Study On Integration Of Natural Gas-fired Combined Heat And Power And Heat Pump Based On Active Heat Balance

Buildings have constitute three main social energy consumption bodies in China together with industries and traffic,so energy conservation for buildings is an irresistible trend of historical development.Optimization of building body design,adoption of efficient energy utilization devices in buildings and establishment of efficient energy supply systems for buildings are three important ways for realizing energy conservation for buildings,wherein the distributed energy system has become one kind of novel efficient energy supply system for buildings due to its multiple advantages,and the distributed energy system is extensively applied since the combined cooling,heating and power(CCHP)system is taken as a main utilization mode.Buildings in a hot-summer and cold-winter zone have a characteristic of meanwhile demand on cold,heat and electricity energy,so that a CCHP distributed energy system is one way to provide building energy and realize energy conservation for buildings in such zone.On the other hand,due to its high energy efficiency and less environmental influence,the ground-source heat pump utilizing shallow geothermal energy is extensively applied.Furthermore,coupled application of the CCHP distributed energy system and shallow geothermal energy can combine advantages of the two,which is more beneficial to energy conservation of buildings.Main characteristics of the existing NG-based distributed energy system and shallow geothermal energy coupled application system are as follows:an NG-based distributed energy system and the ground-source heat pump are in simple parallel operation or series operation,and are not applied through an integration system,so that problems of high exergy loss,incapability of recycling after-heat from flue gases containing sulphur at low temperature,underground heat imbalance caused by the ground-source heat pump,and so on during the utilization process of middle-temperature flue gases can not be well solved.Therefore,based on the thermal energy stepped utilization theory,the study puts forwards an active thermal balance control technology so as to construct an NG-based distributed energy system and ground-source heat pump integrated coupled system,key underground heat exchange devices are researched,and the rules in influences of key parameters of the system on its performances are researched.The main research contents of this study are as follows:(1)An NG-based combined heating and power(CHP)and heat pump coupled system based on active thermal balance is put forward.Based on parallel temperature and integrated energy stepped utilization theory,utilization methods of middle-temperature flue gases and flue gases containing sulphur at low temperature are researched and changed as follows:the middle-temperature flue gases are recycled for directly producing domestic hot water,and the flue gases containing sulphur at low temperature are utilized for heating inflow water at the source water side of the ground-source heat pump.Furthermore,one reference system is adopted,and the essential differences between the two systems are stated.(2)One kind of PE heat exchanger underground is researched and developed.In such heat exchanger,flue gases containing sulphurat low temperaturefrom the CHP system can be utilized to heat inflow water at the source water side of the ground-source heat pump,so that the system efficiency is improved.Furthermore,since soil is not used as a main heat source any more,but serves as a heat transfer chamber for water to absorb heat from flue gases,so that the problem of underground thermal imbalance in the ground-source heat pump is solved.The underground heat exchanger and an experimental system thereof are designed,an underground heat exchanger model is established,the heat-resistant property,heat transfer performance and sulphur content of the PE heat exchanger underground are tested in experiment,and by virtue of comparing the model simulation result with the experimental result,it is shown that the performance of the heat exchanger is good.(3)Study shows rules in influences of key parameters of the NG-based CHP and heat pump coupled system based on active thermal balanceon its performances.Performance calculation and heat economy analysis on a total energy system are finished by taking the first law of thermodynamics,the second law of thermodynamics and heat economy evaluation indexes as system performance evaluation criteria and adopting Aspenplus as a system performance calculation tool.Study shows rules in influences of key parameters such as heat pump source water inlet temperature and final exhaust flue gas temperature of the system on system performances,thereby laying a theoretical foundation for the optimization design of the system.Research results show that:(1)An integration method for an NG-based CHP and heat pump coupled system based on active thermal balance is put forward,which embodies the advancement of an active thermal balance control technology in thermal energy stepped utilization degree and makes up the defect of thermal perfection degree during an energy production process of a traditional coupled system.(2)The novel PE heat exchanger underground can meet the temperature requirement of flue gasescontaining sulphurat low temperature;a PE pipe has no obvious surface deformation at a temperature of 202 DEG C without pressure bearing;the PE pipe has no obvious rigidity change at a temperature not higher than 100 DEG C;furthermore,the PE heat exchanger underground can be used for recovering after-heat from the flue gases containing sulphur at a temperature not higher than 100 DEG C.The heat exchanger has the comprehensive heat transfer coefficient of 22.6 and thus can meet the requirement of heat transfer between water and flue gas which have great heat capacity difference.(3)By virtue of a method of utilizing middle-temperature flue gases and flue gases containing sulphur at a low temperature,the exergy loss of after-heat recovery of middle-temperature flue gases can be reduced by 3%,and the energy efficiency of the heat pump system can be improved by 3-5%.Compared with a reference system,its overall thermal efficiency and exergy loss are respectively 82.7%and 28.8%,which are far higher than those(respectively 73.8%and 25.8%)of the reference system.(4)The rules in influences of key parameters of the underground heat exchanger on system performances are as follows:reducing the final exhaust temperature of flue gases and the transverse spacing of heat exchange pipes,increasing the source water inflow temperature,the flow velocity,the pipe diameter and the screw pitch of the heat exchange pipe and improving the system performances.