Automated shape-transformable self-solar-tracking tessellated crystalline Si solar cells using in-situ shape-memory-alloy actuation

Abstract

Photovoltaic energy systems in urban situations need to achieve both high electricity production and high capacity in restricted installation areas. To maximize power output, solar-tracking systems tilt solar arrays to track the sun's position, and typically flat modules are used to maximize the cross-sectional area. Such tracking systems are complex and expensive, and flat modules cannot utilize omnidirectional incident light. For solar systems in urban environments, we have developed two-dimensional (2D) or three-dimensional (3D) tessellated solar-cell modules that use shape transformation, and combine solar tracking and an arch structure for use in restricted areas. The modules can use scattered and omnidirectional incident light. Simply by attaching shape-memory alloy strips to the surface of the solar panels, the shape of the array can be transformed in response to heat from sunlight. Compared to a perfect solar-tracking system, our simulation results indicate that the modules present a large cross-sectional area perpendicular to the direction of sunlight and provide superior tracking performance, resulting in a 60% increase in electricity production over the course of 1 day. In addition, by using different designs for the tessellation units, dome shaped or other 3D structures are possible, for specific applications and to meet aesthetic requirements.

Figure 1

(a) Schematic illustration of the concept of automated self-solar-tracking using a shape-transformation 2D tessellated solar cell array during 1 day. (b) Schematic illustration of the mechanism of reversible shape transformation via transfer of the heat of the solar-cell surface induced by sunlight.

Figure 2

(a) Differential scanning calorimetry (DSC) analysis of shape memory and (b) the temperatures of the surface of the solar cell and the shape-memory alloy attached to the solar cell. (c) Schematic illustration of the heat-transfer process and shape-memory actuation. (d,e) Heating and cooling times of the surface of the solar cell and of the shape-memory alloy depending on the angle of incidence (AOI) under illuminated and non-illuminated conditions. (f) Photographs of the transformation process of rectangular and equilateral-triangle tessellated solar cells from folded to flattened shapes.

Figure 3

(a) Photographs of the wire-type shape-memory-alloy’s shape transformation in response to an external heat source according to the direction of heating, and (b) schematic illustration of the transformation mechanism using elastic tension and straightening of the memorized shape of a fixed loop and arch shape. (c) Photograph of an installation of rectangular (top left) and right-angled (top right) 2D tessellated solar cells in a restricted area; the front and back sides of the edge part are fixed by a rod (bottom right) to retain the array within a restricted area with free movement (bottom left). (d) Schematic illustration of shape transformation within a restricted installation area by free rotation allowing flattening of the surface of the module and recovery to the arch shape by the elastic backbone. (e) Photographs of the shape transformation of equilateral-triangle and rectangular tessellated solar cells at an AOI of 70°.

Figure 4

(a) Solar-tracking performance of a 2D tessellated solar-cell array according to the AOI and type of backbone. Illustration (b) and simulated values (c) comparing cross-sectional areas of a fixed flat perfect-tracking system and transformation of the arch-shaped tessellated solar cell array depending on AOI. (d) Actual arch geometry simulation results according to the AOI. (e) Simulation power-production results comparing a perfect tracking system and the transforming 2D arch-shaped tessellated solar cell array according to the AOI and (f) the interval between modules.

Figure 5

(a) Photographs of rectangular, right-angled and equilateral-triangle-shaped tessellated solar cells in flat and various 2D arch states in a restricted installation area. Efficiency based on installed area of arches comprised of (b) rectangular and (c) right-angled-triangle 2D tessellated solar cells. (d) Rectangular 2D tessellated solar cell’s accumulated power and power over a day and (e) relative accumulated power and power over a day compared according to a restricted installation area for flat and various 2D arch shapes.