Publications

This report contains the design basis for a generic molten-salt solar power tower. A solar power tower uses a field of tracking mirrors (heliostats) that redirect sunlight on to a centrally located receiver mounted on top a tower, which absorbs the concentrated sunlight. Molten nitrate salt, pumped from a tank at ground level, absorbs the sunlight, heating it up to 565 C. The heated salt flows back to ground level into another tank where it is stored, then pumped through a steam generator to produce steam and make electricity. This report establishes a set of criteria upon which the next generation of solar power towers will be designed. The report contains detailed criteria for each of the major systems: Collector System, Receiver System, Thermal Storage System, Steam Generator System, Master Control System, and Electric Heat Tracing System. The Electric Power Generation System and Balance of Plant discussions are limited to interface requirements. This design basis builds on the extensive experience gained from the Solar Two project and includes potential design innovations that will improve reliability and lower technical risk. This design basis document is a living document and contains several areas that require trade-studies and design analysis to fully complete the design basis. Project- and site-specific conditions and requirements will also resolve open To Be Determined issues.

Solar thermal-to-electric power plants have been tested and investigated at Sandia National Laboratories (SNL) since the late 1970s, and thermal storage has always been an area of key study because it affords an economical method of delivering solar-electricity during non-daylight hours. This paper describes the design considerations of a new, single-tank, thermal storage system and details the benefits of employing this technology in large-scale (10MW to 100MW) solar thermal power plants. Since December 1999, solar engineers at Sandia National Laboratories' National Solar Thermal Test Facility (NSTTF) have designed and are constructing a thermal storage test called the thermocline system. This technology, which employs a single thermocline tank, has the potential to replace the traditional and more expensive two-tank storage systems. The thermocline tank approach uses a mixture of silica sand and quartzite rock to displace a significant portion of the volume in the tank. Then it is filled with the heat transfer fluid, a molten nitrate salt. A thermal gradient separates the hot and cold salt. Loading the tank with the combination of sand, rock, and molten salt instead of just molten salt dramatically reduces the system cost. The typical cost of the molten nitrate salt is $800 per ton versus the cost of the sand and rock portion at $70 per ton. Construction of the thermocline system will be completed in August 2000, and testing will run for two to three months. The testing results will be used to determine the economic viability of the single-tank (thermocline) storage technology for large-scale solar thermal power plants. Also discussed in this paper are the safety issues involving molten nitrate salts and other heat transfer fluids, such as synthetic heat transfer oils, and the impact of these issues on the system design.

Solar Two, a 10MWe power tower plant in Barstow, California, successfully demonstrated the production of grid electricity at utility-scale with a molten-salt solar power tower. This paper provides an overview of the project, from inception in 1993 to closure in the spring of 1999. Included are discussions of the goals of the Solar Two consortium, the planned-vs.-actual timeline, plant performance, problems encountered, and highlights and successes of the project. The paper concludes with a number of key results of the Solar Two test and evaluation program.

Solar Two was a collaborative, cost-shared project between eleven US industry and utility partners and the U. S. Department of Energy to validate molten-salt power tower technology. The Solar Two plant, located east of Barstow, CA, was comprised of 1926 heliostats, a receiver, a thermal storage system and a steam generation system. Molten nitrate salt was used as the heat transfer fluid and storage media. The steam generator powered a 10 MWe, conventional Rankine cycle turbine. Solar Two operated from June 1996 to April 1999. The major objective of the test and evaluation phase of the project was to validate the technical characteristics of a molten salt power tower. This paper describes the significant results from the test and evaluation activities.

A non-conventional type of heating system is being tested at Sandia National Laboratories for solar thermal power tower applications. In this system, called impedance heating, electric current flows directly through the pipe to maintain the desired temperature. The pipe becomes the resistor where the heat is generated. Impedance heating has many advantages over previously used mineral insulated (MI) heat trace. An impedance heating system should be much more reliable than heat trace cable since delicate junctions and cabling are not used and the main component, a transformer, is inherently reliable. A big advantage of impedance heating is the system can be sized to rapidly heat up the piping to provide rapid response times necessary in cyclic power plants such as solar power towers. In this paper, experimental results from testing an impedance heating system are compared to MI heat trace. We found impedance heating was able to heat piping rapidly and effectively. There were not significant stray currents and impedance heating did not affect instrumentation.

A non-conventional type of heating system is being tested at Sandia National Laboratories for solar thermal power tower applications. In this system, called impedance heating, electric current flows directly through the pipe to maintain the desired temperature. The pipe becomes the resistor where the heat is generated. Impedance heating has many advantages over previously used mineral insulated (MI) heat trace. An impedance heating system should be much more reliable than heat trace cable since delicate junctions and cabling are not used and the main component, a transformer, is inherently reliable. A big advantage of impedance heating is the system can be sized to rapidly heat up the piping to provide rapid response times necessary in cyclic power plants such as solar power towers. In this paper, experimental results from testing an impedance heating system are compared to MI cable heat trace. We found impedance heating was able to heat piping rapidly and effectively. There were not significant stray currents and impedance heating did not affect instrumentation.

Experiments have been conducted with a molten salt loop at Sandia National Laboratories in Albuquerque, NM to resolve issues associated with the operation of the 10MW{sub e} Solar Two Central Receiver Power Plant located near Barstow, CA. The salt loop contained two receiver panels, components such as flanges and a check valve, vortex shedding and ultrasonic flow meters, and an impedance pressure transducer. Tests were conducted on procedures for filling and thawing a panel, and assessing components and instrumentation in a molten salt environment. Four categories of experiments were conducted: (1) cold filling procedures, (2) freeze/thaw procedures, (3) component tests, and (4) instrumentation tests. Cold-panel and -piping fill experiments are described, in which the panels and piping were preheated to temperatures below the salt freezing point prior to initiating flow, to determine the feasibility of cold filling the receiver and piping. The transient thermal response was measured, and heat transfer coefficients and transient stresses were calculated from the data. Freeze/thaw experiments were conducted with the panels, in which the salt was intentionally allowed to freeze in the receiver tubes, then thawed with heliostat beams. Slow thermal cycling tests were conducted to measure both how well various designs of flanges (e.g., tapered flanges or clamp type flanges) hold a seal under thermal conditions typical of nightly shut down, and the practicality of using these flanges on high maintenance components. In addition, the flanges were thermally shocked to simulate cold starting the system. Instrumentation such as vortex shedding and ultrasonic flow meters were tested alongside each other, and compared with flow measurements from calibration tanks in the flow loop.

Cold filling refers to flowing a fluid through piping or tubes that are at temperatures below the fluid’s freezing point. Since the piping and areas of the receiver in a molten-nitrate salt central-receiver solar power plant must be electrically heated to maintain their temperatures above the nitrate salt freezing point (430°F, 221 °C), considerable energy could be used to maintain such temperatures during nightly shutdown and bad weather. Experiments and analyses have been conducted to investigate cold filling receiver panels and piping as a way of reducing parasitic electrical power consumption and increasing the availability of the plant. The two major concerns with cold filling are (1) how far can the molten salt penetrate cold piping before freezing closed, and (2) what thermal stresses develop during the associated thermal shock. Cold fill experiments were conducted by flowing molten salt at 550°F (288°C) through cold panels, manifolds, and piping to determine the feasibility of cold filling the receiver and piping. The transient thermal responses were measured and heat transfer coefficients were calculated from the data. Nondimensional analysis is presented which quantifies the thermal stresses in a pipe or tube undergoing thermal shock. In addition, penetration distances were calculated to determine the distance salt could flow in cold pipes prior to freezing closed. © 1995 by ASME.

Three concepts to measure incident flux (1) relative, real-time power measurement, (2) flux mapping and incident power measurement, and (3) real-time flux mapping) and two concepts to measure receiver surface temperatures low and high resolution temperature measurements) on an external central receiver are discussed along with the potential and shortcomings of these concepts to make the desired measurements and the uncertainties associated with the measurements caused by atmospheric and surface property variations. These concepts can aid in the operation and evaluation of the receiver and plant. Tests have shown that the incident flux distribution on a surface can be mapped out using a fixed, narrow white target and a CCD camera system by recording the images of the beam as it is passed over the target and by building a composite image. Tests with the infrared cameras have shown they are extremely valuable tools in determining temperature profiles during startup of the receiver and throughout operation. This paper describes each concept in detail along with the status of testing to determine the feasibility of these concepts.

In one design of molten-salt central receivers, the molten salt flows in a serpentine path, down one panel of tubes then up the next and down again continuing in this fashion through the receiver. There have been concerns about this design because in the down flow sections, the heat flux incident on the tubes can cause flow instability since the flow is in direct opposition to the buoyant forces. In extreme cases the buoyant forces can cause flow stagnation or reversal. An analysis of flow stability within individual tubes and down flow sections of receiver panels is presented. When the partial derivative of the pressure drop with respect to mass flow rate is negative ({partial_derivative}{Delta}P/{partial_derivative}{sup {lg_bullet}} < 0), the flow is unstable and could cause serious damage to the receiver. Stability maps are developed that show safe operating regimes where inertial forces dominate over buoyant forces. The data is then normalized using the Grashof and Reynolds numbers.

A solar photocatalytic process has been under development at both Sandia National Laboratories and the National Renewable Energy Laboratory (formerly the Solar Energy Research Institute). This process uses solar ultraviolet light to activate a titanium dioxide catalyst which oxidizes organic contaminants in water. In the summer of 1991, a solar photocatalytic detoxification of water system was installed and tested at a California Superfund Site located at Lawrence Livermore National Laboratory. The site was designated a Superfund Site because of widespread groundwater contamination which resulted from the release of chlorinated solvents, principally trichloroethylene, when the site was a Naval Air Station in the early 1940s. The objectives of these experiments were to measure the effects of process variables and the process efficiency in an actual remediation setting, to collect experimental data and operating experience in photocatalytic oxidation of organic contaminants, to develop accurate models of the system operation and to develop control strategies.

This report contains a summary of large-scale experiments conducted at Sandia National Laboratories under the Solar Detoxification of Water project. The objectives of the work performed were to determine the potential of using solar radiation to destroy organic contaminants in water by photocatalysis and to develop the process and improve its performance. For these experiments, we used parabolic troughs to focus sunlight onto glass pipes mounted at the trough's focus. Water spiked with a contaminant and containing suspended titanium dioxide catalyst was pumped through the illuminated glass pipe, activating the catalyst with the ultraviolet portion of the solar spectrum. The activated catalyst creates oxidizers that attack and destroy the organics. Included in this report are a summary and discussion of the implications of experiments conducted to determine: the effect of process kinetics on the destruction of chlorinated solvents (such trichloroethylene, perchloroethylene, trichloroethane, methylene chloride, chloroform and carbon tetrachloride), the enhancement due to added hydrogen peroxide, the optimal catalyst loading, the effect of light intensity, the inhibition due to bicarbonates, and catalyst issues.