Over the course of the year the angle of the sun will change. The earth revolves on an axis that passes through the North and South poles. This axis is tilted in respect to the sun. This tilt is what causes the seasonal changes in the length of the day. It also affects the angle at which the sun hits the earth. In the summer the sun is high in the sky, shinning close to straight down. During a New England winter the sun is much lower in the sky, shining on a more horizontal angle. When the sun hits a collector panel at an angle, as opposed to straight on, the energy collected is reduced.
In the summer the sun is more directly overhead, while in the winter the angle is lower. This change has a direct impact on the amount of solar radiation absorbed by a solar thermal or PV collector. Most PV panels convert less than 20% of solar radiation to power. To get the best performance the panels need to face true south. If they are placed East/West you can expect a 20-25% reduction in output. PV panels can also “track” the sun using mechanical means (at additional cost), in effect maximizing the angle and hours of collection. Solar thermal panels are more forgiving as they can collect and covert up to 90% of the radiation that strikes them. With solar thermal, tracking is unnecessary. But if you are using solar thermal for heating applications winter performance is very important. Here in New England we have short winter days and for year round applications we site the collector to gather as much winter sun as possible. This means the collector will be at about a 45-55° angle. Since summer brings long days, your system will have plenty of time to create hot water despite the fact the panel is not directly pointed at the sun.
The changes in the sun’s angle can also create seasonal shading issues. As the sun gets lower in the sky through the winter it is common for roof lines to create shadow areas. In the summer trees are covered with leaves that might cause shading issues. We use a tool called a Solar Pathfinder to check for shading issues whenever we have any concerns. It can identify shade areas, throughout the seasons, and throughout the day. Since the hours of 9:00-3:00 give the most energy, it is important that the collectors are relatively shade free during this time of day. If trees are causing a problem often just topping them can let the needed sun through. In some cases summer shading can be an advantage to a solar thermal system, especially if it is sized for space heating, reducing excess summer production. If rooflines are the issues it is often possible to install extra collectors. With PV systems we use micro inverters, such as Enphase, so that one shaded module will not bring down the performance of the entire array as happens with a single string inverters
We use a tool called a Solar Pathfinder to determine the sun exposure for a location. A Pathfinder is oriented and leveled using its compass. The clear dome projects the reflections of nearby objects onto a special chart, marked with months, so shading issues can be identified. If you have a tree or roof line that may shade the area you plan to place your collector a Pathfinder can determine if, and to what extent, the shading will be a problem, in each of the seasons and throughout the times of day.
A handheld solar pathfinder is set up for the geographical location and reflects all the surrounding trees and buildings onto a special screen to determine seasonal and hourly shading issues.
Solar Azimuth angles refer to “true” north line. This is the line at the earth’s surface this is parallel with the earth’s polar axis. The earth’s magnetic field is not aligned parallel with its polar axis. In some US locations a compass needle is off as much as 20° east or west or a true polar N/S line. Portland is off about 16° west. If you were to orient a solar collector to a south compass you would be facing about 16 ° east of true south. This would impact the collection of solar radiation. In addition the magnetic poles are in constant minimal movement.
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We also take into consideration day light savings time. For truly critical applications it is even possible to consider the geographic displacement within the time zone. For example the sun placement at noon in Boothbay ME is different than noon in Corning NY despite the fact the hour on the clock is the same.
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