System Design and Analysis of Heat Supply Using Waste Heat from Data Centers

In recent years, with the accelerated process of social informatization and intelligence, the amount of data communication, processing, and storage are increasing rapidly. Data center is an important node in digital services, its data communication traffic has reached about 11 ZB, data storage scale is about 26 EB. data center energy consumption is also growing rapidly, the global total energy consumption of data centers in 2020 will be about 200~250 TW-h, accounting for about 1.1% of the global total electricity consumption. China's overall power consumption of data centers is increasing at an annual rate of more than 10%, and its power consumption will exceed 200 billion kW-h in 2020, accounting for about 2.71% of the whole society's power consumption. About 90% of data center power consumption is converted into low-temperature waste heat, which creates the potential for waste heat recovery and reuse.

The total global centralized heating heat in 2020 will be about 16 EJ, of which the total centralized heating in China will be about 5.9 EJ, accounting for about 37% of the total global centralized heating. The total area of centralized heating in the north of China is about 15.6 billion ^m2, the total energy consumption is about 214 million tons of standard coal, and the emission of _CO2 is about 550 million tons, and the carbon emission per unit of heating heat is about 1.29 times of the world average. In the context of energy saving and emission reduction, the use of renewable energy and waste heat as heat source is the development trend of centralized heating. Therefore, the use of low-temperature waste heat in data centers in northern China is an effective way to reduce centralized heating energy consumption and carbon emissions, and improve the efficiency of data center energy use.

Researchers at home and abroad have made various attempts to utilize waste heat from data centers, including district heating/hot water supply, auxiliary power generation, absorption/adsorption refrigeration, organic Rankine cycle, thermoelectricity, biomass conversion, and seawater desalination/wastewater treatment, etc. Huang et al. reviewed the literature related to the utilization of waste heat from data centers, and concluded that the direct heat supply is the most prospective way of utilizing waste heat from data centers.Yu et al. used a water source heat pump to recover waste heat from a data center to supply heat to an annex building, and simulated the heat load and heating energy consumption of the annex building; compared with the air source heat pump system, the operating cost and initial investment of waste heat supply in data centers are lower, which is economically beneficial. he et al. proposed a distributed data center cooling scheme using a heat pipe system, and carried out the application of waste heat recovery and heat supply in a data center in Hohhot; at a chilled water supply temperature of 18°C, the waste heat recovery and supply were applied to a data center. In the case of chilled water supply temperature of 18 ℃ and heating temperature of 54 ℃, compared with the coal-fired boiler heating system, the data center waste heat heating system saves about 18,000 t of coal annually, and the annual power saving rate is nearly 10%.Oró et al. recovered the waste heat from the data center to supply heat for the nearby swimming pool, and the payback period was 15 a, and the indoor swimming pool operation cost was reduced by 18%.
 

The systems reported in the domestic and international data center waste heat heating literature generally suffer from low waste heat utilization and long payback periods. For example, the data center waste heat of the system reported by Yu et al. is about 72.26 MW, while the total heat load of the affiliated building is 15.57 MW, and the waste heat utilization rate is only 22%. When using data center waste heat for small-scale, short-distance heating, the huge gap between the server power density and the building heat load inevitably leads to low waste heat utilization. The power density of high-density communication servers has reached 8~15 ^kW/m2, and the power of a single fully loaded blade server will even reach more than 35 kW, which is much larger than the winter heating load of various buildings. The common air-cooled data center cooling system has a low cooling temperature, and the heating requires the use of heat pumps to raise the temperature, which consumes more energy. The low utilization of waste heat and the low temperature level of the cooling system are the main reasons for the long payback period of waste heat heating investment in data centers. The cooling system temperature of liquid-cooled and two-phase flow-cooled data centers can reach more than 60 ℃, which is capable of directly supplying heat, but the investment is high and the operation and maintenance are difficult. The relevant waste heat heating system is usually outside the data center and then set up an additional set of water source heat pump system, duplication of equipment has led to a higher initial investment, longer payback period.

Waste heat supply in data centers should consider both changes in building heat load and changes in data center energy consumption. The building heat load is mainly affected by changes in outdoor air temperature, which can be predicted by heating energy consumption and meteorological data in previous years. Common loads in data centers are: network workloads, high-performance computing loads and data workloads, and server power consumption varies with load intensity. The intensity and energy consumption of high-performance computing loads and data workloads are almost constant, while the intensity and energy consumption of network workloads change over time. Data center waste heat heating is difficult to simultaneously couple data center power consumption and user heat load changes, requiring the data center heat discharge is greater than the heat load. Adding a heat storage device in the heating system can improve both the utilization rate of data center waste heat and safety.

This paper proposes a long-distance waste heat supply system for air-cooled data centers using a multistage heat pump, which has better economy and energy saving and emission reduction effects, improves data center energy utilization efficiency and reduces heating carbon emissions under the premise of guaranteeing data center safety.