Solar panels are most effective when they face the sun and their surface is perpendicular to the sun’s rays. How do you determine the position of the solar panels so that they will produce the maximum amount of energy in a day? What is the best orientation for solar panels?
The sun moves across the sky from east to west. The Sun’s position in the sky is determined by 2 coordinates – declination and azimuth. Declination is the angle between the line connecting the observer and the Sun and the horizontal surface. Azimuth is the angle between the direction to the Sun and the direction to the south (see figure at right).
Declination and azimuth. Solar panel orientation
Note also that the direction to magnetic south (i.e., according to the compass) is not always the same as the direction to true south. There are true and magnetic poles that do not coincide with each other. Accordingly, there are true and magnetic meridians. And from both of them you can read off the direction to the desired object. In one case we will deal with the true azimuth, in the other – with magnetic. True azimuth is the angle between the true (geographical) meridian and the direction to the object. Magnetic azimuth is the angle between the magnetic meridian and the direction to a given object. It is clear that the true and magnetic azimuths differ by the same amount by which the magnetic meridian differs from the true meridian. This value is called magnetic declination. If the compass needle swings eastward from the true meridian, magnetic declination is called eastward; if the needle swings westward, declination is called westward. Eastern declination is often referred to as “+” (plus) and western declination as “-” (minus). Magnetic declination varies from place to place. So, for Moscow region declination is +7, +8° and in general on the territory of Russia it varies within larger limits. See also “How to calculate true azimuth by declination and magnetic azimuth”.
Generally speaking, there are only three options to increase solar exposure to direct sunlight:
- Installation of solar panels on a fixed structure at the optimum angle
- Installation on a two-axis tracker (a rotating platform that can rotate behind the sun in two planes)
- Installation on a single-axis tracker (the platform can change only one axis, most often the one responsible for the tilt)
Options #2 and #3 have their own advantages (a significant increase in solar panel lifetime and some increase in energy production), but there are also disadvantages: higher price, lower reliability of the system due to the introduction of moving elements, the need for additional maintenance, etc.). We will consider the feasibility of trackers in a separate article, for now we will talk only about option 1 – a fixed structure, or a fixed structure with a variable angle of inclination.
Solar panels are usually located on the roof or supporting structure in a fixed position and cannot track the position of the sun during the day. Therefore, solar panels are usually not at the optimum angle (90 degrees to the sun’s rays) throughout the day. The angle between the horizontal plane and the solar panel is usually called the tilt angle.
Due to the Earth’s movement around the Sun, there are also seasonal variations. The sun does not reach the same angle in winter as it does in summer. Ideally, solar panels should be more horizontal in summer than in winter. Therefore, the tilt angle for summer operation is chosen less than for winter operation. If there is no possibility to change the angle twice a year, the panels should be located at the optimum angle, the value of which lies somewhere in the middle between the optimum angles for summer and winter. For each latitude has its own optimal angle of inclination of the panels. Only for areas near the equator, the solar panels should be placed almost horizontally (but even there they are installed at a slight angle to let the rains wash away dirt from the solar panel).
Usually for spring and fall, the optimal angle of inclination is taken equal to the latitude value of the area. For winter, 10-15 degrees is added to this value, and in summer 10-15 degrees is taken away from this value. That is why it is usually recommended to change the angle of slope from summer to winter twice a year. If there is no such an opportunity, the angle of slope is chosen approximately equal to the latitude of the terrain. Moreover, the angle of inclination also depends on the latitude of the area.
Dependence of solar panel output on the direction to the sun
Let’s calculate the amount of solar energy produced by the solar panels when the sun rays fall at an angle other than 90° with the following example:Example: the solar panels are oriented to the south, with no longitudinal tilt. The sun is shining from the southeast. A line drawn perpendicular between the solar panels and the direction to the Sun has an angle of 360/8=45 degrees. The width of one beam of incident solar radiation will be equal to tan (|90-45|) / sin (|90-45|) = 1.41, and the amount of solar energy received by the solar panels will be 1/1.41=71% of the power that would be received if the Sun was shining exactly from the south.