A closed organic Rankine cycle power system arranged to produce power from an alternative energy heat source such as solar energy or geothermal energy resource fluid, comprising: a vaporizer in which liquid intermediate organic working fluid is vaporized using heat from a solar energy heat source fluid or geothermal energy resource liquid so that intermediate organic working fluid vapor and heat depleted solar energy or geothermal energy resource liquid is produced; a turbine for expanding said intermediate organic working fluid vapor and producing power and expanded intermediate organic working fluid vapor; a condensing and vaporizing means for condensing said expanded vaporised intermediate organic working fluid to produce intermediate organic working fluid condensate and for vaporizing further organic working fluid to produce further organic working fluid vapor; a further turbine for expanding the further organic working fluid vapor and producing power and expanded further organic working fluid vapor; a condenser to which expanded further organic working fluid vapor is supplied, said condenser producing further organic working fluid condensate; means for supplying said further organic working fluid condensate to said condensing and vaporizing means; a preheater for preheating said further organic working fluid condensate prior to supplying it to said condensing and vaporizing means; and a recuperator for receiving said intermediate expanded working fluid vapor and transferring heat to said intermediate working fluid condensate producing heated intermediate working fluid condensate, characterized in that said intermediate organic working fluid is a highly thermally stable working fluid suitable for operating in the range of about -50° C. up to 350° C., comprising as the major component at least one compound of formula C n H 2n+2 wherein n is between 8 and 20, wherein the compounds of formula have at least three branched methyl groups, and wherein part or all the methyl groups present in said compound are substituted with one or more halogens or halogen containing substituents, wherein said halogens are selected from the group consisting of fluorine, chlorine, and bromine, wherein said intermediate organic working fluid comprises an additive selected from the group consisting of fire-retardant agents, flow aids, corrosion inhibitors, lubricants, anti-freezing agents, antioxidants, and process oils and mixtures thereof. a vaporizer in which liquid intermediate organic working fluid is vaporized using heat from a solar energy heat source fluid or geothermal energy resource liquid so that intermediate organic working fluid vapor and heat depleted solar energy or geothermal energy resource liquid is produced; a turbine for expanding said intermediate organic working fluid vapor and producing power and expanded intermediate organic working fluid vapor; a condensing and vaporizing means for condensing said expanded vaporised intermediate organic working fluid to produce intermediate organic working fluid condensate and for vaporizing further organic working fluid to produce further organic working fluid vapor; a further turbine for expanding the further organic working fluid vapor and producing power and expanded further organic working fluid vapor; a condenser to which expanded further organic working fluid vapor is supplied, said condenser producing further organic working fluid condensate; means for supplying said further organic working fluid condensate to said condensing and vaporizing means; a preheater for preheating said further organic working fluid condensate prior to supplying it to said condensing and vaporizing means; and a recuperator for receiving said intermediate expanded working fluid vapor and transferring heat to said intermediate working fluid condensate producing heated intermediate working fluid condensate, characterized in that said intermediate organic working fluid is a highly thermally stable working fluid suitable for operating in the range of about -50° C. up to 350° C., comprising as the major component at least one compound of formula C n H 2n+2 wherein n is between 8 and 20, wherein the compounds of formula have at least three branched methyl groups, and wherein part or all the methyl groups present in said compound are substituted with one or more halogens or halogen containing substituents, wherein said halogens are selected from the group consisting of fluorine, chlorine, and bromine, wherein said intermediate organic working fluid comprises an additive selected from the group consisting of fire-retardant agents, flow aids, corrosion inhibitors, lubricants, anti-freezing agents, antioxidants, and process oils and mixtures thereof.