Techno-economic study of solid cycles for thermochemical energy storage and carbon capture

Abstract

A techno-economic analysis of the calcium looping cycle for combined thermochemical energy storage and carbon capture is presented within this work. A steady-state process model was developed using Aspen+ software, following the simulation of both an open-loop cycle with continual system clearing, and a closed-loop cycle with a different targeted number of particle cycles as well as different carbon capture rates. The model computes the mass and energy balances of the system under different conditions, which are consecutively used to evaluate the economic potential of the different cases studied. For the cases presented within this thesis, cycling the solid material 9 times prior to discharge provided the best economic results, with the most favourable NPV output over the plant lifetime, although the net energy conversion was lower (~16% reduction) than that of a system with 1 solids cycle prior to system discharge. The trend indicates that past a certain point - in this work Ncyc=3 - the NPV of the system rises with increasing particle cycle number. To maintain particle activity whilst increasing solids cycling number a continual make up flow is required, the purge and make-up flows increase in size with rising cycle number. The costs associated with these flows can be compared to the cost of clearing the system following a lesser number of cycles, demonstrating that increasing the number of solid-cycles within the system should eventually reduce the operational expenditure. A further sensitivity analysis was undertaken to determine the impact of varying carbon capture rates on the economic potential of the process. Of the studied cases a carbon capture rate of 0.95 presented the best economic results for a particle cycle number of 1, whilst a carbon capture rate of 0.9 proved more profitable for the higher particle cycle number of 9, given the economic assumptions made within the work. The trends presented within the carbon capture sensitivity analysis indicate the importance of the economic inputs, such as carbon tax and electricity prices, on determining the optimum capture rate, similarly indicating that the optimum capture rate will vary depending on the number of solids cycles the material undergoes. The variation in carbon capture rate presented within this work highlights the economic potential of solids storage (and hence energy storage) versus carbon tax profits, the model can be adjusted to develop further research determining optimum values for different cases