A review of the case for modern-day adoption of hydraulic air compressors

Abstract

Despite 18 reported commercial large scale deployment cases of hydraulic air compressors (HACs), the technology has fallen into demise. This paper opens by explaining that many of the reasons for this are no longer relevant in a modern context. The operating principles of HACs are reviewed and a hydrodynamic formulation is outlined so that HAC performance can be assessed by means of simulation. Simulation results confirm that HACs practically offer a close-to-isothermal gas compression and so still offer large scale gas compression utility with lower energy consumption in comparison to modern-day compression plant, even if decoupled from their opportunistic utilization of natural hydropower resources. Failure to properly account for the pressure solubility of gases in water leads to the erroneously high estimates of around 83% for the HAC mechanical efficiency for the two largest HACs built, at Victoria, MI, USA and at Ragged Chutes, ON, Canada. While still remaining only weakly coupled to the hydrodynamic simulator, simulations based on reported performance of the historical HACs, that now account for gas solubility, result in downward revision of mechanical efficiency to around 64%. Simulation results further indicate that for HACs where water is recirculated, mechanical efficiencies may be able to be increased from the new, lower estimates by i) prior solution of a salt in the circulating water or ii) increasing the temperature of HAC operation. Both measures have the effect of reducing gas solubility and hence increasing compressed air yield. An examination of the modern day economic case for HAC technology using discounted cash flow techniques concludes that they may be able to compete economically with large scale multi-stage centrifugal compressors. Two new potential application areas for HACs are presented. Firstly a deep mine ventilation air cooling concept is explained and secondly, gas solubility, that hitherto has proved problematic for HACs, is turned to advantage in a carbon capture concept.