Configuration Optimization And Operation Of District Heating System Based On Medium-deep Geothermal Energy With Enhanced Ejector Heat Exchangers

Exploiting renewable geothermal heat resources for space heating contributes to increasing heating capacity of central heat source and reducing emission of atmospheric pollutants in winter.Due to higher temperature and greater density of heat flux,the medium-deep geothermal heat will be a kind of ideal heat source of district heating.At present,some large-scale geothermal fields are far from urban heating load zones.Due to shorter economic geothermal heat transmission distance,the conventional medium-deep geothermal heating technology cannot be used to develop some large-scale geothermal fields.Therefore,transmitting geothermal heat over a long distance is the key to district heating systems based on medium-deep geothermal heat with longer geothermal heat transmission distance.Lowering return temperature of the primary heating network(PHN)not only helps to increase temperature difference between supply and return of the PHN for longer economic heat transmission distance,but also contributes to utilizing medium-deep geothermal heat and waste heat of low-temperature flue gas.A new enhanced ejector heat exchanger consisted of a new enhanced ejector refrigeration machine and a water-water heat exchanger,can be used to greatly reduce return temperature of PHN,and it is featured by simple system and small size.According to the characteristics of the enhanced ejector heat exchanger,new principles for refrigerant screening are proposed,and it is used to screen current refrigerants for the enhanced ejector heat exchanger under normal-temperature heating mode and low-temperature one.In view of the characteristics of the medium-deep geothermal energy and the small size demand of heat exchanger with bigger temperature difference,a new district heating mode based on medium-deep geothermal energy with new enhanced ejector heat exchanger was proposed.According to the principle of energy cascade utilization,three configurations of district heating system based on medium-deep geothermal energy with new enhanced ejector heat exchanger were presented.As for three proposed district heating system,heating substations are set up the new enhanced ejector heat exchangers.For the No.1 proposed district heating system,the component of the heating station comprises a water-to-water heat exchanger,a gas-to-water heat exchanger and gas-fired boilers.For the No.2 one,the heating station consists of water-to-water heat exchanger,a gas-to-water heat exchanger,a gas-fired absorption heat pump and gas-fired boilers.For the No.3 one,the heating station is made up of a gas-to-water heat exchanger,gas-fired boilers and aⅡtype absorption heat exchanger which consists of a water-to-water heat exchanger and aⅡtype absorption heat pump.To disclose optimization regular of district heating system based on medium-deep geothermal energy,configurations of these proposed district heating systems are analyzed from the perspective of thermodynamics and economics.The results show that R141b is the best choice for the enhanced ejector heat exchanger under the normal-temperature heating mode,and R227ea is the most suitable for one under low-temperature heating mode.By using the enhanced ejector heat exchanger,return temperature of PHN can be reduced to 25℃,and thus temperature difference between supply and return of PHN is increased by about 40%.From the perspective of thermodynamics,the No.3 proposed district heating system is with the highest thermodynamic performance,and thus its configuration is the best.From the aspect of economics,the No.2 one has best financial benefit,and thus its configuration is preferred.As for the No.3 one under the low-temperature heating mode,the fossil fuel utilization efficiency,system product exergy efficiency,heating cost and economical heat transmission distance are about 1.97,23.19%,53.23￥/GJ,and 32 km,and it can reduce consumption of natural gas by6.8 Nm~3 per year for unit heating area.As for the No.2,the fossil fuel utilization efficiency,system product exergy efficiency,heating cost and economical heat transmission distance are about 1.88,22.97%,45.14￥/GJ,and 55 km,and it can lower consumption of natural gas by about 6.5 Nm~3 per year for unit heating area.