Geometry optimisation of an industrial thermocline Thermal Energy Storage combining exergy, Life Cycle Assessment and Life Cycle Cost Analysis
Résumé
Recently, packed-bed storage has been considered as a promising alternative solution for thermal energy storage
especially for waste heat recovery in industrial plants. This work aims to optimise environmental footprint, costs
and exergy efficiency of a thermocline thermal energy storage through two optimisation variables. These vari-
ables describe the tank shape and the particle grain size. Two solid filler materials are compared: machined
ceramic and ceramic from fly ashes. The reference storage is an existing industrial high-temperature air/bauxite
packed-bed storage called Eco-Stock®. A one-dimensional two-phase (fluid and solid) model is used to determine
the energy and exergy performance of the thermocline tank. For the life cycle assessment, four indicators are
selected: cumulative energy demand, global warming potential, abiotic depletion potential and particulate
matter. Finally, a life cycle cost analysis is performed to determine levelised cost of energy used as economic
criterion. This multi-objective problem is solved by the multi-criteria genetic algorithm available on the Matlab®
platform. A Pareto set is obtained, bounded by the single exergy and environmental optimisation solutions. The
economic optimisation is found on the Pareto set, close to the environmental optimal solution. Favouring eco-
nomic performance reduces the environmental footprint of the storage. Despite better exergy performance and
smaller tank volume, the exergy-optimised tank increases environmental impacts and costs due to higher
pumping work. The environmental and economic optimisations lead to stocky tank shapes while a tapered tank is
obtained for the exergy optimisation. According to the TOPSIS method, the economic optimal solution appears to
be the best trade-off for both fillers tested. Despite poorer thermophysical properties, the solution with ceramic
from fly ashes shows similar exergy and economic performance as the machined ceramic solution (more than 96
% of exergy efficiency for 3.1 c€/kWhth), while the environmental footprint is greatly reduced (61 vs 87 ca.year).
This tank has a diameter of 2.6 m and a height of 1.7 m. The particle diameter is 11 mm.