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My bright idea ... engineer Michael Crowley discusses an isothermal compression method that could reduce the power needed to compress gases.

The diagram on this page shows a new method of compression. In figure 1A, the piston is at the end of the suction stroke about to start compression and in figure IB, the compression stroke has just been completed. The feature that makes the compressor isothermal is a heat-absorbing-and-releasing structure (HARS) attached to the piston, which is inserted into the hydraulic fluid (usually water) during the compression stroke.

The HARS is made from 0.15mm sheets of aluminium. Each sheet is at 2mm spacing, and is held in position on the end of the piston with an epoxy resin moulding. Each sheet is curved around the same centre. This gives them much higher stiffness, so they are not deflected by hydraulic or other loads as they move in and out of the hydraulic fluid at speed.

During compression, the gas is contained inside the HARS, which has a large surface area. The thermal heat capacity of the HARS is higher than the gas, so as the gas is compressed the heat of compression is rapidly transferred into the HARS, thus stabilising the gas temperature and providing isothermal compression. The HARS is subsequently cooled by the hydraulic fluid in the bottom of the cylinder, and then an external cooling circuit is used to cool this fluid.

Unlike with other isothermal compressors, in this method the fluid is almost stationary and held in the bottom of the cylinder by gravity. It is the HARS attached to the piston that moves in and out of the fluid. The HARS is designed so that its sheets are parallel to the direction of travel of the piston, allowing the hydraulic fluid to flow freely in and out of the HARS with the minimum hydraulic resistance. These two design features allow the device to operate at relatively high speeds, with the piston successfully operated at speeds up to l,500rpm.

Increasing speed or reduced spacing between the HARS sheets will cause increased turbulence in the hydraulic fluid. Eventually, the fluid turbulence increases to the point where the gas and fluid mix. Gas bubbles are formed at the bottom of the cylinder, and fluid is lost from the cylinder as a fluid mist with the compressed gas. Loss of fluid from the cylinder and fluid gas mixing can reduce the efficiency of the device.

However, the compressor can be arranged to address these issues, as illustrated in figure 2A/B. It shows an air compressor where the working hydraulic fluid is water. Figure 2 A shows the compressor with the piston and attached HARS retracted at the end of the suction stroke, and figure 2B shows the compressor with the piston and attached HARS fully inserted at the end of the compression stroke.

Fixed to the bottom of the cylinder are a series of baffles, interleaved between the sheets of the HARS. The baffles have the same physical form as the HARS. These baffles reduce turbulence and prevent air bubbles penetrating below the water surface. They resist the movement of the water in the bottom of the cylinder, and hold the working fluid in place. This arrangement allows the compressor to operate at higher speeds.

In figure 2, the cooling coils inside the cylinder (shown in figure 1) have been replaced with an external cooling jacket, which helps to reduce the free surface area inside the cylinder and so helps with the fluid stability in the cylinder. But it is possible to use cooling coils in other configurations, if required for heat transfer purposes.

A small amount of water mist is expelled from the cylinder with the compressed gas on each compression stroke. This water is recovered in a coalescer and recirculated back into the cylinder through a metering restrictor.

The gas leaving the coalescer will have 100% humidity, which will take water out of the system. The gas entering the compressor will also have humidity, but the volume flows are higher. So there will be a balance between water vapour entering and leaving, which usually results in a net flow of water in or out.

The system shown in figure 2 has been designed to accommodate either a loss or a gain in net water. If there is a net flow of water out of the system, then there is a mains water top-up via a flow regulator and check valve. The water mains pressure needs to be only a little higher than the suction pressure, as the check valve will prevent reverse flow when the cylinder is at high pressure, and the top-up fill can occur on the suction stroke. An automatic water drain is provided inside the coalescer, so that if too much water collects, it can be drained away.

I am investigating several geometry changes to improve system efficiency. Reducing the gap between the HARS sheets improves system efficiency, but a reduced gap can also limit the maximum speed of operation. If alternative sheets have staggered lengths, then the shorter lengths are in the hydraulic fluid only for a reduced part of the cycle, which allows the system to operate faster. This technique has been shown to give a good compromise between sheet gap and system speed.

If the gas velocity in the HARS is increased so the gas flow is turbulent, efficiency could be improved. The gas velocity can be increased by using a spiral HARS. In this configuration, the gas in the centre of the spiral has to travel a greater distance for every stroke of the compressor, compared with the original configuration, so the average gas velocity in the HARS is increased. However, the velocity of insertion and removal of the HARS from the hydraulic fluid is unchanged.

The method described here can reduce the power required for compressing gases, helping to reduce operator costs. It could also improve the round-trip efficiency of compressed-air energy storage systems (CAES).

The isothermal compressor could be used as the compression stage in an Otto cycle engine, enabling the use of a higher compression ratio and improving efficiency.

Liquid air energy storage is an emerging technology. Dearman has developed an engine that runs on liquid air, while Highview Power Storage has a system that uses liquid air for grid energy storage.

However, these companies use liquid air or liquid nitrogen provided by the big gas suppliers. If there is any growth in liquid air as an energy storage vector, using the method described here in its manufacture would be more cost-effective and would improve total system efficiency.
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Title Annotation:Engineering extras: Innovation: Ideas: Careers: Gadgets: Books: Internet
Author:Crowley, Michael
Publication:Professional Engineering Magazine
Date:Jul 1, 2015
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