Thermal storage systems that preferably do not create substantially any additional back pressure or create minimal additional back pressure and their applications in combined cycle power plants are disclosed. In one embodiment of the method for efficient response to load variations in a combined cycle power plant, the method includes providing, through a thermal storage tank, a flow path for fluid exiting a gas turbine, placing in the flow path a storage medium comprising high thermal conductivity heat resistance media, preferably particles, the particles being in contact with each other and defining voids between the particles in order to facilitate flow of the fluid in a predetermined direction constituting a longitudinal direction, arrangement of the particles constituting a packed bed, dimensions of the particles and of the packed bed being selected such that a resultant back pressure to the gas turbine is at most a predetermined back pressure.