| As energy and environmental issues are receiving increasing attention,the Organic Rankine Cycle(ORC)has proven to be one of the viable technical routes for utilization of low-and medium-temperature energy.Because low-grade energy has the characteristics of sparse distribution,large local reserves,and difficulty in grid connection,small ORC systems have inherent advantages for the utilization of distributed energy.Based on the previous experiences and research results,this paper designed and analyzed a 10 kW ORC system.A performance evaluation model under steady-state boundary conditions was established.Given the conditions of heat sink and heat source,thermal efficiency,exergy efficiency and economic function were used as evaluation criteria.The preferred process of working fluid was conducted among seven candidate organic fluids.The comprehensive evaluation function was used to optimize the evaporation temperature and condensation temperature of the system,so as to design other parameters of the system.Because the static research could not meet the performance prediction requirements of the deviation from the design condition during the actual operation of the ORC system,a dynamic model was established based on the ?-NTU algorithm combined with the moving boundary method to consider the heat transfer in the twophase region.More advanced models had been developed for expander and working fluid pump.The dynamic model had a fast calculation speed,good stability and accuracy,which could meet the actual engineering requirements.It had a good prediction of the performance under the off-designed operating condition.For the ORC dynamic model,new types of evaporator,condenser,working fluid pump and expander had been developed under the TRNSYS simulation software.It had good dynamic analog response characteristics,which could be quickly connected with other existing types to construct digital experimental platform for engineering applications.Since small ORC could be easily applied to all aspects of industrial processes.In the TRNSYS,the ORC recycled industrial waste heat for district heating system,and analyzed two application methods of retrofitting the original heating system by using the designed small ORC unit:(1)The ORC was installed in the front end of the primary heating network.It was divided into two modes of operation: a)The 125 °C industrial waste heat was recycled by the ORC,then the ORC waste heat supplied to the users of the floor radiant heating after long-distance transmission.b)The industrial waste heat was directly supplied for heating in the heating season,and the ORC unit was used to recycle the industrial waste heat in the non-heating season.(2)The ORC was installed in the front end of the secondary heating network,and used 95 °C heat exchange station hot water as heat source to supply heat after shortdistance transmission.The results showed that various systems could maintain the indoor temperature of the user.In the first mode of long-distance transmission,the annual net output of the system was 29825 kW·h.It was found that due to the low temperature difference between the waste heat of the ORC and the outside,quantities of heat loss could be avoided in the long-distance transmission every year.The payback period calculated by the Module Costing Technique method was 5.3 years.In the second mode,there was no need to install pumps on the heating pipes,the annual net output was 26213 kW·h.There was not to avoid heat loss but could be used in radiator heating system.The payback period was 9.1 years.The total output of shortdistance transmission was 2277 kW·h during the heating season.It had no promotion significance under the existing technical conditions. |
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