Configuration Optimization Of Deep Geothermal District Heating System Based On Compression Heat Exchangers

Recently,with the rapid development of urbanization,the heating load and energy consumption for space heating in Northern China are increasing year by year,which leads to the further deterioration of the atmospheric environment quality in winter.The worse atmospheric environment quality has threatened people’s health,to a large extent.Compared with district heating mode based on coal-fired boiler or coal-fired cogeneration,district heating mode based on gas-fired boiler or gas-fired cogeneration can improve the atmospheric environment to a certain extent,but its heating cost is higher,and it would result in insufficient supply of natural gas in winter.Therefore,the application and development of district heating based on gas-fired boiler or gas-fired cogeneration would be restricted in the future.In order to win the "key battle for prevention and control of air pollution emission",the State Council requires that local governments,especially for the "2+26 city" located in the air pollutant transmission channel,should follow the feasible measures to develop clean energy district heating according to available energy resources and actual conditions.For example,develop geothermal energy for space heating is a way to optimize energy structure of the district heating section.Hydrothermal geothermal is one kind of renewable energy,which is featured by high heat flux,high temperature and good thermal stability.Thus,it is an ideal concentral heat source.However,the spatial distribution of hydrothermal geothermal resources is uneven due to the complex geological structure movement.Some large-scale hydrothermal geothermal fields are far away from urban heating load area,which leads to long distance of transporting geothermal heat and difficulty in developing geothermal resource.In order to solve the above problems of geothermal resources development,the central heating mode of deep geothermal based on compression heat transfer is proposed.In this central heating mode,the absorption heat exchanger is set in the heating station,and the compression heat exchangers are set in the heat substations.The absorption heat exchanger is composed of absorption heat pump type Ⅱ and a water-to-water heat exchanger,which is used to reduce the irreversible loss in the process of heat transfer between geothermal water and the circulating water in the primary heating network,and improve the supply temperature of the primary heating network;The compression heat exchanger consists of an electric compression heat pump and a water-to-water heat exchanger,and it is used to utilize thermal energy in of circulating water step by step.In this way,the return temperature of primary heating network is lowered greatly,and temperature difference between supply and return of primary heating network increases.In order to clearly express the optimization rule for configuration of central heating system,three kinds of medium-deep geothermal district heating system systems are proposed,and its thermodynamic model is established.Configuration optimization of medium-deep geothermal district heating system based on compression heat exchangers is studied from the aspects of thermal performance,economic benefit and environmental protection benefit.The first central heating system is that the medium-deep geothermal district heating system based on direct gas fired absorption heat pump.The second one that the medium-deep geothermal district heating system based on compression heat exchangers.The third one is that the medium-deep geothermal central heating system based on compression heat exchangers and an absorption heat exchanger.The results show that reducing the return temperature of the primary heating network of the compression heat exchangers not only helps to increase the temperature difference between the supply and return of the primary heating network,but also contributes to developing geothermal resource efficiently.Compared with the water-to-water heat exchanger,absorption heat exchanger has smaller irreversible loss in the heat transfer process,and higher outlet temperature of the circulating water in the primary heating network.As the temperature of geothermal water supply becomes lower,the ratio of capacity of compression heat pump to that of the water-to-water heat exchanger increases in the heating substation,while the ratio of capacity of heating absorption heat pump and to that of water-to-water heat exchanger is almost the same in the heating station.The thermal performance and energy saving potential of the three heating systems are reduced with the decreasing of geothermal water temperature.When the geothermal water temperature is 75 ℃,and the distance of transporting geothermal heat is20 km,the annual fossil energy utilization rate and annual product exergy efficiency of the third proposed heating system are 0.33% and 0.68% higher than that of the second proposed one,and0.53% and 3.77% higher than that of the first proposed one,respectively.Compared with the district heating system based on gas fired boiler,the natural gas consumption of the first,second and third proposed heating systems can be reduced by 90.25%,91.03% and 91.46% respectively.When the geothermal water temperature is 65 ℃,and distance of transporting heat is 20 km,the annual fossil energy utilization rate and annual product exergy efficiency of the third proposed heating system are 0.55% and 3.35% higher than that of the second proposed one,respectively.Annual product exergy efficiency of the third proposed heating system is 13.79%higher than that of the first proposed one.Compared with the central heating system based on gas fired boiler,the natural gas consumption of the first,second and third heating systems can be reduced by 80.09%%,81.40% and 84.15% respectively.When the geothermal water temperature is 55 ℃ and the distance of transporting heat is 20 km,the annual fossil energy utilization rate and annual product exergy efficiency of the third proposed heating system are0.33% and 3.54% higher than that of the second proposed one,and 0.30% and 19.24% higher than that of the first proposed one,respectively.Compared with the district heating system of gas fired boiler,the natural gas consumption of the first,second and third proposed heating systems can be reduced by 66.27%,73.57% and 77.03% respectively.It can be seen that in the three proposed heating systems,the third proposed heating system has the highest thermal performance,greatest potential of energy saving and emission reduction,and the best economic benefit.Therefore,the third proposed heating system has advanced energy utilization process,and its system configuration is optimal.When the supply temperature of geothermal water is 75 ℃,the economic heat transportation distance of the primary heating network is as long as 42 km for the third proposed heating system.When the supply temperature of geothermal water is 65 ℃,the economic heat transportation distance of the primary heating network is as long as 30 km of the third proposed heating system.When supply temperature of geothermal water is 55 ℃,it is difficult to develop geothermal energy for the third proposed heating system.In this case,the utilization rate of geothermal heat becomes lower,the capital investment of the central heating system becomes more,and a large amount of high-grade energy would be consumed for developing geothermal energy.The running cost also becomes higher,and the payback period becomes longer.The medium-deep geothermal central heating system based on compression heat exchangers has the highest thermal performance,the greatest potential of energy saving and emission reduction,and the best economic benefit in the proposed central heating systems.Thus,its application is promising in the "2+26 cities" area with rich geothermal resources.The studied results could provide a new idea for the development of clean energy heating in Northern China,and also provide technology support for development of China’s "carbon peak,carbon neutral".