AIR SOLUBILITY AND ITS IMPACT ON THE BUSINESS CASE FOR HYDRAULIC AIR COMPRESSORS
The mechanical efficiency of a hydraulic air compressor is defined as the indicated work done on the air between the air intake and the air delivery, which may be taken to follow a polytropic process, divided by the input hydropower. For open and closed-loop hydraulic air compressors the input hydropower is replaced by electrically driven pump work input. The mechanical efficiency is thus a key technical determinant of the economic viability of hydraulic air compressors, and yet is one that analysis of the literature on the 18+ installations made worldwide shows is poorly understood.
Due to the solubility of atmospheric gases in water, as air passes through a hydraulic air compressor, so a fraction of the air dissolves in the liquid phase, and defines a parameter described as the compressed air yield which represents the fraction of input gas recovered at the compressor delivery. Across the historical fleet of hydraulic air compressors, the yield parameter ranges from 70% to 97%, and leads to mechanical efficiencies in the range 40% to 75%.
For closed-loop hydraulic air compressors, dissolving sodium sulphate in water a priori will reduce the solubility of atmospheric gases in water by a factor ~2 which in turn leads to yield values far closer to 100%, and mechanical efficiencies over 80%. Raising the operating temperature of the hydraulic air compressor from 25°C to 80°C also leads to a reduction of atmospheric gas solubility by a factor ~2, that is, it has a similar effect on solubility. Controlling the temperature controls the solubility which controls the efficiency. In effect, the mechanical efficiency can be chosen.
Comparative discounted cash flow analyses of hydraulic air compressors and multi-stage centrifugal compressors suggest that there is no credible price for compressed air at which a modern centrifugal compressor will economically outperform a hydraulic air compressor with a mechanical efficiency at or above 80%.