Research On Fundamental Problems Of Collaborative Design Of Reversible Heat Pump-Heat Engine

Electrical energy storage（EES）is the key technology to overcome the defects of renewable energy systems and balance power fluctuations.There are obvious shortcomings in the existing technologies for long-term electrical energy storage;therefore,the development of new long-term energy storage technologies with high cycle life and low capital cost is of great significance for the further development of EESs.Pumped thermal electricity storage（PTES）is an emerging EES developed in recent years.It has received extensive attention due to its advantages of large storage capacity,sufficient power,high efficiency,low capital cost and long storage cycle.Compressed heat energy storage（hereinafter referred to as reversible heat pump–heat engine,RHP-HE）is a derivative technology of PTES.In addition to the advantages of traditional PTES,it can also overcome the disadvantages of high temperature and high pressure of traditional PTES.At the same time,it also has the ability to recover low-grade waste heat,which is conducive to the transformation of the energy structure and the efficient use of energy.At present,most of the research on RHP-HE focuses on the analysis and optimization of thermodynamic parameters,the selection of working fluids and the prospects of technical applications,which are all based on thermodynamic methods.However,the actual operation of RHP-HE is a transient process involving multi-step and multi-type energy conversion,storage and release.There are still many fundamental theoretical problems to be solved urgently in the collaborative design between discharging process and charging process.Such as the main influencing factors and changing rules of the‘double phase change’heat transfer process between the working fluid and latent storage/release medium（LS/RM）in the latent storage/release unit（LS/RU）,as well as the collaborative design of working fluid and LS/RM.To supplement and improve the theory of collaborative design of RHP-HE,this thesis proposes a basic configuration RHP-HE that couples the thermodynamic cycle process with the‘double phase change’heat transfer process.Aiming at several fundamental theoretical problems of collaborative optimization design of RHP-HE,the main tasks and conclusions are summarized as follows:（1）Thermodynamic performance analysis and optimization of RHP-HE.The collaborative design method of RHP-HE at the thermodynamic level are proven.The performance that RHP-HE can achieve under ideal conditions is clarified,which provides a cycle parameter design criterion for subsequent research.The specific work is as follows.Based on the coupling relationship between the energy release and storage process of RHP-HE,the thermodynamic analysis method is innovated.The applicability of the working fluid is explored in combination with the thermophysical properties of the working fluid.The influence of the key parameters of the cycle on the thermodynamic performance of the system is studied in detail considering the working fluid.Then,the multi-objective optimization of the thermodynamic parameters is studied.Through multi-objective decision-making,the optimal performance of the system under ideal conditions and the exergy loss distribution of the system under optimal performance are obtained,and the optimization method of the working fluid for realizing collaborative design of the RHP-HE at thermodynamic level are clarified.（2）Research on the working mechanism of the heat storage/release process of RHP-HE.According to the changing rules of the‘double phase change’heat transfer process,focusing on the factors such as the mass flow of working fluid,the main size of the LS/RU,and the volume of the LS/RM,a collaborative optimization criterion is carried out.It provides theoretical support for the collaborative design of the RHP-HE considering the heat transfer process.Specifically,a heat transfer simulation model of LS/RU is established.The evolution of different types of LS/RM during heat transfer with different working fluids under the guidance of thermodynamic optimization results is explored.The effects of different types of LS/RM and different working fluid combinations on the heat storage performance and heat release performance are clarified.At the same time,the effects of the cascaded LS/RU configuration on heat storage performance and heat release performance is proven.（3）Research on collaborative design of RHP-HE system.The collaborative design criterion of RHP-HE considering the‘double phase change’heat transfer process is clarified.Specifically,the distribution of the LS/RM temperature and liquid phase rate at the end of the HP is taken as the initial condition of the HE.The effects of different types of LS/RM,different working fluids under the guidance of thermodynamic optimization results and different configurations of LS/RU on the performance of the RHP-HE system are summarized and explored.The collaborative design of the flow of HP,the flow of HE and the length of LS/RU is carried out.The Pareto optimal solution set and its fitting surface determined by the three parameters are obtained by multi-objective optimization.The Pareto optimality of each performance index of the RHP-HE system under transient conditions is clarified,and the collaborative optimization design criteria of the RHP-HE system is proven.（4）Design,development and experimental research of HP.Considering that the numerical research cannot simulate the operation of the system after the addition of lubricating oil,and the existing technology level cannot efficiently integrate the expander and compressor into one,as the first step to realize the energy storage/release function of the RHP-HE,test rig of HP is firstly built.The influence of key factors on the process and performance of heat storage are explored,then the results of numerical research are supplemented and improved.Specifically,the test rig of HP is built;the paraffin wax-expanded graphite composite LS/RM is self-made and its thermal physical properties are tested.Then,the experimental study on the influence of waste heat source temperature and flow rate of cooling water on the actual operation performance of HP are carried out.The results show that increasing the waste heat source temperature will reduce COP_HP and ,while increase/and COP_TES.The COP_HP,,/and COP_TES decrease with the decrease of the flow rate of cooling water.（5）Case study of a distributed energy system based on RHP-HE.Aiming at a distributed energy application scenario,a hybrid power generation system integrated with RHP-HE and diesel generator is proposed.The performance of the hybrid system is compared with that of pure diesel generator under a certain power demand.The results show that under three different control strategies,compared with pure diesel generator,the hybrid system saves 10.25%,9.83%and 11.00%of fuel in summer,and saves 10.95%,10.96%and 5.93%of fuel in winter,respectively.