A focus in the development of the next generation of concentrating solar power (CSP) plants is the integration of high temperature particle receivers with improved efficiency supercritical carbon dioxide (sCO2) power cycles. The feasibility of this type of system depends on the design of a particle-to-sCO2 heat exchanger. This work presents a finite element analysis (FEA) model to analyze the thermal performance of a particle-to-sCO2 heat exchanger for potential use in a CSP plant. The heat exchanger design utilizes a moving packed bed of particles in crossflow with sCO2 which flows in a serpentine pattern through banks of microchannel plates. The model contains a thermal analysis to determine the heat exchanger's performance in transferring thermal energy from the particle bed to the sCO2. Test data from a prototype heat exchanger was used to verify the performance predictions of the model. The verification of the model required a multitude of sensitivity tests to identify where fidelity needed to be added to reach agreement between the experimental and simulated results. For each sensitivity test in the model, the effect on the performance is discussed. The model was shown to be in good agreement on the overall heat transfer coefficient of the heat exchanger with the experimental results for a low temperature set of conditions with a combination of added sensitives. A set of key factors with a major impact on the performance of the heat exchanger are discussed.
The National Solar Thermal Test Facility (NSTTF) at Sandia National Laboratories is conducting research on a Generation 3 Particle Pilot Plant (G3P3) that uses falling sand-like particles as the heat transfer medium. The system will include a thermal energy storage (TES) bin with a capacity of 6 MWht¬ requiring ~120,000 kg of flowing particles. Testing and modeling were conducted to develop a validated modeling tool to understand temporal and spatial temperature distributions within the storage bin as it charges and discharges. Flow and energy transport in funnel-flow was modeled using volume averaged conservation equations coupled with level set interface tracking equations that prescribe the dynamic geometry of particle flow within the storage bin. A thin layer of particles on top of the particle bed was allowed to flow toward the center and into the flow channel above the outlet. Model results were validated using particle discharge temperatures taken from thermocouples mounted throughout a small steel bin. The model was then used to predict heat loss during charging, storing, and discharging operational modes at the G3P3 scale. Comparative results from the modeling and testing of the small bin indicate that the model captures many of the salient features of the transient particle outlet temperature over time.
Realizing cost-effective, dispatchable, renewable energy production using concentrated solar power (CSP) relies on reaching high process temperatures to increase the thermal-to-electrical efficiency. Ceramic based particles used as both the energy storage medium and heat transfer fluid is a promising approach to increasing the operating temperature of next generation CSP plants. The particle-to-supercritical CO2 (sCO2) heat exchanger is a critical component in the development of this technology for transferring thermal energy from the heated ceramic particles to the sCO2 working fluid of the power cycle. The leading design for the particle-to-sCO2 heat exchanger is a shell-and-plate configuration. Currently, design work is focused on optimizing the performance of the heat exchanger through reducing the plate spacing. However, the particle channel geometry is limited by uniformity and reliability of particle flow in narrow vertical channels. Results of high temperature experimental particle flow testing are presented in this paper.
The National Solar Thermal Test Facility (NSTTF) at Sandia National Laboratories is conducting research on a Generation 3 Particle Pilot Plant (G3P3) that uses falling sand-like particles as the heat transfer medium. The system will include a thermal energy storage (TES) bin with a capacity of 6 MWht¬ requiring ~120,000 kg of flowing particles. Testing and modeling were conducted to develop a validated modeling tool to understand temporal and spatial temperature distributions within the storage bin as it charges and discharges. Flow and energy transport in funnel-flow was modeled using volume averaged conservation equations coupled with level set interface tracking equations that prescribe the dynamic geometry of particle flow within the storage bin. A thin layer of particles on top of the particle bed was allowed to flow toward the center and into the flow channel above the outlet. Model results were validated using particle discharge temperatures taken from thermocouples mounted throughout a small steel bin. The model was then used to predict heat loss during charging, storing, and discharging operational modes at the G3P3 scale. Comparative results from the modeling and testing of the small bin indicate that the model captures many of the salient features of the transient particle outlet temperature over time.
Solid particle receivers provide an opportunity to run concentrating solar tower receivers at higher temperatures and increased overall system efficiencies. The design of the bins used for storing and managing the flow of particles creates engineering challenges in minimizing thermomechanical stress and heat loss. An optimization study of mechanical stress and heat loss was performed at the National Solar Thermal Test Facility at Sandia National Laboratories to determine the geometry of the hot particle storage hopper for a 1 MWt pilot plant facility. Modeling of heat loss was performed on hopper designs with a range of geometric parameters with the goal of providing uniform mass flow of bulk solids with no clogging, minimizing heat loss, and reducing thermomechanical stresses. The heat loss calculation included an analysis of the particle temperatures using a thermal resistance network that included the insulation and hopper. A plot of the total heat loss as a function of geometry and required thicknesses to accommodate thermomechanical stresses revealed suitable designs. In addition to the geometries related to flow type and mechanical stress, this study characterized flow related properties of CARBO HSP 40/70 and Accucast ID50-K in contact with refractory insulation. This insulation internally lines the hopper to prevent heat loss and allow for low cost structural materials to be used for bin construction. The wall friction angle, effective angle of friction, and cohesive strength of the bulk solid were variables that were determined from empirical analysis of the particles at temperatures up to 600°C.
The use of solid particles as a heat-transfer fluid and thermal storage media for concentrating solar power is a promising candidate for meeting levelized cost of electricity (LCOE) targets for next-generation CSP concepts. Meeting these cost targets for a given system concept will require optimization of the particle heat-transfer fluid with simultaneous consideration of all system components and operating conditions. This paper explores the trade-offs in system operating conditions and particle thermophysical properties on the levelized cost of electricity through parametric analysis. A steady-state modeling methodology for design point simulations dispatched against typical meteorological year (TMY) data is presented, which includes computationally efficient submodels of a falling particle receiver, moving packed-bed heat exchanger, storage bin, particle lift, and recompression supercritical CO2 (sCO2) cycle. The components selected for the baseline system configuration presents the most near-term realization of a particle-based CSP system that has been developed to date. However, the methodology could be extended to consider alternative particle receiver and heat exchanger concepts. The detailed system-level model coupled to component cost models is capable of propagating component design and performance information directly into the plant performance and economics. The system-level model is used to investigate how the levelized cost of electricity varies with changes in particle absorptivity, hot storage bin temperature, heat exchanger approach temperature, and sCO2 cycle operating parameters. Trade-offs in system capital cost and solar-to-electric efficiency due to changes in the size of the heliostat field, storage bins, primary heat exchanger, and receiver efficiency are observed. Optimal system operating conditions are reported, which approach levelized costs of electricity of $0.06 kWe-1hr-1
Imponenti, Luca; Albrecht, Kevin; Kharait, Rounak; Sanders, Michael D.; Jackson, Gregory S.
Redox cycles of doped calcium manganite perovskites (CaMnO3−δ) are studied for cost-effective thermochemical energy storage at temperatures up to 1000 °C for concentrating solar power and other applications. If the thermodynamics and kinetics for heat-driven reduction can be tailored for high temperatures and industrially accessible low O2 partial pressures (PO2⩾10-4 bar), perovskite redox cycles can offer high specific energy storage at temperatures much higher than state-of-the-art molten-salt subsystems. To this end, a range of A-site and B-site doped CaMnO3−δ were screened for their reducibility at 900 °C and PO2≈10-4 bar via thermogravimetric analysis. For compositions with high reducibility, notably A-site doped Ca1−xSrxMnO3−δ (x=0.05 and 0.10) and B-site doped CaCryMn1−yO3−δ (y=0.05 and 0.10), oxygen non-stoichiometry δ with respect to temperature and PO2 were measured and used to fit thermodynamic parameters of a two-reaction, point-defect model of the redox process for the two prominent crystalline phases (orthorhombic and cubic) that the perovskites occupy during the cycle. The fits compare favorably to differential scanning calorimetry measurements with the magnitude of the overall reduction enthalpies decreasing as the degree of reduction increases and the perovskites shift from orthorhombic to cubic crystalline phases. Based on thermodynamic limits, redox cycles of both Ca1−xSrxMnO3−δ compositions between air at 500 °C and PO2≈10-4 bar at 900 °C can store and release up to ≈700 kJ kg−1 with over 50% of the total energy stored as chemical energy. This is approximately 140 kJ kg−1 more chemical energy than the thermodynamic limits for CaCryMn1−yO3−δ compositions under the same cycle conditions. Approaching these thermodynamic limits for the specific energy storage of these redox cycles in a concentrating solar plant requires fast kinetics for perovskite reduction in the solar receiver and for reoxidation in the heat recovery reactor. Isothermal packed-bed redox cycling experiments of Ca1−xSrxMnO3−δ and CaCryMn1−yO3−δ compositions at temperatures up to 1000 °C show that reoxidation is fast compared to reduction. Thus, specific thermochemical energy storage is limited by residence times available for high-temperature reduction. The Sr-doped compositions approach higher fractions (≈90% or more) of the specific energy storage equilibrium limit after 300 s of reduction in the packed bed configuration above 800 °C and completely reoxidize in ⩽20 s in air. Non-isothermal cycling with heating from 500 °C to 900 °C in low PO2≈10-4 bar and subsequent reoxidation during cooling in air back to 500 °C demonstrate excellent chemical stability over 1000 cycles for all doped CaMnO3−δ compositions tested. The results suggest that these redox cycles may offer a viable energy storage subsystem with long-term stability for future concentrating solar plants and other high-temperature energy storage applications.
Experiments for measuring the heat transfer coefficients and visualization of dense granular flows in rectangular vertical channels are reported. The experiments are directed at the development of a moving packed-bed heat exchanger to transfer thermal energy from solar-heated particles to drive a supercritical carbon dioxide (sCO2) power cycle. Particle-wall heat transfer coefficients are found to agree with Nusselt number correlations for plug flow in a parallel plate configuration. The plate spacing and particle properties in the prototype design result in experimentally measured particle-wall heat transfer coefficients of 200 W/m2-K at intermediate temperature and are expected to be higher at elevated temperature due to improved packed bed thermal conductivity. The high-temperature (600°C) visualization experiments indicate that uniform particle flow distribution through the vertical channels of a shell-and-plate heat exchanger can be achieved through a mass flow cone particle feeder. Uniform drawdown was experienced for both 77° and 72° feeder angles over a range of particle mass flow rates between 0.05 and 0.175 kg/s controlled by a slide gate to modulate the outlet flow cross-sectional area.
Solar thermochemical hydrogen (STCH) production is one avenue for converting sunlight into hydrogen through concentrating solar thermal technology. STCH is a two-step redox process that begins with concentrated sunlight to thermally reduce a metal oxide around 1500 °C leaving it in an oxygen deficient form. Subsequent exposure of the reduced metal oxide to steam at lower temperature reoxidizes the material and produces hydrogen. The efficiency of this process is dependent on the metal oxide material thermodynamic properties and cycle operating conditions.
This report summarizes the results of a two-year project funded by the U.S. Department of Energy's Solar Energy Technologies Office (SuNLaMP 1506) to evaluate the performance of high-temperature (>700 °C) particle receivers for concentrating solar power (see Appendix A for project information). In the first year, novel particle release patterns were designed and tested to increase the effective solar absorptance of the particle curtain. Modeling results showed that increasing the magnitude and frequency of different wave-like patterns increased the effective absorptance and thermal efficiency by several percentage points, depending on the mass flow rate. Tests showed that triangular-wave, square-wave, and parallel-curtain particle release patterns could be implemented and maintained at flow rates of ~10 kg/s/m. The second year of the project focused on the development and testing of particle mass-flow control and measurement methods. An automated slide gate controlled by the outlet temperature of the particles was designed and tested. Testing demonstrated that the resolution accuracy of the slide-gate positioning was less than ~1 mm, and the speed of the slide gate enabled rapid adjustments to accommodate changes in the irradiance to maintain a desired outlet temperature range. Different in-situ particle mass-flow measurement techniques were investigated, and two were tested. The in-situ microwave sensor was found to be unreliable and sensitive to variations in particle flow patterns. However, the in-situ weigh hopper using load cells was found to provide reliable and repeatable measurements of real-time in-situ particle mass flow. On-sun tests were performed to determine the thermal efficiency of the receiver as a function of mass flow rate, particle temperature, and irradiance. Models of the tests were also developed and compared to the tests.
ASME 2018 12th International Conference on Energy Sustainability, ES 2018, collocated with the ASME 2018 Power Conference and the ASME 2018 Nuclear Forum
This paper presents an evaluation of alternative particle heat-exchanger designs, including moving packed-bed and fluidized-bed designs, for high-temperature heating of a solardriven supercritical CO2 (sCO2) Brayton power cycle. The design requirements for high pressure (> 20 MPa) and high temperature (> 700 °C) operation associated with sCO2 posed several challenges requiring high-strength materials for piping and/or diffusion bonding for plates. Designs from several vendors for a 100 kW-thermal particle-to-sCO2 heat exchanger were evaluated as part of this project. 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. An analytical hierarchy process was used to weight and compare the criteria for the different design options. The fluidized-bed design fared the best on heat transfer coefficient, structural reliability, scalability and inspection ease, while the moving packed-bed designs fared the best on cost, parasitics and heat losses, manufacturability, compatibility, erosion and corrosion, and transient operation. A 100 kWt shell-and-plate design was ultimately selected for construction and integration with Sandia's falling particle receiver system.
ASME 2017 11th International Conference on Energy Sustainability, ES 2017, collocated with the ASME 2017 Power Conference Joint with ICOPE 2017, the ASME 2017 15th International Conference on Fuel Cell Science, Engineering and Technology, and the ASME 2017 Nuclear Forum
Particle-based concentrating solar power (CSP) plants have been proposed to increase operating temperature for integration with higher efficiency power cycles using supercritical carbon dioxide (sCO2). The majority of research to date has focused on the development of high-efficiency and high-temperature particle solar thermal receivers. However, system realization will require the design of a particle/sCO2 heat exchanger as well for delivering thermal energy to the power-cycle working fluid. Recent work has identified moving packed-bed heat exchangers as low-cost alternatives to fluidized-bed heat exchangers, which require additional pumps to fluidize the particles and recuperators to capture the lost heat. However, the reduced heat transfer between the particles and the walls of moving packed-bed heat exchangers, compared to fluidized beds, causes concern with adequately sizing components to meet the thermal duty. Models of moving packed-bed heat exchangers are not currently capable of exploring the design trade-offs in particle size, operating temperature, and residence time. The present work provides a predictive numerical model based on literature correlations capable of designing moving packed-bed heat exchangers as well as investigating the effects of particle size, operating temperature, and particle velocity (residence time). Furthermore, the development of a reliable design tool for moving packed-bed heat exchangers must be validated by predicting experimental results in the operating regime of interest. An experimental system is designed to provide the data necessary for model validation and/or to identify where deficiencies or new constitutive relations are needed.
