Sequencing Control Optimization For Ground Source Heat Pump And Chiller Hybrid System

With the growth of energy demand in China,greater requirements are being placed on energy-saving performance of building energy systems.Air conditioning systems,which account for a third of building energy consumption,play a significant role on energy conservation.Unit sequencing control technology capable of achieving real-time matching of energy demand and supply has good potential of energy conservation,so it has drawn strong interests of scholars and engineers.Studies on unit sequencing control have simply focused on chillers in recent years.However,the operation efficiency of the chiller is always affected by the outdoor wet bulb temperatures and bears the energy efficiency degeneration at some area with higher web bulb temperatures.Compared with chillers,ground source heat pumps(GSHPs)with advantages of high efficiency,stable operation,and additional heating function,are more promising to be equipped with sequencing control.However,the GSHPs have some disadvantages such as possible underground thermal imbalance,the high initial investment for ground heat exchangers(GHEs)installation and the insufficient land area for GHE installation.In face with this situation,GSHP hybrid system appears as the promising air conditioning solution especially in large commercial buildings with profiles of dominating cooling load.Taking a large commercial building as the research object,this paper proposes a stable and energy-efficient unit sequencing control strategy for GSHPs hybrid system.Firstly,the simulation platform DeST is used to simulate the real-time load of the target building.Results show the peak cooling load and heating load are 4305 kW and 3195 kW,respectively.Based on characteristics of the load profile,two heating and cooling source schemes are proposed,which are single GSHPs and hybrid system with chillers.Their corresponding system models are built in TRNSYS platform.Secondly,four different unit sequencing control strategies are proposed for the two systems,which are:strategy 1: based on the single GSHP system,the hybrid control method of total load and return water temperature control is used to on/off control of the four heat pump units;strategy 2: based on the GSHP hybrid system,t the hybrid control method of total load and return water temperature control is used to on/off control of two heat pump units and two chillers;strategy 3: based on the optimal sequencing control of strategy 2,the restriction control of return temperature from ground source side is added,which means the sequencing control strategy takes into account the influence of underground temperature on the operation efficiency of GSHP;strategy 4: based on control strategy 3,the cooling time control for chillers is added.The group control strategy takes into account the effect of outdoor wet bulb temperature on the efficiency of chillers during cooling season.These four strategies are correspondingly applied to the two systems proposed,and a full year simulation is carried out in TRNSYS platform,respectively.The results show that the operation with all strategies can meet indoor comfort,although the deployments of sequencing control strategies 1 and 2 cause the underground imbalance rate to be 53.9% and 29.04% respectively.Then sequencing control strategies are optimized by calling the TRNOPT module in the TRNSYS platform.Finally,10 years of operation driven by each optimized strategy is simulated in the TRNSYS platform,respectively.With a comprehensive comparison of underground heat balance,unit energy efficiency,total cost of ten years operation and control complexity,strategy 4 shows optimal energic and economic performance.The underground average temperature rises only 2.4?,and the COP of heat pump unit keeps high efficient operation for systems controlled by strategy 4.Besides,the system power consumption and total cost under strategy 4 for ten years' operation are lower than systems under all other strategies.The research in this paper has a good reference value for the realization and optimization of unit sequencing control of the ground source heat pump hybrid system.