A positive-displacement unitary pump and turbine comprising: a rotor casing defining a rotor chamber having a contoured wall forming a plurality of lobes (50,52,54,56) 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 (50,52,54,56); 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 (50,52,54,56); 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 (50,52) at respective ones of said inlet ports and the charging fluid exiting said first and second lobes (50,52) 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 (54,56) via respective ones of said inlet ports into a higher energy state upon exiting said third and fourth lobes (54,56) via respective ones of said outlet ports; characterised in that said lobes (50,52,54,56), said inlet and outlet ports (58,60), 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 (50,52,54,56) 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 (50,52,54,56); 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 (50,52,54,56); 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 (50,52) at respective ones of said inlet ports and the charging fluid exiting said first and second lobes (50,52) 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 (54,56) via respective ones of said inlet ports into a higher energy state upon exiting said third and fourth lobes (54,56) via respective ones of said outlet ports; characterised in that said lobes (50,52,54,56), said inlet and outlet ports (58,60), 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.