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.
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.
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.
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
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.
Imponenti, Luca; Albrecht, Kevin J.; 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.