Dynamic flow control and performance comparison of different concepts of two-phase on-chip cooling cycles

A hybrid on-chip two-phase cooling cycle specifically designed to cool server boards with chips of high performance computers was experimentally evaluated considering steady-state and transient operation of two parallel pseudo-chips and auxiliary electronics mimicking a real server board. Control strategies were developed and evaluated by reference tracking and disturbance rejection tests considering several setpoints of controlled variables. The hybrid cycle, operating with a common refrigerant R134a as the working fluid, was energetically and exergetically compared with two other cooling cycles experimentally evaluated in a previous study, one driven by an oil-free gear pump and another by an oil-free mini-compressor. The results showed that, for a specific steady state condition and heat load, respectively 28.9%, 51.9% and 62.5% of the energy out of the pump, compressor and hybrid cycles were associated with heat losses. The differences observed between the three cycles were justified firstly due to the concept of the cycles, i.e. cycles with the compressor showed as expected lower thermal performance than that with pump since its appeal is for energy recovery (benefitting from a higher condensing temperature) and secondly due to the irreversibilities observed in drivers, condenser and piping (thermal insulation). In summary, the three cycles proved to be efficient, simple and reliable concepts to cool server boards (CPUs, DIMMs etc.), showing high thermal performance and potential for heat recovery when compared with traditional air-cooling systems in current use in data centers. It can also be said that the pump cycle showed the best results in terms of energy and exergy, with the cooling and heat recovery performances reaching a maximum of about 5 and 1.8 times higher than the other cycles (worth noting that the focus in the present study was two-phase flow control and proof-of-concept of different cooling loops, meaning that no “optimal” system design was attempted and the differences above can be reduced).