| At present,with the development of rail transit,the energy consumption of metro stations is also increasing.Ventilation and air conditioning system as an important part of the subway station,its energy consumption occupies a large proportion.In order to achieve the goal of building energy saving,the research of air conditioning system has become the focus of energy saving research.Li Guoqing and Meng Xin of BEIJING URBAN ENGINEERING DESIGN&RESEARCH INSTITUTE CO.,LTD puts forward a new energy-saving direct expansion evaporative condensation air conditioning system for Metro and applies it to practice.In this paper,the performance of the system is measured and the optimal operation strategy is studied.HVAC system needs to solve the worst conditions to ensure the indoor thermal environment.So the system is designed according to the maximum load,and most of the time in the actual operation is in partial load conditions.If we can determine the optimal operating point of the air conditioning system under different conditions and optimize the operation through the control system,the energy consumption of the air conditioning system will be greatly reduced.In this paper,the direct expansion evaporative condensation air conditioning system of Metro Station on Beijing Metro Line 14 is taken as the research object.Station is a second-floor underground station with a 12-meter island platform.The structure of the station is 249.6 meters in length,the width of the standard section is 21.3 meters and the building area is 19184 square meters.The main work and research are as follows:(1)Analysis of heat and mass transfer mechanism:The heat and mass transfer process of evaporative condenser is analyzed.The results show that the main driving force of the heat and mass transfer process of evaporative condenser is the temperature difference between refrigerant and water film and the enthalpy difference between water vapor layer and air on the surface of water film.The heat transfer coefficients of each heat transfer process are analyzed and summarized.It is concluded that the main influencing factors of heat transfer process are air wet bulb temperature and water-gas ratio of evaporative condenser when the structure of evaporative condenser is fixed and the load in the station is constant.(2)Measurements and data analysis:In the air-conditioning season of 2016,a comparison test was made between station A with new refrigeration system and station B with traditional water-cooled system.The test results show that the energy consumption of refrigeration system in station A is lower.The COP of refrigeration system is higher than that in station B.The transportation energy consumption of refrigeration system in station A accounts for a small proportion of the total energy consumption.Because the refrigeration room of station B is located at one end of the platform and ground cooling tower is installed,the energy consumption of refrigerated water and cooling water transportation is high.Long-term testing of the system was conducted in the air-conditioning seasons of 2016,2017 and 2018(July-September).The test contents include the parameters of each equipment,energy consumption of each equipment in evaporative condensation refrigeration system,relevant parameters of refrigeration system,system performance coefficient and operation status.Through testing,it is found that there is energy-saving space for the fan of the system which can not be adjusted according to the load frequency conversion.According to the characteristics of the system,the main factors affecting energy efficiency in the air conditioning system are selected as decision variables:fresh air temperature and humidity,supply air volume,evaporative condenser side air volume and spray water volume.The physical model of the air conditioning system and equipment is established.Compared the measured energy consumption with the simulated energy consumption under the same equipment power and outdoor temperature and humidity,it is found that the error is less than 10%.The accuracy of the model is verified.The influence of the above variables on COP of the system is studied through the model.The remaining variables are excluded as decision variables of the model through reasonable analysis and Consideration Based on the operation of the optimization program.At the same time,the constraints are determined to ensure the normal operation of the system.The energy consumption optimization model of the system is established.(3)Modeling and validation:In order to study the combined operation strategy of various equipment in the system,a mathematical model of evaporative condensation air conditioning system is established to analyze the influencing factors of energy consumption in the operation of evaporative condensation air conditioning system.The direct expansion evaporator and evaporative condenser in the model are modeled by distributed parameter method.Considering the different phase states of refrigerants in the refrigerant tube,which lead to different heat transfer modes,the single-phase region and the two-phase region are divided into several nodes based on temperature difference and enthalpy difference.Each node is regarded as an independent micro-heat exchanger.An iterative algorithm is constructed and solved according to the energy and mass conservation relationship between the nodes.The system model solves the total power of different outdoor air parameters,station load and equipment operating frequency through the simultaneous models.On this basis,the model is validated by the measured energy consumption data of the air conditioning system in the Metro station.The results show that the error between the simulation results and the measured system power is less than 10%.(4)System optimization research:The influence of different main air supply fan,induced draft fan and pump frequencies and COP of outdoor air wet bulb temperature system is studied through the model.It is concluded that main air supply fan,induced draft fan and outdoor air wet bulb temperature are the main factors affecting the total power of the system.The objective of energy-saving operation of the system is determined:the frequencies of the main blower,the induced blower and the compressor corresponding to the minimum total power of the system are obtained.At the same time,in order to ensure the normal operation of the system,the constraints are determined.The energy consumption optimization model of the system is established.The optimization model of air conditioning system is solved,and the optimal operation scheme under each working condition of air conditioning system is obtained.By comparing the optimized operation scheme with the simulated energy consumption of the original operation scheme,it is found that the energy consumption of the refrigeration system is reduced by 6%-20%,which proves that the optimized model has the effect of reducing the energy consumption of the refrigeration system.This paper puts forward a concrete implementation plan of using the temperature and humidity of the automatic control system of the metro station and the monitoring data of the fan frequency to calculate the refrigeration capacity in the station,and using the optimization program to adjust the refrigeration capacity according to the change of working conditions. |
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