Air Compression Performance Improvement via Trajectory Optimization: Experimental Validation

Abstract

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.

Optimal Efficiency-Power Tradeoff for an Air Compressor/Expander

“…The benefits of applying optimized compression trajectories have been validated experimentally in Refs. [25] and [26] for the low pressure (10 bar) and the high pressure (200 bar) settings, respectively. Optimal trajectories were shown to double the power densities for the same efficiency or to increase efficiency by 4-5% for the same power.…”

Automation and Electrical Actuation of a Compressed Air Energy Storage Installation

“…Instalațiile de scară mare au o eficiență foarte redusă, fiind viabile comercial numai în regiunile în care prețul energiei electrice variază în mod dramatic. Ciclurile de comprimare și detentă a aerului introduc pierderi prin efecte termice, rezultând o eficiență limitată [8,9], care poate fi însă îmbunătățită prin introducerea unor sisteme de stocare termică [5,6]. Construcția acestor instalații este limitată și de topografia terenului, având cerințe speciale în ce privește structura geologică [10].…”