Summer ShadingConcentrating Solar Thermal
The sun has been used to heat buildings in two basic ways for many years. Active and Passive strategies reduce the use of fossil fuels in buildings. The main performance advantage of an active system is that there is no heat loss at night through the windows like in a passive solar design.
There are about six hours per day that the sun can effectively heat a building whether it is through south facing windows or solar panels. Since only one quarter of the time the building is being heated, capturing this heat is critical in maintaining a comfortable temperature.
The most cost effective way to reduce the carbon footprint of a building is to minimize its heat loss before designing a heating system. A buildings shell can be compared to a thermos. Technology has advanced in recent years to the point where heat loss is greatly reduced. Roof insulation of R-75, wall and floor insulation of R-40 is not overkill in most of the United States where freezing temperatures are the norm in winter.
Air tightness is important, as this amount of insulation would go to waste if cold air were to leak into the building. This level of tightness creates its own challenge as the building must have fresh air. An Energy Recovery Ventilator becomes necessary to provide fresh air, while capturing most of the heat of the outgoing air.
The windows and doors complete the high performance envelope. Windows that let the sun’s energy in also let it out. Low E can help but very little compared to an R-40 wall. Modern windows and frames are R-9 or more. They block much of the heat of the sun, but are essential to balance the heat loss of the building components.
Sun Angle
The angle of the collecting surface is important in maximizing the energy absorbed by the system. The ideal angle is perpendicular to the sun during the heating season. Panels are sometimes laid flat on the roof for aesthetic reasons. While the look is less intrusive, the roof is rarely steep enough for appropriate seasonal collection. The winter heating and hot water requirements are three times that of summer. In order to match this seasonal energy demand, a steeper panel angle is required to provide the additional heat in winter. This is even more pronounced in higher latitudes as the sun is lower in the sky. A rule of thumb is latitude plus 15-20 degrees to maximize solar collection during the heating season. At this angle there is extra heat in summer beyond the needs of the domestic hot water system. This heat has to be transferred somewhere or the panels will continue to get hotter until the pressure relief valve blows. Summer over heating can be avoided by adding a summer dump loop. This adds complexity and cost. Another way to deal with the extra heat is by tipping the panel more toward the lower mid winter sun at 30-35 degrees plus the latitude, but this also gives up some collection efficiency in winter as well. I have used this strategy for many years to simplify the system, always looking for a better way to balance the solar collection with the seasonal heating requirements.
Concentrating
Concentrating the sun’s energy with a reflector is a way to increase winter collection while providing only enough heat in summer for the domestic hot water. This is done by tilting the panel perpendicular to the winter sun (15-20 degrees plus latitude) and adding a mirror to the top of the panel. The effective collection area is increased in winter and the overhanging reflector casts a partial shadow on the panel in summer. Correct proportions enable the system to produce enough heat in summer for domestic hot water and double the energy of the panels alone, in winter.
A mirror placed at the bottom of the panel can also increase the collection area of the system. If a reflector is used on both top and bottom at the same time a problem arises. The high summer sun bounces off the bottom reflector and hits the top one. At certain sun angles a bright light reflects out horizontally. In most locations this would not be acceptable. This mirror-like material made by Reflectech is 94% efficient. It would be like the low west sun shining deep into your neighbor’s house.
A bottom reflector can be used instead of a top one to provide a significant increase in solar gain. If mounted on a flat roof a 3’ high panel and a bottom reflector may be completely invisible from the street. This design does not produce a shadow in summer to protect from summer overheating but with a panel angle of 70 degrees in Santa Fe, NM for example it is safe from overheating. With a bottom reflector added it will produce almost as much energy year round as a top reflector design and besides being less visible it also has less wind loading. A drawback of the lower reflector is the accumulation of dirt, which is insignificant on the top one.
A small amount of dirt does not affect the performance very much, but as more dirt accumulates the reflective performance diminishes. Rain is effective in cleaning this bottom reflector but varies depending on location and season. In the large concentrated solar electric power plants they use a special truck that washes the reflective surface to get every one percent of power possible.
We are currently looking into a tracker system to continuously optimize the angle of the reflector. Even manual adjustment of the reflector once or twice a year as is often done on solar electric systems produces more annual heat per square foot of collector. This option also gives added control as any one year may differ from another.
As the incident angle of the sun with the collecting surface increases (closer to perpendicular) two things change. One is the projected area that the sun sees of the panel. As the panel is tilted away from the sun the solar window gets smaller. At 30 degrees the solar window is half of the collection area at 90 degrees. Thirty degrees is also an important number in terms of the amount of energy absorbed. At 75 degrees most of the sun’s energy penetrates into the panel and is absorbed. This is true for the rays directly striking the panel from the sun and also the reflected rays from the mirror. At 45 degrees there is still considerable heat gain, but much below this it drops off quickly and by 30 degrees it’s very little.
Concentrating the sun’s energy can be dangerous. Most of us have seen what can happen when the sun passes through a magnifying glass. A curved surface is needed to concentrate the energy. Since I use flat reflectors there is actually no concentration or the associated fire hazard. The added collection area and seasonal balance are the benefits of this system.
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