Study On The Optimal Control Technology For Outdoor Pipe Network In District Heating And Cooling System

Energy is the foundation and driving force for the progress of human civilization.It matters to the economy,to people’s lives,to national security.Building energy conservation is the key of the energy conservation and emission reduction in our country.In District Heating and Cooling System,There are the long pipeline,high energy consumption and huge potentiality for conservation of outdoor pipe network.Study on the optimal control technology for outdoor pipe network in district heating and cooling system which can meet the load demand of all users and reduce energy consumption of pumps to the most extent is very significant to save energy and improve its running performance.Firstly,this paper proposes the optimum running temperature difference of the terminal and the most unfavorable thermal terminal.According to the mathematical principle of heat exchanger,the method for determining the optimum running temperature difference of the terminal is introduced.On this basis,combined with the hydraulic characteristics of the outdoor pipe network,the method for determining the most unfavorable thermal terminal is presented.And the differential pressure set point of outdoor pipe network can be obtained by using the differential pressure demand of the most unfavorable thermal terminal.The variable differential pressure control strategy for outdoor pipe network in district cooling and heating system is established.Based on the variable differential pressure control strategy,considering the influence of different pump operation schemes on the energy consumption,this paper also proposes a joint control strategy combining the variable differential pressure control strategy and the optimal control scheme of parallel variable speed pumps.By using the optimal control scheme of parallel variable speed pumps,an actual case is simulated.And it is found that the energy consumption of pumps doesn’t increase monotonically with differential pressure and flow of the pipe network increasing when the pressure difference of the pipe network is far lower than the rated head of pump.Therefore,the paper further optimizes the operation of pumps,and presents an optimal control strategy based on the characteristics of pipe network and pump which is the most energy-saving strategy in principle.Secondly,through the investigation of a regional energy station in Chongqing,it is found that there are serious problems of " large flow,small temperature difference" and "bypass countercurrent" in the pipe network.The rated head of pump is too large,and the running efficiency is low.Study found that some rerminals dosen’t use motor regulating valve,and the effect of motor regulating valve is poor which are main reasons for the problems of " large flow,small temperature difference".Then,taking this energy station as the research object,the paper establishes an optimal control simulation model of ring-shaped network based on digital twin though Matlab software to compare the control effect of four strategies.The results show that the control effects of the variable differential pressure control strategy and the joint control strategy combining the variable differential pressure control strategy and the optimal control scheme of parallel variable speed pumps are the best in the aspect of pipe network running performance,and the optimal control strategy based on the characteristics of pipe network and pump performs best in the aspect of pump running performance.The effect of the constant differential pressure control strategy is the worst.The variable differential pressure control strategy,the joint control strategy combining the variable differential pressure control strategy and the optimal control scheme of parallel variable speed pumps and the optimal control strategy based on the characteristics of pipe network and pump can save energy by 18.57%,29.13% and 34.27% respectively under heating mode,and by 16.22%,21.86% and 27.72% respectively under cooling condition than the constant differential pressure control strategy.The results also show too large pump selection affects the running efficiency and increases energy consumption of pump.When the rated head of pump is reduced to 14 m,the running performance of pipe network and pump is significantly improved,and each strategy can save energy by 43.3%,51.8%,46.3% and42.1% respectively.Finally,this paper compares the control effect of four strategies by testing.The test results are consistent with the simulation results.If the number of pumps is not limited in the experiment,the optimal control strategy based on the characteristics of pipe network and pump can save the most energy,and the energy saving rate can reach 29.9%.