Investigation on energy consumption characteristics in different time scales of a metro station

. As a big energy consumer in metro station, the air-conditioning system is responsible for air-cooling and ventilation in cooling season while only for ventilation in other seasons. Understanding the hourly energy consumption characteristics of the system contributes to renewable energy integration and achieving carbon emission reduction in metro stations. Thus, energy consumption characteristics in different time scales of a Beijing metro station is investigated in this study. The results reveal that the air-conditioning system accounts for 61.7% of the total electricity consumption for the entire year, while the proportion rises to 73.7% for the cooling season. In the daily scale, the electricity consumption for air-conditioning increases with the outdoor air temperature rising, while it is nearly free from the effect of the passenger flow rate changing. In the hourly scale, the electricity consumption for air-conditioning shows a weather-condition-related characteristic, the electricity consumption for lighting shows a time-related characteristic, while the electricity consumption for others is nearly constant during the operation period. It is anticipated that the study helps to understand the hourly energy consumption characteristics and provides reference for energy-saving strategy selection in metro stations.


Introduction
By the end of 2020, there have been 45 cities on the Chinese mainland owing urban rail transit systems, and the total length of Chinese metro lines has reached to 6280.8 km [1]. Chinese urban rail transit systems consumed 17.2 billion kWh of electricity in 2020, showing a remarkable year-on-year growth rate of 6.3% [1]. Saving energy in metro station contributes to achieving national energy conservation and carbon emission reduction. Thus, more and more researches and engineers have payed attention to energy consumption of metro stations in recent years [2][3][4].
As a big energy consumer in metro station, the airconditioning system is responsible for air cooling and ventilation in cooling season while only for ventilation in other seasons. Recently, more efforts have been made to save energy consumed by air-conditioning systems in metro stations. Zhang et al. [5] proposed an innovative environment control system for metro stations, and the on-site experimental data revealed that the system could satisfy the requirements of thermal comfort and the energy consumption could be reduced by 20.6%-60.4% by adopting the system. Wang et al. [6] conducted filed studies and developed autonomous control system for Beijing metro stations. It is indicated that, compared with the conventional control strategy, the developed autonomous control system helped the metro stations reduce energy by 20%-38%. Zhang and Li [7] investigated and compared effects of platform bailout door, platform screen door, and bailout door-screen door * Corresponding author: zt2015@mail.tsinghua.edu.cn combined systems on energy consumption of airconditioning systems in metro stations. The results demonstrated that the combined system could achieve the best energy performance among the systems. Yin et al. [8] analysed the variation law of the ventilation and air-conditioning system of a deep-buried subway station in sub-tropical climates. Seven energy-saving strategies were put forward and the results showed that the energy consumption of the air-conditioning system could be saved by over 30% after optimization. He et al. [9] focused on the effective utilization of unorganized fresh air induced by piston effect in metro stations in the transitional season, and found that the new environmental control system has the potential to save energy compared with traditional platform screen doors system, indicating an energy-saving rate of 42.7%.
Understanding the energy consumption characteristics in different time scales in the metro station contributes to renewable energy integration and achieving carbon emission reduction in the further. Thus, energy consumption characteristics in different time scales in a metro station is investigated in this study. It is anticipated that the results could provide reference for energy-saving strategy selection in metro stations.

Methodology
A metro station in Beijing is selected as the objective station in this study. The station is a typical underground station, with a construction area of 11273 m 2 . In terms of the public area, there are two floors and four entrances in this station as shown in Fig. 1. Trains in the opposite directions pass though the island type platform. Hourly electricity consumption of the metro station during the entire 2021 was monitored, in addition to the passenger flow rate and outdoor air temperature. The partial correlation coefficient is introduced as Eq. (1) to evaluate the correlation strength between the electricity consumption and different influencing factors.
where r ij(k) denotes the partial correlation coefficient between variables x i and x j when the effect of x k is controlled, r ij denotes the correlation coefficient between x i and x j , cov(x i ,x j ) denotes the covariance between x i and x j , and var(x i ) and var(x j ) denote variances of x i and x j , respectively. The strength of correlation can be classified according to Eq.

Electricity consumption in monthly scale
The total electricity consumption of the station is 2.1 million kWh in 2021, based on the record data. According to different purposes, the electricity consumption can be divided into the following three categories: (i) air-conditioning, (ii) lighting, and (iii) others. With respect to air-conditioning, the airconditioning system is responsible for air-cooling and ventilation in cooling season while only for ventilation in other seasons. With respect to lighting, the lighting system is responsible for station lighting, advertising lighting, and section lighting. With respect to others, elevator/escalator device, station signal device, and etc. are abbreviated as others.
As shown in Fig. 2 (a), the electricity consumption of the station significantly increases when the cooling season comes. Except the electricity consumption for air-conditioning, the electricity consumption for lighting and others is nearly constant for different months. For the entire year, the air-conditioning system accounts for 61.7% of the total electricity consumption, the lighting system accounts for 18.0%, while the others accounts for the remaining 20.3%. As the air-conditioning system dominates the total electricity consumption, the increasing cooling demand in summer results in the significant station's energy consumption rise in Fig. 2  (a). Moreover, because the air-conditioning system is responsible for cooling besides ventilation, the airconditioning system responsible for a greater proportion of the total electricity consumption in the cooling season, as shown in Fig. 2 (c). The air-conditioning system accounts for 73.7% of the total energy consumption in the cooling season, while it only accounts for 50.2% in other seasons.

Electricity consumption in daily scale
To further illustrate the reason for the energy consumption variation of the air-conditioning system in the cooling season, its energy consumption in daily scale will be investigated in this section.
The partial correlation analysis results are listed in Table 2. The electricity consumption for airconditioning shows a significantly moderate correlation strength with the outdoor weather condition, while it nearly shows no correlation with the passenger flow rate. As shown in Fig. 3. The daily electricity consumption of the station increases with the rise of outdoor air temperature as shown in Fig. 3 (a), due to the cooling load increasing and the energy-efficiency decreasing of the air-conditioning system in the cooling season. As depicted in Fig. 3 (b), the daily electricity consumption is nearly free from the effect of the passenger flow rate. The reason can be illustrated as follow: (i) the mechanical ventilation subsystem always operates at the maximum frequency according to the regulation for pandemic prevention, and thus the mechanical fresh air volume can be approximately regarded as constant under different passenger flow rates; (ii) the unorganized infiltration induced by the piston effect is mainly affected by the train behaviour instead of the passenger behaviour [11,12], and thus the infiltration air volume can be approximately regarded as constant under different passenger flow rates; (iii) the fresh air load dominates the cooling load of the air-conditioning system for the public area in metro stations during the cooling season [13,14], and the passenger flow rate changing can hardly affect the total cooling load. In summary, the daily electricity consumption is nearly free from the effect of the passenger flow rate.

Electricity consumption in hourly scale
The hourly electricity consumption characteristics of the air-conditioning system, lighting system, and others will be interpreted in this section.
The hourly electricity consumption for airconditioning is depicted as Fig. 4(a). As shown in Fig. 4, the first trains for the opposite directions depart at 5:25 and 6:25, respectively, while the last trains for the two directions depart at 22:45 and 23:49, respectively. Seen from the figure, the hourly electricity consumption for air-conditioning in the cooling season is significantly higher than that in other seasons, due to the extra cooling mission in the cooling season. Furthermore, the electricity consumption for air-conditioning at noon is greater than that in morning or evening. The electricity consumption tends to increase with the outdoor air temperature increasing. In summary, the hourly electricity consumption for air-conditioning shows a weather-condition-related characteristic during the operation period (6:25-22:45).
The hourly electricity consumption for lighting is depicted as Fig. 4(b). Seen from the figure, the electricity consumption for lighting shows a timerelated characteristic: (i) only part of the lighting devices are open in daytime (daytime mode), while more devices turn on when night approaching (night time mode); (ii) from May to September, the daytime mode starts in advance of 1-2 hours in morning and ends with 1-2 hours delay in the evening. The above arrangement aims to make full use of the nature light for station illumination.
Moreover, the hourly electricity consumption for others is depicted as Fig. 4 (c). Observed from the figure, the value is approximately constant during the operation period (6:25-22:45). The value slightly rises when the evening rush hour comes. The rise is induced by the utilization rates of the elevator/escalator and other devices increasing. However, the rise is insignificant owing to others accounting for a little proportion among the total electricity consumption (Fig. 2).

Conclusion
Energy consumption characteristics in different time scales in a Beijing metro station is investigated in this study. The air-conditioning system is responsible for aircooling and ventilation in cooling season while only for ventilation in other seasons. The air-conditioning system accounts for 61.7% of the total electricity consumption for the entire year, while the proportion rises to 73.7% for the cooling season. In the daily scale, the electricity consumption for air-conditioning increases with the outdoor air temperature rising, while it is nearly free from the effect of the passenger flow rate changing. In the hourly scale, the electricity consumption for air-conditioning shows a weather-condition-related characteristic, the electricity consumption for lighting shows a time-related characteristic, while the electricity consumption for others is nearly constant during the operation period.