A method of producing and utilizing thermal energy in a combined heat and power plant including the steps of: a) generating a flow of steam in a steam generator from a flow of feed water; b) directing a generated flow of steam from the steam generator into a steam engine to produce mechanical power and discharging a flow of exhaust steam from the steam engine; c) directing either a flow of exhaust steam from the steam engine or a flow comprising a combination of an exhaust steam from the steam engine and supplementary steam from a steam generator as process steam to provide thermal energy for a downstream process; d) in at least a vessel, wherein the vessel includes a primary space and a secondary space in heat communication with, but with fluid separation from, each other, said secondary space containing a quantity of a secondary fluid in liquid phase; introducing the flow of process steam of step (c) into the primary space as primary steam to vaporize the secondary fluid in liquid phase as a vaporized secondary fluid and resulting in precipitation of the primary steam as a primary condensate simultaneously introducing the secondary fluid in liquid phase as make-up fluid into the secondary space directing the vaporized secondary fluid to impart thermal energy for the downstream process whilst delivering the primary condensate from the primary space characterized in that the method further includes the steps of: e) during periods of surplus vaporized secondary fluid, increasing a flow rate of introduction of the make-up fluid into the secondary space, thus reducing the surplus vaporized secondary fluid and accumulating a transitionally confined reserve quantity of the secondary fluid in liquid phase in a heated state in a reservoir space, and during periods of deficit vaporized secondary fluid, decreasing the flow rate of introduction of the make-up fluid into the secondary space, thus increasing the output of the vaporized secondary fluid and depleting the transitionally confined reserve quantity of the secondary fluid in liquid phase in the heated state in the reservoir space by continued vaporization, wherein the temperature of the make-up fluid introduced into the secondary space is lower than the temperature of the vaporized secondary fluid. a) generating a flow of steam in a steam generator from a flow of feed water; b) directing a generated flow of steam from the steam generator into a steam engine to produce mechanical power and discharging a flow of exhaust steam from the steam engine; c) directing either a flow of exhaust steam from the steam engine or a flow comprising a combination of an exhaust steam from the steam engine and supplementary steam from a steam generator as process steam to provide thermal energy for a downstream process; d) in at least a vessel, wherein the vessel includes a primary space and a secondary space in heat communication with, but with fluid separation from, each other, said secondary space containing a quantity of a secondary fluid in liquid phase; introducing the flow of process steam of step (c) into the primary space as primary steam to vaporize the secondary fluid in liquid phase as a vaporized secondary fluid and resulting in precipitation of the primary steam as a primary condensate simultaneously introducing the secondary fluid in liquid phase as make-up fluid into the secondary space directing the vaporized secondary fluid to impart thermal energy for the downstream process whilst delivering the primary condensate from the primary space characterized in that the method further includes the steps of: e) during periods of surplus vaporized secondary fluid, increasing a flow rate of introduction of the make-up fluid into the secondary space, thus reducing the surplus vaporized secondary fluid and accumulating a transitionally confined reserve quantity of the secondary fluid in liquid phase in a heated state in a reservoir space, and during periods of deficit vaporized secondary fluid, decreasing the flow rate of introduction of the make-up fluid into the secondary space, thus increasing the output of the vaporized secondary fluid and depleting the transitionally confined reserve quantity of the secondary fluid in liquid phase in the heated state in the reservoir space by continued vaporization, wherein the temperature of the make-up fluid introduced into the secondary space is lower than the temperature of the vaporized secondary fluid. e) during periods of surplus vaporized secondary fluid, increasing a flow rate of introduction of the make-up fluid into the secondary space, thus reducing the surplus vaporized secondary fluid and accumulating a transitionally confined reserve quantity of the secondary fluid in liquid phase in a heated state in a reservoir space, and during periods of deficit vaporized secondary fluid, decreasing the flow rate of introduction of the make-up fluid into the secondary space, thus increasing the output of the vaporized secondary fluid and depleting the transitionally confined reserve quantity of the secondary fluid in liquid phase in the heated state in the reservoir space by continued vaporization, wherein the temperature of the make-up fluid introduced into the secondary space is lower than the temperature of the vaporized secondary fluid.