Volker Zagolla

Proof of principle demonstration of a self-tracking concentrator

We present to the best of our knowledge the first successful demonstration of a planar, self-tracking solar concentrator system capable of a 2-dimensional angular acceptance of over 40°. The light responsive mechanism allows for efficient waveguide coupling and light concentration independently of the angle of incidence within the angular range. A coupling feature is created at the focal spot of the optical system by locally melting a phase change material which acts as an actuator due to the large thermal expansion. A dichroic prism membrane reflects the visible light so that it is efficiently coupled into a waveguide at the point of the created coupling feature. We show simulation results for concentration and efficiency, validated by an experimental proof of concept demonstration of a self-tracking concentrator array element. Simulations show that a system based on this approach can achieve 150X effective concentration by scaling the system collecting area to reasonable dimensions (40 x 10 cm²).

Self-tracking solar concentrator with an acceptance angle of 32°

Solar concentration has the potential to decrease the cost associated with solar cells by replacing the receiving surface aperture with cheaper optics that concentrate light onto a smaller cell aperture. However a mechanical tracker has to be added to the system to keep the concentrated light on the size reduced solar cell at all times. The tracking device itself uses energy to follow the suns position during the day. We have previously shown a mechanism for self-tracking that works by making use of the infrared energy of the solar spectrum, to activate a phase change material. In this paper, we show an implementation of a working 53 x 53 mm(2) self-tracking system with an acceptance angle of 32° ( ± 16°). This paper describes the design optimizations and upscaling process to extend the proof-of-principle self-tracking mechanism to a working demonstration device including the incorporation of custom photodiodes for system characterization. The current version demonstrates an effective concentration of 3.5x (compared to 8x theoretical) over 80% of the desired acceptance angle. Further improvements are expected to increase the efficiency of the system and open the possibility to expand the device to concentrations as high as 200x (C(geo) = 400x, η = 50%, for a solar cell matched spectrum).

Light induced fluidic waveguide coupling.

We report on the development of an opto-fluidic waveguide coupling mechanism for planar solar concentration. This mechanism is self-adaptive and light-responsive to efficiently maintain waveguide coupling and concentration independent of incoming lights direction. Vapor bubbles are generated inside a planar, liquid waveguide using infrared light on an infrared absorbing glass. Visible light focused onto the bubble is then reflected by total internal reflection (TIR) at the liquid-gas interface and coupled into the waveguide. Vapor bubbles inside the liquid are trapped by a thermal effect and are shown to self-track the location of the infrared focus. Experimentally we show an optical to optical waveguide coupling efficiency of 40% using laser light through a single commercial lens. Optical simulations indicate that coupling efficiency > 90% is possible with custom optics.

Efficiency of a micro-bubble reflector based, self-adaptive waveguide solar concentrator

State of the art solar concentrators use free-space, non-imaging optics to concentrate sunlight. Mechanical actuators keep the focal spot on a small solar cell by tracking the sun’s position. Planar concentrators emerged recently that employ a waveguide slab to achieve high concentration by coupling the incident sunlight into the waveguide. We report on the development of an opto-fluidic waveguide coupling mechanism for planar solar concentration. The self-adaptive mechanism is light-responsive to efficiently maintain waveguide coupling and concentration independent of incoming light’s direction. By using an array of axicons and lenses, an array of vapor bubbles are generated inside a planar, liquid waveguide, one for each axicon-lens pair. The mechanism uses the infrared part of the solar spectrum on an infrared absorbing medium to provide the energy needed for bubble generation. Visible light focused onto the bubble is then reflected by total internal reflection (TIR) at the liquid-gas interface and coupled into the waveguide. Vapor bubbles inside the liquid are trapped by a thermal effect and are shown to self-track the location of the infrared focus. We show experimental results on the coupling efficiency of a single bubble and discuss the effect of angular coupling. Furthermore the effect of an array of bubbles inside the waveguide (as produced by a lensarray) onto the coupling efficiency and concentration factor is analyzed.

Self-tracking planar concentrator using a solar actuated phase-change mechanism

In this paper we discuss optical considerations and present design simulation results for a self-tracking (passive) solar concentrator. The self-tracking mechanism uses a reversible in-plane paraffin thermal actuator to couple shortwavelength light into a lightguide at the position of the solar focus. By splitting the solar spectrum using a longpass dichroic faceted reflector for actuation energy, this device adaptively self-tracks and concentrates solar light into a planar waveguide.

Trackfree planar solar concentrator system

Solar concentrator systems rely on focusing a large collecting aperture onto a small one where a high efficiency solar cell is placed. The main drawback of concentrator systems is the need to track the direction of the sun. We report on the development of a trackfree planar solar concentrator which employs a self-adaptive light responsive mechanism. The working mechanism is based on optical trapping reflective particles dispersed into a liquid waveguide. The trapping effect experienced by the reflective particle at the focus of the collecting aperture effectively forces the particle to follow the direction of sunlight. The role of the reflective particle is to couple the cone of focused sunlight into a waveguide by total internal reflection (TIR) towards a solar cell placed at its edges. We report on preliminary experiments on metallic reflective particles and on gas-filled hollow glass particles that exhibit reflective properties by total internal reflection. We show promising results on vapor bubbles generated by the focused light which couple nearly 100% of the incoming light into waveguiding modes. The generated bubbles are stable and track the focal point.

Demonstration of a 5x5 cm(2) self-tracking solar concentrator

Solar concentration is using optics in order to minimize the amount of expensive photovoltaic cell material needed. For concentration factors higher than approximately 4, tracking the sun’s position is needed to keep the focal spot on the solar cell. Based on recent developments using a waveguide slab to concentrate sunlight we propose and demonstrate a light responsive, self-tracking solar concentrator. Using a phase change material acting at the focal spot, it is possible to maintain efficient coupling into the waveguide, up to an angular range of +/- 20 degrees. The system uses the unused infrared part of the solar spectrum as energy for the phase change actuator to achieve its high acceptance angle. With a spectrally matched custom silicon solar cell attached to the waveguide slab, in which light is coupled, the visible part of the solar spectrum can be efficiently converted to electricity. A proof-of-concept single lens device was demonstrated in our previous work. Here we extend the principle to a 3x3 lens array demonstration device. The current demonstration device features an acceptance angle of +/- 16 degrees and an effective concentration factor of up to 20x.

Demonstration and future potential of a self-tracking phase change actuator

State-of-the-art concentrators use free-space optics to concentrate sunlight onto photovoltaic cells. To achieve high concentration factors it is necessary to track the sun’s position. In current systems, mechanical actuators keep the focal spot in the solar cell. Planar concentrators have recently emerged, which use a waveguide slab to concentrate the sunlight. Here we demonstrate the development of a phase-change actuator (PCA) for self-adaptive tracking. The demonstrated mechanism is light-responsive and provides self-adaptive light concentration in a planar waveguide while maintaining efficient concentration over an angular range of +/- 16 degrees. The proposed system consists of a lens array to focus the sunlight, a waveguide slab acting as a concentrator, a dichroic prism membrane, splitting the solar spectrum into a visible (VIS) and infrared (IR) part, and the phase-change actuator. The actuator undergoes a phase change upon absorption of the IR light and vertically expands, creating a coupling feature upon contact with the waveguide. Visible light is then reflected off the prism membrane and efficiently coupled into the waveguide. As the focus spot moves, so does the coupling feature due to the light responsiveness of the actuator. We show an experimental proof-of concept prototype, highlighting the desired features of the concept. This is then further expanded by simulations of a full system achieving high effective concentrations (>100X) and first experimental results expanding the prototype to a full system.

Self-Tracking Solar Concentration: Improvements to the Demonstrator

Solar concentration requires tracking to function effectively by following the sun’s position. The proposed self-tracking system greatly reduces the constraints on tracking. We built a demonstration device and report on the results and improvements.

Self-tracking phase change concentrator device demonstrator

Concentration photovoltaic systems uses free-space optics to concentrate sunlight onto photovoltaic cells, using mechanical trackers to accurately track the sun’s position and keep the focal spot on the PV cell. We recently proposed and demonstrated a proof-of concept of a self-tracking mechanism using a phase-change material to greatly extend the acceptance angle of the concentration device. The light responsive mechanism allows for efficient waveguide coupling and light concentration independent of the angle of incidence inside its angular range of acceptance. The system uses a lens pair to achieve a flat Petzval field curvature over the acceptance angle range. A waveguide slab acts as the concentrating device. The use of a dichroic prism membrane separates the solar spectrum into two parts, lower wavelengths (<750nm) are coupled into the waveguide, while the energy of higher wavelengths (>750nm) is used to power the self-tracking mechanism. The energy in this part of the spectrum is absorbed by a carbon black paraffin wax mixture that undergoes a phase change and subsequently creates a coupling feature due to thermal expansion which allows the lower wavelength part to be coupled into the waveguide. We show the extension of our proof-of concept to a device-like demonstrator, featuring the extension to a lens array and much larger dimensions. All parts of the proof-of-concept device have been upscaled to meet the requirements of a larger scale demonstrator. The demonstrator has an acceptance angle of +/- 16 degrees and can achieve an effective concentration factor of 20x. We present experimental and simulation results of the demonstration device.