Passive Solar Exposure

Passive exposure to solar irradiation as a means of destroying airborne pathogens is being investigated by the Penn State Architectural Engineering Department. The principle is that ultraviolet, and other, radiation from the sun is sufficient to sterilize most pathogens within the space of about 30-60 seconds. This is the primary reason most infectious microorganisms die in the outdoor air. In the diagram shown at the right, the spectrum of light produced by the sun is illustrated. The ultraviolet component of sunlight includes the range of 2050 - 3020 Angstroms, which is biocidal to microorganisms. UVGI systems operate at 2537 Angstroms, and can be highly effective.

In the design for a ten-story office building shown at left, a portion of the windows serves as the outside face of the duct, and the rest of the duct can be inexpensive plexiglas. The vertical red bars represent very long runs of transparent ductwork, or the Passive Solar Exposure (PSE) plenum. These faces can be oriented east and west, and as the air is mixed on the roof, the same sterilizing effect can be had moring or afternoon. Air exhausted from each room passes into the PSE plenum and travels down to the first floor before turning back up towards the roof. The air handling unit on the roof then filters (and processes) the air for return, also through the PSE plenum.

In the alternate design depicted below the entire window area of each face serves as a return or supply air plenum. As it is transparent on both sides the view is preserved for the occupants. As in the design above, the air is reheated, or cooled, and mixed at the zone itself. The red rectangles penetrating the window space each represent a zone inlet and zone exhaust. Except for the local zone equipment, this building has no ductwork at all.

The fact that the duct occupies window space means there is no additional cooling or heating load on the building as the result of this design. One innovation incorporated in this design is the intake of outside air at the individual room, where it is mixed with return air in a ceiling plenum. In addition, the return air is heated (reheated) in the ceiling plenum prior to mixing with the outside air, which has improved germicidal effects.

Cooling is accomplished with chilled water coils in the plenum and chilled water is supplied from basement chillers.

The main advantage of this design is that the increased cost of operating this system is very small, and also, the first cost of construction can be integrated into the building design at a minor add-on cost.

One potential enhancement to this design is the incorporation of titanium oxide PCO (Photocatalytic Oxidation) units into the PSE plenum. The ultraviolet light from the sun would activate the titanium dioxide and oxidize any microorganisms in the plenum air. The effectiveness of such a design is currently being explored at Penn State with a computer simulation of the ten story building depicted in the wire frame drawing above. Other enhancements and innovations are being investigated and will be presented here upon project completion. For more information see the section on Photocatalytic Oxidation.

References

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