Field of the invention: This invention relates primarily to refilling stations for vehicles using gaseous fuels. Such vehicles may include aircraft, road vehicles and trains. It also has relevance to vehicles that simply carry gases.

Background:

There is a move towards fuelling vehicles with compressed natural gas (CNG) and hydrogen in preference to fuelling them from liquid hydrocarbon fuels like petrol, diesel and kerosene. Although CNG is a hydrocarbon that does release C02 on combustion like the liquid hydrocarbons, it C02 intensity is lower. The main motivation here is hydrogen gas which is completely carbon-free.

The most straightforward way to carry gaseous fuels in vehicles is within pressurised tanks. Hydrogen tanks typically operate at maximum pressures of 700bar and CNG tanks more typically operate with a maximum pressure of 250bar. As the gaseous fuel is consumed, the tank pressure gradually falls and heat naturally flows into the tank through the tank walls to maintain the tank temperature which naturally tends to drop. Because fuel is normally consumed over periods of hours, these heat flows are not problematic and the temperature differences between the external air and the tank are negligible.

For obvious reasons, it is usually desirable that the filling operation should take place quickly. In all of the cases of relevance here, the vehicle remains stationary during filling and clearly the vehicle is not fulfilling its primary purpose at such times. Filling or emptying a gas tank quickly causes effects to become relevant that would be negligible for slow filling or slow emptying. In particular, during filling, the temperature of the gas within the tank tends to rise because gas already in the tank at any one time is being compressed. This invention addresses that issue directly.

One further issue is important that is even less obvious than the temperature problem and that further issue relates to how much work is required to drive the fuel into the tank. When vehicle tanks are filled with liquid hydrocarbon fuels like petrol, diesel or kerosene, the work required to pump the fuel into the tank is negligible compared with the thermal energy value of the fuel. For example, 1kg of petrol can release about 45MJ of heat and a pump lifting this mass of petrol from a tank 10m below would require only -100J of work (about two-millionths of 45MJ). With hydrogen as a fuel, a 10Olitre tank at 700bar would contain ~5.9kg of hydrogen and -830MJ of heat value. The minimum amount of work required to raise the pressure of that much gaseous hydrogen from 1bar to 700bar is -38.8MJ which is -4.7% of the heat value. That minimum amount of work is only achieved if the gas in the tank remains at ambient temperature. In short, the work done to drive hydrogen fuel into tanks is far from negligible and the efficiency of this operation is therefore quite important. The present invention provides for much more efficient operation than the simplest approach.

A very simple approach to refilling vehicle gas tanks involves keeping a number of stationary reservoir tanks holding gas at different pressures and connecting each one in turn to the vehicle tank so that the gas pressure in the vehicle tank first matches the pressure in the first tank connected, then it matches the pressure in the second tank connected - and so on. In such an arrangement, the vehicle tank would be connected to a manifold that would then have connections to each one of the different pressure level tanks which can be isolated by valves. In the most basic incarnation of this simple approach, there would be one single reservoir tank. In this most basic incarnation, the vehicle tank is connected to the manifold, the isolating valve to the main tank is opened slowly and gas rushes through this isolating valve. Initially, there is a large pressure drop across the valve but as the tank pressure comes closer to the pressure of gas in the connected reservoir tank, that pressure drop obviously reduces. Allowing gas pressure to fall across a valve is a well-known and well-understood waste of energy. For hydrogen in particular, the very low density causes this loss of energy to be significant relative to the chemical energy value (the calorific value).

Summary of this invention.

An aspect of the present invention provides a system for refilling gas tanks on vehicles which enables the following sequence of operations to take place:

A. The vehicle tank is connected to a filling system with a double connection: one part for liquid flow and one part for gas flow. A “syphon tube” is connected to the lowest point in the vehicle tank for transferring liquid and a gas connection is made to a high point in the vehicle tank for transferring gas.

B. The part of the gas connection joined externally to the vehicle tank is a manifold which includes connections to one or more stationary tanks - each one containing gas. The mass of air that can become trapped in the manifold as that manifold is connected to the vehicle tank is minimised by design. The gas pressure in one of the stationary tanks, a pressure balancing tank, begins at a value slightly above atmospheric pressure. A purge outlet valve on the gas manifold is opened and then the valve separating the pressure balancing tank is opened slightly so that air is purged from the gas manifold. A very small amount of gas is lost through the purge outlet before the purge outlet valve is closed again. Then the manifold contains relatively pure gas at the same pressure as the gas in the pressure balancing tank and the valve separating this from the gas manifold is opened fully.

C. The pressure balancing tank contains some hydraulic compensation liquid in its bottom portion. More hydraulic compensation liquid is then pumped into this pressure balancing tank so that the gas pressure in that tank and in the gas connection manifold rises up to match the pressure of residual gas in the vehicle tank. Then the valve separating the gas manifold from the vehicle tank is opened and initially there is no gas flow through this valve.

D. Now hydraulic compensation liquid is pumped into the vehicle tank through the “siphon tube” hydraulic compensation liquid is removed from the pressure-balancing tank at a similar rate. The effect is to drive gas out of the vehicle tank through the gas manifold and for that gas to flow into the pressure balancing tank. Since there is an open gas connection between the vehicle tank and the pressure balancing tank, no significant difference in gas pressure exists between the two tanks during this operation.

E. When the vehicle tank is (largely) empty of gas, the pressure in the gas manifold is raised by pumping more hydraulic compensation liquid into the pressure balancing tank. The pressure balancing tank contains features that enable very good heat transfer between the gas and liquid in that tank so that gas temperature rise is very low even when the rise in pressure is substantial. Because very little gas remains in the vehicle tank itself, only a negligible amount of heat must be removed from that as its pressure rises. The final gas pressure reached in the gas manifold is identical to the target final tank pressure - normally 700bar for hydrogen. This pressure is the same as the pressure in at least one main gas reservoir also connected to the gas manifold but isolated by a valve.

F. The valve between the pressure balancing tank and the gas manifold is then closed. The valve between the (appropriate) main gas reservoir and the gas manifold is opened. Now the vehicle tank is in pressure equilibrium with the/a main gas reservoir but the vehicle tank is filled with hydraulic compensation liquid. This liquid is pumped from the vehicle tank into the main gas reservoir and at the same time, gas flows naturally from the main gas reservoir into the vehicle tank to balance the volumes. At the end of the operation, the vehicle tank has been emptied of all but a few drops of the liquid and it is filled with gas to the desired pressure. This invention addresses the filling of pressurised gas tanks in vehicles. In most cases, the tanks of interest are fuel tanks for the respective vehicles but this is not exclusive. Specifically, this invention is also applicable to tanks on vehicles that are expressly designed for carrying that fuel.

The composition of the system is fairly implicit from the sequence described above but some components are:

(1) A set of stationary closed tanks containing the gas. This set typically includes at least one pressure balancing tank and at least one main gas reservoir. Each one of the stationary closed tanks includes a port at the top for inducting/expelling gas and a separate port, normally at the base, for inducting/expelling hydraulic compensation liquid. The pressure balancing tank contains provisions for enhanced heat transfer to the contained gas so that temperature changes in that gas are minimised whilst its pressure is raised or lowered.

(2) A gas connection manifold that attaches to the vehicle tank and has connections to the stationary closed tanks.

(3) A purge outlet valve on the gas connection manifold

(4) A set of gas isolation valves with one gas isolation valve located in each connection between the gas manifold and a stationary closed tank.

(5) A liquid connection manifold that has connections to the lower ends of each of the one or more stationary closed tanks

(6) A set of liquid isolation valves with one gas isolation valve located in each connection between the liquid manifold and a stationary closed tank.

(7) A siphon tube that connects the liquid connection manifold to the lowest point in the vehicle tank. In some instances the siphon tube may be integral with the vehicle tank and in other instances, it may be attached permanently to the stationary frame.

(8) A volume of hydraulic compensation liquid that is distributed at any one time between the liquid connection manifold, the siphon tube, the connections between the liquid connection manifold and the stationary closed tanks, the stationary closed tanks themselves, the vehicle tank and a liquid reservoir for accommodating variations in the amount of hydraulic compensation liquid in the other places which variations arise mainly from varying states of charge of the stationary main gas reservoir(s).

(9) The liquid reservoir mentioned in the above point. This would be typically unpressurised in most cases and at most would be designed to sustain a low gauge pressure. (10) A set of liquid pumps with one pump in each connection from the liquid connection manifold. In all cases, these pumps would be bi-directional. In some cases, these pum3s would be positive-displacement devices capable of recovering work from the flow of liquid as well as doing work to drive the liquid.

(11) A feature for managing the recovery of dissolved gas from the hydraulic compensation liquid when its pressure is reduced in a flow of that liquid from the liquid connection manifold into the liquid reservoir.

A high-level description of the system is as follows:

(1) In an aspect, this invention is a system for filling gas tanks in vehicles which avoids (in part or in full) the compression of gas within the vehicle tank by using simultaneous liquid and gas exchanges with the tank to gas flowing at constant pressure into the vehicle tank whilst liquid is simultaneously transferred from the tank at a similar pressure.

(2) The invention is optionally operable such that it avoids expansion of gas within the vehicle tank by using simultaneous liquid and gas exchanges with the tank to gas flowing at constant pressure from the vehicle tank whilst liquid is simultaneously transferred into the tank at a similar pressure.

(3) The invention includes at least one pressure balancing tank and at least one main gas reservoir. Each one of these is a stationary closed tank and each one includes a port, normally at the top, for inducting/expelling gas and a separate port, normally at the base, for inducting/expelling a hydraulic compensation liquid. The pressure balancing tank contains provisions for enhanced heat transfer to the contained gas so that temperature changes in that gas are minimised whilst gas pressure is raised or lowered. During the operation of the system, whenever significant mechanical work is done on/by the gas in compression/expansion actions respectively, that work is done mainly in the pressure balancing tank.

(4) The invention includes a gas connection manifold that can be connected directly to the vehicle tank. Said gas connection manifold optionally includes a purge valve operable so that air trapped when the vehicle tank is connected can be purged from the gas connection manifold before gas transfers take place between the stationary system and the vehicle tank.

(5) The invention includes a set of gas isolation valves isolating all of the closed tanks (including the vehicle tank, the main gas reservoir and the pressure balancing tank) from the gas connection manifold. A key feature of the invention is that any one of these valves is normally opened only when there is a small pressure difference across that valve. An alternative way to express this is that there is never a substantial loss of exergy due to throttling a gas flow through one of the gas isolation valves.

(6) The invention includes a liquid connection manifold that can be connected directly to the lowest point in the vehicle tank. Said liquid connection manifold enables hydraulic compensation liquid to flow into/from any of the stationary closed tanks, into/from the vehicle tank and into/from a stationary non-pressurised hydraulic liquid reservoir.

(7) A liquid isolation valve and a bi-directional pump are fitted in the connection between the main gas reservoir and the liquid connection manifold. Similarly, a liquid isolation valve and a bi-directional pump are fitted in the connection between the pressure balancing tank and the liquid connection manifold. Similarly a liquid isolation valve and a bi-directional pump are fitted in the connection between the non- pressurised hydraulic liquid reservoir and the liquid connection manifold.

In an aspect, there is provided a system for filling gas tanks in vehicles uses a hydraulic compensation liquid to minimise or eliminate the loss of exergy that happens when gas is throttled through valves in the filling operation and to avoid changing the pressure of any substantial mass of gas within the vehicle tank. The filling operation involves moving hydraulic compensation liquid in the vehicle tank and in the stationary gas tanks as necessary such that there is no pressure difference across the isolation valve or valves through which the gas will pass as those valves are opened. When the gas isolation valves are open, any remaining gas present in the vehicle tank is first transferred back into stationary tanks in order that pressure changes of that gas occur there and the vehicle tank is largely (or completely) filled with hydraulic compensation liquid. Provisions for enhanced heat transfer are present in at least one of the stationary tanks where all major changes of gas pressure take place. Once gas in the stationary tanks is at the pressure intended as the final pressure for the vehicle tank, gas is driven into the tank and the hydraulic compensation liquid is allowed to re-emerge so that the gas pressure in the tank remains constant at the intended fill pressure. The system enables rapid filling of the tank without concerns over heat transfer or temperature changes and without the irreversibilities of throttling the gas flow.

According to an aspect, there is provided a system for re-fuelling vehicle gas tanks comprises a first fluid connection (manifold) through which the fuel gas flows, a second fluid connection (manifold) through which a hydraulic compensation liquid flows, a pressure balancing tank coupled to both of these fluid connections (manifolds) and equipped with a heat transfer provision to suppress temperature fluctuations of the gas within that pressure balancing tank as the its gas pressure changes, a main fuel gas reservoir also coupled to both fluid connections a holding tank for the hydraulic compensation fluid together with a set of reversible pumps/hydraulic-motors and valves.

The system may comprise a demountable joint that can be made between the first fluid connection (manifold) and the gas volume present in the vehicle tank and wherein a second demountable joint can be made between the second fluid connection (manifold) and the volume of hydraulic compensation liquid present in the vehicle tank.

According to another aspect, there is provided a method for re-fuelling gas tanks within vehicles involves joining the vehicle tank to the re-fuelling system via a first fluid connection (manifold) through which the fuel gas flows and a second fluid connection (manifold) through which a hydraulic compensation liquid flows and progressing through these steps: (i) matching the fuel gas pressure within the first fluid connection (manifold) with the gas pressure within the vehicle tank before opening a valve separating these two, (ii) pumping hydraulic compensation liquid into the vehicle tank via the second fluid connection (manifold) so that most or all of the remaining gas passes out to the first fluid connection (manifold), (iii) raising the pressure in the first fluid connection (manifold) by pumping hydraulic compensation liquid into a pressure balancing tank coupled to both fluid connections (manifolds) until the pressure matches that of a main fuel gas reservoir and (iv) then drawing out the hydraulic compensation liquid from the vehicle tank so that full-pressure fuel gas naturally enters the vehicle tank.

In an embodiment, no gas ever flows across a significant pressure drop (i.e. there is no significant throttling) and wherein the hydraulic compensation liquid flows experience significant pressure changes as they pass through a hydraulic pump/motor unit where there is efficient power conversion between flow energy and electrical power.

Description of the Figures

Figure 1 provides a description of the first embodiment. This figure shows a main gas reservoir (10) for containing the pressurised gas and a pressure balancing tank (20) which also contains pressurised gas but whose pressure varies during each vehicle tank fill. The figure also shows the vehicle tank (30) and the liquid reservoir (40) used for holding hydraulic compensation liquid that is not inside the pressurised system. The gas connection manifold (50) and the liquid connection manifold (60) are present. The gas connection manifold (50) has an integral purge valve (51) to enable small quantities of air trapped in the gas manifold when a new vehicle tank is connected to be purged out. A siphon-tube (63) connects the liquid connection manifold (60) to the vehicle tank (30) via a siphon-tube isolation valve (62). The main gas reservoir (10) has several objects associated with it comprising: a gas isolator valve (11) separating it from the gas connection manifold (50), a liquid isolator valve (12) separating it from the liquid connection manifold (60) and an associated bi-directional liquid pump (13).

The pressure balancing tank (20) has several objects associated with it comprising: a gas isolator valve (21) separating it from the gas connection manifold (50), a liquid isolator valve (22) separating it from the liquid connection manifold (60) and an associated bi-directional liquid pump (23).

The liquid reservoir (20) has several objects associated with it comprising: a gas isolator valve (21) separating it from the gas connection manifold (50), a liquid isolator valve (22) separating it from the liquid connection manifold (60) and an associated bi-directional liquid pump (23).

Hydraulic compensation liquid (70) is present in several distinct locations including the unpressurised liquid reservoir (40), the main gas reservoir (10), the pressure balancing tank (20), the vehicle tank (30) and the liquid connection manifold (60).

Figure 2 provides for the description of a second embodiment. Most of the items present in Figure 2 are also present in Figure 1. The key difference here is that in Figure(2), the siphon tube (63) remains attached to the vehicle tank (30) and travels with the vehicle. This necessitates the introduction of an additional siphon tube isolation valve (64) that can prevent ejection of the small quantity of hydraulic compensation fluid remaining in the siphon tube (63) when the vehicle is disconnected from the filling installation.

A first embodiment.

There are two main embodiments of this invention. In the first embodiment, the siphon tube (63) remains attached to the stationary system. This first embodiment is described here with reference to Figure 1.

The stationary system comprises a main gas reservoir (10) for containing the pressurised gas at a relatively constant pressure, a pressure balancing tank (20) in which gas pressure undergoes a large cycle for each vehicle refill and a gas connection manifold (50) as well as other components discussed subsequently. The pressure balancing tank (20) has a volume similar to the largest expected vehicle tank (30) and much larger than the volume trapped in the gas connection manifold (50) when the connection is made between that and the vehicle tank (30). The volume of the main gas reservoir (10) would greatly exceed the volume of the pressure balancing tank (20) because this main gas reservoir (10) would be capable of filling a vehicle tank (30) many times over without having its own charge of gas replenished. The remaining aspects of this first embodiment are explained through a description of the process that takes place to fill a vehicle tank (30) with pressurised gas.

When the vehicle arrives at the refilling station, the gas connection manifold (50) is connected to the vehicle tank (30) and all of the gas isolation valves into that gas connection manifold (50) are closed. Those valves include the main gas reservoir gas valve (11), the pressure balancing tank gas valve (21) and the vehicle tank gas valve (31).

The gas pressure in the pressure balancing tank (20) is brought down to a value slightly above atmospheric pressure by drawing hydraulic compensation liquid (70) out of the bottom of the pressure balancing tank (20). This liquid flows through the pressure balancing tank liquid isolation valve (22) and the associated pump (23) to enter the liquid connection manifold (60). To accommodate that incoming liquid, liquid is transferred liquid connection manifold (60) into the unpressurised liquid reservoir (40) via the unpressurised liquid reservoir isolation valve (42), the associated pump (43) and the degassing provision (44) for managing the escape of gas from de-pressurised hydraulic compensation liquid.

The gas connection manifold purge valve (51) is opened and the pressure balancing tank gas valve (21) is opened slightly to fill the gas connection manifold (50) with gas and drive out air through the gas connection manifold purge valve (51). A very small quantity of gas is lost in this process. Then the gas connection manifold purge valve (51) is closed and the gas pressure in the gas connection manifold (50) rises quickly to match the pressure in the pressure balancing tank (20) as the pressure balancing tank gas valve (21) is opened fully.

Hydraulic compensation liquid (70) is then pumped into the pressure balancing tank (20) by operating the pressure balancing tank pump (23). This has the effect of raising gas pressure in that tank. Gas pressure in the gas connection manifold (50) rises at the same time. If no heat transfer provision was present, gas temperature would tend to rise as its pressure is raised. However, the pressure balancing tank (20) contains an internal heat transfer enhancement feature (25) which might comprise either a set of long vertical metal rods or a porous metal foam or similar which provides for very good thermal contact with the gas contained within the pressure balancing tank (20). This internal heat transfer enhancement feature (25) ensures that very little temperature rise does actually occur in the gas as its pressure is raised. When the gas within the pressure balancing tank (20) and the gas connection manifold (50) matches the pressure of gas within the vehicle tank (30), the vehicle tank gas isolation valve (31) is opened. No gas flow results as a consequence of this action since pressures are balanced on either side of the valve. In this first embodiment, the siphon tube (63) previously contained within the gas connection manifold (50) is then advanced through the open vehicle tank gas isolation valve (31) and into the vehicle tank (30) until it reaches the lowest point in the vehicle tank (30). The siphon tube isolation valve (62) is then opened with the effect that the pressure in the hydraulic compensation liquid (70) present in the hydraulic liquid connection manifold (60) quickly changes to match the pressure of the gas in the vehicle tank (30).

Now the pressure balancing tank liquid isolation valve (22) is opened and hydraulic compensation liquid (70) is driven from the pressure balancing tank (20) into the vehicle tank (30) through the hydraulic liquid connection manifold (60) and the siphon tube (61) by operating the pressure balancing tank pump (23). As hydraulic compensation liquid (70) flows from the pressure balancing tank (20) into the vehicle tank (30), gas flows in the opposite direction until eventually the vehicle tank is substantially filled with hydraulic compensation liquid (70) and has very little (or no) remaining gas. Most or all of the residual charge of gas that had been in the vehicle tank (30) when it arrived for filling has been transferred into the pressure balancing tank (20) at this stage.

Further hydraulic compensation liquid (70) is then pumped into the pressure balancing tank (20) by operating the pressure balancing tank pump (23) and the liquid reservoir tank pump (43). This has the effect of further raising gas pressure in the gas connection manifold (50). As before, the internal heat transfer enhancement feature (25) within the pressure balancing tank (20) ensures that very little temperature rise does actually occur in the gas as its pressure is raised further. Gas pressure continues to rise until it matches the pressure of the gas within the main gas reservoir (10). That pressure will be the final pressure in the vehicle tank (30). Then the main gas reservoir gas isolation valve (11) is opened.

Next all of the gas remaining in the pressure balancing tank (20) is expelled by pumping more hydraulic compensation liquid (70) from the vehicle tank (30) into the pressure balancing tank. Gas flows in the opposite direction to balance volumes. This achieves a partial fill of the vehicle tank (30) at the final target gas pressure. The pressure balancing tank gas isolation valve (21) is closed and so is the pressure balancing tank liquid isolation valve (22). The main gas reservoir liquid isolation valve (12) is then opened. To complete the filling operation, hydraulic compensation liquid (70) is pumped from the vehicle tank (30) into the main gas reservoir (10) via the liquid connection manifold (60). At the same time, gas flows in the opposite direction to balance volumes. When the vehicle tank (30) has been emptied of hydraulic compensation liquid (70), the siphon tube (63) is withdrawn from the vehicle tank (30) and the vehicle tank gas isolation valve (31) is closed. The siphon tube isolation valve (62) is also closed.

The main gas reservoir gas isolation valve (11) is then closed again and the main gas reservoir liquid isolation valve (12) is also closed. The pressure balancing tank gas isolation valve (21) is opened and so is the pressure balancing tank liquid isolation valve (22). By pumping hydraulic compensation liquid (70) out of the pressure balancing tank, the gas pressure in the gas connection manifold (50) is reduced. Typically, but not necessarily, this pressure reduction continues until the pressure is at or below atmospheric pressure.

Finally, the pressure balancing tank gas isolation valve (21) and the pressure balancing tank liquid isolation valve (22) are both closed. The vehicle tank is disconnected from the gas connection manifold (50) and the filling operation is complete.

A second embodiment.

In the second embodiment, a part of the siphon tube (63) is integral with the vehicle tank (30). An additional siphon tube valve (64) is mounted with the vehicle tank (30) so that at the end of a filling operation when this valve is closed, the small amount of hydraulic compensation liquid (70) remaining in the siphon tube (63) cannot be driven out of the siphon tube (63) by the pressure of the contained gas. This second embodiment is described here with reference to Figure 2.

The stationary system comprises a main gas reservoir (10) for containing the pressurised gas at a relatively constant pressure, a pressure balancing tank (20) in which gas pressure undergoes a large cycle for each vehicle refill and a gas connection manifold (50) as well as other components discussed subsequently. The pressure balancing tank (20) has a volume similar to the largest expected vehicle tank (30) and much larger than the volume trapped in the gas connection manifold (50) when the connection is made between that and the vehicle tank (30). The volume of the main gas reservoir (10) would greatly exceed the volume of the pressure balancing tank (20) because this main gas reservoir (10) would be capable of filling a vehicle tank (30) many times over without having its own charge of gas replenished. The remaining aspects of this second embodiment are explained through a description of the process that takes place to fill a vehicle tank (30) with pressurised gas.

When the vehicle arrives at the refilling station, the gas connection manifold (50) is connected to the vehicle tank (30) and all of the gas isolation valves into that gas connection manifold (50) are closed. Those valves include the main gas reservoir gas valve (11), the pressure balancing tank gas valve (21) and the vehicle tank gas valve (31). The liquid connection manifold (60) is also connected to the vehicle tank (30) via the siphon tube (63) through the first siphon tube isolation valve (62) and the additional siphon tube isolation valve (64) that travels with the vehicle. Both of the siphon tube isolation valves ((62) and (64)) are opened so that the pressure of hydraulic compensation liquid (70) in the liquid connection manifold (60) and in the siphon tube (63) matches the gas pressure in the vehicle tank (30).

The gas pressure in the pressure balancing tank (20) is brought down to a value slightly above atmospheric pressure by drawing hydraulic compensation liquid (70) out of the bottom of the pressure balancing tank (20). This liquid flows through the pressure balancing tank liquid isolation valve (22) and the associated pump (23) to enter the liquid connection manifold (60). To accommodate that incoming liquid, liquid is transferred liquid connection manifold (60) into the unpressurised liquid reservoir (40) via the unpressurised liquid reservoir isolation valve (42), the associated pump (43) and the degassing provision (44) for managing the escape of gas from de-pressurised hydraulic compensation liquid.

The gas connection manifold purge valve (51) is opened and the pressure balancing tank gas valve (21) is opened slightly to fill the gas connection manifold (50) with gas and drive out air through the gas connection manifold purge valve (51). A very small quantity of gas is lost in this process. Then the gas connection manifold purge valve (51) is closed and the gas pressure in the gas connection manifold (50) rises quickly to match the pressure in the pressure balancing tank (20) as the pressure balancing tank gas valve (21) is opened fully.

Hydraulic compensation liquid (70) is then pumped into the pressure balancing tank (20) by operating the pressure balancing tank pump (23). This has the effect of raising gas pressure in that tank. Gas pressure in the gas connection manifold (50) rises at the same time. If no heat transfer provision was present, gas temperature would tend to rise as its pressure is raised. However, the pressure balancing tank (20) contains an internal heat transfer enhancement feature (25) which might comprise either a set of long vertical metal rods or a porous metal foam or similar which provides for very good thermal contact with the gas contained within the pressure balancing tank (20). This internal heat transfer enhancement feature (25) ensures that very little temperature rise does actually occur in the gas as its pressure is raised.

When the gas within the pressure balancing tank (20) and the gas connection manifold (50) matches the pressure of gas within the vehicle tank (30), the vehicle tank gas isolation valve (31) is opened. No gas flow results as a consequence of this action since pressures are balanced on either side of the valve.

Now the pressure balancing tank liquid isolation valve (22) is opened and hydraulic compensation liquid (70) is driven from the pressure balancing tank (20) into the vehicle tank (30) through the hydraulic liquid connection manifold (60) and the siphon tube (63) by operating the pressure balancing tank pump (23). As hydraulic compensation liquid (70) flows from the pressure balancing tank (20) into the vehicle tank (30), gas flows in the opposite direction until eventually the vehicle tank is substantially filled with hydraulic compensation liquid (70) and has very little (or no) remaining gas. Most or all of the residual charge of gas that had been in the vehicle tank (30) when it arrived for filling has been transferred into the pressure balancing tank (20) at this stage.

Further hydraulic compensation liquid (70) is then pumped into the pressure balancing tank (20) by operating the pressure balancing tank pump (23) and the liquid reservoir tank pump (43). This has the effect of further raising gas pressure in the gas connection manifold (50). As before, the internal heat transfer enhancement feature (25) within the pressure balancing tank (20) ensures that very little temperature rise does actually occur in the gas as its pressure is raised further. Gas pressure continues to rise until it matches the pressure of the gas within the main gas reservoir (10). That pressure will be the final pressure in the vehicle tank (30). Then the main gas reservoir gas isolation valve (11) is opened.

Next all of the gas remaining in the pressure balancing tank (20) is expelled by pumping more hydraulic compensation liquid (70) from the vehicle tank (30) into the pressure balancing tank. Gas flows in the opposite direction to balance volumes. This achieves a partial fill of the vehicle tank (30) at the final target gas pressure. The pressure balancing tank gas isolation valve (21) is closed and so is the pressure balancing tank liquid isolation valve (22).

The main gas reservoir liquid isolation valve (12) is then opened. To complete the filling operation, hydraulic compensation liquid (70) is pumped from the vehicle tank (30) into the main gas reservoir (10) via the liquid connection manifold (60). At the same time, gas flows in the opposite direction to balance volumes. When the vehicle tank (30) has been emptied of hydraulic compensation liquid (70), the vehicle tank gas isolation valve (31) is closed.

The main gas reservoir gas isolation valve (11) is then closed again and the main gas reservoir liquid isolation valve (12) is also closed. The pressure balancing tank gas isolation valve (21) is opened and so is the pressure balancing tank liquid isolation valve (22). By pumping hydraulic compensation liquid (70) out of the pressure balancing tank, the gas pressure in the gas connection manifold (50) is reduced. Typically, but not necessarily, this pressure reduction continues until the pressure is at or below atmospheric pressure.

Finally, the pressure balancing tank gas isolation valve (21) and the pressure balancing tank liquid isolation valve (22) are both closed. The vehicle tank is disconnected from the gas connection manifold (50) and the filling operation is complete.

In the first embodiment, it may include, optionally, a “rose-head” end for the siphon-tube (63) so that surface tension of the hydraulic compensation liquid (70) would prevent that liquid from falling out of the siphon-tube (63) as that is being withdrawn from the vehicle tank (30).

In the second embodiment, optionally, a purge valve may be included for allowing air to be removed from the connection between the liquid connection manifold (60) and the siphon tube (63).

In either embodiment, an optional mode of operation that might be faster than the process described above (but less efficient) is that gas might be compressed partially within the vehicle tank (30) at the same time that pressure is being raised in the gas connection manifold.

In either embodiment, another optional mode of operation that might be faster than the process described above (but less efficient) is that gas might be compressed initially in the pressure balancing tank up to a pressure fractionally above the pressure in the vehicle tank (30) of the incoming vehicle. This is slightly more lossy than the standard procedure outlined initially for two reasons (i) some throttling of gas occurs as the pressure balancing tank gas connection valve is opened first and (ii) more of the valuable fill gas probably escapes through the purge valve (51) than might otherwise have escaped.