Technology: Application of Water-side Evaporative Cooling in Data Centers

In recent years, evaporative cooling has been widely applied in data centers and communication base stations. Some early-built domestic data centers have adopted renovation schemes of evaporative cooling air conditioning systems due to high operating energy consumption. There are two main types of evaporative cooling applied in data centers: air-side evaporative cooling and water-side evaporative cooling. Air-side evaporative cooling represented by indirect evaporative cooling air handling units has been put into effective service at home and abroad. Water-side evaporative cooling technology utilizes cooling towers and evaporative chillers to supply chilled water for terminal air conditioning units in computer rooms. As the heat load per cabinet in data centers keeps rising, this technology features strong cooling capacity, small floor area and high concentration degree, showing great application potential in data centers.

The former national standard Code for Design of Electronic Information System Rooms (GB 50174-2008) stipulated the thermal and humidity requirements for Class A and Class B computer rooms: the temperature of main equipment rooms shall be maintained at 23±1°C with relative humidity ranging from 40% to 55%, and condensation is prohibited. The new national standard Code for Design of Data Centers (GB 50174-2017) revises the requirements as follows: the temperature of cold aisles or cabinet air intake areas shall be 18°C to 27°C, the dew point temperature shall be 5.5°C to 15°C, and the relative humidity shall not exceed 60%. If IT equipment allows wider ambient temperature and relative humidity ranges, the temperature of cold aisles or air intake areas can be extended to 15°C to 32°C.

When the national standard Code for Design of Computer Rooms (GB 50174-93) was formulated, servers could not operate stably at relatively high temperatures. With over two decades of development in material science and refrigeration technology, the performance of IT equipment and computer room air conditioning systems has been greatly improved. Practical operation proves that servers can run safely and stably with inlet air temperature up to 27°C. The raised upper temperature limit specified in GB 50174-2017 has ushered in a new era of high supply air temperature control for data center cooling solutions. Water-side evaporative cooling technology can produce high-temperature chilled water with low energy input, delivering superior performance in data center applications.

Supply and return water temperatures are key indicators for centralized chilled water systems. The higher allowable inlet air temperature under the new standard means the supply and return water temperatures for terminal units can also be appropriately increased. On the one hand, the cooling efficiency of traditional vapor-compression refrigeration systems is significantly enhanced. On the other hand, the applicable scope of water-side evaporative cooling is expanded, effectively alleviating the high energy consumption of data centers.

In recent years, the chilled water supply temperature of newly built data centers has been greatly increased, and the traditional 7°C/12°C supply and return water temperature mode has been phased out. GB 50174-2017 specifies that the chilled water supply temperature for computer rooms is 7°C to 21°C, and the return water temperature is 12°C to 27°C. Many cloud computing data centers operated by internet companies adopt high supply temperature design. Some large-scale data centers that deploy high-temperature resistant servers have raised the chilled water temperature to 15°C to 18°C. The high-temperature chilled water generated by water-side evaporative cooling matches well with corresponding terminal air conditioning units.

According to installation positions, terminal air conditioning units for computer rooms are classified into room-level, row-level and rack-level types. Row-level and rack-level units are installed close to servers, featuring short air delivery distance, low energy consumption and low sensitivity to the heat transfer temperature difference between chilled water and return air. Therefore, rack-level and row-level terminal units are more suitable for water-side evaporative cooling air conditioning systems.

For water-side evaporative cooling systems dominated by cooling towers, the outlet water temperature is higher than the ambient wet-bulb temperature, which is evaluated by the approach temperature. When the outdoor wet-bulb temperature is 5°C lower than the required outlet water temperature of the cooling tower, the air conditioning system can be switched to the direct free cooling mode. For systems mainly equipped with evaporative chillers, the outlet water temperature falls between the ambient wet-bulb temperature and dew point temperature, and is characterized by the cold depression. The applicability of evaporative chillers can be judged according to the outdoor wet-bulb temperature.

For mainstream air conditioning products, the temperature difference between terminal supply air and chilled supply water is 6°C to 8°C, and the typical temperature difference between chilled supply and return water is around 5°C. In accordance with the new national standard, if the cold aisle inlet air temperature of a data center is set to 26°C, the required chilled water temperature for water-side evaporative cooling shall be 16°C to 19°C.

There are numerous successful cases of adopting evaporative chillers to produce chilled water in domestic civil buildings. Many enterprises have optimized the materials and structure of evaporative chillers, enabling their outlet water temperature to approach the outdoor dew point temperature.

As stated in the test results from the literature: Experimental Study on Outlet Water Temperature Characteristics of Indirect-Direct Combined Evaporative Chillers (Huang Xiang, Sun Tiezhu, Bai Yanbin, et al., Fluid Machinery, 2012, Vol.40, No.7: 52-55), the cold depression of evaporative chillers remains stable with a fluctuation range of 2°C to 3°C. This means water-side evaporative cooling can produce chilled water 2°C to 3°C lower than the ambient wet-bulb temperature. Based on the prediction of outdoor wet-bulb temperature, when the outdoor wet-bulb temperature is 22°C, water-side evaporative cooling can fully meet the chilled water demand of data centers.

Under the requirements of the old standard, water-side evaporative cooling can independently supply chilled water for data centers only when the outdoor wet-bulb temperature is below 19°C. Based on the statistics of outdoor meteorological parameters specified in Code for Design of Heating, Ventilation and Air Conditioning of Industrial Buildings (GB 50019-2015), this paper analyzes the summer outdoor design wet-bulb temperature of regions across the country where the wet-bulb temperature is below 22°C, so as to discuss the applicable scope of water-side evaporative cooling.

Regions with summer outdoor design wet-bulb temperature below 19°C mainly include Xizang, Xinjiang, Inner Mongolia and Qinghai. Regions with the wet-bulb temperature ranging from 19°C to 22°C cover Xizang, Xinjiang, Qinghai, Yunnan, Guizhou, Sichuan, Gansu, Ningxia, Shaanxi, Shanxi, Inner Mongolia and Heilongjiang. For humid regions, water-side evaporative cooling can be combined with mechanical refrigeration. The evaporative cooling mode can be operated for most of the year to reduce the overall operating cost.

Water-side evaporative cooling presents a viable cooling solution for data centers and can be well applied in such facilities.

An open cooling tower is adopted as the free cooling solution for the air conditioning system of a data center in Inner Mongolia. The system configuration consists of an open cooling tower, water chiller and plate heat exchanger. The water-cooled chiller is connected in series with the plate heat exchanger and further linked to terminal air conditioning units of the computer room, while the open cooling tower is connected in series with the plate heat exchanger and the water-cooled chiller on the other side. The cooling system operates in three modes:
First, the tower free cooling mode in winter. The refrigeration compressors are shut down, and the cooling tower delivers cooling capacity to terminal units via the plate heat exchanger.
Second, the tower pre-cooling mode in transition seasons. Cooling water from the tower pre-cools return water of the air conditioning system through the plate heat exchanger. This maximizes the utilization of natural cold sources and improves the efficiency of main chillers.
Third, the conventional refrigeration mode in summer, where chillers supply cooling capacity to terminal units.

This configuration is widely used in data centers. The system runs on tower free cooling for part of the year, and the energy-saving effect depends on the annual operating duration of this mode. With a designed chilled water supply temperature of 18°C and taking into account temperature rise in plate heat exchangers and water delivery pipelines, full free cooling via cooling towers is achievable when the ambient wet-bulb temperature drops to 11°C. Taking Hohhot as an example, the annual hours with wet-bulb temperature below 11°C total 6254 hours, accounting for 71% of the whole year. Mechanical refrigeration can be disabled during this period, with water-side evaporative cooling acting as the primary cooling method and mechanical refrigeration as the supplement.

A data center in Jiangsu adopts a design combining closed cooling towers and water-cooled chillers. The system is composed of water-cooled chillers, closed cooling towers, water pumps, valves and other components. Valves regulate pipeline switching to enable three operating modes: mechanical refrigeration, free cooling and hybrid pre-cooling.

Compared with open cooling towers, closed cooling towers eliminate the need for plate heat exchangers. Cooling coils are installed inside the tower, and circulating cooling water is sprayed directly onto coil surfaces to achieve high heat exchange efficiency. Distilled water is used as the cooling medium, which prevents scale formation and ensures unobstructed fluid flow inside coils. Nevertheless, anti-freezing measures are still required for applications in northern regions. Taking Nanjing as an example, with a designed water supply temperature of 18°C and consideration of temperature rise in water pipelines, the system can operate fully in free cooling mode when the ambient wet-bulb temperature is lower than 12°C. This mode accounts for 46.8% of the annual operating hours, proving the energy-saving potential of this technology in high-humidity areas.

A data center in Xinjiang takes evaporative chillers as the primary cold source all year round. In accordance with local meteorological conditions and design requirements, the evaporative cooling system is indirectly connected to computer room terminals via plate heat exchangers, with three operating modes available.

Mode I: Combined air-side and water-side evaporative cooling for extreme weather. Chilled water produced by evaporative chillers cools return water from terminal units through plate heat exchangers, and the computer room operates under the full fresh air mode.

Mode II: Standalone water-side evaporative cooling in summer and transition seasons. Chilled water from evaporative chillers cools terminal return water via plate heat exchangers, and the computer room runs under the full return air mode.

Mode III: Glycol free cooling mode in winter. The glycol free cooling section is connected to terminal units through glycol solution pipelines, and heat generated inside the computer room is transferred by the glycol solution.

Cooling data centers with evaporative chillers represents an innovative air conditioning solution. Different from conventional cooling towers, evaporative chillers are equipped with an inlet air pre-cooling section, enabling the outlet water temperature to drop below the ambient wet-bulb temperature. This feature expands the application scope of water-side evaporative cooling. The designed outlet water temperature of the cold source in the aforementioned Xinjiang data center is 16°C. Water-side evaporative cooling fully meets the summer cooling demand and enables year-round free cooling operation.

Data centers adopt various free cooling methods. Technical and economic comparison is essential to verify the energy-saving performance of water-side evaporative cooling. Annual energy consumption is a core indicator for energy efficiency evaluation. The applicable duration of water-side evaporative cooling varies across regions and climatic zones. A test bench for variable operating conditions is required. Comprehensive economic analysis shall be conducted by combining simulation calculation and actual energy consumption measurement.

Evaporative cooling air conditioning systems are highly dependent on meteorological conditions, and ambient parameters greatly affect the refrigeration performance throughout the year. Priority shall be given to mode switching between water-side evaporative cooling and mechanical refrigeration as seasons change. During daily operation, focus shall be placed on adjusting the operating quantity of evaporative cooling equipment and regulating the cooling capacity of individual units.

Liquid cooling is regarded as a revolutionary technology for data centers. Water-side evaporative cooling can satisfy the temperature requirements of liquid cooling under natural conditions, bringing new opportunities for the development of this technology. Further discussion is needed on the matching between cold source equipment for water-side evaporative cooling and liquid cooling terminals, as well as the upper temperature limit applicable to liquid cooling systems.

Water serves as the heat transfer medium for water-side evaporative cooling systems, which realize refrigeration based on water evaporation. Continuous water consumption is inevitable during operation, making water consumption a key concern. During system design, comprehensive evaluation shall be carried out based on local power and water resources. A reasonable cooling solution shall be selected by comparing water-side evaporative cooling equipment with traditional water-cooled devices.

Components such as packing towers, high-temperature surface coolers, sensible heat terminals and water supply and return pipelines are filled with circulating water in water-side evaporative cooling systems. Water quality affects heat and mass transfer as well as energy delivery in all heat exchange processes, and also determines the service life and operating efficiency of heat exchange equipment. Further research is required on regional water quality investigation and corresponding water treatment solutions.

Advancements in materials for high-temperature resistant servers and new types of air conditioning terminals have raised the allowable upper limit of inlet air temperature in data center rooms. The chilled water supply temperature has been increased to 18°C to 20°C, which further expands the applicable scope of water-side evaporative cooling. This technology can independently provide cooling for data centers when the ambient wet-bulb temperature is below 22°C.

The annual operating duration of water-side evaporative cooling is critical to the overall energy saving of data centers. Air conditioning systems shall be designed to maximize the running time of free cooling throughout the year, with mechanical refrigeration serving only as auxiliary support under extreme weather conditions. Among all solutions adopting water-side evaporative cooling, evaporative chillers deliver the longest duration of annual free cooling operation.