This paper describes the optical and illumination design of a CPV solar energy system. The challenges of creating a highly efficient yet low-cost system architecture come from many sources, but are primarily limited by the photoelectron conversion efficiency of the cells and the illumination performance of the system for on-axis and off-axis pointing scenarios. Furthermore, the need for high solar spectral throughput, evenly concentrated sunlight, and tolerance to offaxis pointing places strict illumination requirements on the optical design. To be commercially viable, the cost associated with all components must be minimized so that when taken together, the absolute installed cost of the system in kWh is lower than any other solar energy method. We present two low-cost optical design embodiments of a dishbased concentration photovoltaic (CPV) system that utilize Köhler illumination to achieve good illumination uniformity across an array of solar cells. Further optimization for active shadowing compensation and compound electrical I-V curve modeling for the solar cell array is performed that allows realistic off-axis performance scenarios to be modeled with the correct power response sensitivity.
Conventional concentrating photovoltaic (CPV) systems track the sun with high precision dual-axis trackers. The emergent field of tracking-integrated optics has the potential to simplify the mechanics of CPV systems by loosening or eliminating the need for dual-axis tracking. In a tracking-integrated scheme, external module tracking is complemented or entirely replaced by miniature tracking within the module. This internal tracking-integration may take the form of active small-motion translation, rotation of arrayed optics, or by passive material property changes induced by the concentrated light. These methods are briefly reviewed. An insolation weighting model is presented which will aid in the design of tracking-integrated optics by quantifying the tradeoff between angular operation range and annual sunlight collection. We demonstrate that when tracking-integrated optics are used to complement external module tracking about a horizontal, North-South oriented axis, truncating the operational range may be advantageous. At Tucson AZ latitude (32.2°N), 15.6% of the angular range may be truncated while only sacrificing 3.6% of the annual insolation. We show that modules tracked about a polar-aligned axis are poorly-suited for truncation.
Line-focus parabolic trough mirrors for solar thermal generation cannot produce the high concentration required for concentrating photovoltaic (CPV) systems. We describe a freeform lens array with toroidal symmetry which intercepts the low-concentration line focus to produce a series of elongated, high-concentration foci. The design employs 2D Kӧhler illumination to improve the acceptance angle in one direction. The two-stage concentrator has 1000X average geometric concentration with an acceptance angle of +/-1.49° in the azimuthal direction and +/-0.29° in the elevation direction. Preliminary results of a prototype roll-forming process are shown in thermoplastics and B270 glass.
A solar concentrator with a highly asymmetric acceptance cone is investigated. Concentrating photovoltaic systems require dual-axis sun tracking to maintain nominal concentration throughout the day. In addition to collecting direct rays from the solar disk, which subtends ~0.53 degrees, concentrating optics must allow for in-field tracking errors due to mechanical misalignment of the module, wind loading, and control loop biases. The angular range over which the concentrator maintains <90% of on-axis throughput is defined as the optical acceptance angle. Concentrators with substantial rotational symmetry likewise exhibit rotationally symmetric acceptance angles. In the field, this is sometimes a poor match with azimuth-elevation trackers, which have inherently asymmetric tracking performance. Pedestal-mounted trackers with low torsional stiffness about the vertical axis have better elevation tracking than azimuthal tracking. Conversely, trackers which rotate on large-footprint circular tracks are often limited by elevation tracking performance. We show that a line-focus concentrator, composed of a parabolic trough primary reflector and freeform refractive secondary, can be tailored to have a highly asymmetric acceptance angle. The design is suitable for a tracker with excellent tracking accuracy in the elevation direction, and poor accuracy in the azimuthal direction. In the 1000X design given, when trough optical errors (2mrad rms slope deviation) are accounted for, the azimuthal acceptance angle is +/- 1.65°, while the elevation acceptance angle is only +/-0.29°. This acceptance angle does not include the angular width of the sun, which consumes nearly all of the elevation tolerance at this concentration level. By decreasing the average concentration, the elevation acceptance angle can be increased. This is well-suited for a pedestal alt-azimuth tracker with a low cost slew bearing (without anti-backlash features).
Methods developed to maximize the overall reflectance of the second-surface silvered glass used in concentrating solar power (CSP) and concentrating photovoltaics (CPV) solar systems are reported. The reflectance at shorter wavelengths is increased with the aid of a dielectric enhancing layer between the silver and the glass, while at longer wavelengths it is enhanced by
use of glass with negligible iron content. The calculated enhancement of reflectance, compared to unenhanced silver on standard low-iron float glass, corresponds to a 4.5% increase in reflectance averaged across the full solar spectrum, appropriate for CSP, and 3.5% for CPV systems using triple junction cells. An experimental reflector incorporating these improvements, of
drawn crown glass and a silvered second-surface with dielectric enhancement, was measured at National Renewable Energy Laboratory to have 95.4% solar weighted reflectance. For comparison, nonenhanced, wet-silvered reflectors of the same 4-mm thickness show reflectance ranging from 91.6% to 94.6%, depending on iron content. A potential drawback of using iron-free
drawn glass is reduced concentration in high concentration systems because of the inherent surface errors. This effect is largely mitigated for glass shaped by slumping into a concave mold, rather than by bending. Finally, an experiment capable of determining which junction limits the triple junction cell is demonstrated.
Using illumination modeling, we provide a comparison of glass Total Internal Reflection Concentrators (TIRC) and
metal Hollow Reflective Concentrators (HRC) used as secondary concentrator elements in a dish-based highconcentration photovoltaic (CPV) system. Comparisons of optical efficiency, flux uniformity, off-axis acceptance angle, and cost are vital to choosing an ideal secondary concentrator element for a CPV system employing multi-junction (MJ) cells. In many CPV systems, a free-form optic or sharp-cornered rectangular TIRC composed of glass is used to increase the geometrical flux concentration at the surface of the MJ cell, and may also serve as homogenizers to mix the light to increase flux uniformity. We have demonstrated in on-sun testing that an electroformed metal HRC can be used in place of a glass TIRC of the same geometry, eliminating the need for polymeric bonding to the MJ cell surface, and providing a side-contact surface pathway for active cooling. Although geometrically equivalent, we show that glass TIRC’s achieve superior off-axis performance (higher etendue from surface refraction) and are generally acknowledged to have less degradation than optics with over-coated silver, yet metal HRC’s employing over-coated silver are superior in spectral absorption characteristics under high solar flux (no losses from glass absorption or Fresnel surface reflections) and don't require accurate glass pressing into many shapes. To better understand the trade-offs between optical efficiency, off-axis performance, mechanical tolerances, cost and reliability, metal (HRC) and glass (TIRC) tapered funnels are analyzed at the surface of equal irradiance in a Kohler-Illumination concentrator system, and a trade study is presented.
The University of Arizona has developed a new dish-based High Concentration Photovoltaic (HCPV) system which is in
the process of being commercialized by REhnu, Inc. The basic unit uses a paraboloidal glass reflector 3.1 m x 3.1 m
square to bring sunlight to a high power point focus at a concentration of ~20,000x. A unique optical system at the focus reformats the concentrated sunlight so as to uniformly illuminate 36 triple junction cells at 1200x geometric
concentration<sup>1</sup>. The relay optics and cells are integrated with an active cooling system in a self-contained Power Conversion Unit (PCU) suspended above the dish reflector. Only electrical connections are made to the PCU as the active cooling system within is completely sealed. Eight of these reflector/PCU units can be mounted on a single two axis tracking structure<sup>2</sup>. Our 1st generation prototype reflector/PCU unit consistently generated 2.2 kW of power normalized to 1kW/m<sup>2</sup> DNI in over 200 hours of on-sun testing in 2011<sup>3</sup>. Here, we present on-sun performance results for our 2<sup>nd</sup> generation prototype reflector/PCU unit, which has been in operation since June 2012. This improved system consistently generates 2.7 kW of power normalized to 1kW/m<sup>2</sup> DNI and has logged over 100 hours of on-sun testing. This system is currently operating at28% DC net system efficiency with an operating cell temperature of only 20°C above ambient. Having proven this system concept, work on our 3<sup>rd</sup> generation prototype is underway with a focus on manufacturability, lower cost, and DC efficiency target of 32% or better.
We describe the construction and application of innovative deformable mirrors for adaptive optics (AO) being developed
at the University of Arizona's Center for Astronomical Adaptive Optics. The mirrors are up to 1 m in diameter, with high
actuator stroke, and are optically powered. Scientific motivations for the work include the detection of earthlike planets
around other nearby stars, as well as non-astronomical applications such as directed energy and horizontal imaging for
defense and security. We describe how high resolution imaging is delivered over an unusually wide field of view by
ground-layer AO. This technique employs multiple laser guide stars to sense the instantaneous three-dimensional
distribution of atmospheric turbulence. Imaging with high signal-to-noise ratio in the thermal infrared is enabled by
embedding the deformable mirror directly in the telescope. We also describe recent work to develop a new generation of
these mirrors with lighter weight and improved robustness by use of replicated composite materials which shows
promise for greatly reducing the cost of AO and broadening its appeal, particularly for non-astronomical applications as
well as for a new generation of extremely large ground-based telescopes of 30 m diameter now under construction.
This paper reports methods developed to maximize the overall reflectance second-surface silvered glass. The reflectance
at shorter wavelengths is increased with the aid of a dielectric enhancing layer between the silver and the glass, while at
longer wavelengths it is enhanced by use of glass with negligible iron content. The calculated enhancement of reflectance,
compared to unenhanced silver on standard low-iron float glass, corresponds to a 4.4% increase in reflectance averaged
across the full solar spectrum, appropriate for CSP, and 2.7% for CPV systems using triple junction cells. An
experimental reflector incorporating these improvements, of drawn crown glass and a silvered second-surface with
dielectric boost, was measured at NREL to have 95.4% solar weighted reflectance. For comparison, non-enhanced, wetsilvered
reflectors of the same 4 mm thickness show reflectance ranging from 91.6 - 94.6%, depending on iron content. A
potential drawback of using iron-free drawn glass is reduced concentration in high concentration systems because of the
inherent surface errors. This effect is largely mitigated for glass shaped by slumping into a concave mold, rather than by
Carbon-fiber reinforced polymer (CFRP) composite is an attractive material for fabrication of optics due to its high
stiffness-to-weight ratio, robustness, zero coefficient of thermal expansion (CTE), and the ability to replicate multiple
optics from the same mandrel. We use 8 and 17 cm prototype CFRP thin-shell deformable mirrors to show that residual
CTE variation may be addressed with mounted actuators for a variety of mirror sizes. We present measurements of
surface quality at a range of temperatures characteristic of mountaintop observatories. For the 8 cm piece, the figure
error of the Al-coated reflective surface under best actuator correction is ~43 nm RMS. The 8 cm mirror has a low
surface error internal to the outer ring of actuators (17 nm RMS at 20°C and 33 nm RMS at -5°C). Surface roughness is
low (< 3 nm P-V) at a variety of temperatures. We present new figure quality measurements of the larger 17 cm mirror,
showing that the intra-actuator figure error internal to the outer ring of actuators (38 nm RMS surface with one-third the
actuator density of the 8 cm mirror) does not scale sharply with mirror diameter.
Carbon fiber reinforced polymer (CFRP) composites provide several advantages as a substrate for thin-shell adaptive
secondary mirrors, including high stiffness-to-weight ratio and low coefficient of thermal expansion (CTE). We have
addressed some of these concerns using a prototype CFRP mirror under actuation. Using 4D and Newton interferometry,
we present measurements of surface quality at a range of temperatures. Under actuator relaxation at room temperature,
its surface error is low (92 nm RMS) and dominated by edge curvature. This error is reduced further under best actuator
correction to 43 nm RMS, placing it into consideration for use in near-IR astronomy. The low surface error internal to
the outer ring of actuators - 17 nm RMS at 60°F and 33 nm RMS at 20°F - suggests that larger mirrors will have a
similar figure quality under actuator correction on ground-based AO systems. Furthermore, the actuator forces required
to correct the figure are small compared to the dynamic range of voice coil actuators (~0.1 N). In addition, surface
roughness is characterized to address the effects of high spatial frequency errors.