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MAE PhD Defense – Corey Misenheimer
July 25, 2017 @ 12:00 pm - 2:00 pm
TITLE: Modeling Chilled-Water Storage System Components for Coupling to a Small Modular Reactor in a Nuclear Hybrid Energy System
DATE: Tuesday, July 25, 2017
TIME & LOCATION: 12:00 PM EB3 – 2302
ABSTRACT:
The intermittency of wind and solar power puts strain on electric grids, often forcing carbon-based and nuclear sources of energy to operate in a load-follow mode. Operating nuclear reactors in a load-follow fashion is undesirable due to the associated thermal and mechanical stresses placed on the fuel and other reactor components. Various methods of Thermal Energy Storage (TES) can be coupled to nuclear (or renewable) power sources to help absorb grid instabilities caused by daily electric demand changes and renewable intermittency. This is known as a Nuclear Hybrid Energy System (NHES).
During the warmer months of the year in many parts of the country, facility air-conditioning loads are significant contributors to the increase in the daily peak electric demand. Previous research uncovered that a stratified chilled-water storage tank can help displace peak cooling loads to off-peak hours. Based on these findings, the objective of this work is to evaluate the prospect of using a stratified chilled-water storage tank as a potential TES reservoir for a nuclear reactor in a NHES. This is accomplished by developing time-dependent models of chilled-water system components, including absorption chillers, cooling towers, a storage tank, and facility cooling loads synonymous with a large office space or college campus, as a callable FORTRAN subroutine. The resulting TES model is coupled to a high-fidelity mPower-sized Small Modular Reactor (SMR) Simulator, with the goal of utilizing excess reactor capacity to operate several sizable chillers in order to keep reactor power constant.
Chilled-water production via single effect, lithium bromide (LiBr) absorption chillers is primarily examined in this study, though the use of electric chillers is briefly explored. Absorption chillers use hot water or low-pressure steam to drive an absorption-refrigeration cycle. The mathematical framework of a high-fidelity dynamic absorption chiller model is presented. The transient FORTRAN model is grounded on time-dependent mass, species, and energy conservation equations. Due to the vast computational costs of the high-fidelity model, a low-fidelity absorption chiller model is formulated and calibrated to mimic the behavior of the high-fidelity model.
Stratified chilled-water storage tank performance is characterized using Computational Fluid Dynamics (CFD). The geometry employed in the CFD models is synonymous with a 5-million-gallon storage tank currently in use at a North Carolina college campus. Simulation results reveal the laminar numerical model most closely aligns with actual tank charging and discharging data. A subsequent parametric study corroborates storage tank behavior documented throughout literature and industry.
Two absorption chiller configurations are considered. The first involves bypassing low-pressure steam from the low-pressure turbine to absorption chillers during periods of excess reactor capacity in order to keep reactor power near steady-state. Simulation results demonstrate that steam conditions downstream of the turbine control valves are a strong function of turbine load, and absorption chiller performance is hindered by reduced turbine impulse pressures at reduced turbine demands.
A more suitable configuration entails integrating the absorption chillers into a flash vessel system that is thermally coupled to a sensible heat storage system. The sensible heat storage system is able to maintain reactor thermal output constant at 100% and match turbine output with several different electric demand profiles. High-pressure condensate in the sensible heat storage system is dropped across a let-down orifice and flashed in an ideal separator. Generated steam is sent to a bank of absorption chillers. Simulation results show enough steam is available during periods of reduced turbine demand to power four large absorption chillers to charge a 5-million-gallon stratified chilled-water storage tank, which is used to offset cooling loads in an adjacent facility. The coupled TES systems operating in conjunction with an SMR comprise the foundation of a tightly coupled NHES.
BIOGRAPHY:
Corey enrolled at North Carolina State University in 2009. At NC State, he matriculated into the Mechanical Engineering degree program, where he developed a keen interest for heat transfer, thermodynamics, and energy systems. After graduating summa cum laude with a Bachelor of Science in Mechanical Engineering in 2013, Corey continued his graduate studies at NC State under the direction of Dr. Stephen D. Terry investigating potential Thermal Energy Storage methods for use in Nuclear Hybrid Energy Systems. In 2017, he earned his Masters of Science in Mechanical Engineering while pursuing his Ph.D. His research focuses on developing time-dependent models of chilled-water storage system components to be used in conjunction with next-generation Small Modular Reactors (SMRs), with the goal of reducing thermal and mechanical stresses placed on the fuel elements and other reactor components during power maneuvers.
