Daylighting and Solar Heat Gain
When interiors are illuminated with daylight, this requires electrical energy, and usually it also has a positive effect on people's well-being. Daylight is solar radiation, which is ultimately absorbed in the room and warms it in the process. Diffuse sky light causes rather little heating, direct sunlight on the other hand can act like a powerful heater and lead to overheating. These factors must be taken into account in a daylighting concept.
Overcast skies and direct sunlight occur about equally frequently in the Central European climate. Lighting with daylight must therefore function well under both conditions.
Diffuse sky light
In the case of an overcast sky and also in the case of unsunny windows, the aim is
to get as much as possible of the diffuse sky light available outside into the room.
The extent to which this is possible depends on the geometry of the building,
including the surrounding buildings, and on the arrangement and size of the windows.
If - as an experiment - a desk is placed outdoors and a fisheye lens is used to photograph upwards from its surface, the photo will show the entire upper hemisphere illuminating the workstation.
Graphics: D.Hennings
If we repeat the experiment with a desk in an office room, the photo shows a small section of the sky as a light source, which is limited by the window opening and the horizon, here by the neighboring building.
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The two graphs show that only a small portion of the luminous hemisphere of the sky can contribute to the illumination of an interior, even under the favorable conditions shown in the graph on the right, such as large, lintel-free windows reaching to the ceiling and a workplace close to the window. The daylight factor is a number describing the proportion of daylight available outside that is effective inside the room. Ideally, it should amount to a few percent near the window, so that office workplaces can be illuminated with natural light during bright daylight hours. With distance from the window, the daylight factor decreases, in deep rooms to far below 1%.
Direct solar radiation
In contrast to the overcast sky, direct sunlight often causes an excess of daylight and glare.
At the same time, the solar radiation absorbed in the room has a significant heating effect.
When the atmosphere is clear and the position of the sun is high, the sun's radiant intensity
is up to about 1 kiloWatt per square meter, of which up to 500 W/m² can arrive behind windows,
depending on the glazing and geometry. For comparison: a person heats with about 100 W in the
office, an energy-saving notebook computer with 20 W or less, and lighting with diffuse daylight or
with economical LEDs with a few W/m². Solar radiation can therefore quickly become by far the
dominant heat source and overheat rooms.
When direct sunlight enters a room through windows, it can cause glare from the bright light and overheating from the high heat input. The contrasts between sunlit and unsunlit surfaces become very large and can strain the eyes.
Graphics: D.Hennings
A venetian blind provides an adjustable sunshade that, when in the 'cut-off' position, blocks out direct sunlight in a way that maintains the visual relationship to the outside. A light colored blind also diffuses some of the sunlight to the ceiling of the room, improving the illumination in the depth of the room.
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To protect against both glare and overheating, sunlit windows therefore require effective solar shading that can be adjusted to the variable direction of sun incidence and removed for unsunny periods.
Only on north-facing facades the sun exposure is so low, even in summer, that sun protection is usually not required. At all other orientations, strong sunning can occur at times and sun protection may be required. The sun position diagram shows hourly positions of the sun for each month, where the hours are in local solar time, so that the sun is exactly at 12 o'clock in the south. The diagram shown is valid for 51° northern latitude (e.g. Cologne, Erfurt, Dresden). Sun chart diagrams for any latitude can be generated here.
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The approaches presented here for good daylighting without the risk of overheating interiors should be embedded in an overall concept for a pleasant indoor thermal climate with very low energy consumption at the same time. Ingredients of such a concept can be:
• Daylight-oriented geometry of the building structure and the interior floor plans as well as daylight-optimized window geometry with moderate aperture
• Sun protection adjustable to the need, light directing functions, neutral solar control glass
• Internal heat sources, no more than necessary for the use of the building
• Effective thermal storage masses for thermal day-night balancing
• Effective and energy-efficient removal of excess heat via ventilation, geothermal heat exchangers, component tempering or other processes
• Good air quality with adequate air exchange; heat recovery, if necessary cold recovery
• A very good thermal insulation
• Energy-efficient lighting and technical building equipment.
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by D. Hennings • www.eclim.de •