Air Compression Performance Improvement via Trajectory Optimization: Experimental Validation

In an isothermal compressed air energy storage (CAES) system, it is critical that the high pressure air compressor/expander is both efficient and power dense. The fundamental trade-off between efficiency and power density is due to limitation in heat transfer capacity during the compression/expansion process. In our previous works, optimization of the compression/expansion trajectory has been proposed as a means to mitigate this trade-off. Analysis and simulations have shown that the use of optimized trajectory can increase power density significantly (2–3 fold) over ad-hoc linear or sinusoidal trajectories without sacrificing efficiency especially for high pressure ratios. This paper presents the first experimental validation of this approach in high pressure (7bar to 200bar) compression. Experiments are performed on an instrumented liquid piston compressor. Correlations for the heat transfer coefficient were obtained empirically from a set of CFD simulations under different conditions. Dynamic programming approach is used to calculate the optimal compression trajectories by minimizing the compression time for a range of desired compression efficiencies. These compression profiles (as function of compression time) are then tracked in a liquid piston air compressor testbed using a combination of feed-forward and feedback control strategy. Compared to ad-hoc constant flow rate trajectories, the optimal trajectories double the power density at 80% efficiency or improve the thermal efficiency by 5% over a range of power densities.

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Optimal Control Experimentation of Compression Trajectories for a Liquid Piston Air Compressor

Volume 1: Heat Transfer in Energy Systems; Thermophysical Properties; Theory and Fundamental Research in Heat Transfer ◽

10.1115/ht2013-17613 ◽

2013 ◽

Cited By ~ 1

Author(s):

Farzad A. Shirazi ◽

Mohsen Saadat ◽

Bo Yan ◽

Perry Y. Li ◽

Terry W. Simon

Keyword(s):

Heat Transfer ◽

Heat Transfer Coefficient ◽

Transfer Coefficient ◽

Power Density ◽

Heat Transfer Model ◽

Transfer Model ◽

Compression Efficiency ◽

Air Compressor ◽

Compression Time ◽

Liquid Piston

Air compressor is the critical part of a Compressed Air Energy Storage (CAES) system. Efficient and fast compression of air from ambient to a pressure ratio of 200–300 is a challenging problem due to the trade-off between efficiency and power density. Compression efficiency is mainly affected by the amount of heat transfer between the air and its surrounding during the compression. One way to increase heat transfer is to implement an optimal compression trajectory, i.e., a unique trajectory maximizing the compression efficiency for a given compression time and compression ratio. The main part of the heat transfer model is the convective heat transfer coefficient (h) which in general is a function of local air velocity, air density and air temperature. Depending on the model used for heat transfer, different optimal compression profiles can be achieved. Hence, a good understanding of real heat transfer between air and its surrounding wall inside the compression chamber is essential in order to calculate the correct optimal profile. A numerical optimization approach has been proposed in previous works to calculate the optimal compression profile for a general heat transfer model. While the results show a good improvement both in the lumped air model as well as Fluent CFD analysis, they have never been experimentally proved. In this work, we have implemented these optimal compression profiles in an experimental setup that contains a compression chamber with a liquid piston driven by a water pump through a flow control valve. The optimal trajectories are found and experimented for different compression times. The actual value of heat transfer coefficient is unknown in the experiment. Therefore, an iterative procedure is employed to obtain h corresponding to each compression time. The resulted efficiency versus power density of optimal profiles is then compared with ad-hoc constant flow rate profiles showing up to %4 higher efficiency in a same power density or %30 higher power density in a same efficiency in the experiment.

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Effect of Moisture on the Efficiency and Power Density of a Liquid Piston Air Compressor/Expander

10.1115/dscc2016-9884 ◽

2016 ◽

Cited By ~ 1

Author(s):

Anirudh Srivatsa ◽

Perry Y. Li

Keyword(s):

Heat Transfer ◽

Heat Capacity ◽

Power Density ◽

Water Evaporation ◽

Water Droplets ◽

Flow Rates ◽

Trade Off ◽

Air Compressor ◽

Apparent Heat Capacity ◽

Effect Of Moisture

For a compressed air energy storage (CAES) system to be competitive for the electrical grid, the air compressor/expander must be capable of high pressure, efficient and power dense. However, there is a trade-off between efficiency and power density mediated by heat transfer, such that as the process time increases, efficiency increases at the expense of decreasing power. This trade-off can be mitigated in a liquid (water) piston air-compressor/expander with enhanced heat transfer. However, in the past, dry air has been assumed in the design and analysis of the compression/expansion process. This paper investigates the effect of moisture on the compression efficiency and power. Evaporation and condensation of water play contradictory roles — while evaporation absorbs latent heat enhancing cooling, the tiny water droplets that form as water condenses also increase the apparent heat capacity. To investigate the effect of moisture, a 0-D numerical model that takes into account the water evaporation/condensation and water droplets have been developed. Results show that inclusion of moisture improves the efficiency-power trade-off minimally at lower flow rates, high efficiency cases, and more significantly at higher flow rates, lower efficiency cases. The improvement is primarily attributed to the increase in apparent heat capacity due to the increased propensity of water to evaporate.

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Effects of porous media insert on the efficiency and power density of a high pressure (210 bar) liquid piston air compressor/expander – An experimental study

Applied Energy ◽

10.1016/j.apenergy.2017.12.093 ◽

2018 ◽

Vol 212 ◽

pp. 1025-1037 ◽

Cited By ~ 13

Author(s):

Jacob Wieberdink ◽

Perry Y. Li ◽

Terrence W. Simon ◽

James D. Van de Ven

Keyword(s):

Experimental Study ◽

High Pressure ◽

Porous Media ◽

Power Density ◽

Air Compressor ◽

Liquid Piston

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Combined Optimal Design and Control of a Near Isothermal Liquid Piston Air Compressor/Expander for a Compressed Air Energy Storage (CAES) System for Wind Turbines

Volume 2: Diagnostics and Detection; Drilling; Dynamics and Control of Wind Energy Systems; Energy Harvesting; Estimation and Identification; Flexible and Smart Structure Control; Fuels Cells/Energy Storage; Human Robot Interaction; HVAC Building Energy Management; Industrial Applications; Intelligent Transportation Systems; Manufacturing; Mechatronics; Modelling and Validation; Motion and Vibration Control Applications ◽

10.1115/dscc2015-9957 ◽

2015 ◽

Cited By ~ 5

Author(s):

Mohsen Saadat ◽

Perry Y. Li

Keyword(s):

Energy Storage ◽

Power Density ◽

Compressed Air ◽

Compressed Air Energy Storage ◽

Expansion Chamber ◽

Trade Off ◽

Air Compressor ◽

Energy Storage Technology ◽

And Control ◽

Liquid Piston

The key component of Compressed Air Energy Storage (CAES) system is an air compressor/expander. The roundtrip efficiency of this energy storage technology depends greatly on the efficiency of the air compressor/expander. There is a trade off between the thermal efficiency and power density of this component. Different ideas and approaches were introduced and studied in the previous works to improve this trade off by enhancing the heat transfer between air and its environment. In the present work, a combination of optimal compression/expansion rate, optimal chamber shape and optimal heat exchanger material distribution in the chamber is considered to maximize the power density of a compression/expansion chamber for a given desired efficiency. Results show that the power density can be improved by more than 20 folds if the optimal combination of flow rate, shape and porosity are used together.

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Heat Transfer in a Long, Thin Tube Section of an Air Compressor: An Empirical Correlation From CFD and a Thermodynamic Modeling

Volume 7: Fluids and Heat Transfer, Parts A, B, C, and D ◽

10.1115/imece2012-86673 ◽

2012 ◽

Cited By ~ 3

Author(s):

Chao Zhang ◽

Mohsen Saadat ◽

Perry Y. Li ◽

Terrence W. Simon

Keyword(s):

Heat Transfer ◽

Heat Flow ◽

Flow Rate ◽

Dimensionless Number ◽

Transfer Model ◽

Heat Flow Rate ◽

Air Compressor ◽

Thin Tube ◽

Linear Ode ◽

Liquid Piston

Heat transfer during compression of air in a long, thin tube is studied by CFD. The tube represents one of the many in a honeycomb geometry inserted in a liquid piston air compressor to minimize temperature rise. A dimensionless number for the heat flow rate that includes the changing heat transfer area between the tube wall and air during compression is used. From the CFD results, alinear relation between the inverse of this dimensionless heat flow rate and the Stanton number is found. Using thisrelation, the transient volume-averaged temperature, and heat flow rate from the air can be well predicted by thermodynamic modeling.With the heat transfer model, a non-linear ODE is solved numerically todetermine the average temperature and pressure. The application of this study can be found in liquid piston air compressors for compressed air energy storage systems.

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Optimal Efficiency-Power Tradeoff for an Air Compressor/Expander

Journal of Dynamic Systems Measurement and Control ◽

10.1115/1.4037652 ◽

2017 ◽

Vol 140 (2) ◽

Cited By ~ 2

Author(s):

Andrew T. Rice ◽

Perry Y. Li ◽

Caleb J. Sanckens

Keyword(s):

Heat Transfer ◽

Ad Hoc ◽

Expansion Chamber ◽

Compression Efficiency ◽

Air Compressor ◽

Air Volume ◽

Compression And Expansion ◽

Optimal Efficiency ◽

The Heat Transfer Coefficient

An efficient and power dense high pressure air compressor/expander (C/E) is critical for the success of a compressed air energy storage (CAES) system. There is a tradeoff between efficiency and power density that is mediated by heat transfer within the compression/expansion chamber. This paper considers the optimal control for the compression and expansion processes that provides the optimal tradeoff between efficiency and power. Analytical Pareto optimal solutions are developed for the cases in which hA, the product of the heat transfer coefficient and heat transfer surface area, is either a constant or is a function of the air volume. It is found that the optimal trajectories take the form “fast-slow-fast” where the fast stages are adiabatic and the slow stage is either isothermal for the constant-hA assumption, or a pseudo-isothermal (where the temperature depends on the instantaneous hA) for the volume-varying-hA assumption. A case study shows that at 90% compression efficiency, power gains are in the range of 500−1500% over ad hoc linear and sinusoidal profiles.

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