Optimisation of power modulation for Hall-Héroult cells: Process operability and constraints as virtual energy storage

Aluminium is manufactured through the Hall-Héroult process, which is very energy intensive. Power modulation, as an industrial-scale demand-side power management approach, allows aluminium smelters to operate with variable power consumption rates and, as such, be powered by renewable energy sources. In this way, aluminium smelting cells can be used as a large virtual energy storage system to balance power demand and supply, and stabilise electrical grids. This paper studies the potential optimal power modulation operating conditions, including time-varying line current and anode-cathode distance (ACD) profiles, to maximise the profitability of the aluminium reduction cell, subject to constraints on the cell thermal balance. To deal with the complex cell dynamics which are spatially distributed and multi-timescale, a novel optimisation approach that utilises both reduced-order and detailed models is developed. The results yield insights into the optimal line current and ACD profiles for different power modulation scenarios, including the time of use electricity tariff and spot price. These results can form the foundation for further studies into online control policies of aluminium reduction cells.