As outlined in the work programme, “Europe needs to develop and mature the next generation of competitive technologies and services for the distribution grid (…) beyond the state of the art and will be ready to integrate the market in five to ten years' time”. It is crucial that such storage technologies has the potential to scale up for decentralized, safe, efficient and affordable application. The BAoBaB project introduces a new generation of competitive electrical energy storage technologies, based on pH and salinity gradients, that allows to address this specific challenge. Salinity gradients were already recognized as a renewable energy source back in the 1950s, as the power due to osmotic force and pH-gradients is an order of magnitude larger than obtained from the same volume in the existing hydropower storage technology. While the membrane-based conversion technologies were not available at that time, technologies to convert salinity gradient energy into electricity are now available, which nowadays is called “Blue Energy”. A reversible or rechargeable technology for electrical energy storage in pH and salinity gradients, however, has not yet been developed.
In specific, the BAoBaB technology has the groundbreaking potential for widespread, decentralized electricity storage at timescales of days, thereby enabling an increasing share of fluctuating renewable energy sources without compromising the grid stability and security. Compared to existing storage technologies it has:
- Higher availability and scalability. Pumped Hydropower Storage and Compressed Air Energy Storage – the only commercially available technologies capable of providing very large storage capacities – are obviously limited by geographical requirements. Moreover, they can hardly be applied decentralized, thereby requiring an intensified electricity grid. At the same time, the operational safety of large traditional batteries is a concern for their deployment in urban areas or in proximity. Salt water reservoirs needed for BAoBaB are not site-specific and can be placed anywhere (no safety issues and no geographical constraints, except for the space needed) and in different scales (from kWh to hundreds of MWh).
- Higher durability. Except for Pumped Hydropower Storage with typical lifetimes of 40-60 years, the lifetime of existing energy storage systems such as batteries is short, with corresponding limitations for life cycle energy/resource consumption and generating revenues. Metal-ion batteries suffer intrinsically from electrode pulverization, recrystallization, or crossover of metal ions in redox flow batteries, which all cause irreversible degradation of the electrolytes and hence loss in capacity. In BAoBaB, degradation of the salt solutions is not possible as they do not differ in composition, only in concentration. Moreover, the system is tolerant to contaminants (e.g., dissolved air or other salts). From laboratory experiments, we know that high material lifetimes (>10 years) are achieved for membranes, electrodes and current collectors, due to the controlled environment in a closed system.
- Tuneable capacity to power ratio. As the BAoBaB system is a flow battery, the energy storage capacity can be tuned independent of the conversion power, in contrast to traditional Li-ion, metal-hydride, lead-acid or metal-air batteries. In particular, because the storage medium of the BAoBaB system (e.g., water and salt) is orders of magnitude cheaper than that of any other batteries including redox flow batteries, the regime for medium- to long-term electricity storage becomes available. This market segment is especially relevant for smart grids as renewable energy sources feature fluctuations at typical time scales of several days to weeks.
- Higher performance. The BAoBaB system has an energy density (7 kWh/m3) which is at least one order of magnitude larger than that of Pumped Hydropower Storage. The energy can be stored over a long time, virtually without self-discharge as the majority of the salt solutions are stored in separated reservoirs. Conversion efficiencies of 80% can be obtained, comparable to Pumped Hydropower Storage. The BAoBaB system can be used for time shifting loads as it is capable of discharge times in hours to multiple days. A ramp rate in MW/minute is obtainable, making BAoBaB suitable for moderating the variability of wind and PV systems. The operational window in terms of kinetic and mass transfer limitations (e.g., the temperature dependency) will be addressed in this project.
- Higher safety. Generally, the operational safety of large storage systems is a concern and will be a barrier in their deployment in urban areas or in proximity of other grid resources such as substations. The risks of a BAoBaB system are extremely low, as the system cannot be over-charged or over-discharged, the salt solutions and the electrodes are thermally stable, no exothermic reactions are involved and hence there is no risk of a thermal runaway. The risk and impact of chemical spills is low, as water and salt are the main components. Environmental risks of the BAoBaB system are minimal and will be addressed in this project.
Lower costs. Regarding the salt water storage, the BAoBaB system has the potential to achieve very low capital cost levels per kWh storage (10-100 €/kWh). The capital cost levels per kW will depend on component development (low-cost membranes and stacks) as addressed in this project. The aimed potential levelized cost is <0.05 €/kWh/cycle (i.e., economically scalable without subsidies).