Passive solar heating system with high-temperature heat-store wall and air-to-air heat exchanger

 I have just posted a new document to my web site:

"Passive solar heating system with high-temperature heat-store wall and
air-to-air heat exchanger"

http://www.geocities.com/davidmdelaney/solar-ah-tm-he/ah-storage-wall-he.ht=
ml

I would be grateful for comments.

David Delaney

The text follows. The figures and graphs may be seen at the site:

Passive solar heating system with high-temperature heat-store wall and
air-to-air heat exchanger

David M. Delaney
ddelaney@sympatico.ca
Ottawa, Canada
June 7, 2006

Fig. 1 -ah-he-concrete-stones-tm-fig-1.GIFThe entirely passive
residential solar heating system described here was conceived with
particular attention to indoor air quality and earthquake hazard. The
proposed heating system could provide all of the space 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,  a high temperature heat store
built into the south wall, and a passive air-to-air 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 interest rates and fuel costs. It may
recommend itself to those who expect both interest rates and fuel costs
to rise to unreasonable levels, and to those who expect shortages of
heating fuel and failures of electricity supply to coincide.

One significant cost of this system is that it precludes south windows.
On a big house a 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 house that has good southern sun exposure -- at the cost of
blocking south windows.

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.  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 to a
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 space may accumulate in the mass of stones and be
reintroduced later into the living space air.  It is impractical to
clean such deposits from a mass of 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 heating system  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 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 metal 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 sheet metal 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 separated.
The heat store
concrete-support-structure-3D-fig-2.gif 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.  Each 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 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, 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 between the thermal
mass and the sheet metal wall of the heat exchanger.  When the
descending hot air reaches a level at which the adjacent air in the
thermal mass  is cooler than the descending air,  the descending hot
air permeates south 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 sheet metal
wall, passing down past the north end of the horizontal concrete
projection that supports the columns of stones,  passes under the
projection, down through  the cold trap, and past the one-way flapper
back into the air heater.
concrete-support-structure-3D-fig-3.gif
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 cold
trap.

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.
ah-tm-he-discharge-Fig-4.GIF The 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 given
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
Heat is transferred from the heat store to the sheet metal wall by
radiation from the thermal mass of the heat store and by contact
between the air of the heat store and the sheet metal wall.  The sheet
metal wall has a large surface area equal to the area of the north face
of the thermal mass. The surfaces of the sheet metal 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 sheet metal wall to the
living space air by contact between the sheet metal 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 sheet metal wall. The two surfaces of the sheet
metal wall and the south surface of the 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 metal 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.hs-he-equiv-circuit-Fig-5.gif .he-equations.gif



























he-performance-fig-6.gif
Fig. 6





















Readings

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,

Tools used
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

Key words: 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.

 
davidmdelaney wrote:

I have just added an appendix describing the creation and maintenance
of thermal stratification, at
http://www.geocities.com/~dmdelaney/solar-ah-tm-he/air-flow-while-charging.html

The text follows, drawings may be seen at the web site.

Appendix 1 Air flow in heat store while charging, Or, How thermal
stratification is preserved when the heat store must accept air cooler
than its hottest air David M. Delaney, June 12, 2006

ah-tm-he-charging-flow-fig-7.GIF When there is enough sun to make the
column of air in the air heater hotter and lighter than the column of
air in the heat store, there will be a flow of warm air from the air
heater into the top of the heat store, and a flow of cool air from the
heat store into the bottom of the air heater.  There are no fans in the
air heater or heat store, which operate entirely by natural (unforced)
convection.

The design of the heat store is intended to promote the formation,
maintenance, and maximization of thermal stratification, in which each
part of the thermal mass is always at least as warm as any lower part.
In order to meet this goal, the following statement must be true, or
approximately true: Air descends through the thermal mass of small
stones only when, and because, it is being cooled by the stones, or is
at least not being heated by them. Or, alternatively and equivalently:
Air descends through a part of the thermal mass of small stones only
when, and because, the descending air is at least as warm as the stones
in that part of the thermal mass.

If all air that flowed down through the heat store had to pass through
the thermal mass of small stones from its top to its bottom, these
statements would not be true, since convective forces could force
cooler air through a very hot top layer of stones, provided the stones
below were cooler than the entering air. The result would be to
decrease top-to-bottom temperature differences in the thermal mass, to
decrease thermal stratification.

The air space just north of the thermal mass has an important function
in  preserving thermal stratification in the thermal mass. It provides
an easy flow path, much easier than the path through the stones of the
thermal mass. Air that is cooler than air at a given height of the
thermal mass may fall in this air space past the warmer part of the
thermal mass before entering it through the wire mesh that retains the
north face of the thermal mass of small stones.
ah-tm-he-charging-flow-fig-8.GIF 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.  A baffle on top of the mass of stones prevents air from
entering it through its top. Air enters the thermal mass only via the
air space north of the thermal mass.

There are two general patterns for the flow of air within the heat
store. In the first case, Fig. 7, the air coming from the air heater is
hotter than the air in the top of the thermal mass of small stones.  In
the second case, Fig. 8, the air coming from the air heater is cooler
than the air in the top of the thermal mass of small stones.  The
incoming air is always warmer than the air in the lower part of the
thermal mass of small stones, of course, or there would be no flow into
the heat store. Return to main document