Using various embodiments, methods and systems for cooling a data center using recovered thermal energy are described. In one embodiment, a data center cooling system comprises a first channel conveying a cooling fluid received from an outlet of a ventilation system of the computer data center at a first temperature, a second channel conveying the cooling fluid provided into an inlet of the ventilation system to cool the data center at a second temperature, and a heat transfer subsystem (HTS) configured to operate in an organic Rankine cycle (ORC) or a gas compression cycle (GCC) to change the temperature of the cooling fluid from the first temperature to the second temperature. In another embodiment, the system includes a monitoring subsystem monitoring a temperature, pressure, or flow of a working-fluid of the HTS and a controller subsystem to determine whether to operate the HTS in the ORC or GCC modes.