This invention relates to a storage vessel for cryogenic liquid. A cryogenic liquid is defined herein as a liquid having a boiling point (at

1 bar) of -100°C or less.

A cryogenic liquid is conventionally stored in a thermally-insulated vessel. The storage vessel may comprise an inner container and an outer jacket spaced from the inner container. The space between the jacket and the container is closed. This space is typically evacuated so as to provide vacuum insulation to the inner container. The space may also contain insulating material such as superinsulation. The storage vessel has a first outlet for liquid from the inner container, the first outlet terminating outside the confines of the outer jacket. The storage vessel also has a second outlet for vapour that communicates with the ullage space within the inner container. Both outlets are typically provided with valves. The valve in the second outlet is typically set to open if the pressure in the ullage space exceeds a chosen value.

Notwithstanding the insulation of the storage vessel, the difference in temperature between the ambient atmosphere surrounding the vessel and the cryogenic liquid causes a flow of heat from the surrounding atmosphere into the cryogenic liquid stored in the vessel. This causes the cryogenic liquid to vaporise with the consequence that the pressure in the storage vessel gradually builds up to a level at which the vent valve opens. Vapour is therefore lost to the atmosphere. Such venting is particularly undesirable if the vapour is of a "greenhouse" gas. If there are frequent withdrawals of cryogenic liquid from the storage vessel, vaporisation in the vessel of the cryogenic liquid tends to be enhanced. There is an increasing demand for liquefied natural gas (LNG) as a vehicle fuel. The storage of this fuel gives rise to the problems identified above. It is therefore desirable to keep down vapour emissions from a vessel storing a cryogenic liquid.

According to the present invention there is provided a cryogenic storage vessel for a first cryogenic liquid, comprising an inner container defining a liquid holding volume, the inner container being spaced from an outer jacket, wherein there is a closed evacuated space between the inner container and the outer jacket, the vessel has in the liquid-holding volume or in heat exchange contact with an outer surface of the inner container a heat exchange member defining a passage for a second cryogenic liquid that boils at atmospheric pressure at a lower temperature than the first cryogenic liquid.

A cryogenic storage vessel according to the invention is able to be operated such that the first cryogenic liquid is stored in subcooled state. The said passage may be arranged to vent vapour of the second cryogenic liquid to atmosphere.

The heat exchange member may be in the form of a coil. The coil may be disposed around the inner container in physical and thermal contact therewith.

The heat exchange member may be associated with at least one valve responsive to the temperature of the first cryogenic liquid in the liquid-holding volume. The valve may therefore be arranged to open and close so as to keep the first cryogenic liquid in a sub-cooled state. In one example, a temperature signal is generated by a temperature sensor located in a thermowell that terminates against an outer surface of the inner container. The cryogenic storage vessel according to the invention typically has a first outlet for the first cryogenic liquid in sub-cooled state and a second outlet for vapour communicating with the ullage space within the inner container. The outlet for sub-cooled cryogen is typically provided with a flow control valve. The outlet for vapour is typically provided with a valve which is set to open in the event of the pressure in the inner vessel exceeding a chosen value.

A cryogenic storage vessel according to the present invention may form part of a vehicle refuelling station. The refuelling station may be of the pressure-decant kind. In such refuelling stations, the outlet for sub-cooled first liquid communicates with a refuelling nozzle adapted to inject cryogenic liquid into a fuel tank of a vehicle to be refuelled.

In such examples, the fuel may be liquefied natural gas. If the fuel is liquefied natural gas, the second cryogenic liquid is typically liquid nitrogen or liquid air.

A cryogenic storage vessel according to the present invention may be operated so as to maintain the first cryogenic liquid in sub-cooled state, thereby keeping to a minimum, occasions on which vapour of the first cryogenic liquid is vented from the vessel.

A cryogenic storage vessel according to the invention will now be described by way of example with reference to the accompanying drawing which is a schematic diagram of a vehicle refuelling station.

The drawing is not to scale.

Referring to the drawing, the vehicle refuelling station includes a storage vessel 1 in accordance with the invention. The storage vessel 1 comprises an inner container 2 spaced from an outer container 4. The outer container 4 provides a jacket for the inner container 2. A closed space 6 is defined between the inner container 2 of the outer container 4. The space 6 is evacuated. The space 6 also contains insulating material, for example super insulation. The storage vessel 1 is provided with a fill pipe 36, typically fitted with a fill valve 38, which may be connected to a secondary vessel (not shown) in order to fill the inner container 2 with a chosen cryogenic liquid, typically liquefied natural gas (LNG). The evacuated space 6 and the thermal insulation 8 within the space 6 keep down the rate of absorption of heat by the LNG when stored within the storage vessel 1.

The storage vessel 1 has a first outlet pipe 10 for the first cryogenic liquid communicating with the bottom of the inner container 2. The first outlet pipe 10 serves a pipeline 11 in which may be located a stop valve 12 and a flow control valve 14. In a refuelling station, the pipeline 11 comprises a flexible, thermally insulated, length of hose terminating in a refuelling nozzle 16. The refuelling nozzle 16 is adapted to be inserted into a fill port (not shown) of a vehicle (not shown) to be refuelled and is provided in a conventional manner with a trigger or lever (not shown) which when activated causes the stop valve 12 to open and LNG to be delivered from the storage vessel 1 to the vehicle to be refuelled.

The storage vessel 1 has a second outlet pipe 18 which communicates with the top of the inner container 2. The outlet pipe 18 is fitted with a vent valve 20 which is arranged to open in the event of the pressure in the ullage space of the inner container 2 reaching a chosen level. Typically, the inner container 2 is maintained at an elevated pressure, that is a pressure above atmospheric pressure. The pressure is sufficient to provide an adequate flow of LNG to the refuelling nozzle 16 without the need to operate a mechanical pump to enhance the flow rate. In other words, the LNG is, in operation, decanted under pressure from the storage vessel 1. If the pressure in the ullage space of the inner container 2 exceeds the desired decanting pressure by a chosen amount, the vent valve 20 automatically opens. Since natural gas is a greenhouse gas and since it is readily combustible, it is generally undesirable for the vent valve 20 to open. In accordance with the invention, the storage vessel 1 is provided with a heat exchange member 22 in thermal contact with the inner container or a volume of LNG held within the inner container, the heat exchanger member 22 having a passage therethrough for the flow of a coolant, thereby enabling the LNG to be held in sub-cooled state notwithstanding the absorption of heat by the storage vessel 1 from the surrounding environment. By holding the LNG in sub-cooled state, vaporisation of the LNG is kept down and the creation of an excess pressure within the inner container 2 such that the vent valve 20 automatically opens may generally be avoided. As shown in the drawings an exchange member takes the form of a heat exchange coil typically made of a thermally conductive material such as copper in thermal and physical contact with the outer surface of the inner container 2. In an alternative embodiment (not shown), the heat exchange member 22 may be located within the inner container 2 so that it comes into direct contact with the LNG held within the storage vessel 1.

The heat exchange member 22 has an inlet pipe 24 connected to a pipeline 26, typically thermally insulated which may be located a stop valve 28 and a flow control valve 30. The pipeline 26 communicates with a storage vessel 31 for a sacrificial cryogenic liquid which boils at a lower temperature (at atmospheric pressure) than the cryogenic liquid held in the storage vessel 1. In the example of LNG, the second cryogenic liquid is typically liquid nitrogen although liquid air is, for example, an alternative. The heat exchange member 22 has a valved outlet pipe 40 outside the storage vessel 1 , which outlet 40 is typically arranged to vent vapour of the second cryogenic liquid to atmosphere, the vapour being formed by heat exchange with the LNG or other cryogenic liquid held within the storage vessel 1 as the second cryogenic liquid flows through the heat exchange member 22. The flow rate of the second cryogenic liquid through the heat exchange member 22 may be controlled so as to maintain the LNG held within the storage vessel 1 at a chosen temperature below its boiling point. In one arrangement, the storage vessel 1 is provided with a thermowell 32 that terminates in thermal contact with the inner container 2 and houses a thermistor 34 or other temperature sensing device also in thermal contact with the outer wall of the inner container 2, the thermistor 34 being able to generate signals that can be used to control the opening and closing of the valve 28 (and/or to adjust the position of the flow control valve 30) and through such means to maintain the temperature of the LNG in sub-cooled state. The storage vessel 1 is thus able to keep to a minimum the venting of natural gas vapour therefrom.

The first outlet pipe 10, the second outlet pipe 18, the fill pipe 36, the inlet pipe 24 of the heat exchange member 22, the outlet pipe 40 of the heat exchange member 22 and the thermowell 32 each pass through the outer container 4 through a vacuum-retaining fitment (not shown).

In an alternative embodiment of the apparatus shown in Figure 1 , the vapour of the second cryogenic liquid may be collected instead of being vented to the atmosphere.