UNDERSTANDING SOLAR ENERGY for BUILDING DESIGN – Part-1

Post 536  by Gautam Shah

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SUN > Wikipedia image from NASA by Author NASA/SDO

Sun is a powerful source of energy as a result of its internal fusion reactions. Part of this energy is transmitted to the Earth, through space by electromagnetic radiation. The strength of solar radiation at the outer edge of the Earth’s atmosphere (the solar constant), is 1.37 kW per sqm. The intensity of energy actually available at the Earth’s surface is less than the solar constant. This is due to shielding effect of the Earth’s atmosphere, and also for the absorption and scattering of radiant energy.

Wikipedia image by Author Christian Thiergan

The process, climate starts with the arrival of radiant energy (radiation) from the Sun, to our planet. Energy enters into the precinct of Earth from many sources and in different forms. Earth receives electromagnetic energy from other bodies in space, and it also experiences gravitational energy associated with their masses. However the most significant of all, is the energy received from Sun.

Smog cutting off the Sun in Peking Wikipedia image by Author Berserkerus

During its passage through the space, the solar radiation loses little energy, but on entering the atmosphere it encounters molecules of gases, liquids and solids, Ozone and water vapour. These absorb radiation, but are affected by different sections of the solar spectrum. Ozone absorbs ultraviolet radiation, having a profound effect on the development of life. Water vapour absorbs an infrared sector. Gases and suspended matter disperse the incident solar radiation, into multi-directional radiation, some of which passes back to the space. In the visible spectrum the blue light is scattered to a greater extent than other wave lengths, resulting in predominantly blue sky. Scattering by suspended materials is termed as diffused selection. The amount of scattering that takes place depends on the size of the particles, particle density in the air and the distance radiation travels in the atmospheric layer containing particles. Sahara dust storms can reduce the solar radiation transmission by 30% and causing a fall of 6.0 C.

Earth energy budget diagram Wikipedia image by NASA

Atmosphere absorbs approximately 17 units of the total 100 units of the solar radiation. This small absorbed component (17 units) contributes to an increase in the internal energy store of the atmosphere. Of the remaining 83 parts, roughly 29 units are lost to the space by reflection, of which 6 units are lost by scattering and 23 units are lost by cloud reflection. The other 54 units are transmitted to the Earth’s surface, of which 36 units arrive as direct radiation, and 18 units by diffuse radiation through the scattering.

Wikipedia image by Author neuro + copyright holder of this image, Christopher Down

  • Energy sources and energy stores:                             Energy x 1020 J
  • Total annual receipt of solar energy by the Earth         54385
  • Energy released in Chinese Earth quake in 1976          5006
  • Combustive energy stored in Earth’s coal reserves       1952
  • Combustive energy stored in Earth’s oil reserves            179
  • Combustive energy stored in Earth’s gas reserves          179
  • Annual consumption of energy in USA                                 3
  • Heat flux from Earth’s interior                                               0.027
  • Total radiation from the Moon                                              0.006

Earth receives only 0.002% of the total radiation emitted by the sun, and, yet the Sun is the main energy provider for Earth. The solar radiation consists of, on average 7% ultraviolet (short wavelengths), 50% of visible wave bands, and 43% of infrared (long wavelengths) radiation. The radiation that penetrates the surface, is absorbed and heats up the surface. It evaporates the water, thaws the snow, drives the air currents, and causes a variety of chemical reactions. Earth’s atmosphere, oceans, and plant life are major collectors of solar energy.

solar radiation spectrum for direct light at both the top of the Earth’s atmosphere and at sea level wikipedia image by Author Nick84

The Earth receives varied amounts of solar energy. At macro level this is due to diurnal exposure, solar flares and solar spots, inclination or tilt of the Earth’s axis of movement, and elliptical orbit around the Sun. Regions beyond 23 N and 23 S, are exposed to Sun only for a part of the year.

Wikipedia image by Author Muhammad Mahdi Karim

The upper surface of cloud is a good reflector of solar radiation. The amount of reflection depends on the degree of cloud cover, type and thickness. A dense cloud may reflect 50% whereas a heavy storm cloud may reflect 90% of the radiation. If there is persistent cloud cover, as exists in some equatorial regions, substantial part of the incident solar radiation is reflected back to space. Water surfaces have low reflectivity (4-10℅) and so are very efficient in absorbing. Snow surfaces, on the other hand, have high reflectivity (40-80℅) and are poor in absorption. High-altitude desert regions absorb more solar radiation because of the reduced effect of the atmosphere above them.

….to continue in next part …..

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