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.
This paper provides an overview of a next-generation particle-based concentrating solar power (CSP) system. The Gen 3 Particle Pilot Plant (G3P3) will heat particles to over 700 °C for use in high-temperature air or supercritical CO2 Brayton cycles with 6 hours of storage. The particles, which are inert, non-corrosive, durable, and inexpensive, are used as both the heat-transfer and storage media. Details of the operation, requirements, and design basis for the G3P3 system are presented, including a description of expected operational states and major components. Operational states include start-up, transients, steady-state operation, off-design conditions, and idling. The key components include the particle receiver, storage bins, heat exchanger, lift, and tower structure subsystems. Design bases and innovative features of each component are presented that will aid in achieving the desired cost and performance metrics.
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.
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.