Patent ID: 12259090

DETAILED DESCRIPTION

The thermal management methods disclosed herein are configured to manage the thermal gradient that arises due to pressurizing a conformable pressure vessel including pressure vessel segments.

Heat gradients typically do not develop within the interior of traditional gas storage tanks because the gas is mixed throughout the entire storage tank instead of split between pressure vessel segments. The gas has the freedom to move and mix, so any rise in temperature is diffused evenly throughout the storage tank. Unlike conformable pressure vessels, traditional storage tanks are not sectioned, so there is no beginning or end.

In one example, a conformable pressure vessel includes seven pressure vessel segments. A last pressure vessel segment is capped with a plug and a first pressure vessel segment is connected to a valve to fill the conformable pressure vessel. Gas continuously enters the first pressure vessel segment and then gets pushed into the following pressure vessel segments as more gas enters. There is a pressure drop occurring in each consecutive pressure vessel segment because gas must flow through a section of each pressure vessel segment having a lesser diameter to reach the next pressure vessel segment. Therefore, the gas in the last pressure vessel segment is at a lower pressure than the gas in the first pressure vessel segment. Pressure tends to equalize throughout a traditional storage tank, but due to the pressure vessel segments included in the conformable pressure vessel, this equilibrium takes longer to achieve. Therefore, the pressure of the gas in the last pressure vessel segment will always lag behind the pressure of the gas in the first pressure vessel segment.

A magnification of this phenomenon occurs when the fill rate of the conformable pressure vessel is increased. Adding gas at a faster rate causes the temperature difference between pressure vessel segments to be much greater and leads to heat building up within the conformable pressure vessel more quickly. This increase in heat within the conformable pressure vessel occurs because more gas molecules are compressed in a smaller amount of time.

To manage the thermal gradient present during fill or extraction of gas from conformable pressure vessels, several methods are disclosed. The first method involves using thermal conductive materials in the conformable pressure vessel. Thermal conductive materials leverage the increased surface to volume ratio of the pressure vessel to transfer heat between the different parts of the pressure vessel to achieve a thermal equilibrium and avoid extreme temperatures within the conformable pressure vessel. Another method recirculates the gases in the conformable pressure vessel. Another method uses phase change materials within the conformable pressure vessel.

Thermally conductive material transfers heat from the pressure vessel segments having a higher temperature to the pressure vessel segments having a lower temperature due to heat transfer caused by conduction and convection. Each pressure vessel segment is defined by a liner material (e.g. a thermoplastic liner). A reinforcement layer surrounds the liner. The reinforcement layer can be surrounded by a thermally conductive material, such as a thermally conductive foam. A shell encloses the thermally conductive foam. Thermally conductive material, such as a heat pipe, can couple two or more pressure vessel segments. Thermal conduction allows for more thermal consistency throughout the conformable pressure vessel, where heat transfer can reduce the temperature extremes and achieve faster thermal equilibrium.

The method of recirculating the gas is accomplished by connecting different pressure vessel segments to allow the gas to travel in a continuous loop through the conformable pressure vessel. To enable this continuous gas flow, the pressure vessel may include a connecting tube connecting an outlet of the conformable pressure vessel to an inlet. The connecting tube may include a Tesla valve, which is a valve allowing unidirectional flow. Continuous gas flow through the pressure vessel segments enables temperature diffusion throughout the conformable pressure vessel as hot gas flows from the last pressure vessel segment into the cooler first pressure vessel segment. Simulations using this method show that, compared to conformable pressure vessels lacking continuous flow, the temperature of the first pressure vessel segment increases, and the temperature of the last pressure vessel segment decreases to reach a state of thermal equilibrium between the pressure vessel segments.

Another method includes using a phase change material (PCM) applied to the liner of the conformable pressure vessel. PCMs have been widely studied as thermal energy storage options in gas storage tanks. PCMs absorb heat generated during fueling of gas storage tanks in the form of latent heat. As the PCM absorbs heat, it begins to melt, but it does not heat up. In one example, the PCM includes paraffin wax embedded with a graphite matrix. The PCM works most efficiently when the mixture is applied to the inside of the liner. This configuration allows a faster fill rate of the conformable pressure vessel. The PCM also reduces the degree of pre-cooling required for a given fill time by 10-20° C.

FIG.1shows a cutaway perspective view of a conformable pressure vessel100. The conformable pressure vessel100contains seven connected pressure vessel segments102configured to receive and store a gas in a compressed state. Each of the pressure vessel segments102includes a liner (e.g. a thermoplastic liner) underneath a reinforcement layer (e.g. a fiber layer). The pressure vessel segments102are surrounded by a thermally conductive material, in this example, a thermally conductive foam104. The pressure vessel segments102and the thermally conductive foam104are enclosed by a shell110.FIG.1also shows an end fitting including a cap106and a stem108. The end fittings connect the conformable pressure vessel100to valves, adapters, plugs, or couplings, and assist in containing the pressurized gas within the conformable pressure vessel100. There is an end fitting on each end of the pressure vessel segments102that reaches outside the shell110.

FIG.2shows a sectional view of the same seven-segment conformable pressure vessel100. Each pressure vessel segment102includes a liner212that defines a cavity240. The liner212can include a thermoplastic liner. Each liner212includes a first section242having a first diameter and a second section244having a second diameter smaller than the first diameter. The pressure vessel segments102are coupled together at the respective second sections244of the liner212such that the cavities240of each liner212are fluidly coupled. The conformable pressure vessel100includes an inlet246in fluid communication with the cavities240of the pressure vessel segments102through the second section244of a first pressure vessel segment228. The inlet246is configured to receive a gas from a gas source. The conformable pressure vessel100also includes an outlet248in fluid communication with the cavities240of the pressure vessel segments102through the second section244of a second pressure vessel segment232. The outlet248is configured to output the gas from the pressure vessel segments102. Though the outlet248is shown as being in fluid communication with the last pressure vessel segment in the seven-segment conformable pressure vessel100(i.e., the second pressure vessel segment232) in this example, either the inlet246or the outlet248could be fluidly coupled with alternate pressure vessels segments.

A reinforcement layer214surrounds the liner212. The reinforcement layer214can include a fiber reinforcement layer. The thermally conductive foam104surrounds the reinforcement layer214. The shell110encloses the thermally conductive foam104and the pressure vessel segments102. One cap106connects to the liner212and the reinforcement layer214at the inlet246. Another cap106connects to the liner212and the reinforcement layer214at the outlet248. The caps106function to attach the stems108to the respective pressure vessel segments102. The thermally conductive foam104and the shell110, which provide structure, support, and protection for the pressure vessel segments102, are also shown.

Materials such as plastic and high strength fibers typically used in lightweight and high-pressure gas storage tanks and conformable pressure vessels do not have high thermal conductivities. However, by using thermally conductive materials such as thermally conductive foams and heat pipes as indicated herein, the high surface to volume ratio in conformable pressure vessels can be used to transfer heat from areas of high temperature to areas of low temperature. This can significantly reduce temperature extremes within the conformable pressure vessel. An example is described in reference toFIG.3.

FIG.3is a side view of a six-segment pressure vessel300. The pressure vessel300includes the pressure vessel segments302. The pressure vessel segments302are connected by heat pipes330, also referred to as thermally conductive members. The heat pipes330allow heat to flow between the pressure vessel segments302. In this configuration, when different pressure vessel segments302are heated or cooled to different temperatures, the heat pipes330can assist in creating temperature equilibrium between the different pressure vessel segments302. The heat pipes330are connected to thermally conductive materials331which are disposed around the top-most and bottom-most pressure vessel segments302in the example ofFIG.3. The thermally conductive materials331can further assist in creating the temperature equilibrium between the pressure vessel segments302. Though the heat pipes330are shown as extending between first and last pressure vessel segments302, the heat pipes330can be applied between any respective pressure vessel segments302in the same pressure vessel300to achieve temperature equilibrium. Additionally, though the thermally conductive materials331are shown as surrounding the first and last pressure vessel segments302, the thermally conductive materials331can surround any of the pressure vessel segments302in the pressure vessel300.

FIG.4shows a side view of a conformable pressure vessel400with six pressure vessel segments employing one thermal-mitigation method, recirculation. While gas is circulating through the conformable pressure vessel400, a pressure drop can occur in sections of the pressure vessel segments having a lesser diameter because these sections can restrict flow between the pressure vessel segments. During a fill event, the last pressure vessel segment, that is, the pressure vessel segment furthest downstream from the location of an inlet, is therefore at a lower pressure than the first pressure vessel segment. Pressure tends to equalize throughout gas storage tanks, but due to the segmented nature of the conformable pressure vessel400, this equilibrium can take some time to achieve.

The method of recirculation can be used to more quickly achieve a pressure and/or temperature equilibrium in the conformable pressure vessel400. This method involves connecting the first pressure vessel segment and the last pressure vessel segment. This connection can include a heat pipe connected to the first pressure vessel segment and the last pressure vessel segment to permit the transfer of heat between the pressure vessel segments. This connection can also include a passage for gas to flow between the first and the last pressure vessel segments. In embodiments of the conformable pressure vessel400including the passage, gas is able to flow from the last pressure vessel segment into the first pressure vessel segment. This makes the conformable pressure vessel400act as if it were one continuous segment. The increase in temperature of the conformable pressure vessel400is diffused throughout the entire pressure vessel as hot gas flows from the warmer last pressure vessel segment into the cooler first pressure vessel segment.

InFIG.4, an arrow shows gas entering the conformable pressure vessel400in the bottom right corner. The gas leaves the valve of its reservoir and travels through a connector416, through the stem408of the end fitting and into the first pressure vessel segment418of the conformable pressure vessel400. Gas then circulates through each pressure vessel segment and out of the last pressure vessel segment420until it reaches the elbow422. The elbow422attaches to a connecting tube424. The connecting tube424is in fluid communication with an inlet446of the first pressure vessel segment418and with an outlet448of the last pressure vessel segment420. The connecting tube424is configured to receive gas from the outlet448and to supply the gas from the outlet448to the inlet446. In this configuration, the gas is recirculated through the pressure vessel segments in response to the inlet446receiving gas from a gas source (e.g., gas traveling from a reservoir and through the connector416).

For gas to flow in the direction desired, the last pressure vessel segment420must be at a higher pressure than the first pressure vessel segment418. To achieve this pressure differential, the conformable pressure vessel400may include a pressure delta to help motivate flow from the last pressure vessel segment420to the first pressure vessel segment418. In one example, the pressure delta includes a nozzle.

FIG.5shows a sectional view of a nozzle500used to create a pressure differential in the conformable pressure vessel400ofFIG.4. The nozzle500includes a first end550downstream from a gas source and a second end552downstream from the first end550and upstream from the pressure vessel segments. The nozzle500includes a middle portion554between the first end550and the second end552. A cross-sectional area of the middle portion554is smaller than cross-sectional areas of the first end550and the second end552. The nozzle500can replace the connector416shown inFIG.4. Gas enters from the right side of the nozzle500, as shown by the arrow. As the gas moves further through the nozzle500, the cross-sectional area through which the gas travels decreases. By the principle of mass conservation, the mass flow rate must remain constant throughout the entire nozzle500. Since mass flow rate is equal to density times velocity times cross-sectional flow area, velocity and cross-sectional area are inversely proportionate.
{dot over (m)}=ρAv

Where, {dot over (m)} is the mass flow rate, ρ is the density of the fluid and v is the velocity of the fluid. This means that as the cross-sectional area decreases in the middle of the nozzle, the velocity increases to conserve mass flow.

Then, by the Venturi Effect, when the velocity of a fluid or gas increases in a nozzle, pressure decreases. That means that at point526, the middle of the nozzle500inFIG.5, where the cross-sectional area is the smallest, velocity is largest, and pressure is lowest according to Bernoulli's Principle. Assuming the fluid experiences a negligible change in height while travelling through the nozzle, Bernoulli's equation can be re-written to say that pressure plus (one half times density times velocity squared) remains constant throughout the nozzle500. This means that if velocity increases, pressure must decrease so that the equation still equals the same constant. Therefore, a low-pressure area can be created in the middle of the nozzle500. As the gas moves past the middle portion554of the nozzle500, the gas expands and the pressure remains low. This location was selected as the connection point between the first pressure vessel segment418and the last pressure vessel segment420in the conformable pressure vessel400ofFIG.4to maintain the validity of the narrow cross-section in the middle of the nozzle500.

FIG.6shows a partial sectional view of a pressure vessel600similar to the conformable pressure vessel400shown inFIG.4. The gas comes out of a valve and enters the assembly at the bottom right as shown by the arrow. The gas goes straight through the nozzle500toward the first pressure vessel segment. While the gas travels through the nozzle500, the gas reaches its lowest pressure in the middle of the nozzle500, where the cross-section is smallest. The gas then travels through each consecutive pressure vessel segment, though only the first and last pressure vessel segments are shown inFIG.6. Once the gas reaches the end of the last pressure vessel segment and enters the elbow622, the low-pressure area inside the nozzle500causes the gas to flow down through the connecting tube628toward the nozzle500. The nozzle500connects back to the first pressure vessel segment, so the gas is recirculated throughout the entire pressure vessel600. During the recirculation, the gas diffuses the heat, allowing the pressure vessel600as a whole to reach one equilibrium temperature.

Additionally and/or alternatively, one or more other devices may be used to enable recirculation of gas through the pressure vessel in a preferred flow direction. For example, an interior profile of the connecting tube424shown inFIG.4can be shaped to enable flow of gas through the connecting tube424from the outlet448to the inlet446and to prevent flow of the gas through the connecting tube424from the inlet446to the outlet448. This interior profile may be achieved using a Tesla valve, shown inFIG.7. The Tesla valve causes gas to flow preferentially one way, shown by the arrow729. For example, in the sectional view shown inFIG.7, if gas enters the valve from the left side, when it reaches the first intersection, some of the gas splits to follow the top channel and some goes down through the lower channel. The gas that travelled up then follows the path around and ends up looping so far as to be re-directed leftward. This tendency of a fluid or gas to follow the shape of a convex object it contacts is known as the Coanda Effect. This effect causes the gas to interfere with the rest of the flow that was initially travelling from left to right. This occurs at every junction, so the overall flow from left to right is slowed. However, if gas enters the valve from the right side, it experiences a different path. In this case, there are no junctions, due to the angle of the shapes and channels within the cavity of the Tesla valve, so the gas does not split and interfere with its own flow.

The Tesla valve700is shown incorporated into the pressure vessel inFIG.8. It is oriented so that gas flows preferentially from the last pressure vessel segment to the first pressure vessel segment. This configuration causes the gas to recirculate throughout the conformable pressure vessel and therefore diffuse the thermal gradient that would otherwise form. This may be used independently of or in conjunction with another device used to create a pressure delta, such as the nozzle500shown inFIG.8.

Phase change materials (PCMs) may also be implemented in a conformable pressure vessel to further mitigate the development of temperature gradients. PCMs can store and release large amounts of latent heat energy. As the PCM changes phase, its temperature remains constant even as it absorbs heat. This is because the heat absorbed by the PCM energizes molecules of the PCM to a point where the PCM changes phase. PCMs, such as paraffin wax, can be used to coat an interior of the liner of the conformable pressure vessel. Then, as the coated pressure vessel segment is filled with gas and the temperature rises, the PCM will absorb the heat, preventing the temperature from rising. As the PCM absorbs heat, it begins to melt, but does not heat up. When the area surrounding the PCM becomes cool, the PCM releases the heat, moderating the temperature of the conformable pressure vessel. A PCM may coat an interior surface of the liner of one or more pressure vessel segments. For example, the PCM may coat an interior surface of the liner of every pressure vessel segment individually or may coat the liner of only a first pressure vessel segment and a second (e.g. last) pressure vessel segment. Coating an interior surface of the liner of the first and last pressure vessel segments may assist to moderate the temperature of the conformable pressure vessel since the temperature is most extreme in these segments.

To verify that the methods discussed above are effective, a study was done using physics modeling and pressure and temperature simulations. A first analysis was done to establish a baseline pressure vs. temperature curve for a pressure vessel having seven pressure vessel segments. The first and last pressure vessel segments were not connected, and no additional temperature mitigation device was included aside from the thermal conductivity of the pressure vessel itself, which includes thermally conductive foam. A fill rate of 30 grams per second was used. The results of this analysis are shown inFIGS.9and10.

FIG.9includes a graph of pressure vs. temperature curves for the first pressure vessel segment of different conformable pressure vessels including various temperature mitigation devices for the first 30 seconds of gas fill. Separate curves are shown to indicate performance of pressure vessels having the following features: a baseline pressure vessel including thermally conductive foam; a gas recirculating pressure vessel similar to that shown inFIG.4; a heat pipe; a gas recirculating pressure vessel including a Tesla valve similar to that shown inFIG.8; a pressure vessel including a phase change material (PCM); and a pressure vessel including all of these listed features.

FIG.10includes a graph of pressure vs. temperature curves for the last pressure vessel segment of the same conformable pressure vessels described with respect toFIG.9. for the first 30 seconds of gas fill.

Table 1 shows the minimum temperature of the first pressure vessel segment and the maximum temperature of the last pressure vessel segment of each conformable pressure vessel described with respect toFIGS.9and10.

TABLE 1Test Results of Conformable Pressure VesselsUsing Different Thermal Mitigation MethodsMinimumMaximumTemperatureTemperatureThermal Mitigationfor Firstfor LastTechniqueSegment (° C.)Segment(° C.)Baseline Thermal15.7582.0ConductivityHeat pipe15.7106.3Simple Recirculation15.7515.0Recirculation with Tesla valve16.176.6Phase Change Materials17.043.8All Techniques Together18.9110.2

In the baseline pressure vessel including thermally conductive foam, the temperature of the first pressure vessel segment cooled from an initial temperature of 20° C. to a minimum temperature of 15.7° C. The temperature of the last pressure vessel segment increased from 20° C. to a maximum temperature of 582° C. These temperatures exceed the maximum capabilities of most materials used in typical gas storage tanks and would result in insufficient fill due to post-fill cooling.

In another tested configuration, a series of heat pipes were connected to the first and last pressure vessel segments. This modification significantly reduced the temperature gradient between the first and last pressure vessel segments. As shown inFIG.10, the maximum temperature of the last pressure vessel segment was reduced from the baseline of 582° C. to 106.3° C.

The next method evaluated was a recirculation method, for example, the recirculation method shown inFIG.4, where the last pressure vessel segment is connected to the first pressure vessel segment with a connecting tube. The pressure vessel segments form a continuous chain, and hot gas from the last pressure vessel segment mixes with cooler gas in the first pressure vessel segment.FIGS.9and10show the pressure vs. temperature curves for the recirculation method in the first and last pressure vessel segments relative to the baseline. While the maximum temperature of the last pressure vessel segment was reduced from 582° C. to 515° C., the minimum temperature of the first pressure vessel segment of 15.7° C. was not significantly changed. The recirculation method does allow the gases from the first and last pressure vessel segments to mix and provides some temperature mitigation; however, the recirculation method does not prevent the inflowing gas from initially traveling in the unintended direction from the first pressure vessel segment, through the connecting tube, and to the last pressure vessel segment. This extends the time required to establish a circulating flow path within the conformable pressure vessel.

By including a Tesla valve in the connecting tube, the initial flow of gases in the unintended direction is reduced, and a recirculation flow can be more quickly established in the conformable pressure vessel. As shown inFIG.10, after an initial increase in temperature of the last pressure vessel segment, recirculating flow moved hot gases from the last pressure vessel segment to mix with the incoming cool gas from the fill port. This resulted in a maximum temperature of the last pressure vessel segment of only 76.6° C. Additionally, the minimum temperature of the first pressure vessel segment increased from 15.7° C. of the baseline to 16.1° C. This further demonstrates that hot gases from the last pressure vessel segment mixed with the cool gases in the first pressure vessel segment.

In another tested configuration, a PCM was used to coat the liner of the baseline conformable pressure vessel. As shown inFIG.10, the maximum temperature of the last pressure vessel segment was reduced to 43.8° C. As shown inFIG.9, the minimum temperature of the first pressure vessel segment was increased to 17° C.

Finally, a final gas recirculating pressure vessel model including heat pipes, a Tesla valve, thermally conductive foam, and a PCM was tested. As shown inFIG.9, the minimum temperature of the first pressure vessel segment increased to 18.9° C. As shown inFIG.10, the maximum temperature of the last pressure vessel segment was 110.2° C.

The above studies illustrate the significant improvement in the fill-related temperature gradient achieved by employing the devices and methods disclosed herein. These devices and methods enable faster filling and extraction of gas from conformable pressure vessels while at the same time maintaining temperatures within the operating limits of the materials of the conformable pressure vessel.