China’s urbanization rate increased from 36% in 2000 to 60% in 2019. With the rapid growth of urbanization, the efficiency of the centralized energy supply is receiving increasing attention. It becomes more important to coordinate different energy sectors, i.e., electricity, heating, cooling, and gas systems to increase the overall energy efficiency. With the development of electricity market reform and action plans for the energy internet, integrated energy service is open for public investment and will grow rapidly in the near future.
Compared with the United States and the European Union, the population density of urban areas in China is much higher and, as a result, so is the density of the energy supply. Such heavy energy supply density makes centralized heating and cooling much more economical than distributed options, and makes the coupling of different energy sectors quite beneficial in district energy systems. One important question for building urban integrated energy systems is, what would the best design for such energy systems be in terms of energy efficiency and cost effectiveness?
Our group at Tsinghua University has conducted a series of empirical studies in future urban integrated energy systems. Apart from the widely recognized components such as cogeneration, heat pumps, and absorption chillers, we identified other key components that will play an important role in future urban integrated energy systems in China.
Variable renewable energy: Higher penetration is possible
Along with urbanization, China is experiencing a quick energy transition, from a coal-based energy system to a variable renewable energy (VRE)–based one. Wind and solar power constituted more than 9% of China’s electricity generation in 2019, and this percentage is expected to rise to 20% in 2035 and 30% in 2050. The high curtailment rate of VRE now in China calls into question the possibility of higher levels of renewable energy penetration.
By coupling different energy systems, the flexibility from other energy sectors can be used to accommodate the VRE that might otherwise have been curtailed. For example, heating and cooling storage facilities and storage in pipelines can be used as virtual batteries for wind and solar power. Our research shows that the energy penetration of VRE in district energy systems can reach up to 40% of the overall energy provided with curtailment less than 10% . Under such high penetration, the power system alone cannot fully balance the fluctuation of VRE. The help of heating, cooling, and gas systems is indispensable. Although in such a scenario the levelized cost of energy would be significantly higher, it is expected to be reduced rapidly, along with the reduction of the renewable energy capital investment.
Compressed air energy storage: A hub of integrated energy systems
Compressed air energy storage (CAES) is a cost-efficient, large-scale energy storage technology. It looks promising for satisfying the huge demand of energy storage capacity in future power systems. Although the electricity-to-electricity round-trip efficiency of CAES is lower than other storage technologies, it has a much higher efficiency when used in integrated energy systems. The reason is that the charge and discharge of CAES involve the conversion among internal energy, electric energy, and mechanical energy. Integrated energy systems facilitate a wise use of the heat and the cooling energy that cannot be converted back to electricity. Furthermore, the CAES plant can act as a hub of integrated energy systems that allows flexible conversion among different energy forms. Our study shows that CAES is able to reduce the operational cost of integrated energy systems by up to 10%. Diabatic CAES is more beneficial than adiabatic CAES in integrated energy systems because the heat and cooling can be more efficiently used.
Seasonal energy storage: Ironing out seasonal energy imbalance
Urban integrated energy systems in China have a great seasonal imbalance. In southern China, the energy needs in summer are twice those in winter because of the air conditioning load. In contrast, in northern China the energy needs in winter are doubled because of the heating load. Although in central China, the energy needs in winter for heating and in summer for cooling are similar, the load in spring and autumn is lower and shows a double peak yearly energy load curve. The result would be a serious seasonal supply-demand imbalance under high penetration of VRE. We find that seasonal storage, i.e., pit thermal energy storage , will play an important role in the decarbonization of integrated energy systems. Given the current investment cost of pit thermal energy storage—around $40/m3—the energy penetration of VRE can increase by 10% without increasing the levelized cost of energy through the use of pit thermal energy storage.
In summary, energy systems integration—including energy system coupling, CAES, and seasonal energy storage—will play an important role in China’s urban energy systems in the decades ahead.
Department of Electrical Engineering
 Yaohua Cheng, Ning Zhang, Daniel S. Kirschen, Wujing Huang, Chongqing Kang. Planning multiple energy systems for low-carbon districts with high penetration of renewable energy: An empirical study in China. Applied Energy, 2020, 261, 114390.