TECHNOLOGY INSIGHTSA Report from EPRI’s Innovation Scouts

concentrating photovoltaics: technology for reducing the cost of concentration

the technology

New glass-shaping process and optical design leverage experience from large astronomical telescopes to reduce the cost of concentration

the value

Advancements could enable higher conversion efficiencies, lower capital costs, and better reliability than competing products

epri’s focus

Independent assessments, including field testing, are being explored to characterize performance and reliability

technology overviewConventional solar photovoltaic (PV) technology employs large-area cells constructed of silicon and other semiconductor materials to convert natural sunlight directly into electrons that flow to produce direct-current (DC) electricity. Concentrating PV (CPV) systems generally use lenses or mirrors with tracking systems to focus the Sun’s energy on multijunction cells that convert a larger portion of the solar spectrum into electricity than the single-junction cells used in conventional flat-plate PV. By concentrating sunlight on higher-efficiency converters, CPV also increases energy production per unit area. Until recently, CPV’s lower silicon requirement was viewed as the technology’s major potential advantage over flat-plate PV, particularly for utility-scale applications. That argument has become less relevant with the dramatic reduction in silicon prices over the last several years.

To make market inroads, CPV products must deliver superior performance at sufficiently low cost. Because multijunction PV cells are commercially available and relatively inexpensive when used at high (1000x) concentration levels, technology developers are focusing increased attention on reducing the cost of concentration. CPV innovations that offer significant cost reduction have the potential to be game-changers by capturing market share in a rapidly growing industry, particularly in arid regions with the high levels of direct-normal insolation (DNI) required for optical concentration.

This brief reviews a CPV system being developed by REhnu, Inc., that incorporates several unique features: paraboloidal (dish-shaped) mirrors fabricated using a new glass molding process, a novel method of coupling high-efficiency solar cells to the concentrated light, and active

cooling of the compact cell assemblies. This combination of technology advancements could enable increased efficiencies and better reliability at lower capital cost than competing PV and CPV products. In 2012, REhnu’s 2nd-generation prototype system (Figure 1) demonstrated 28.1% DC net system efficiency at standard conditions (scaled to 25°C cell temperature, and after accounting for parasitic cooling losses). REhnu expects its 3rd-generation design to achieve 32% efficiency in 2014 field demonstrations [1].1

basic science

Under concentration, the multijunction cells utilized in most CPV systems are roughly twice as efficient as the single-junction cells used

Figure 1 – REhnu 2nd-generation unit in operation (foreground) and 1st-generation system (rear).

1 Much of the information reported herein is based on personal communication with REhnu representatives Roger Angel and Thomas Stalcup.

Technology Insights 2 December 2013

in conventional flat-plate PV systems, reducing the amount of collector area and semiconductor material required to achieve a given level of energy production. On balance, however, the potential cost-performance advantages of CPV have been eroded by the steep decline in the price of silicon-based PV modules. The major challenge REhnu addresses is the cost of concentration, through the use of large, low-cost glass mirrors and small relay optics to clustered cells.

The mirrors require innovation in glass shaping to achieve a highly precise point focus using a single sheet of glass. Developing an advanced mirror molding technique is the objective of a $1.5M U.S. Department of Energy (DOE) SunShot project awarded to the University of Arizona in 2012. Paraboloidal mirrors will be formed in a process described by REhnu as “softening and slumping” to mold the back-silvered mirrored surface into the desired shape [2]. The mold itself is machined with grooves to minimize contact with the polished front and back surfaces of the glass mirror while still providing the required shaping accuracy. The new molding process is expected to reduce mirror-shaping time from more than 5 hours for the prototype to a few minutes at high volume. According to REhnu, the process could potentially lower commercial manufacturing costs by 40%. Developmental work is also aimed at boosting the reflectivity of the glass through the use of glass compositions free of iron absorption and at preventing mirror soiling with the application of dust-resistant coatings. Mirrors fabricated using the new shaping process with a reflector dimension of 1.65 m x 1.65 m will be demonstrated in REhnu’s 3rd-generation prototype in early 2014 [1].

In parallel with its mirror development activities, REhnu is taking a fresh look at best ways to couple clustered cells to the high-power point focus provided by the mirrors. A separate $1M DOE SunShot Incubator Award in 2012 [3] is funding a new power conversion unit (PCU), in which reflected sunlight is concentrated onto a fused-silica ball lens with an anti-reflective coating that distributes the light through optical funnels, or secondary concentrators, to 36 triple-junction cells (Figure 2 and Figure 3). The square cells are attached to small circuit cards [1]. The ball and funnel optics allow for obscuration by the PCU ahead of the large reflector, and they are designed to ensure that the cells uniformly receive 920x concentrated sunlight and thus all generate equal electric current. The hermetically sealed, coffee-can-sized PCU’s compact design builds on packaging experience from the power electronics and computer industries to offer potential for a 20-year lifetime without routine maintenance. It is optimized to be tolerant of tracking error, with on-sun testing demonstrating production of 90% of maximum power at a pointing error of 0.6 degrees.

REhnu’s technology also is designed to mitigate heat generation at the focal point where cells are clustered, a challenge facing all high-concentration systems. REhnu has implemented an active cooling system that keeps cells at a constant 25°C above ambient air temperature to minimize thermal cycling [1]. Cooler cells are not only more efficient, but also they are more reliable as shown by the U.S. National Renewable Energy Laboratory (NREL), which found that 67% of damage to CPV cells was caused by thermal cycles greater than 30°C [4]. REhnu’s closed-

loop system circulates antifreeze coolant behind the cell assemblies and discharges the heat through a centrally located fan-cooled radiator [5].

Most CPV designs utilize small polymer-based Fresnel lenses, each one focusing sunlight onto an individual cell that is cooled passively by an aluminum heat sink. Such passive cooling designs must accommodate relatively large convective surface area directly behind each cell, whereas active cooling allows for small, high-power-density cell packages with minimal cell temperature rise. In REhnu’s 3rd-generation PCU, a cell array small enough to fit in the palm of one’s hand is rated at 800 W (Figure 3b shows the secondary optic that feeds the cell array); this compact design will facilitate the swapping out of cell arrays if justified by future cost-performance improvements. The cooling pump and fan currently use about 2% of the generated power [1]. The REhnu design team expects to reduce parasitic losses to 1% in future designs—perhaps comparable to performance degradation at the higher temperatures associated with passive cooling in conventional CPV designs, but without the adverse impacts on reliability.

REhnu has also developed a lightweight dual-axis tracker designed to eliminate the need for concrete foundations [5]. The 3rd-generation

Figure 2 – Schematic of REhnu’s optical system (top); prototype system during on-sun operation (bottom) [1].

Technology Insights 3 December 2013

design supports the reflectors from behind using a torsion tube and ribs. Tracking accuracy for the 2nd-generation prototype is ± 0.2 degrees, which is comparable to other dual-axis CPV technologies.

value to the industryThe CPV industry is largely pre-commercial, and the technology currently represents a small fraction of the total PV market—approximately 90 MW of cumulative global deployment was achieved by end of year 2012 [6]. CPV’s primary competitor is flat-plate PV, which holds roughly 98% of worldwide solar market share. Greentech Media reports that global solar PV deployment more than doubled from 50 GW in January 2011 to over 100 GW in mid-2013, and it is expected to double again by January 2015 [7]. Such rapid overall solar market growth offers an enormous opportunity for CPV if it can demonstrate bankability and cost-competitiveness.

Most CPV systems are able to leverage recent advances in commercial high-efficiency multijunction PV technology—the NREL-verified world records are 44% for a commercial-ready production cell (2012) and 35.9% for an Amonix module (2013). Additional beneficial characteristics include capacity factors of up to 31% (versus 27% for flat-plate PV), stable performance in high-temperature environments, and potential reductions in water and land requirements. These features, combined with low-cost and locally sourced commodity materials and the ability to utilize existing manufacturing infrastructure for fast scale-up, may allow the CPV industry to more quickly capture market share. In fact, IMS Research estimates CPV may represent 27% of the U.S. market by 2016 [8]. Other market research firms project that global CPV deployment will be a more modest 300 MW to 750 MW annually by 2016 [9].

The prospects for CPV are uncertain, but advanced technologies that offer significant cost reduction have the potential to be game-changers in a rapidly growing market segment. REhnu estimates hardware costs (reflector, PCU, tracker, cooling system, foundation, and inverter) of $0.70/W for their commercial product in 2017, assuming production of 10,000 units per year (~75 MW), with units rated at 7.9 kWp based on cell efficiency of 45% and net system DC efficiency of 36% [10]. For comparison, average installed costs for utility-scale flat-plate PV systems in early 2013 were hovering just above $2/W.

Cost, validated system performance and reliability data, solid financial backing, and other criteria will be important in determining market success. The potential benefits of the REhnu technology are summarized below:• Achievement of cost and performance goals could potentially result

in levelized cost of electricity as low as $0.05/kWh without subsidy [5], considerably lower than competing solar products available today. REhnu’s 2020 target for installed system cost is $1/Wp.

• The large mirrors and compact PCU are designed for quick scale-up in existing high-volume factories [11]. Supply chains for curved glass mirrors, cell assemblies, and trackers are mature and currently operating at high volume.

• Compared to flat-plate PV plants, the sunlight collection surface area is roughly halved [11], possibly resulting in more efficient land use. Estimates for the 3rd-generation technology are 60 to 75 MW/km2 (160 to 200 MW/square mile) or 1.3 to 1.6 hectares/MW (3.2 to 4 acres/MW) [5].

• Use of raw materials common to CPV systems, i.e., glass, steel, concrete, and wiring, is minimized through the use of compact, lightweight arrays [11]. Currently, the specific mass of the system is expected to be 160 kg/kW [5]. In contrast, alternative CPV approaches tend to be heavier with large modules, larger sealing surfaces, concrete foundations, and cells with interconnecting wires distributed over large areas [12].

• Similar to any CPV system, the greatest operations and maintenance (O&M) effort is expected to be regular washing of the mirrored concentrators to restore reflectivity [5]. REhnu has designed the CPV controls to turn units parallel to a roadway, such that a cleaning vehicle can wash the mirrors. Washing frequency will depend on location and the efficacy of the mirrors’ dust-resistant coating. The PCU itself is expected to require minimal maintenance.

• All PV and CPV systems experience various rates of performance degradation over their 20- to 30-year lifetimes. The REhnu product can potentially offer lower degradation rates due to reduced thermal cycling and subsequent mechanical distortion [13], along with the option to substitute higher-efficiency cell assemblies at a later date to restore or boost system output over extended lifetimes up to 40 years. Cell replacement may be feasible in other CPV system designs as well, but the compact nature of the REhnu cell assembly is expected to facilitate replacement at significantly lower cost.

Figure 3 – REhnu’s third-generation power conversion unit components: (a) Focused sunlight enters through a ball lens and is equally distributed onto an array (b) of tapered funnel apertures that impart the light on a concave array (c) of multijunction solar cells [1].

Technology Insights 4 December 2013

state of the technologyThe CPV industry has suffered as a result of the global recession and falling prices for traditional flat-plate PV systems. Nonetheless, several organizations, including EPRI, see the long-term potential benefits of CPV and are actively scouting developments and pursuing field-testing and reliability programs. Some CPV systems have achieved a technology readiness level (TRL) of 8 (commercial availability), while numerous pre-commercial CPV system concepts and components are being pursued in the United States and around the world.

A $1M 2012 DOE Incubator award to REhnu is funding development of a 32% efficient production-ready PCU, while a $1.5M DOE SunShot award to the University of Arizona is aimed at developing a fast mirror-shaping process. The furnace for the mirror-shaping process is compatible with mass production of 75 MW capacity annually [10]. The newly designed REhnu reflector and the 3rd-generation PCU are both currently at TRL4 and are expected to move to TRL5 by end of year 2013. Early demonstration of an integrated system (TRL6) is scheduled for early 2014. A few months after installation, the 2nd-generation prototype had demonstrated 100 hours of on-sun operation without failure [1].

The REhnu design leverages the experience of several other industries, which may accelerate its development. For example, extensive mirror design expertise was borrowed from the inventors of large astronomical telescopes, and REhnu is working with established curved mirror suppliers to develop a production-ready manufacturing process. Multijunction cells will be sourced from commercial suppliers, and the active cooling system utilizes mature, off-the-shelf technology developed for the automobile and heating, ventilation, and cooling industries [1].

intellectual property and public literature

Three patents have been issued to the University of Arizona with two additional patents in process, and REhnu has the exclusive license to commercialize this patented technology. U.S. Patent No. 8,082,755 was awarded in 2011 for a “Method of Manufacturing Large Dish Reflectors for a Solar Concentrator Apparatus.” U.S. Patent No. 8,350,145, issued in 2013, covers the “Photovoltaic Generator with a Spherical Imaging Lens for Use with a Paraboloidal Solar Reflector.” U.S. Patent No. 8,430,090, also awarded in 2013, is for a “Solar Concentrator Apparatus with Large, Multiple, Co-Axial Dish Reflectors.”

Additional information about REhnu’s CPV technology, including links to a video of the operating prototype system, presentations, and conference proceedings, is publically available at: www.rehnu.com/.

next milestone

REhnu plans to demonstrate a 3rd-generation prototype design (Figure 4) with modular 6.4-kWp pedestals in early 2014. Dish-shaped mirrors will focus sunlight onto 800-Wp PCUs that contain compact arrays of high-efficiency multijunction cells at the focal points [1]. A lightweight, electric dual-axis tracker will support eight mirror-PCU pairs while

eliminating the need for concrete foundations. Net DC system efficiency is anticipated to be 32% or better. REhnu intends to produce 140 systems for a 1-MW deployment in 2015 [10].

independent assessmentsThe performance and cost of the REhnu technology have not yet been verified by a third party. However, the technology was reviewed by DOE as part of the competitive solicitation processes for the DOE Incubator and SunShot awards, and three U.S. patents have been issued.

collaborationREhnu, Inc. was formed in 2009 by a team of researchers at the University of Arizona’s Department of Astronomy and Steward Observatory in Tucson, Arizona. Under the direction of Roger Angel, REhnu’s chief technology officer and founder of the Steward Observatory Mirror Lab and the Center for Astronomical Adaptive Optics, close collaboration between the two organizations continues. Through the DOE SunShot project, REhnu has partnered with Rioglass Solar, S.A. to provide the raw float glass material and glass-mounting expertise and with Glasstech, Inc. to evaluate the mirror-shaping process [2]. Tucson Electric Power has supported field testing at the nominally 20-MW University of Arizona Tech Park in Tucson, Arizona.

REhnu is currently looking for investment to begin manufacturing of the 3rd-generation unit and support long-term commercialization. Most of the leading CPV companies today are backed by large companies that can underwrite projects, e.g., Semprius/Aerojet Rocketdyne, SunPower/Total, and Morgan Solar/Iberdrola/Nypro. REhnu plans to initially test smaller 1.6-kW (two-mirror) units for evaluation, potentially in collaboration with EPRI at SolarTAC. Independent testing and validation of the integrated 3rd-generation system also are under consideration.

A combination of laboratory and field testing is typically required to gain the confidence of investors and future CPV end users. For example, mirrored reflectors fabricated using REhnu’s new molding process must undergo standard testing for durability, including wind loading and hail stress tests. Stress testing for the newly designed PCU also must be performed to identify potential failure modes. These failure modes

Figure 4 – Artist rendering of REhnu’s 3rd-generation CPV system to be tested in early 2014.

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would then need to be addressed before commercial production begins. Accelerated laboratory testing (temperature cycling, humidity tests, etc.) is required to build confidence in predictions of long-term performance.

On-sun field testing of the integrated CPV system for an extended period of time will be necessary to establish real-world performance, O&M requirements, and reliability characteristics. For example, the cooling subsystem must demonstrate the ability to perform across different climate conditions. If REhnu develops its own advanced dual-axis tracker technology, field testing will be essential to assess the tracking accuracy, foundation requirements, and other characteristics.

references1. T. Stalcup, R. Angel, B. Coughenour, B. Wheelwright,

T. Connors, W. Davison, D. Lesser, J. Elliott, and J. Schaeger, “On-Sun Performance of an Improved Dish-Based HVPV System,” SPIE (2012).

2. U.S. DOE SunShot Concentrating Solar Power: Advanced Manufacture of Reflectors, DOE/GO-102012-3678 (Sept. 2012).

3. http://www1.eere.energy.gov/solar/sunshot/incubator_projects.html

4. N. Bosco and S. Kurtz, “Quantifying the Weather: an analysis for thermal fatigue,” NREL PV Module Reliability Workshop (2010).

5. http://www.rehnu.com/

6. Solar Industry Update 2012. EPRI, Palo Alto, CA: 2012. 1025406.

7. https://www.greentechmedia.com/articles/read/chart-2-3rds-of-global-solar-pv-has-been-connected-in-the-last-2.5-years

8. “The World Market for Concentrated PV (CPV) – 2012,” IMS Research, September 2012.

9. SPV Market Research/Strategies Unlimited, Dec. 2012.

10. REhnu Foldable Flyer for Email.

11. REhnu Investor Brochure, Feb. 2013.

12. V. M. Fthenakis and H. C. Kim, “Photovoltaics: Life-Cycle Analyses,”Solar Energy 85 2011, pp. 1609–1628

13. Review of Photovoltaic System Reliability Challenges and Opportunities. EPRI, Palo Alto, CA: 2012. 1024002.

contactCara Libby, Project Manager Environment and Renewable Energy 650.776.6009 , clibby@epri.com