Publications

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Concentrating solar power (CSP) plants with integrated thermal energy storage (TES) have successfully been coupled with photovoltaics (PV) + chemical battery energy storage (BES) in recent commercial-scale projects to balance system cost and diurnal power availability. Sandia National Laboratories has been tasked with designing an advanced solar energy system to power Kirtland Air Force Base (KAFB) where Sandia is co-located in Albuquerque, NM, USA. This design process requires optimization of individual components and capacities of the hybrid system. Preliminary modeling efforts have shown that a hybrid CSP+TES/PV+BES in Albuquerque, NM is sufficient for net-zero power generation for Sandia/KAFB for the next decade. However, the ability to meet the load in real-time (and minimize energy export) requires balance of generation and storage assets. Our results also show that excess PV used to charge TES improves resilience and overall renewables-to-load for the system. Here we will present the results of a parametric study varying the land use proportions of CSP and PV, and TES and BES capacities. We evaluate the effects of these variables on energy generation, real-time load satisfaction, site resilience to grid outages, and LCOE, to determine viable hybrid solar energy designs and their cost implications.

The need for reliable, cost-effective, utility scale energy storage that is universally applicable across different regions is becoming evident with the global transition towards non-polluting renewable energy resources. The operations and management of these energy storage technologies introduces a unique challenge that is inherently different from the conventional energy storage in the form of fossil fuel. The investigation into the business model, value proposition and economic viability of a utility scale thermal energy storage was part of a program sponsored by the United States Department of Energy, called Energy I-Corps. During this program, the project team reached out to a series of industry stakeholders to conduct interviews on the topic of thermal energy storage for utility scale power generation. Specific focus was placed on the business model based on the market needs in the context of the power grid in the United States. The utilization and re-use of infrastructure at existing thermo-electric power plants yielded the most viable business model for the implementation of the form of thermal energy storage discussed here.

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A comprehensive control strategy is necessary to safely and effectively operate particle based concentrating solar power (CSP) technologies. Particle based CSP with thermal energy storage (TES) is an emerging technology with potential to decarbonize power and process heat applications. The high-temperature nature of particle based CSP technologies and daily solar transients present challenges for system control to prevent equipment damage and ensure operator safety. An operational controls strategy for a tower based particle CSP system during steady state and transient conditions with safety interlocks is described in this paper. Control of a solar heated particle recirculation loop, TES, and a supercritical carbon dioxide (sCO2) cooling loop designed to reject 1 MW of thermal power are considered and associated operational limitations and their influence on control strategy are discussed.

This study investigated the durability of four high temperature coatings for use as a Gardon gauge foil coating. Failure modes and effects analysis have identified Gardon gauge foil coating as a critical component for the development of a robust flux gauge for high intensity flux measurements. Degradation of coating optical properties and physical condition alters flux gauge sensitivity, resulting in flux measurement errors. In this paper, four coatings were exposed to solar and thermal cycles to simulate real-world aging. Solar simulator and box furnace facilities at the National Solar Thermal Test Facility (NSTTF) were utilized in separate test campaigns. Coating absorptance and emissivity properties were measured and combined into a figure of merit (FOM) to characterize the optical property stability of each coating, and physical coating degradation was assessed qualitatively using microscope images. Results suggest rapid high temperature cycling did not significantly impact coating optical properties and physical state. In contrast, prolonged exposure of coatings to high temperatures degraded coating optical properties and physical state. Coatings degraded after 1 hour of exposure at temperatures above 400 °C and stabilized after 6-24 hours of exposure. It is concluded that the combination of high temperatures and prolonged exposure provide the energy necessary to sustain coating surface reactions and alter optical and physical coating properties. Results also suggest flux gauge foil coatings could benefit from long duration high temperature curing (>400 °C) prior to sensor calibration to stabilize coating properties and increase measurement reliability in high flux and high temperature applications.

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Particle heat exchangers are a critical enabling technology for next generation concentrating solar power (CSP) plants that use supercritical carbon dioxide (sCO₂) as a working fluid. This report covers the design, manufacturing and testing of a prototype particle-to-sCO₂ heat exchanger targeting thermal performance levels required to meet commercial scale cost targets. In addition, the the design and assembly of integrated particle and sCO₂ flow loops for heat exchanger performance testing are detailed. The prototype heat exchanger was tested to particle inlet temperatures of 500 °C at 17 MPa which resulted in overall heat transfer coefficients of approximately 300 W/m²-K at the design point and cases using high approach temperature with peak values as high as 400 W/m²-K

This paper summarizes the evolution of the Gen 3 Particle Pilot Plant (G3P3) receiver design with the goal of reducing heat losses and increasing thermal efficiencies. New features that were investigated included aperture covers and shrouds, active airflow, multistage catch-and-release devices (stairs), and optimization of receiver cavity geometry. Simulations and ground-based testing showed that a reduced receiver volume and aperture shroud could reduce advective heat losses by ∼40 - 50%, and stairs could increase opacity and reduce backwall temperatures. The reduced volume receiver and stairs were selected for on-sun testing, and receiver efficiencies up to 80 - 90% were achieved in the current test campaign. The receiver thermal efficiency generally increased as a function of incident power and particle mass flow rates. In addition, particle outlet temperatures were maintained to within ±10 °C of a prescribed setpoint temperature up to ∼700 °C using a PID controller that adjusted the particle mass flow rate into the receiver in response to the measured particle outlet temperatures.

Design and construction of a particle-to-sCO2 heat exchanger test station is described in this paper. The purpose of this test station is to make steady-state measurements of thermal performance with sCO2 as the working fluid. While the test station was initially constructed to test a 20 kW heat exchanger developed under the Gen3 Particle pilot plant (G3P3) project, it also was designed to accommodate testing of other heat exchanger configurations. Improvements for this test station design is based on lessons learned from prior heat exchanger testing. Maximum pressure and temperature ratings are based on the desire to use primarily stainless steel in the construction to reduce cost and lead time of components. Construction of the test station was completed and commissioning and initial testing took place during the October to November 2020 timeframe.

This paper summarizes the evolution of the Gen 3 Particle Pilot Plant (G3P3) receiver design with the goal of reducing heat losses and increasing thermal efficiencies. New features that were investigated included aperture covers and shrouds, active airflow, multistage catch-and-release devices (stairs), and optimization of receiver cavity geometry. Simulations and ground-based testing showed that a reduced receiver volume and aperture shroud could reduce advective heat losses by ∼40 - 50%, and stairs could increase opacity and reduce backwall temperatures. The reduced volume receiver and stairs were selected for on-sun testing, and receiver efficiencies up to 80 - 90% were achieved in the current test campaign. The receiver thermal efficiency generally increased as a function of incident power and particle mass flow rates. In addition, particle outlet temperatures were maintained to within ±10 °C of a prescribed setpoint temperature up to ∼700 °C using a PID controller that adjusted the particle mass flow rate into the receiver in response to the measured particle outlet temperatures.

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This paper describes the development of a facility for evaluating the performance of small-scale particle-to-sCO2 heat exchangers, which includes an isobaric sCO2 flow loop and an electrically heated particle flow loop. The particle flow loop is capable of delivering up to 60 kW of heat at a temperature of 600°C and flow rate of 0.4 kg/s. The loop was developed to facilitate long duration off-sun testing of small prototype heat exchangers to produce model validation data at steady-state operating conditions. Lessons learned on instrumentation, control, and system integration from prior testing of larger heat exchangers with solar thermal input were used to guide the design of the test facility. In addition, the development and testing of a novel 20-kWt moving packed-bed particle-to-sCO2 heat exchanger using the integrated flow loops is reported. The prototype heat exchanger implements many novel features for increasing thermal performance and reducing pressure drop which include integral porting of the sCO2 flow, unique bond/braze manufacturing, narrow plate spacing, and pure counter-flow arrangement. The experimental data collected for the prototype heat exchanger was compared to model predictions to verify the sizing, thermal performance, and pressure drop which will be extended to multi-megawatt heat exchanger designs in the future.

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This report describes the design, development, and testing of a prototype 100 kWt particle-to-supercritical CO₂ (sCO2) heat exchanger. An analytic hierarchy process was implemented to compare and evaluate alternative heat-exchanger designs (fluidized bed, shell-and-plate moving packed bed, and shell-and-tube moving packed bed) that could meet the high pressure (≥ 20 MPa) and high temperature (≥ 700 °C) operational requirements associated with sCO2 power cycles. Cost, heat-transfer coefficient, structural reliability, manufacturability, parasitics and heat losses, scalability, compatibility, erosion and corrosion, transient operation, and inspection ease were considered in the evaluation. A 100 kWt shell-and-plate design was selected for construction and integration with Sandia’s falling particle receiver system that heats the particles using concentrated sunlight. Sandia worked with industry to design and construct the moving packed-bed shell-and-plate heat exchanger. Tests were performed to evaluate its performance using both electrical heating and concentrated sunlight to heat the particles. Overall heat transfer coefficients at off-design conditions (reduced operating temperatures and only three stainless steel banks in the counter-crossflow heat exchanger) were measured to be approximately ~25 - 70 W/m²-K, significantly lower than simulated values of >100 W/m²-K. Tests using the falling particle receiver to heat the particles with concentrated sunlight yielded overall heat transfer coefficients of ~35 – 80 W/m²-K with four banks (including a nickel-alloy bank above the three stainless steel banks). The overall heat transfer coefficient was observed to decrease with increasing particle inlet temperatures, which contrasted the results of simulations that showed an increase in heat transfer coefficient with temperature due to increased effective particle-bed thermal conductivity from radiation. The likely cause of the discrepancy was particle-flow maldistributions and funnel flow within the heat exchanger caused by internal ledges and cross-bracing, which could have been exacerbated by increased particle-wall friction at higher temperatures. Additional heat loss at higher temperatures may also contribute to a lower overall heat-transfer coefficient. Design challenges including pressure drop, particle and sCO2 flow maldistribution, and reduced heat transfer coefficient are discussed with approaches for mitigation in future designs. Lessons learned regarding instrumentation, performance characterization, and operation of particle components and sCO2 flow loops are also discussed. Finally, a 200 MWt commercial-scale shell-and-plate heat-exchanger design based on the concepts investigated in this report is proposed.

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