Joint project "Electricity storage via adiabatic air compression"

The integration of fluctuating renewable energy sources requires bulk storage of electricity to ensure grid stability and to match supply and demand. At present, PHES covers 99% of the world's bulk storage as measured by power. The main advantages of PHES are high efficiencies of 80-90% and comparatively low environmental impacts as measured by global warming potential and high ratios of energy stored over energy invested. The main disadvantages are environmental opposition, site-dependent capital costs, and uncertainty about the profitability in markets with high shares of fluctuating renewable energy sources. An alternative to PHES is CAES, which in its diabatic form is proven through the plants in Huntorf, Germany (321 MWel, since 1978) and McIntosh, USA (110 MWel, since 1991). Both plants store the compressed air in salt caverns. The main drawbacks of diabatic CAES are low efficiencies of 45-50% caused by rejecting the heat of compression and the need to resupply the rejected heat by the burning of fossil fuels. AA-CAES retains the heat of compression in a thermal-energy storage (TES), avoiding the need to burn fossil fuels and allowing efficiencies of 65-75% to be attained.

To study AA-CAES with a combined sensible/latent TES and rock caverns through simulations and experiments and to carry out a sustainability analysis in terms of environmental and economic factors.

1. The technical feasibility of AA-CAES was demonstrated through experiments with the world's first pilot plant. The plant used a combined sensible/latent TES and a rock cavern and attained estimated efficiencies of 63-74%.
2. The combined TES was tested at temperatures of up to 566°C. The latent section reduced the drop in the outflow temperature during discharging.
3. The degradation of the encapsulated phase-change material (PCM) was experimentally and numerically characterised and mitigation strategies based on a ceramic diffusion barrier were developed.
4. A numerical model that simulates the dynamic behaviour of a complete AA-CAES plant was developed. The model includes realistic sub-models for the main plant components.
5. AA-CAES and PHES have similar global warming and ecosystem quality scores. The economic performance of AA-CAES plants depends strongly on assumptions.

Implications for research

1. AA-CAES is technically feasible, but further experiments on cavern tightness and long-term TES cycling studies are required.
2. The numerical model shows that sliding-pressure operation increases the plant efficiency, but also poses challenges for the operation of existing turbines.
3. Alternative encapsulation/PCM couples should be investigated to understand if additional or different approaches can suppress or reduce degradation.

Implications for practice

1. The reduced drop in the outflow temperature of the combined TES simplifies the operation of the turbine in an AA-CAES plant.
2. The economic performance of AA-CAES plants requires further investigation.
3. The encapsulation/PCM degradation mitigation strategies can be applied to other industrial processes.

High-Temperature Combined Sensible/Latent-Heat Storage Based on Novel Materials for Electricity Storage Using Advanced Adiabatic Compressed Air Energy Storage

The joint project consists of three research projects

Design and Optimization of High-Temperature Combined Sensible/Latent-Heat Storage

• Dr. Andreas Haselbacher, Departement für Maschinenbau und Verfahrenstechnik, ETH Zürich; Dr. Peter Burgherr

Analysis of AA-CAES cycles exploiting Combined Sensible/Latent Thermal Energy Storage and Novel Materials

• Dr. Maurizio Barbato, Dipartimento Tecnologie Innovative (DTI), Scuola universitaria professionale della Svizzera italiana, Manno; Dr. Peter Burgherr, Paul Scherrer Institut

Aluminium-silicon based phase change material structures for high-temperature latent heat storage

• Prof. Sophia Eva Martha Haussener, Laboratoire de la science et de l'ingénierie de l'énergie renouvelable, EPF Lausanne; Dr. Peter Burgherr, Prof. Andreas Mortensen, Dr. Ludger Weber