With the development of global digitalization, data centers, as the core infrastructure, have increasingly prominent energy consumption problems, and the energy consumption of their cooling systems accounts for 40% to 50% of the total electricity consumption, and the traditional air conditioning and refrigeration mode causes energy waste and restricts their sustainable development.
At the same time, the direct emission of low-grade waste heat from data centers causes energy waste and thermal pollution, and energy gradient utilization through cooling energy saving and waste heat recovery is the key to building green data centers and helping the goal of “double carbon”. This paper focuses on discussing related technical solutions to provide reference for designers specializing in water supply and drainage and HVAC.
Understanding Data Center Cooling Energy-saving Technology System
Natural Cooling Technology
Natural cooling technology is an important means of energy saving and consumption reduction in data centers, and its core principle is to use outdoor natural cooling source to replace or assist mechanical refrigeration, so as to reduce chiller operation time and energy consumption.
According to the different cooling media and working principles, natural cooling technology is mainly divided into two categories: water-side natural cooling and air-side natural cooling, which have their own emphasis on the applicable scenarios, design points and energy-saving effects.
Water-side natural cooling technology mainly includes direct water cooling, air cooling and cooling tower cooling three ways.
- The direct water cooling method can effectively reduce energy consumption and improve the energy use efficiency (PUE) by directly accessing external cold water as the cold source. It is suitable for areas with stable low-temperature water sources, such as areas near lakes, rivers, or areas rich in groundwater, and the design should focus on water quality treatment, corrosion prevention of pipelines, and reliability of the cold source.
- Air-cooled technology to air cooling device as the core, the circulating water cooling, and with auxiliary cooling system, its key components include secondary coil and dry cooling device, can be maintained at higher ambient temperatures under the cooling effect, suitable for water-scarce areas, but by the greater impact of the ambient temperature.
- Cooling tower cooling technology using cooling towers to maintain the cooling water and refrigerant independent circulation, and through the heat transfer between the two to achieve room cooling, with plate heat exchanger equipment can reduce the chiller running time, improve system efficiency, the design should pay attention to the cooling tower selection, arrangement and noise control.
Air-side natural cooling to outdoor cold air as a cold source, through the air ducts introduced into the machine room heat transfer, without additional cooling medium, energy saving is significant, suitable for year-round low-temperature areas; the design needs to optimize the air ducts to enhance the efficiency of heat transfer, and do a good job of dust prevention, anti-condensation treatment.
Cold Plate Liquid Cooling Technology
Cold plate liquid cooling technology contacts the core components of the server through the cold plate, utilizing the coolant to conduct heat and dissipate heat, with significant advantages, divided into single-phase and two-phase routes: single-phase cooling through the coolant cycle, and two-phase use of the phase change to enhance the effect of heat conduction, applicable to high heat load scenarios.
This technology has outstanding energy-saving effect when applied to data center related systems, which can realize natural cooling throughout the year, significantly reduce air conditioning and fan power consumption, liquid cooling part PUE <1.1, and at the same time stabilize equipment temperature, improve performance and stability, and extend equipment life.
Submerged Liquid Cooling Technology
Submerged liquid cooling technology is to submerge all the hardware inside the server directly in the special electronic coolant, through the direct contact between the coolant and the heating components to achieve efficient heat exchange, is currently one of the highest cooling efficiency of the liquid cooling technology, mainly divided into single-phase and two-phase two categories.
Single-phase submerged liquid cooling, the coolant maintains a liquid state and direct heat exchange with heating components, energy saving and noise reduction, is the preferred choice for high-performance computing and large-scale data centers; two-phase submerged liquid cooling using the coolant vaporization of latent heat transfer to further enhance the heat dissipation effect, to ensure that the components of the temperature stability.
Submerged liquid cooling is a fanless design with zero fan power consumption and noise ≤ 45dB, which can realize natural cooling throughout the year, with PUE controlled at 1.03～1.1, independent of the environmental temperature and humidity, and with strong stability, suitable for ultra-high-density, high-heat-load data centers.
Spray-type Liquid Cooling Technology
The core of spray liquid cooling technology is to spray the coolant directly onto the surface of electronic components to dissipate heat through direct convection. Compared with submerged liquid cooling, its coolant dosage is small, cost-effective, and easy to maintain, and there is no obvious phase change in the coolant, which only dissipates heat through the temperature change cycle.
In practical application, a large data center adopts this technology, with summer PUE≤1.13 and annual PUE of 1.05-1.10, with remarkable energy-saving effect, which is suitable for energy-saving transformation of medium and high-density data centers.
Technology Comparison and Selection
Analyzing from the perspective of technical characteristics and application applicability, although traditional data center cooling technology and liquid cooling technology both aim at improving energy efficiency and reducing energy consumption, there are significant differences in principle, applicable environment and long-term benefits, and designers need to make reasonable choices according to the scale of the data center, load density, siting conditions and other factors.
- Traditional cooling technology adopts airflow optimization and intelligent control, with the advantages of mature technology, low transformation cost and flexible deployment, applicable to data centers with low load density and small scale, but with limited energy-saving potential, it is difficult to meet the demand for high-density arithmetic, and the energy-saving effect will be attenuated with the arithmetic enhancement.
- Liquid cooling technology innovates the heat dissipation path and directly takes away server heat through coolant, which can reduce PUE, support high-power chips, and provide significant long-term energy-saving benefits. It is applicable to data centers with high heat loads, but there are problems of equipment compatibility, complex maintenance, high initial costs, and high requirements for operation and maintenance.
Applicability analysis shows that for small and medium-sized data centers with stable loads, optimized airflow and intelligent temperature control are more cost-effective; for high-performance computing, large-scale clusters and other high-heat-load scenarios, liquid-cooling solutions have more potential to achieve long-term energy-efficiency gains and green and sustainable operation.
Synergistic Design of Liquid Cooling System with Building Plumbing and Air Conditioning Systems
Data center liquid cooling system, building drainage system and air conditioning system is not independent operation, the three in the complex heat transfer and exchange mechanism to work together, the rationality of its synergistic design directly affects the cooling efficiency of the data center, the level of energy consumption and operational stability.
Therefore, to do a good job of the three synergistic design, is to realize the key link of data center energy saving and consumption reduction.
Key Roal of Synergistic Operation in the System
Core functions of each system
- Liquid cooling system: directly contacting the server, absorbing a large amount of heat generated by the operation of the equipment, and is the core link of heat dissipation in the data center.
- Building water supply and drainage system: provides coolant circulation for the liquid cooling system and assumes the carrier function of waste heat recovery.
- Air-conditioning system: responsible for regulating the overall temperature and humidity of the server room and assisting the liquid cooling system to realize efficient heat dissipation.
The synergistic operation of liquid cooling system, building water supply and drainage system and air conditioning system is essentially a process of reasonable heat transfer and utilization.
Take submerged liquid cooling system as an example
The temperature of the coolant rises after absorbing heat, and it is transported to the heat exchanger (the key component of heat transfer) through circulation, so that the high-temperature coolant of the liquid cooling system exchanges heat with the circulating medium of the building water supply and drainage system or air-conditioning system:
- Synergize with water supply and drainage system: the high-temperature coolant transfers heat to the water source of the water supply and drainage system, which is used in waste heat recovery scenarios such as domestic hot water preheating.
- Synergize with air-conditioning system: high-temperature coolant will transfer heat to the refrigerant of air-conditioning system, and the heat will be discharged to the outdoor through the air-conditioning refrigeration cycle.
Factors such as the material and structure design of the heat exchanger and the temperature difference between the coolant and the circulating medium will affect the heat exchange efficiency.
So in the synergistic design, it is necessary to choose high-efficiency heat exchanger, optimize the heat exchange structure, and reasonably control the temperature difference in order to improve the efficiency of heat transfer and control the energy consumption.
Temperature and Humidity Control Synergy
The long-term stable operation of data center equipment is directly related to the temperature and humidity of the liquid cooling system, building water supply and drainage system and air conditioning system.
The liquid cooling system assumes the core function of lowering the temperature of the equipment, but its long-term operation may have an impact on the indoor humidity. For example, if the pipeline sealing is poor or the indoor humidity is too high in the cooling state of the cold plate liquid cooling system, it may lead to the generation of condensate, which may damage the server hardware.
The air conditioning system is responsible for accurately regulating the indoor temperature and humidity, through cooling, heating, dehumidification and other functions, to ensure that the data center environment always meets the operational requirements of the equipment.
When the two operate together, it is necessary to establish a linkage control mechanism to dynamically adjust the air conditioning system operation mode according to the real-time conditions of the liquid cooling system:
- If the liquid cooling system continues to take away a large amount of heat resulting in a sudden drop in indoor temperature, the air conditioning system will promptly regulate the cooling capacity to prevent localized overcooling and avoid equipment failure due to low temperature;
- If the liquid cooling system causes condensation problems, the air conditioning system will intensify dehumidification efforts to reduce indoor humidity and create a suitable operating environment for the equipment.
Synergy in Energy Utilization
In order to achieve the data center energy saving goals, liquid cooling, water supply and drainage, air conditioning systems need to be synergistic in energy use. Liquid cooling system refrigeration energy consumption advantages are significant, including submerged liquid cooling can reduce refrigeration energy consumption by 90% to 95%, laying the foundation for synergistic energy saving.
In cooperation with the water supply and drainage system, the residual heat of liquid cooling can be recovered and converted into domestic hot water, heating and other usable thermal energy;
The air conditioning system can be combined with the liquid cooling conditions to optimize the operating parameters to reduce consumption, and the introduction of a natural cooling source to reduce the mechanical refrigeration when the temperature is low, so as to realize the synergistic energy saving of the multi-systems.
Key Points of Water Supply and Drainage System Design
The water supply and drainage system is the key to ensure the efficient and stable operation of the cold plate liquid cooling system, and it needs to be designed in an integrated manner for the coolant circulation, piping arrangement, redundancy control, water quality management, etc., so as to avoid system failure or increased energy consumption due to unreasonable design.
- Coolant selection needs to take into account thermal conductivity, insulation, safety and other requirements, giving priority to the selection of high specific heat capacity, low viscosity and insulation fluid compatible with the cold plate; water quality control needs to be set up filtration systems and precision filters, adding non-toxic bacteriostatic agents, to prevent clogging and bacterial growth.
- Piping design needs to optimize the layout and materials: galvanized steel pipes for the primary side of the outdoor piping, stainless steel materials and fittings for the secondary side; scientific calculation of the pipe diameter, reduce elbows, the use of Y-type filters, and control the pressure drop in order to reduce the energy consumption of the pumping unit.
- The pumping unit needs to do redundancy, frequency conversion and UPS power supply design, the main and backup pumps are connected in parallel, the pump speed is adjusted by frequency conversion, and the UPS guarantees the operation in case of power failure; at the same time, sensors are installed to monitor the leakage of liquids, and infusion tanks, liquid collection trays and antistatic floor drains are set up to realize the emergency drainage.
In order to ensure the water saving effect of the water supply and drainage system, the primary side can adopt the closed cooling tower, which saves about 80% more water than the open system.
At the same time, it sets up the water softening device and the reverse osmosis module, and reasonably determines the replenishment rate and the capacity of the replenishment tank, so as to enhance the comprehensive water use efficiency, and to realize the dual goals of energy saving and water saving.
Data Center Waste Heat Recovery Technology
The data center operates uninterruptedly throughout the year, and most of the electricity consumed is converted into heat energy and distributed, which is a low-temperature heat source with stable heat production throughout the year.
If the waste heat can be fully recycled, it not only reduces the waste of secondary energy, but also reduces carbon emission, realizes the energy gradient utilization of the data center, and provides important support for the realization of the goal of “double carbon”.
Necessity and Feasibility of Waste Heat Recovery
Necessity: The direct emission of low-grade waste heat from data centers wastes energy and causes thermal pollution, and waste heat recovery can convert it into usable thermal energy for heating and domestic hot water, reducing dependence on fossil energy, lowering operating costs, and providing both economic and environmental benefits.
Feasibility: Liquid cooling technology optimizes the conditions for waste heat utilization, cold plate liquid cooling can still ensure the normal operation of the chip at 60℃ or even 75℃ inlet temperature, and the high temperature waste heat can be directly utilized to reduce the recovery cost; ASHRAE TC 9.9 also verifies the feasibility of the liquid cooling return water as the source of heat supply.
Waste Heat Recovery Methods
At present, the waste heat recovery technologies available to data centers mainly include district heating, absorption refrigeration, direct power generation, indirect power generation, seawater desalination and biochemical energy refining, etc. However, there are big differences in the technical maturity, applicable conditions and promotion difficulty of different methods.
Direct power generation, indirect power generation and other waste heat recovery methods, subject to site selection, heat recovery efficiency and cost constraints, it is difficult to promote a large area in the short term; absorption refrigeration due to the large size of the equipment, the high temperature of the required waste heat, the need for additional warming, increasing energy consumption and costs.
In contrast, district heating is the most mature technology with the widest promotion prospects, which can significantly improve the energy utilization rate. Recovering waste heat from data centers as its heat source is a key focus of the current research because of its mature technology, controllable cost, and the realization of energy synergy, as well as multiple benefits.
Realization of Waste Heat Recovery for District Heating
Heat pump heating is the main way to recover waste heat from data centers for district heating, and the core is to upgrade low-grade waste heat to high-grade heat energy through heat pumps. Theoretically, absorption, air source, heat pump water heater and water source heat pump can be used, but their applicability varies significantly.
Similar to absorption refrigeration, the low-temperature waste heat of the data center can not be directly recycled by absorption heat pumps, additional warming is required, the practicality is low. Air source heat pump waste heat recovery system available in data centers usually has two ways:
- one is a centralized collection of high-heat exhaust air from the server cabinets, through a special duct led to the heat pump evaporator;
- the other is the heat pump evaporator is arranged directly in the data center. These two ways are required in the data center to implement the relevant supporting projects, supporting facilities are cumbersome, and the heat capacity of the air is low, the heat recovery benefit is not high, applicable to small data centers or waste heat recovery needs lower scenarios.
The heat pump water heater uses waste heat to prepare domestic hot water in situ, which is suitable for hot water centralized area; the water source heat pump is the main force of waste heat recovery, and uses the circulating water of water supply and drainage to raise the temperature for district heating, which is highly efficient and low-consumption; the water-cooled chiller with heat recovery can supply cooling, heating and hot water synchronously to improve the comprehensive energy efficiency.
Liquid-cooled Data Center Heat Recovery Program
Take the secondary side centralized circulation direct contact cold plate liquid cooling system as an example, the rest of the heat recovery program design needs to be combined with the heat dissipation characteristics of the liquid cooling system and air-cooled system to achieve efficient recovery and utilization of waste heat, the specific design points are as follows:
- The main equipment of the system includes chip heat dissipation cold plate, heat exchange module, closed cooling tower or dry cooler, etc.. Cold plate liquid cooling server heat dissipation is divided into two parts: 20% to 35% of the air-cooled, the rest of the liquid cooling system, need for different heat dissipation methods and media temperature design heat recovery program.
- Liquid cooling system with high temperature resistance, outdoor use of dry cooler direct cooling to reduce consumption; liquid cooling primary side of the supply and return water temperature is designed for 45/50 ℃, high temperature return water first through the heat recovery heat exchanger and 44 ℃ regional heat network heat exchange, warming to 49 ℃ for district heating, liquid cooling waste heat recovery.
- Air-cooled using chilled water precision air conditioning, chilled water supply and return water temperature design for 12/18 ℃, the cold source selection of the full heat recovery chiller, the configuration of a closed cooling tower to balance the amount of heat dissipation and heat recovery, the addition of natural cooling heat exchanger to enhance the energy-saving effect of the low-temperature season.
- Liquid cooling water temperature is constant, heat recovery is controlled by adjusting the water valve opening, convenient operation and maintenance; air-cooled need to start and stop the full heat recovery unit according to the heat recovery mode, chilled water, hot water side according to the corresponding temperature cycle, closed cooling tower to ensure the stability of the system.
Heat Recovery Benefit Analysis
Taking an IDC data center in Shanghai as an example, with an IT load of 18,000kW, the liquid-cooled part directly recovers waste heat through plate heat exchangers without additional power consumption, and the air-cooled part is equipped with a full heat recovery chiller unit with a high overall energy efficiency ratio.
The total heat recovery capacity of the center reaches 19,687kW, which can be used to heat designated standard residential apartments at an average annual load rate of 70%, saving about 3,657 tons of standard coal per heating season, and the energy-saving benefit is even more significant if domestic hot water recovery is taken into account.
The heat recovery efficiency of the data center is as high as 23, which is far better than the 8.27 level of the whole air-cooled system, fully indicating that the cold plate liquid cooling system is effective in energy saving and waste heat recovery, and has a very high value of popularization and application.
Dual Source Independent Control Cooling and Waste Heat Recovery System
In order to further improve the cooling efficiency and waste heat recovery capacity of the data center, combined with the internal and external source load characteristics of the data center and the indoor positive pressure demand of the server room, the dual-source independently controlled refrigeration and air conditioning and waste heat recovery system can be designed to achieve synergistic optimization of refrigeration and waste heat recovery to maximize the energy utilization efficiency.
System Design Concept
Taking a data center in Jinan as the research object, the design goal of the system is to maximize the recovery of waste heat, and for its internal source cooling and waste heat recovery part, the core goal is to obtain 48℃ hot water with full heat recovery and 65℃ hot water with partial heat recovery, taking into account the efficiency of refrigeration and operational stability.
The system adopts dual-source independent control mode, which can be dynamically adjusted with the load to guarantee efficient refrigeration and waste heat recovery; the COP reaches 9.69 at 54% load rate, and the energy consumption of a single set of working days is 2488.82kWh, which can produce 1,936.3 tons of hot water at 48℃ and 71.39 tons of hot water at 65℃.
Optimization Analysis of System Parameters
Optimization of system parameters is the key to improve the energy efficiency of the system and the benefits of waste heat recovery, by changing the total heat recovery hot water outlet temperature, chilled water parameters, etc., to analyze its impact on the system energy consumption and the amount of waste heat recovery, to provide a basis for the design and operation of the system.
For every 1℃ increase in full heat recovery hot water outlet temperature, the system energy consumption rises by 3.47%, and the amount of full heat recovery hot water and part of the heat recovery hot water flow rate increase by 0.27% and 3.1%, respectively, which needs to be combined with the waste heat demand to balance the energy consumption and recovery amount.
Fixed chilled water outlet temperature, the temperature difference between the import and export increases can reduce the chilled water pump energy consumption; fixed temperature difference, every 1 ℃ increase in export temperature, the system energy consumption decreased by 3.4%, the two types of recovery of hot water slightly reduced, need to optimize the two to achieve the optimal balance of energy consumption and recovery.
Energy Saving and Emission Reduction Benefits
The system has significant energy saving and emission reduction benefits, compared with the conventional refrigeration system, annual emission reduction of 3.28 tons of sulfur dioxide, 0.967 tons of soot, 84.34 tons of nitrogen oxides, carbon dioxide 36,982.83 tons, to help the “double carbon” goal.
In terms of water saving, it can save 94774.14 tons of cooling tower make-up water throughout the year; in terms of economic benefits, the waste heat gain is 5,547,600 yuan, and the annual value of the cost is reduced by 37.83% compared with that of the conventional system, which is a significant benefit.
Waste Heat Utilization Expansion
Currently, there are obvious seasonal limitations in the recovery of waste heat in data centers, and the recycling is mainly concentrated in the heating season, and the utilization rate of waste heat resources is low in the non-heating season;
Combined with the fifth-generation district heating pipeline network technology, it can expand the diversified utilization paths such as constant temperature heating of swimming pools, commercial drying, and cross-seasonal heat storage, which can effectively improve the utilization efficiency of waste heat resources and further enhance the comprehensive benefits of waste heat recovery in data centers.
Magnetic Levitation Phase Change Multi-Link Waste Heat Recovery Technology
Magnetic Levitation Phase Change Multi-Link Waste Heat Recovery Technology is an innovative waste heat recovery solution that combines the advantages of magnetic levitation technology, phase change heat transfer technology and multi-link system to achieve efficient recovery and utilization of waste heat in data centers while improving cooling efficiency, which is applicable to data centers of various scales, especially high-density and high heat load scenarios.
System Composition
The core technical points of the system: build an independent return air room above the air supply room, and configure air-refrigerant heat exchange device and multi-row EC fan to form a return air wall, which can guide the hot channel of the machine room back to the air, enhance the heat recovery and cooling efficiency, and guarantee the stability and energy saving of the system.
The refrigerant is directly exchanged with the hot return air in the evaporator to warm up and improve the heat exchange efficiency; the multi-row EC fan can even out the airflow and reduce the local hot spots to improve the energy efficiency of the system and reduce the energy consumption of the fan.
Parallel Connection of Evaporation and Heat-absorbing Wind Wall
Parallel connection of evaporation heat-absorbing wind wall can realize the synergy of multiple wind walls and improve the efficiency of waste heat recovery in large-scale data center; the refrigerant substance in the wind wall absorbs the waste heat from the return air of the server room and evaporates into gaseous state, which rapidly absorbs the latent heat to complete the waste heat recovery.
The gaseous refrigerant enters the magnetic levitation compressor and is compressed into a high temperature and high pressure gas, which then flows into the water-refrigerant heat exchanger and transfers the heat to the cooling water; the cooling water is transported by the pump to the heat storage tank for storage and regulation, and ultimately supplies hot water to the end-end heat-loaded rooms to realize the waste heat resourceization.
Valve and control
The side wall of the waste heat recovery room is installed with an electric air valve (key control device), whose operation status affects the efficiency of waste heat recovery and energy consumption; when the system is running stably, the air valve is closed and the hot air is guided to the waste heat recovery wall to complete heat exchange and ensure efficient recovery.
When the equipment is shut down, the bypass air valve opens automatically, and the hot air flow bypasses the waste heat recovery device and is processed by the indirect evaporative cooling system, which can reduce the resistance of heat exchanger, avoid the rise of power consumption of the fan, maintain the energy efficiency of the system, and realize the flexible adjustment of the working conditions and efficient operation.
The intelligent control system can monitor the operation status of the bypass electric air valve in real time and automatically realize the opening and closing switch of the valve to ensure that the ideal effect of heat exchange and the energy consumption level of the fan can be maintained under various working conditions without manual intervention, thus enhancing the operation and maintenance efficiency of the system.
Design Optimization Path
In order to realize the maximum benefits of data center cooling energy saving and waste heat recovery, it is necessary to optimize the design from the aspects of equipment interface, fault diagnosis, and technology application, solve the synergistic pain points, and improve the system stability, efficiency and maintainability.
- Unified equipment interface standards: Liquid cooling, water supply and drainage, air conditioning system interface differences affect the synergy and operation and maintenance, need to develop a unified specification. Hardware to clarify the interface size, material and specifications, the implementation of standardized quick connector; software to unify the communication protocol and data format, to achieve seamless linkage and intelligent optimization of equipment.
- Formulate a unified fault diagnosis process and standards: standardized diagnosis can quickly locate faults, reduce downtime losses, and improve the efficiency of operation and maintenance, covering key equipment and parameters, establishing a fault database, real-time monitoring and automatic diagnosis, and standardizing the standard to reduce operation and maintenance costs.
- Reasonable adoption of energy-saving technologies: priority is given to energy-saving pumps, frequency conversion air conditioning and other high-efficiency equipment, configured according to demand; with the help of frequency conversion and intelligent control to achieve precise energy control, combined with the use of natural cooling sources at the site, optimize the airflow, and set up an energy monitoring system, to enhance the level of energy saving.
Conclusion
Under the background of digital development, the energy consumption problem of data centers is highlighted, restricting the green and low-carbon development.
The application of liquid cooling, natural cooling and other advanced cooling energy-saving technologies can significantly reduce consumption and improve efficiency; the use of waste heat recovery technology can realize waste heat resource utilization, convert low-grade waste heat into usable heat energy, and realize energy ladder utilization.
