Study On Control Strategy Of Dynamic Distributed District Heating System With Water-mixing And Variable Frequency Pump

Low efficiency, disorders of the hydraulic and thermal conditions, too large energy consumption of the pipe network and other issues the district heating business facing have been researched in this paper. Heating energy consumption is the largest component of the building energy consumption. For the positive response to the national call of the energy-saving and emission reduction, energy-saving in the heating system is extremely urgent. As the proportion of the initial investment and the dynamic energy consumption of the heating transmission and distribution system in the heating system is very large, therefore, a comparative study is made on the heating transmission and distribution system while the rational design and operation of the heating pipe network system as the starting point. A comparison is made in the economic and energy interests by analyzing the structure and the adjustable and stability of the heating pipe network. Through the simulation of regulation characteristics of the system, the rationality and feasibility of the dynamic distributed district heating system with water-mixing and variable frequency pump is proposed, and the corresponding control strategy has been given. It was studied mainly from the following aspects.1、The topology of the heating pipe network was analyzed using the graph theory, and the mathematical model and algorithm of the heating pipe network was established through the fluid network analysis.2、The energy efficiency of the water-mixing connection in the heating system was analyzed. According to law of conservation of the energy and mass, the structure of the pipe network was introduced and the theoretical analysis was made in the heating system with the water-mixing connection, and the dynamic simulation of mathematical models were established with the boiler, the pipe network system, radiator, the indoor temperature of the user and pump motors respectively. The models of various parts were simulated by programming with SIMULINK, and the time delay curves of the boiler outlet temperature and the indoor air temperature reaching the steady-state were simulated. The delay time of the water-mixing heat exchange is3hours or less, far below the indirect heat exchange. 3、The economic of water-mixing heat exchange was analyzed. In aspects of initial investment, operating costs, maintenance costs, etc. was compared, and the way of water-mixing heat exchange is superior to traditional heat exchange way in the economy.4、The design of the dynamic distributed district heating system with water-mixing and variable frequency pump was proposed by analyzing the energy efficiency of frequency conversion technology and the structure of dynamic distributed pipe network in the heating system. In addition to the circulation pump with smaller lift was set at the heat source, more than one pressurized circulation variable frequency pump were set up along the heating out-let in the design, and the way of water-mixing heat exchange was adopted by heat users. Although more pumps were used in the system, but the total power of the pressurized circulating pump was reduced, and this is the advantage of the design program of the dynamic distributed district heating system with water-mixing and variable frequency pump.5、Based on the comparative analysis on different settings of the main circulating pump and pressurized pump, it was respectively compared from the theoretical energy consumption, investment costs, running electricity and equipment recovery period. Therefore, the development of the dynamic distributed district heating system with water-mixing and variable frequency pump should be encouraged, for the dominance of the economic and energy interests.6、The control strategy of the system was proposed by analyzing the regulation characteristics of the dynamic distributed district heating system with water-mixing and variable frequency pump.The adjustability of the system was analyzed. The adjustability of the heating system was the basis of the system during normal operation. Applying variable frequency pump instead of regulating valve in the system, the phenomenon of a valve shock and shutdown was avoided. Therefore, there’s better adjustability in the system.The stability of the system was analyzed. The hydraulic stability is an important indicator to measure the regulation performance of the transmission and distribution system. The hydraulic stability of the dynamic distributed district heating system with water-mixing and variable frequency pump was analyzed by the definition of the hydraulic stability.Finally, programming operation was made with VC++by analyzing the regulation characteristics of the system, in order to get the performance curves of the water-mixing ratio, water-mixing flow, and the flow and temperature of the primary network changing over time of different users. Then the correctness of the control strategy was analyzed and demonstrated through analyzing and solving the mathematical model.