Azimuth And Altitude Of The Sun At A Site

The angle with which the sun strikes at a location is represented by the terms 'altitude' and 'azimuth'. Altitude is the vertical angle in the sky (sometimes referred to as height); azimuth is the horizontal direction from which it comes (also referred to as bearing). Altitude angles range from 0° (horizontal) to 90° (vertical: directly overhead). Azimuth is generally measured clockwise from north so that due east is 90°, south 180° and west 270° (or -90°).

North Horizontal Sun Path Diagram

Because the Earth revolves around the sun once a year, we have four seasons. The Earth's axis remains in a constant alignment in its rotation so twice a year the incoming solar radiation is perpendicular to the latitude of the equator and only once a year is it perpendicular to the tropics of Cancer and Capricorn, as shown in Figure 7.2.

The changing values of azimuth and altitude angles are predominantly a reflection of the changes in the relative positions of Earth and sun. These are governed by:

• the rotation of the Earth around the sun;

• the rotation of the Earth about its axis.

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Solar Azimuth And Altitude
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One of the simplest tools we can use for the derivation of altitude and azimuth angles is a graph using Cartesian coordinates (Figure 7.3). Figure 7.3 incorporates two types of line. Firstly, those representing the variation in altitude and azimuth over the period of a day (given for the 21 st or 22nd day of each month). Secondly, those joining the points on the altitude-azimuth lines for a specific hour. Thus the solar angles for 11 a.m. on 21 March may be read off on the horizontal and vertical axes where these two lines meet (altitude 36°, azimuth 19°). Values for other days may be read by interpolating between these lines.

How Read Sun Chart

Sunpath diagram for 52°N on cartesian axes (Andrew Bairstow).

Equidistant vertical sunpaths, LAT = 52°

Equidistant vertical sunpaths, LAT = 52°

Andrew Bairstow

Sunpath diagram for 52°N on cartesian axes (Andrew Bairstow).

It may appear that these are the only determinants of angular position, however we are actually concerned with the direction of the sun's radiation rather than the Earth-sun position. Also, radiation does not travel in an entirely straight line but is bent slightly by the Earth's atmosphere.

The distance between the Earth and the sun is approximately 150 million km, varying slightly through the year with the variation of the azimuth and altitude angles with time.

The sun emits solar radiation that arrives at the Earth in the form of a number of different wavelengths ranging from infrared, the hot end, through the visible light spectrum to ultraviolet light (http://www. fridge.arch.uwa.edu.au/).

Solar Incidence

Solar incidence at different latitudes. Closer to the equator, where the sun is more directly overhead, the sun is 'hotter' because the energy density is determined by the angle of incidence (Mazria, 1979).

All passive solar features involve the transmission of solar radiation through a protective glazing layer(s) on the sun side of a building, into a building space where it is absorbed and stored by thermal mass (for example thick masonry walls and floors or water-filled containers). The typical processes involved are:

Solar incidence at different latitudes. Closer to the equator, where the sun is more directly overhead, the sun is 'hotter' because the energy density is determined by the angle of incidence (Mazria, 1979).

• collection - to collect solar energy, double-glazed windows are used on the south-facing side of the house;

• storage - after the sun's energy has been collected, some heat is immediately used in the living spaces and some is stored for later use. The storage, called thermal mass, is usually built into the floors and/or interior walls. Mass is characterized by its ability to absorb heat, store it and release it slowly as the temperature inside the house falls. Concrete, stone, brick and water can be used as mass;

• distribution - heat stored in floors and walls is slowly released by radiation, convection and conduction. In a hybrid system, fans, vents and blowers may be used to distribute the heat.

There are several types of passive solar system that can be used in homes. The most common are direct gain, indirect gain and isolated gain.

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Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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Responses

  • jago
    How to read a sun path chart?
    8 years ago
  • rhoda gamgee
    What is the angle of incidence and altitude of the sun in the equator?
    8 years ago

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