Timothy A. Moss

Experimental Analysis of Overall Thermal Properties of Parabolic Trough Receivers

The heat loss of a receiver in a parabolic trough collector plays an important role in collector performance. A number of methods have been used to measure the thermal loss of a receiver tube depending on its operating temperature. This paper presents methods for measuring receiver heat losses including field measurements and laboratory set-ups both based on energy balances from the hot inside of the receiver tube to the ambient. Further approaches are presented to measure and analyze the temperature of the glass envelope of evacuated receivers and to model overall heat losses and emissivity coefficients of the receiver. Good agreement can be found between very different approaches and independent installations. For solar parabolic trough plants operating in the usual 390°C temperature range, the thermal loss is around 300W/m receiver length.

Testing of a CR5 Solar Thermochemical Heat Engine Prototype

Sandia National Laboratories (SNL) is investigating thermochemical approaches for reenergizing CO2 and H2 O feed stocks for input to synthetic liquid hydrocarbon fuels production. Key to the approach is the Counter-Rotating-Ring Receiver/Reactor/Recuperator (CR5), a novel solar-driven thermochemical heat engine concept for high-temperature carbon dioxide and water splitting based on two-step, nonvolatile metal oxide thermochemical cycles. The CR5 integrates two reactors, recuperators, and solar receiver and intrinsically separates the product gases. The CR5 thermochemical heat engine concept and the underlying thermodynamics and kinetics have many uncertainties. While results from laboratory scale material tests are promising, they are different than what occurs in a CR5. To evaluate the potential of the CR5 we have designed and built a CR5 prototype. The overall objective of the SNL Sunshine to Petrol (S2P) project is to show a solar thermochemical pathway for the efficient production of liquid fuels from CO2 and H2 O feed stocks. To achieve the overall long-term goal of 10% efficient conversion of sunlight to petroleum, the thermochemical solar conversion of sunlight to CO needs to be 20% efficient. The short-term goal for the CR5 prototype is to demonstrate a solar to chemical conversion efficiency of at least 2%. In this paper, we present initial test results for the CR5 prototype in the 16 kWt National Solar Thermal Test Facility (NSTTF) solar furnace in Albuquerque, NM. Lessons learned from the initial tests and approaches for improving performance to achieve our goals are also presented.Copyright

Field Survey of Parabolic Trough Receiver Thermal Performance

This paper describes a technique that uses an infrared (IR) camera to evaluate the in-situ thermal performance of parabolic trough receivers at operating solar power plants. The paper includes results to show how the glass temperature measured with the IR camera correlates with modeled thermal losses from the receiver. Finally, the paper presents results of a field survey that used this technique to quickly sample a large number of receivers to develop a better understanding of how both original and replacement receivers are performing after up to 17 years of operational service.

Practical Field Alignment of Parabolic Trough Solar Concentrators

In this paper a new technique for parabolic trough mirror alignment based on the use of an innovative theoretical overlay photographic (TOP) approach is described. The technique is a variation on methods used to align mirrors on parabolic dish systems. It involves overlaying theoretical images of the heat collection element (HCE) in the mirrors onto carefully surveyed photographic images and adjustment of mirror alignment until they match. From basic geometric principles, for any given viewer location the theoretical shape and location of the reflected HCE image in the aligned mirrors can be predicted. The TOP approach promises to be practical and straightforward, and inherently aligns the mirrors to the HCE. Alignment of an LS-2 mirror module on the rotating platform at the National Solar Thermal Test Facility (NSTTF) with the TOP technique along with how it might be implemented in a large solar field is described. Comparison of the TOP alignment to the distant observer approach on the NSTTF LS-2 is presented and the governing equations used to draw the theoretical overlays are developed. Alignment uncertainty associated with this technique is predicted to be less than the mirror slope error.

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

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

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

FREEZE-THAW TESTS OF TROUGH RECEIVERS EMPLOYING A MOLTEN SALT WORKING FLUID

Several studies predict an economic benefit of using nitrate-based salts instead of the current synthetic oil within a solar parabolic trough field. However, the expected economic benefit can only be realized if the reliability and optical performance of the salt trough system is comparable to today’s oil trough. Of primary concern is whether a salt-freeze accident and subsequent thaw will lead to damage of the heat collection elements (HCEs). This topic was investigated by experiments and analytical analysis. Results to date suggest that damage will not occur if the HCEs are not completely filled with salt. However, if the HCE is completely filled at the time of the freeze, the subsequent thaw can lead to plastic deformation and significant bending of the absorber tube.Copyright

Test Results and Status of the TOP Alignment System for Parabolic Trough Solar Collectors

Parabolic trough solar power plants produce the lowest cost solar electricity, yet unsubsidized electricity from parabolic trough power plants costs about twice that from conventional sources. To make parabolic trough electricity more competitive, we are developing an innovative approach for rapidly and effectively evaluating the alignment of mirrors in parabolic trough power plants and prescribing corrective actions as needed. The Theoretical Overlay Photographic Collector Alignment Technique (TOPCAT) system could be used during construction, to improve the performance of existing power plants, or for routine maintenance. It is also an enabling technology for higher concentration ratio and lower cost trough solar collector designs needed to make solar electricity more competitive with conventional sources. In this paper a truck-mounted TOPCAT field characterization system is described. Test results from mirror alignment of an LS-3 loop in a commercial parabolic trough power plant in southern California are also presented. The performance improvements were measured using a comparative calorimetric technique which inherently accounts for variations in insolation levels, sun incident angle, and mirror and heat collection element (HCE) glass envelope cleanliness. Measurements indicate a 3.5% increase in thermal performance of an LS-3 loop aligned with the TOPCAT system. Benchmarking results of labor hours and materials show that the TOPCAT system is an extremely cost effective tool for improving the performance of existing parabolic trough power plants.Copyright

Test Results of a Schott HCE Using a LS-2 Collector

Sandia National Laboratories has completed thermal performance testing on the Schott parabolic trough receiver using the LS-2 collector on the Sandia rotating platform at the National Solar Thermal Test Facility in Albuquerque, NM. This testing was funded as part of the US DOE Sun-Lab USA-Trough program. The receiver tested was a new Schott receiver, known as H eat C ollector E lements (HCEs). Schott is a new manufacturer of trough HCEs. The Schott HCEs are 4m long; therefore, two were joined and mounted on the LS-2 collector module for the test. The Schott HCE design consists of a 70mm diameter high solar absorptance coated stainless steel (SS) tube encapsulated within a 125mm diameter Pyrex® glass tube with vacuum in the annulus formed between the SS and glass tube to minimize convection heat losses. The Schott HCE design is unique in two regards. First, the bellows used to compensate for the difference in thermal expansion between the metal and glass tube are inside the glass envelope rather than outside. Second, the composition of materials at the glass-to-metal seal has very similar thermal expansion coefficients making the joint less prone to breakage from thermal shock. Sandia National Laboratories provided both the azimuth and elevation collector module tracking systems used during the tests. The test results showed the efficiency of the Schott HCE to be very similar to current HCEs being manufactured by Solel. This testing provided performance verification for the use of Schott tubes with Solargenix trough collector assemblies at currently planned trough power plant projects in Arizona and Nevada.Copyright

Features of Fabrication Technology and Properties of Wicks of Heat Pipe Receivers for Solar Dish/Stirling Systems

Elaboration of robust and reliable capillary systems for solar energy heat pipe receivers is the important step for future application of this product for systems with thermal power of 30–80 kW. The paper considers a new approach to fabrication of capillary structure of heat pipe receivers on the basis of discrete metal fibers with diameter of 30 microns made of stainless steel 316L, and describes some of methods of wicks characterization as well. This technology has been demonstrated by fabrication of porous 4 mm thick wicks with bulk porosity about 0.82 applied to convex surfaces of dome-shaped receivers with radius 178 mm/height 119 mm (thermal power 36 kW) and radius 247 mm/height 173 mm (thermal power 68 kW) and for inner surface of tube with length 450 mm and diameter 73 mm (thermal power 14 kW). The distinction of the proposed technology is in the use of discrete fibers, which are felted on the curved surface in a special way and in the combination of procedures of the felt formation and their sintering to the surface (substrate material is Haynes Alloy 230). Execution of an extensive program of experimental characterization of a wick layer attached to the substrate has been developed and completed. The characterization of applied wicks determines a definition of their structural (local porosity, thickness of porous layer), mechanical (quality of wick bonding to substrate) and hydrodynamic properties (pumping diameter, one-phase and two-phase permeability). Initial estimation of wick performance was performed on the basis of methods developed at the National Technical University of Ukraine for two main modes of receiver operation — with return of sodium to a point on the dome (reflux) and without it. Prediction of receiver thermal performance, when they operate as a part of solar concentration assembly, was determined by specialized heat pipe performance software /Sandia National Laboratories, C. Andraka, 1999/.Copyright