Research On Control Mechanism Of Hybrid Ground Source Heat Pump System With Auxiliary Cooling For Energy-saving

Ground source heat pump is an effective technology to use geothermal energy Which has grown fast in recent years However,the performance of ground source heat pump is constrained by problems of low efficiency during continuous operation and soil heat accumulation.particularly in areas with unbalanced heating and cooling load.The problem of soil heat accumulation not only affects the performances of system to meet the peak load for a short period,and even cause the annual unbalance of geothermal energy.To solve these problems,a hybrid ground source heat pump system has been proposed,and a lot of research results have been achieved.However,a complete set of control theory has not been developed because of the system complexity and the coupling issues among the system components.Auxiliary cooling system is the primary type of hybrid systems where the cooling load is in dominance.The research works on auxiliary cooling systems has been increasing in recent years,in particular those focusing on control strategy and operation parameters.However,those works still fail to address the optimization of input capacity from auxiliary cooling facilities under the conditions of different types of systems,control strategies and operating periods,etc.In detail,the problems are if there is the the optimum capacity of auxiliary cooling in varies running programs,what is the influence mechanism between optimum capacity of auxiliary cooling and different factors,and how to determine the optimum capacity of auxiliary cooling.This thesis,therefore,aims to address the above stated problems.Models have been established including computation model of air conditioning cooling load,heat transfer model of all system components,parameter-coupling model of the components,and computation model of system energy consumption.It has explored the relationships between auxiliary heat dissipation and energy consumption,soil heat accumulation on TRNSYS.The simulation model of the pump system developed on TRNSYS is used to obtain data of energy consumption and soil temperature.The results of various operation plans has revealed that the optimum heat dissipation ratio exists in both series and parallel system forms,regardless of control strategy and operating periods.The results are verified by experiment.And optimization capacity of auxiliary cooling is much lower than the design capacity of the auxiliary cooling facility obtained by the traditional design method.Given the soil heat balance,the optimal auxiliary heat dissipation capacity should be used,which cannot only save the operation cost,but also reduce the installed capacity of auxiliary cooling equipment and reduce the upfront investment.The results answered the problem if there is the the optimum capacity of auxiliary cooling in varies running programs.Simulations under various operation conditions for both series and parallel connected auxiliary cooling systems are carried out on TRNSYS in order to identify the optimum auxiliary heat dissipating capacity,this capacity's relation to system form,control strategies,operating periods,operating load rate,and meteorological parameters is also studied.According to the simulation studies,load changes caused by different operations influence the energy consumption without affecting the distribution of the optimum heat dissipation ratio.In this case,we need to optimize unit operation,and then get the optimum input capacity of auxiliary cooling facilities according to the system type,control strategy and operating periods.Simulation in three climatic regions of Beijing,Shanghai and Guangzhou reveal that,the optimum auxiliary cooling ratio gradually falls with the temperature rises.This trend is mainly due to the difference of wet bulb temperature in different climates.In the south,the cooling tower is less efficient due to higher ambient temperature,resulting in less heat dissipating capacity at the same input of auxiliary cooling facility capacity.This part of the results reveals the control mechanism for energy-saving of hybrid ground source heat pump system with auxiliary cooling,which provide a theoretical basis for the optimal design and operation control.Designed and constructed the experiment facility of hybrid source heat pump system with auxiliary cooling with the capacity of 42.5 kW.Experiments were done on the relationship between the auxiliary cooling ratio and the total energy consumption of system,soil heat accumulation under the condition of series system,synchronous control strategy,8 hours operating periods.The experiments provide a large amount of useful data,and verified the simulation conclusion with the same working conditions.Based on a large number of real-time operating data,the influencing factors of the variations of soil temperature,heat exchange of buried tubes and unit characteristic parameters are explored,and the dependability of each factor are studied.The correlation among these parameters is established.The conclusions from dependability research can be used to guide the optimization research of hybrid ground source heat pump system with auxiliary cooling.The real-time parameters correlation can be used as reference in similar projects as the project that designed in this thesis,Which provides a feasible method to determine the optimum capacity of auxiliary cooling in operation,and solves the problem of how to determin the optimum capacity of auxiliary cooling.