Theoretical and experimental development of a quality-matched energy flow strategy for on-site energy harvesting in buildings

Abstract

In many commercial building settings, however, the actual solar energy incident on the building is far greater than the building’s energy consumption profile, suggesting a radically alternative paradigm for the effective use of solar energy: to capture, transform, buffer, and redistribute this energy to building systems. This thesis tests the ability of a transparent, multifunctional envelope collector to source both electricity and high quality thermal energy while transmitting diffuse light, providing views, and almost entirely eliminating unwanted solar heat gain. To make the best use of harvested thermal resources the envelope is integrated into a building’s systems where thermal storage banks control the flow of collected heat, and thermal-driven chillers and dehumidifiers extract work from that flow before it is adaptively applied on-demand for zonal cooling, heating, hot water and plug loads. This cascading configuration of systems is an example of an alternative paradigm, which is described here as Quality-Matched Energy Flows—or Q-MEF.

The Q-MEF hypothesis is tested in two broad stages: an operational prototype of the photovoltaic and thermal solar collector is characterized, and the experimental results are incorporated into a building energy model with distributed thermal systems that leverage the collector’s high-temperature output. The collector, while generating electricity and thermal energy, is largely transparent which allows for diffuse daylighting and views.

Experiments with the prototype have demonstrated cogenerative solar collection array efficiencies of up to 25% (exergy) and 43% (energy) in conventional installation, relative to direct normal irradiance transmitted through exterior glazing. By simulating nominal enhancements to optics, cell type, heat exchangers, and exterior glazing, cogenerative efficiency increased to 41% (exergy) at 62 °C, and 73% (energy) at 60°C, which is sufficient to drive an adsorption chiller at its nominal COP of 0.55. If driven at lower temperatures suitable for service water heating, the combined system efficiency is projected to reach over 70%, with over 30% electrical efficiency. Exergetic collection efficiency remains constant, both in measured and modeled data, relative to increasing outlet temperatures, which is notable for collectors with a photovoltaic component. In addition to generating power, the collector, if modeled to be installed in a south-facing envelope, transmits only 4% of direct solar gains into the interior, a significantly greater reduction than the 46% transmittance from existing solar-control glazing (SHGC=0.65).

Integrating Q-MEF systems into an energy model of a medium-sized office-type building resulted in net-zero energy use in two climates, hot desert and dry-summer subtropical (Phoenix and Mountain View). Electricity consumption in a humid-continental climate (New York City) was reduced by 87% relative to the highly glazed baseline commercial building type. These high-performance behaviors accompany projected gains in leasable floor area and building volume due to the reduction of mechanical systems sizing and ducting, while peak demands for grid power were reduced in all climates save for New York. As a demonstrative subset of possible technologies within the Q-MEF rubric, this developed model shows good potential for net-zero and energy-positive behavior in a range of situations, with parallel benefits from increasing useable (and valuable) indoor space.

The widespread achievement of on-site net-zero commercial buildings has been stalled by the prevalent strategy of designing building envelopes to reject incoming solar energy and glare through insulating, shading, and controlling the emissivity of glazing surfaces. Simultaneously, solar harvesting systems are often added-on, single function systems with low efficiencies; while façade-integrated collectors are gaining some traction in the form of opaque air heaters, ventilated double-skin glazed façades, and semi-transparent photovoltaics, the power these methods produce is often a fraction of the building’s electrical requirements.