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FOCUS

Prof. Gang Chen (MIT), PI is the Head of the Department of Mechanical Engineering, the Carl Richard Soderberg Professor of Power Engineering, and is the director of the "Solid-State Solar-Thermal Energy Conversion Center (S3TEC Center)" - an Energy Frontier Research Center funded by the US Department of Energy. He was an assistant professor at Duke University, a tenured associate professor at UC Los Angeles, before moving to MIT. He is a recipient of a K.C. Wong Education Foundation fellowship and a John Simon Guggenheim Foundation fellowship. He received an NSF Young Investigator Award, an R&D 100 award, and an ASME Heat Transfer Memorial Award. He is a fellow of AAAS, APS, and ASME. In 2010, he was elected a member of the US National Academy of Engineering. Prof. Evelyn Wang (MIT), PI is an Associate Professor in ME at MIT. She has extensive experience in developing efficient liquid and two-phase heat and mass transport systems, and in the synthesis and characterization of nanostructured materials and aerogels.  Prof. Zhifeng Ren (University of Houston) is an M. D. Anderson Chair Professor of the Department of Physics and The Texas Center for Superconductivity of the University of Houston (TcSUH). He has extensive experience on materials synthesis and characterizations in the fields of superconductors, carbon nanotubes, zinc oxide nano structures, thermoelectric materials, photovoltaic materials, nano bio materials, transparent electrical conductors, and selective solar absorbers. Dr. Svetlana Boriskina (MIT) is a Research Scientist at the Department of Mechanical Engineering at MIT. She has an extensive experience in engineering of nanostructured photonic and plasmonic spectrally-selective absorbers and emitters. Her expertise also includes design of thermophotovoltaic energy conversion systems. 

Project PI – Dr. Wei Pan, Sharp Labs of America.  Dr. Pan has more than 17 years industrial experience developing practical manufacturing processes for magnetic storage, non-volatile memories, image sensors, solar cells, and other electronic devices.  Dr. Pan has 55 awarded U. S. patents. His main job function for the project is to lead the team to design and integrate the state-of-the art CSP-CPV system and meet the performance specifications. Project Co-PI – Dr. Douglas Tweet, Sharp Labs of America.  Dr. Tweet has more than 20 years of industrial research experience in theoretical analysis, fabrication and characterization of optical and semiconductor thin films and new materials for display, LED, and energy-efficiency applications.  Dr. Tweet has 73 awarded U. S. patents. He works closely with UA team for optical system design, characterization, and integration. Project Co-PI – Mr. Greg Stecker, Sharp Labs of America. Mr. Stecker has more than 25 years in semiconductor industry.  His expertise covers the area of mechanical parts and system design and fabrication.  He has built many research tools from concept to accommodate project needs. He works closely with Solargenix and UM to design secondary optics and PV array to fit into Solargenix’s trough. Project Co-PI – Dr. Robert A. Norwood, University of Arizona, Optical Science. Dr. Norwood is a globally recognized expert in optics.  He is a fellow of the OSA and the SPIE, and has been an Associate Editor of IEEE Photonics Technology Letters and Optical Materials Express.   He was previously CTO and VP of Photon-X, Inc., a VC backed fiber optic startup, and spent 11 years working in staff and leadership positions in corporate research and development positions at Honeywell and Hoechst Celanese. He has more than 100 publications in refereed journals, has 30 issued US patents, and has given 65 invited talks at major international meetings. He is responsible for optical design and testing and work closely with Sharp Labs of America for the whole system integration. Project Co-PI – Dr. Chung-Lung Chen, University of Missouri.  Dr. Chen is the Endowed Chair of Mechanical & Aerospace Engineering at the University. He is a recognized expert in the field of thermal management, flow control, micro/nano technology, and renewable energy. Dr. Chen has more than 110 scientific publications and received many DARPA/ONR awards and honors.  He develops thermal management schemes for the hybrid system. Project Co-PI – Mr. John F. Myles, Solargenix LLC. Mr. Myles is CEO and Chairman of the Board and the Founder of Solargenix LLC.  He is a licensed General Contractor with more than 40 years general construction experience with particular interest in solar thermal energy generation. He is past Chairman of the American Solar Energy Society Board of Trustees and served as a member of the Board of Directors for Acciona Solar Power, Inc. Solargenix has participated in the development of several utility-scale CSP projects worldwide.  Solargenix will design and build the hybrid trough system for the project.

PI - Todd Otanicar, PhD, PE, The University of Tulsa: Dr. Otanicar is leading the effort from the University of Tulsa. His research focuses on heat transfer and thermodynamics centered primarily on the use of nanoscale structures to enhance thermal transport and energy conversion. Dr. Otanicar has been at the forefront of the use of nanoparticles for solar energy absorption with over 10 publications in this field alone. Co-PI – Kenneth Roberts, PhD, The University of Tulsa: Performs research on the synthesis and optical/electron characterization of metallic, semiconductor, and oxide nanoparticles. Moreover, he investigates numerous nanoparticle surface ligands to tailor solubility and surface reactivity.  Co-PI – Parameswar Hari, PhD, The University of Tulsa: Conducts research on nanomaterials, especially on materials used in third generation photovoltaics and is also the associate director of the nanotechnology institute at the University of Tulsa. Dr. Hari is the author of over 60 publications. Co-PI – Ratson Morad, Cogenra Solar: Serves as COO and VP R&D at Cogenra. He will have overall responsibility for the project at Cogenra.  Mr. Morad developed multiple advanced technology systems, including MRI, semiconductors deposition systems and PV systems. Mr. Morad has 30 granted US patents. Co-PI – Aaron Saunders, PhD, nanoComposix: Dr. Saunders is a Senior Scientist at nanoComposix, and conducts and directs research using nanomaterials in optical coatings, diagnostic assays, and defense-related applications.  He has researched nanomaterials for more than 12 years, and is author of more than 20 publications and patents.

PI Minjoo Larry Lee is an associate professor of electrical engineering at Yale University. He is the author or coauthor of over 100 technical papers and conference proceedings and holds nine patents. He has received numerous recognitions including: the Lange Lectureship (UCSB), North American conference on MBE (NAMBE) Young Investigator Award, DARPA Young Faculty Award, NSF CAREER award, MRS gold award for graduate student research, and the IEEE Electron Device Society George E. Smith award. His advisees have won best presentation or poster prizes at the MRS fall meeting, IEEE Photovoltaic Specialists Conference, Electronic Materials Conference, and NAMBE. Myles Steiner is a senior scientist in the High Efficiency Crystalline Photovoltaics group at NREL, working primarily on III-V multijunction solar cells for concentrator applications. He has worked on photon recycling and luminescent coupling in multijunction cells, growth and ordering of lattice-mismatched GaInP and GaInAs solar cells, mechanical stacking and lateral spectrum splitting multijunction designs, and the design and measurement of solar cells at elevated operating temperatures. He is the author or co-author of over 50 Journal and conference papers, and holds two patents.   Dr. Paul Sharps is the Chief Scientist for SolAero Technologies Corporation. Dr. Sharps received his PhD from Stanford University in 1990, studying CdTe thin films for photovoltaic applications. Dr. Sharps worked on research and development of III-V multi-junction solar cells for Research Triangle Institute from 1990 until 1998. In 1998 Dr. Sharps joined Emcore Photovoltaics, and has held a number of positions in R&D. He has managed a number of development programs, including one that developed the first GaInP2/GaAs/Ge triple junction device to achieve an efficiency of greater than 27.5%. Dr. Sharps has also been involved in transitioning the technology into commercial products. Dr. Sharps won an R&D 100 award in 2008 (Along with Dr. Mark Wanlass from NREL) for development of the IMM cell. Dr. Sharps has 15 granted patents, with 3 more pending, and has over 100 technical presentations and papers. He has been on the organizing committee for the IEEE PVSC for the past 8 years. In late 2014 Emcore Photovoltaics became SolAero Technologies Corporation.  

The PVMirror project is currently on track to successful completion of its third-quarter milestones. Several technical risks to the project were assessed in Q2, including PV cell efficiency loss due to bending to the curvature of the trough, and fabrication of optical coatings with the target performance. A large-area glass curving process developed at UA was modified to enable small pieces of glass to be slumped to the right curvature to make PVMirror prototypes, and a process for making PVMirror prototypes was successfully established using the curved glass and six-inch silicon heterojunction PV cells fabricated at ASU. As expected, curving the PV cells to the large trough radius (1.6 meters) has no deleterious impact on their power output. Optical coatings were designed with software and fabricated both on glass and on the silicon PV cells themselves (two routes to the same end—the optical coating should be between the glass and the cells). The coatings consisted of SiO2/TiO2 stacks deposited by electron-beam evaporation performed by a commercial vendor in the US, and the measured reflectance and transmittance spectra were reasonably close to the target spectra. The team also evaluated a free-standing polymer film that behaves like an optical filter; this option was not anticipated at the start of the project and appears to be an exciting possibility from both a performance and manufacturing-convenience perspective. Economic analysis of the PVMirror concept is an integral part of the project, and the main focus of Q3 is to determine which of the three spectrum-splitting options is most techno-economically promising. This will guide the final design of the first PVMirror prototype ready for outdoor testing, which will be completed at the end of Q4.  Progress UpdatesMay 2015 Progress Update 

A modular hybrid PVT/CSP collector design was developed in a CAD framework with part and assembly details enabling structural support of optical, photovoltaic, and thermal elements.  The relative positioning of optical elements was derived from an analysis of the design trade space with respect to efficiency (via raytracing), cost, and availability of components.  The assembly was further disaggregated into a BOM for preliminary cost modeling of the system.  The design includes the advantage of leveraging existing supply chains for the CSP industry and potential compatibility with existing CSP installed capacity as an output-boosting upgrade.  A dichroic filter was developed to the specifications of the project and manufactured. The accuracy and tunability of the UV and IR band edges was established, and angle of incidence (AOI) effects were measured and quantified for transmission and reflection.  The shift in band edge towards lower wavelengths with increased AOI was characterized and the effects were incorporated into the system model. Optimization analysis of design configurations has been greatly facilitated by the development of a multi-scale physics-based simulation of energy flows through a generalized system model of the Double FOCUS architecture.  Parametric sweeps of design and operating parameters established performance maps and provided a deeper level of insight into the fundamental nature of the spectrum splitting proposition.  Tradeoffs between exergy efficiency and receiver operating temperatures, exergy efficiency and thermal fraction, and exergy efficiency as a function of band gap and filter band edge were explored.  A detailed energy balance between exergy and losses was constructed for the proposal base case (operating at 190/350C in the PVT/CSP receivers respectively) and for exergy maximized cases with alternative operating temperatures, band edges, and band gaps.  The platform from which to explore the design space further is now functional and providing high quality data. Progress UpdatesMay 2015 Progress Update 

Significant progress has been made on developing the particle heat capture, transport and storage concept, the back-reflecting gallium arsenide cells and the hybrid solar converter. Tests carried out on a tube representing a full scale solar receiver have demonstrated the ability of the selected solid particles to effectively absorb and transport heat while maintaining stable pressure drop and thermal absorption levels for well over a thousand heating and cooling cycles. In a parallel effort, suitable gallium arsenide cell prototypes have been manufactured and tested at elevated temperatures and radiation levels over a one week period showing stable performance and negligible physical degradation. Additionally, a 1 m long section of a full scale hybrid solar collector has been designed and fabricated together with a high temperature test loop for on-sun testing of the solar converter, and techno-economic analysis of a full scale hybrid solar system has also been completed.  During the next quarter, the team expects to complete testing of the particle heat storage concept and the hybrid solar collector, update the techno-economic analysis, and secure interested industrial end user for its full scale field demonstration.    

MicroLink and NRL’s project to develop a high-temperature solar cell has been underway since June 2014.  Initial development has focused primarily on investigating fundamental material science questions, as there is little published information on how the relevant materials will perform at 400 °C.  Additionally, the program has focused in the early stages on developing techniques for measuring device performance at high temperature, and methodologies for performing accelerated life testing. One of the first issues to be examined has been to determine how to protect the top surface of the solar cell from dissociation of volatile elements at high temperature, which would have a detrimental effect on the crystalline structure and thus device performance.  The metal contacts on the solar cell are anticipated to present another likely cause of device failure at high temperature, and therefore preliminary investigations of various metal stacks have been undertaken to search for a stable, conductive contact. During the upcoming quarters, the team will continue to focus on designing stable metallization.  The diffusion of elements within the solar cell structure at high temperature will also be investigated, to inform the choice of the most thermally-stable materials for the design.  Demonstrations of solar cell device measurements at 400 °C will commence. Progress Updates: May 2015 Progress Update 

A high fidelity integrated system model has been developed and optimized which predicts exergy performance of the HEATS receiver (includes optical, thermal, and pumping losses). This model is vital for accurate receiver design, as the individual components interactions cannot be captured if modeled separately. The model tracks different spectral bands separately, which allows for the tracking of the three primary bands of interest (PV band, and the two solar bands above and below). In-house simulation tools for modeling concentrated solar flux distribution on a central receiver from multi-reflector array geometries such as heliostats and linear Fresnel reflectors (LFRs), were also developed as part of the integrated model. From a fabrication standpoint, a variety of high reflectance selective absorber fins have been tested and found to achieve reflectance targets in the pass band using fewer nanostructured layers than our original design (resulting in reduced fabrication time and system cost). Likewise, optically transparent and thermally insulating aerogels have been successfully synthesized, meeting thickness-corrected transmittances and thermal conductivity metrics. Moving forward, in addition to continued optimization of spectral properties, we are beginning to start extensive durability testing on the individual HEATS receiver components (SSTC, aerogels, etc.). Concurrently, an outdoor testing site in Cambridge is being secured for future setup of a prototype collector, and scaling up of various aerogel synthesizing methods are in progress. 

Finished optical modeling of the whole hybrid trough system.  First round of System specifications are determined. Finished mechanical design and FEA of hybrid trough collector and dichroic mirror attachment. Designed and fabricated dichroic thin film to the specification. A full secondary concentrating lens unit is fabricated and tested.  Results proved optical simulation results. Established a comprehensive system power model based on dichroic band, CPV types, and bandgap selection. System overall exergy peak output is greater than 37%.  Designed and fabricated special 2J CPV cells and cell cards for the project.  Planned higher risk research tasks for CPV design and concentration lens configuration. Finished passive and active thermal management design, simulation. Results indicate that the CPV cell temperature can be well controlled below 70C during normal operation. A TEA model is under construction. Progress UpdatesMay 2015 Progress Update 

Currently the project is undergoing detailed design of the liquid filter receiver, including the necessary thermal packaging to achieve high efficiency with the current Cogenra platform. An optimized mix of nanoparticles has been selected, a combination of gold and semiconducting nanoparticles, maximize infrared absorption and visible transmission. Additionally synthesis methods for these nanoparticles have been established and used to fabricate suspensions for characterization. In the near term we plan to complete the thermal packaging design focused on maximizing exergy and percent of exergy that can be stored as heat. Specifically we are looking at the role different heat transfer minimization strategies have on the overall device performance, ranging from low-E optical coatings to low thermal conductivity transparent insulation materials. Further, we are developing new stabilization routes to ensure particle stability at temperatures up to 300 C within the working heat transfer fluid. The team is also working to integrate the higher efficiency GaAs cell into the Cogenra platform. We also plan to install the Cogenra T-14 hybrid solar energy system on the University of Tulsa campus to serve as the working platform for demonstrating on-sun performance. Progress UpdatesMay 2015 Progress Update 

The project is currently in its second quarter and off to a strong start. In the first quarter, we worked on tasks including: computer modeling of solar cells at elevated temperature; developing metal contacts and anti-reflection coatings that function well at elevated temperature; reliability studies; growth of solar cell materials predicted to offer high efficiency at 400 °C; and Technology to Market (T2M). One particular task was in testing our custom model that predicts the performance of single-junction solar cells at elevated temperature. Measurements of dark current in III-V solar cells up to 350 °C indicate that the model accurately captures the evolution of dark current as a function of temperature; while measurements up to 200 °C had been attempted before, no data at these high temperatures had been previously available. Another major area of work has been on AlGaInP materials development for the top-cell in our dual-junction device. A range of growth conditions were examined using both metallorganic vapor phase epitaxy (MOVPE) and molecular beam epitaxy (MBE), and initial cell results will be ready by the end of the 2nd quarter. We have also begun to investigate the electrical and structural properties of metal contacts that are thermally stable while providing low resistance, in addition to dual-layer anti-reflection coatings that are stable and effective at high temperature. Considerable effort has also been put into developing test equipment for assessing both the efficiency and the reliability of the proposed solar cells at 400 °C.  Progress UpdatesMay 2015 Progress Update

The goal of the PVMirror project is to create a hybrid photovoltaic (PV) and concentrating solar thermal power (CSP) system that combines the high efficiency of a PV system with the energy storage benefits of a CSP system. The result will be a PVMirror power plant with an efficiency 50% higher than that of a conventional CSP plant for only 10% additional cost. This is achieved by splitting the broad solar spectrum and sending select wavelengths to PV cells while sending the rest to the CSP system. A PVMirror looks like a conventional CSP trough mirror, but with an optical coating and PV cells lining the curved surfaces of the trough in place of the silver mirror coating. Incident near-infrared (NIR) light passes through the optical coating to the silicon PV cells behind, where it is absorbed and converted directly to electricity with >40% efficiency. The shorter visible wavelengths and longer infrared (IR) wavelengths—which cannot be efficiently used by silicon PV cells—are reflected by the coating. Because the coating is conformal to the curved glass trough (which tracks the sun from East to West throughout the day), this light is concentrated at the trough’s line focus where a black tube (thermal absorber) is placed. The heat that is transferred to oil passed through this tube powers a steam turbine to generate electricity, or it is stored until the sun sets for nighttime electricity generation. The aim of this project is the development of silicon PV cells and optical coatings that are optimized for use in PVMirrors; the lamination of the curved glass, optical coating, and PV cells into PVMirror prototypes; and a techno-economic analysis of these prototypes to demonstrate the viability of PVMirrors in the solar energy market. 

In this project Cogenra Solar develops DoubleFOCUS (DF), an innovative solar power system that integrates multiple state-of-the-art technologies from a broad cross-disciplinary fields, in order to maximally exploit energy from the sun. The DF concept combines concentrating photovoltaic (CPV) and thermal concentrating solar power (CSP) in a novel way that leverages the strengths of each technology, more optimally utilizes the solar spectrum, and shares system components to generate more exergy and energy value with minimal incremental hardware. DF is a platform technology ideal for both small, distributed applications as well as utility-scale generation, and it can easily incorporate complementary advances in PV and CSP, thereby creating a new learning curve that will drive down the cost of dispatchable solar power faster than PV or CSP alone. Exergy efficiency of the DF converter will be ~42% despite its moderate temperature. DoubleFocus is conceptually similar to a conventional parabolic trough CSP system. Parabolic mirrors track the sun and concentrate direct normal incident energy to 40-50 suns. The CSP absorber tube runs along the focal line of the system.  However, in this innovative optical design, a dichroic filter (fabricated inexpensively from polymer layers with alternating high and low index of refraction) preferentially reflects the short wavelength energy (<950 nm) onto one of two photovoltaic/thermal receivers.  The PV/T receiver incorporates inexpensive single-junction, high-efficiency, single-crystal GaAs cells lifted off from reusable substrates.  The cells will be laminated directly onto an aluminum baseplate that is extruded with an internal void; heat transfer fluid circulates through this conduit and collects the energy that the PV cells do not convert directly to electricity.  The two PV/T receivers are deliberately offset from the mirrored focal line, reducing the effective concentration to 20-25 suns.  The system delivers heat at two different temperatures to two separate thermal storage/conversion systems. For utility scale deployments, the CSP absorber tube will deliver heat at 350-400°C to a steam turbine and the PV/T receiver will deliver heat at 175-200°C to an organic Rankine cycle (ORC) engine. For industrial, commercial, and district heating deployments, the lower temperature loop can alternatively provide industrial process heat, heat domestic water or air, or drive a double-effect adsorption chiller air conditioner. 

There are two primary methods for capturing and using sunlight today: direct conversion of sunlight to electricity using photovoltaic (PV) solar panels, or focusing sunlight onto a fluid that is used to drive a steam turbine in concentrated solar power (CSP) systems. PV uses just part of the solar spectrum at high efficiency, while CSP systems use the entire solar spectrum but at low efficiency. GTI is developing a hybrid solar system combining the best elements of these two technologies that could provide a means to get the most out of the full solar spectrum, generating both electricity and storable heat (for later use) within the same system. The solar converter focuses sunlight onto solar cells with a reflective backside mirror that convert most visible wavelengths of sunlight to electricity while reflecting the unused wavelengths to heat a stream of flowing particles. The particles are used to store the heat for use immediately or at a later time to drive a turbine and produce electricity. GTI's design integrates the parabolic trough mirrors, commonly used in CSP plants, into a dual-mirror system that captures the full solar spectrum while storing heat to dispatch electricity when the sun does not shine. Current solar cell technologies capture limited portions of the solar spectrum to generate electricity that must be used immediately. By using back-reflecting gallium arsenide (GaAs) cells, this hybrid converter is able to generate both electricity from specific solar wavelengths and capture the unused light as heat in the flowing particles. The particle-based heat storage system is a departure from standard fluid-based heat storage approaches and could enable much more efficient and higher energy density heat storage. GTI's converter could be used to provide solar electricity whether or not the sun is shining.

MicroLink Devices and the US Naval Research Laboratory (NRL) are collaborating to develop a solar cell for operation at the very high temperature of 400 °C.  This photovoltaic technology will be suitable for integration into a hybrid solar converter to produce both electricity and heat.  If realized, a category of hybrid solar systems with the photovoltaic topping device operating at high temperature will provide solar energy conversion with both high efficiency and dispatchable power.  This will allow an increased grid penetration compared with other solar energy technologies. The solar cell structure is a dual-junction design, with subcells made from compounds including indium gallium phosphide and gallium arsenide grown epitaxial layer by layer on a gallium arsenide substrate.  Simulations of the solar cell structure under development predict that such a device will be able to convert more than 25% of the power from the incident sunlight to electricity at the operating temperature of 400 °C under optical concentration of 500-suns.  Accelerated lifetime testing will be undertaken using NRL’s extensive thermal testing facilities to develop a cell with a field lifetime greater than 25 years at the proposed operating conditions.  To substantially reduce the final cost of the solar cell devices, the cost of the GaAs substrate, which accounts for up to 50% of the epitaxial bill of materials, will be reduced using MicroLink’s proprietary epitaxial lift-off and substrate reuse process.  The solar cell wafer is grown on a release layer, which is selectively etched, leaving the substrate and the epitaxial layers intact.  The substrate may then be reused multiple times.

The joint MIT/UH research team is working on a three-year development project to demonstrate a high-performance stacked thermal and photovoltaic (PV) solar receiver comprising of a hot thermal absorber and a near ambient temperature PV cell (Figure. 1) – dubbed the Hybrid-Electric-And-Thermal Solar (HEATS) receiver. The innovative stack comprises, from top to bottom, a top aerogel layer that is optically transparent and thermally insulating (OTTI); a spectrally selective thermal absorber (SSTC) in which the working fluid is heated; a bottom OTTI layer; and a PV cell. The transparent aerogels provide the desired optical transparency and thermal insulation. The top OTTI layer serves to reduce the radiation and convection heat losses from the internally hot region to the ambient, and the bottom OTTI layer keeps the PV cell operating near ambient temperature. The SSTC captures infrared solar radiation below the bandgap of the PV cell, as well as the short wavelength photons that are not efficient for PV energy generation. The absorber is made of a high thermal conductivity material and is thermally linked with the heat exchanger, which efficiently transfers the absorbed solar heat to the pumped fluid. The stacked configuration enables the sharing of the same concentrating optics mechanism for both thermal and PV components, reducing the complexity and cost of the system. Furthermore, the splitting of the solar spectrum minimizes thermalization losses in the PV cell, thus helps to avoid excessive cell heat up. Based on these innovations and preliminary modeling, the proposed receiver has the ability to achieve maximum solar-to-exergy conversion efficiencies of 36.9% at an optical concentration of 20 suns, providing both highly dispatchable thermal energy at a heat fraction fth of 0.62 from the thermal absorber and highly efficient variable electricity from the PV cell, at a reduced total system cost per unit exergy of $0.9/Wx, and an estimated lifetime of > 25 years.

Starting with a concentrated parabolic trough system, we propose to split initially concentrated solar radiation into two components, one which is further concentrated and directed onto high performance PV cells and the other which is directed to an efficient heat collection device.  In addition, collection and management of thermal energy associated with PV cells will also be optimized.  The output of the system will be a high quality thermal stream (400°C) and electrical current.  In order to achieve the proposed performance, Sharp Labs of America (SLA) and University of Arizona (UA) will work together on optical system development, particularly focusing on optical design, material selection, and device fabrication.  The first task is to design/fabricate/characterize dichroic mirrors with characteristics similar to classic cold mirrors.  At least two designs for each case will be analyzed, comparing performance vs. cost.  The secondary concentrating optical system, further focus the reflected PV light down to CPV cells, will also be designed and fabricated.  Issues dealing with integration of the dichroic and secondary optics into the overall system will require special attention and is handled by Solargenix LLC, an experienced CSP builder.  Concurrent with optical system development, SLA and University of Missouri (UM) will design a thermal management system to maintain PV cells at the optimum temperature and/or harvest as much associated waste heat as is economically feasible; subsystem integration will also be carried out.  SLA will coordinate fabrication of prototype optical and thermal components and Solargenix will fabricate a 5x8 m2 trough prototype which will be installed and characterized in Tucson, AZ using University of Arizona facilities there.  The target is a robust design able to provide exergy greater than 35% at cost less than $1/W.

In this project we plan to utilize ultra-small nanoparticles with controllable properties directly suspended in the working fluid of the thermal absorber to selectively absorb incoming solar energy that cannot be used by the photovoltaic (PV) cell. This direct absorption liquid filter serves two purposes: the direct capture of thermal energy in the working fluid as well as filtering off of the infrared portion of incoming sunlight before striking the PV cell. The fluid filter will be integrated into a two-pass concentrating PV and thermal architecture. The first pass, on the back side of the PV cell, is used to limit temperature rise in the PV cell while capturing any waste heat generation within the PV cell. The second pass, in front of the PV cell, is used to achieve spectral filtering and direct thermal energy absorption within the working fluid. Our prior work has demonstrated the efficiency improvements of this design over conventional hybrid conversion schemes while also not requiring major redesigns of concentrating solar architecture. Additionally this approach allows for the direct photothermal energy conversion within the working fluid eliminating the need for additional heat transfer steps which limit the efficiency. The liquid filter is integrated with Cogenra’s hybrid solar energy system designed to utilize a high efficiency GaAs PV cell coupled with concentrated solar energy from the flat glass concentrator. This system creates a 50% improvement in overall performance of current hybrid solar conversion systems while costing less than $0.65/watt.

The Yale/NREL/SolAero team aims to demonstrate an efficient and reliable dual-junction concentrator photovoltaic (CPV) cell that can operate at temperatures as hot as the inside of a brick oven. While CPV cells efficiently convert much of the solar spectrum directly into electricity, they become significantly less efficient upon heating — an inevitable side effect of absorbing highly concentrated sunlight. In addition, any heat generated during the photovoltaic energy conversion process and any light not absorbed by the cells must be rejected as waste. Unlike CPV cells, which maintain their efficiency by dispersing the heat away, our team will design and fabricate cells from III-V materials that can generate electricity reliably at temperatures exceeding 400 °C. This “hot CPV cell” will be integrated with a solar thermal collector that absorbs the unused portion of the solar spectrum as well as excess heat from the cells. The otherwise-wasted energy will be transferred to high-temperature fluids that can be used to power a steam turbine and generate electricity in a manner analogous to concentrating solar power (CSP). The high-temperature fluids can also be economically stored so that the heat energy can be dispatched when the sun is not shining or whenever electrical demand rises. This method of storing solar energy is projected to be significantly more cost-effective than battery storage. The current high cost of storing solar electricity in batteries, combined with the natural variation of available sunlight, will weaken the economic drive for photovoltaic market growth unless novel solutions are developed. The Yale/NREL/SolAero project addresses both these challenges by combining the high efficiency of CPV with the low-cost energy storage offered by CSP.

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