Small-Scale Compressed Air Energy Storage: a Techno-Economic and Environmental Perspective

The increasing penetration of variable renewable energy sources required to meet international decarbonization targets, such as those established by the Paris Agreement, calls for energy storage technologies capable of providing flexibility, reliability, and long-duration energy shifting. While electrochemical batteries are widely deployed for short-term storage and fast-response applications, they cannot represent the sole solution to global energy storage needs. In particular, the large-scale deployment of stationary battery systems may be constrained by the limited availability and production capacity of critical raw materials. Consequently, the development of a diversified portfolio of energy storage technologies is essential. Within this context, Compressed Air Energy Storage (CAES) represents a potentially attractive option, especially for decentralized and resource-constrained energy systems. This thesis presents a comprehensive assessment of small-scale CAES systems, focusing on realistic system architectures based on commercially available components and operated under time-varying conditions. A dynamic modeling framework was developed in the OpenModelica environment and validated against experimental data obtained from an industrial micro-CAES prototype. Building on this framework, the analysis was extended through an integrated sensitivity analysis and surrogate-based optimization approach implemented using the Dakota toolkit. This methodology enabled a systematic exploration of the design space and an evaluation of the influence of key design parameters on overall system performance. The results show that small-scale CAES systems are generally characterized by moderate round-trip efficiencies, typically in the range of 30–40%. Moreover, the analysis demonstrates that realistic load variability has a significant impact on system efficiency, with reductions of up to 10–15 percentage points observed under highly fluctuating demand profiles. These findings highlight the importance of accounting for realistic operating conditions in the design and assessment of CAES systems. To assess the feasibility of small-scale CAES, this thesis presents a comprehensive techno-economic and life-cycle assessment of micro-CAES systems based on standard volumetric machines, integrated into hybrid photovoltaic–diesel microgrids for remote and off-grid communities. Parametric life cycle inventories were developed as a function of system size, enabling consistent and scalable environmental evaluations across different configurations and contributing to fill the gap in environmental assessments for small-scale CAES identified in the literature. The results indicate that the integration of CAES can substantially reduce greenhouse gas emissions, with CO2 emissions decreasing by more than 60% compared to systems where photovoltaic generation is not coupled with energy storage. However, economic competitiveness remains strongly dependent on local solar resource availability and fuel prices. In the locations analyzed, the levelized cost of energy associated with storage integration ranges from an increase of approximately 0.03 $/kWh to a reduction of about 0.05 $/kWh, with the most favorable outcomes observed in scenarios characterized by high solar availability and elevated fuel costs. Alternative system configurations based on radial turbomachinery were also investigated to assess their potential at small scale. The analysis indicates that turbomachinery-based solutions may achieve higher round-trip efficiencies, on the order of 50–60%, and lower storage costs when deployed at sufficiently large system sizes. However, a scale-dependent transition is identified, below which performance degradation and economic penalties limit their applicability. The analysis is finally extended to grid-connected small-scale CAES systems to assess their role under variable electricity market conditions. Price-arbitrage-oriented operating strategies are evaluated as a function of system size and market dynamics. The results show that price arbitrage alone is generally insufficient to ensure economic viability for small- and medium-scale CAES systems, which becomes achievable only in electricity markets with extreme intraday price spreads or when combined with cost-reduction measures and regulatory incentives. These findings point to the need for alternative operational paradigms to improve system profitability. Overall, this work provides an integrated modeling, optimization, and life-cycle assessment framework for the analysis of small-scale CAES systems, offering quantitative insights into their performance, environmental impacts, and design trade-offs. By explicitly accounting for dynamic operation, off-design component behavior, and realistic operating conditions, the thesis contributes to a clearer understanding of the potential role of micro-CAES in decentralized energy systems with high shares of variable renewable energy sources.