Heat recovery steam generator comprises a casing, low-pressure evaporator coils, preheater booster coils upstream thereof and feedwater heater coils downstream thereof, a water-to-water heat exchanger having low and high temperature paths; a first conduit from the preheater to the high-temperature path, and a second conduit from the feedwater heater to the preheater. A conduit can extend from feedwater heater to low-pressure evaporator. A conduit can extend from the water-to-water heat exchanger to the feedwater heater. High-pressure economizer coils can be upstream of the preheater, with a conduit exiting the feedwater heater to the high-pressure economizer. Additional coils can be upstream of the high-pressure economizer. The feedwater heater can comprise first and second sections, or first, second and third sections; or more sections. The connections among the various components and sections can be near their upstream and downstream faces.

Heat recovery steam generator comprises a casing, low-pressure evaporator coils, preheater booster coils upstream thereof and feedwater heater coils downstream thereof, a water-to-water heat exchanger having low and high temperature paths; a first conduit from the preheater to the high-temperature path, and a second conduit from the feedwater heater to the preheater. A conduit can extend from feedwater heater to low-pressure evaporator. A conduit can extend from the water-to-water heat exchanger to the feedwater heater. High-pressure economizer coils can be upstream of the preheater, with a conduit exiting the feedwater heater to the high-pressure economizer. Additional coils can be upstream of the high-pressure economizer. The feedwater heater can comprise first and second sections, or first, second and third sections; or more sections. The connections among the various components and sections can be near their upstream and downstream faces.

A boiler system (1) according to one aspect of the present invention includes a water electrolysis device (20) that electrolyzes electrolysis target water with electric power supplied from a natural energy power generation device (10) to generate hydrogen and oxygen, a boiler (30) that heats makeup water by combusting fuel to generate steam, a heat exchange device (40) that exchanges heat between the electrolysis target water and a heat medium, and a control device (70) having a cooling controller (71) that cools the electrolysis target water by supplying the makeup water as the heat medium to the heat exchange device when a preset cooling start condition is satisfied.

A boiler system (1) according to one aspect of the present invention includes a water electrolysis device (20) that electrolyzes electrolysis target water with electric power supplied from a natural energy power generation device (10) to generate hydrogen and oxygen, a boiler (30) that heats makeup water by combusting fuel to generate steam, a heat exchange device (40) that exchanges heat between the electrolysis target water and a heat medium, and a control device (70) having a cooling controller (71) that cools the electrolysis target water by supplying the makeup water as the heat medium to the heat exchange device when a preset cooling start condition is satisfied.