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Optical Design & Engineering

TwinFocus: a compact photovoltaic concentrator

Technology modeled on car headlamps provides the basis for efficient collection of sunlight.
26 March 2013, SPIE Newsroom. DOI: 10.1117/2.1201303.004779

Concentrated photovoltaics (CPV) generate electricity using optics to focus sunlight on a small area of high-efficiency PV cells. The technology is an attractive alternative to standard PV as a power generation source because it requires less space to produce the same (or greater) electrical current.1–3 The most recent CPV systems are based on triple-junction (3J) solar cells, which contain multiple p-n junctions (the interface of p-type and n-type silicon within a semiconductor) to increase the cell's efficiency.3–5 While CPV is simple in principle, using inexpensive mirrors or lenses to concentrate sunlight onto small solar cells, in practice the technology faces challenges. The greatest of these is how best to manage the system's capacity to withstand high temperatures (its ‘ thermal budget’). Also for consideration is how to include high-quality optics in a form that is easily reproduced at low cost.

Here we present a scalable and cost-effective CPV prototype, known as the TwinFocus ® unit: see Figure 1.6 The design is a collaboration between research groups at AtemEnergia SRL, the Enrico Fermi Centre in Rome, and the University of Padua, and Italian companies Unitek, Unica, and Piazzarosa.

Figure 1. (Top) Photo of a TwinFocus solar concentrator. (Bottom) Schematic diagram of a cross section, showing the ‘twin’ triple junction (3J) solar cells.

The module exploits the mature technologies of automotive headlamps. The polycarbonate concentrator uses a single molding process, which has both optical and structural functions, and contains two symmetrical concentrating mirrors. It holds a pair of 3J cells and their respective passive coolers (which reduce temperature without using power). Each half of the ‘ twin’ primary optics concentrates the sunlight onto the 3J cell of the farther receiver: see Figure 1.

To achieve a uniform illumination profile using a reflector of small depth, we divided the mirrors into four sections, each illuminating the whole solar cell. We could then control potential hot spots on the target caused by deformations in the mirror. We could not completely avoid these irregularities, so we measured the real primary surface using a profilometer, and reconstructed the optics using ray-tracing software, a computer graphics tool that produces highly accurate images.

We then introduced a reflective secondary optical element (SOE) to collect marginal rays and to extend the angle at which incoming sunlight could be captured. Figure 2 shows the illumination profile after the reconstructed primary optics and the introduction of SOE. The ‘ Tilt x/Tilt y’ planes show angular acceptance: see Figure 2. This is defined as the maximum tilt angle at which the collected rays are 90% of those at the perfectly aligned concentrator.

One of the greatest challenges in designing the module was how to manage the high temperature of solar cells exposed to the intense sunlight. With a concentration ratio above ∼500×, there are more than 15W of heat to dissipate. We used a heat ‘ sink’: a passive device that conducts heat away from the cell to maintain a temperature of less than 90° under all conditions. However, this approach is not cost-effective, and we continue to research and test thermal and electrical interfaces in the receiver as a means to dissipate heat. Specifically, we studied three interfaces: the first connecting the solar cell to the conductive support; the second between the conductor and the receiver base; and the third connecting the receiver and the heat sink. Our aim was to minimize the surfaces' thermal resistance, to draw away heat. We ran a flash test (using simulated sunlight) of the first interface, in which we compared open-circuit voltages (Voc): see Figure 3(a). The lower ΔVoc indicates the lower thermal resistance. Changing the timescale and observing the same voltage it was possible to deduce other thermal resistances: see Figure 3(b). We tested interface 3 by placing it between two metallic bars, heating one side, and taking the temperature difference to calculate the thermal resistance: see Figure 3(c). Analyzing the data, we were able to select materials that would minimize both cost and cell temperature.

Figure 2. Results of research into optical design of concentrated photovoltaics. (Top) Illumination profile after optical adjustment. (Bottom) Angular acceptance of collected sun rays in x- and y-axes.

Figure 3. Testing of thermal resistance in three interfaces in the receiver. (a, b) Plots of open-circuit voltages against time: short time-scale and long time-scale, respectively. (c) Heat sink temperature immediately after the third interface. (d) Sketch of the interfaces structure, from the 3J cell to the heat sink.

The concentrator prototype described is the basis of four 32m2 demonstration trackers we installed in summer 2012: see Figure 4. Each has capacity 4.64kWp under concentrated standard operating conditions, and 5.3kWp under concentrated standard test conditions (where the power is measured at a theoretical 1000Wm−2). In designing the concentrator, our key considerations were compactness and modularity, since the system needs to be light enough to attach to a high-precision sun tracker.

Figure 4. A 32m2 tracker at work. Four systems have undergone testing since August 2012.

In future, AtemEnergia will focus on integrating the system's mechanics with the heat sinks as part of a strategy to minimize costs. Our aim is to make CPV financially viable for private energy developers.

This study was funded by the Enrico Fermi Centre and Polo Fotovoltaico Veneto. The companies Unitek, Unica and Piazza Rosa supported AtemEnergia in the development of prototypes.

Sandro Centro, Alessandro Saccà
University of Padua
Padua, Italy

Sandro Centro has been professor of experimental physics since 1987. He has been active in the field of particle physics since 1968, and was among the founders of AtemEnergia, a spin-off of the University of Padua. He has been a member of the CERN finance committee since 2000.

Alessandro Saccà received his masters degree in Physics in 2011 and is currently a PhD student of the Industrial Engineering Doctoral School. His research focuses on design, manufacturing and characterization of non-imaging optics, particularly for CPV.

Piergiorgio Antonini
Enrico Fermi Centre
Rome, Italy

Piergiorgio Antonini obtained a masters degree in physics at the University of Padua. He obtained his PhD at the University of Düsseldorf, performing a measurement of the isotropy of the speed of light. Since 2008 he has focused on realizing high-efficiency concentrating photovoltaic modules.

Stelvio Golfetto
AtemEnergia SRL
Treviso, Italy

Stelvio Golfetto taught physics and mathematics in high schools for more than 10 years. Since 2011 he has been an employee of Unitek SRL, where he is involved in the research and development of AtemEnergia's CPV system. His studies include heat dissipation and characterization of 3J solar cell.

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