Study On Optimal Control Strategy Of The VRF-ice Storage Combined Air Conditioning System

The multi-split air-conditioning system,also called VRF(Variable Refrigerant Flow System)has firstly launched by DAKIN in the 1980 s in Japan,and because of its excellent performance it has been widely used throughout the world.However,since this system directly conveys cooling capacity to room by refrigerant,it has an inherent defect which is its lacking of fresh air supply.Therefore,the lacking of fresh air supply for this system is the important issue that researchers have been trying to solve.In this paper,we firstly put forward the VRF-ice storage combined air conditioning system.This system could provide fresh and cold air in the daytime and store cooling at night.The cold storage tank in composite system used to provide cold capacity for indoor air processing.The VRF unit is used to provide cold capacity for cold storage tank at night,and take the rest indoor cooling load in the working hours of daytime.Based on the TRNSYS software,we have set up the needed models,connected every parts of the VRF system and also built simulation models for VRF subsystem.Since the ice storage tank is carried out by the same compressor cold storage,ice storage tank can also be seen as a VRF indoor machine.Thus,we should set up a VRF subsystem model and through using test bench to test the subsystem specially on dynamic characteristics and steady state.After verification,it shows that the subsystem simulation platform has shown a better simulation performance.Based on the built VRF subsystems,other controlling components and building models,we expand connections and construct compound dynamic simulator air conditioning systems.And we have also studied operational control of the simulator.Firstly,we focus on the study of indoor temperature control characters,from which,we found that in cooling and heating mode,regardless of setting values of the air conditioning area is same or not,the indoor temperature could stably control over the set value.Furthermore,while some parts of the indoor machine closed,the temperature controls of other parts are not affected and this indicates that system has relatively flexibility on independent control in each region.Secondly,the water flow of cold storage tank,air flow rate and fresh air supply temperature are all effectively controlled,besides indoor air quality could also been guaranteed.Thus,it's proved that the fresh air problem in VRF system could be effectively solved through composite system.In addition,about the system energy consumption characteristics,system running parameters change will significantly affect system operation energy consumption characteristics,among which,fresh air temperature is the key variable,the changeable fresh air temperature causes VRF units and ice storage tank system bear the load appearing reciprocal relationship and ultimately affect the system energy consumption.Hereby,it turned out that through optimizing real-time fresh air temperature to coordinate VRF units and the operation of ice storage system,so as to achieve the purpose of reducing system energy consumption.Last point is system energy efficiency features.We found that under different conditions,there is a strong correlation between COP units and parts of the load rate.When part of the coincidence rate is within the range of 0.4 ~ 0.7,the highest COP unit shows.Therefore,based on the character of high COP value under VRF partial load,we used load distribution to optimize fresh air temperature of the composite system,and finally proved the system efficiency had improved.Meanwhile,we had put forward a new cost function,which is considering the economy and energy consumption of the system operation performance.While the weight coefficient is optimizing through choosing different values,it means optimized process has different preferences on economy and efficiency,so there will be different optimization results.In the paper,we studied on the load forecasting,and in this study,we used time series forecasting method to calculate indoor total load in daytime and fresh air load.And then it proved that the prediction method has more accurate prediction effect.And upon the results of the load forecast,we have optimized ice-cold storage process at night to make the cold storage process run efficiently.Therefore,through using VRF ice-storage compound air conditioning system,this article has solved VRF system fresh air problem and optimized economy and efficiency of compound system and found a new path for VRF promotion as well.