The invention relates to a method for heat exchange. At a first point in time, in a thermal energy storing device, at least part of the previously accumulated thermal energy is discharged to a first fluid by heat exchange, and subsequently, at a second later point in time, when the first fluid has been heated to a temperature higher than the temperature of a second fluid, the second fluid receives heat energy by heat exchange with said first fluid which then circulates on the other side of a first wall that prevents the first and second fluids from mixing.