Hot Water Cooling Technologies

This article is quite interesting and you may want to read it "Water Cooling Comes of Age, Again" by Larry Vertal

Here are some interesting excerpts

Many “water cooled” data centers are using rear door or in-row coolers. While rear door and in-row coolers are a step in improving cooling efficiency they provide only intermediate benefits as they still require cold water.  These systems in effect bring the CRAH unit closer to the server, reducing the energy needed to move air through the data center.  The servers are still air cooled.

Bringing liquid inside the server to directly cool high heat flux components such as CPUs, GPUs and memory provides even greater benefits as is done with Asetek’s Direct-to-Chip (D2C) technology.  These benefits include:

• Enabling much higher rack densities.
• Reducing overhead power requirements of the data center for cooling.
• Thereby reducing operational costs and green house emissions.
• Lowering acoustic noise.

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Hot Water Cooling is Energy Efficient and EffectiveThe term “hot water cooling” appears to be an oxymoron, but this method is one of the most effective. Consider that in order to operate properly the surface temperature of a CPU (case temp) needs to be maintained in the 67°C to 85°C (153°F to 185°F) range, depending on CPU model.  The operating surface temperatures for memory chips, GPUs and co-processors is even higher, in the 90°to 95°C (194°F to 203°F) range.  Cooling with air requires a high initial temperature difference between the cooling air and the desired surface temperature (high Delta T).  The cooling efficiency of water allows it to maintain the required case temps with a much smaller delta T.  This means the water used for cooling the components can be hot.
 
The use of hot water to directly cool the high heat flux components within servers such as CPUs, GPUs and memory is the foundation of an effective Direct-to-Chip, liquid cooling system. This technology replaces air heat sinks and puts cold plates directly on the hot spots inside the server.  Liquid is flowed over the cold plates, collects the heat and is pumped out of the server into the CDU (Cooling Distribution Unit).  At the CDU, heat is exchanged with facilities liquid and pumped out of the data center.
 
Cooling with hot water means that, even in summer, outdoor air is cold enough to cool the very hot return water from the data center back down to the temperature need to cool these components. Using outdoor or ambient temperature air to ultimately cool the data center is the basis of "free cooling." Outdoor air cools the water, generally by using dry coolers or cooling towers, meaning no power is used to actively chill the water. Additionally, the water leaving the data center is hot enough to enable waste heat recycling, further increasing energy cost savings and resulting in cooling Energy Reuse Efficiencies (EREs) below 1.