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ECS-HAC How it Works


Hydraulic air compressors (HAC) were developed in the late 19th century to supply compressed air to mining companies.

The basic principle employed was to divert a watercourse into a vertical shaft via a device that entrained air, in the form of bubbles into the downflowing water.

As the water pressure increased the air bubbles were equally pressurised. At the bottom of the shaft the air/water mixture was discharged into a horizontal chamber, designed to allow the air to separate from the water and also to act as a large receiver.

Water was discharged under the pressure of the air at a lower level to re-join the watercourse. The difference in height between the water inlet level and its discharge level represented the renewable energy being used to compress the air.

Unlike mechanically compressed air, any heat produced is absorbed by the water flow, resulting in virtually isothermal air.

Counter intuitively, the compressed air is also very dry, improving the maintenance of the mining machinery.

Some of the river driven HAC units were in constant use for 60+ years, as they were very economical to operate, but eventually they went into decline with the advent of modern electrical or diesel driven compressors.

For anyone interested in the history of these gigantic compressors, a detailed review can be seen at:

In more recent times, with increasing energy costs, interest in HACs has been revived. Research into closed circuit units, where the water is pump circulated, in place of the river power is being undertaken in Canada by:


The schematic shows a simplified circuit of a closed- circuit HAC.

In a manner similar in action to that of ECS-DM units, the gravitational base pressure created by the weight of the water flow in column 1, displaces the lighter, downflowing mixture of air and water in column 2, causing it to ‘back up’ and rebalance at a greater height.

The maximum practical volume of air that can be entrained with the downflowing water, by the mixer unit, is circa 50% vol/vol, making the mixture a nominal 0.5sg. (Water, is of course 1 sg, so the ratio of water to air in a HAC is 1: 0.5).

This compares to a working ratio in the ECS-DM (Dense Media) variant of 1: 2 or four times larger, which enables much greater base pressures and differential water heads to be developed. (This explains why the DM units are more suitable for multi-MW output and the HAC units for lower power outputs.)

The pump now requires considerable energy to lift and circulate water from its static level up to the top of the level in column 2, which the mixture of air/water reaches as it backs up to rebalance, under the influence of the gravitational base pressure.

The patented ECS technology, which is used to prevent the backup developing and is detailed in the ECS-DM explanation, can also be utilised with ECS- HAC units.

This leaves a significant margin for the generation of export power, by converting the compressed air into either shaft or electrical power, using conventional air expander-generators.

In addition, ECS-HAC units have additional advantages:

ECS-HAC units have the potential to be simultaneously used for Direct Air Capture of atmospheric CO2, by the addition of amine or caustic based solvents to the circulating water. The air, being continuously entrained in the mixing unit, has a large surface area in the form of the bubbles, resulting in intense contact with the solvents. Conventional technology enables CO2 recovery and return of water to the unit. (NB. For 2023 the UK Emission Trading System (ETS) carbon price is £83.03 per tonne of CO2.)

ECS-HAC units also have the potential to produce sub-zero exhaust air, depending on the type of turbine expander used in the electricity generation process.

The large volumes of cold air produced will have many applications in refrigeration, air conditioning and computer centres, adding to the revenue stream.

These features, combined with electrical generation, significantly add to the versatility of ECS-HAC and open up a wide range of markets for this inexpensive, 24/7, renewable energy technology.

Schematic Layout of a Closed Circuit Hydraulic Air Compressor - ECS-HAC
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