Solar energy for heating is an idea that is as old as people getting cold. The Greeks are credited with designing buildings to use the sun’s heat in the 4th or 5th century BC, using what are now called passive techniques, such as rooms facing south to capture the heat, but with overhanging eaves to prevent too much sunshine going in during summer. Passive techniques also include using heavy masonry construction on the south facing surfaces, which heat up during the day and release the heat slowly later.
This sums up the main challenge with solar heating. When the sun is shining, during the day, is the time when you’re least likely to need heating. At night, the sun is nowhere to be found, and you’re most likely to want some extra heat. Thus, all successful solar heating installations have to incorporate some sort of thermal storage, and there have been several different approaches to achieve this and give a uniform temperature night and day.
Passive solar heating systems
One early development of the passive system is called the Trombe wall, which is a heavy masonry wall with glass on the outside and an air space. The wall is usually dark, to get the most heat, and the glass ensures that the air space is also much hotter than the outside, capturing the maximum heat available. The Trombe wall is typically concrete, and 8 or 9 inches thick. It’s been found that heat goes through concrete at about one inch per hour, so heat going into this wall at lunchtime would only get through by the evening. Obviously, there is much less heat coming out into the room than went in, but this system of delaying the effect of the heat is automatic, needing no machinery or controls.
Active solar heating systems
Active solar heating systems use some sort of moving parts, such as fans or pumps, to control the system’s performance. These fall into two main types, heating either water or air with the sun. The solar collector is black, and has glass on the face for maximum effect. Water is pumped through tubes, or air blown through the collector. Hot water can be stored in a collection tank, and pumped out later to heat the room, whereas the heat in air is commonly stored by blowing it over a bed of rocks, and used later in the same way.
Heating directly with the sun’s heat is effective, and is used today in many areas of the world with adequate sunshine during winter.