Abstract
Vertical space bears great potential of solar energy especially for congested urban areas, where photovoltaic (PV) windows in high-rise buildings can contribute to both power generation and daylight harvest. Previous studies on sun-tracking PV windows strayed into the trade-off between tracking performance and mutual shading, failing to achieve the maximum energy generation. Here we first build integrated models which couple the performance of sun-tracking PV windows to the rotation angles. Secondly, one-degree-of-freedom (DOF) and two-DOF sun tracking are mathematically proven to be not able to gain either maximum power generation or non-glare daylighting under reasonable assumptions. Then we derive the optimum rotation angles of the variable-pivot-three-degree-of-freedom (VP-3-DOF) sun-tracking elements and demonstrate that the optimum VP-3-DOF sun tracking can achieve the aforementioned goals. When the restriction of the proposed model is relaxed, the same performance can be achieved by the optimum one-DOF sun tracking with extended PV slats and particular design of cell layout, requiring less complicated mechanical structures. Simulation results of nine global cities show that the annual energy generation and average module efficiency are improved respectively by 27.40% and 19.17% via the optimum VP-3-DOF sun tracking over the conventional perpendicular sun tracking. The proposed optimum sun-tracking methods also reveal better protection against sun glare. The optimum VP-3-DOF sun tracking is also demonstrated to be applicable to horizontal PV windows, as those applied in the sun roof of a glass greenhouse.
Highlights
IntroductionPV windows take full advantage of vertical space in congested urban areas, where available horizontal lands are scarce, and local energy consumptions are tremendous
Comparing with the optimum VP-3-DOF sun tracking, the optimum one-DOF sun tracking with the improved layout of horizontal stripes achieve the same performance with simpler mechanical structures
We have investigated the performance of the one-degree-offreedom, two-DOF, and three-DOF sun tracking using our proposed irradiance model
Summary
PV windows take full advantage of vertical space in congested urban areas, where available horizontal lands are scarce, and local energy consumptions are tremendous. To evaluate the equivalent horizontal area (EHA) of available vertical surfaces, we define Rv/h as the ratio of the annual solar energy received on the sunward (e.g. equator-facing for temperate zones) vertical unit area to that received on the horizontal unit area, i.e., R Gv,global (t) dt Rv/h = R. where Gv,global (t) indicates the global irradiance on a sunward vertical plane; and Gh,global (t) indicates the global irradiance on a horizontal plane. The EHA of the highest skyscraper (632 m) in Shanghai equals to the area of 3.5 standard football fields, which occupy 15.6-fold horizontal areas as the building does (see Supplementary Note 1). Considering all the urban high-rise buildings around the world, vertical area holds enormous potential for the utilization of solar energy, especially the window area, which is relatively large in modern buildings
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