Concentrated Solar Thermal

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.

The energy consuming elements of a building

Much of the energy consumed by buildings is used during construction including the embodied energy of the materials. Transportation is becoming an even larger piece of this equation. There is also the energy consumed during the life of the building. This can be greatly reduced if we make energy consumption important during the design of the building.  

Strategies for designing an energy efficient building in a sunny cold climate.

 A.  Let the energy in (Solar exposure)

 B.  Store it (Mass/Batteries)       

C.   Keep it in (Insulation)

Passive Solar Design 

A no-moving-parts-strategy where the form of the house is a solar collector in winter but not in summer.  

Add insulation around some mass (stone, adobe the more natural the better) and in this climate the house will stay comfortable year round with little or no help from fossil fuels.  

Active Solar systems 

  Collect the suns energy and distribute it in the   building either with wires in the case of electric or plumbing for heating systems.                                                                                        

“Solar Thermal systems” are the most widely used and are very effective in a cold climate.  A typical space heating system pumps a fluid through a solar panel and then into storage in a radiant high mass floor or a tank for domestic hot water.  

In Search of Cheap Mass

This first solar project I did was in the late 70’s.  It was an addition / remodel of an old stone farmhouse.  We enclosed it inside a solar envelope design.  With this much mass the night day swings were small for a passive solar house.  

It’s well understood that large amounts of thermal mass store the suns energy to keep the house warm at night and through cloudy days, and it’s often expensive.  The stone house was already there so enclosing it was more cost effective.  

Water is an excellent heat medium but takes up space and the potential leaks can be avoided by using solid materials.  Adobe is a good choice, especially in the southwest.  It has low embodied energy (if un-stabilized) and performs well as a heat sink though not nearly as well as stone or concrete.  It holds almost as much heat but moves it much more slowly.  To get a thermal performance comparable to concrete, an adobe building would need about twice as much surface area of exposed mass.  Adobe has some advantages.  It balances the humidity and softens sound, but is expensive compared to wood framing so using it only on inside walls (exposed on both sides to the inside air) would be more cost effective. 

Mass wall with radiant heat tubing

Another variable to be considered when designing mass into a solar building is getting the heat into and out of the storage medium.  If the sun’s direct rays strike a massive surface there will be good heat transfer.  If the mass is in the same room with the sunshine but only gets indirect radiation, about 40% is absorbed into the exposed mass.  One strategy to get deeper penetration of direct solar gain into a house is to have an internal mass wall struck on one side by the sun.  Again mass walls are expensive.

Some years ago I started to look at thicker mass floors to save some expense and retain the solar thermal performance of a high mass building.  This was done by excavating some extra soil below where the slab would be poured, placing the insulation deeper, and compacting several inches of gravel and fines below the slab.  This is a cost effective way of containing more mass within the insulated envelope.  As the solar heated fluid raises the temperature of the concrete the heat also migrates into the gravel below adding to the stored heat in the floor.  Data loggers placed at the bottom and top of the mass have shown the daily pulses of heat and also when clouds have blocked the heat gain.

As time went on I increased the thickness of the mass to 16” with 3” of insulation below.  A newer project has 32” of mass over load bearing insulation that is continuous under the footing, to eliminate the thermal leak.  Four inches of perimeter foundation insulation is used as the temperature of the ground at the edge is much colder in winter than under the house.  There is a second set of radiant tubes placed in the gravel 8” above the insulation.  They are also fed by the solar collectors as a surplus dump.  This lower set of tubes will continue to be fed slightly during the summer to maintain the 270 tons of mass inside the insulated envelope at about 70 degrees.  As long as the floor surface temperature is kept below the room temperature no heat will be transferred to the living space.  Temperatures are monitored by data loggers at 4 different levels.  One was placed below the insulation, one just above the insulation, another in the middle of the mass and one in the slab.  The heat transfer through the mass is working as predicted, allowing the stored heat to migrate to the slab during cloudy spells.

A heat recovery ventilator is a vital part of an energy efficient home.  Heat recovery ventilators have a minor problem in this cold climate.  They tend to frost up below 12 degrees and use a 1000 watt heater to prevent frost.  In this climate where temperatures often go below this at night, there is a fair amount of energy consumed.  To prevent this, a 6” dia. pre-heater is cast in the mass below the slab.

Much of the thermal performance of this house is below the floor and the energy conserving elements above complete the package.  Compressed earth block is used for most of the walls both inside and out.  Closed cell spray foam is used on the exterior walls and is the only efficient option for insulating an external mass wall.  Twenty inches of cellulose is used in the roof and R-9 quad windows and doors are used throughout.