Ammonia Driven Reversible Solid Oxide Cell As Large-Scale Grid Energy Storage System

The global agreement on the reduction of greenhouses entails increased use of intermittent renewable energy sources such as wind and solar. In this contest, there is a huge need for efficient and cost-effective energy storage systems. Currently, different types of grid energy storage systems are under operation worldwide.

However, few of them have the capacity of storing several MWhs of electricity. Large-scale energy storage systems will play a vital role in grid stabilization by reducing imbalances between energy production and demand. Solid oxide cells (SOC) are promising and efficient technologies operating both on electrolyzer and fuel cell modes applicable for energy storage and production.

The efficiency of SOC can be further increase by utilization of excess high temperature steam from other processes like ammonia synthesis when it is operating as electrolyzer. In this way, part of the energy needed for electrlozer is covered and less input energy is required, which leads to higher efficiency.

This is the main reason behind combining solid oxide cells with the ammonia synthesis process. In the present work, we propose a large-scale electricity storage system operating with reversible solid oxide cells (RSOC) to store electricity as synthetic ammonia. Ammonia can be easily produced, when RSOC operates as an electrolyzer, and stored in the liquid form during off-peak demand.

During the peak demand, stored ammonia can be fed into the RSOC, when RSOC operates as a fuel cell, to produce green electricity. Besides the RSOC, the proposed system mainly consists of an air separation unit (ASU) and a Haber-Bosch loop (HBL). Besides large-scale storing of grid electricity, such a system can create an efficient link between electricity and ammonia markets.

The system performance is evaluated at a component level thermodynamic analysis showing that a round-trip storage efficiency higher than 50% is achievable.