Operational Parameters Prediction And Optimization Research Of District Heating System Based On Pipe Network Dynamic Model

District heating rapidly develops in China. Whether district heating scale, radius or pipe network type, all have greatly changed. In order to further improve energy efficiency of district heating system and better bring its characteristic of energy saving and environmental protection into play, an operational parameter prediction and optimized operation method of district heating system, which is based on pipe network dynamic model, is proposed in this paper. This method is systematically researched, and the effectiveness and accuracy of the proposed dynamic model and algorithm have been verified. The results show that the proposed method has guiding significance to promote development of operational adjustment and optimal control technology, and to improve operational management level in district heating system.Firstly, based on network graph theory and Kirchhoff s law, the hydraulic model of district heating network is established, and solved by improved square root method. Aiming to two different constant pressure forms, pump lift adopts fitting formula and constant value, the hydraulic model of a primary simulated network is established. The results show that initial adjustment is an important prerequisite to guarantee the pipe network hydraulic balance.Secondly, according to the energy conservation equation, the thermal dynamic model of hot water heating pipe is derived. This dynamic model is hyperbolic equation, so that the characteristics method can be used to establish its characteristic equation. Then its corresponding finite difference equation along the characteristic line is set up by the inverse step method, first-order Taylor series expansion and forward explicit difference scheme. Then the least squares method is proposed to fit linear equations of pipe heat loss and water supply temperature for different diameter and different insulation materials, then these fitting equations can be used by the thermal dynamic model. Through the above, the thermal model that considers heat dissipation individually is deduced and called "dissipation model". At the same time, the thermal model that both consider heat dissipation and heat accumulation is deduced and called "regenerative model"Then, the dissipation model and regenerative model of the primary simulated network are established respectively. Through simulating a working condition by the dissipation model, the influence on simulating result is analyzed from error limit, space step and initial value. In order to ensure the convergence of the differential format, the temperature difference, which is the difference of two neighbor iterative temperature values on each computing node, is substituted by the temperature gradient, which is the ratio of the above temperature difference with the current time step, to determine whether the program is convergence or not, and it is proved that the method is good. At the same time, two different operating conditions are respectively simulated by the dissipation model and the regenerative model. It is found that the pipeline heat dissipation leads to lower the entrance temperature of remote users, and to further exacerbate the thermal imbalance and adjustment difficulty of pipe network. The regenerative effect makes the delay time of network from any disturbance to stabilization increase, then it will affect when the operational strategy is adopted to adjust the pipe network. Therefore, the regenerative model is suggested to be used when the delay time of network stability is more important. Otherwise, the dissipation model will be firstly selected because of its relatively simple and fast convergence.Aiming to the node leak failure of district heating pipe network, The flow and pressure distribution of three different leak conditions are simulated by the simulated network, which is one leak point in the supply pipes, two leak points in the supply pipes and one leak point in the return pipes, different leakage have been compared either. Furthermore, the thermal dynamic of the first leak condition are simulated. Finally, the pressure, flow and temperature variation of each point on the pipe network is summarized when the node leak happens, and it could provide corresponding reference for diagnosis of pipe network leak failure and leak point position.A dynamic model of an indirect connection pipe network is established, and it includes a simulated primary network,10secondary heat exchanger stations and secondary networks which consist of100building users. Each building user is overall described by a radiator model. This model is used to simulate the case that users independently regulate valve to control indoor temperature when the method of heat metering is adopt. The results show that the model well reflects the influence on the indoor temperature, the user load and the operating conditions of the primary and secondary network from the user's independent regulation. Then, part of a real city heating pipe network is modeled by the dissipation model. And it is taken as validation data and compared with simulated data that the monitoring data average of primary supply water temperature and pressure in half an hour are collected from6selected heat exchanger stations. The results show that the relative errors of supply water temperature and pressure in5stations are all in the range of2%, in addition to one station whose relative error is more than3%. And the results verify validity and accuracy of the proposed model and algorithm in this paper.For the operational optimization problem of supply temperature and flow adjustment (known as "quality-quantity" regulation) in district heating system, an operational cost equation of primary network is set up, taking the supply water temperature and the water flux as variables. The optimization objective of this equation is to minimize operational cost. Then, based on the nonlinear programming theory, a constraint problem is translated into an unconstrained one by general constrained multiplier method, the conjugate gradient and linear search method are integrated together to solve the optimization problem. Finally, the presented method and the step-by-step method are respectively applied to a real urban district heating system. The result proves that the former is more effective and accurate than the later; the energy saving effect of quality-quantity regulation which is guided by the presented method is compared with the one of quality regulation, it proves that the former is prior to the later and further verifies effectiveness of the presented method.