Separating Direct From Diffuse

Observations of Visible Transmittance Through a Tracking Photovoltaic Envelope



An Enclosure-Integrated, Daylighting, Tracking Solar Collector (EIDTSC) has been developed for market which integrates into a building’s fenestration (windows, curtain walls, skylights, canopies) and contributes daylighting, solar gain control, and power. The fundamental means of this active system is optical concentration, by which direct beam irradiance is blocked (and focused into photovoltaic generation). Off-axis irradiance is not focused, and is transmitted. Because transparent optics are a major fraction (by area) of the system’s components, diffuse light passes through, while beam luminance is either blocked or diffused, depending on the chosen tracking mode. The enclosure’s glazing lites can be specified highly transparent and neutrally-colored. The EIDTSC therefore performs daylighting with good color rendering, while reducing contrast-triggered glare and solar heat gain to indoor spaces.

A pre-production EIDTSC assembly has been evaluated for visible light and solar energy transmittance, as installed in a horizontal glazing cassette (skylight configuration). Tests were conducted over intervals of 2.5 to 5 hours, under variable-sky and clear-sky conditions, and operating for either maximum power generation (tracking) or enhanced diffuse daylighting (off-tracking). While tracking, the daily average transmittance ranged from 0.11 (during clear-sky tests) to 0.15 (during variable-sky tests). Steady-state transmittance peaked during cloudy conditions (up to 0.25), indicating beam luminance was preferentially blocked. The average illuminance on a work plane under the cassette was stable, shifting from 10,700lx during variable-sky tests, to 11,100lx under clear skies (a 4% change), indicating the EIDTSC moderated shifts in the sunlight resource. With off-tracking control, mean hemispherical transmittance rose to 0.25, a 108% increase. These results suggest new affordances from dynamic fenestration, such as improved light-to-solar gain (LSG) ratios, greater glazing ratios, and dynamic daylighting control in side-lit spaces. These opportunities could grant more freedom for architectural design, while improving building energy performance and occupant comfort relative to other fenestration strategies.




IDN Direct normal irradianceIGH Global horizontal Hemispherical irradiance measured after test articleEGH Global horizontal Hemispherical illuminance measured after test articleTvis Visible light transmittanceTsol Solar energy transmittance

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Discussions of measurable performance continue to permeate the practice of architecture—performance according to environmental, health, comfort, and economic metrics. Active envelope elements—though their application has long been restricted by questions

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Active envelopes can improve the energy use of commercial building stock (Heiselberg 2012), and innumerable concepts and approaches have been investigated and developed. Objectives for adding dynamic (or otherwise active)

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A pre-production assembly of the EIDTSC was installed in a glazed cassette, outfitted with light sensors and solar sensors, and observed in different modes of operation, in different weather conditions

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Results and Discussion

The visible and solar transmittances through the EIDTC are characterized as averages of the ratio between simultaneous measurements at sensors underneath the array (to the interior) and the sensor suite

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Conclusion and Future Work

A pre-production EIDTSC device was tested for visible and solar transmittance. It was observed that average visible transmittance was 0.12 or 0.25, depending on the chosen mode of operation, and

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The contributing research and support from the Rensselaer Center for Architecture, Science and Ecology, the Yale Center for Ecology and Architecture, and SHoP Architects are gratefully acknowledged.

Funding was provided internally.

Rights and Permissions

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