sun
Pattern 1 is defined as storage mass that has one surface exposed to the living space and a back surface that is insulated. The exposed surface is further defined as being in direct sun for at least six hours a day. Architecturally, this pattern combined with Pattern 2 is useful in sunspaces or solariums.
The mass can be either a directly lit floor slab, as shown in the illustration, or a directly lit outside wall (inside walls are considered in Pattern 3). As with Patterns 2 and 3, the mass element is one-sided; that is, heat moves into and out of the mass from the same surface.
Example: The design procedure has resulted in 100 square feet of south-facing glass in a room with 200 square feet of floor and 380 square feet of windowless wall. Does a 4-inch concrete slab provide enough thermal mass to prevent overheating?
According to the table, you should provide 4 square feet of 4-inch concrete slab for each square foot of glazing. That would require 400 square feet of slab. Increasing the slab thickness to 8 inches would still require 300 square feet of slab. Therefore, you must either reduce the glazed area or add more mass, using one of the other four patterns.
Pattern 2: Floor, Wall or
Ceiling in Indirect Sun
The mass in Pattern 2 is like that in Pattern 1: The mass is one-sided and insulated on the back side. The distinction here is that the mass is receiving reflected sun rather than direct radiation.
In a simple direct-gain space, some of the mass will fit Pattern 1 (a floor slab near the solar glazing, for example) and some mass will fit Pattern 2 (the ceiling, for example). Much of the mass in such a space will be irradiated directly some of the day and irradiated indirectly the rest of the day. In these cases, an interpolation between Pattern 1 and Pattern 2 must be carried out, as described in Pattern 1.
Example: You have decided to use an 8-inch concrete slab for the room described in Pattern 1. This leaves 33 square feet of glazing unaccounted for. How many square feet of 8-inch brick wall will be required to provide this mass?
According to the table, 10 square feet of 8-inch brick wall is required to balance each square foot of glazing. You therefore need 330 square feet of brick wall. Since the total wall area is 380 square feet, this is a practical solution.
Pattern 3: Floor, Wall or Ceiling
Remote From Sun
As in Patterns 1 and 2, the storage mass in this pattern is one-sided. The difference is that the mass receives neither direct radiation nor reflected radiation. It is instead heated by the room air that is warmed as a result of solar gains elsewhere in the building.
This pattern is useful for storage materials in spaces deeper within a passive building, away from the rooms receiving solar gains. However, the solar-heated air must reach the remote mass either by natural or forced air circulation.
Reasonable judgment is required here. A hallway open to a south room could be included; a back room totally closed off from the solar-heated space should be excluded.
Example: Your remodeling plan calls for removing half of a wood-framed gypsum wall to open the south-facing kitchen to the living room. The remaining wall has an 80-square-foot fireplace of 8-inch brick on the living room side. You plan to add a south-facing window in the kitchen. How many square feet of window will the mass of the fireplace balance?
According to the table, the fireplace alone will account for only 4 square feet of window. Obviously, you must look elsewhere for thermal mass.
Pattern 4: Full Masonry Wall or
Water Wall in Direct Sun
This pattern is useful for isolated sunspaces and greenhouses. The storage wall may have high and low vents or be unvented, as shown, without affecting the values in the table.
The performance of the wall improves with thickness up to about 18 inches but is not very sensitive to variations in thickness within normal buildable ranges. For brick walls, higher-density bricks (with water absorption of less than 6 percent) are recommended over bricks of lower density. Note that the mass surface area refers to the area of the sunlit side only.
Example: You are considering adding an attached solar greenhouse. The primary purpose of the greenhouse will be to grow plants. The greenhouse structure should therefore be isolated to avoid excess humidity in the living space.
As the table above shows, 1 square foot of 8-inch brick, 12-inch concrete or 8-inch water wall (water containers) for each square foot of glazing will provide all of the required thermal mass.
In the next installment, we?ll turn our attention to the building?s systems, starting with electrical wiring