Along with technological progress and rapid economic development, the demand for energy has rapidly increased. The total energy consumption has experienced a rapid growth, jumping by 1.92 times between the year 2000 and 2015, and reached 4.299 billion tons of coal equivalents in 2015 . An excessive reliance on traditional energy sources (coal, oil, and natural gas) makes us face significant problems of energy shortage and environmental deterioration . However, the total amount of waste heat and renewable energy is substantial, and it is essential and meaningful to integrate waste heat and renewable energy into future energy systems in order to save resources, improve energy efficiency, reduce emissions, and protect the environment. Currently, organic Rankine cycle (ORC) systems which convert low grade waste heat and renewable energy to useful power, have aroused a widespread concern of scholars due to their simple structure, high efficiency and environmental friendliness .
|q||flow rate/t·h−1; m3·h−1)||V||volume|
|W||Power/W||1||inlet state of working fluid pump|
|specific work/kJ·kg−1||2s||ideal outlet state of working fluid pump|
|Greek letters||2||actual outlet state of working fluid pump|
|Efficiency/%||3||inlet state of expander|
|Subscript||4s||ideal outlet state of expander|
|exp||expander||BWR||back work ratio|
|m||mass||ORC||organic Rankine cycle|
ORC systems are designed for numerous energy sources, including geothermal [4-6], biomass [7,8], solar sources [9,10], waste heat [11-13], and many other areas . Currently, both theoretical analysis [15,16] and experimental tests [17-19] were conducted to improve the system performance. Shu et al.  designed a dual-loop ORC system and compared six working fluids to search for a proper working fluid with high thermal performance. They proposed that R1234yf was a better working fluid for high operating load in theory. Wang et al.  evaluated the performance of five different ORC configurations based on the first and second laws of thermodynamics to obtain the maximum thermal efficiency for each ORC configuration by theoretical analysis. They indicated that the ORC with an internal heat exchanger had the best thermodynamic performance. Yang et al.  theoretically established thermodynamic, economic and optimization models to investigate a dual loop ORC system for waste heat recovery by comparing the superheat degree and exhaust outlet temperature. They found that a higher evaporation pressure and a lower condensation pressure exhibited a positive effect on the performances of the ORC system. Zhang et al.  experimentally tested the effects of expander torque and diesel engine loads on the performance of an ORC system for waste heat recovery from a diesel engine exhaust gas. They concluded that single-screw expanders were suitable for small/medium scale ORC systems, which can obtain a good performance at low-medium rotational speed. Pu et al.  conducted a small scale ORC experiment system using a single stage axial turbine expander coupled with a permanent magnet synchronous generator. They indicated that the electric power output of R245fa was greater than that of HFE7100 at the same pressure drop. Kang  also designed and developed an ORC that generated electric power with a radial turbine directly connected to the high-speed synchronous generator. A review of literature in the past decades revealed that many current studies focus on configuration improvement, thermo-economic analysis, expander selection, and etc.
A working fluid pump forces the working fluid to circulate and raise its pressure from the value at the condenser to that required in the evaporator . In an ORC system, the working fluid pump determines the mass flow rate and the evaporation pressure. In addition, the two parameters have significant effects on the overall performance of the ORC system [27-29]. However, the study on working fluid pumps in the ORC system can be categorized as follows: a) Ignoring the electric power input of working fluid pumps; b) Theoretical calculating
Table 1 Study methods for working fluid pumps in the ORC system
|Pump||Research methods for working fluid pumps||Condensation temperature||Heat-source temperature||Output power range|
|Li et al. ||R123||Hydraulic|
|Calculated by enthalpy difference||300-314 K||403K||Maximum:|
|Zheng et al. ||R245fa||Diaphragm|
|Not considered||296 K||363 K||0.05-0.35 kW|
|Zhou et al. ||R123||Multistage|
|Calculated by enthalpy difference||323-363 K||363-493 K||Maximum: 0.645 kW|
|Calculated by enthalpy difference||310-313 K||Maximum:|
|Pei et al. ||R123||Multistage|
|Calculated by enthalpy difference||302-303 K||3.75 kW|
|Li et al. ||R123||Multistage|
|Calculated by enthalpy difference||303 K||373-343 K|
|Miao et al. ||R123||Gear pump||Calculated by enthalpy difference||288-297 K||413 K, 433 K||Maximum:|
2.35 kW, 3.25 kW
of the electric power input of working fluid pumps. Table 1 gives detailed research methods for working fluid pumps in ORC systems [30-35]. As can be seen from the table, little research on working fluid pumps took the operating conditions into account. Borsukiewicz-Gozdur  stated that the pumping work should usually be taken into account in the calculations of the ORC power plant output and efficiency. Bianchi et al.  presented that pumping work in energy recovery units based on ORC can severely affect the net power output recovered. Quoilin et al.  presented that the power consumption of the pump should be considered in the calculations of the thermal efficiency and net power output of the ORC system. Furthermore, multistage centrifugal pumps were widely used in the ORC systems for their superior performance and a wider range of heat-source temperature.
A review of the previous literature revealed that very few studies specifically focused on the performance of working fluid pumps and their effects on the overall performance of the ORC systems. Moreover, the effects of the condensation conditions on the performance of working fluid pumps were rarely studied. Further investigation is needed to improve the system efficiency and reduce the electric power consumption of working fluid pumps. In this paper, a multistage centrifugal pump [38,39] was selected as the working fluid pump since its wide operating range, high efficiency, low cavitation, high reliability, compact structure and convenient maintenance. R245fa [40,41] was selected as the working fluid in the ORC system. To illuminate the effect of the operating performance of the pump, key parameters at various speeds under different condensation conditions were analyzed. Furthermore, a theoretical analysis based on the experimental data was performed to assess the effects of pump operating conditions on the performance of the ORC system, and identify optimum operating conditions.