Solar Radiation

And it’s General Applications

Nitin Jayswal

Solar RadiationSolar radiation describes the visible and near-visible (ultraviolet and near-infrared) radiation emitted from the sun.

Radiation contains following rays of wavelengths:Ultraviolet: 0.20 - 0.39µmVisible: 0.39 - 0.78 µmNear-Infrared: 0.78 - 4.00 µmInfrared: 4.00 - 100.00 µm

Solar RadiationPart of the solar radiation entering the earth’s atmosphere is scattered and absorbed by air and water vapor molecules, dust particles and water droplets in clouds and thus the solar radiation incident on earth’s surface is less than the solar constant.

Solar RadiationThe part of solar radiation that reaches the earth’s surface without being scattered or absorbed is the direct radiation. Solar radiation that is scattered or reemitted by the constituents of the atmosphere is the diffused radiation.

Solar Heat Gain Through WindowsWhen solar radiation strikes a glass surface part of it (about 50% for uncoated clear glass) is reflected back to outdoors, part of it (5 to 50%, depending on composition and thickness) is absorbed within the glass and remainder is transmitted indoors.

Solar Heat Gain Through WindowsThe conservation of energy principle requires that

τs + ρs + αs = 1Where:

τs is Transmissivity

ρs is Reflectivity

αs is Absorbptivityof glass for solar energy.

Solar Heat Gain Of The BuildingThe solar radiation absorbed by the glass is transferred to the indoors and outdoors by convection and radiation. The sum of transmitted solar radiation and the portion of the absorbed radiation that flows indoors constitutes the solar heat gain of the building.

Saving Electricity By Using DrapesDraperies reduces the annual heating and cooling loads of a building by 5 to 20 percent.Light-colored draperies made of closed of tightly woven fabrics maximize the back reflection and minimize the solar gain.Dark-colored draperies made of open or semi-open woven fabrics minimize the back reflection and maximize the solar gain.

Shading CoefficientThe shading coefficient of drapes depend on the way they are hung. The width of drapery used is usually twice the width of the draped area to allow folding of the drapes and give them full or wavy appearance.A flat drape behaves like an ordinary windows shade thus having a higher reflectance and lower shading coefficient .

External Shading DevicesExternal shading devices such as overhangs and tinted glazings do not require any operation, and provide reliable service over a long time without significant degradation.A properly sized overhangs blocks off the sun’s rays completely in summer while letting them in in winter.

Indoor Shading DevicesThe primary function of an indoor shading device is to provide thermal comfort for the occupants.The emitted radiation and the transmitted direct sunlight may bother the occupants near the window.In winter, the temperature of the glass is lower than the room air temperature, causing excessive heat loss by radiation from the occupants.

Low-e Film in cold ClimatesIn cold climate where the heat load is much larger than the cooling, the windows should have the highest Transmissivity for the entire solar spectrum. Low-e windows are well suited for such heating-dominated buildings. Properly design and operated windows allow more heat into the building over a heating season than it loses, making them the energy contributors rather than the energy losers.

Low-e Films in Warm Climates In warm climate where the cooling load is much larger than the heating load, windows should block off the infrared solar radiations. Low-e windows can reduce the solar heat gain by 60% with no appreciable loss in day lighting.

References:Heat and Mass Transfer (A Practical Approach)by Yunus A. Cengelwww.wikipedia.org