Article 正式发布 Versions 2 Vol 28 (4) : 621-643 2019
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Performance Analysis of a Multistage Centrifugal Pump Used in an Organic Rankine Cycle (ORC) System under Various Condensation Conditions; 变冷凝工况下有机朗肯循环系统中多级离心泵运行性能分析
: 2019 - 07 - 05
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Abstract & Keywords
Abstract: In an organic Rankine cycle (ORC) system, the working fluid pump plays an important role in the system performance. This paper focused on the operating characteristics of a multistage centrifugal pump at various speeds and condensation conditions. The experimental investigation was carried out to assess the influence of the performance of the pump by the ORC system with special attention to actual net power output, thermal efficiency as well as back work ratio (BWR). The results showed that an increase in the pump speed led to an increase in the mass flow rate and expand in the operating range of the outlet pressure. The mass flow rate decreased nonlinearly with the increase of the outlet pressure from 0.22 to 2.41 MPa; the electric power consumption changed between 151.54 and 2409.34 W and the mechanical efficiency of the pump changed from 7.90% to 61.88% when the pump speed varied from 1160 to 2900 r/min. Furthermore, at lower pump specific speed the ORC system achieved higher thermal efficiency, which suggested that an ultra-low specific speed pump was a promising candidate for an ORC system. The results also suggested that the effects of condensation conditions on the pump performance decreased with the pump speed increasing and BWR was relatively sensitive to the condensation conditions, especially at low pump speed. 在有机朗肯循环系统(ORC)中,工质泵性能对系统性能以及其他各部件性能均起着尤为重要的影响作用。本研究主要关注,在变工质泵转速和变冷凝工况下多级离心泵运行特性的变化情况。本文重点实验研究了工质泵性能变化对ORC系统实际净输出功率、热效率和泵功指数等ORC系统性能参数的影响情况。研究结果表明:多级离心泵转速的增加导致工质质量流量的增加,并且泵出口压力的运行范围也随之扩大;随着多级离心泵出口压力从0.22 MPa变化到 2.41 MPa,工质质量流量呈现出非线性下降的趋势;当多级离心泵转速在1160 r/min到2900 r/min范围内变化时,相应的泵输入功率在151.54 W到2409.34 W范围内变化、泵机械效率从7.90% 变化到61.88%。此外,较低的泵比转速可以使ORC系统获得较高的热效率,这表明较低的比转速泵更适合于ORC系统。研究结果也表明随着多级离心泵转速的升高,冷凝工况的变化对泵性能影响程度降低;当多级离心泵在低转速下运行时,泵功指数受冷凝工况的变化影响程度较大。
Keywords: waste heat recovery, organic Rankine cycle, multistage centrifugal pump, operating characteristics, various condensation conditions, back work ratio (BWR)
1. Introduction
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 [1]. An excessive reliance on traditional energy sources (coal, oil, and natural gas) makes us face significant problems of energy shortage and environmental deterioration [2]. 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 [3].
 
Nomenclature
hEnthalpy/kJ·kg−1netnet
HHead/mPpump
nSpeed/r·min−1Sspecific
pPressure/MPaththermal
qflow rate/t·h−1; m3·h−1)Vvolume
WPower/W1inlet state of working fluid pump
specific work/kJ·kg−12sideal outlet state of working fluid pump
Greek letters2actual outlet state of working fluid pump
Efficiency/%3inlet state of expander
Subscript4sideal outlet state of expander
acactualAcronyms
expexpanderBWRback work ratio
mmassORCorganic 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 [14]. Currently, both theoretical analysis [15,16] and experimental tests [17-19] were conducted to improve the system performance. Shu et al. [20] 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. [21] 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. [22] 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. [23] 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. [24] 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 [25] 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 [26]. 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
 
AuthorsWorking
fluid
PumpResearch methods for working fluid pumpsCondensation temperatureHeat-source temperatureOutput power range
Li et al. [30]R123Hydraulic
Diaphragm Pump
Calculated by enthalpy difference300-314 K403KMaximum:
6 kW
Zheng et al. [31]R245faDiaphragm
Metering Pump
Not considered296 K363 K0.05-0.35 kW
Zhou et al. [32]R123Multistage
Centrifugal Pump
Calculated by enthalpy difference323-363 K363-493 KMaximum: 0.645 kW
Kang [25]R245faMultistage
Centrifugal Pump
Calculated by enthalpy difference310-313 KMaximum:
32.7 kW
Pei et al. [33]R123Multistage
Centrifugal Pump
Calculated by enthalpy difference302-303 K3.75 kW
Li et al. [34]R123Multistage
Centrifugal Pump
Calculated by enthalpy difference303 K373-343 K
Miao et al. [35]R123Gear pumpCalculated by enthalpy difference288-297 K413 K, 433 KMaximum:
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 [26] 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. [36] presented that pumping work in energy recovery units based on ORC can severely affect the net power output recovered. Quoilin et al. [37] 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.
2. Methodology
2.1. Experimental setup


 
Fig. 1 Schematic layout of the experimental setup
Table 2 Specifications of the multistage centrifugal pump used in the paper
 
ItemValue
ModelCR3-29
Rated speed2900 r/min
Rated volume flow rate3 m3/h
Rated head139.5 m
Rated power input2200 W
Table 3 Parameters of the sensors
 
SensorTypeMeasurement rangeAccuracy
Inlet
pressure
SMP131
(Diffusion Silicon)
-0.1 to 1.0 MPa±0.5%
Outlet
pressure
SMP131
(Diffusion Silicon)
0.0 to 3.0 MPa±0.5%
TemperatureLG100 (Thermal Resistance)73 to 873 K±0.5 K
Mass flow meterCoriolis Mass Flowmeter0–10 t/h±0.15%
Power meterFrequency conversion power meter0–5000 W±0.5%
2.2   Testing procedure
Table 4 The condensation conditions
 
CaseTemperaturePressure
1303 K0.18 MPa
2313 K0.25 MPa
3323 K0.34 MPa
2.3   Theoretical analysis


 
Fig. 2 The T-s diagram of the basic ORC system


 
Fig. 3 Theoretical analysis on the specific pumping work with the outlet pressure
3. Experimental Results and Discussion
3.1   Mass flow rate
3.2   Electric power consumption


 
Fig. 4 Variation of the mass flow rate for various condensation conditions


 
Fig. 5 Variation of the mass flow rate at various pump speeds


 
Fig. 6 Electric power consumption at various pump speeds
3.3   Specific speed


 
Fig. 7 Electric power consumption at various condensation condition


 
Fig. 8 Variation of the specific speed for various condensation conditions
3.4   Mechanical efficiency
3.4.1. Effects of the outlet pressure on the mechanical efficiency of the pump


 
Fig. 9 Variation of the specific speed for various pump speeds


 
Fig. 10 The effects of the outlet pressure on the mechanical efficiency of the pump
3.4.2. Effects of the specific speed on the mechanical efficiency of the pump


 
Fig. 11 The effects of the specific speed on the mechanical efficiency of the pump
Table 5 The optimum specific speed under various pump speed.
 
CasePump speed/
r·min−1
Specific
speed
Mechanical efficiency
of the pump/%
Mass flow rate/
t·h−1
Head/
m
Outlet pressure/
MPa
Electric power
Input/W
111608.4044.951.6819.990.45203.11
17408.1457.472.5046.670.81552.17
23208.7559.793.4677.311.201218.30
29008.4358.354.13123.051.812370.17
211608.7445.641.7119.500.50199.24
17408.1357.202.4246.410.85534.06
23208.1561.883.2182.051.311159.37
29008.7460.914.26121.481.812311.51
311608.0047.321.5921.190.60193.52
17408.4059.192.4545.550.90513.91
23207.4260.702.8386.701.431099.91
29009.1860.224.35116.991.812298.17
4. Thermodynamic Analysis for the ORC System
4.1   Effects of the evaporation temperature on BWR
4.2 Effects of the evaporation temperature on the actual net power output


 
Fig. 12 The effects of the evaporation temperature on the BWR


 
Fig. 13 The effects of the evaporation temperature on the actual net power
4.3 Effects of the specific speed on the thermal efficiency


 
Fig. 14 The effects of the specific speed on the thermal efficiency
5. Conclusions
In this paper, a multistage centrifugal pump was used as the working fluid pump for an ORC system and R245fa was selected as the working fluid. Experimental studies were conducted to evaluate the pump characteristics including mass flow rate, electric power consumption and mechanical efficiency. The effects of the condensation condition, outlet pressure and pump rotational speed were investigated. In addition, the experimental results were used to assess the performance of the pump on the ORC system which provided a basis for the optimization of pump operation. The main findings are summarized as follows:
(1) The operating range of the mass flow rate and the outlet pressure is broadened when the multistage centrifugal pump runs at a high speed, the variation of the mass flow rate is between 0.38 and 5.55 t/h with the outlet pressure in the range from 0.22 to 2.41 MPa, which suggests that the matched ORC system can have a wider range of high temperature heat sources. Moreover, the pump can reach a maximum mechanical efficiency of 61.88%.
(2) The condensation conditions have a weak impact on the performance of the multistage centrifugal pump. However, increasing the condensation temperature can improve the performance of the ORC system.
(3) A larger electric power consumption of working fluid pumps should not be ignored in the ORC system. The electric power consumption of the multistage centrifugal pump increases first and then decreases with the increase of outlet pressure, ranging between 151.54 and 2409.34 W.
(4) The optimum specific speed, corresponding to a high pump mechanical efficiency, about 8, is weakly affected by condensation conditions and pump rotational speeds. The process of matching a suitable working fluid pump with the ORC system should focus on a working fluid pump with an ultra-low specific speed.
The experimental results can be helpful for the control of working fluid pumps used in the ORC system and the optimization of the system performance. Further work will be conducted with a view to decreasing the electric power consumption of the pump and increasing the net power output of the ORC system by adjusting the pump structure, optimizing the ORC system layout.
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Article and author information
Downlaod:
YANG Yuxin1,2,3
ZHANG Hongguang1,2*
zhanghongguang@bjut.edu.cn
TIAN Guohong3*
g.tian@surrey.ac.uk
XU Yonghong4
WANG Chongyao1,2
GAO Jianbing3
This work was sponsored by the National Natural Science Foundation of China (Grant No. 51776005), the National Key R&D Program of China (Grant No. 2016YFE0124900).
Publication records
Published: July 5, 2019 (Versions2
References
Journal of Thermal Science