Date

8-1-2018

Document Type

Dissertation

Degree

Doctor of Philosophy

Department

Mechanical Engineering

First Adviser

Oztekin, Alparslan

Abstract

Thermochemical energy storage (TCES) is an attractive mode of storing the solar energy in the form of heat to be used with concentrated solar power (CSP) plants. Thermal energy is stored/released using a reverse endothermic/ exothermic reaction. TCES offers clear advantages over the other thermal energy storage (TES) options with sensible heat storage (SHS) and latent heat storage (LHS) in terms of energy density, storage time and the temperature range for thermal energy storage. However, this technique of storing the solar power at higher temperatures is still at laboratory or pilot scale, and researchers are trying to develop viable storage systems using potential candidate reaction systems for TCES. The reversible reaction system involving CaO/Ca(OH)2 has great potentials to be used as TCES with certain advantages, in terms of cost, availability, industrial feedback, long-term storage and charge-discharge temperature range, over the other reaction systems. Despite these advantages, the reaction system suffers from drawbacks which include lower thermal conductivity of the solid reaction materials. In this study, various reactor configurations are investigated with particular regard to the heat transfer to and from the bed during charging and discharging processes respectively. A mathematical model is developed including reaction kinetics and energy and mass transport within the reaction bed and heat transfer fluid (HTF). The model is solved numerically using finite elements and is applied to simulate various possible reactor configurations with fixed reaction beds. A reactor with a rectangular reaction bed heated and cooled by flat plate heat exchanger is considered first. The model is validated against the available experiments for this reactor configuration, and parametric studies are performed involving bed porosity and HTF flow velocity. The model is then applied to various other configurations with a circular reaction bed. Different heat exchanger designs for the circular reaction bed are studied and compared for various bed sizes and HTF flow conditions. Heat transfer enhancements (HTE) are introduced within the circular reaction bed to overcome the problem of lower thermal conductivity. It is found that the heat transfer within and to/from the reaction bed are important aspects in the design of any storage system, especially for the CaO/Ca(OH)2 reaction system where the thermal conductivity of the storage material is low. A circular bed designed with the heat transfer enhancements and heated/cooled with air flowing perpendicular to the bed axis is the most efficient configuration in terms of power ratings and simplicity of the operation. This design also allows easy upscaling of the reactor by increasing the number of the reaction beds and their size. The findings from this study would be quite useful in designing and optimizing TCES based on CaO/Ca(OH)2 or any other gas-solid reactions involving similar kinetics.

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