Theoretical And Experimental Study On Low-grade Energy Improvement And Conversion

China is rich in low-grade heat but it is not efficiently recoveried for lack of advanced technology.It can relieve the contradiction between development and energy supply to utilize the low-temperature heat efficiently and deeply.There are mainly three ways for low-temperature heat utilization: heat exchanging technology,heat pump technology and heat to power conversion technology.The last two ways can relize energy improvement and conversion and they are research focus in this field.This study focuses on power generation technology based on Organic rankine cycle(ORC)and compression heat pump.The ORC power generation technology is firstly investigated.Then heat pump technology and ORC technology are integrated into one bifunctional system,which can use the low-temperature heat soure to relize two functions of power generation and heat pump.The principle and method of improvement and conversion technology of low-grade energy are firstly proposed.The technical characteristics of compression heat pump and ORC power generation are summarized.And the critical technical problems are analyzed including screening of the organic fluids,fluid-structure interaction,optimization of working parameters,which provides the guildlines for the development of the key equipments and construction of the experimental bench in ORC system and bifunctional system.The ORC system applied for heat recovering with temperature range of 60 to 100 ℃ is carried out.It adopts exchange type of “preheater + evaporator” and twin screw expander to improve the system efficiency.An experimental bench of a 50-kW ORC power generation plant driven by low-grade heat source is established to assess the ORC system and get the performance map at off-design operating conditions in a typical year.The maximum electricity production and thermal efficiency were 46.5 kW and 6.52% and the highest exergetic efficiency reached 36.3% at the optimal operating condition.Furthermore,by fitting the actual plant data obtained in different months,an empirical model has been developed to predict the net power output and thermal efficiency with acceptable accuracy.The integrated bifunctional system of compression heat pump and power generation system based on ORC has been proposed to develop the specific application of single function ORC system.The bifunctional system can use one set of device to relize heat pump and power generation based on ORC.The thermodynamic model is established and the cycle parameters are calculated with four different working fluid(R245fa、R152a、R142b and R134a)under the working condition in which evaporation temperature is variable to screen efficient working fluid and obtain the performance of the system to provide basis for the design of the equipment.The experimental bench of the bifunctional system is established in which a inverter scroll machine is used as the compressor and expander in different running modes.The results prove that the modification is successful.In expansion process,the maxmun isentropic efficiency is 82.56 %,eletromechenical efficiency ranges from 34.5 % to 51.5 %.In compression process,the volumetric efficiency ranges from 0.83 to 0.96 and eletromechenical efficiency ranges from 47 %-57.4 % and the the maximum isentropic efficiency is 89.3 %.As for the global performance,the maximum power output and thermal efficiency is 2.95 kW and 3.75 % respectively.In heat pump mode,the maximum heating temperature is 90 ℃ and the COP is greater than 2.25 when heating temperature is less than 84 ℃.The experimental investigation on a type of bifunctional system is carried out in which the twin screw mahine coupled with permanent-magnet moter is used as the compressor and expander.Performance test of the permanent-magnet moter is first conducted and results show that it can realize electric production and driving functions.The power generation efficiency ranges from 33.8 %-84.8 % and the motor efficiency ranges from 75.0 %-90.0 %.According the experimental date,this type of bifunctional system can achieve the goal of heat pump and power generation.Comparing with the scroll type bifunctional system,the stability and Robustness of the twin screw are better,which is beneficial for the heat utilization application characterized by high power consumption or production and variable heat source temperature.