Solar Energy Storage Wall for Passive Survivability
David M. Delaney
June 10, 2006
Key words: passive survivability, solar
air heater, thermosyphon, passive solar, heat store, thermal closet,
thermal storage wall, heat store
wall, thermal mass, air-to-air heat exchanger, solar heat, solar
thermal, natural convection, forced convection, plastic film flapper, plastic film damper,
dampers, rock bed, bed of stones, packed bed, gabion, column of
stones, attic, basement, high temperature heat store, segregated heat
store, passive collection, passive charging, passive
discharging, indoor air quality, IAQ, mould, mold, earthquake.
entirely passive residential solar heating system described here was
conceived for passive survivability, with particular attention to
air quality and earthquake hazard. The proposed heating system could
provide all of the
heat needed by a well insulated house in a cold winter, even during a
prolonged absence of electricity and other non-solar energy. The
system, Fig. 1
, consists of a solar air heater on the south wall,
temperature heat store built into the south wall, and a passive
heat exchanger that separates the air of the heating system from the
air of the living space.
This system would not justify itself by energy cost savings based
on reasonable projections of future
and fuel costs. It may recommend itself to those who expect both
rates and fuel costs to rise to unreasonable levels, and to those who
expect shortages of heating fuel and failures of electricity supply to
cost of this system is that it precludes south windows. On a big house
relatively narrow central part of the south wall might be reserved for
windows with two heating systems positioned to the sides of the
windows. The system could be added as a south extension to almost any
that has good southern sun exposure -- at the cost of blocking south
To obtain all necessary space heat from the sun, a house in a cold
high-latitude climate (40°N to 50°N) must collect large amounts
of heat during short periods of sun and store it for subsequent slow
release into the living space. In the climates of use intended
here, periods of several days or a week without sun are common. It is
impractical to store the necessary amount of heat in the living space
of the house, making a separate high temperature store necessary. In this
context, "high temperature" means from 35C
to 65C (95F to 150F). An advantage of air as the working fluid of
a residential solar heating
system is that an air system can be entirely passive, while a water
system cannot. [I was wrong here. See Buckley's Thermic Diode. DMD] A disadvantage of air is that water can be used as
both the heat collection fluid and the heat storage medium, while air
cannot. Heat in the air of a solar air heater must be transferred
separate storage medium. To obtain adequate efficiency of heat
transfer, the storage medium must present a very large surface area to
the hot air. A mass of small stones has good characteristics for the storage medium. The hot air passes
through the stones to give up its heat to them. Cool air may be heated by passing it
through the mass of stones at a later time. Unfortunately, the use of a
mass of small stones to heat air for a
living space raises health concerns. House dust, mold spores,
cooking vapors and smoke, and other pollutants in the air of the living
accumulate in the mass of stones and be reintroduced later into the
space air. It is impractical to clean such deposits from a mass
stones, but the mass must serve for the life of the house.
The heat exchanger
In the design
proposed here, an air-to-air heat exchanger keeps the air of the
separate from the air of the living space. House dust
and other material in the air of the living space cannot
accumulate in the heating system. Nor can any noxious material that
might happen to be present in the heating system enter the air of the
living space. The heat exchanger consists of a sheet metal or plastic film wall (the heat exchange wall) , the
two air spaces on its north and south sides, the north face of the
thermal mass, and the south face of the insulated wall that divides the
living space from the heat exchanger. The space between the heat exchange wall
and the insulated wall forms a channel through which cool air from the
living space can enter at the bottom and leave at the top after having
been heated. The heat exchange wall also serves
as an air barrier and a vapor barrier keeping the house air separate
from the heating system air. See Fig. 1.
Openings in the outer wall of the air
heater provide ventilation for the heating system. The
openings are blocked with a filter material that stops the incursion of
outdoor dust while permitting a slow exchange of water vapor and air
with the out of doors. These openings keep the absolute humidity
of air in the heating system close to
the absolute humidity of outside air, thereby keeping the relative humidity of
the air in the heating system very low, and preventing condensation on
the inner surface of the air heater glazing no matter how cold the air
heater becomes at night.
The extremely low relative humidity and
high temperature of the air of the heat store provide a powerful deterrent to
life forms that might otherwise be attracted by its quiet protection. The relative
humidity of the air in the living space can be maintained at a level
that is comfortable for the inhabitants without regard to the operation
of the heating system.
The temperature drop through the heat exchanger reduces the ability of the energy in the heat store to heat the house.
This reduction must be compensated by a larger air heater than would be
required if the air of the heating system and living space were not
The heat store
The thermal mass of the heat store consists of multiple columns of small stones. See Fig. 1.
The columns have a rectangular horizontal cross section. They are
impermeable to air on their east, south, and west surfaces, where they
are supported by a concrete structure that also provides additional thermal
mass. The concrete supporting structure consists of an east-west
shear wall and multiple short fin walls that extend north from it. See Fig. 2. and Fig. 3.
adjacent pair of fin walls supports the east and west sides of a column
of stones. The north surface of each column of stones is supported by
wire mesh, making the north surfaces of the columns permeable to air
for the full height of the columns. The wire mesh is bolted to
the north ends of the fin walls. The columns sit on a floor consisting of a horizontal
northward projection from the bottom of the shear wall. The
concrete supporting structure can easily be made strong enough to retain its
integrity and uprightness in a violent earthquake.
Heat moves from the solar air heater to the heat store by natural
convection -- air movement
due to the difference of weight between equal volumes of air having
different densities because of their different temperatures. See
Fig. 1. The heat collection system has only one moving part, a passive
flapper, or damper, made of thin plastic film. When the air heater is
warmer than the heat store, the flapper permits cool air from the
bottom of the heat store to pass into the air heater to be warmed. When
the air heater is cooler than the heat store, (see Fig. 4) the flapper prevents a
backward flow of cool air into the bottom of the heat store.
Air enters the heat store from the air heater through a hot trap at the
top. The buoyancy of the light hot air in the air heater pushes
the hot air up into the heat store and down the gap just north of the
thermal mass between the thermal
mass and the heat exchange wall. The
descending hot air diffuses southward into the thermal mass. The cooler
air in the thermal mass falls out of the mass to descend in the gap.
Air falls from the bottom of the gap between the thermal mass and the
heat exchange wall, passing down past the north end of the concrete
floor that supports the columns of stones, passes
under the floor supporting the columns, through the cold trap,
and past the
one-way flapper back into the air heater.
The air space just north of the thermal mass has an important
function in preserving thermal stratification in the
mass. It provides an easy flow path, much easier than the path through
the stones of the thermal mass, by which air that is cooler than air at
a given height of the thermal mass may fall past the warmer part of the
thermal mass before entering it. This bypassing flow
pattern preserves existing thermal stratification in the heat store
when air entering at the top of the heat store is not as hot as the
hottest air in the heat store. See Appendix 1. In general, air descends through the thermal mass of small stones only when, and because, it is being cooled by the stones.
The cold trap is the short horizontal section of duct just north of the
one-way flapper. When the air heater is colder than the heat store, the
flapper prevents air movement from the air heater into the heat
store, but considerable heat loss may occur through the thin plastic of
the flapper. When the air in the cold trap becomes cold because
of the heat loss through the thin flapper, the heat loss through the
flapper drops to a very low level. The cold heavy air in the cold
trap does not mix with the warmer lighter air in the heat store above the
The hot trap is the short vertical section of duct that rises
from the top of the solar air heater to the top of the heat store. See Fig. 1.
When the air heater is colder than the heat store, its cold air
is heavier than the air above it in the upper section of the heat
store, so there can be no movement of the cold air up into the heat
store. There will be no downward movement of hot air into the air
heater through the hot trap as long as the heat store is air tight except for the hot trap.
Another plastic film flapper could be configured in the upper duct
between the air heater and the heat store. This upper flapper would
provide redundancy to protect against failure of the lower
flapper. Its location would make it inconvenient to
service. Its placement and access would require much detailed
design, and result in additional requirements for structural complexity
and space. I prefer to rely on making the lower flappers easy to
inspect and service.
The low rate of dust accumulation and the absence of an upper flapper permit the heat store to be
built as a sealed unit that can operate for the life of the house
without internal maintenance or decreased performance.
Heating the house.
process of discharging the heat store to warm the house can also
operate entirely passively by natural convection, as shown in Fig. 4.
The temperature of the heat store will usually be strongly stratified,
the temperature increasing from the bottom of the thermal mass to the
top. Stratification maximizes the availability of a given quantity of
stored energy to heat the living space. The rising motion of the living
space air in the heat exchanger as it is being heated preserves
both the temperature stratification of the heat store and the
readiness of the heat in the store to heat the living
space. The living space air is heated first by the coolest
part of the heat store, and then by progressively warmer parts of the
store, which therefore deliver less energy than if the air they were
warming were cooler. At each level of its rise the living space air air is
heated by only a relatively small temperature difference compared to
the difference between the average temperatures of the heat store and
the living space. This process extracts a great deal of the heat being
delivered to the living space from the cooler parts of the heat store,
preserving a higher temperature in the warmer parts. (In the language
of thermodynamics, the heating process uses a smaller amount of
"availability", or "exergy", or "creates less entropy", for a
delivery of heat to the living space when the thermal mass is
stratified in this way.)
Most users will
want an automatic temperature control system consisting of thermostats
and fans, or
thermostats and damper motors, to avoid having to control the heating
system manually. An automatic temperature control system should be
designed so that it can
be replaced easily by manual control during electricity failures.
The performance of heat delivery from the heat store
transferred from the heat store to the heat exchange
wall by radiation from the thermal mass of the heat store and
contact between the air of the heat store and the heat exchange
wall. The heat exchange wall has a large surface area equal to
area of the north face of the thermal mass. The surfaces of the heat
exchange wall have a high emissivity at the wavelength of the
temperature of the thermal mass to facilitate reception of radiative
heat transfer from the thermal mass and re-radiation north to the south
wall of the insulated wall of the living space.
Heat is transferred from the heat exchange wall to the living space air
contact between the sheat exchange wall and the air of the
living space in the heat exchanger, and by contact between the air of
the living space inside the heat exchanger and the south surface of the
insulated wall that divides the living space from the heat exchanger.
This south surface of the insulated wall is heated by radiation from
the heat exchange wall. The
two surfaces of the heat exchange wall and the south surface of
insulated wall separating the heat exchanger from the living space
should be painted
with paint known to have very high emissivity (emissivity = 0.9) at
thermal wavelengths. For the same reason, the north surfaces of the
stone columns and their steel retaining mesh may be sprayed
with the same paint to raise their emissivity to about 0.9.
The heat exchange wall might be corrugated or folded to increase its surface area.
See the MathCAD file (rendered to PDF form) in which equations are solved and graphs produced here (PDF)
Fig. 5. .
Appendix 1. How stratification is preserved during charging
Appendix 2. Comparison of stratified and unstratified heat stores
Appendix3. Calculations and graph plotting.
Steve Baer, "Sunspots - An exploration of solar energy through fact and fiction", 1979, Cloudburst Press, Seattle. 127 pp.
Nick Pine and Paul Bashus, "Solar closets and sunspaces",
Proceedings of the 1996 Fourth World Renewable Energy Congress in
Denver. Also at http://vu-vlsi.ee.vill.edu/~nick/solar/solar.html.
Donald Pitts and Leighton Sissom, "Schaum's outline of heat transfer", second edition,1998, McGraw-Hill.
S. Robert Hastings and Ove Morck, eds., "Solar air systems, a design handbook", 2000, James and James Science Publishers,
The 2D images were drawn with Autosketch 9.
The 3D perspective images were rendered from a Google Sketchup 3D model.
The calculations and graphs were produced with MathCAD 13