Field of the invention: The present invention relates to a system and method for storing gas, and in particular to a system and method for storing gas at constant pressure and monitoring the integrity of same.

Background:

It is very common for gases of various sorts to be stored in tanks - usually called “cylinders”. These cylinders are normally relatively stiff compared with the internal gas. That is to say, if a small increment is made in the mass of stored gas, the proportionate increase in the volume of the cylinder is typically negligible compared with the increase in volume that would occur in the gas if that gas was allowed to expand back to its original pressure whilst being retained at the same temperature. In such cases, the pressure of the stored gas rises as the mass of stored gas increases. Conversely, the pressure of the stored gas falls as mass of stored gas reduces again. These cylinders are approximately “isochoric” - i.e. constant volume.

There are several disadvantages to isochoric containments (cylinders) for gas. One obvious disadvantage is that the machinery used to compress and expand the air must deal with a range of different storage pressures and it is difficult to design machines that have optimal, or near-optimal, performance over a range of different storage pressures. A second disadvantage is it will usually be impractical to withdraw more than about 50% of the maximum charge of the gas from the store. The state of stress of the walls of the containment changes is affected by the internal pressure and large oscillations in this state of stress can cause material damage in the walls. A third disadvantage relates to heat transfer. If storage pressure falls as gas is withdrawn during a discharging process, the temperature of the stored air also tends to fall and heat leaks into the stored air through the cylinder walls. The opposite happens during charging where the stored air tends to rise in temperature and heat leaks back out. The net effect of this heat transfer over a complete charge-discharge cycle is a loss of exergy and this loss of exergy translates itself into a loss of turnaround efficiency in the energy storage process.

Isobaric (constant pressure) storage of the gas resolves all of the above problems. The compression/expansion machinery can operate with the same pressures on inlet and exhaust at all states of charge and discharge. The state of stress of the containment walls stays (almost) constant and the temperature of the stored gas is not inclined to change.

The attractions of isobaric storage are widely understood and there are several ways in which this can be achieved. One commonly-proposed method is to use so-called hydraulic compensation. Hydraulic compensation involves pumping an incompressible liquid such as water into the containment to displace gas as gas is being withdrawn and conversely allowing the liquid to be driven back out of the containment as gas is being re-introduced into the containment. We will describe this liquid as the hydraulic compensation liquid (HCL). A pump drives the HCL into the cylinder whilst pressurised gas is being discharged and clearly this pump absorbs mechanical power but the amount of power used by the pump is far lower (typically 5-10 times lower) than the work done by the gas being exhausted. When the system is charging again, the HCL is driven out again and most of the work done to pump the HCL into the containment can be recovered as that liquid flows back from the high-pressure containment into a holding tank at or near to ambient-pressure. The rating of the pump must obviously be sufficient to drive the HCL against the pressure difference between the gas storage pressure in the cylinder and the external pressure (normally ambient pressure) at which the HCL is held when it is not within the pressure cylinder.

Summary of this invention.

The present invention relates to a system and method as defined in the claims.

This invention may be applicable to all systems where a gas is intended to be stored within one or more tanks and where the pressure of the stored gas is required to remain fairly constant. A common instance of this requirement occurs in compressed air energy storage systems where the pressurised air is stored in tanks. There are also applications in the storage of other gases such as methane, propane and hydrogen. In all cases, the integrity of the tanks is a critical concern. The present invention provides a method for conducting safe tests on the tanks at regular intervals.

The structural integrity of pressure vessels is always of paramount concern. A sudden rupture of a cylinder containing a large volume of pressurised gas could release a highly destructive quantity of energy in an extremely short timespan. Before any pressure vessel is put into service, it is standard practice to do a “water test” on that vessel using water or another relatively incompressible liquid to raise the internal pressure in that pressure vessel to a level significantly above the rated working pressure. If the vessel should fail during this test, this failure would be very safe because there would be a negligible amount of energy stored in the liquid. In most circumstances, conducting a “water test” on a cylinder requires that the cylinder be taken out of normal service and the fluid connections to that cylinder would normally be changed. It transpires that for cylinders that are equipped with the capability for isobaric storage using a HCL, most of the facilities to conduct a “water test” are already present.

The present invention is a method for ensuring the ongoing safety of cylinders holding a pressurised gas where those cylinders are being operated isobarically. The method is applied on occasions when the cylinder of interest is completely emptied of gas and is instead filled with the HCL. At such times (and on a reasonably regular basis), a “water test” is conducted by closing the isolation valve connecting that cylinder to the main gas manifold and employing the HCL pump to raise the pressure within the cylinder to a level above the normal working pressure for the cylinder.

A pressure vessel (cylinder) is operated to store gas at a pressure that remains relatively steady by pumping in a hydraulic compensation liquid (HCL) to displace gas is it is withdrawn. The HCL flows back out as the cylinder is recharged with gas. The pressure of the HCL (and hence the pressure of the gas within the cylinder) is controlled by a positive displacement pump whose pressure rating exceeds the normal working pressure of the cylinder. At times when the cylinder is largely or completely emptied of its gas charge, an isolation valve disconnects the cylinder from the main part of the main gas manifold and a “water test” is conducted on the cylinder by using the positive displacement pump to raise the pressure of the HCL to a level above the normal operating pressure of the cylinder and hold it at that elevated pressure for a suitable (short) period. Through the regular conduct of such “water tests”, there is very low probability that a catastrophic failure of the cylinder can happen when the cylinder is filled with pressurised gas. Description of the Figures

Figure 1 shows one cylinder (10) for containing the pressurised gas (20) and normally also containing some amount of HCL (40). Most of the remainder of the HCL is held in a holding tank (50) which is normally open to ambient pressure at the top. A small amount of the HCL is also present in the pipe connecting the holding tank (50) to the cylinder (10) and in a positive-displacement pump (60) present along that pipe whose pressure rating is above the normal rated pressure of the cylinder (10).

A port at the top of the cylinder connects the main gas manifold (30) directly to the upper section of the cylinder (10). An isolation valve (70) is present in the main gas manifold so that at any given time, that cylinder can be decoupled from the other provisions connected to the main gas manifold.

Embodiment.

Figure 1 explains the invention. The hydraulic compensation liquid (HCL) (40) rests in the bottom part of the cylinder beneath the stored pressurised gas (20). In normal operating conditions when the quantity of gas (20) in the cylinder (10) is not changing, the level of HCL (40) in the cylinder (10) is not changing either. When the quantity of gas (20) stored in the cylinder (10) is decreasing, some HCL (40) is simultaneously being pumped into the cylinder (10). Conversely, when the quantity of gas (20) stored in the cylinder (10) is increasing, some HCL (40) is simultaneously being allowed out of the cylinder (10) and it passes backwards through the positive-displacement pump (60).

A main concept of this patent is that at times when the quantity of stored gas (40) being held in the cylinder (or cylinders) (10) is stationary at a very low or zero percentage level relative to the complete volume of the tank, the isolation valve (70) is closed and the positive displacement pump (60) is called upon to raise the pressure of the HCL present inside the cylinder (10) to some pressure above normal gas storage pressure and then hold that pressure for a short while.

The actual choice of over-pressure and the frequency at which such a “water test” is conducted will vary from case to case. In general, slightly larger over-pressure values will be used in cases where the frequency of this testing is lower.