Charles E. Andraka

Dish-Stirling Systems: An Overview of Development and Status

Dish-Stirling systems have demonstrated the highest efficiency of any solar power generation system by converting nearly 30% of direct-normal incident solar radiation into electricity after accounting for parasitic power losses [1]. These high-performance, solar power systems have been in development for two decades with the primary focus in recent years on reducing the capital and operating costs of systems. Even though the systems currently cost about

Solar heat pipe testing of the Stirling Thermal Motors 4-120 Stirling engine

10,000 US/kW installed, major cost reduction will occur with mass production and further development of the systems. Substantial progress has been made to improve reliability thereby reducing the operating and maintenance costs of the systems. As capital costs drop to about

Solar Dish Field System Model for Spacing Optimization

3000 US/kW, promising market opportunities appear to be developing in green power and distributed generation markets in the southwestern United States and in Europe. In this paper, we review the current status of four Dish-Stirling systems that are being developed for commercial markets and present system specifications and review system performance and cost data. We also review the economics, capital cost, operating and maintenance costs, and the emerging markets for Dish-Stirling systems.

Rapid Reflective Facet Characterization Using Fringe Reflection Techniques

Stirling-cycle engines have been identified as a promising technology for the conversion of concentrated solar energy into usable electrical power. A 25 kW electric system takes advantage of existing Stirling-cycle engines and existing parabolic concentrator designs. In previous work, the concentrated sunlight impinged directly on the heater head tubes of the Stirling Thermal Motors (STM) 4-120 engine. A Sandia-designed felt-metal-wick heat pipe receiver was fitted to the STM 4-120 engine for on-sun testing on Sandias Test Bed Solar Concentrator. The heat pipe uses sodium metal as an intermediate two-phase heat transfer fluid. The receiver replaces the directly-illuminated heater head previously tested. The heat pipe receiver provides heat isothermally to the engine, and the heater head tube length is reduced, both resulting in improved engine performance. The receiver also has less thermal losses than the tube receiver. The heat pipe receiver design is based on Sandias second-generation felt-wick heat pipe receiver. This paper presents the interface design, and compares the heat pipe/engine test results to those of the directly-illuminated receiver/engine package.

Status of the Advanced Dish Development System Project

Dish Stirling power generation systems have been identified by DOE, Sandia National Laboratories, and Stirling Energy Systems (SES) as having the capability of delivering utility-scale renewable energy to the nation’s electrical grid. SES has proposed large plants, 20,000 units or more (0.5 GW rated power) in one place, in order to rapidly ramp up production automation. With the large capital investment needed in such a plant it becomes critical to optimize the system at the field level, as well as at the individual unit level. In this new software model, we provide a tool that predicts the annual and monthly energy performance of a field of dishes, in particular taking into account the impact of dish-to-dish shading on the energy and revenue streams. The Excel-based model goes beyond prior models in that it incorporates the true dish shape (flexible to accommodate many dish designs), multiple-row shading, and a revenue stream model that incorporates time-of-day and time-of-year pricing. This last feature is critical to understanding key shading tradeoffs on a financial basis. The model uses TMY or 15-minute meteorological data for the selected location. It can incorporate local ground slope across the plant, as well as stagger between the rows of dish systems. It also incorporates field-edge effects, which can be significant on smaller plants. It also incorporates factors for measured degraded performance due to shading. This tool provides one aspect of the decision process for fielding many systems, and must be combined with land costs, copper layout and costs, and O&M predictions (driving distance issues) in order to optimize the loss of power due to shading against the added expense of a larger spatial array. Considering only the energy and revenue stream, the model indicates that a rectangular, unstaggered field layout maximizes field performance. We also found that recognizing and accounting for true performance degradation due to shading significantly impacts plant production, compared with prior modeling attempts.

COST/PERFORMANCE TRADEOFFS FOR REFLECTORS USED IN SOLAR CONCENTRATING DISH SYSTEMS.

Mirror facets for Concentrating Solar Power (CSP) systems have stringent requirements on slope accuracy in order to provide adequate system performance. This paper presents a newly developed tool that can characterize facets quickly enough for 100% inspection on a production line. A facet for a CSP system, specifically a dish concentrator, has a parabolic design shape. This shape will concentrate near-parallel rays from the sun to a point (or a line for trough systems). Deviations of surface slope from the design shape impact the performance of the system, either losing power that misses the target, or increasing peak fluxes to undesirable levels. Three types of facet slope errors can impact performance. The first is a focal length error, typically caused by springback in the facet forming process. In this case, the wavelength of the error exceeds the size of the facet, resulting in a parabola, but with the wrong focal length. The results in a slope error that is largely systematic across the facet when the measured slope is compared to the design slope. A second shape error, in which the period of the error is on the order of the length of the facet, manifests also as a systematic slope error. In this case, the facet deviates from a parabolic shape, but can be modeled with a higher order curve. Finally, the residual errors after a model is proposed are usually lumped through a Root Mean Square (RMS) process and characterized as the 1-sigma variation of a normal distribution. This usually characterizes the small-scale imperfections in the facet, and is usually called “slope error”. However, all of these deviations from design are in facet errors in the slope of the manufactured facet. The reported characterization system, named SOFAST (Sandia Optical Fringe Analysis Slope Tool) has a computer-connected camera that images the reflective surface, which is positioned so that it views the reflection of an active target, such as an LCD screen. A series of fringe patterns are displayed on the screen while images are captured. Using the captured information, the reflected target location of each pixel of mirror viewed can be determined, and thus through a mathematical transformation, the surface normal map can be developed. This is then fitted to the selected model equation, and the errors from design are characterized. The reported system currently characterizes point focus mirrors (for dish systems), but extensions to line focus facets are planned. While similar approaches have been explored, several key developments are presented here. The combination of the display, capture, and data reduction in one system allows rapid capture and data reduction. An “electronic boresight” approach is developed accommodating physical equipment positioning errors, making the system insensitive to setup errors. A very large number of points are determined on each facet, providing significant detail as to the location and character of the errors. The system is developed in MatLab, providing intimate interactions with the data as techniques and applications are developed. Finally, while commercial systems typically resolve the data to shape determination, this system concentrates on slope characterization and reporting, which is tailored to the solar applications. This system can be used for facet analysis during development. However, the real payoff is in production, where complete analysis is performed in about 10 seconds. With optimized coding, this could be further reduced.Copyright

Improved alignment technique for dish concentrators.

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.© 2003 ASME

Reduction of radiative heat losses for solar thermal receivers

Concentrating Solar Power (CSP) dish systems use a parabolic dish to concentrate sunlight, providing heat for a thermodynamic cycle to generate shaft power and ultimately, electricity. Currently, leading contenders use a Stirling cycle engine with a heat absorber surface at about 800°C. The concentrated light passes through an aperture, which controls the thermal losses of the receiver system. Similar systems may use the concentrated light to heat a thermochemical process. The concentrator system, typically steel and glass, provides a source of fuel over the service life of the system, but this source of fuel manifests as a capital cost up front. Therefore, it is imperative that the cost of the reflector assembly is minimized. However, dish systems typically concentrate light to a peak of as much as 13,000 suns, with an average geometric concentration ratio of over 3000 suns. Several recent dish-Stirling systems have incorporated reflector facets with a normally-distributed surface slope error (local distributed waviness) of 0.8 mrad RMS (1-sigma error). As systems move toward commercialization, the cost of these highly accurate facets must be assessed. However, when considering lower-cost options, any decrease in the performance of the facets must be considered in the evaluation of such facets. In this paper, I investigate the impact of randomly-distributed slope errors on the performance, and therefore the value, of a typical dish-Stirling system. There are many potential sources of error in a concentrating system. When considering facet options, the surface waviness, characterized as a normally-distributed slope error, has the greatest impact on the aperture size and therefore the thermal losses. I develop an optical model and a thermal model for the performance of a baseline system. I then analyze the impact on system performance for a range of mirror quality, and evaluate the impact of such performance changes on the economic value of the system. This approach can be used to guide the evaluation of low-cost facets that differ in performance and cost. The methodology and results are applicable to other point- and line-focus thermal systems including dish-Brayton, dish-Thermochemical, tower systems, and troughs.Copyright

Development and Characterization of a Color 2F Alignment Method for the Advanced Dish Development System

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.© 2003 ASME

Dish/Stirling hybrid-heat-pipe-receiver design and test results

Solar thermal receivers absorb concentrated sunlight and can operate at high temperatures exceeding 600°C for production of heat and electricity. New fractal-like designs employing light-trapping structures and geometries at multiple length scales are proposed to increase the effective solar absorptance and efficiency of these receivers. Radial and linear structures at the micro (surface coatings and depositions), meso (tube shape and geometry), and macro (total receiver geometry and configuration) scales redirect reflected solar radiation toward the interior of the receiver for increased absorptance. Hotter regions within the interior of the receiver also reduce thermal emittance due to reduced local view factors in the interior regions, and higher concentration ratios can be employed with similar surface irradiances to reduce the effective optical aperture and thermal losses. Coupled optical/fluid/thermal models have been developed to evaluate the performance of these designs relative to conventional designs. Results show that fractal-like structures and geometries can reduce total radiative losses by up to 50% and increase the thermal efficiency by up to 10%. The impact was more pronounced for materials with lower inherent solar absorptances (< 0.9). Meso-scale tests were conducted and confirmed model results that showed increased light-trapping from corrugated surfaces relative to flat surfaces.