Producing power from a cryogenic liquid

Claim 1

A method of producing power from a store of a cryogenic liquid containing methane, the method comprising (a) compressing the cryogenic liquid to supercritical pressure; (b) providing first and second closed and independent power cycles employing, respectively, first and second methane-containing heat exchange media each of which undergoes isobaric condensation over a range of temperatures; (c) in said first closed power cycle, compressing said first heat exchange medium in condensed form to a superatmospheric pressure P 2 , evaporating it, expanding the evaporated medium to a lower superatmospheric pressure P 1 in a first expansion engine, condensing the expanded vapour and recycling the condensate so formed for recompression; (d) in said second closed power cycle, compressing said second heat exchange medium in condensed form to a superatmospheric pressure P 4 , evaporating it, expanding the evaporated medium to a lower superatmospheric pressure P 3 in a second expansion engine, condensing the expanded vapour and recycling the condensate so formed for recompression; and wherein (e) the temperature range at which the condensed first heat exchange medium evaporates at P 2 is lower than the temperature range at which the expanded second heat exchange medium condenses at P 3 ; (f) the first heat exchange medium is condensed by indirect heat exchange with said compressed cryogenic liquid in a first heat exchange step at subambient temperature; (g) the second heat exchange medium is condensed by indirect heat exchange at subambient temperature in a second heat exchange step with evaporating first heat exchange medium and with compressed cryogenic liquid recovered from said first heat exchange step; and (h) power is taken from said first and second engines.

Claim 2

A method of producing power from a store of a cryogenic liquid containing methane, the method comprising (a) compressing the cryogenic liquid to supercritical pressure; (b) providing first and second closed and independent power cycles employing, respectively, first and second methane-containing heat exchange media each of which undergoes isobaric condensation over a range of temperatures; (c) in said first closed power cycle, compressing said first heat exchange medium in condensed form to a superatmospheric pressure P 2 , evaporating it, expanding the evaporated medium to a lower superatmospheric pressure P 1 in a first expansion engine, condensing the expanded vapour and recycling the condensate so formed for recompression; (d) in said second closed power cycle, compressing said second heat exchange medium in condensed form to a superatmospheric pressure P 4 , evaporating it, expanding the evaporated medium to a lower superatmospheric pressure P 3 in a second expansion engine, condensing the expanded vapour and recycling the condensate so formed for recompression; and wherein (e) the temperature range at which the condensed first heat exchange medium evaporates at P 2 is lower than the temperature range at which the expanded second heat exchange medium condenses at P 3 ; (f) the first heat exchange medium is condensed by indirect heat exchange with said compressed cryogenic liquid in a first heat exchange step at subambient temperature; (g) the second heat exchange medium is condensed by indirect heat exchange at subambient temperature in a second heat exchange step with evaporating first heat exchange medium and with compressed cryogenic liquid recovered from said first heat exchange step; and (h) power is taken from said first and second engines.