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 solar-driven supercritical CO 2 (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 kW t shell-and-plate design was ultimately selected for construction and integration with Sandia’s falling particle receiver system.

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 (sCO 2 ). 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/sCO 2 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.

The implementation of efficient and cost effective thermal energy storage in concentrated solar power (CSP) applications is crucial to the wide spread adoption of the technology. The current push to high-temperature receivers enabling the use of advanced power cycles has identified solid particle receivers as a desired technology. A potential way of increasing the specific energy storage of solid particles while simultaneously reducing plant component size is to implement thermochemical energy storage (TCES) through the use of non-stoichiometric perovskite oxides. Materials such as strontium-doped lanthanum cobalt ferrites (LSCF) have been shown to have significant reducibility when cycling temperature and oxygen partial pressure of the environment [1]. The combined reducibility and heat of the oxidation and reduction reactions with the sensible change in temperature of the material leads to specific energy storage values approaching 700 kJ kg −1 . A potential thermochemical energy storage system configuration and modeling strategy is reported on, leading to a parametric study of critical operating parameters on the TCES subsystem performance. For the LSCF material operating between 500 and 900°C with oxygen partial pressure swings from ambient to 0.0001 bar, system efficiencies of 68.6% based on the net thermal energy delivered to the power cycle relative to the incident solar flux on the receiver and auxiliary power requirements, with specific energy storage of 686 kJ kg −1 are predicted. Alternatively, only cycling the temperature between 500 and 900°C without oxygen partial pressure swings results in TCES subsystem efficiencies up to 76.3% with specific energy storage of 533 kJ kg −1 .

One potentially attractive application of solid oxide fuel cells (SOFCs) is for combined heat and power (CHP) in light commercial buildings. An SOFC-based CHP system can be employed to efficiently serve building thermal and electric loads, thereby lowering utility bills and offering many distributed generation benefits. It is often desirable to operate SOFCs in a predominately base load manner from a hardware viewpoint. However, systems in practice will experience some load dynamics during their lifetime and furthermore, optimal economic dispatch of CHP systems frequently recommends a load-following strategy. Thus, the present work is motivated by the need to understand the dynamic response capabilities of SOFC-CHP systems. Part-load performance and dynamic load-following capabilities of a 24 kW planar SOFC system for light commercial applications was investigated through computational modeling. The SOFC and balance-of-plant component models were implemented in gPROMS modeling software. The modeling strategy of each system component and associated transients are discussed. A dynamic SOFC channel-level model, which has been verified against experimental cell data, was integrated with additional balance-of-plant (BOP) component models consisting of a fuel reformer, tail gas combustor, turbomachinery, heat exchangers, and bypass valves. The performance of the system at part-load operation displays increases in electrical efficiency and decreases in CHP efficiency, as well as a more uniform PEN temperature profile. Modeling comparisons between the responses of systems consisting of either dynamic or steady-state BOP component models are reported. A fully dynamic system-level model displays anodic fuel depletion effects and waste heat recovery transients not captured by the steady-state models. The dynamics influence the ability of an SOFC system to load follow indicating when thermal and electric storage may be necessary.