SOLAR HEATING AND COOLINGSub-Sections5.3.1 Other Glazing5.3.2 Solar heating5.3.3 Shading

Dr. Samer as’ad

Khaled Al Ashqar

Haytham AlShaer Hussam AlZahrany

5.3.1 Other Glazing'sFiberglass-reinforced plastics, polycarbonate (Lexan) and polyvinyl chlorides (PVCs) are available for windows, passive solar systems, daylighting, greenhouses, and so on. They can be obtained in single sheets or as component systems of two sheets plus insulation for higher R-values. Previous disadvantages were weathering and coloring with age due to UV radiation. Sun-Lite has a Tedlar film on the exterior surface to reduce UV damage.

Material Transmission (depends on thickness)Sun-Lite 80–90%Lexan 64–84%

5.3.2 Solar heatingThe amount of solar energy (insolation) that comes into a building through the windows (size, orientation, glazing, and shading) depends on time of year, location, and percentage sunshine (clouds and haze). The solar heating can be estimated from tables of clear-day insolation by latitude and percentage sunshine by month (Figure 5.2 and Table 5.3). Average insolation values by month that take into account cloud cover for different angles of surfaces are available on the Internet. The data for vertical insolation indicate that in most locations vertical windows on the south can be net heat gainers, depending on climate and Rvalue of the windows. Surprisingly, even single-pane windows on the south can be net energy gainers in temperate climates. Also, during the summer vertical windows let in less solar

FIGURE 5.2  Clear-day insolation, average for month, for latitude 32° N.

Example: 5.1Calculate the amount of solar heat that comes through a south-facing window (single pane) for January (70% sunshine)Vertical window, 1.2 by 2.5 m, single pane, transmission = 90%Area = 3 m2, insolation for January is 6 kWh/m2 per clear dayEnergy hitting window per day = 3 m2* 6 kWh/m2= 18 kWh/dayEnergy transmitted = 0.9 * 18 kWh/day = 16 kWh/day = 55,000 Btu/dayEnergy for month = 16 kWh/day * 31 days * 0.70 = 350 kWh = 1.2 * 106 Btu

Maps of solar insolation for the United States by month are available from the National Renewable Energy Laboratory (NREL) for different types of collectors and orientation (http://rredc.nrel.gov/solar/old_data/nsrdb/redbook/atlas/). The two-axis concentrating collector gives the normal to the sun. The maps also take into account the percentage sunshine (Table 5.4), so the average values of energy per day do not have to be adjusted.

Example: 5.2Calculate the amount of solar heat that comes through a south-facing window (double pane) for January for Amarillo, Texas.Vertical window, 1.5 by 2 m, area = 3 m2, double pane, transmission = 80%Average insolation for January for vertical window is 4.5 kWh/m2 per day (takes into account cloudy weather).Energy hitting window per day = 4.5 kWh/m2* 3 m2= 13.5 kWh/dayEnergy transmitted per month = 0.80 * 13.5 kWh/day * 31 days = 335 kWh

The double-pane window will transmit less solar heat into the house; however, it will reduce the heat loss. Therefore, in colder climates double-pane windows are better. During the winter, vertical windows on the south (Northern Hemisphere) capture most of the solar energy available (Figure 5.3) because of the position of the sun. On December 21 (35° N, 102° W), there are 9 h of sunshine with around 6 h for solar heating (Figure 5.3). At 0900 and 1500, the altitude is 15° with azimuths of 120° and 217°, respectively. The maximum elevation is 32°, so during that 6 h, the vertical windows let in most of the sunlight. On July 21, sunrise is at 0549 and sunset at 1958. That means there are 14 h of sunshine, with the sun due east at 0900 and due west at 1650. There are 4 h when the elevation angle is above 60°, so more solar radiation is reflected by the window. However, it is still better to have the south-facing windows shaded during the summer months. The sun path across the sky can be plotted for any latitude and longitude for any day.

FIGURE 5.3  Path of sun for December 21 and July 21, location 35° N, 102° W. Solar noon occurs at different times primarily because of daylight savings time.

5.3.3 Shading The south-facing windows need to let sunlight in during the heating season and be shaded during the cooling season so that little direct sunlight enters the house. The simplest shading devices are exterior to the house, such as overhangs and awnings. Deciduous trees that provide shade on the east and west sides of the house are also effective. Even though the sun is highest and the longest day is on June 21, shading needs to be provided through August (Figure 5.4). There can be a problem with awnings as they require manual control, and if you live in a windy area they tend to get torn. South-facing windows transmit more energy during the winter than the summer due to the solar position of the sun, even though there are more hours of sunshine during the summer. The primary factor is the angle, such that less area is perpendicular to the sun, and the other factor is that there is more reflection for angles above 60°.Sites are available for determining the sun angle for shading for buildings. VRSolar, University of Southern California, is a Web site for teaching site solar access (http://www.usc.edu/dept/architecture/mbs/tools/vrsolar/index.html).For the Southern Hemisphere, the sun angles are symmetric, so shading needs to be on the north. For the tropics, the days are all about the same, 12 h of sunshine. However, now you need shading for windows on both the south and north sides of the house.

FIGURE 5.4 Overhang gives shade in summer and permits direct solar radiation into the structure in winter.

FIGURE 5.5 Direct gain with solar heat stored in direct mass (direct sunlight), indirect mass (reflected sunlight), or remote mass through convection.

FIGURE5.6 Direct gain through south-facing windows and clerestory. Clerestory gives direct gain into rooms that are not on the south side of the house. Overhangs shade windows in the summer.