Optimal trajectories for a liquid piston compressor/expander in a Compressed Air Energy Storage system with consideration of heat transfer and friction

Abstract

Abstract-For a Compressed Air Energy Storage (CAES)approach to be viable, the air compressor/expander must be sufficiently powerful and efficient. Since efficiency is governed by heat transfer, there is a generally a tradeoff between efficiency and compression/expansion time (or power). In this paper, we determine Pareto optimal compression/expansion profiles for a liquid piston air compressor/expander that maximizes efficiency (power) for a given power (efficiency). Compared to previous works, a numerical optimization approach is proposed that allows for more general heat transfer model, the consideration of the viscous friction, and system limitations in the optimization. The resulting optimal profiles are compared to other trajectories. At compression ratio of 10, the optimal profile results in 10%-40% increase in power density relative to other methods. Optimal geometries that trades off friction and heat transfer improvement can also be determined using this method.

Iterative optimal and adaptive control of a near isothermal liquid piston air compressor in a compressed air energy storage system

“…The interested reader is referred to [5] for a detailed mathematical modeling. Calculating the optimal compression profile can be addressed as a functional optimization problem (or optimal control problem) for which the cost function is the input work defined as follows.…”

“…Optimal trajectories have also been numerically obtained for cases with varying heat transfer coefficient, and considering the effect of liquid friction and flow rate constraints of the system [5]. Application of such optimal trajectories has been verified for a liquid piston C/E using CFD and the results have been compared with AIA and APIA trajectories.…”

“…Gas compression and expansion has many applications in pneumatic and hydraulic systems, including in the Compressed Air Energy Storage (CAES) system for offshore wind turbine that has recently been proposed in [2], [5]. In the proposed CAES system, high pressure (∼20-30MPa) compressed air is stored in a dual chamber storage vessel with both liquid and compressed air.…”

“…For high compression/expansions ratios 200:1-350:1, such Pareto optimal trajectories have been shown, theoretically, to increase the power of the C/E by 200-500% at the same efficiency, over ad-hoc trajectories such as linear and sinusoidal trajectories [3], [4], [5]. In [3], [4], the optimal trajectories were derived analytically based on simple heat transfer models and by considering thermodynamic losses alone.…”

“…In previous papers, we have shown that the compression or expansion trajectory can be optimized so that for a given power, the efficiency can be optimized and vice versa. Theoretically, for high compression ratios, the improvement over ad-hoc trajectories can be significant-for example, at the same efficiency of 90%, the power can be increased by 3-5 folds [2], [3], [4], [5]. Yet, the optimal trajectories depend on the heat transfer coefficient profile that is often unknown.…”