The geometry of the sun
Due to the revolution and rotation of the earth, the position of the sun relative to the photovoltaic array is always changing. Designers can use several geometric techniques to make the photovoltaic array capture as much solar energy as possible. The position of the sun can be specified by two angles, which change every day and every year.
In the northern hemisphere, since the sun generally appears in the southern sky, photovoltaic arrays are usually installed facing south; in the southern hemisphere, photovoltaic arrays are usually facing north. This is not the case in the area between the Tropic of Capricorn and the Tropic of Cancer. At certain times of the year, the sun appears in the southern sky in the southern hemisphere and in the northern sky in the northern hemisphere. Due to the natural tilt of the earth, the position of the sun in the sky in summer is generally higher than that in winter.
The longest and shortest days of the year are the summer solstice and winter solstice respectively, usually around June 21 and December 21. The solar altitude angle is highest on the summer solstice and lowest on the winter solstice. The midpoint between the summer and winter solstices is the vernal and autumnal equinox, usually around March 20 and September 23. Many prehistoric cultures learned to recognize and predict the summer solstice, winter solstice, vernal equinox, and autumnal equinox. These dates are very useful for harvesting and religious celebrations.
In the sky at any given location, the path of the sun can be depicted as a two-dimensional diagram of the path of the sun.
This picture is used to determine the position of the sun in the sky on any day of the year and at any time of the day. With this information, the occlusion time of this area can be determined, and in turn, the annual radiation can be calculated.
The composition of the solar path diagram includes:
●Azimuth angle, expressed as the circle in the figure.
●Elevation angle, expressed as concentric circles.
●The path curve of the sun from east to west on different days of the year.
●The timeline of the day that crosses the path of the sun.
●On-site information related to latitude.
The solar path diagrams in different regions may look completely different. At the equator, the path of the sun is symmetrical from north to south. Outside the tropic, the solar path diagram looks generally downward, while the solar path diagram in the northern hemisphere is the reverse. Note that the path of the sun has seasonal variations. Outside the tropic, the solar altitude angle is low in winter, usually north or south of the observer (depending on which hemisphere); the solar altitude angle is higher in summer. In all countries and regions, the sun is usually in the northernmost or southernmost position at the summer or winter solstice, in the north in June, and in the south in December.
The geometry of photovoltaic array installation
The position of the photovoltaic module is also called the orientation. The orientation of the photovoltaic array is very important because it affects the amount of light received by the array and therefore the amount of output power. Orientation generally includes the direction (such as true south) and the inclination angle that the module faces. The inclination angle is the angle between the photovoltaic module and the ground plane. Due to the movement of the sun in the sky, the light received by the array will also change with the time of the day.
Then it is clear that if the sun is directly above the sky and the photovoltaic module is flat on the ground, all the sunlight can be captured, as shown in Figure 1~Figure 3.
The installation location of photovoltaic modules should collect as much solar radiation as possible. To achieve this goal, photovoltaic modules should be installed facing true south (northern hemisphere) or true north (southern hemisphere). Depending on the site environment (for example, photovoltaic arrays installed in valleys in the southern hemisphere may not face true north), there will be some exceptional installation locations. In order to keep the photovoltaic modules facing the sun at all times, a set of sun tracking brackets needs to be installed. This method is more expensive, so it is not commonly used in most photovoltaic applications.
The international standard (SI) unit of energy is joule (J). The value of 1J is very small, so a large amount of energy such as solar radiation is usually expressed as megajoules (MJ). 1MJ is equal to 1000000 MJ and the conversion factor for kWh is 1kWh=3.6MJ, or 1MJ=1/3.6kWh.