EP1447614A2 - Station for supplying pressurized gas to tanks, in particular tanks installed on vehicles - Google Patents

Abstract

Station for supplying pressurized gas to tanks, inparticular to tanks installed on vehicles, includes anelectric unit (1), fed by an electric main (L) and aimedat supplying at the outlet, in accordance with thecontrol and command signals of a command and control unit(C), to which it is connected, an electric voltage ofadjustable effective value (V₀) and frequency (f₀). Anelectric motor (M) is powered by the electric unit. Acompression unit (Y) is operated by the shaft of themotor (M), and its aspiration is connected to a lowpressure gas main (AM). A reference tank (P) is connectedto the delivery of the compression unit (Y). A device (D)for delivering the pressurized gas is connected to thereference tank (P), and it has connectors (U1, U2, ...,Un), which can be connected to as many users (K). Atleast one sensor (R) for measuring pressure inside saidtank (P) is connected to the command and control unit(C), which compares a selected pressure value with thepressure value measured by the sensor, and commands theelectric unit (I) in relation to the pressure differencebetween the selected and measured values, with aconsequent activation of the compression unit (Y), inwhich the working cycles in time unit depend on theeffective value (V₀) and frequency (f₀) of the motor (M)power voltage.

Description

• There are known stations for supplying pressurizedmethane to the tanks of users of such stations, forexample tanks installed on motor-vehicles.
• These stations include substantially an electric motor(usually an asynchronous motor with a squirrel-cagerotor), aimed at operating a compressor unit (a multistageone), fed by the methane delivery main (for examplewith interposition of an expansion tank) feeding the gasat a pressure of about 8 - 10 bar, and for supplying, inturn, a battery of tanks connected in parallel, with apressure, which is usually about 250 bar.
• As it is known, during the compression, the temperatureof methane increases, therefore it is necessary to placeheat exchangers between one stage of the compression unitand another, and between the last stage and the tanks.
• Known stations use gas/air exchangers, in particularconsisting of a fan, which sends a jet of air, incounter-flow, on a serpentine, through which methane ismade to pass.
• The motor - compression unit group is activated when thedelivery pressure of methane, at the outlet correspondingto the tanks (that is the user side), is lower than aprefixed value (for example 220 bar) and it isdeactivated, when a selected value (for example 250 bar)is reached.
• The group is activated again as soon as the pressure islower than the above mentioned selected value.
• Consequently, the above group is subjected to an "on/off"regulation, which results in many drawbacks.
• When activated, the electric motor works to deliver itsmaximum power, therefore the corresponding electricstation, supplying it, must be designed with acorresponding capacity.
• As it is known, an asynchronous motor delivers a ratherlimited torque at the pickup moment, thus it is necessaryto take measures against loading the motor shaft with anexcessive resisting torque.
• What above makes the aforementioned group reach its fullcapacity with a certain delay, with all consequentproblems; in other words, the above mentioned group meetsthe delivery requests with a delay.
• This aspect is partially compensated by the receivereffect of the battery of tanks, however all this resultsin an increase of time necessary to fill the users' tanks(bottles).
• In order to reduce this negative aspect, in some knownstations, the battery of tanks is divided in two parts,respectively at medium pressure (between 170 and 220 bar)and high capacity, and at high pressure (between 220 and250 bar) and capacity reduced with respect to the firstone.
• The presence of at least one battery of tanks in thestation forces the constructor to place the group and thebattery inside a kind of close parallelepiped structure,having apertures for the flow of air generated by thecooling fans of the radiators, through which methanepasses.
• The conformation of this structure depends on the numberof tanks, to comply with the relative safetyrequirements.
• Another drawback derives from the current transients(surge current), occurring to the asynchronous motorduring the pickup, partially limited by the well knownstar-delta starter.
• A further drawback lies in the fact that in knownstations, the temperature of the methane at the outlet ofthe last compression stage, which, in a station intendedfor one or more users, practically coincides with thetemperature of methane delivered to the users' bottles,depends on the external air temperature; in hot monthsthe temperature of methane introduced into the bottlescan be excessive (for example 35°C), which reduces thedegree of bottles filling.
• The object of the present invention is to propose astation for supplying tanks with gas, in particularmethane, which avoids the above mentioned drawbacks, dueto its conformation allowing to modulate the delivery ofgas in accordance with the users' needs.
• Another object of the present invention is to propose agroup electric motor - compression unit, which fitsitself rapidly to the users' needs.
• A further object of the present invention is to propose astation, in which the electric motor - compression unitgroup does not provoke surge currents during the pickupand which allows adjustments of the driving torque outputby the motor shaft in accordance with the resistingtorque generated by the compressor unit.
• A still further object of the present invention is topropose a station, which does not need the receiver orbank effect.
• Yet a further object of the present invention is topropose a station, in which the temperature of themethane introduced into the users' bottles can beadjusted and selected, so as to optimize filling of thelatter independently from the climatic conditions andfrom the number of bottles fed contemporarily.
• Another object of the present invention is to propose astation, which does not need the presence of a battery oftanks.
• The above mentioned objects are obtained by the presentinvention, in accordance with the contents of the claims.
• The characteristic features of the invention are pointedout in the following, with particular reference to thedrawing, in which:
• Fig. 1 shows a block diagram of the station proposedby the invention.
• The diagram does not indicate, even schematically, meansand/or devices related to the systems of control andsafety of the station, as they are not connected to theinvention.
• With reference to the figure, the reference letter Lindicates an electric main (preferably three-phase),which feeds, in accordance with the condition of a switchQ, operated by a command and control unit C, a group I(known to those skilled in the art as inverter), aimed atsupplying at the outlet a sinusoid voltage (in this casea three-phase tern), whose effective value V₀ andfrequency f₀ vary continuously.
• The group I is controlled by the command and control unitC.
• The group I feeds an electric motor M, for example anasynchronous three-phase induction motor (preferably withsquirrel-cage rotor), which drives into rotation a multistagecompression unit Y with angular speed Ω; accordingto the shown example two stages A, B are considered, forexample including two alternative compressors.
• The aspiration of the first stage A is connected to themethane delivery main AM, in particular to an expansiontank S.
• The delivery of the first stage A is connected to theaspiration of the second stage B; a first heat exchangergas/liquid S_A is interposed therebetween. The temperatureT_B1 of the flow of methane exiting the heat exchanger ismeasured by a sensor H_A connected to the command andcontrol unit C.
• The cooling liquid of the first heat exchanger S_A,preferably water, circulates in acircuit 100, whichincludes: achamber 110 with the serpentine S1_A of theexchanger S_A, in which the gas passing through theserpentine is cooled; a liquid/air heat exchanger 120,for example a cooling tower (known also as evaporationtower), which receives the liquid from thechamber 100 bymeans of a pump W controlled by the command and controlunit C; achannel 130 connecting the liquid/air heatexchanger 120 with thechamber 110, allowing the liquid,cooled in the exchanger, to return to thechamber 110.
• The delivery of the second stage B is connected to areference tank P and a second gas/liquid heat exchanger S_Bis interposed therebetween. The temperature T_f of themethane at the delivery of the second stage B is measured by a sensor H_B connected to the command and control unitC.
• In the shown example, the serpentine S1_B of the secondexchanger S_B is connected to saidcooling circuit 100; forthis purpose it is situated inside thechamber 110.
• It is understood that the serpentines S1_A , S1_B of thegas/liquid heat exchangers S_A, S_B, first and second, can beconnected to corresponding circuits, independent one fromanother, so as to cool the liquid circulating in thecircuits, for example, by means of gas/liquid heatexchangers of the type described with reference to thecircuit 100.
• The pressure of methane inside the tank P is measured bya sensor R connected to the command and control unit C.
• The tank feeds a supplying device D, having attachmentsof known type, indicated with U1, U2, ...Un, which allowthe connection with the bottles of a corresponding numberof users K.
• It is known that the mechanical characteristics of anasynchronous three-phase induction motor are function ofthe effective value V₀ and of the frequency f₀ of thesupplying voltage; consequently, it is possible to changethe torque and the angular speed Ω of the motor shaft bycontinuous adjusting the above mentioned parameters.
• Accordingly, it is possible to change the number ofrevolutions in time unit (rpm) of the motor shaft, andobviously, of the number of working cycles in time unitof the compression unit Y.
• It is to be pointed out that, the temperature T_B1 of themethane at the inlet of the second stage B (measured bythe sensor H_A) can be adjusted to the desired value by changing the speed of the pump W disposed in thecoolingcircuit 100 of the liquid flowing through the firstexchanger S_A, by means of the command and control unit C,while the other conditions remain unchanged.
• Likewise, the temperature T_f of the methane sent to thetank P can be adjusted to the desired value (e.g. 25°C)by changing the speed of the pump W through the commandand control unit C, while the other conditions remainunchanged.
• In the example taken into consideration, there is onlyone liquid/air cooling circuit for both gas/liquidexchangers; however, this does not create a limitation,because the temperature to be controlled and adjusted tothe desired value is the T_f temperature of the gas sent tothe tank P.
• The operation of the station proposed by the presentinvention results to be obvious from the abovedescription.
• In rest conditions, that is when the device D does notdeliver any methane, the station must be ready tooperate, that is, it must be able to serve the users.
• For this purpose, the reference tank P must containmethane at a selected pressure (for example: 240 - 250bar) obtained in the way explained later on.
• The selected value is stored in the command and controlunit C.
• The connection of at least one user tank (bottle) to thesupply device D makes the tank P deliver methaneimmediately.
• Consequently, the pressure in the tank P tends to lower,which is picked up by the sensor R.
• The command and control unit C compares the selectedvalue with the picked up value, and since the selectedstored value in higher, it activates the inverter, whichin turn operates the motor M.
• The motor pickup occurs gradually and, at the same time,rapidly, while the value of the delivered torque has asubstantially linear path with respect to the angularspeed Ω.
• During this pickup, there are practically no surgecurrents, since the motor is fed in such a way, as toavoid them.
• What above causes the activation of the compressor unit Yand of the means connected thereto; in particular, thecommand and control unit C adjusts the speed of the pumpW, so as to impose the selected value of T_f temperature ofmethane, sent in parallel to the tank P and to the supplydevice D.
• The delivery of the "pumped" methane must satisfy theusers K needs.
• When the quantity of delivered methane increases (forexample: the number of users K increases), the pressureof the delivered methane tends to decrease, which ispicked up by the sensor R, and consequently by thecommand and control unit C.
• The command and control unit C controls in a suitable waythe inverter I, which increases the effective value V₀ andfrequency f₀ of the voltage supplied to the motor M.
• Consequently, the motor shaft increases the deliveredtorque and speed, which results in the increase of thenumber of working cycles of the compressor unit Y in thetime unit.
• Thus, the station sends a required quantity of methane tothe supply device D.
• In these conditions, the decrease of the quantity ofdelivered methane causes the increase of the pressurewith respect to the previous situation, which is pickedup by the sensor R.
• The command and control unit C, by means of the inverterI, changes the electric parameters of the motor poweringvoltage, so as to reduce the number of working cycles ofthe compressor unit, to adjust the quantity of "pumped"methane in accordance to the contingent situation ofrequired delivery.
• Consequently, the proposed station "pumps" such aquantity of methane, which depends on the quantity ofmethane delivered by the device D, in other words, itdepends on the number of users K connected to the deviceD.
• When the last user K is disconnected from the device D,the station continues to "pump" methane to the tank P,until a pressure value equal to the prefixed one isreached inside the tank P.
• In these conditions, the motor M is de-activated and thestation is ready to deliver methane as soon as a user Kis connected to the device D.
• The compressors used during the compression stages A, Bare alternative, therefore the pressure of the "pumped"methane is not continuous and have pulses with respect tothe medium value.
• The pulses inside the reference tank P are minimized tonegligible values; consequently, the value measured bythe sensor R is not subjected to the pulses.
• The proposed technical solution has various technical-functionaladvantages.
• The motor startup does not cause surge currents, which ispositive for the motor itself, as well as for all themeans situated upstream of the inverter.
• The motor is started rapidly and under loaded condition,which is positive for the compliance (elasticity) of thestation to the effective methane request.
• The motor M - compression unit Y group must satisfy themaximum load conditions, that is with all the attachmentsU1, ..., Un connected to a corresponding number of users;in other conditions, the group exploits only a part ofits capacity.
• The proposed station adapts rapidly to the increases ordecreases of the methane request; in these conditions themotor - compression unit group is activated, so as to"auto-adapt" to the changing delivery requests.
• The motor - compression unit group is deactivated, whenthe pressure in the tank P is equal to the selectedvalue; this occurs when the device D stops deliveringmethane.
• It is to be pointed out that the reference tank P has alimited capacity, because its function is to allow thedetermination of a reference pressure, which is extremelypositive for the station costs, as well as the dimensionsof the structure containing it.
• The station, proposed by the invention, allows anadjustment of the methane temperature Tf to a value,which optimizes the users' bottles filling.
• It is to be pointed out that the proposed station hasonly onecircuit 100 for cooling the liquid of both gas/liquid exchangers S_A, S_B; which results in thesimplification of the station construction, withconsequent costs reduction, as well as of the stationmaintenance.
• Another advantage of the invention derives from the factthat it uses a particular heat liquid/air exchanger tocool the liquid of thecooling circuit 100, moreprecisely, an evaporation tower.
• Thus, the production of thecircuit 100 is simplified,and the control of the speed of only one pump W allowsadjustment of the temperature Tf of the methane "pumped"to the delivery device D.
• The shown example takes into consideration twocompression stages; it is understood that the proposedtechnical solution does not depend on the number ofstages.

Claims (4)

1. Station for supplying pressurized gas to tanks, inparticular to tanks installed on vehicles,characterizedin that it includes:

   an electric unit (1), fed by an electric main (L)and aimed at supplying at the outlet an electricvoltage of variable effective value (V₀) andfrequency (f₀), in accordance with control andcommand signals issued by a command and control unit(C), to which it is connected;

   an electric motor (M) powered by said electric unit;

   a compressor unit (Y), which is operated by theshaft of said motor (M), and whose aspiration isconnected to a low pressure gas main (AM) feedingsaid gas;

   a reference tank (P), connected to the delivery ofsaid compression unit (Y);

   a device (D) for delivering said gas under pressure,connected to said reference tank (P), and havingconnectors (U1, U2, ..., Un), which can be connectedto a corresponding number of users (K);

   at least one sensor (R) for measuring pressureinside said reference tank (P), the sensor beingconnected to said command and control unit (C),which compares a selected pressure value with thepressure value measured by the sensor, and controlssaid electric unit (I) in relation to the pressuredifference between said selected and measuredvalues, with a consequent activation of saidcompression unit (Y), in which the working cycles in time unit depend on said effective value (V₀) andsaid frequency (f₀) of said motor (M) power voltage.
2. Station according to claim 1,characterized in thatsaid compressor unit is defined by at least onecompression stage (A,B), whose delivery is connected tosaid reference tank (P), with at least one heat exchangerdisposed therebetween for cooling said gas, in which saidexchanger (S1_A, S1_B) is a gas/liquid exchanger, and theliquid, in turn, is cooled by a liquid/air heat exchanger(120).
3. Station according to claim 2,characterized in thatthe circuit (100), containing said cooling liquid,includes at least one pump (W) and at leas one sensor (H_A,H_B), which measures the gas temperature at the delivery ofsaid compression stage, said sensor being connected tosaid command and control unit (C), which controls saidpump (W), in relation to the difference between aselected temperature value and the value measured by saidsensor (H_A, H_B), with consequent maintaining of the gastemperature (Tf) at the above mentioned selected value.
4. Station, according to claim 2,characterized in thatsaid liquid/air heat exchanger (120) is a cooling tower.

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