何海
中国空气动力研究与发展中心工程师
DKV（Deep Knowledge Volunteer)计划精英成员
数据中心热管理及创新冷却策略综述Ⅳ
建筑围护结构
A review of thermal management and innovative cooling strategies for data centerⅣ
Building envelope
前篇回顾
集装箱数据中心
Container-based data center
集装箱数据中心 (CDC)由集装箱内置机架组成。这些集装箱被放置在仓库或露天。集装箱装有传统的冷却系统。Endo等人对直接新风冷却的集装箱数据中心能耗进行了研究，如图27所示。与使用机房空调的 CDC在一年内的估算能耗相比，直接新风冷却的CDC能耗节省了20.8%。
Container data center (CDC) is a concept which consists of putting racks in containers. These containers are then placed in warehouses or outside. Containers are equipped with traditional cooling systems. Endo et al. worked on energy consumption in a direct fresh-air container data center, illustrated in Fig. 27. The direct-fresh-air-cooled CDC exhibited a 20.8% reduction in the total energy use compared with the estimated use of a CRAC-cooled CDC over one year.
Fig. 27. Fresh-air container data center.
图27.新风自然冷却的集装箱数据中心
相变材料
Phase change material
相变材料(PCMs)采用了潜热储存的概念。当物质由固体变为液体或相反时，热量被吸收或释放。通过降低温度波动，提高建筑围护结构的热惯量，是控制建筑室内环境温度的一种有效方法。Akeiber等人将建筑中相变材料的运用分为有源和无源两种技术。在有源技术中，相变材料由传统冷却系统充电，白天放电，以冷却室内环境。在无源技术中，根据昼夜温差采取充放电流程。Sun等人构建了一个将相变材料与自然冷源相结合的原型，以降低电信基站的空间冷却能量。采用调整后的能效比(AEER)对该机组进行了性能评价，并与常规空调进行了比较。AEER为14.04，明显高于制冷量小于4500 W空调的3.2的限值。
The use of phase change materials (PCMs) consists of employing the latent heat storage concept. Heat is absorbed or released when the material changes from solid to liquid and vice versa. It is an efficient way to increase the thermal inertia of building envelopes and controls the indoor environment of a building by reducing temperature fluctuations. Akeiber et al. classified two techniques for PCM operation in buildings: active and passive. In the active technique, the PCM is charged by a conventional cooling system and discharged during the day time to cool the indoor environment. In the passive technique, the temperature difference between night and day cause the charging and discharging processes. Sun et al. built a prototype that combines PCMs with a natural cold source to reduce the space cooling energy of telecommunications base stations. The adjusted energy efficiency ratio (AEER) was used to evaluate the performance of this unit and to compare it with conventional air conditioners. The AEER was 14.04, which is considerably higher than the limiting value of 3.2 for air conditioners with a cooling capacity of less than 4 500 W.
估计原型机的平均节能潜力为50%。Sundarama等人开发了一种新的被动冷却系统，包括相变材料和两相闭式热虹吸换热器，为位于电信舱室中的电信设备提供热管理。新开发的热系统在白天最热的时候吸收设备散发的热量，将其储存为潜热，并在夜间通过热虹吸器将其释放到环境中。图28显示了这种被动冷却系统，每年可以节省大约14吨的碳排放。因此，用被动冷却系统替代传统的空调系统，使电信舱室成为一种绿色舱室。
The estimated average energy savings potential of the prototype was 50%. Sundarama et al. developed a new passive cooling system incorporating PCM and two-phase closed thermosyphon heat ex-changers to provide thermal management for telecommunication equipment housed in telecom shelters. The newly developed thermal system absorbs the equipment dissipated heat during the hottest part of the day, stores it as latent heat and releases it through thermosyphons during the night to the ambient. Fig. 28 shows this passive cooling system that can save approximately 14 tons of carbon foot print every single year. Therefore, the replacement of conventional air-con-ditioning system with passive cooling system makes the telecom shelter as a green shelter.
Fig. 28. Passive cooling system combining PCM and two-phase closed ther-mosyphon .
图28.采用相变材料和两相封闭式热虹吸系统的被动冷却系统
与被动冷却方案的比较
Comparison with passive cooling solutions
这四种主要的冷却技术都有利于减少电力消耗和提高能源效率。表3总结了上面介绍的所有冷却技术的性能标准。每一种方法都附带一个各自解决方案的应用系统示例、它的优缺点以及节能效果。
These four main cooling technologies present benefits to reduce electricity consumption and to improve energy efficiency. Table 3 summarizes the criteria of performance of all cooling technologies in-troduces above. Each of them is accompanied by an example of system applying the respective solution, the benefits and drawbacks it offers and the energy savings.
表3还表明，被动冷却技术在过去几年得到了广泛的发展。2011年绿色网格的一项调查显示，直接风侧节能器是最受欢迎的，其次是水侧节能器。然而，外部空气条件必须在指定的设定值内。英特尔公司的一项调查显示，水侧节能器比风侧节能器更划算。与风冷数据中心相比，液冷系统可以将数据中心的总能耗降低30%，但成本较高，存在流体泄漏风险。
Table 3 also indicates that passive cooling technologies have been widely developed over the past few years. According to a survey of the Green Grid in 2011 , the direct airside economizer is the most popular, and the waterside economizer is the second. However, the outside air conditions have to be within specified set point. A survey of Intel indicates the waterside economizers are more cost-effective than the airside economizers. The liquid-cooled systems can reduce the overall data center consumption up to 30% in comparison with air cooled data centers , but they are expensive and presents fluid leakage risks.
到目前为止，热虹吸循环冷却技术具有较高的散热能力。它提供了简单，紧凑，并通过与微通道换热器，环路热虹吸在较小的质量流量的冷却剂工作情况下，能够去除较高的热通量。如果设计合理，热虹吸回路可以提供更均衡的设备温度。不过，大多数应用仍处于人工气候实验室，而不是真正的数据中心。此外，由于它们与风侧间接自然冷却相结合，因此它们依赖于室外条件，从而需要增加机械制冷或风扇来提供足够的冷却能力。下一个挑战是使两相冷却系统不再受气候条件制约，实现完全的无源，并将其应用到实际的数据中心。
So far, the cooling technology with the higher heat removal capacity is thermosyphon loop. It offers simplicity, compactness and, by asso-ciating with micro-channels heat exchangers, the loop thermosyphon is capable of removing higher heat fluxes while working with smaller mass flow rate of coolant. If it is properly designed, the thermo-syphon loop could provide a more uniform equipment temperature. Still, most are applied into climatic chambers, not in a real data center. Furthermore, they are combined with indirect airside free cooling, so they depend of outdoor conditions, leading to an addition of mechan-ical refrigeration or fans to provide a sufficient cooling power. The next challenges is to make two-phase cooling system independent of climatic constraints, making them 100% passive, and to apply them into a real data center.
目前，被动冷却系统为余热回收提供了可能。它支持服务器散热产生的能量再利用。Oró等人设计了基于芯片的服务器的液体冷却配置，并以室内游泳池为例进行了数值计算。室内游泳池运营方减少了18%的运行开支。Ebrahimi等人研究了不同的系统，这些系统可将冷却系统热排放进行再利用。详细介绍了吸收式制冷系统和朗肯循环组织，认为这两种技术是数据中心余热再利用最有前景的技术，如图29所示。
Nowadays, passive cooling systems offer the possibility to the waste heat recovery. It consists of reusing the energy produced by server heat dissipation. Oró et al. designed the liquid cooling configuration of on-chip servers evaluated numerically for a case study of an indoor swimming pool. The indoor swimming pool operator reduces its operational expenses 18%. Ebrahimi et al. reviewed the different system capable of reusing the heat removed by the cooling system. The authors detailed particularly the absorption refrigeration system and organic Rankine cycle, and claimed that these techniques are the most promising technologies for data center waste heat reuse. These systems are represented in Fig. 29.
Fig. 29. Waste heat recovery system:
(a) Absorption refrigeration system; (b) Schematic diagram of organic Rankine cycle.
图29. 余热回收系统：（a）吸收式制冷系统；（b）朗肯循环组织原理图
总
结
本文指出了降低电力消耗最有希望的解决方案:
This review points out the most promising solutions to decrease the electrical power consumption:
自然冷却技术是提高能效的最适宜的技术。风侧节能采用自然空气对数据中心进行冷却，水侧节能则采用自然冷水排放热量;
The free cooling technology is the most suitable technology to improve energy efficiency. While the airside economization uses natural air to cool the data center, the water economization remove the ejected heat by using the natural cold water ;
建筑围护结构技术包括将机架放入集装箱中，通过传统的冷却系统进行冷却，或者利用相变材料的原理吸收或释放热量，从而减小温度变化;
he building envelope technology consists of either putting racks into containers and cool them by traditional cooling systems, or employing the principle of phase change materials by absorbing or releasing heat and thus reducing temperature fluctuations ;
高功率密度的数据可采用液冷技术：直接液冷在CPU上附加一个冷板；机架级液冷机架背面安装一个液冷门；也可以将服务器浸入液体中，并将热量转移到外部回路;
The liquid cooling technology is employed when data centers pro-vide high power density. The direct liquid cooling consists of attaching the CPU with a cold plate, and the rack-level liquid cooling consists of installing a liquid-cooled door on the back of the rack. It is also possible to submerge servers in liquid to transfer the heat to an external loop ;
两相冷却技术能够消除高热通量。该方案利用制冷剂的蒸发/冷凝原理，吸收排出的热量并将其释放到环境中。热管系统利用毛细作用，而热虹吸换热器与综合系统不同，主要由蒸发器和冷凝器组成，不包括机械部件。两相浸没系统浸没所有部件，并利用液体的温度控制各部件的温度。
The two-phase cooling technologies are capable of removing high heat flux. This solution uses the principle of evaporation and condensation of a refrigerant to absorb the ejected heat and release it into environment. The heat pipe system uses the capillary action while the thermosyphon heat exchanger is composed mainly by an evaporator and a condenser and do not include mechanical components, unlike integrated system. The two-phase immersion consists of immerging all components and controlling the temperature of these components thanks to the temperature of the liquid.
由于其产生的经济效益，这些技术可能会引起数据中心运营商的兴趣。它们可能会减少机械设备使用并消除较高的热通量。
These technologies may be of interest data center operators, since they offer specific benefits. They can potentially allow the reduction of using mechanical equipment and the removing of higher heat fluxes.
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