Two principal approaches to lowering solar concentrator costs are through improved geometry (lower profile, simpler shape), and through simplified methods to make concentrator substrates. The paper discusses issues relating to fabrication of solar concentrator mirrors from inexpensive elastically bent float glass mirrors. Protection from hail damage by a novel shock absorbing back layer is presented. New low-profile concentrator geometry is presented which utilize the low-cost mirror substrates. One new geometry uses a stationary cylindrical reflector with novel arc-motion tracking absorber that appears suitable for mid-sized thermal applications. Another uses the cylindrical mirror with an azimuth-elevation tracking mechanism appears to be suitable for small-scale applications. To get even higher concentration, some low-cost secondary designs are briefly discussed which use refractive tubes or lenses as secondary concentrating elements.

The objective of this paper was to provide an overview of current ground-based efforts in the design and development of hybrid lighting systems, combining solar irradiance and electric lighting, to effect reduced electrical power demand for growing plants in a bioregenerative life support system (BLSS) for future space applications on the Moon or Mars. Two experimental Hybrid Solar and Artificial Lighting (HYSAL) systems have been developed: one employing xenon-metal halide (XMH) lamps and the other adopting light-emitting diodes (LEDs) as the electric-lighting components, and both using a mirror-based, fiberoptic-based solar collection system. The results showed that both HYSAL systems provided acceptable spectral quality and lighting uniformity. The apparent electrical conversion efficiency of the HYSAL system with LEDs, however, exceeded that of the HYSAL system with XMH by five-fold. Preliminary simulations of a Martian BLSS indicated that the lighting power required for plant production could be supplemented by the available extraterrestrial solar irradiance by as much as 50%. The use of solar stationary orbits over Mars for an orbiting Martian BLSS to reduce the required electrical power was also suggested.

This paper reports on the methodology and results of a preliminary analysis of a hybrid solar lighting concept that has the potential to provide cost-effective topside daylighting to the core areas of commercial buildings of all types. The presented hybrid lighting concept also includes a “full spectrum” dish-type concentrating solar collector. This collector uses both the incident visible and infrared solar radiation to provide both daylighting and electric power. The incident solar radiation is separated using cold mirror technology. Visible light is sent to a fiber optic bundle for routing to integrated fiber optic/fluorescent luminaires within the building. Separated infrared solar radiation is focused on a photovoltaic cell and converted to electricity. The electricity produced provides self-powering of the collector solar tracking system and the excess electricity produced is sent into the grid-connected commercial building electric system. The paper presents the potential for performance increases/cost reductions in the technology and the current and future cost competitiveness of this promising solar technology.

Dish/Stirling systems rely on accurate mirrors and alignment. Inaccuracies in alignment can lead to flux “hotspots” that can reduce performance and even damage the solar receiver. To improve the alignment process a procedure that can be used to align solar dish concentrator facets was developed on the second-generation (Mod 2) Advanced Dish Development System (ADDS) at Sandia National Laboratories in Albuquerque, NM. The previously used facet alignment method was based on the distant light source technique and required the dish to be pointed near the horizon while an observer located approximately 100 meters away views the image of a target and calls in adjustments to an aligner. At high sun elevation angles structural deflections inherent in any dish concentrator result in facet misalignments. To address these issues while allowing improved access to facet adjustments, an alignment technique that permits alignment at any elevation angle was developed and evaluated. This technique utilized a video camera and color target affixed on a lightweight tower at a distance near twice the dish focal length (2f) from the vertex of the dish, hence the name “color 2f”. The camera and target are centered on the optical axis of the concentrator. During the initial evaluation of the color 2f facet alignment procedure on the ADDS Mod 2 concentrator, no facet adjustments were made. Instead concentrator facet deflections were measured by taking and evaluating images of the dish reflecting the target while the dish was pointed at the horizon, 45 degrees elevation, and vertical. These images were compared to each other, to determine gravity induced pointing errors as the dish changes in elevation. Future implementations of the color 2f system could be used to align the system rather than simply diagnose it. The proposed approach has the potential to accurately align concentrators without large open land spaces, and to account for elevation-induced structural deflections. In this paper, the color 2f alignment technique is described along with the results from the gravity induced structural deflection study. Issues and recommendations for improvement of the 2f alignment system are also presented.

Almost all life on Earth has been using solar energy in many ways, but when high temperatures are desired, concentration of the incident solar radiation (insolation) becomes necessary. The present work is an attempt to improve and experimentally compare alternative beam delivering and focusing energy systems of a small solar concentrator. The researched solar energy concentrator (SEC) facility consists of modified two mirror Cassegrainian solar concentrator, two-speed sun-tracking manual and automatic control, concentrated insolation transmitting and continuous beam focusing systems. A number of system modifications during the development of the two stage, point focusing type solar concentrator arrangement for solar thermal power utilization have also been explored and are reported in this paper. Some of the experimental testing results obtained using single polymer fiber 14 mm in diameter, a truncated conical concentrator, and auxiliary lens system alternatives, have been compared. In addition, some details about various improvements of the sun-tracking sensor and automatics, beam transmission and continuous focusing capabilities of the SEC facility have been described.

For solar lighting systems employing fiber optic waveguides 1,2 , to conduct the collected light, paraboloidal mirrors are the preferred reflector choice. To achieve optimum performance in systems with relatively small collection apertures, both the quality of the mirror and the quality of optical system alignment must be well controlled. In systems employing multiple waveguides with a single paraboloid, the focus of the paraboloid must be segmented into several separate focal points directed into individual fibers. Each waveguide entrance aperture must be accurately co-located with its designated focal point so that the image that is formed on the fiber will have the fewest possible aberrations and thus, the smallest possible focused spot size. Two methods for aligning individual optical waveguides in a multi-aperture paraboloidal collection system are described. The first method employs a commercially available collimation tester to incrementally improve the alignment. The second, a deterministic method, employs a cube corner retro-reflector and an easily constructed imaging system to reliably align the fibers to their respective segments of the parent paraboloid. The image of the focused spot formed by the light that is returned from the retro-reflector reveals alignment information that is easily interpreted to enable pitch, yaw and focus errors to be systematically removed. This ensures that the alignment of the system is optimized to reduce aberrations prior to final adjustment of the system “on-sun”.

Parabolic dish concentrators have shown significant promise of generating competitive electric energy for grid and off-grid applications. The efficiency of a dish-electric system is strongly affected by the quality of the concentrator optics. Most parabolic systems consist of a number of facets mounted to a support structure in an approximate parabolic arrangement, where the individual facets have spherical or parabolic optical shapes. The individual facets must be accurately aligned because improper alignment can compromise performance or create hot spots that can reduce receiver life. A number of techniques have been used over the years to align concentrator facets. In the Advanced Dish Development System (ADDS) project, a color look-back alignment approach that accurately aligns facets (mirror panels) and in addition indicates quantitative information about the focal length was developed. Key factors influencing the alignment, some of which had very large effects on the quality of the alignment, were also identified. The influence of some of the key factors was characterized with a flux mapping system on the second-generation ADDS concentrator. Some of these factors also affect other alignment approaches. The approach was also successfully applied to two other concentrators with differing facet arrangements. Finally, we have extended the method to a 2-f approach that eliminates the need for a distant line-of-sight to the dish and permits alignment at near vertical dish attitudes. In this paper, we outline the color look-back alignment approach, discuss the key alignment factors and their effect on flux distribution, and discuss extensions to non-gore dishes. A companion paper discusses the 2-f color alignment approach in detail.

The Advanced Dish Development System (ADDS) project is a system-level dish/engine development activity aimed at the extensive but challenging remote power market. The ADDS project involves integration and test of advanced dish/Stirling systems. The ADDS designs utilize the WGAssociates solar concentrator and controls, and the SOLO 161 Stirling Power Conversion Unit. Development has focused on extending the application of dish/Stirling systems to water pumping, and reliability and performance improvement. Testing includes unattended, automatic operation of stand-alone dish/Stirling solar power generation systems in both on and off-grid modes at the National Solar Thermal Test Facility (NSTTF) in Albuquerque, NM. In 1999, a first generation (Mod 1) system was fielded at the NSTTF and routine unattended operation initiated. In 2000, a system reliability tracking system was implemented on the Mod 1 system and an upgraded, second-generation (Mod 2) system, including a stand-alone water-pumping capability was developed. In 2001 and 2002 system performance and reliability were improved. Overall, the ADDS project has been successful with most of the original system specifications and objectives having been met or exceeded. The ADDS designs are efficient and maintainable and have proven the ability to operate autonomously in a remote environment. The Mod 1 system net power rating was increased from 9 to 10 kWe even while the concentrator mirror area was reduced by over 14%. The Mod 2 design is the first modern dish/engine system to operate independent of the utility grid and is capable of interfacing with standard three-phase, 480-volt, water-pump or other single motor applications. In this paper, the ADDS project plan and history, technical approach, and the major system components and features are briefly described. Project milestones and status along with test results are also presented.

The Advanced Dish Development System (ADDS) project is an advanced, system-level dish/Stirling solar power system development activity aimed at the remote power marketplace. Unlike any of the other U.S. Department of Energy (DOE) Concentrating Solar Power (CSP) program sponsored activities, the ADDS project is being integrated by Sandia National Laboratories (SNL). Although SNL and the National Renewable Energy Laboratory (NREL) help the DOE manage the CSP program, previous system development activities were all integrated by industry partners, with SNL and NREL providing technical support. The ADDS project is a technology development activity with the specific objective of fielding a remote, off-grid, water-pumping dish/Stirling system on an Indian reservation in the Southwestern U.S. To meet this objective, the WGAssociates (WGA) solar concentrator and controls, and the SOLO 161 Stirling Power Conversion Unit (PCU) were selected for the major components. The WGA concentrator and controls technology are well advanced and utilize structural facet mirror technology and concentrator structure and controls technology developed over decades with DOE and private investment. The SOLO 161 Stirling PCU, including solar receiver and controls, has also benefited from decades of development. Two ADDS designs have been developed. A first-generation (Mod 1) grid-connected prototype system was fielded at the National Solar Thermal Test Facility (NSTTF) in Albuquerque, NM in the summer of 1999, primarily for the purpose of reliability improvement. In 2000, an upgraded, second-generation (Mod 2) system, including a standalone water-pumping ADDS was developed, and in 2001 it was installed and demonstrated at the NSTTF. While the value of remote power electricity is greater than for utility power, (estimates range from 20 to more than 50 cents per kilowatt-hour) the remote environment presents additional requirements. The ADDS project has been a first step in the development and demonstration of a dish/Stirling system that can potentially address this large commercial opportunity. Integration of the ADDS has also been challenging and rewarding. As a result of the ADDS project, insights beyond specific component expertise have been obtained. Even though we have not been able to achieve our original objective of fielding a dish/Stirling system on an Indian reservation because of reduce budgets, the ADDS project has been successful in that most of the original system specifications and technical objectives were met or exceeded. It has also served as a valuable test bed for the evaluation of advanced technology in a system context. The ADDS technology and results are presented in other papers. In this paper, the integration philosophy and approaches utilized in the various stages of the ADDS project are presented and described. Insights gained from integration of the ADDS are also discussed.

Concentrating solar power (CSP) technologies use large mirrors to collect sunlight to convert thermal energy to electricity. The viability of CSP systems requires the development of advanced reflector materials that are low in cost and maintain high specular reflectance for extended lifetimes under severe outdoor environments. The long-standing goals for a solar reflector are specular reflectance above 90% into a 4-mrad half-cone angle for at least 10 years outdoors with a cost of less than $13.8/m 2 (the 1992 $10.8/m 2 goal corrected for inflation to 2002 dollars) when manufactured in large volumes. Durability testing of a variety of candidate solar reflector materials at outdoor test sites and in laboratory accelerated weathering chambers is the main activity within the Advanced Materials task of the CSP Program at the National Renewable Energy Laboratory (NREL) in Golden, Colorado. Test results to date for several candidate solar reflector materials will be presented. These include the optical durability of thin glass, thick glass, aluminized reflectors, frontsurface mirrors, and silvered polymer mirrors. The development, performance, and durability of these materials will be discussed. Based on accelerated exposure testing the glass, silvered polymer, and front-surface mirrors may meet the 10-year lifetime goals, but at this time because of significant process changes none of the commercially available solar reflectors and advanced solar reflectors have demonstrated the 10-year or more aggressive 20-year lifetime goal.

As a possible alternative to glass mirrors, the use of composite-based reflector facets has been investigated for use on parabolic trough concentrators. The approach evaluated here is a composite-based parabolic facet (steel and fiberglass) with a separately attached reflector. This paper describes the basic design of these structural facets and reports on the optical tests that have been used to determine the optical accuracy of the facets.

This paper presents the structure and application of a sun tracking photo-sensor, which controls the direction of a reflected solar radiation to be constant. The sensor has a very simple structure, in which two photo-cells are placed side by side in the bottom of a box with an aperture. When the reflected solar radiation passes through the aperture, the photo-cells are fractionally illuminated, and a electric current, which is proportional to the illuminated area, is generated on each photo-cell. The direction of the reflected solar radiation is kept to be constant by controlling the angle of the reflection mirror to make the two currents equal. The performance of the photo-sensor was tested with a mirror-telescope system and the sun tracking error was estimated to be less than 0.6 mrad on sunny day. To ascertain the applicability of the photo-sensor, a sensor-controlled heliostat, in which no computer-control system and rotary encoders were installed, was manufactured. The performance of the heliostat was evaluated by observing the radiation on the target screen which was placed 70 m south side of the heliostat, and the angular error within 2 mrad was achieved on sunny days, while the tracking error increased in various ways in cloudy weather. This photo-sensor is applicable for small solar furnace, sun lighting system, and sun tracking photovoltaic system.

Concentrating solar power (CSP) companies have deployed thin-glass mirrors produced by wet silver processes on ∼1-mm-thick, relatively lightweight glass. These mirrors have been bonded to metal substrates in commercial installations. Initial hemispherical reflectance is ∼93% to 96%, and the cost is ∼$16.1/m 2 to $43.0/m 2 . These mirrors have the confidence of the CSP industry. However, corrosion was observed in mirror elements of operational solar systems deployed outdoors for 2 years. NREL’s advanced optical materials team was assigned to investigate the problem. First, it was noted that this corrosion is very similar to the corrosion bands and spots observed on small (45 mm × 67 mm) thin-glass mirrors laminated to metal substrates with several different types of adhesives and subjected to accelerated exposure testing (AET) at NREL. These samples exhibited corrosion at the unprotected edges and along cracks, and the choice of adhesive affected the performance of weathered thin-glass mirrors. Secondly, two significant changes in mirror manufacture have occurred in the wet-chemistry process because of environmental concerns. The first is the method of forming a copper-free reflective mirror, and the second is the use of lead-free paints. A test matrix of 84 combinations of sample constructions (mirror type / back protective paint / adhesive / substrate) was devised for AET as a designed experiment to identify the most promising mirrors, paints, and adhesives for use with concentrator designs. Two types of accelerated exposure were used: an Atlas Ci5000 WeatherOmeter (WOM) and a damp-heat chamber. Based on an analysis of variance (ANOVA), the various factors and interactions were modeled. These samples now have almost 24 months of accelerated exposure. Analysis of the thin-glass mirror matrix indicated that the Glaverbel mirror with a copperless formulation demonstrates slightly better performance compared to the Naugatuck standard copper-containing mirror and new copperless constructions although most results are within experimental uncertainty. Analysis of the thin-glass mirror matrix indicates commercial (non-mirror) back-protective paint applied after mirror manufacturing is not beneficial. Degradation of the samples exposed to date in the damp-heat chamber is similar, but at a rate 10 times faster than observed for samples in the WOM. We will discuss the results of the continued exposure testing of these mirror samples. Although glass mirrors with copper back-layers and heavily leaded paints have been considered robust for outdoor use, the new copperless back-layer and lead-free paint systems were designed for interior mirror applications and the outdoor durability must be determined.

Solargenix Energy is currently constructing a 64-MWe parabolic trough solar plant in Eldorado Valley, Nevada, just south of Las Vegas. As part of the preparation for construction and operation of the new utility-scale solar plant, Solargenix has collaborated with UNLV and NREL to build a collector test row. The test row is serving as a platform for field testing advanced parabolic trough components before their large-scale deployment. The test row consists of two Solargenix Solar Collector Assemblies (SCAs); each SCA has 12 collector modules (space frames and mirrors). This facility has been used to field test new Solargenix designs for first and second generation collector space frames, advanced reflectors, advanced local controllers (AdLoCs), a hydraulic-based drive system, receiver support arms, low-cost injection-molded bearings, ball joints and collector support pylons. The test-row facility also has equipment for monitoring the following weather data: direct normal irradiance, dry bulb temperature, relative humidity, wind speed and precipitation. Data logging equipment is used to record and track weather data as well as SCA parameters. Site instrumentation is solar-powered (photovoltaics) and uses cellular technology to transmit data to a web-based data collection system. This paper describes construction of this facility, the installation of the data-collection system and some data collected to date.

Monte Carlo radiative transfer analysis is applied to a cylindrical cavity-receiver containing an array of high-temperature tubular absorbers directly exposed to concentrated solar power entering through a spectrally selective window. The cavity walls are assumed either diffusely or specularly reflective. The relative dimensions, the number of tubes, and their position are optimized for maximum energy transfer efficiency or maximum absorber temperature. A single-tube absorber operating at 2000 K performs best when located at 60% relative distance to the cavity’s aperture. Higher absorber temperatures are attained for a specularly reflective cavity that serves as internal infrared mirror, but at the expense of lower energy transfer efficiencies. In contrast, diffuse reflecting cavity walls promote a more uniform temperature distribution around the tubular absorber. Decreasing the window-to-cavity areas ratio further results in an increase of the absorber temperature, which peaks for an optimum absorber-to-cavity radii ratio. This optimum ratio shifts to lower values for multiple-tube absorbers. However, the average absorber temperature is not significantly affected by using multiple-tube absorbers of constant total cross sectional area.

Two advanced optical systems and a highly efficient thermal cycle suitable for beam-down power tower with thermal storage are presented. (1) To increase field efficiency, the “cross beam” heliostat array concept is proposed. Using continuum optical model, the characteristics of the cross beam concept and its economy were investigated. (2) To protect the central reflector (CR) against wind force, a “multi-ring CR” concept is proposed. The concentration performance of multi-ring CRs is calculated using the ray-tracing method. It shows no worse results than the case with a single hyperboloid mirror. (3) The potential of a closed gas turbine cycle with supercritical carbon dioxide as a working fluid was investigated. An optimal cycle configuration involves a regenerative cycle with pre-cooling and inter-cooling cycles, in which theoretically achievable cycle thermal efficiency is 47% at the turbine inlet temperature of 800 K and turbine inlet pressure of 20 MPa. Detailed thermal design of a critical component, regenerative heat exchanger (RHX) is carried out using a newly developed printed heat exchanger (PCHE). It proved to be a feasible design.