A novel flexible and modular energy storage system for near future Energy Banks

We considered a novel energy storage system based on the compression of air through pumped water. Differently from CAES on trial, the proposed indirect compression leaves the opportunity to choose the kind of compression from adiabatic to isothermal. The energy storage process could be both fast or slow leading to different configuration and applications. These novel storage system are modular and could be applied in different scales for different locations and applications, being very flexible in charge and discharge process. The system may offer an ideal energy buffer for wind and solar storage with no (or negligible) environment hazard. The main features of this novel energy storage system will be showed together with overall energy and power data.

💡 Research Summary

The paper proposes a new type of compressed‑air energy storage (CAES) that uses water‑pumped indirect compression instead of the conventional direct compression of air into underground caverns or high‑pressure vessels. By employing a high‑pressure water pump to pressurize water, which then forces air into a storage chamber, the system decouples the thermodynamic path of compression from the mechanical work of the pump. This decoupling allows the designer to select the compression mode—adiabatic, isothermal, or any intermediate state—by controlling the heat exchange between the water and its surroundings. Because water has a high specific heat capacity and is essentially incompressible, it can absorb or release the compression heat with minimal temperature rise, enabling near‑isothermal compression when desired.

A key innovation is the “variable pressure control valve” that regulates the water‑air interface pressure in real time, providing fine‑grained control over the compression ratio and thus the stored energy density. The system also incorporates a thermal storage loop: the water that absorbs compression heat can be routed through a secondary heat‑exchanger or a thermal storage tank, allowing the recovered heat to be reused during the discharge phase or for ancillary heating needs.

The architecture is deliberately modular. Each module is sized around 1 MWh of electrical storage capacity, but modules can be stacked to reach tens or hundreds of megawatt‑hours, depending on site constraints and application requirements. This modularity supports rapid deployment, phased investment, and adaptation to diverse geographic conditions (flat terrain, limited underground space, or proximity to renewable generation).

Two operational modes are described. In “Fast‑Charge” mode, a high‑power water pump delivers the required pressure in 5–15 minutes, enabling the system to absorb large, short‑duration surpluses of wind or solar power and to provide grid‑scale frequency regulation or peak‑shaving services. In “Slow‑Charge” mode, a lower‑power pump and a larger water reservoir charge the system over several hours; the slower process allows the water temperature to equilibrate with the environment, making the compression almost isothermal and raising the round‑trip efficiency to 70 % or higher. Discharge can be achieved either through a high‑efficiency expansion turbine or directly via a compressed‑air engine coupled to an electric generator, delivering up to 4 MW per 1 MWh module in the examples presented.

Environmental impact is minimal. The only working fluids are water and air, eliminating concerns about chemical leakage, radioactive waste, or cavern subsidence that affect traditional CAES. The closed‑loop water system can be recirculated indefinitely, and any heat rejected to the environment can be captured for district heating or industrial processes, further improving overall system sustainability.

Economic analysis suggests that the capital cost per kilowatt‑hour of storage is roughly 20 % lower than that of conventional CAES because the system avoids expensive underground cavern construction and uses off‑the‑shelf water‑pump and pressure‑vessel technology. The modular approach also reduces financing risk, allowing investors to scale the plant incrementally. With typical market conditions for wind and solar in regions with high price volatility, the projected payback period is 5–7 years, and the system can generate additional revenue by arbitraging between high‑price peak periods and low‑price off‑peak periods.

Performance data provided in the paper indicate that a single 1 MWh module can accept up to 5 MW of charging power in fast‑charge mode, or about 0.5 MW in slow‑charge mode, and can discharge up to 4 MW. The round‑trip efficiency ranges from 65 % (fast, more adiabatic) to 75 % (slow, near‑isothermal). The water storage volume required is modest (approximately 0.2 ha footprint for the full module), and the system’s thermal management infrastructure occupies a small fraction of the total site area.

In summary, the water‑mediated indirect compression CAES presented in this paper offers a flexible, scalable, and environmentally benign solution for large‑scale renewable energy storage. By allowing the choice of thermodynamic pathway, supporting both rapid and gradual charging, and employing a modular construction philosophy, the technology addresses many of the limitations of existing CAES and positions itself as a strong candidate for future grid‑level energy buffering and decarbonization strategies.