Introduction to solar energy/Solar resources and availability
On average the solar radiation arriving at the top of the Earth's atmosphere is approximately 1361 W/m². No matter what time of day or year it is, the Earth, seen from the Sun as a circular disk, will receive approximately 1361 W/m² at all times. For a specific location on the Earth's surface, the amount of solar irradiance received, will depend on many factors, some of which are periodic, and others, variable.
The periodic variables, are those that repeat cyclically with periodicity measured in hours, i.e. the day, or in days, i.e. the month or the year, and others in much larger time scales, as the orbit of the Earth around itself slowly changes and as the orbit of the Earth around the Sun slowly changes.
Relevant periodic timescales for solar engineering are:
- Day/night cycles
- Seasonal cycles
Other variable, and difficult to predict factors are:
- Cloud coverage
- Other natural weather events
Luckily, there is a plethora of equations designed to predict with a high-level of accuracy, the daily paths of the sun over the horizon.
Latitude and longitude
[edit | edit source]In addition to the position of the planet around the sun, the amount of sunlight available at any point on the Earth's surface will also depend on the longitude and latitude of the geographic position. Latitude is a measure of how far north or south you are, and varies from -90° (90° south) to +90° (90° north). The longitude is a measure of how far east or west one is, and varies from -180° (180° west) to +180° (180° east). Thus, at the north pole, the latitude is 90°, at the equator, the latitude is 0°, and at the south pole, the latitude is exactly -90°.
True solar time
[edit | edit source]True solar time (TST) is used to convey the relative position of the Sun compared to the Earth, irrespective of the position on Earth. Similar to the 24:00 clock, TST is measured in hours. However, specific times, such as noon, also known as, solar noon, conveys a specific and precise definition of the position of the Sun in the sky. At solar noon, the sun is at its highest point in the sky. The following equation returns the TST in hours:
Where UTC stands for Universal Time Coordinates, longitude is measured in degrees, and EoT represents the equation of time in minutes, defined later in this chapter.