Masato Fukagaya

Energy consumption in data centers has seen a drastic increase in recent years. In data centers, server racks are cooled down in an indirect way by air-conditioning systems installed to cool the entire server room. This air cooling method is inefficient as information technology (IT) equipment is insufficiently cooled down, whereas the room is overcooled. The development of countermeasures for heat generated by IT equipment is one of the urgent tasks to be accomplished. We, therefore, proposed new liquid cooling systems in which IT equipment is cooled down directly and exhaust heat is not radiated into the server room. Three cooling methods have been developed simultaneously. Two of them involve direct cooling; a cooling jacket is directly attached to the heat source (or CPU in this case) and a single-phase heat exchanger or a two-phase heat exchanger is used as the cooling jacket. The other method involves indirect cooling; heat generated by CPU is transported to the outside of the chassis through flat heat pipes and the condensation sections of the heat pipes are cooled down by coolant with liquid manifold. Verification tests have been conducted by using commercial server racks to which these cooling methods are applied while investigating five R&D components that constitute our liquid cooling systems: the single-phase heat exchanger, the two-phase heat exchanger, high performance flat heat pipes, nanofluid technology, and the plug-in connector. As a result, a 44–53% reduction in energy consumption of cooling facilities with the single-phase cooling system and a 42–50% reduction with the flat heat pipe cooling system were realized compared with conventional air cooling system.

Energy utilization in data centers, especially cooling systems for server racks, needs extensive improvement. The present authors proposed advanced cooling network systems for data centers, and R & D activities have been conducted under the so-called Green IT Project sponsored by NEDO (New Energy and Industrial Technology Development Organization). In the present concept, CPUs in servers are cooled down by either direct liquid cooling system or heat pipes with liquid cooling systems in the condensation region. The liquid cooling systems are integrated in each server rack and among server racks. A series of studies on both single phase and two phase narrow channel heat exchangers, high performance heat pipes with self-rewetting fluids and nanofluids for heat transfer enhancement are ongoing. In addition, a prototype server rack with the cooling network systems is also under development toward commercial products. This paper reports the updated status of the present R & D.

For a drastic energy conservation in data centers (more than 30%), a new advanced concept of thermal management systems of integrated cooling network in server racks is proposed, and a series of developmental studies toward the realization of the concept have been conducted. The concept consists of the following technological items: - Plug-in thermal network cables, - Narrow channel heat exchangers (single-phase and two-phase), - Thin flat-type heat pipes, - Development of nano-fluids for heat transfer enhancement. In this concept, CPUs in server racks are directly cooled with the aid of advanced 1.5 mm thick flat-type heat pipes or 3 mm thick narrow channel heat exchangers, though CPUs in server racks, at present, are cooled down with massive heat sinks by forced air flow in the racks. In the case of heat pipes, condensation regions are cooled by either single-phase or two-phase narrow channel heat sink instead of air-cooled finned heat sink. These cooling units are accommodated in server racks. In addition, plug-in thermal network cables, in which a working fluid for narrow channel heat exchangers is running, are integrated among server racks, and thermal energy dissipated in server racks are inclusively merged and managed by the thermal network cables. The working fluid heated up by the dissipated heat is recuperated in an outdoor cooling system. An attempt at the applications of silver nano-fluids as a working fluid in heat pipes and thermal network is also conducted in the present study. Silver nano-fluids are thermochemically synthesized by a microwave heating technique, which allows for superior stable suspension characteristics.