使用兩相熱虹吸設計應用於資料中心之節能研究

Investigation of Free Cooling in Datacenters Using Separated Two-Phase Thermosiphon Loop

哈飛思(Hafiz M. Daraghmi)

指導教授 ： 王啟川

國立交通大學/工學院/機械工程系所/博士(2019年)

摘要

This experimental study aims to investigate the energy saving potential by using a separated two-phase thermosiphon loop (STPTL) as a free cooling technique in datacenter. The thermosiphon heat exchanger takes the form as fin-and-tube configuration and is installed at the exit of a data rack. R-134a is used as the working fluid, and a water-cooled shell and tube heat exchanger is used as the condenser. The connected adiabatic piping, i.e., riser and downcomer, are made via PFA for easier installation and mobility consideration. The fin-and-tube thermosiphon heat exchanger is made of copper pipes and aluminum fins. The surface area of the thermosiphon is 1.11 × 0.69 m² including collector and header, and it consists of 48 vertical copper tubes with 9.52 mm nominal diameter (10.3 mm collar diameter after expansion). Tests are conducted with filling ratio ranging from 0 to 90% and the ambient temperatures varies from 21 C to 25 C. Based on the foregoing discussions, the following conclusions can be derived:
1. The experimental results indicate that the energy saving increases with the rise of filling ratio. However, the optimum energy savings can be achieved at a filling ratio of 70%, a further increase of filling ratio leads to a reduction in energy saving. A substantial energy savings of 38.7% can be achieved at the filling ratio of 70%.
2. At a low filling ratio like 10%, the evaporator starves for refrigerant and a very uneven air temperature distribution occurring at the exit of data rack. The uneven temperature distribution is relieved considerably when the evaporator is fully flooded.
3. It is also found that the energy saving is in line with the rise of system pressure. Overfilling of the evaporator may lead to a decline of system pressure. This is because the evaporator is fully flooded and only nucleate boiling prevails.
4. A lower overall thermal resistance occurs at high filling ratios and a higher ambient temperature. However, the effect of ambient temperature on the overall thermal resistance is negligible at high filling ratios. Yet appreciable increase in overall thermal resistance is encountered at low filling ratios due to the starvation of working fluid.
5. It is found that the overall thermal resistance is in line with the predictions especially at the high filling ratios.

關鍵字

資料中心之 ； 兩相 ； 熱虹吸設 ； 節能

並列摘要

In this study, the current status of the free cooling technologies applicable for datacenters has been discussed. By introducing the free cooling technologies, the compressor loading of refrigeration system can be partially or completely relieved. Utilization of airside or waterside free cooling relies strongly on the ambient conditions, yet either airside free cooling or waterside free cooling may be integrated with other systems such as absorption, solar system, adsorption, geothermal, evaporative cooling, and the like to extend its performance. However, mechanical piping system of such techniques requires long distance transportation of air or water. In other words, they consume a lot of electrical energy through fans or pumps. One alternative for energy-saving technique is based on heat pipe for its passive feature. Heat pipes such as thermosiphon heat exchangers feature unique characteristics to transfer heat at small temperature difference are quite promising for datacenter free cooling. This study investigates the feasibility of using R-134a filled separated two-phase thermosiphon loop (STPTL) as a free cooling technique in datacenters. Two datacenter racks one of them is attached with fin and tube thermosiphon were cooled by a computer room air conditioning unit (CRAC) individually. Thermosiphon can help to partially eliminate the compressor loading of the CRAC; thus, energy saving potential of thermosiphon loop was investigated. The condenser is a water-cooled design and perfluoroalkoxy pipes were used as adiabatic riser/downcomer for easier installation and mobile capability. Tests were conducted with filling ratio ranging from 0 to 90%. The test results indicate that the energy saving increases with the rise of filling ratio and an optimum energy savings of 38.7% can be achieved at filling ratios of 70%, a further increase of filling ratio leads to a reduction in energy saving. At a low filling ratio like 10%, the evaporator starves for refrigerant and a very uneven air temperature distribution occurring at the exit of data rack. The uneven temperature distribution is relieved considerably when the evaporator is fully flooded. It is also found that the energy saving is in line with the rise of system pressure. Overfilling of the evaporator may lead to a decline of system pressure. A lower thermal resistance occurs at high filling ratios and higher ambient temperature.

並列關鍵字

data center ； free cooling ； two-phase flow ； thermosiphon ； separated loop ； PFA pipe

參考文獻

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