Water is well suited to carry and remove heat. It also happens to be readily available and cheap (often due to subsidies). That is why it is used widely for cooling in data centers. However, as data centers become ever denser, their water consumption will begin to compete with its other uses and come under greater scrutiny. It is estimated that a 15MW data center (small by today’s standards), consumes enough water to meet the needs of 100,000 households. Efficient use of water is therefore important for data centers.
While most data centers measure and monitor their energy efficiency (PUE), many are now also tracking their ‘Water Usage Effectiveness’ (WUE), which is a ratio of the total water consumed to the energy consumed by its IT equipment. A more holistic measure of water efficiency is WUESource which also takes into account water used in generation of electricity consumed by a data center.
So, how does one limit the use of water without increasing the risk of a shutdown? Data centers use a combination of chillers (which use refrigeration cycle) and cooling towers (which use evaporative cooling). Some water is lost through evaporation, which is inevitable since that is what leads to cooling. The rest of the water is circulated back for the next round of cooling. However, if the water has impurities, they begin to accumulate in each subsequent run. Eventually, the water needs to be ‘blown down’, and replaced with fresh, ‘make-up’ water. The number of times water can be run through the cooling system is called ‘cycles of concentration’ (CoC). This can range from less than 2, if water has impurities like silica, to as high as 15-20. By increasing the CoC, water consumption can be dramatically reduced. Nalco Water, a global leader in water management, helped a data center improve the quality of its make-up water through reverse-osmosis and disinfection system. This led to an improvement in its CoC, resulting in water saving of nearly 100K m3/year. Improving water quality also increases the life of assets by preventing fouling, scaling, corrosion and microbial growth. Better quality water also leads to lower energy consumption through improved heat transfer efficiency. The other strategy for water stewardship is to harvest rainwater and treat the blowdown water for alternative uses, such as irrigation.
The data center cooling space is bubbling with other innovative ideas, some of which are gaining traction. These include indirect evaporative cooling, state point liquid cooling, direct- to-chip cooling, immersion cooling, and seawater cooling. Each of these has its own pros-and-cons and may work better in certain conditions. Immersion cooling, which has a lot of potential is also held back by the reluctance of insurers to underwrite he risk. Seawater cooling requires investment in water purification. As we move towards greater adoption of AI, we need more and more innovation in water and energy sustainability.
Based on an APAC Sustainability Advisory Council session on Water Sustainability in Data Centres with Janice Liang and Dr Kelly Lu of Nalco Water, a global leader in water treatment and sustainability.