Optimal Control Of District Heating System With Variable Frequency Multistage Distributed Pumps Based On Operational Parameter Identification

The techniques of distributed variable frequency pumps have advantages in reducing distribution cost of district heating systems.However,the increase of district heating network scale requires higher pump heads for heating substations far from the heat source,which may raise the pressure level of the primary network.In this thesis,new type of district heating system with variable frequency multistage distributed pumps was established,the hydraulic and thermal characteristics were analyzed,and the operation control strategies were studied.For the district heating system with variable frequency multistage distributed pumps,the by-pass pipe is replaced by the hydraulic decoupler,and the primary supply temperature is maintained as the boiler outlet temperature.The operation scheme of constant boiler flow and variable network flow can be realized,which resolves the operation delay of district heating network,relieves the “large flow rate and small temperature difference” operation condition and reduces the distribution energy.The secondary pump head design technique and range were proposed,which can reduce the pressure level of primary network.The range of zero pressure difference point locations was also obtained.The ideal zero pressure difference point is defined as the location within the range,which minimizes the total pump power of the district heating network,for the guidance of system design.For the existing district heating network,reducing of the pressure level will be beneficial to the reformation of the existing district heating network.The hydraulic resistance identification technique of the single-heat-source tree-topology district heating network based on the second order extremum condition of multivariable function.This technique utilizing the operating parameters of the flow rates and pressures in heating substation nodes derives the analytical expression of the best square approximation of the hydraulic resistances and the hydraulic conditions avaliable for identification,which resolves the problem of only using heating substation parameters for hydraulic resistance identification of the single-heat-source tree-topology district heating network.The method has been applied to identifying hydraulic resistances of real district heating networks,and the maxmum relative error can be restricted within ±5%,which is in satisfactory range for engineering application.The optimal pressure control strategy of the district heating system with variable frequency multistage distributed pumps were proposed,which can minimize the total pumping power of district heating system,while ensures the load requirement of the heating substations.The operation performances of optimal pressure control strategy were compared with the constant pressure difference control under the condition of equal and inequal flow rate ratios of all the heating substations.When the flow rate ratio is 0.3,the total pump power of district heating network under the optimal control strategy can be reduced by 57% conmpared with the constant pressure difference control.With the heating load varying,the optimal control strategy can maintain the location of zero pressure difference point almost unchanged,which will provide stable operation conditions for the tertiary pumps.Hence,for large heating area,the district heating system with variable frequency multistage distributed pumps has advantages over the traditional district heating system on reducing the distribution energy.