Passive Solar Building Design in Shenzhen
Comparing shading techniques throughout the year

Sephir Hamilton

12 April 2000

These shading design suggestions assume that a primary design goal is minimizing solar heat load to reduce energy use. Designers should balance these suggestions with other design issues, such as daylighting and aesthetics, to produce a good passive solar building.

Eastern and Western Windows

·        Minimize window exposure by orienting the building to face south and by using small windows.

·        Either use deep overhangs (depth approximately equal to the window height), or exterior sun screens (SC = 0.3) on all windows.

Southern Windows

·        Use shallow overhangs (depth approximately equal to 30% of the window height), and shallow fins on both sides (depth approximately equal to 30% of the window width).

·        Use interior blinds (e.g. venetian blinds) to give occupants control over low wintertime sun.

Northern Windows

·        Use shallow fins on the western side of windows (depth approximately equal to 30% of the window width) to block late day sun in the summertime.

This paper justifies the shading design suggestions stated above for an apartment building in Shenzhen, China.

Solar energy travels through the windows of a building, increasing the energy used by air-conditioners and degrading occupant comfort. An ideal passive solar building minimizes internal energy use (e.g. electricity and heating fuel) by optimizing its design to suit the annual path of the sun and the local climate.

In temperate climates (such as Beijing and Boston), buildings use energy to heat during winter and to cool during summer. A good passive solar design in a temperate climate captures the energy of the sun during winter but keeps the sun off during the summer.

Buildings in the warm climate of Shenzhen, however, should keep the sun off for most of the year. Designing a building that minimizes solar heat gain throughout the year is crucial to an energy-efficient design.

Optimizing building shape, orientation, and shading design will produce an energy-efficient solar building. This paper suggests an optimum shape and orientation based on annual solar data (see figures 5-7). It also compares several shading schemes using solar energy calculations for each window direction (north, south, east, and west). A discussion of the shading schemes indicates advantages and disadvantages of each scheme, and leads to the shading design suggestions stated above.

The Sun in Shenzhen

Building designers should understand the path of the sun at the site. Obviously the sun rises in the east and sets in the west; days in the summer have more daylight hours than do days in the winter; the sun is higher at noon than early or late in the day.

However, a good passive design dictates that designers know more details such as sun path, elevations during different days, and azimuth angles throughout each day of the year. For Shenzhen (22.18 N latitude), Figures 1 shows elevations at noon on three dates, and Figure 2 shows azimuth angles (sun-path from a bird’s-eye view) throughout the day on those dates.

Figure 1 – Sun elevation at solar noon on June 21, September 21, and December 21

Figure 2 – Azimuth sun angles on June 21, September 21, and December 21

People living in “northern” cities immediately note that the sun looks very different in Shenzhen than in cities such as Boston or Beijing. At 42N latitude (Boston), the maximum noon-time sun elevation is 72 degrees in summer; the minimum noon-time elevation is 25 degrees in winter; the sun is north of a building (absolute azimuth angle greater than 90 degrees) only during early morning and late afternoon in June and July. In Shenzhen the maximum sun-elevation is 89 degrees in summer, the minimum noon-time elevation is 45 degrees in winter, and the sun is north of a building throughout the entire day from June 4 to July 10 (and during much of the day the rest of summer).

Sun paths and angles are important in understanding the sun, but designers must also know how much solar energy will hit a building. Figure 3 shows that each surface (north, south, east, and west) sees different sun levels during the day. The plot shows the solar power per area of window (kW/m²) incident on each surface on August 21. The western window curve (not shown) is a mirror image about solar noon of the eastern window curve (shown). Note also that near June 21st there will be no southern exposure and more northern exposure, but near December 21st the opposite is true.

Figure 3 – Solar energy during a single day incident on windows in Shenzhen

Orienting a Building for the Sun

The following general Shenzhen information is useful when designers orient a building to optimize solar heat gain:

• During winter months, southern windows receive more solar energy than do other windows at any time.
• During summer months, southern windows receive very little solar energy while northern windows receive some (though relatively little) solar energy.
• Eastern and western windows receive consistent solar energy throughout the year.

Shenzhen’s climate is hot and humid during most of the year. Minimizing solar heat gain through windows, especially during the warmest months, will reduce air-conditioning loads and improve the energy-efficiency of a building.

Eastern and western windows see significantly more solar gain during the summer months. Southern windows, however, see very large solar gains during winter months. Opening windows during winter (when air is cooler than comfort levels) balances the excess wintertime solar gains and solves the problem. Reason then dictates this optimum orientation and shape for a Shenzhen building:

A building should have minimum window (and surface) exposure on its eastern and western faces. Therefore, a slender building with its long sides facing north and south is desirable.

Figure 4 – Orientation and shape of a building to minimize solar exposure

Using Shading to Keep the Sun Off

Once a building is oriented properly, designers should add adequate shading to improve its energy-efficiency.

Two fundamental shading strategies exist for reducing solar energy that passes directly into a building. The first method places obstructions between the sun and the window (e.g. trees, external shades, or adjacent buildings). The second method uses the window itself to reflect excess sun (e.g. reflective coatings or interior blinds).

>This discussion focuses on these four practical techniques to shade windows: exterior shading screens (similar to insect screens), interior shades (such as venetian blinds), window overhangs (above the window), and window fins (on the sides of the window). Another practical shading technique (not considered) uses vegetation, adjacent structures, or other natural obstructions.

Figure 5 – Practical shading techniques discussed in this paper

Figure 6 – This MIT building uses inset windows to create deep overhangs and fins

The effectiveness of a window’s shading properties is measured by the window’s shading coefficient (S.C.). The amount of sun energy (Q in Joules) a window lets in is given by:

Q = 0.87*S.C.*E

,where E is the solar energy hitting the window (in Joules).

The shading coefficient compares how much sun a piece of tinted glass lets through compared to a standard clear piece of glass. A clear piece of glass has a shading coefficient of 1.0, while an opaque wall has a shading coefficient of zero. Shading coefficient values are found experimentally. Overhangs and fins can not be quantified using shading coefficients because the amount of shading they provide changes dramatically throughout the day and year.

To find the best method for shading windows in Shenzhen, this paper compares eight shading cases (using combinations of the above techniques). The table below describes each case while graphs show the performance of each case throughout a year for each window direction (north, south, east, and west). The total solar energy gain per area of window (MWh/m²) for the year (and for the nine-month period excluding winter) further shows how each design performs (shown as bar charts).

Figure 7 – Description of the eight shading cases

The graphs below present the results of each case as the solar energy per area of window area (kJ/m²) over one day (the 21st day of each month) for a northern window, a southern window, and an eastern-western window.

Northern windows see minimal solar gains, and only during the warmer months (April through August). Southern windows see large gains during winter months but relatively little during the summer. Eastern-western windows see relatively even gains throughout the year.

Figure 8 – Daily energy gain for a Northern window

Figure 9 – Daily energy gain for a Southern window

Figure 10 – Daily energy gain for an East or West window

The total solar energy blocked by each shading case is of most interest. The charts below show the total direct solar energy per area of window (MWh/m²) for each case summed over the year and over the nine-month period between March and November.

Note that the total energy for the building will be found by adding the north window energy, south window energy, and twice the east window energy (to account for the east and west windows together).

Figure 11 – Annual energy gain per unit area of window

Figure 12 – Nine-month energy gain per unit area of window