ME 258 Johann Karkheck

The ability to store energy has become a necessity due to the intermittency of renewable energy sources that are gaining presence on the grid. Various technologies exist to accommodate a wide variety of storage needs.

CAES is capable of high power output with long discharge times. Issues with CAES Existing configurations are not very efficient 321 MW Huntorf, Germany ~ 42% 110 MW McIntosh, Alabama ~ 50% Storage Vessel Energy Losses

Modeled each system component separately to achieve a modular layout Analyze different CAES configurations to achieve maximum efficiency Configuration 1 – Single-stage turbo machinery components with polytropic processes Configuration 2 – Double-stage compression and heat exchangers Configuration 3 – Triple-stage compression and heat exchangers Configuration 4 – Single-stage turbo machinery w/isentropic compression/expansion and ideal motor and generator

Inter and after-cooling have the most detrimental effect on system efficiency The highest efficiency, 52-62% depending on heating and cooling load, is achieved in a two-stage adiabatic CAES configuration If cooling is done via a natural source (i.e. river) an efficiency of 60% is realistic and would not require supplemental fuel The key element to improve efficiency is development of a high temperature thermal storage (>600°C) and temperature resistant compressor materials

CAES can be implemented to mitigate the intermittency of renewable energy sources Thermal energy storage coupled with CAES eliminates the need for fossil fuel in reheating Evaluate the effect of thermal storage on the efficiency of CAES using thermodynamic modeling

Initial parameters were used to study the effect of thermal storage on CAES Inlet temperature of compressor and expander has greatest effect on thermal and power efficiency

The ratio of high storage pressure to the ambient pressure effects all work and heat transfer parameters Number of charge/discharge cycles does not effect these parameters Selection of appropriate pressure limits can raise power and thermal efficiency

Charge and discharge processes induce fluctuations in the pressure and temperature within the storage cavern Predictions of these fluctuations are required for proper cavern design and selection of turbo-machinery Numerical and approximate analytical solutions were used to model the T&P of the air cavern Sensitivity analysis was conducted to determine the dominant parameters that affect the T&P fluctuations and the required storage volume.

Heat transfer through cavern walls greatly effects T&P variations Preference should be given to caverns with high rock effusivity Losses can be reduced through reducing the injected air temperature Longer durations of charge and discharge can also aid in reducing losses

Numerical modeling was performed on coupled thermodynamic, multiphase fluid flow and heat transfer associated with CAES in lined rock caverns Using concrete lined caverns at a relatively shallow depth can reduce construction and operational costs if air tightness and stability can be assured

Numerical modeling was performed on coupled thermodynamic, multiphase fluid flow and heat transfer associated with CAES in lined rock caverns Using concrete lined caverns at a relatively shallow depth can reduce construction and operational costs if air tightness and stability can be assured

Models of both tight and leaky caverns show that the leakage rate increases over time resulting in increasing gas saturation of the lining and cavern wall Leaky storage caverns continue to diminish the achievable storage pressure over time

Use of lined rock caverns at a shallow depth is only feasible if air tightness and stability can be assured The key parameter to assure long term air tightness is the permeability of the concrete and surrounding rock Increasing the moisture content of the lining can also decrease air leakage Keeping the injection temperature close to the ambient cavern temperature can nearly eliminate thermal losses through the cavern walls

To avoid the deterioration of the cavern over time, configurations of a constant pressure water-compensated CAES system was studied The constant pressure system with a compensating water column requires a very deep air storage cavern to produce the required pressure, resulting in high construction costs

Using a hydraulic pump rather than elevation difference reduces necessary storage depth, but the pump consumes approximately 15% of the generated power Coupled compressed air and hydraulic storage tanks allow the compressed air to remain at constant pressure and energy to be produced by both CAES and hydraulic storage

System is independent of storage depth Exergy loss of the air in hydraulic storage during discharge can be reduced by spraying water into the air to achieve a quasi- isothermal process

Thermal storage improves CAES system efficiency and can negate the need for supplemental fuel during expansion Research is needed to find high temperature (>600°C) thermal storage and compressor materials Keeping the cavern inlet temperature near the storage temperature reduces losses due to heat transfer Air tightness of storage cavern is essential to retain long term system efficiency CAES coupled with hydraulic storage can make the depth of the storage cavern irrelevant and improve start up time of discharge cycle

1. The thermodynamic effect of thermal energy storage on compressed air energy storage system Zhang, Yuan (Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing , China); Yang, Ke; Li, Xuemei; Xu, Jianzhong Source: Renewable Energy, v 50, p , February 2013 Database: Compendex 2. Operating characteristics of constant-pressure compressed air energy storage (CAES) system combined with pumped hydro storage based on energy and exergy analysis Kim, Y.M. (ECO Machinery Division, Korea Institute of Machinery and Materials, 171 Jang-dong, Yuseong-gu, Daejeon , Korea, Republic of); Shin, D.G.; Favrat, D. Source: Energy, v 36, n 10, p , October 2011 Database: Compendex 3. Exploring the concept of compressed air energy storage (CAES) in lined rock caverns at shallow depth: A modeling study of air tightness and energy balance Kim, Hyung-Mok (Korea Institute of Geoscience and Mineral Resources (KIGAM), Daejeon , Korea, Republic of); Rutqvist, Jonny; Ryu, Dong-Woo; Choi, Byung-Hee; Sunwoo, Choon; Song, Won-Kyong Source: Applied Energy, v 92, p , April 2012 Database: Compendex 4. Temperature and pressure variations within compressed air energy storage caverns Kushnir, R. (School of Mechanical Engineering, Tel Aviv University, Tel Aviv 69978, Israel); Dayan, A.; Ullmann, A. Source: International Journal of Heat and Mass Transfer, v 55, n 21-22, p , October 2012 Database: Compendex 5. Simulation and analysis of different adiabatic Compressed Air Energy Storage plant configurations Hartmann, Niklas (University of Stuttgart, Institute of Energy Economics and the Rational Use of Energy (IER), Hebrühlstr. 49a, Stuttgart, Germany); Vöhringer, O.; Kruck, C.; Eltrop, L. Source: Applied Energy, v 93, p , May 2012 Database: Compendex