Concentrated Solar Thermal
Heating a building with the sun has been done in two basic ways. Active and Passive strategies can reduce the use of fossil fuels. The main performance advantage of an active system is that there is no heat loss from the solar collector at night. In a passive solar home there is heat loss in winter through the windows 24 hours a day. Even when the radiant energy of the sun is penetrating the glass and heating the house, the window is conducting heat to the cooler outside air. There are only about 6 hours per day that the sun can effectively heat a building through the south façade or panels. Another advantage of an active system is the heat can easily be distributed through the house usually in a massive radiant floor.
Vertical windows project a somewhat smaller collection area to the sun than a sloped window and more of the sun’s energy bounces off. Sloped windows were used extensively in the early days of Passive Solar design. They do provide more heat in winter but create major overheating at other times of the year. Combining these disadvantages with the large heat loss of a window in winter make an active thermal system more cost effective. Adding tight insulating shades can offset much of this heat loss but cost and the sometimes short life minimize their desirability.
The angle of the collecting surface has an important role to play in maximizing the energy absorbed by the system. This angle is basically perpendicular to the sun during the heating season. Panels are sometimes laid flat on the roof so they are less visible. While most of us prefer the look, the roof is rarely steep enough for appropriate seasonal collection. The winter heating and hot water requirements are 3 times that of summer. In order to match the annual heat and hot water demands of a home a steeper panel angle is required. In northern New Mexico an angle of 60 degrees will provide maximum energy through the heating season. A negative effect of this angle is summer overheating. This can be avoided by tipping the panel up more toward the mid-winter sun to 70 degrees but this gives up some collection efficiency at other times of the winter. I had been using 70 degrees here in northern NM for many years. Another way to deal with the extra heat is by adding a summer dump loop. This adds complexity and cost. A shading device can be added which casts a shadow on the panel in summer. This also adds cost but if the underside is a mirror it can increase the heat collected by the low angled sun of winter thus lowering the overall cost of the system.
Concentrating the sun’s energy with a reflector is a way to more closely match the heat collected with the seasonal heating needs. This is done by mounting the panel perpendicular to the winter sun and adding a reflector. This adds heat in winter and casts a partial shadow on the panel in summer to protect from overheating. Correct proportions enable the system to collect enough heat in summer for domestic hot water while increasing the heat collected in winter.
The mirrors typically cost half or less than the collectors per square foot. This increases the solar window (collection area) and can provide more power at a lower cost. Reflective material specifically designed for this purpose is used in large parabolic concentrated solar electric plants. It is 94% efficient and can be applied to a variety of substrate.
This patent pending solar thermal application uses the same material but is applied to flat
aluminum sheets. A curved surface actually concentrates the suns energy and can be a fire
hazard. Since I use a flat reflector it only increases the collection area and has no associated fire
issue.
With computer modeling I’ve been able to minimize the building of prototypes while optimizing the appropriate angles. The first one installed in December 2008 doubled the solar window and nearly doubled the heat coming out of the panel. The reflector is at a fixed angle, so as the sun gets higher in the sky the there is less additional energy added by the mirror. By mid-April there is no reflected sun on the panel and as the sun continues to get higher in the sky a shadow cast by the overhanging mirror begins to limit the amount of solar collection. There are three lines coming down from above on the computer simulation. They represent the sun angle at 10 am, 11 am and noon. On systems designed to provide both space heating and hot water, the mid-summer shading is set to provide only enough heat for hot water.
The original prototype has a reflector below and one above. An unforeseen
problem occurs with this design. During the summer shading season the sun
bounces off the bottom reflector hits the top reflector and a very bright
reflection is projected forward that in many locations would be unacceptable.
For this reason I use only the overhead reflector in most applications.