Abstract:
The CNG system comprises a gas inlet line and a reciprocating compressor arranged and configured for compressing gas from the gas inlet line and delivering compressed gas towards a dispenser. A Stirling engine is drivingly connected to the reciprocating compressor. A burner receives gas from the gas inlet line and generates thermal energy for powering the Stirling engine.

Description:
FIELD OF THE INVENTION 
       [0001]    The present disclosure relates in general to a system and method for compressing gas. More specifically, the present disclosure relates to a system and method for dispensing compressed natural gas in a refueling station. 
       BACKGROUND 
       [0002]    Traditionally, internal combustion engines have been fueled by one or more distillates of fuel oil, such as gasoline or diesel. Gasoline or diesel is at atmospheric pressure during filling. Recently a growing number of vehicles have been manufactured, or converted, so their engines operate on natural gas instead of the longer chain hydrocarbons. The availability, low cost, and lower emissions of combusting natural gas over fuel oil distillates have garnered interest in continuing to increase the number of natural gas powered vehicles. Typically, natural gas fills a vehicle at a pressure exceeding 200 bar, which greatly exceeds the atmospheric pressure conditions of traditional fuels. The high filling pressure of natural gas requires compressing the natural gas prior to dispensing it to the vehicle. Thus while there are incentives to power vehicles with natural gas, obstacles exist in its delivery. 
       BRIEF DESCRIPTION 
       [0003]    According to one aspect, the present disclosure relates to a system for compressing a gas including a gas source, for instance a gas pipeline or a gas distribution grid, which can be connected to a gas inlet line for delivering gas to a reciprocating compressor. The reciprocating compressor is arranged and configured for compressing gas from the gas source and delivering compressed gas towards a utility. In some embodiments the utility can include a dispenser, for instance a dispenser for vehicle fueling. In some embodiments the utility can include a compressed-gas storage tank. A combination of compressed-gas storage tank(s) and one or more dispensers can be provided, gas being compressed by the reciprocating compressor and delivered to the storage tank and therefrom upon request to the dispenser, e.g. for vehicle re-fueling. The system further includes a Stirling engine drivingly connected to the reciprocating compressor. A burner receives gas from the gas source, and gas burned in the burner is used to provide thermal power to the Stirling engine for converting thermal power into mechanical power and driving the reciprocating compressor. The burner can be connected to the same gas supply line which supplies gas at the suction side of the reciprocating compressor. Gas treatment equipment, such as dryers, filters and the like can be provided for treating the gas from the gas source before feeding the gas to the gas burner and to the reciprocating compressor inlet. 
         [0004]    A compressed gas station can thus be designed, which can be installed in any location where a source of hydrocarbon gas is available, e.g. a gas pipeline. The gas is used as a source of energy for operating a gas compressor and dispensing compressed gas to a dispenser, for example for fueling a vehicle. The Stirling engine provides efficient power conversion to operate the compressor, and can easily be operated with a minimum or virtually no maintenance. Gas can e.g. be diverted from a gas pipeline and delivered partly to the burner for generating thermal energy powering the Stirling engine, and partly to the reciprocating compressor. This latter compresses the gas and delivers compressed gas to the compressed-gas storage tank and/or the dispenser, for example for vehicle fuelling purposes. 
         [0005]    According to a further aspect, the disclosure concerns a method of supplying compressed gas, e.g. hydrocarbon gas. The method includes providing a supply of hydrocarbon gas; providing a reciprocating compressor; connecting the reciprocating compressor to the supply of hydrocarbon gas; providing a Stirling engine drivingly connected to the reciprocating compressor; generating thermal power by burning hydrocarbon gas from the supply; at least partly converting the thermal power into mechanical power in the Stirling engine; driving the reciprocating compressor with mechanical power generated by the Stirling engine; compressing hydrocarbon gas from the supply in the reciprocating compressor; and delivering the compressed hydrocarbon gas to a utility, e.g. a dispenser and/or a compressed-storage tank in turn connected to a dispenser. 
         [0006]    Features and embodiments are disclosed here below and are further set forth in the appended claims, which form an integral part of the present description. The above brief description sets forth features of the various embodiments of the present invention in order that the detailed description that follows may be better understood and in order that the present contributions to the art may be better appreciated. There are, of course, other features of the invention that will be described hereinafter and which will be set forth in the appended claims. In this respect, before explaining several embodiments of the invention in details, it is understood that the various embodiments of the invention are not limited in their application to the details of the construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein are for the purpose of description and should not be regarded as limiting. 
         [0007]    As such, those skilled in the art will appreciate that the conception, upon which the disclosure is based, may readily be utilized as a basis for designing other structures, methods, and/or systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    A more complete appreciation of the disclosed embodiments of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
           [0009]      FIG. 1  schematically illustrates a CNG refueling station with a reciprocating compressor driven by a Stirling engine; 
           [0010]      FIG. 2  illustrates a cross-sectional view of a Stirling engine in the “alpha” configuration, for driving the reciprocating compressor of a CNG refueling station; and 
           [0011]      FIG. 3  schematically illustrates an arrangement of a reciprocating compressor driven by a free-piston Stirling engine arrangement. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    The following detailed description of the exemplary embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. Additionally, the drawings are not necessarily drawn to scale. Also, the following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims. 
         [0013]    Reference throughout the specification to “one embodiment” or “an embodiment” or “some embodiments” means that the particular feature, structure or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrase “in one embodiment” or “in an embodiment” or “in some embodiments” in various places throughout the specification is not necessarily referring to the same embodiment(s). Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments. 
         [0014]      FIG. 1  is a schematic illustration of a compressed natural gas (CNG) system  10  shown having an inlet line  12  for delivering gas to the CNG system  10 . The inlet line  12  attaches to a supply line  14 ; which in an example is in communication with a natural gas pipeline  14 A or a natural gas utility distribution system that distributes natural gas to residential and commercial customers of natural gas, and operates at example pressures of from about 0.03 bar to about 14 bar. Alternatively, the supply line  14  can be in communication with a transmission line and having example operating pressures of from about 14 bar to about 105 bar. Example gases include hydrocarbons that are a gas at standard temperature and pressure, such as but not limited to methane, ethane, propane, butane, and mixtures thereof 
         [0015]    In an example, the hydrocarbons can be saturated or unsaturated, and the gas can include trace amounts of non-hydrocarbons, such as nitrogen, hydrogen, oxygen, sulfur. A shut-off valve  16 , which may optionally be automated or manual, is shown at the connection between the inlet line  12  and supply line  14  for selectively blocking communication between the inlet line  12  and supply line  14 . Optionally, an additional valve  18  may be provided in the inlet line  12  downstream of valve  16 . Inlet line  12  terminates at a filter  20 , which may be used for removing particles and other non-desirable matter from within a stream of gas flowing within the inlet line  12 . Filter  20  connects via line  22  to a dryer  24 , which may include a desiccant for removing moisture from the gas stream. 
         [0016]    Optionally, dryer  24  can be empty and provide an open space to operate as a knockout drum thereby removing moisture by gravity separation. Valve  26  is disposed in line  22  for selectively blocking flow between filter  20  and dryer  24 . An outlet line  28  connects dryer  24  to a second filter  30  for additional filtering downstream of the dryer  24 . Valve  32  is shown in line  28  and selectively blocks communication between dryer  24  and filter  30 . Optional regeneration lines  34 ,  36  are shown connecting respectively to line  22  and line  28  between the dryer  24  and valves  26 ,  32 . Desiccant in the dryer  24  can be regenerated by closing valves  26 ,  32  to isolate dryer  24 , opening valves in regeneration lines  34 ,  36 , and circulating a hot and/or dry gas through regeneration lines  34 ,  36  and dryer  24 . A line  38  connects to filter  30  on one end and to a compressor package  40  on another for transmitting gas from the filter  30  to be compressed within the compressor package  40 . A pressure control valve  42  is shown in line  38  for controlling the flow of gas within line  38 . 
         [0017]    The example compressor package  40  of  FIG. 1  is shown having a multistage reciprocating compressor  41 . In the exemplary embodiment of  FIG. 1  reciprocating compressor  41  includes four stages, each including a cylinder  43 , housing a reciprocatingly moving piston, slidingly arranged therein, driven by crankshaft  45  powered by compressor driver  47 . Reciprocating compressor  41  can further include a flywheel  46 . Intercoolers, or interstage coolers (not shown) can be provided between two sequentially arranged cylinders of reciprocating compressor  41 , to reduce the temperature and increase the density of the compressed gas delivered by the upstream stage towards the downstream stage. Reciprocating compressor  41  can be a double-effect compressor, including a cross-head and a piston rod for each piston slidingly housed in a respective cylinder  43 . Each cross-head is connected to crankshaft  45  by a tie rod. 
         [0018]    An exit line  94  connects the last compressor cylinder and provides a transmission line for discharging compressed gas from the compressor package  40 . Thus, in one example, the compressor package  40  receives gas at about the pressure in the supply line  14  and compresses the gas to pressures in excess of about 210 bar, and alternatively to pressures in excess of about 250 bar. Optionally, the discharge pressure in exit line  94  can be in excess of about 300 bar, and alternatively to pressures in excess of about 400 bar. Compressors for use with the method and system described herein are not limited to four stage compressors. Alternative embodiments exist wherein the gas is compressed with a compressor having, one stage, two stages, three stages, five stages, or more than five stages. Also, in further embodiments, more than one reciprocating compressor can be provided, each driven by a respective compressor driver  47 . 
         [0019]    Still referring to  FIG. 1 , the filters  20 ,  30 , dryer  24 , and compressor package  40  are schematically illustrated as being within container  120 , wherein valve  18  is disposed just inside of container  120 . As known for instance from WO 2013/134344, the content whereof is incorporated herein by reference, example containers may include those manufactured to an international standards organization (ISO) and more specifically to ISO standard  6346 . In an embodiment, a standardized container housing the CNG system  10 , after the CNG system  10  is installed in the container  120 , can be readily transported with its contents as a single modular unit. This is because most shippers of freight use vehicles (e.g. trains, tractor trailer rigs, cargo ships) equipped to receive and stow a standardized shipping container. Moreover, attachment points provided on a readily available ISO container enable them to be safely secured in or on a shipping vehicle. 
         [0020]    The CNG system  10  of  FIG. 1  further includes lines  122 ,  124 ,  126  that branch from a portion of exit line  94  downstream of the compressor package  40 . Lines  122 ,  124 ,  126  respectively connect to an inlet of compressed-gas storage tanks  128 ,  130 ,  132 . Although three storage tanks  128 ,  130 ,  132  are illustrated, embodiments exist of the CNG system  10  disclosed herein having zero, one, two, four, and more than four storage tanks. Schematically shown in  FIG. 1 , the storage tanks  128 ,  130 ,  132  are substantially elongate and cylindrical members that in one example are arranged in parallel and for instance mounted on an upper surface of container  120 . In an alternative arrangement, the tanks  128 ,  130 ,  132  can be provided on side or lower surfaces of the container  120 , or separate from the container  120 , such as at grade. Valves  134 ,  136 ,  138  are respectively provided in lines  122 ,  124 ,  126  and are for selectively regulating flow to tanks  128 ,  130 ,  132 . 
         [0021]    Gas compressed in CNG system  10  can be accessible to end users of the compressed gas via dispensers  140 ,  142 . Nozzles  144 ,  146  on dispensers  140 ,  142  provide a flow path for gas compressed in the CNG system  10  to a vehicle (not shown) or other storage vessel for compressed gas purchased by a consumer. Thus, dispensers  140 ,  142  may be equipped with card readers or other payment facilities so that a consumer may purchase an amount of compressed gas at the dispensers  140 ,  142 . Although two dispensers  140 ,  142  are shown, the CNG system  10  can have one, three, or more than three dispensers. 
         [0022]    Lines  94 ,  148 ,  150 ,  152  provide example flow paths between the CNG system  10  and dispensers  140 ,  142 . In the example of  FIG. 1 , lines  148 ,  150 ,  152  have an inlet end connected to lines  122 ,  124 ,  126  and downstream of valves  134 ,  136 ,  138 . Valves  154 ,  156 ,  158  are provided respectively in lines  148 ,  150 ,  152 ; selective opening and closing of valves  154 ,  156 ,  158  in combination with selective opening and closing of valves  134 ,  136 ,  138 ,  159  selectively deliver compressed gas to storage tanks  128 ,  130 ,  132  or directly to dispensers  140 ,  142 . Optionally, gas stored within tanks  128 ,  130 ,  132  can be selectively delivered through one of lines  148 ,  150 ,  152  by the closing of valves  154 ,  156 ,  158 . In one example, compressed gas can flow directly from the compressor package  40  through exit line  94  to the dispensers  140 ,  142 . In this example, valve  159  in line  94  is open to allow flow through exit line  94 . 
         [0023]    As mentioned, compressor driver  47  of compressor package  40  includes a Stirling engine. The Stirling engine can be of any known configuration. According to some embodiments Stirling engine  47  is of the α-type, as illustrated in  FIG. 2  and described in greater detail here below. In other embodiments, not shown, Stirling engine  47  can be a β-type or a γ-type Stirling engine. 
         [0024]    Referring to  FIG. 2 , a Stirling engine  47  of the so called α-type includes a first cylinder  251 , wherein a first piston  253  is slidingly movable. A second cylinder  255  is further provided, oriented at e.g. 90° with respect to the cylinder  251 . A second piston  257  is slidingly arranged in the second cylinder  255 . 
         [0025]    A first connecting rod  259  connects the first piston  253  to a crank pin  261  forming part of an output  263 . A second connecting rod  265  connects the second piston  257  to the same output  263 . A flywheel  267  can be mounted on the output shaft  63 . 
         [0026]    The Stirling engine  47  can include a hot end with a heater  269  which receives heat from the burner  48 . The heater is in flow communication with the interior of the first cylinder  251 . A flow path connects the heater  269  to a regenerator  273 , a cooler  275  and the interior of the second cylinder  255 . The cooler  275  can be in thermal contact with a cold source or heat sink, and forms a cold end of the Stirling engine  47 . The heat sink can be the ambient air. In some embodiments, a cooler with a cooling circuit, for example a water cooling circuit can be used as a heat sink. In  FIG. 2  a cooling circuit is schematically represented by inlet and outlet manifolds  277  and  279 . 
         [0027]    The operation of the Stirling engine is known to those skilled in the art and will not be described in detail herein. In general terms, a working gas contained in the closed system formed by the inner volumes of cylinder-piston system  251 ,  253 , cylinder-piston system  255 - 257 , heater  269 , regenerator  273 , cooler  275  and relevant piping is subject to a thermal cycle including cyclic compression, heating, expansion and cooling. The thermodynamic cycle performed by the working gas in the Stirling engine  47  converts part of the thermal energy delivered by the thermal source  271  to the hot end of the Stirling engine into useful mechanical power available on the output shaft  263 . 
         [0028]    The α-type Stirling engine shown in  FIG. 2  is only one of several possible configurations of Sterling engines. Other useful Sterling engine arrangements are of the β-type and γ-type of Stirling engines, which will not be described herein and which are known to those skilled in the art. 
         [0029]    The various embodiments of the system disclosed herein can utilize an α-type Stirling engine  47  as schematically shown in  FIG. 2 , or else any other suitable Stirling engine configuration, suitable for converting thermal energy available from the thermal energy source or heat source  271  into mechanical power, which is used to drive the reciprocating compressor  1  and/or to produce electric power, as will be described here below. 
         [0030]    The output shaft of Stirling engine  47  can be directly connected to crankshaft  45  of reciprocating compressor  41 . 
         [0031]    The hot end of Stirling engine  47  receives heat from a burner  48 , where natural gas from supply line  14  is burned. The gas delivered to burner  48  can be taken upstream of the filter and drier arrangement  20 ,  24 ,  30 . In some embodiments, gas is however taken from line  38 , after filtering and drying. A gas diverting line  50  connects line  38  to burner  48 . A valve  52  can be provided along diverting line  50 , to shut down fuel delivery to the burner. 
         [0032]    According to some embodiments, the compressor package can be designed so that the compressing piston of the compressor is directly acted upon by the working piston of a free-piston Stirling engine, thus avoiding a crankshaft. An embodiment using a free-piston Stirling engine of this kind for driving the reciprocating compressor of the CNG system  10  is illustrated in  FIG. 3 . The Stirling engine is labeled again  47  and the reciprocating compressor is labeled again  41 . The Stirling engine  47  can include a cylinder  301  wherein a displacer  302  and a power piston  303  are slidingly arranged. The interior of the cylinder  301  is divided into an expansion chamber  301 E at the hot end of the engine and into a compression chamber  301 C at the cold end of the engine. A heater  305  and a cooler  307  are provided along a flow passage connecting the compression chamber and the expansion chamber  301 E. A regenerator  308  is arranged between the heater  305  and the cooler  307 . A bouncing volume or another resilient system, e.g. a set of springs, are provided to bias the power piston  303 . Operation of the free-piston Stirling engine described so far is known and will not be described herein. 
         [0033]    The power piston  303  can be directly connected with a piston rod  313  to a reciprocating piston  315  slidingly housed in a cylinder  317  of the reciprocating compressor  41 . In some embodiments the reciprocating compressor  41  can be a double-effect compressor. The interior of the cylinder  317  can be divided by piston  314  into a first chamber  317 A and a second chamber  317 B. Each chamber  317 A,  317 B can be provided with at least one automatic suction valve  318 A,  318 B and one automatic discharge valve  319 A,  319 B. The suction valves  318 A,  318 B selectively connect the two chambers  317 A,  317 B with a suction duct  321  wherefrom low-pressure gas is sucked. The suction duct  321  can be connected e.g. to the gas supply line  38 . The discharge valves  319 A,  319 B selectively connect the two chambers  317 A,  317 B with a discharge duct, which can be in direct or indirect fluid communication with one or more dispensers  140 ,  142  and/or with one or more storage tanks  128 ,  130 ,  132 . 
         [0034]    The gas supply line  38  also supplies gas to a burner  48  which provides heat to the hot end of the Stirling engine  47 . 
         [0035]    The free-piston Stirling engine provides a direct link between the power piston  303  of the engine and the reciprocating piston  315  of the reciprocating compressor  41 , thus further simplifying the structure of the system. 
         [0036]    Compressor package  40  and optional filter and dryer arrangement  20 ,  24 ,  30  can be arranged in a container for shipping or transportation, e.g. an ISO container. The storage tanks  128 ,  130 ,  132  can be mounted in or on the container or installed separately therefrom. A transportable modular compression system is thus obtained, which can easily be transported to a location having a supply of hydrocarbon gas and which can quickly be connected to the gas supply. The same gas which is compressed by the system is also used as a fuel to operate the Stirling engine. Need for an electric power distribution grid is avoided. 
         [0037]    According to some embodiments, the Stirling engine can also be provided for driving an electric generator  160 , for generating electric power available on a local electric power grid G. The electric power generated by electric generator  160  can be used to power auxiliaries of the system  10  and/or external additional users. 
         [0038]    The use of an external combustion engine, such as a Stirling engine, rather than an internal combustion engine makes the compressor package more reliable and requiring virtually no maintenance. Among the potential benefits of a Stirling engine vs. an internal combustion engine the following are worth noting: limited or no need (depending upon the configuration of the Stirling engine) for lubrication oil, which reduces or eliminates the need of lubrication circuit topping-up, periodical oil and oil-filter replacement; no need for spark plugs, air filters, timing chains and other components of the timing system; no need for fuel injection systems; and consequent reduction of management and maintenance costs. This makes the system particularly suitable for use in locations which are difficult to reach e.g. by suppliers of spare parts. 
         [0039]    In an embodiment, Stirling engines are moreover particularly useful in combination with a CNG reciprocating compressor as the rotational speed rate of a Stirling engine is substantially the same as the rotational speed of the reciprocating compressor and thus gearboxes can be avoided, which increases the overall efficiency of the system and the reliability thereof. 
         [0040]    The reduced vibrations and noise of the Stirling engine vis-à-vis an internal combustion engine make the use thereof even more attractive. 
         [0041]    While the disclosed embodiments of the subject matter described herein have been shown in the drawings and fully described above with particularity and detail in connection with several exemplary embodiments, it will be apparent to those of ordinary skill in the art that many modifications, changes, and omissions are possible without materially departing from the novel teachings, the principles and concepts set forth herein, and advantages of the subject matter recited in the appended claims. Hence, the proper scope of the disclosed innovations should be determined only by the broadest interpretation of the appended claims so as to encompass all such modifications, changes, and omissions. Different features, structures and instrumentalities of the various embodiments can be differently combined.