Energy-Saving Evaluation And Optimization Of Heat Pump District Heating And Cooling System

Heat pump system can utilize various low grade natural or unused energy efficiently. So it usually has distinguished advantages of energy-saving and environmental protection. However, the energy efficiency of the heat pump district heating and cooling system is always lower than the heat pump system used in an individual building, and the system is very likely to lose its energy-saving characteristics because of the existence of the district pipe network. Thus the energy-saving characteristics of the heat pump district heating and cooling system has to be evaluated before its utilization in a project. Meanwhile, as an energy intensive system, it is especially necessary for the heat pump district heating and cooling system to be optimized with regard to the energy consumption. So the energy-saving evaluation and the energy optimization of the heat pump district heating and cooling system was carefully studied in the dissertation.Though the energy optimization of the heat pump district heating and cooling system can improve the energy efficiency of the system, it cannot guarantee the energy-saving effect of the system. The energy-saving pre-evaluation was studied on the basic research concept of ’pre-evaluation first and optimization later’. In the study of energy-saving pre-evaluation of the system, the conventional boiler house district heating system and the individual fresh water cooling tower system that is commonly used in independent buildings were chosen to be the comparison systems here. Then the energy-saving pre-evaluation indexes in both heating and cooling modes are deduced after detailed comparison of the heat pump district heating and cooling system with the above mentioned conventional systems. Further, the methodologies of energy-saving pre-evaluation for the heat pump district heating and cooling system are summarized accordingly.The energy optimization of he heat pump district heating and cooling system can be further proceeded if the pre-evaluation of the system shows its energy-saving potential. It is the district pipe network that mainly prevent the energy efficiency of the heat pump district heating and cooling system from increasing. So a pipe diameter optimization model is established for the main pipeline of the district pipe network. This provides the vital basis for the selection of the circulation pump of the district pipe network. It is found that only a very slight correlation exists between the pipe diameter optimization and the energy efficiency ratio of the whole heat pump district heating and cooling system. Therefore the pipe diameter optimization of the main pipeline of the district pipe network can be done separately from other parts of the system.The heat pump units consume the largest portion of energy in the whole heat pump district heating and cooling system. So the selection of the most energy efficient combination of heat pump units becomes the core of the energy optimization of the whole system. In the research, an optimization model was set up for the selection of the most energy efficient combination of heat pump units. Firstly, a determinant mathematical model to search the optimized heat pump loading strategy for a given combination of heat pump units was established. That is, the dynamic heating/cooling load of the system can be allocated to each heat pump unit in the most energy efficient way by the mathematical model. And the minimum annual energy consumption of the given heat pump units combination can be calculated at he same time. Then all the feasible combinations of heat pump units that meet the system heating/cooling load were searched out from the candidate heat pump units set for an actual project. So the most energy efficient combination of heat pump units can be found out by comparing the minimum annual energy consumption of all the feasible heat pump units combination schemes.The energy optimization strategy was put forward for the heat pump district heating and cooling system by integrating the above research results, and the accurate energy-saving evaluation of the heat pump district system can then be made. In the end, after the analysis of the advantages and disadvantages of the heat pump district system over the individual heat pump system, three key indexes that influence the energy-saving evaluation result of the heat pump district cooling system were brought out. They are the average part-load ration (PLRa), district radius of the system (R11) and the total design cooling load of the system (Qc) respectively in the order of their impacts on the energy efficiency of the heat pump district system.