Development of High Temperature, Corrosion Resistant Sensors for Concentrating Solar Power Systems

Solar power is a sustainable resource which can reduce the power generated by fossil fuels, lowering greenhouse gas emissions and increasing energy independence. The U.S. Department of Energy’s SunShot Initiative has set goals to increase the efficiency of concentrating solar power (CSP) systems. One SunShot effort to help CSP systems exceed 50% efficiency is to make use of high-temperature heat transfer fluids (HTFs) and thermal energy storage (TES) fluids that can increase the temperature of the power cycle up to 1300°C. Sporian has successfully developed high-temperature operable pressure, temperature, thermal flux, strain, and flow sensors for gas path measurements in high-temperature turbine engines. These sensors are based on a combination of polymer derived ceramic (PDC) sensors, advanced high-temperature packaging, and integrated electronics. The overall objective is the beneficial application of these sensors to CSP systems. Through collaboration with CSP industry stakeholders, Sporian has established a full picture of operational, interface, and usage requirements for trough, tower, and dish CSP architectures. In general, sensors should have accurate measurement, good reliability, reasonable cost, and ease of replacement or repair. Sensors in contact with hot salt HTF and TES fluids will experience temperature cycling on a daily basis, and parts of the system may be drained routinely. Some of the major challenges to high-temperature CSP implementation include molten salt corrosion and flow erosion of the sensors. Potential high-temperature sensor types that have been identified as of interest for CSP HTF/TES applications include temperature, pressure, flow, and level sensors. Candidate solar salts include nitrate, carbonate, and chloride, with different application temperatures ranging from 550°C-900°C. Functional ceramics were soaked for 500 hours in molten nitrate, carbonate, and chloride salts, showing excellent corrosion resistance in chloride salts and good resistance in nitrate salts. The demonstration of functional ceramics in relevant HTFs laid the foundation for full prototype sensor and packaging demonstration. Sporian has developed a packaging approach for ceramic-based sensors in various harsh gaseous environments at temperatures up to 1400°C, but several aspects of that packaging are not compatible with corrosive and electrically conductive HTFs. In addition to consulting published literature, a 300 hour soak test in molten chloride salt allowed the authors to identify suitable structural metals and ceramics. Based on discussions with stakeholders, molten salt corrosion testing and room-temperature water flow testing, suitable for CSP sensor/packaging concepts were identified for future development, and initial prototypes have been built and tested.

Preliminary Feasibility Study of Groundwater Source Geothermal Heat Pumps in Mason County and the American Bottoms Area, Illinois

Groundwater source heat pumps exploit the difference between the ground surface temperature and the nearly constant temperature of shallow groundwater. This project characterizes two areas for geothermal heating and cooling potential, Mason County in central Illinois and the American Bottoms area in southwestern Illinois. Both areas are underlain by thick sand and gravel aquifers and groundwater is readily available. Weather data, including monthly high and low temperatures and heating and cooling degree days, were compiled for both study areas. The heating and cooling requirements for a single-family house were estimated using two independent models that use weather data as input. The groundwater flow rates needed to meet these heating and cooling requirements were calculated using typical heat pump coefficient of performance values. The groundwater in both study areas has fairly high hardness and iron concentrations and is close to saturation with calcium and iron carbonates. Using the groundwater for cooling may induce the deposition of scale containing one or both of these minerals.

Volume Sizing for Thermal Storage With Phase Change Material for Concentrated Solar Power Plant

With a large capacity thermal storage system using phase change material (PCM), Concentrated Solar Power (CSP) is a promising technology for high efficiency of solar energy utilization. In a thermal storage system, a dual-media thermal storage tank is typically adopted in industry for the purpose of reducing the use of the heat transfer fluid (HTF). While the dual-media sensible heat storage system has been well studied, a dual-media latent heat storage system (LHSS) still needs more attention and study; particularly, the sizing of volumes of storage tanks considering actual operation conditions is of significance. In this paper, a strategy for LHSS volume sizing is proposed, which is based on computations using an enthalpy-based 1D model. One example of 60MW solar thermal power plant with 35% thermal efficiency is presented. In the study, potassium hydroxide (KOH) is adopted as PCM and Therminol VP-1 is used as HTF. The operational temperatures of the storage system are 390°C and 310°C, respectively for the high and low temperatures. The system is assumed to operate for 100 days with 6 hours charge and 6 hours discharge every day. From the study, the needed height of the thermal storage tank is calculated from using the strategy of tank sizing. The method for tank volume sizing is of significance to engineering application.

Development of High Temperature Liquid Metal Heat Transfer Fluids for CSP Applications

In response to the DOE Sunshot Initiative to develop low-cost, high efficiency CSP systems, UCLA is leading a multi-university research effort to develop new high temperature heat transfer fluids capable of stable operation at 800°C and above. Due to their operating temperature range, desirable heat transfer properties and very low vapor pressure, liquid metals were chosen as the heat transfer fluid. An overview of the ongoing research effort is presented. Development of new liquid metal coolants begins with identification of suitable candidate metals and their alloys. Initial selection of candidate metals was based on such parameters as melting temperature, cost, toxicity, stability/reactivity Combinatorial sputtering of the down selected candidate metals is used to fabricate large compositional spaces (∼ 800), which are then characterized using high-throughput techniques (e.g., X-ray diffraction). Massively parallel optical methods are used to determine melting temperatures. Thermochemical modeling is also performed concurrently to compliment the experimental efforts and identify candidate multicomponent alloy systems that best match the targeted properties. The modeling effort makes use of available thermodynamic databases, the computational thermodynamic CALPHAD framework and molecular-dynamics simulations of molten alloys. Refinement of available thermodynamics models are performed by comparison with available experimental data. Characterizing corrosion in structural materials such as steels, when using liquid metals, and strategies to mitigate them are an integral part of this study. The corrosion mitigation strategy we have adopted is based on the formation of stable oxide layers on the structural metal surface which prevents further corrosion. As such oxygen control is crucial in such liquid metal systems. Liquid metal enhanced creep and embrittlement in commonly used structural materials are also being investigated. Experiments with oxygen control are ongoing to evaluate what structural materials can be used with liquid metals. Characterization of the heat transfer during forced flow is another key component of the study. Both experiments and modeling efforts have been initiated. Key results from experiments and modeling performed over the last year are highlighted and discussed.

Granular Flow and Heat Transfer Study in a Near-Blackbody Enclosed Particle Receiver

Concentrating solar power (CSP) is an effective means of converting solar energy into electricity with an energy-storage capability for continuous, dispatchable, renewable power generation. However, challenges with current CSP systems include high initial capital cost and electricity price. The U.S. Department of Energy’s (DOE) SunShot program aims to reduce cost and improve performance of CSP technology. To this end, NREL is developing a solid-particle based CSP system projected to have significant cost and performance advantages over current nitrate-based molten salt systems. The design uses gas/solid, two-phase flow as the heat transfer fluid and separated solid particles as the storage medium. A critical component in the system is a novel near-blackbody (NBB) enclosed particle receiver with high-temperature capability developed with the goal of meeting DOE’s SunShot targets for receiver cost and performance. Development of the NBB enclosed particle receiver necessitates detailed study of the dimensions of the receiver, particle flow conditions, and heat transfer coefficients. The receiver utilizes an array of absorber tubes with a granular medium flowing downward through channels between tubes. The current study focuses on simulation and analysis of granular flow patterns and the resulting convective and conductive heat transfer to the particulate phase. This paper introduces modeling methods for the granular flow through the receiver module and compares the results with an in-situ particle flow test.

Low-Cost Lightweight Thin Film Solar Concentrators

A low-cost rigid foam-based concentrator technology development program was funded by the DOE SunShot Initiative to meet installed cost goals of $75/m2 vs. current costs of $200–250/m2. The cost reduction in this approach focuses primarily on designing a mirror module with a rigid foam center with stainless steel facesheets and reflective film. The low mechanical strength of the foam is compensated by optimizing the densities and dimensions to meet pointing accuracy requirements of 4 milliradians (mrad) in 27mph winds. Two alpha concentrators were built to validate the mirror module manufacturing process and one of them was accurate to 0.15 mrad RMS vs. the design requirement of 1 mrad RMS. To understand the lifetime reliability of the panels, fifteen 4-inch square samples were exposed to various environmental conditions including acid rain, bird droppings, thermal cycling, and the final results indicated no loss in reflectivity of 95%. UV testing will be performed in the next phase. Three mechanical structure options covering the range of large multi-faceted heliostats with diagonal load carrying elements, small single facet heliostats low to the ground and optimized truss-based deep structure designs were analyzed with FEA and analytically; results indicated a significant cost benefit (>2×) for the truss-based design over the other options. Other elements such as the controls, actuators were also considered in th analysis with vendor data. Cost trades were performed for heliostats ranging from 10m2 to 250m2. The results indicated a broad installed cost minimum around $113/m2 for heliostat sizes ranging from 80 m2 to 130 m2. Additional cost saving approaches will be considered in Phase 2 of the project.

Oxidation Rate Analysis of Carbon Nanoparticles for a Small Particle Heat Exchange Receiver

A new experimental set-up has been introduced at San Diego State University’s Combustion and Solar Energy Lab to study the thermal oxidation characteristics of in-situ generated carbon particles in air at high pressure. The study is part of a project developing a Small Particle Heat Exchange Receiver (SPHER) utilizing concentrated solar power to run a Brayton cycle. The oxidation data obtained will further be used in different existing and planned computer models in order to accurately predict reactor temperatures and flow behavior in the SPHER. The carbon black particles were produced by thermal decomposition of natural gas at 1250 °C and a pressure of 5.65 bar (82 psi). Particles were analyzed using a Diesel Particle Scatterometer (DPS) and scanning electron microscopy (SEM) and found to have a 310 nm average diameter. The size distribution and the complex index of refraction were measured and the data were used to calculate the specific extinction cross section γ of the spherical particles. The oxidation rate was determined using 2 extinction tubes and a tube furnace and the values were compared to literature. The activation energy of the carbon particles was determined to be 295.02 kJ/mole which is higher than in comparable studies. However, the oxidation of carbon particles bigger than 100 nm is hardly studied and almost no previous data is available at these conditions.

Novel Controlled-Velocity Wind Turbine Testing Apparatus to Simulate Turbulent, Non-Return Flow

This paper presents the work done by the authors to analyze the method of performance characterization of a 100W scale vertical axis wind turbines using a controlled-velocity test apparatus. The design of the power transfer system containing a gearbox and generator requires test data to determine the peak and operating range of wind speed, corresponding to RPM and torque. Multiple methods of turbine testing were considered, including in situ, wind tunnel, and control-velocity. Controlled-velocity, a method where the turbine is moved through a fluid, was selected based on lack of test location wind speeds or access to a wind tunnel of sufficient size. The test apparatus is designed to be effective for VAWT turbines of a diameter range from 1.45 to 4.2 meters in a wind velocity range of 1 to 17 m/s. This covers a Reynolds number range between (2.5 × 10^5 < Re < 4.2 × 10^6). A change from previous control-velocity test apparatus is the use of a separate truck and trailer compared to a flatbed truck, which allows greater distance between the truck cab and the turbine, to decrease any flow interference of the cab. This previous work and testing has shown to be a valid test method in that the turbine is in similar turbulent conditions as near the ground and buildings which the turbine is designed for. The main advantage of this test apparatus is the ability to test turbines in a region with low average wind speeds and minimum infrastructure.

Development and Design Prototype 300 kW-Thermal High-Temperature Particle Heating Concentrator Solar Power System Utilizing Thermal Energy Storage

The advantages of high temperature central receiver particle heating solar heat supply systems in concentrator solar power (CSP) have been recognized in recent years. The use of particulate as the collection medium provides two critical advantages: (1) Ordinary particulate minerals and products will allow higher collection temperatures approaching 1000°C compared with conventional molten salts, which are limited to around 650°C, and (2) the low cost high temperature particulate material can also be used as the storage medium in a highly cost effective thermal energy storage (TES) system. The high operating temperature allows use of high efficiency power conversion systems such as supercritical steam in a vapor power cycle or supercritical carbon dioxide in a Brayton cycle. Alternatively, a lower cost gas turbine can be used for the power conversion system. High conversion efficiency combined with inexpensive TES will yield a highly cost effective CSP system. The 300 kW-th prototype is being constructed as a solar heat supply system only, deferring the power conversion system for later demonstration in a larger integrated CSP system. This paper describes the general design and development efforts leading to construction of the 300 kW prototype system located in the Riyadh Techno Valley development near King Saud University in Riyadh, Saudi Arabia, which is the first sizeable solar heat supply system purposely designed, and constructed as a particle heating system. An important component in a particle heating system is the particle heating receiver (PHR), which should be durable and efficient while remaining cost-effective. A critical enabling technology of the PHR being implemented for this project was invented by researchers on our team. In our version of the PHR, the particulate flows downwards through a porous or mesh structure where the concentrated solar energy is absorbed. The porous structure will reduce the speed of the falling particulate material allowing a large temperature rise on a single pass. The new design will also increase the absorption of solar energy and mitigate convective heat loss and particle loss. Other innovative aspects of this design include low cost thermal energy storage bins and a cost effective particle to working fluid heat exchanger. Certain features of these design elements are subjects of ongoing patent applications. Nevertheless, the overall design and the development process of the prototype system is presented in this paper.

Optical Modeling of Reflectivity Loss Caused by Dust Deposition on CSP Mirrors and Restoration of Energy Yield by Electrodynamic Dust Removal

For large scale CSP power plants, vast areas of land are needed in deserts and semi-arid climates where uninterrupted solar irradiance is most abundant. These power facilities use large arrays of mirrors to reflect and concentrate sunlight onto collectors, however, dust deposition on the optical surfaces causes obscuration of sunlight, resulting in large energy-yield losses in solar plants. This problem is compounded by the lack of natural clean water resources for conventional cleaning of solar mirrors, often with reflective surface areas of large installations exceeding a million square meters. To investigate the application of transparent electrodynamic screens (EDS) for efficient and cost effective dust removal from solar mirrors, both optical modeling and experimental verifications were performed. Prototype EDS-integrated mirrors were constructed by depositing a set of parallel transparent electrodes into the sun-facing surface of solar mirrors and coating electrodes with thin transparent dielectric film. Activation of the electrodes with a three-phase voltage creates an electrodynamic field that charges and repels dust electrostatically by Coulomb force and sweeps away particles by a traveling electrodynamic wave. We report here brief discussions on (1) rate of deposition and the properties of dust with respect to their size distribution and chemical composition in semi-arid areas of the southwest US and Mojave Desert and their adhesion to solar mirrors, (2) optical models of: (a) specular reflection losses caused by scattering and absorption by dust particles deposited on the surface based on Mie scattering theory, and (b) reflection loss by the integration of EDS on the mirror surface, computed by FRED ray-tracing model. The objective is to maintain specular reflectivity of 90% or higher by frequent removal of dust by EDS. Our studies show that the incorporation of transparent EDS would cause an initial loss of 3% but would be able to maintain specular reflectivity more than 90% to meet the industrial requirement for CSP plants. Specular reflection measurements taken inside a climate controlled environmental chamber show that EDS integration can restore specular reflectivity and would be able to prevent major degradation of the optical surface caused by the deposition of dust.