Abstract:
A method and system for recycling permeated gas is disclosed. A container encapsulating a pressure vessel defines a containment volume. Gas permeating through the pressure vessel is captured in the containment volume. When a sensor detects a threshold level of permeated gas captured within the containment volume, a control module sends a command to open a purge valve. The open purge valve allows permeated gas captured within the containment volume to be supplied to an engine, a repressurization unit, or a secondary container.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application claims priority to U.S. Provisional App. No. 62/198,833 for a “Compressed Gas Capture and Recovery System” filed Jul. 30, 2015 and U.S. Provisional App. No. 62/217,165 for a “Compressed Gas Capture and Recovery System” filed Sep. 11, 2015, the contents of which are incorporated herein by reference in their entireties. 
    
    
     BACKGROUND 
     Compressed gas can be used as a fuel to provide benefits such as lower pollution levels and lower refining costs than, for example, liquid fuels such as gasoline or diesel fuel. Using compressed gas as a vehicle fuel or for industrial applications requires that the gas be transportable, refillable, and safely stored. 
     The storage of compressed gas, such as hydrogen or compressed natural gas (CNG), is particularly challenging, as the gas must be stored at a very high pressure in order to achieve acceptable storage density. Given the high pressure level needed to store a sufficient amount of gas, leaks can occur at fittings, or gas can permeate through the walls of the pressure vessel used to store the gas. 
     The amount of permeation of gas through the walls of a pressure vessel is based on the product of a material permeation coefficient, the surface area of the pressure vessel walls in contact with the gas, and the pressure level of the gas divided by the sectional thickness of the pressure vessel wall material. Thus, the resistance to permeation of a pressure vessel constructed of a particular material is proportional to the surface area of the walls and pressure of the gas and inversely proportional to the material thickness of the pressure vessel. 
     Gas permeation levels are regulated to reduce environmental and user impact. The current NGV2 standard allows a steady state permeation rate of fuel lost to the atmosphere of 0.25 cc of natural gas per hour per liter of water capacity. Given a 40 DGE pressure vessel (550 liters of water capacity), 3.5 cubic feet, or approximately one percent, of the natural gas within the pressure vessel can permeate the walls of the pressure vessel each year. Though this is a relatively small amount, gas permeation near the NGV2 standard levels can still cause a safety concern or an olfactory nuisance in enclosed spaces such as homes or garages, forcing some compressed gas vehicle owners to park vehicles in open spaces. Mercaptans from natural gas are designed to be detected by the human nose at 1,000 ppm, so even small amounts of permeation can be undesirable to, for example, users of CNG pressure vessels. 
     Compressed gas pressure vessels are currently designed to restrict permeation to meet or exceed regulatory levels using expensive materials or other suboptimal solutions such as increased material thickness to improve mechanical and chemical resistance to permeation. Increasing material thickness adds weight to pressure vessels that are already quite heavy based on design requirements to withstand high pressures from the stored compressed gas. This is undesirable for the vehicle manufacturer. 
     Another existing solution to address gas permeation is to use an unsealed, vented cabinet to hold a pressure vessel. However, these cabinets can release permeated gas in an uncontrolled manner or the user is required to occasionally vent the compressed gas from the cabinet to the atmosphere to avoid high concentrations in the enclosed cabinet. This type of cabinet cannot safely contain permeated gas for any period of time. 
     SUMMARY 
     A compressed gas permeation recovery system is disclosed. The system includes a two-stage permeation barrier. First, a pressure vessel includes a traditional barrier of reinforced pressure vessel walls configured to contain a high-pressure gaseous fuel. Next, a sealed container including a vapor barrier surrounds the pressure vessel and blocks lower pressure gas that escapes from the pressure vessel by leakage or permeation from entering the atmosphere. Finally, a recovery system is coupled to the sealed container, allowing safe recovery of the captured gas present at the lower pressure level within the container. 
     In one aspect of the disclosure, a method of recycling permeated gas is disclosed. The method includes capturing permeated gas in a containment volume defined by a container. The permeated gas escapes from a pressure vessel supplying compressed gas to an engine through a main fuel line. The container encapsulates the pressure vessel. The method also includes detecting, using a sensor, a threshold level of permeated gas captured within the containment volume of the container. Based on detecting the threshold level of permeated gas, permeated gas is supplied from the containment volume to the engine. Supplying the permeated gas includes sending a command, from a control module, to modify a position of a purge valve. 
     In another aspect of the disclosure, another method of recycling permeated gas is disclosed. The method includes capturing permeated gas in a containment volume defined by a container. The permeated gas escapes from a pressure vessel supplying compressed gas to an engine through a main fuel line. The container encapsulates the pressure vessel. The method also includes detecting, using a sensor, a threshold level of permeated gas captured within the containment volume of the container. Based on detecting the threshold level of permeated gas, permeated gas is supplied from the containment volume to a repressurization unit. Supplying the permeated gas includes sending a command, from a control module, to modify a position of a purge valve. 
     In yet another aspect of the disclosure, a system for recycling permeated gas is disclosed. The system includes a container defining a containment volume for capturing permeated gas escaping from a pressure vessel. The pressure vessel supplies compressed gas to an engine through a main fuel line. The container encapsulates the pressure vessel within the containment volume. A sensor detects a threshold level of permeated gas captured within the containment volume of the container. A purge valve is in fluid communication with the containment volume. A control module is operable to send commands to modify a position of the purge valve to vent the permeated gas captured within the containment volume based on the sensor detecting the threshold level of permeated gas within the containment volume. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The description makes reference to the accompanying drawings wherein: 
         FIG. 1  is a perspective partial cutaway view of a sealed container including a compressed gas pressure vessel; 
         FIG. 2  is sectional view through the sealed container of  FIG. 1 ; and 
         FIG. 3  is a schematic view of a compressed gas capture and recovery system for use with an internal combustion engine. 
     
    
    
     DETAILED DESCRIPTION 
     The compressed gas capture and recovery system described herein is configured to retain and recover gas that leaks from or permeates through the walls of a pressure vessel. In one embodiment, a container including a vapor-resistant barrier or liner is designed to surround the pressure vessel. In addition to a main fuel line allowing gaseous fuel to exit the pressure vessel, the container includes a recovery fuel line and purge valve designed to redirect captured gas from the space within the barrier and outside pressure vessel. Both the recovery fuel line and main fuel line can provide gaseous fuel under the direction of a control module. In the example of an internal combustion engine, the gaseous fuel can be CNG and the controller can provide a mix of CNG with air from an intake manifold to fuel an internal combustion engine. 
       FIG. 1  is a perspective cutaway view of a sealed container  100  including a compressed gas pressure vessel  102 . The container  100  can be formed, for example, of a metallic or plastic enclosure. The pressure vessel  102  can be a conformable tank comprised of a a continuous chain of cylindrical, elongated, main vessel portions and alternating reduced diameter intermediate portions that allow the pressure vessel  102  to flex and bend to fit within a variety of container shapes, including the sealed container  100 . 
     The pressure vessel  102  shown in  FIG. 1  is configured for use in a 3,600 PSI natural gas vehicle application. In alternative embodiments, the pressure vessel  102  can include main vessel portions of varying shapes, such as ovoid shapes or spherical shapes. In other alternative embodiments, the pressure vessel  102  can include a singular main vessel portion without flexible reduced diameter intermediate portions. 
       FIG. 2  is sectional view through the sealed container  100  of  FIG. 1 . The sealed container  100  includes a gaseous barrier  200  encapsulating the pressure vessel  102 . The gaseous barrier  200  serves to block gas from exiting the container  100 . The gaseous barrier  200  can be, for example, a vapor resistant layer that has material properties selected specifically to resist permeation of a specific form of gas, for example, CNG. In some examples, the vapor resistant layer can be formed from materials similar to the rest of the pressure vessel  102 , such as hytrel or nylon, but need only be designed for installation within the container  100  using a material thickness sufficient to improve permeation performance at the low pressure level present in the container  100 . 
     The main vessel portions of the pressure vessel  102  within the sealed container  100  can include a reinforcing layer  202 . The reinforcing layer  202  can be a braided treatment applied to the exterior of the main vessel portions but not to the reduced diameter intermediate portions of the pressure vessel  102 . The braiding process can include encasing the pressure vessel  102  in high strength fiber material, such as rayon, nylon, glass, or Kevlar® (aramid), or a combination thereof to form the reinforcing layer  202 . Other treatments for the exterior reinforcing layer  202 , such as carbon fiber or glass fiber overbraids or sleeves are also possible. 
     The walls of the pressure vessel  102  and the reinforcing layer  202  serve to limit the amount of permeation of gas from the pressure vessel  102 . Further, the pressure level outside the pressure vessel  102 , but within the gaseous barrier  200 , is significantly lower than the pressure within the pressure vessel  102 , in fact, approaching ambient pressure. Because of the lower pressure level and the small amount of gas present, the gaseous barrier  200  is able to contain most, if not all, of the gas that escapes the pressure vessel  102  (a result of the permeation function of the gaseous barrier  200  described above). The escaped gas held within the container  100  by the gaseous barrier  200  is referred to below as captured gas  204 . The captured gas  204  is shown in  FIG. 2  using a stippling shading to represent the gaseous molecules. 
     In other embodiments, the volume of the external container  100  can be reduced or the container  100  can be wholly eliminated when the main sections of a conformable pressure vessel  102  are tightly packed and/or include sufficient interstitial spaces to act as a containment volume to capture gas escaping the pressure vessel  102 . In the example where the container  100  is eliminated, a gaseous barrier  200  can be disposed directly around the tightly packed main sections of the conformable pressure vessel  102 . A recovery apparatus, relying on a pressure differential in the case of use with an internal combustion engine (further described below) or using a pump in other cases, can be coupled to the sealed container  100  to collect and redirect the captured gas  204 . Once collected, the captured gas  204  can then be re-pressurized and, for example, stored back in the pressure vessel  102 , stored in a secondary tank or vessel, or used in an internal combustion engine in the case of a vehicle fuel application. 
       FIG. 3  is a schematic view of a compressed gas capture and recovery system  300  for use with an internal combustion engine  302 . The system  300  can include the container  100  and pressure vessel  102  of  FIGS. 1 and 2 . The pressure vessel  102  can provide gaseous fuel directly to the engine  302  through a main fuel line  303 . The pressure vessel  102  can also provide gaseous fuel to the engine  302  through a recovery fuel line  304 . A purge valve  305  is disposed along the recovery fuel line  304 , the purge valve  305  being configured to block or vent captured gas  204  from the container  100  depending on the position of the purge valve  305 , that is, whether the purge valve  305  is open or closed. The purge valve  305  can be controlled by a control module  306 . The control module  306  can also be configured to control an intake manifold  308 , the intake manifold  308  providing air to the internal combustion engine  302  for mixing with gaseous fuel from both the main fuel line  303  and from the recovery fuel line  304 . 
     The control module  306  can be any type or form of computing device, or can be composed of multiple computing devices. The control module  306  can include or be coupled to a conventional central processing unit (CPU) or any other type of device, or multiple devices, capable of manipulating or processing information. The control module  306  can also include or be coupled to a memory for storing data and program instructions used by the CPU. The memory can be a random access memory device (RAM) or any other suitable type of storage device. The memory can include data that is accessed by the CPU using a bus. The memory can also include an operating system and installed applications, the installed applications including programs that permit the CPU to perform the gas capture and recovery methods described here. 
     When the control module  306  receives a signal, for example, from a sensor disposed within the container  100  and configured to detect a predetermined threshold level amount of captured gas  204  within the container  100 , the purge valve  305  can be sent a command to vent the captured gas  204 , allowing the gas to flow to the engine  302 . The predetermined threshold level can be based on the pressure level within the container  100  and chosen to prevent over pressurization, for example, 3 psi. Other predetermined threshold levels are also possible. The flow of gas to the engine  302  can be driven by the pressure differential between the container  100  (e.g., higher pressure) and the pressure level in the intake manifold  308  (e.g., lower pressure or vacuum), the intake manifold  308  being in fluid communication with the recovery fuel line  304 . This venting process effectively empties the container  100  of any captured gas  204 . 
     The compressed gas capture and recovery system  300  described here improves performance of gaseous-fueled vehicles by increasing fuel economy (based on less fuel being lost to gaseous emissions from the pressure vessel  102 ) and allows compressed gas vehicles to be safely stored indoors. This reduction in gas lost to permeation or leaking is significant, improving overall safety of any vehicle using the system  300  as well as reducing operating costs for that vehicle. Additionally, the use of exotic or expensive materials to completely eliminate permeation from the pressure vessel  102  can be reduced, leading to a more cost effective fuel system for a vehicle. Though the example described in respect to  FIG. 3  is based on an internal combustion engine  302  and vehicle application, other uses, such as industrial uses, are also possible for the compressed gas capture and recovery system  300 . 
     While this disclosure includes what is presently considered to be the most practical and preferred embodiments, it is to be understood that the disclosure is not to be limited to the disclosed embodiments but, on the contrary, is intended to cover various modifications and equivalent arrangements.