Conceptual Design of a Circulating Fluidized Bed Reactor as a Thermochemical Thermal Energy Storage System for Applications in Concentrated Solar Thermal Power Plants

Thesis (Master's)--University of Washington, 2016-08

The use of fossil fuels as an energy resource results in both the introduction of longline infrared-active gases into the atmosphere, and in the depletion of a finite energy resource. Neither of these results make the fossil energy option desirable in the long term. Clean renewable energy sources provide one means of avoiding these problems. Many renewable energy resources, however, suffer from the problem of varying in power during diurnal and annual cycles. Solar energy suffers from both predictable variations (e.g., the daily solar cycle) and unpredictable variations (e.g., cloud cover). This problem can be addressed by any of several proposed energy storage options. In the present work, we develop a thermochemical storage concept where a reversible Ca(OH)2/CaO reaction is used to store energy. The system is integrated with a central receiver tower solar power plant using two circulating fluidized bed reactors. One reactor is used for an energy absorption process during the day, where the excess energy is used to drive the endothermic Ca(OH)2 to CaO reaction. The other releases energy during the night using the reverse exothermic reaction. The energy absorbing reactor is placed in the tower where it receives heat reflected from the heliostat field where extended surfaces (fins) are used to improve heat flow into the fluidized bed. The energy discharging reactor is insulated using refractory with heat being absorbed by water/steam flowing inside water-walls that are in contact with the fluidized bed material. Conceptual design work suggests that one reactor will have a 7.2 m diameter and a the other will have a 8 m diameter, with a height of 37.5 m for both. It is estimated that the levelized cost of electricity (LCOE) will be 19 cents/kWh, which is higher than LCOE of a concentrated solar power (CSP) plant with no storage system, however, electricity with the thermochemical thermal energy storage (TES) system will be produced consistently for 24 hours overcoming the intermittency problem of renewable energy source.