Solar Energy Storage Wall for Passive Survivability

David M. Delaney
Ottawa, Canada
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.

Fig. 1 -ah-he-concrete-stones-tm-fig-1.GIF

The entirely passive residential solar heating system described here was conceived for passive survivability, 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. [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 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 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 separated.

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. 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 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 thermal 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 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.

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 heat exchange wall by radiation from the thermal mass of the heat store and by contact between the air of the heat store and the heat exchange wall. The heat exchange wall has a large surface area equal to the 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 by 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 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 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.

Fig. 6

Appendices

Appendix 1. How stratification is preserved during charging

Appendix 2. Comparison of stratified and unstratified heat stores

Appendix3. Calculations and graph plotting.

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