number 1 in fig1 indicates as a whole a fuel cell system for earth vehicles , which comprises a fuel cell electrochemical conversion unit 2 ; a fuel tank 3 for supplying unit 2 ; and a recirculating assembly 4 for recirculating surplus fuel not used in the electrochemical conversion . unit 2 , which is known and therefore only described herein as required for a clear understanding of the present invention , may operate according to any of various known construction designs and technologies , and , in the preferred embodiment described , is a proton exchange membrane ( pem ) type employing hydrogen as fuel , and oxygen in atmospheric air as the oxidizing agent . the present invention , however , is not limited to the above type of electrochemical conversion unit , and may be used with electrochemical conversion units of any construction design or technology . unit 2 is defined in known manner by a number of stacked fuel cells 5 connected electrically in series , and defines as a whole a cathode 7 and an anode 8 . an electric circuit 13 connects cathode 7 and anode 8 of unit 2 , and can thus power an electromechanical load 15 — in this case , an electric motor for driving a vehicle — by means of the energy generated by system 1 . unit 2 is supplied at the inlet with atmospheric air by a conduit 9 , and with hydrogen from tank 3 and from recirculating assembly 4 by a conduit 10 , as explained in detail later on . more specifically , the atmospheric air feed conduit 9 enters unit 2 on the cathode 7 side , supplies the individual cells 5 with oxygen , and , on exiting cathode 7 , conducts the waste products , defined by steam and low - oxygen air , to recirculating assembly 4 . the hydrogen feed conduit 10 enters unit 2 on the anode 8 side , supplies the individual cells 5 with hydrogen , and , on exiting anode 8 , conducts the surplus hydrogen to recirculating assembly 4 . with reference to fig1 and 2 , recirculating assembly 4 substantially comprises a compressor 12 for compressing the hydrogen from unit 2 ; and a drive member for driving compressor 12 . an important aspect of the present invention lies in the drive member being defined by a turbine 11 powered by the energy of the waste produces from unit 2 . more specifically , turbine 11 and compressor 12 are angularly connected by a permanent - magnet or electromagnetic joint 16 described in detail later on , and define as a whole a turbocharger 6 of axis a . turbine 11 comprises a spiral casing 18 , and an impeller 19 rotating about axis a inside casing 18 . more specifically , casing 18 defines a circumferential inlet section 18 a for the waste products from unit 2 ; and an outlet section 18 b for expelling the products expanded by turbine 11 from system 1 . more specifically , impeller 19 of turbine 11 is fitted to a shaft 14 projecting from turbine 11 . in a preferred embodiment of the present invention , impeller 19 may be coated with , or made entirely of , corrosion - resistant material , e . g . teflon , to safeguard against corrosion by the deionized steam in turbine 11 . alternatively , impeller 19 may be surface - treated to resist such corrosion . compressor 12 comprises a spiral casing 20 , and an impeller 21 rotating about axis a inside casing 20 . more specifically , casing 20 defines an axial inlet section 20 a for the feedback hydrogen from unit 2 ; and a circumferential outlet section 20 b for feeding the compressed hydrogen to unit 2 along a recirculating conduit 25 connected to conduit 10 . more specifically , impeller 21 is fitted to a shaft 17 projecting from compressor 12 and angularly connectable to shaft 14 by means of joint 16 . in a preferred embodiment of the present invention , impeller 21 may be coated with , or made entirely of , corrosion - resistant material , e . g . teflon , to safeguard against corrosion by the deionized steam in compressor 12 . alternatively , impeller 21 may be surface - treated to resist such corrosion . joint 16 will be described , purely by way of example , with reference to a known permanent - magnet , axial - flow , electric machine not described in detail . joint 16 transmits motion from shaft 14 to shaft 17 , while keeping them a predetermined axial distance apart , and with no contacting mechanical parts , and , more specifically , comprises a disk 22 a fitted to the opposite end of shaft 14 to turbine 11 , and a disk 22 b fitted to the opposite end of shaft 17 to compressor 12 . disks 22 a and 22 b have a number of permanent magnets , and are maintained a predetermined axial distance apart . using joint 16 as described above , which provides for transmitting motion between shafts 14 and 17 with no mechanical contact , the waste product expansion environment can be separated in fluidtight manner from the recirculated hydrogen compression environment , so that shaft 14 and relative disk 22 a are housed in a first casing 23 integral with casing 18 of turbine 11 , and shaft 17 and relative disk 22 b are housed in a second casing 24 separate from casing 23 and integral with casing 20 of compressor 12 . as such , the hydrogen compressed in compressor 12 and the waste products can in no way react chemically with each other to impair the efficiency of recirculating assembly 4 and , therefore , of system 1 . in actual use , impeller 19 of turbine 11 , powered by the waste products from conduit 9 , rotates shaft 14 and disk 22 a integral with shaft 14 ; disk 22 a rotates disk 22 b by means of joint 16 , which thus makes rotation about axis a of shaft 14 of turbine 11 angularly integral with similar rotation about axis a of shaft 17 of compressor 12 ; and disk 22 b rotates impeller 21 of compressor 12 by means of shaft 17 , so as to increase the pressure of the feedback hydrogen fed to compressor 12 by conduit 10 . the waste products from turbine 11 constitute the exhaust of system 1 , whereas the hydrogen from compressor 12 is fed by recirculating conduit 25 into conduit 10 where it mixes with the hydrogen from tank 3 to supply unit 2 . to better regulate system 1 , provision may be made , immediately downstream from tank 3 , for a regulating valve 26 and a flow gauge 27 for regulating the amount of fuel required to supplement the hydrogen made available to unit 2 by recirculating assembly 4 . the advantages of recirculating assembly 4 according to the present invention will be clear from the foregoing description . in particular , recirculating assembly 4 employs the residual energy in the waste products to drive shaft 14 by means of turbine 11 , and , by means of joint 16 , to drive compressor 12 to bring the feedback hydrogen to the pressure necessary to supply unit 2 , and therefore requires no additional energy sources . recirculating assembly 4 therefore provides for enhancing the efficiency of fuel cell system 1 , with no increase in cost or complexity . clearly , changes may be made to recirculating assembly 4 as described and illustrated herein without , however , departing from the scope of the accompanying claims . the invention has been described in detail with respect to preferred embodiments , and it will now be apparent from the foregoing to those skilled in the art , that changes and modifications may be made without departing from the invention in its broader aspects , and the invention , therefore , as defined in the appended claims , is intended to cover all such changes and modifications that fall within the true spirit of the invention .