DATA CENTRE COOLING
preparation, comprehensive calculations are first performed with respect to the performance of the systems. The external conditions - the climatic influences at the planned location - are recorded using statistical data, namely temperature ranges and the number of hours at defined dry bulb and wet bulb temperatures.
Using the required or desired temperatures in the data centre, various alternative system technologies can then be proposed and the corresponding resource consumption and operating costs calculated. In Europe, it is often customary for data centres to have cooling systems with a water-cooled refrigeration chiller and system separation for the refrigeration chiller/free cooling operation. Modern chillers have an energy efficiency ratio (EER) at full load of five to 10. As the consumer circulation pumps and dry coolers must also be taken into consideration in order to improve the efficiency of a cooling system, this configuration theoretically yields three ‘adjusting screws’: The condensation temperature and thus the cooling water temperature are designed to be as low as possible. As a guide value, one can assume that for each 1K that the condensation temperature is lowered, a saving on electrical power consumption of approximately. 3% is made at the refrigeration chiller’s motor. At cooling water temperatures of 35/30°C, an EER of five is thus achieved for a refrigeration chiller, or at 1,000kW refrigerating capacity, a motor power of 200kW. If for this machine, however, the cooling water temperature is lowered by 5K to a level of 30/25°C, the EER is six, and for 1,000kW refrigeration capacity a motor power of 167kW is required. This difference of 33kW also reduces the quantity of heat that needs to be supplied at the dry cooler.
The evaporation temperature and therefore the inlet temperature of the cold water to the server room’s heat exchanger are set as high as possible, which is made easy by new IT technology.
At low external temperatures, the cold water is provided via free cooling, so that the refrigeration chiller remains switched off or can be run in low partial load operation.
Hybrid dry cooling
The tried and trusted free cooling technology can be made significantly more efficient by using hybrid dry coolers, as these combine the advantages of dry and water-cooled cooling thus conserving valuable resources. With a corresponding system set-up, the cooler can, at low ambient temperatures, directly take over cooling the cold water from 20°C to 15°C.
Free cooling mode at low external temperatures without chiller operation in dry and wet operation.
Chiller operation mode at high external temperatures in dry and wet operation.
The refrigeration chiller remains switched off for this. In dry mode, this is possible up to an ambient temperature of approximately 7°C, wetted up to a wet bulb temperature of around 9.5°C - 15°C at relative humidity of 49%. The power requirement reduces with this mode of operation to the electricity demand for the dry cooler. If both the air temperature of the server room supply air and the cold-water temperature required for this are raised at the same time, this yields significantly longer operating times in free cooling mode than in purely dry free cooling operation. At a data centre temperature of +32°C, the efficient free cooling mode can be used at external temperatures of up to +25°C. For energy-efficient operation, an intelligen controller which independently regulates the desired system temperature level and performs the required wetting water management, is indispensable. The controller must assume, on the one hand, the monitoring of the wetting water
circuit and, on the other, the output regulation of the dry cooler. Safer operation and the economical use of water and electricity throughout the entire cooling circuit are likewise significant. The task of the controller is to determine the relevant optimal operating point from the initially contradictory requirements.
Furthermore, it should be possible to exchange operating data, error messages and approvals or floating setpoints via defined interfaces (bus connection, contacts) with a superordinate building management system.
Comparison of wet and hybrid cooling In order to compare the efficiency of a hybrid cooler with that of a wet cooling tower, two systems were examined at the place of installation in Frankfurt/Main. The first is a production plant which requires a room temperature in a workshop of 18°C around the clock. A traditional wet cooling tower is used here.
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