An energy storage system is disclosed. The energy storage system includes a turbo train drive, a hot heat sink, and a reservoir. The turbo train drive is in mechanical communication with a compressor and an expander. The hot heat sink is in thermal communication between an output of the compressor and an input of the expander. The reservoir is in thermal communication between an output of the expander and an input of the compressor. The compressor and the expander, via the turbo train drive, are operable between a charging function for charging the hot heat sink and a discharging function for discharging the hot heat sink.