Optimal Efficiency-Power Tradeoff for an Air Compressor/Expander

Abstract

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.

Compressed Air Energy Storage as a Battery Energy Storage System for Various Application Domains: A Review

“…This is a common problem with hydropneumatic devices when adapted for energy system applications. The trade-off effects grow worse when the pressure source is variable, as described in [85]. Instead of improving the efficiency, the authors of [86] developed maximum power point tracking (MPPT) for hydraulic pumps and motors for accumulator based CAES systems.…”

“…Compression/expansion pressure profile control is a strategy that can ensure that both the power density and energy converter efficiency metrics fall within the optimal Pareto front. The authors of [85] developed a compression trajectory that uses a generalized heat transfer coefficient h and the surface area A as constants. In reality, the LP's heat transfer does not operate with such a constant hA parameter.…”

“…As fluid enters or leaves the chamber, it covers or exposes part of the porous media's contact area, which changes the heat transfer rate in relation to the dynamic hA value. To account for these changes, the authors of [131] took into consideration a varying volume to use an adiabatic-isothermal-adiabatic (AIA) trajectory and predicted a power increase of 5-15 times at 90% efficiency over ad hoc sinusoidal or linear trajectories that used constant hA values in an earlier study [85]. In [132], the focus of trajectory control was the varying heat coefficient, the effect of fluid friction through the porous medium, and other flow constraints.…”

Experimental Investigation on the Performance of Compressors for Small-Scale Compressed Air Energy Storage in Parallel Mode

“…Experimental results indicated that when the plate inserts with a height of 0.2 m, 0.35 m, and 0.5 m, the compression efficiency could be increased by approximately 3%, 4%, and 8%, respectively. Rice et al [25] proposed an optimal trajectory control method to solve the trade-off between power density and efficiency. The results showed that the proposed solutions could improve the power density and keep efficiency unchanged.…”