The
Crestone Eagle, December 2008:
The 100% solar heated home
story & photos by Paul Shippee
unless otherwise noted
 On
a clear sunny day in southern Colorado the sky is turquoise
and there is enough solar radiation falling on your home to
supply all your domestic energy needs.
To examine the feasibility of a 100% solar heated home, we’ll
focus here on how to achieve that for your space heating needs
and hot water supply, leaving aside solar electric for now.
While it is possible to supply 100% of your heat energy needs
with solar, conventional economics says this is not feasible.
However, “conventional economics” has been famously
termed by radical economist Hazel Henderson as a form of brain
damage. To me, this statement has been an invitation to think
outside the box and approach solar design as an innovative
art, to design with an expansive mind and practical skills
applied to architecture.
 The
economic feasibility question has to do with the problem of
laying out extra money (or financing) for the extra solar
collectors to get you to 100% solar heating during the two
coldest months of the year, then leaving them idle for the
other ten months, unused and not needed. Therefore, one is
advised to provide only 70% solar heating fraction, with the
remaining 30% supplied by fossil fuel. While this is reasonable
when narrowly applied to solar “equipment” (i.e.,
active systems), it does not strictly apply to passive solar
homes where the building itself is a solar collection and
thermal storage system, as well as displaying attractive architectural
features.
The basic principles of solar home design are quite simple,
cost effective, and can be arranged in architecturally pleasing
ways. These principles require solar designers to pay close
attention to a few basics:
- The siting, shape, and orientation of the home
- The quantities of heat loss from walls, windows, roof,
floors, and unsealed cracks
- The selection from a variety of passive solar system
types that allow solar warmth and heat to naturally enter
the building
- The type and quantity of thermal mass used to store the
daytime solar heat collected for night time use, without
overheating the rooms during the day
- The type and effectiveness of moveable insulation or
thermal shades employed as night insulation on large south-facing
windows
To these passive solar building components can then be added
considerations for active solar thermal equipment: flat plate
collectors, pumps, controls, piping, heat exchangers, thermal
storage tanks, etc. These “active” components
can supplement the passive solar heat supply and provide domestic
hot water year round. Solar collectors are typically mounted
on the roof, or on the ground near the home, and tilted at
an angle to optimize winter and summer heating needs. They
can utilize either plain water, anti-freeze liquid, or air
as the heat transport medium.
Here are some solar heating and energy conservation design
guidelines for the Colorado climate, including an emphasis
on the two keys to passive solar design: large volumes of
thermal mass, and night insulation for large south windows.
These are qualitative and general guidelines. For specific
rules of thumb and engineering quantitative numbers please
consult my Crestone Solar School website: http://www.crestonesolarschool.com
Siting, shape, and orientation:
The optimal house shape will generally be twice as long as
it is wide, with the long side facing solar south (or a few
degrees east of south) with no shading obstructions year round
to block solar rays.
 House
heat losses:
It is usually 2-3 times more cost effective to prevent excessive
heat loss by providing good insulation (high R value), rather
than to provide extra solar gain. Low E glass should be used
on the smaller north, east and west windows, but not on large
south-facing windows. Sealing all building cracks is good
energy conservation practice. As opposed to burning fuels,
you pay only once for insulation, and it works beautifully
for you year after year.
Passive solar types:
There are four basic passive system types; direct gain, water
wall, Trombe wall, and sunspace. These are all techniques
for using the building as a solar collector. Direct gain simply
lets the sun shine in through large south windows. Overheating
and fading fabrics are a liability here unless heat storage,
also known as thermal mass, is provided.
To block some of the intense direct gain from the sun, heavy,
dense materials (thermal mass) can be placed behind south-facing
glass to absorb the sun’s heat. Containers of water
are used to make a water wall. Earthen materials like adobe,
concrete, or rammed earth can be used in various configurations
to make a Trombe wall. An attractive indoor climate—a
sunspace or food and heat producing greenhouse—can be
created by placing the thermal mass materials several feet
away from the solar glass.
 Thermal
mass:
The function of heavy thermal mass materials (water or earth)
placed inside the building is to moderate day-night temperature
swings, and to store daytime solar heat gain for slow release
on cold winter nights. However, most people are skeptical
and are surprised to learn how much thermal mass heat storage
is actually required for a high-performance passive solar
home system that approaches 100% solar heating fraction. (Again,
consult my website for engineering numbers and quantitative
rules of thumb.)
Consider this question: How much is it worth to you to eliminate
the use of high-energy fossil fuels (oil, gas, electricity,
coal) for your low-energy, low-temperature home heating and
hot water needs? A general rule I like to consider is: you
can’t have too much thermal mass. Everything has its
price, and life-cycle cost/benefit analysis should always
be used when evaluating solar investments, comparing first
cost with long term benefit.
 Night
insulation:
Moveable insulation placed on large south windows on cold
winter nights is the key to obtaining high solar performance
in passive solar homes. There are many types of moveable night
insulation ranging from foam boards, shutters, the popular
and attractive cellular (honeycomb) thermal shades, bubble
wrap, blanket layers, draperies, beadwall (see photo), sliding
doors, and roman shades—with R values ranging from R1
to R20.
In some solar homes I have designed and built, I’ve
found two different ways that work to approach very close
to 100% solar heating; both employ lots of thermal mass for
the interior and exterior walls of various configurations.
One way is to open up the long south side of the building
with windows to solar south, and deploy night insulation there
with R 15-20 (e.g., beadwall). This is not easily done but
technical innovations will make it a more attractive option
in the near future.
The other way is to combine passive and active solar heating
components to make a hybrid system, using easily available
R 4-5 night insulation such as thermal shades. In this system,
passive south windows let in sunshine, while roof-mounted
collectors actively pump solar heated water directly into
pipes embedded in a thick radiant floor slab, from which heat
is released slowly into rooms at night.
This active system is highly efficient because the low, low
operating temperatures of a floor slab allow the roof collectors
to maximize their operating efficiency by running cooler.
Domestic hot water can then be heated as a parallel circuit
off the main radiant floor thermal system. For a schematic
diagram of this system please consult my website: http://www.crestonesolarschool.com
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