A positive-displacement unitary pump and turbine comprising: a rotor casing defining a rotor chamber having a contoured wall forming a plurality of lobes of said chamber, said lobes comprising at least a first lobe , a second lobe , a third lobe and a fourth lobe ; an inlet port and an outlet port defined in said contoured wall at each of said lobes ; a rotor positioned in said rotor chamber , said rotor having an outer rotor surface spaced inwardly from said contoured wall at said at least four lobes ; and a plurality of vanes mounted at said rotor and spaced circumferentially around said outer rotor surface , said vanes having distal end portions configured to slidably engage said contoured wall ; wherein said rotor is rotatably drivable by a charging fluid at a higher energy state entering said first and second lobes at respective ones of said inlet ports and the charging fluid exiting said first and second lobes at a lower energy state via respective ones of said outlet ports ; wherein said rotor is operable to convert a feed fluid at a lower energy state entering said third and fourth lobes via respective ones of said inlet ports into a higher energy state upon exiting said third and fourth lobes via respective ones of said outlet ports ; characterised in that said lobes , said inlet and outlet ports , and said vanes are arranged so that each of (i) the higher energy charging fluid , (ii) the lower energy feed fluid , (iii) the lower energy charging fluid , and (iv) the higher energy feed fluid , acting in combination, apply a net radial force of substantially zero to said rotor during operation thereof. a rotor casing defining a rotor chamber having a contoured wall forming a plurality of lobes of said chamber, said lobes comprising at least a first lobe , a second lobe , a third lobe and a fourth lobe ; an inlet port and an outlet port defined in said contoured wall at each of said lobes ; a rotor positioned in said rotor chamber , said rotor having an outer rotor surface spaced inwardly from said contoured wall at said at least four lobes ; and a plurality of vanes mounted at said rotor and spaced circumferentially around said outer rotor surface , said vanes having distal end portions configured to slidably engage said contoured wall ; wherein said rotor is rotatably drivable by a charging fluid at a higher energy state entering said first and second lobes at respective ones of said inlet ports and the charging fluid exiting said first and second lobes at a lower energy state via respective ones of said outlet ports ; wherein said rotor is operable to convert a feed fluid at a lower energy state entering said third and fourth lobes via respective ones of said inlet ports into a higher energy state upon exiting said third and fourth lobes via respective ones of said outlet ports ; characterised in that said lobes , said inlet and outlet ports , and said vanes are arranged so that each of (i) the higher energy charging fluid , (ii) the lower energy feed fluid , (iii) the lower energy charging fluid , and (iv) the higher energy feed fluid , acting in combination, apply a net radial force of substantially zero to said rotor during operation thereof. <EOS>