Lab-Scale Prototype of a Thermochemical Energy Storage System: Assembly and Experimental Investigation

Abstract

Thermal Energy Storage (TES) has the potential to enable 24/7 production of clean, and infinitely abundant solar energy. Vast efforts are being made to achieve cost-competitiveness with other forms of energy. In this work, Thermochemical Energy Storage (TCS), which stores thermal energy in the form of chemical bonds, is experimentally investigated for its potential for improving the efficiency and economics of the traditional TES systems. The investigation involved the design optimization and construction of a lab-scale prototype of a thermochemical energy storage system based on the reduction - oxidation reactions of the copper oxide at ~1025°C in air. Copper oxide is selected for its relatively high energy density, and used in the form of granules mixed with an anti-sintering agent. The lab-scale prototype of a TCS was designed in a process that simulates the heat supply from a solar receiver, using either air or nitrogen both as heat-transfer fluid (HTF) and reactive gas. The lab-scale prototype was operated over 30 charging and discharging cycles by switching between the flow of "hot" 1100°C and "cold" 870°C air through a packed bed of CuO-based granules. 106.8 grams of CuO-based granules were charged and discharged achieving round-trip efficiencies up to 35.5%. Full conversion of the reactants was consistently accomplished, and the reaction rate did not change for the identically operated cycles; overall displaying no signs of degradation. Subsequent inspection showed that the packed bed sintered into a single block, although it is not clear from the measurements to what extent affected the reaction. Finally, isothermal operation of the lab-scale prototype reduced and oxidized the CuO-based granules by switching the flow between nitrogen and air, respectively, clearly evidencing the heat release and absorption by the produced temperature peaks (~1018.8°C) and valleys (~869°C), on an otherwise isothermal operation (900°C).