Overview 2 Overview The TwinCAT PLC Solar Position Algorithm library (SPA) offers an option for calculating the sun position exactly at almost any time. The times for sunrise, solar apex and sunset can also be determined. In addition to the sun angles an angle of incidence can be issued, if the point of reference has a certain inclination. The sun angles themselves refer to the horizontal at the point of reference. The algorithm is based on a technical report by the U.S. National Renewable Energy Laboratory (NREL). The theoretical inaccuracy of the sun angles between the year -2000 and 6000 is specified as +/-0.0003°.
Based on this the function block of the TwinCAT Solar Position Algorithm library assumes an inaccuracy of +/-0.001° for the sun angles. Sun angles The position of the sun at a fixed observation point is normally determined by specifying two angles. In order to calculated the sun angles using the TwinCAT Solar Position Algorithm library, the date, time, longitude, latitude and further parameters have to be specified, depending on the required accuracy. The graphic illustrates the meaning of the main terms in this context: The sun position represented by two angles.
Longitude and latitude The latitude is specified as the distance of a place on the surface of the earth from the equator to the north or to the south in degrees. The latitude can assume a value from 0° (at the equator) to ±90° (at the poles). A positive sign thereby indicates a northern direction and a negative sign a southern direction. The longitude is an angle that can assume values up to ±180° starting from the prime meridian 0° (an artificially determined North-South line). A positive sign indicates a longitude in an eastern direction and a negative sign in a western direction. Examples: TS3900 8 Version:
1.1 Zenith The zenith angle of the sun is defined as the angle between the vertical above the observer and the connecting line between the observer and the sun. In some cases the altitude is as used to indicate the sun elevation angle. The following applies: 90° - zenith angle = altitude Azimuth The azimuth coincides with the horizon. North is 0°, with the value increasing in clockwise direction (east = 90°, south=180°, west=270°).