The Functionally Compartmentalized Roof

CBS Rooftop CompartmentalizationThe tilted box roof style of cube-based abode geometry is highly conducive to compartmentalization -much like the floors and walls of typical 3D rectilinear buildings. Although this comparison has limits owing to the ever present force of gravity effecting torques on constructs extending outward from the roof, proper bracing guided by CBA geometry enables modest applications of such.

Equinox Solar NoonThe equator-facing roof section in particular  may require careful planning and organizing as it faces the center of all sun positions through- out every day of the year and thus makes the roof ideal for solar applications.

Compartmentalization begins by regarding major solar components separately, starting with the base, which would be the equivalent of the floor slab. In the solar context, the base should generally be composed of a high albedo material that well reflects sunlight. A white roof poses the highest albedo, as exemplified by the roof tops of Bermuda. By reflecting sunlight, the need for artificial cooling may be reduced significantly – as is global warming (a factor largely ignored along with the waste heats of the prevailing steam turbine electrical generation modes in favor of demonizing the CO2 molecule in and of itself).

Photo-voltaic panels that directly convert sunlight into electricity pose their own contribution to the pool of world waste heat, a detriment that plays out in their low (.o5) albedos. Of the total sunlight falling on a panel – the solar constant modified by the angle of incidence  and attenuated by the thickness and changing state of the atmosphere – about 20% is converted to electricity. This represents a 300%+ improvement over the photo-synthesis of plants, but with the remaining 75% of the sunlight becoming waste heat, PV lags the conversion efficiencies of other generating modes. The virtues of PV electricity are that it incurs virtually no transmission loss if used on site, and its detrimental effects can readily be neutralized by simply apportioning roof area to high albedo materials.

The next component of the solar roof might be a hot water panel. Such panels use sun light more efficiently than if the water is heated from electricity provided by the PV panels. Deciding how much roof area to apportion to what type of panel poses a challenge. To lessen it, hybrid PV panels circulate water to cool the panel and retrieve efficiency lost as the exiting water is heated. If nothing else, such panels can preheat water for dedicated hot water panels, reducing the space needed for them. In effecting 2 functions, the PV panel becomes more leaflike.

The next potential roof component is the skylight. Though unduly dismissed as detrimental to passive schemes, skylights can both enhance quality of life and increase passive savings in not uncommon situations if placed thoughtfully and are able to easily accommodate high albedo insulated inserts. Other solar components include solar chimneys (that intentionally create heat to draw low cold air inside into a circulation pattern), clothes dryers, and bread bakers.

There is some debate on 1) the benefits of white “cool roofs” losing heat in winter and 2) the degradation of gains made by passive schemes made by PV. In light of such uncertainty, I believe an approach of balance and flexibity is the best way to go. The example of such below specifies a 16′ x 32′ (512 sq ft) roof over a CBS home housing a family of 4.

CBS Solar Roof Compartmentalization

 

Solar Roof Albedo Calculation

The (221 sq ft) area of the low albedo panels (A of R) is multiplied by their .05 albedo (A of L) to get that area’s effective albedo (A of F). The same is done with the (512 sq ft – 221 sq ft) area of the exposed high  albedo (.70) base. The two effective albedos are added and then the sum is divided by the total area as in the above summation equation.  In this example the total effective albedo is calculated to be .42 for the roof – .07 above the popularly sited average of .35 for earth which includes the variable appearance of white-topped high albedo clouds. The .42 albedo attained in this example is compared to the albedo of the natural ground substance over the same area to determine the roof’s relative cooling effect.

[Note: Because the term “effective albedo” already has a specific meaning, reference to it above (A of F) should probably be changed to Areal Albedo (A of A) – Russell “Loose Brainstorming Cannon” Westfall – Mar 18, 2014]

An element of flexibility might come from an adjustment mechanism that separates the PV panels from the roof to allow air flow in the shade they provide in the summer; and encloses a dead warm air space in the winter. Additionally, total albedo can be increased in the summer with white skylight inserts, and with movable flat porch roof boards painted white.

The metallic business-like machine nature of the compartmentalized solar roof can be softened much as a fiberglass marine pleasure craft is with the natural wood trim arond the entire perimeter as well as each component to make the roof something like a really laid back wall.

Might the other CBA roof have any other solar function? Possibly. While the sun roof exhibits a plane perpendicular to the average direction of the sun’s rays, so the complementary roof parallels those rays. In the depth of penetration suggested, perhaps there is some potential for a storage scheme as with a water well, stacked capacitor discs; or the branches supporting the leafy solar collectors of trees.

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