- Email: [email protected]
Development and experimental study on a single screw expander integrated into an Organic Rankine Cycle
Energy 116 (2016) 43e52
Contents lists available at ScienceDirect
Energy journal homepage: www.elsevier.com/locate/energy
Development and experimental study on a single screw expander integrated into an Organic Rankine Cycle Biao Lei, Wei Wang*, Yu-Ting Wu, Chong-Fang Ma, Jing-Fu Wang, Lei Zhang, Chuang Li, Ying-Kun Zhao, Rui-Ping Zhi Key Laboratory of Enhanced Heat Transfer and Energy Conservation, Ministry of Education and Key Laboratory of Heat Transfer and Energy Conversion, Beijing Municipality, College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 February 2016 Received in revised form 13 July 2016 Accepted 14 September 2016
Expander is a key components in Organic Rankine Cycle (ORC) system. In order to improve the performances of single screw expander in large expansion ratio conditions and give consideration to normal conditions, a new idea called ‘increasing the built-in volume ratio appropriately and converting single screw expander into double-stage machine in large expansion conditions by utilizing the discharge velocity of screw grooves was proposed. By implementing this idea, a new prototype of single screw expander was designed and manufactured. Then the prototype was integrated into an experimental ORC system with R123. Experiments were carried out to analyze the characteristics of the developed single screw expander and the ORC system. The results show that the maximum expander shaft power, shaft efficiency, isentropic efficiency, volume efficiency and expansion ratio were 8.35 kW, 56%, 73%, 83% and 8.5, respectively. In addition, under expansion losses seem to be eliminated by the new structure of the expander. It was also found that 12e17% of the expander power was consumed by the circulation pump, of which the measured efficiency was between 20% and 31%. A maximum ORC efficiency of 7.98% was achieved. © 2016 Published by Elsevier Ltd.
Keywords: Organic Rankine Cycle Single screw expander R123 Circulation pump
1. Introduction Steam Rankine cycle is widely used in the field of heat-power conversion. However, when the temperature of heat source is below 250 C, steam Rankine cycle is not favorable because the high specific volume of steam at low pressure requires large installation and mandatory air removal in condensing mode [1]. In order to address this issue, organic fluids with low boiling point, such as R123 and R245fa, were proposed to substitute water as the working fluids. The Rankine cycle using organic fluids instead of water turns into an Organic Rankine Cycle (ORC). ORC has attracted a lot of attention in recent years [2e4], and it is one of the most promising technologies in the field of converting low grade heat into power [5e7]. The low grade heat can be solar energy [8], geothermal energy [9,10] and industrial waste heat [11,12], which are all widely available. Expander is a key component in ORC system. There are two
* Corresponding author. E-mail address: [email protected] (W. Wang). http://dx.doi.org/10.1016/j.energy.2016.09.089 0360-5442/© 2016 Published by Elsevier Ltd.
types of expander. One is the turbo type, and the other is the positive-displacement type. Large scale ORC systems normally adopt turbo expanders [6,13]. However, for small scale ORC unit, turbo expander might not be favorable. Previous investigations have revealed that the rotational speed of turbo expanders increases with the decrease in turbine output power [14]. The rotational speed of small scale turbo expanders become very high, and the experimental data are normally between 17000 and 65000 r$min1[6,15,16]. In this condition, positive-displacement expanders, such as rolling piston expander [17], scroll expander [18,19] and single screw expander [20,21], are good substitutes for turbo machines due to their relatively high efficiency, high pressure ratio, low rotational speed and tolerance of two-phase fluids [6]. Therefore a lot of experimental small scale ORC units employ positive-displacement expanders. Scroll expander is a research hotspot. Table 1 summarizes recently experimental results which focus on scroll expanders. From the table, it is known that: (1) The expanders were reversely operated scroll compressors, and the expander power was normally smaller than 3.5 kW.
44
B. Lei et al. / Energy 116 (2016) 43e52
2 20 3 4 is sh t th P V m
Nomenclature ·
W h p Dp v N n q m
power [W, kW] enthalpy [J kg1, kJ kg1] pressure [Pa, kPa, MPa] pressure drop [Pa, kPa, MPa] specific volume [m3 kg1] torque [N m] expander rotational speed [r min1] flow rate [kg s1, kg h1, m3 h1, m3 s1] mass
outlet state of expander outlet state of oil separator inlet state of circulation pump outlet state of circulating pump isentropic shaft expander theoretical circulating pump volume mass
Abbreviations ORC Organic Rankine Cycle AC Alternating Current CP Cylinder and Plane
Greek symbol h efficiency [e] εp expansion ratio [e] Subscripts 1 inlet state of expander
Table 1 List of scroll expanders in literature. Authors
Built-in volume ratios
Working fluids
Output power (kW)
Rotational speed (r min1)
Expansion ratio
Efficiency Expander specifications
Lemort et al. [23] Declaye et al. [22] Lemort et al. [24] Chan et al. [25] Chang et al. [25] Chan et al. [19]
4.05
R123
0.4e1.8
1770e2660
2.7e5.4
42e68%
3.95
R245fa
2.1
2000e3500
3e7.4
55e75.7% Reversely operated oil-free scroll air compressor
3.0
R245fa
0.45e2.2
2000e2850
2e6
71.03%a
Reversely operated Refrigeration scroll expander
2.12
R245fa
1.375
Unknown
Unknown
2.95
R245fa
1.77
Unknown
Unknown
60.8 e68.4%a 72e75%a
Reversely operated automotive air conditioning scroll compressor Reversely operated Refrigeration scroll expander
4.05
R245fa
2.3
1535e2970
About 6
73.1%a
Reversely operated oil-free scroll air compressor
a
Reversely operated oil-free scroll air compressor
Note: The authors only provided isentropic efficiency.
(2) The built-in volume ratios of the expanders were between 2 and 4, and the expansion ratios were normally less than 6. Although Declaye et al. [22] increased the expansion ratio to 7.4, obvious under expansion ratio losses were observed. When the output power is larger than 3.5 kW, single screw expander seems more promising. Compared to scroll expander and rolling piston expander, it has many advantages, such as balanced load of the screw, long service life, high volume efficiency, good performances in partial load, low leakage, low noise, low vibration and simple configuration [20,21,26]. A few investigators performed theoretical analyses on single screw expanders. Ziviani et al. [27] reported a comparison between single screw and scroll expander under part-load conditions for low-grade heat recovery ORC systems. It was found that the single screw expander presents higher power output and leads to higher ORC overall efficiency despite a lower maximum value of the isentropic efficiency due to limited expansion ratio. In another paper of Ziviani et al. [28], they reported a geometry-based modeling of single screw expander in ORC system for low-grade heat recovery. Some other investigators conducted experimental studies on single screw expanders. Wang et al. [21] and He et al. [26] performed experimental investigations on single screw expander with compressed air, and it was proved that the single screw structure can work well as expander. However,
only very few investigators presented experimental results about single screw expander in ORC conditions. It is worthy to quote here the small scale experimental ORC system constructed by Zhang et al. [29] In their system, a developed single screw expander with rotor diameter of 155 mm was used, and R123 was adopted as the working fluid. The expander power achieved 10.38 kW. However, the maximum efficiency of the cycle was only 6.48%, mainly because the maximum expansion ratio was 4.6. Both scroll expanders and single screw expanders have fixed built-in volume ratios after they are manufactured, and over or under expansion losses would be thus produced. For an ORC system using R123 with evaporating temperature of 120 C and condensing temperature of 30 C, the expansion ratio would reach up to 10 if the pressure losses are not taken into account. The reversely operated compressors might result in obvious under expansion losses in such conditions [30]. Therefore, it is necessary to develop expanders specifically. Besides the expander, the circulation pump is another key component in ORC system. Several investigators have noticed its significance. For example, Quoilin et al. [30] summarized that the circulation pump should meet the cycle requirements in terms of controllability, efficiency, tightness and net positive suction head. They also reported that the experimental data of circulation pump efficiency in small scale ORC systems were between 7% and 25%
B. Lei et al. / Energy 116 (2016) 43e52
[30]. In another article of Quoilin et al., a small scale ORC system was constructed and they also reported that the efficiency of the system is mainly limited by the poor pump efficiency (about 15%) [31]. This demonstrates that the circulation pump is another important element limiting the efficiency of ORC system, and more attention should be paid to the circulation pump. In the present work, according to ORC operational conditions, a single screw expander was developed. Using the developed single screw expander, an air-cooled serpentine condenser, a multi-stage centrifugal pump and a tube-shell type evaporator, an experimental ORC system was constructed. R123 was selected as the working fluid. Based on this, experiments were conducted to test the performances of the developed single screw expander, the circulation pump and the ORC system under different operating conditions.
2. Description of single screw expander and ORC system In this section, a single screw expander was developed according to ORC operational conditions, and the differences between the machine and typical single screw compressors were presented in detail. Subsequently, the expander was integrated into an experimental ORC system.
2.1. Description of the single screw expander The developed single screw machine was of CP type which has a meshing pair consisting of a screw and two symmetrically collocated planar gate rotors [20,26]. Fig. 1 presents the structure of Cylinder and Plane (CP) type single screw machine. He et al. [26] detailed the working principles of single screw expanders. Compared to typical single screw compressors, the distinguishing features of the developed expander are listed as follows. (1) Exhaust port. In order to get easy installation, the intake port of single screw compressor is always located at the top of the housing. For the developed single screw expander, the exhaust port was located at the bottom of the housing. Therefore the lubricant oil can be easily discharged of the
Fig. 1. Structure of CP type single screw machine.
45
expander with the working fluid and circulate smoothly in the system. (2) Expansion ratio. In order to ensure that the expander work well in large expansion condition and give consideration to normal conditions, the built-in volume ratio of the developed single screw expander was designed to be 4.86. In addition, a new idea was proposed and implemented to reduce under expansion losses. For typical single screw compressors, the working fluid was fed into the screw grooves in radial direction of the screw, and for single screw expanders, the working fluid also left the screw grooves in radial direction of the screw (see Fig. 2). With regard to the developed single screw expander, as shown in Fig. 3, the refrigerant leaves the grooves in opposite direction of screw rotation. In under expansion condition, at the end of expansion process the pressure of working fluid in the grooves is still greater than the discharge pressure, and the leaving velocity of the refrigerant was quite large due to the small discharge port of the grooves. In this situation, the working fluid could rotate the screw, and the expander power would be thus increased. In this situation, the single screw expander actually transformed into a two-stage machine. The first stage is of positive-displacement type, while the second stage is of turbo type. Through two-stage expansion, the under expansion losses might be reduced by this measure. (3) Tightness. One of the key problems about tightness is the shaft seal. For single screw refrigeration compressors, in order to avoid this problem, the compressor components and motor drive are normally assembled with a common shaft and welded into a steel housing. The refrigerant will cool the motor before it is compressed. If a single screw refrigeration compressor is operated reversely and an electric generator is used to substitute the motor, it might be difficult to cool the generator because the temperature of working fluids in expander is high. In addition, the rotational speed and the output torque of expander might be difficult to adjust. As for single screw air compressor, it is normally of open-drive type. The shaft extends out of the compressor housing and connects to the motor. Because there is no requirement of full tightness for air compressor, the air will leak out of the compressor through the gap between the shaft and the housing. Therefore neither air compressor nor refrigeration compressor can be directly operated reversely as ORC expander. Like air single screw compressor, the developed single screw expander is of open-drive type. However, a mechanical seal was used to block the gap between the extending shaft and the expander housing. This construction type is somewhat similar to automotive air-conditioning compressors, of which the size is much smaller.
Fig. 2. Direction of working fluid flow out for typical single screw expanders.
46
B. Lei et al. / Energy 116 (2016) 43e52
Fig. 4 provides a photograph of the developed single screw expander, and its parameters are listed in Table 2. 2.2. Description of the ORC system Fig. 5 presents a schematic diagram of the experimental ORC system. The system consists of three circuits: conductive oil circuit, R123 circuit and lubricant oil circuit.
Fig. 3. Direction of working fluid flow out for the developed single screw expanders.
As for the tightness of the housing, the working fluid within the housing (except the grooves of the screw and the intake port) was designed to work under a low pressure condition, which is approximately equal to the discharge pressure of the expander. Therefore, it is easy for the machine housing to achieve full tightness. (4) Lubrication and cooling. For single screw compressor, the lubricant oil is also used to cool the gas in working chambers, and the consumed power can be decreased in this way. Therefore, it is necessary to cool and atomize the lubricant oil before it is injected into the compressor working chambers. However, cooling the working chamber degrades the performances of expander. Therefore there is no lubricant oil cooling or atomization in the developed single screw expander. In addition, the amount of lubricant injected into the expander was also reduced. (5) Strength. Considering the pressure under ORC condition, the strength of the expander was enhanced compared to air compressors.
(1) Conductive oil circuit. This circuit provides low grade heat for the ORC system. There are four main components in this circuit: conductive oil boiler, conductive oil pump, evaporator and mass flow meter. In the boiler, the conductive oil is first heated by the electrical heater. Then the upper layer of conductive oil, of which the temperature might be higher, is fed into the conductive oil pump, and the pump transports it to the evaporator. The evaporator is of shell and tube type, and the conductive oil goes into the shell side. After an exothermic process in the evaporator, the conductive oil goes back to the bottom of the boiler through the mass flow meter. This is the conductive oil circuit, and it is indicated by dashed lines in Fig. 5. In order to reduce heat losses, all the components and pipes are covered by thermal insulators. (2) R123 circuit. There are seven main components in this circuit: evaporator, expander, oil separator, condenser, R123 reservoir, circulation pump and mass flow meter. Liquid R123 in the reservoir at state 3 is pressurized to state 4 by the circulation pump, and then it is delivered into the tube side of evaporator through the flow meter. R123 absorbs the heat discharged by conductive oil in the evaporator and turns into high-pressure saturated or superheated vapor at state 1. Subsequently the vapor is fed into the singe screw expander to generate power. The vapor leaves the expander at state 2 and flows into the lubricant oil separator and the air-cooled serpentine condenser successively. In the condenser, the refrigerant is condensed to liquid at state 3, which is then collected by R123 reservoir and recirculated by the circulation pump. It is obviously that R123 flows in an Organic Rankine Cycle. This circuit is indicated by solid lines in Fig. 5. Fig. 6 provides a T-s chart of this cycle. The parameters of the circulation pump, evaporator and condenser are presented in Tables 3e5 respectively. For the sake of smooth operation, the installation height of the main components might be significant. In the circuit, the expander exhaust port was installed higher than the inlet of oil separator. Therefore the discharged lubricant of expander can flow smoothly into the separator with refrigerant. In order to avoid cavitation, the circulation pump was installed about 0.5 m lower than the R123 reservoir outlet. The condenser outlet was higher than R123 reservoir and a balance pipe was used to connect the condenser inlet and the reservoir. Therefore the condensate can flow smoothly into the reservoir by gravity. Fig. 7 shows schematic arrangement
Table 2 Parameters of the developed single screw expander.
Fig. 4. Photograph of the developed single screw expander.
Items
Values
Screw diameter Groove number of screw Tooth number of gate rotor Diameter of gate rotor Center distance Built-in volume ratio Intake capacity
117 mm 6 11 117 mm 93.6 mm 4.86 98.3 mL r1
B. Lei et al. / Energy 116 (2016) 43e52
47
Fig. 5. Schematic diagram of the experimental ORC system.
Table 5 Parameters of the condenser.
Fig. 6. T-s chart of the experimental ORC system.
Table 3 Parameters of the circulation pump. Items
Values
Model Rotational speed Flow rate stages Delivery head
CR 3e25 2899 r min1 3 m3 h1 25 122 m
Table 4 Parameters of the evaporator. Items
Values
Number of tube side Number of shell side Heat transfer area Maximum pressure of tube side Maximum pressure of shell side
4 2 20 m2 4 MPa 1 MPa
about installation heights of individual equipment in this circuit. Besides the main components, several accessories were used in this circuit. A filter and a check valve were installed between the circulation pump and the flow meter to catch impurities and prevent back flow, respectively. In order to regulate the R123 flow rate,
Items
Values
Heat transfer area Maximum pressure Air flow rate Total pressure of the fan
90 m2 1.2 MPa 50000 m3 h1 400 Pa
a frequency inverter was used to adjust the rotational speed of circulation pump. In this circuit, the mass flow meter located at the inlet of the evaporator indicated the values of R123 flow rate. An AC electrical dynamometer was used as the expander load, and it can produce electricity utilizing the expander power. In order to obtain the values of expander power, a torque sensor and a rotational speed sensor were installed between the expander and the dynamometer. The electrical energy consumed by the circulation pump was monitored by a power meter. In addition, several pressure and temperature sensors were used to determine the key thermodynamic state points. The details of the sensors are listed in Table 6. (3) Lubricant oil circuit. This circuit was used to supply lubricant for the single screw expander and make it well lubricated and sealed. In the circuit, the mixture of lubricant oil and R123 discharged by the expander flows into the lubricant oil separator where the lubricant oil is extracted. Subsequently the extracted lubricant is injected into the single screw expander by a lubricant oil pump. Then it is mixed with R123 again in the expander. This circuit is indicated by dash dot lines in Fig. 5. The parameters of oil pump are described in Table 7. Pressure drop is an important consideration for R123 circuit and lubricant oil circuit. The pressure drops in oil separator, condenser and the pipes between them increase the expander discharge pressure. From previous investigations, it is known that this exerts an extremely negative influence [27, 33]. Therefore the pressure drops in the separator and condenser were seriously considered in the stage of device design or selection.
48
B. Lei et al. / Energy 116 (2016) 43e52
Fig. 7. Schematic diagram of the equipment heights.
Table 6 Details of the sensors used.
·
Wt ¼
Sensors
Measurement range
Accuracy
p2, p20 , p3 p4, p1 Temperature sensors Rotational speed sensor Torque sensor Mass flow meter Power meter
0.1 to 0.5 MPa 0.1 to 2 MPa PT100, 0e400 C 0e8000 r min1 0e100 N m 0e3000 kg h1 0e5 kW
±0.5% ±0.5% ±0.5 C ±0.5% ±0.5% ±0.2% ±0.5%
Values
Model Rotational speed Flow rate Maximum pressure
Gear pump, CBB 2.5 1450 r min1 2.5 L min1 2.5 MPa
(2)
Here N and n are the output torque and rotational speed of the expander, respectively. They can be obtained by the sensors located at the shaft between the expander and the AC electrical dynamometer. (3) Volume efficiency Volume efficiency is an important parameter of positivedisplacement expanders. For positive-displacement compressors, volume efficiency is defined as the ratio of actual intake volume flow rate to theoretical one. Because leakage is unavoidable, the actual volume flow rate is normally smaller than the theoretical one. Therefore, the volume efficiency of compressor is smaller than 100%. However, for positive-displacement expander, the intake pressure is higher than the discharge pressure, and working fluid leaks though gaps from inlet chambers to discharge rooms. The actual flow rate through the expander is thus increased, and it may become larger than the theoretical flow rate. Therefore it is reasonable to define the volume efficiency of expander as
Table 7 Parameters of the lubricant oil pump. Items
N$n 9:49
Fig. 8 shows the photographs of the experimental facilities.
hV ¼
qV;th 100% qV
(3)
3. Thermodynamic analyses The equations for analyses of experimental results were presented in this section. The involved variables includes expansion ratio, expander shaft power, shaft efficiency, isentropic efficiency, volume efficiency of the expander, efficiency of the circulation pump and efficiency of the ORC system.
qV, th is theoretical volume flow rate through the expander, and qV is the measured one. From the above discussions, it is obvious that hV is probably smaller than 100% for positive-displacement expanders. A small value of volume efficiency denotes an expander with serious leakage. (4) Shaft efficiency
(1) Expansion ratio Expansion ratio is defined as the ratio of the expander intake pressure to the discharge pressure.
εp ¼
p1 p2
Shaft efficiency of expander is defined as the ratio of the obtained expander power to the product of isentropic specific enthalpy drop and mass flow rate. ·
(1)
hsh ¼
·
Wt
m h1 h2; is
Here p1 and p2 are the intake pressure and discharge pressure of the expander, respectively. (2) Expander shaft power Expander shaft power is obtained by
(5) Isentropic efficiency Isentropic efficiency of the expander is obtained by
(4)
B. Lei et al. / Energy 116 (2016) 43e52
49
Fig. 8. Photographs of the experimental facilities (a) oil separator (b) conductive oil boiler (c) R123 reservoir and circulation pump (d) condenser (e) evaporator (f) main body of the system.
his ¼
h1 h2 h1 h2;is
(5)
hP ¼
v3 ðp4 p3 Þ ·
·
(6) Efficiency of the circulation pump The circulation pump efficiency is obtained by
(6)
WP
Here WP is the consumed pumping work of the cycle, given by the power meter. p4 and p3 are the intake pressure and discharge pressure of the pump, respectively. v3 is the specific volume of the liquid working fluid fed to the pump.
50
B. Lei et al. / Energy 116 (2016) 43e52
(7) Efficiency of the ORC system The efficiency of ORC system is obtained by ·
hORC ¼
·
Wt WP ·
mðh1 h4 Þ
(7)
4. Results and discussions During the experiments, the superheat degree of R123 at the expander inlet was controlled at 3e8 C by regulating the refrigerant flow rate. By regulating the electric current through the AC electrical dynamometer, the expander was operated at three different rotational speeds: 2000 ± 30 r$min1, 2500 ± 30 r$min1 and 3000 ± 30 r$min1. The monitored maximum value of pressure drop in filter, check valve, flow meter and evaporator was 250 kPa, while it was lower than 25 kPa in oil separator or condenser. In steady operation conditions, the expander torque and rotational speed, consumed pumping work, R123 flow rate, temperature and pressure of key state points were recorded. The obtained experimental results were categorized into two cases: different expander intake pressure and different expansion ratios.
Fig. 10. Evaluation of the pumping work with different expander intake pressure.
4.1. Effects of expander intake pressure Fig. 9 provides the expander shaft power as a function of intake pressure under different rotational speeds. The figure illustrates that the expander power was improved by increasing intake pressure or rotational speed. The reason is obviously the increasing refrigerant mass flow rate through the expander. The maximum power produced by the expander was 8.35 kW, obtained at a rotational speed of 3000 ± 30 r min1 and an intake pressure of 1.15 MPa. The corresponding work consumed by the circulation pump was presented in Fig. 10. The figure reveals that there was an approximately linear increase of pumping work with increasing expander intake pressure. Combined with Fig. 9, it is known that the pump consumed 12%e17% of the expander power. The efficiency of the circulation pump was presented in Fig. 11. From the figure, it was known that the pump efficiency was between 20% and 31%, and the value was larger under high flow rate and pressure condition. Otherwise it would get lower. This might be caused by the characteristics of the chosen multi-stage centrifugal pump,
Fig. 11. Evaluation of the pumping efficiency with different expander intake pressure.
which has a total stage of 25. Under low flow rate and pressure condition, fewer stages could meet the requirement, and too many stages result in extra friction losses. Therefore the efficiency of the pump was relatively lower.
4.2. Effects of expansion ratio
Fig. 9. Evaluation of the expander shaft power with different intake pressure.
Fig. 12 presents the shaft efficiency as a function of expansion ratio for different rotational speeds. The figure illustrates that the increasing rotational speed would improve shaft efficiency, and the suitable rotational speed of the expander was about 3000 r min1 in the scope of the test. The reason of this might be the leakage problem was alleviated by increasing rotational speed. When the expander operated at about 3000 r min1, the maximum shaft efficiency of 56% was achieved at an expansion ratio of 6.3. Obvious over expansion losses were observed when the actual expansion ratio was smaller than the above value. However, if the expansion ratio was increased to the maximum value of 8.5, the shaft efficiency was only reduced by less than 1%. This indicates that the developed single screw expander performed well under large expansion ratio conditions, and the under expansion losses are almost negligible. This could be credited to the two-stage expansion of the developed single screw expander. Fig. 13 presents a comparison of shaft efficiency and isentropic efficiency when the expander speed was about 3000 r min1. From
B. Lei et al. / Energy 116 (2016) 43e52
Fig. 12. Influences of expansion ratio on shaft efficiency under different rotational speeds.
51
Fig. 14. Influences of expansion ratio on volume efficiency under different rotational speeds.
were about 64% when the expander operated at 2000 ± 30 r min1, and they increased to about 80% at the speed of 3000 ± 30 r min1. The maximum value of volume efficiency was about 83%. This reveals that leakage is still an unavoidable problem for single screw expander. Increasing rotational speed is an effective way to alleviate this problem. In addition, the figure also illustrates that the volume efficiency remained almost unchanged with increasing pressure ratio. Fig. 15 depicts the ORC efficiency as a function of expansion ratio under different rotational speed of the expander. The figure demonstrates that the maximum ORC efficiency was 7.98%, obtained at a pressure ratio of 8.5 and expander rotational speed of about 3000 r min1. With increasing expansion ratio, mainly because the good performance of the expander in large expansion ratio condition, the ORC efficiency presented upward trends. 5. Conclusions Fig. 13. Comparison of n ¼ 3000 ± 30 r min1.
the
shaft
efficiency
and
isentropic
efficiency
for
the Figure, it is obvious that the isentropic efficiency was 15%e20% higher than the shaft efficiency, and the maximum value of isentropic efficiency was about 73%. The differences between shaft efficiency and isentropic efficiency were discovered in previous investigation [32], and the causes of the differences might be the frictions and heat losses of expander. There are two routes of heat losses: the expander housing and the lubricant oil. Both of frictions and heat losses degrade expander power and shaft efficiency. However, heat losses decrease the temperature of working fluid at expander outlet, and the enthalpy drop of working fluid flowing through the expander will thus be increased. This makes the isotropic efficiency appear to be high even when the shaft efficiency is low. When the expander operates at low speed and low intake pressure, the working fluid mass flow rate is small and the influences of heat loss will become protrudent. However, the influences of heat loss will be weakened at high speed and high intake pressure condition, and the difference between shaft efficiency and isentropic efficiency were reduced. In conclusion, the isentropic efficiency is not significant to expanders as compared to the shaft efficiency when heat losses exert influences. The contributions of increasing expander speed were confirmed again by the results of volume efficiency (see Fig. 14). The values
In order to improve the performances of single screw expander in large expansion ratio conditions and give consideration to normal conditions, a new idea called ‘increasing the built-in volume ratio appropriately and converting single screw expander into double-stage machine in large expansion conditions by utilizing the discharge velocity of screw grooves was proposed. By
Fig. 15. Variation of ORC efficiency for various operational conditions.
52
B. Lei et al. / Energy 116 (2016) 43e52
implementing this idea, a new prototype of single screw expander was designed and manufactured. The prototype was then integrated into an experimental ORC system with R123. Experiments were conducted to test the performances of the prototype and the ORC system. Some conclusions were obtained through the experiments. (1) The maximum expander shaft power, shaft efficiency, isentropic efficiency, volume efficiency and expansion ratio were 8.35 kW, 56%, 73%, 83% and 8.5, respectively. It is also found that under expansion losses seem to be eliminated by the new structure of the expander. Increasing the expander rotational speed made positive contributions to its volume efficiency and shaft efficiency. (2) The circulation pump consumed 12e17% of the expander power. The maximum efficiency of the circulation pump was about 31%, achieved at high pressure and high flow rate condition. (3) The maximum efficiency of the ORC system was 7.98%. Acknowledgement This work is supported by the National Basic Research Program of China (973Program, Grant No. 2013CB228306). References [1] Pei G, Li J, Li Y, Wang D, Ji J. Construction and dynamic test of a small-scale organic rankine cycle. Energy 2011;36(5):3215e23. [2] Tchanche BF, Lambrinos G, Frangoudakis A, Papadakis G. Low-grade heat conversion into power using organic Rankine cycles e a review of various applications. Renew Sustain Energy Rev 2011;15(8):3963e79. [3] Guo C, Du X, Yang L, Yang Y. Organic Rankine cycle for power recovery of exhaust flue gas. Appl Therm Eng 2015;75:135e44. [4] Sprouse C, Depcik C. Review of organic Rankine cycles for internal combustion engine exhaust waste heat recovery. Appl Therm Eng 2013;51(1e2):711e22. [5] Chen H, Goswami DY, Stefanakos EK. A review of thermodynamic cycles and working fluids for the conversion of low-grade heat. Renew Sustain Energy Rev 2010;14(9):3059e67. [6] Bao J, Zhao L. A review of working fluid and expander selections for organic Rankine cycle. Renew Sustain Energy Rev 2013;24:325e42. [7] Zhang X, He M, Zhang Y. A review of research on the Kalina cycle. Renew Sustain Energy Rev 2012;16(7):5309e18. [8] Delgado-Torres AM, García-Rodríguez L. Analysis and optimization of the lowtemperature solar organic Rankine cycle (ORC). Energy Convers Manag 2010;51(12):2846e56. [9] Nusiaputra Y, Wiemer H-J, Kuhn D. Thermal-Economic modularization of small, organic rankine cycle power plants for mid-enthalpy geothermal fields. Energies 2014;7(7):4221e40. [10] Madhawa Hettiarachchi HD, Golubovic M, Worek WM, Ikegami Y. Optimum design criteria for an Organic Rankine cycle using low-temperature geothermal heat sources. Energy 2007;32(9):1698e706. [11] Shu G, Zhao J, Tian H, Liang X, Wei H. Parametric and exergetic analysis of waste heat recovery system based on thermoelectric generator and organic
rankine cycle utilizing R123. Energy 2012;45(1):806e16. [12] Long R, Bao YJ, Huang XM, Liu W. Exergy analysis and working fluid selection of organic Rankine cycle for low grade waste heat recovery. Energy 2014;73: 475e83. lez F, Segovia JJ, Martín MC, Antolín G, Chejne F, Quijano A. A technical, [13] Ve economical and market review of organic Rankine cycles for the conversion of low-grade heat for power generation. Renew Sustain Energy Rev 2012;16(6): 4175e89. [14] Song P, Wei M, Shi L, Danish SN, Ma C. A review of scroll expanders for organic Rankine cycle systems. Appl Therm Eng 2015;75:54e64. [15] Hun KS. Design and experimental study of ORC (organic Rankine cycle) and radial turbine using R245fa working fluid. Energy 2012;41(1):514e24. [16] Costall AW, Gonzalez Hernandez A, Newton PJ, Martinez-Botas RF. Design methodology for radial turbo expanders in mobile organic Rankine cycle applications. Appl Energy 2015;157:729e43. [17] Zheng N, Zhao L, Wang XD, Tan YT. Experimental verification of a rollingpiston expander that applied for low-temperature organic Rankine cycle. Appl Energy 2013;112:1265e74. [18] Clemente S, Micheli D, Reini M, Taccani R. Energy efficiency analysis of Organic Rankine Cycles with scroll expanders for cogenerative applications. Appl Energy 2012;97:792e801. [19] Chang J, Hung T, He Y, Zhang W. Experimental study on low-temperature organic Rankine cycle utilizing scroll type expander. Appl Energy 2015;155: 150e9. [20] Wang W, Wu Y, Ma C, Xia G, Wang J. Experimental study on the performance of single screw expanders by gap adjustment. Energy 2013;62:379e84. [21] Wang W, Wu Y, Ma C, Liu L-D, Yu J. Preliminary experimental study of single screw expander prototype. Appl Therm Eng 2011;31(17e18):3684e8. [22] Declaye S, Quoilin S, Guillaume L, Lemort V. Experimental study on an opendrive scroll expander integrated into an ORC (Organic Rankine Cycle) system with R245fa as working fluid. Energy 2013;55:173e83. [23] Lemort V, Quoilin S, Cuevas C, Lebrun J. Testing and modeling a scroll expander integrated into an Organic Rankine Cycle. Appl Therm Eng 2009;29(14e15):3094e102. [24] Lemort V, Declaye S, Quoilin S. Experimental characterization of a hermetic scroll expander for use in a micro-scale Rankine cycle. Proc Inst Mech Eng Part A J Power Energy 2011;226(1):126e36. [25] Chang J, Chang C, Hung T, Lin J, Huang K-C. Experimental study and CFD approach for scroll type expander used in low-temperature organic Rankine cycle. Appl Therm Eng 2014;73(2):1444e52. [26] He W, Wu Y, Peng Y, Zhang Y, Ma C, Ma G-Y. Influence of intake pressure on the performance of single screw expander working with compressed air. Appl Therm Eng 2013;51(1e2):662e9. [27] Ziviani D, Suman A, Lecompte S, De Paepe M, van den Broek M, Spina PR, et al. Comparison of a single-screw and a scroll expander under part-load conditions for low-grade heat recovery ORC systems. Energy Procedia 2014;61: 117e20. [28] Ziviani D, van den Broek M, De Paepe M. Geometry-based modeling of single screw expander for organic rankine cycle systems in low-grade heat recovery. Energy Procedia 2014;61:100e3. [29] Zhang Y, Wu Y, Xia G, Ma C-F, Ji W-N, Liu S-W, et al. Development and experimental study on organic Rankine cycle system with single-screw expander for waste heat recovery from exhaust of diesel engine. Energy 2014;77:499e508. [30] Quoilin S, Broek MVD, Declaye S, Dewallef P, Lemort V. Techno-economic survey of organic rankine cycle (ORC) systems. Renew Sustain Energy Rev 2013;22:168e86. [31] Quoilin S, Lemort V, Lebrun J. Experimental study and modeling of an Organic Rankine Cycle using scroll expander. Appl Energy 2010;87(4):1260e8. [32] Miao Z, Xu J, Yang X, Zou J. Operation and performance of a low temperature organic Rankine cycle. Appl Therm Eng 2015;75:1065e75.
Contents lists available at ScienceDirect
Energy journal homepage: www.elsevier.com/locate/energy
Development and experimental study on a single screw expander integrated into an Organic Rankine Cycle Biao Lei, Wei Wang*, Yu-Ting Wu, Chong-Fang Ma, Jing-Fu Wang, Lei Zhang, Chuang Li, Ying-Kun Zhao, Rui-Ping Zhi Key Laboratory of Enhanced Heat Transfer and Energy Conservation, Ministry of Education and Key Laboratory of Heat Transfer and Energy Conversion, Beijing Municipality, College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 February 2016 Received in revised form 13 July 2016 Accepted 14 September 2016
Expander is a key components in Organic Rankine Cycle (ORC) system. In order to improve the performances of single screw expander in large expansion ratio conditions and give consideration to normal conditions, a new idea called ‘increasing the built-in volume ratio appropriately and converting single screw expander into double-stage machine in large expansion conditions by utilizing the discharge velocity of screw grooves was proposed. By implementing this idea, a new prototype of single screw expander was designed and manufactured. Then the prototype was integrated into an experimental ORC system with R123. Experiments were carried out to analyze the characteristics of the developed single screw expander and the ORC system. The results show that the maximum expander shaft power, shaft efficiency, isentropic efficiency, volume efficiency and expansion ratio were 8.35 kW, 56%, 73%, 83% and 8.5, respectively. In addition, under expansion losses seem to be eliminated by the new structure of the expander. It was also found that 12e17% of the expander power was consumed by the circulation pump, of which the measured efficiency was between 20% and 31%. A maximum ORC efficiency of 7.98% was achieved. © 2016 Published by Elsevier Ltd.
Keywords: Organic Rankine Cycle Single screw expander R123 Circulation pump
1. Introduction Steam Rankine cycle is widely used in the field of heat-power conversion. However, when the temperature of heat source is below 250 C, steam Rankine cycle is not favorable because the high specific volume of steam at low pressure requires large installation and mandatory air removal in condensing mode [1]. In order to address this issue, organic fluids with low boiling point, such as R123 and R245fa, were proposed to substitute water as the working fluids. The Rankine cycle using organic fluids instead of water turns into an Organic Rankine Cycle (ORC). ORC has attracted a lot of attention in recent years [2e4], and it is one of the most promising technologies in the field of converting low grade heat into power [5e7]. The low grade heat can be solar energy [8], geothermal energy [9,10] and industrial waste heat [11,12], which are all widely available. Expander is a key component in ORC system. There are two
* Corresponding author. E-mail address: [email protected] (W. Wang). http://dx.doi.org/10.1016/j.energy.2016.09.089 0360-5442/© 2016 Published by Elsevier Ltd.
types of expander. One is the turbo type, and the other is the positive-displacement type. Large scale ORC systems normally adopt turbo expanders [6,13]. However, for small scale ORC unit, turbo expander might not be favorable. Previous investigations have revealed that the rotational speed of turbo expanders increases with the decrease in turbine output power [14]. The rotational speed of small scale turbo expanders become very high, and the experimental data are normally between 17000 and 65000 r$min1[6,15,16]. In this condition, positive-displacement expanders, such as rolling piston expander [17], scroll expander [18,19] and single screw expander [20,21], are good substitutes for turbo machines due to their relatively high efficiency, high pressure ratio, low rotational speed and tolerance of two-phase fluids [6]. Therefore a lot of experimental small scale ORC units employ positive-displacement expanders. Scroll expander is a research hotspot. Table 1 summarizes recently experimental results which focus on scroll expanders. From the table, it is known that: (1) The expanders were reversely operated scroll compressors, and the expander power was normally smaller than 3.5 kW.
44
B. Lei et al. / Energy 116 (2016) 43e52
2 20 3 4 is sh t th P V m
Nomenclature ·
W h p Dp v N n q m
power [W, kW] enthalpy [J kg1, kJ kg1] pressure [Pa, kPa, MPa] pressure drop [Pa, kPa, MPa] specific volume [m3 kg1] torque [N m] expander rotational speed [r min1] flow rate [kg s1, kg h1, m3 h1, m3 s1] mass
outlet state of expander outlet state of oil separator inlet state of circulation pump outlet state of circulating pump isentropic shaft expander theoretical circulating pump volume mass
Abbreviations ORC Organic Rankine Cycle AC Alternating Current CP Cylinder and Plane
Greek symbol h efficiency [e] εp expansion ratio [e] Subscripts 1 inlet state of expander
Table 1 List of scroll expanders in literature. Authors
Built-in volume ratios
Working fluids
Output power (kW)
Rotational speed (r min1)
Expansion ratio
Efficiency Expander specifications
Lemort et al. [23] Declaye et al. [22] Lemort et al. [24] Chan et al. [25] Chang et al. [25] Chan et al. [19]
4.05
R123
0.4e1.8
1770e2660
2.7e5.4
42e68%
3.95
R245fa
2.1
2000e3500
3e7.4
55e75.7% Reversely operated oil-free scroll air compressor
3.0
R245fa
0.45e2.2
2000e2850
2e6
71.03%a
Reversely operated Refrigeration scroll expander
2.12
R245fa
1.375
Unknown
Unknown
2.95
R245fa
1.77
Unknown
Unknown
60.8 e68.4%a 72e75%a
Reversely operated automotive air conditioning scroll compressor Reversely operated Refrigeration scroll expander
4.05
R245fa
2.3
1535e2970
About 6
73.1%a
Reversely operated oil-free scroll air compressor
a
Reversely operated oil-free scroll air compressor
Note: The authors only provided isentropic efficiency.
(2) The built-in volume ratios of the expanders were between 2 and 4, and the expansion ratios were normally less than 6. Although Declaye et al. [22] increased the expansion ratio to 7.4, obvious under expansion ratio losses were observed. When the output power is larger than 3.5 kW, single screw expander seems more promising. Compared to scroll expander and rolling piston expander, it has many advantages, such as balanced load of the screw, long service life, high volume efficiency, good performances in partial load, low leakage, low noise, low vibration and simple configuration [20,21,26]. A few investigators performed theoretical analyses on single screw expanders. Ziviani et al. [27] reported a comparison between single screw and scroll expander under part-load conditions for low-grade heat recovery ORC systems. It was found that the single screw expander presents higher power output and leads to higher ORC overall efficiency despite a lower maximum value of the isentropic efficiency due to limited expansion ratio. In another paper of Ziviani et al. [28], they reported a geometry-based modeling of single screw expander in ORC system for low-grade heat recovery. Some other investigators conducted experimental studies on single screw expanders. Wang et al. [21] and He et al. [26] performed experimental investigations on single screw expander with compressed air, and it was proved that the single screw structure can work well as expander. However,
only very few investigators presented experimental results about single screw expander in ORC conditions. It is worthy to quote here the small scale experimental ORC system constructed by Zhang et al. [29] In their system, a developed single screw expander with rotor diameter of 155 mm was used, and R123 was adopted as the working fluid. The expander power achieved 10.38 kW. However, the maximum efficiency of the cycle was only 6.48%, mainly because the maximum expansion ratio was 4.6. Both scroll expanders and single screw expanders have fixed built-in volume ratios after they are manufactured, and over or under expansion losses would be thus produced. For an ORC system using R123 with evaporating temperature of 120 C and condensing temperature of 30 C, the expansion ratio would reach up to 10 if the pressure losses are not taken into account. The reversely operated compressors might result in obvious under expansion losses in such conditions [30]. Therefore, it is necessary to develop expanders specifically. Besides the expander, the circulation pump is another key component in ORC system. Several investigators have noticed its significance. For example, Quoilin et al. [30] summarized that the circulation pump should meet the cycle requirements in terms of controllability, efficiency, tightness and net positive suction head. They also reported that the experimental data of circulation pump efficiency in small scale ORC systems were between 7% and 25%
B. Lei et al. / Energy 116 (2016) 43e52
[30]. In another article of Quoilin et al., a small scale ORC system was constructed and they also reported that the efficiency of the system is mainly limited by the poor pump efficiency (about 15%) [31]. This demonstrates that the circulation pump is another important element limiting the efficiency of ORC system, and more attention should be paid to the circulation pump. In the present work, according to ORC operational conditions, a single screw expander was developed. Using the developed single screw expander, an air-cooled serpentine condenser, a multi-stage centrifugal pump and a tube-shell type evaporator, an experimental ORC system was constructed. R123 was selected as the working fluid. Based on this, experiments were conducted to test the performances of the developed single screw expander, the circulation pump and the ORC system under different operating conditions.
2. Description of single screw expander and ORC system In this section, a single screw expander was developed according to ORC operational conditions, and the differences between the machine and typical single screw compressors were presented in detail. Subsequently, the expander was integrated into an experimental ORC system.
2.1. Description of the single screw expander The developed single screw machine was of CP type which has a meshing pair consisting of a screw and two symmetrically collocated planar gate rotors [20,26]. Fig. 1 presents the structure of Cylinder and Plane (CP) type single screw machine. He et al. [26] detailed the working principles of single screw expanders. Compared to typical single screw compressors, the distinguishing features of the developed expander are listed as follows. (1) Exhaust port. In order to get easy installation, the intake port of single screw compressor is always located at the top of the housing. For the developed single screw expander, the exhaust port was located at the bottom of the housing. Therefore the lubricant oil can be easily discharged of the
Fig. 1. Structure of CP type single screw machine.
45
expander with the working fluid and circulate smoothly in the system. (2) Expansion ratio. In order to ensure that the expander work well in large expansion condition and give consideration to normal conditions, the built-in volume ratio of the developed single screw expander was designed to be 4.86. In addition, a new idea was proposed and implemented to reduce under expansion losses. For typical single screw compressors, the working fluid was fed into the screw grooves in radial direction of the screw, and for single screw expanders, the working fluid also left the screw grooves in radial direction of the screw (see Fig. 2). With regard to the developed single screw expander, as shown in Fig. 3, the refrigerant leaves the grooves in opposite direction of screw rotation. In under expansion condition, at the end of expansion process the pressure of working fluid in the grooves is still greater than the discharge pressure, and the leaving velocity of the refrigerant was quite large due to the small discharge port of the grooves. In this situation, the working fluid could rotate the screw, and the expander power would be thus increased. In this situation, the single screw expander actually transformed into a two-stage machine. The first stage is of positive-displacement type, while the second stage is of turbo type. Through two-stage expansion, the under expansion losses might be reduced by this measure. (3) Tightness. One of the key problems about tightness is the shaft seal. For single screw refrigeration compressors, in order to avoid this problem, the compressor components and motor drive are normally assembled with a common shaft and welded into a steel housing. The refrigerant will cool the motor before it is compressed. If a single screw refrigeration compressor is operated reversely and an electric generator is used to substitute the motor, it might be difficult to cool the generator because the temperature of working fluids in expander is high. In addition, the rotational speed and the output torque of expander might be difficult to adjust. As for single screw air compressor, it is normally of open-drive type. The shaft extends out of the compressor housing and connects to the motor. Because there is no requirement of full tightness for air compressor, the air will leak out of the compressor through the gap between the shaft and the housing. Therefore neither air compressor nor refrigeration compressor can be directly operated reversely as ORC expander. Like air single screw compressor, the developed single screw expander is of open-drive type. However, a mechanical seal was used to block the gap between the extending shaft and the expander housing. This construction type is somewhat similar to automotive air-conditioning compressors, of which the size is much smaller.
Fig. 2. Direction of working fluid flow out for typical single screw expanders.
46
B. Lei et al. / Energy 116 (2016) 43e52
Fig. 4 provides a photograph of the developed single screw expander, and its parameters are listed in Table 2. 2.2. Description of the ORC system Fig. 5 presents a schematic diagram of the experimental ORC system. The system consists of three circuits: conductive oil circuit, R123 circuit and lubricant oil circuit.
Fig. 3. Direction of working fluid flow out for the developed single screw expanders.
As for the tightness of the housing, the working fluid within the housing (except the grooves of the screw and the intake port) was designed to work under a low pressure condition, which is approximately equal to the discharge pressure of the expander. Therefore, it is easy for the machine housing to achieve full tightness. (4) Lubrication and cooling. For single screw compressor, the lubricant oil is also used to cool the gas in working chambers, and the consumed power can be decreased in this way. Therefore, it is necessary to cool and atomize the lubricant oil before it is injected into the compressor working chambers. However, cooling the working chamber degrades the performances of expander. Therefore there is no lubricant oil cooling or atomization in the developed single screw expander. In addition, the amount of lubricant injected into the expander was also reduced. (5) Strength. Considering the pressure under ORC condition, the strength of the expander was enhanced compared to air compressors.
(1) Conductive oil circuit. This circuit provides low grade heat for the ORC system. There are four main components in this circuit: conductive oil boiler, conductive oil pump, evaporator and mass flow meter. In the boiler, the conductive oil is first heated by the electrical heater. Then the upper layer of conductive oil, of which the temperature might be higher, is fed into the conductive oil pump, and the pump transports it to the evaporator. The evaporator is of shell and tube type, and the conductive oil goes into the shell side. After an exothermic process in the evaporator, the conductive oil goes back to the bottom of the boiler through the mass flow meter. This is the conductive oil circuit, and it is indicated by dashed lines in Fig. 5. In order to reduce heat losses, all the components and pipes are covered by thermal insulators. (2) R123 circuit. There are seven main components in this circuit: evaporator, expander, oil separator, condenser, R123 reservoir, circulation pump and mass flow meter. Liquid R123 in the reservoir at state 3 is pressurized to state 4 by the circulation pump, and then it is delivered into the tube side of evaporator through the flow meter. R123 absorbs the heat discharged by conductive oil in the evaporator and turns into high-pressure saturated or superheated vapor at state 1. Subsequently the vapor is fed into the singe screw expander to generate power. The vapor leaves the expander at state 2 and flows into the lubricant oil separator and the air-cooled serpentine condenser successively. In the condenser, the refrigerant is condensed to liquid at state 3, which is then collected by R123 reservoir and recirculated by the circulation pump. It is obviously that R123 flows in an Organic Rankine Cycle. This circuit is indicated by solid lines in Fig. 5. Fig. 6 provides a T-s chart of this cycle. The parameters of the circulation pump, evaporator and condenser are presented in Tables 3e5 respectively. For the sake of smooth operation, the installation height of the main components might be significant. In the circuit, the expander exhaust port was installed higher than the inlet of oil separator. Therefore the discharged lubricant of expander can flow smoothly into the separator with refrigerant. In order to avoid cavitation, the circulation pump was installed about 0.5 m lower than the R123 reservoir outlet. The condenser outlet was higher than R123 reservoir and a balance pipe was used to connect the condenser inlet and the reservoir. Therefore the condensate can flow smoothly into the reservoir by gravity. Fig. 7 shows schematic arrangement
Table 2 Parameters of the developed single screw expander.
Fig. 4. Photograph of the developed single screw expander.
Items
Values
Screw diameter Groove number of screw Tooth number of gate rotor Diameter of gate rotor Center distance Built-in volume ratio Intake capacity
117 mm 6 11 117 mm 93.6 mm 4.86 98.3 mL r1
B. Lei et al. / Energy 116 (2016) 43e52
47
Fig. 5. Schematic diagram of the experimental ORC system.
Table 5 Parameters of the condenser.
Fig. 6. T-s chart of the experimental ORC system.
Table 3 Parameters of the circulation pump. Items
Values
Model Rotational speed Flow rate stages Delivery head
CR 3e25 2899 r min1 3 m3 h1 25 122 m
Table 4 Parameters of the evaporator. Items
Values
Number of tube side Number of shell side Heat transfer area Maximum pressure of tube side Maximum pressure of shell side
4 2 20 m2 4 MPa 1 MPa
about installation heights of individual equipment in this circuit. Besides the main components, several accessories were used in this circuit. A filter and a check valve were installed between the circulation pump and the flow meter to catch impurities and prevent back flow, respectively. In order to regulate the R123 flow rate,
Items
Values
Heat transfer area Maximum pressure Air flow rate Total pressure of the fan
90 m2 1.2 MPa 50000 m3 h1 400 Pa
a frequency inverter was used to adjust the rotational speed of circulation pump. In this circuit, the mass flow meter located at the inlet of the evaporator indicated the values of R123 flow rate. An AC electrical dynamometer was used as the expander load, and it can produce electricity utilizing the expander power. In order to obtain the values of expander power, a torque sensor and a rotational speed sensor were installed between the expander and the dynamometer. The electrical energy consumed by the circulation pump was monitored by a power meter. In addition, several pressure and temperature sensors were used to determine the key thermodynamic state points. The details of the sensors are listed in Table 6. (3) Lubricant oil circuit. This circuit was used to supply lubricant for the single screw expander and make it well lubricated and sealed. In the circuit, the mixture of lubricant oil and R123 discharged by the expander flows into the lubricant oil separator where the lubricant oil is extracted. Subsequently the extracted lubricant is injected into the single screw expander by a lubricant oil pump. Then it is mixed with R123 again in the expander. This circuit is indicated by dash dot lines in Fig. 5. The parameters of oil pump are described in Table 7. Pressure drop is an important consideration for R123 circuit and lubricant oil circuit. The pressure drops in oil separator, condenser and the pipes between them increase the expander discharge pressure. From previous investigations, it is known that this exerts an extremely negative influence [27, 33]. Therefore the pressure drops in the separator and condenser were seriously considered in the stage of device design or selection.
48
B. Lei et al. / Energy 116 (2016) 43e52
Fig. 7. Schematic diagram of the equipment heights.
Table 6 Details of the sensors used.
·
Wt ¼
Sensors
Measurement range
Accuracy
p2, p20 , p3 p4, p1 Temperature sensors Rotational speed sensor Torque sensor Mass flow meter Power meter
0.1 to 0.5 MPa 0.1 to 2 MPa PT100, 0e400 C 0e8000 r min1 0e100 N m 0e3000 kg h1 0e5 kW
±0.5% ±0.5% ±0.5 C ±0.5% ±0.5% ±0.2% ±0.5%
Values
Model Rotational speed Flow rate Maximum pressure
Gear pump, CBB 2.5 1450 r min1 2.5 L min1 2.5 MPa
(2)
Here N and n are the output torque and rotational speed of the expander, respectively. They can be obtained by the sensors located at the shaft between the expander and the AC electrical dynamometer. (3) Volume efficiency Volume efficiency is an important parameter of positivedisplacement expanders. For positive-displacement compressors, volume efficiency is defined as the ratio of actual intake volume flow rate to theoretical one. Because leakage is unavoidable, the actual volume flow rate is normally smaller than the theoretical one. Therefore, the volume efficiency of compressor is smaller than 100%. However, for positive-displacement expander, the intake pressure is higher than the discharge pressure, and working fluid leaks though gaps from inlet chambers to discharge rooms. The actual flow rate through the expander is thus increased, and it may become larger than the theoretical flow rate. Therefore it is reasonable to define the volume efficiency of expander as
Table 7 Parameters of the lubricant oil pump. Items
N$n 9:49
Fig. 8 shows the photographs of the experimental facilities.
hV ¼
qV;th 100% qV
(3)
3. Thermodynamic analyses The equations for analyses of experimental results were presented in this section. The involved variables includes expansion ratio, expander shaft power, shaft efficiency, isentropic efficiency, volume efficiency of the expander, efficiency of the circulation pump and efficiency of the ORC system.
qV, th is theoretical volume flow rate through the expander, and qV is the measured one. From the above discussions, it is obvious that hV is probably smaller than 100% for positive-displacement expanders. A small value of volume efficiency denotes an expander with serious leakage. (4) Shaft efficiency
(1) Expansion ratio Expansion ratio is defined as the ratio of the expander intake pressure to the discharge pressure.
εp ¼
p1 p2
Shaft efficiency of expander is defined as the ratio of the obtained expander power to the product of isentropic specific enthalpy drop and mass flow rate. ·
(1)
hsh ¼
·
Wt
m h1 h2; is
Here p1 and p2 are the intake pressure and discharge pressure of the expander, respectively. (2) Expander shaft power Expander shaft power is obtained by
(5) Isentropic efficiency Isentropic efficiency of the expander is obtained by
(4)
B. Lei et al. / Energy 116 (2016) 43e52
49
Fig. 8. Photographs of the experimental facilities (a) oil separator (b) conductive oil boiler (c) R123 reservoir and circulation pump (d) condenser (e) evaporator (f) main body of the system.
his ¼
h1 h2 h1 h2;is
(5)
hP ¼
v3 ðp4 p3 Þ ·
·
(6) Efficiency of the circulation pump The circulation pump efficiency is obtained by
(6)
WP
Here WP is the consumed pumping work of the cycle, given by the power meter. p4 and p3 are the intake pressure and discharge pressure of the pump, respectively. v3 is the specific volume of the liquid working fluid fed to the pump.
50
B. Lei et al. / Energy 116 (2016) 43e52
(7) Efficiency of the ORC system The efficiency of ORC system is obtained by ·
hORC ¼
·
Wt WP ·
mðh1 h4 Þ
(7)
4. Results and discussions During the experiments, the superheat degree of R123 at the expander inlet was controlled at 3e8 C by regulating the refrigerant flow rate. By regulating the electric current through the AC electrical dynamometer, the expander was operated at three different rotational speeds: 2000 ± 30 r$min1, 2500 ± 30 r$min1 and 3000 ± 30 r$min1. The monitored maximum value of pressure drop in filter, check valve, flow meter and evaporator was 250 kPa, while it was lower than 25 kPa in oil separator or condenser. In steady operation conditions, the expander torque and rotational speed, consumed pumping work, R123 flow rate, temperature and pressure of key state points were recorded. The obtained experimental results were categorized into two cases: different expander intake pressure and different expansion ratios.
Fig. 10. Evaluation of the pumping work with different expander intake pressure.
4.1. Effects of expander intake pressure Fig. 9 provides the expander shaft power as a function of intake pressure under different rotational speeds. The figure illustrates that the expander power was improved by increasing intake pressure or rotational speed. The reason is obviously the increasing refrigerant mass flow rate through the expander. The maximum power produced by the expander was 8.35 kW, obtained at a rotational speed of 3000 ± 30 r min1 and an intake pressure of 1.15 MPa. The corresponding work consumed by the circulation pump was presented in Fig. 10. The figure reveals that there was an approximately linear increase of pumping work with increasing expander intake pressure. Combined with Fig. 9, it is known that the pump consumed 12%e17% of the expander power. The efficiency of the circulation pump was presented in Fig. 11. From the figure, it was known that the pump efficiency was between 20% and 31%, and the value was larger under high flow rate and pressure condition. Otherwise it would get lower. This might be caused by the characteristics of the chosen multi-stage centrifugal pump,
Fig. 11. Evaluation of the pumping efficiency with different expander intake pressure.
which has a total stage of 25. Under low flow rate and pressure condition, fewer stages could meet the requirement, and too many stages result in extra friction losses. Therefore the efficiency of the pump was relatively lower.
4.2. Effects of expansion ratio
Fig. 9. Evaluation of the expander shaft power with different intake pressure.
Fig. 12 presents the shaft efficiency as a function of expansion ratio for different rotational speeds. The figure illustrates that the increasing rotational speed would improve shaft efficiency, and the suitable rotational speed of the expander was about 3000 r min1 in the scope of the test. The reason of this might be the leakage problem was alleviated by increasing rotational speed. When the expander operated at about 3000 r min1, the maximum shaft efficiency of 56% was achieved at an expansion ratio of 6.3. Obvious over expansion losses were observed when the actual expansion ratio was smaller than the above value. However, if the expansion ratio was increased to the maximum value of 8.5, the shaft efficiency was only reduced by less than 1%. This indicates that the developed single screw expander performed well under large expansion ratio conditions, and the under expansion losses are almost negligible. This could be credited to the two-stage expansion of the developed single screw expander. Fig. 13 presents a comparison of shaft efficiency and isentropic efficiency when the expander speed was about 3000 r min1. From
B. Lei et al. / Energy 116 (2016) 43e52
Fig. 12. Influences of expansion ratio on shaft efficiency under different rotational speeds.
51
Fig. 14. Influences of expansion ratio on volume efficiency under different rotational speeds.
were about 64% when the expander operated at 2000 ± 30 r min1, and they increased to about 80% at the speed of 3000 ± 30 r min1. The maximum value of volume efficiency was about 83%. This reveals that leakage is still an unavoidable problem for single screw expander. Increasing rotational speed is an effective way to alleviate this problem. In addition, the figure also illustrates that the volume efficiency remained almost unchanged with increasing pressure ratio. Fig. 15 depicts the ORC efficiency as a function of expansion ratio under different rotational speed of the expander. The figure demonstrates that the maximum ORC efficiency was 7.98%, obtained at a pressure ratio of 8.5 and expander rotational speed of about 3000 r min1. With increasing expansion ratio, mainly because the good performance of the expander in large expansion ratio condition, the ORC efficiency presented upward trends. 5. Conclusions Fig. 13. Comparison of n ¼ 3000 ± 30 r min1.
the
shaft
efficiency
and
isentropic
efficiency
for
the Figure, it is obvious that the isentropic efficiency was 15%e20% higher than the shaft efficiency, and the maximum value of isentropic efficiency was about 73%. The differences between shaft efficiency and isentropic efficiency were discovered in previous investigation [32], and the causes of the differences might be the frictions and heat losses of expander. There are two routes of heat losses: the expander housing and the lubricant oil. Both of frictions and heat losses degrade expander power and shaft efficiency. However, heat losses decrease the temperature of working fluid at expander outlet, and the enthalpy drop of working fluid flowing through the expander will thus be increased. This makes the isotropic efficiency appear to be high even when the shaft efficiency is low. When the expander operates at low speed and low intake pressure, the working fluid mass flow rate is small and the influences of heat loss will become protrudent. However, the influences of heat loss will be weakened at high speed and high intake pressure condition, and the difference between shaft efficiency and isentropic efficiency were reduced. In conclusion, the isentropic efficiency is not significant to expanders as compared to the shaft efficiency when heat losses exert influences. The contributions of increasing expander speed were confirmed again by the results of volume efficiency (see Fig. 14). The values
In order to improve the performances of single screw expander in large expansion ratio conditions and give consideration to normal conditions, a new idea called ‘increasing the built-in volume ratio appropriately and converting single screw expander into double-stage machine in large expansion conditions by utilizing the discharge velocity of screw grooves was proposed. By
Fig. 15. Variation of ORC efficiency for various operational conditions.
52
B. Lei et al. / Energy 116 (2016) 43e52
implementing this idea, a new prototype of single screw expander was designed and manufactured. The prototype was then integrated into an experimental ORC system with R123. Experiments were conducted to test the performances of the prototype and the ORC system. Some conclusions were obtained through the experiments. (1) The maximum expander shaft power, shaft efficiency, isentropic efficiency, volume efficiency and expansion ratio were 8.35 kW, 56%, 73%, 83% and 8.5, respectively. It is also found that under expansion losses seem to be eliminated by the new structure of the expander. Increasing the expander rotational speed made positive contributions to its volume efficiency and shaft efficiency. (2) The circulation pump consumed 12e17% of the expander power. The maximum efficiency of the circulation pump was about 31%, achieved at high pressure and high flow rate condition. (3) The maximum efficiency of the ORC system was 7.98%. Acknowledgement This work is supported by the National Basic Research Program of China (973Program, Grant No. 2013CB228306). References [1] Pei G, Li J, Li Y, Wang D, Ji J. Construction and dynamic test of a small-scale organic rankine cycle. Energy 2011;36(5):3215e23. [2] Tchanche BF, Lambrinos G, Frangoudakis A, Papadakis G. Low-grade heat conversion into power using organic Rankine cycles e a review of various applications. Renew Sustain Energy Rev 2011;15(8):3963e79. [3] Guo C, Du X, Yang L, Yang Y. Organic Rankine cycle for power recovery of exhaust flue gas. Appl Therm Eng 2015;75:135e44. [4] Sprouse C, Depcik C. Review of organic Rankine cycles for internal combustion engine exhaust waste heat recovery. Appl Therm Eng 2013;51(1e2):711e22. [5] Chen H, Goswami DY, Stefanakos EK. A review of thermodynamic cycles and working fluids for the conversion of low-grade heat. Renew Sustain Energy Rev 2010;14(9):3059e67. [6] Bao J, Zhao L. A review of working fluid and expander selections for organic Rankine cycle. Renew Sustain Energy Rev 2013;24:325e42. [7] Zhang X, He M, Zhang Y. A review of research on the Kalina cycle. Renew Sustain Energy Rev 2012;16(7):5309e18. [8] Delgado-Torres AM, García-Rodríguez L. Analysis and optimization of the lowtemperature solar organic Rankine cycle (ORC). Energy Convers Manag 2010;51(12):2846e56. [9] Nusiaputra Y, Wiemer H-J, Kuhn D. Thermal-Economic modularization of small, organic rankine cycle power plants for mid-enthalpy geothermal fields. Energies 2014;7(7):4221e40. [10] Madhawa Hettiarachchi HD, Golubovic M, Worek WM, Ikegami Y. Optimum design criteria for an Organic Rankine cycle using low-temperature geothermal heat sources. Energy 2007;32(9):1698e706. [11] Shu G, Zhao J, Tian H, Liang X, Wei H. Parametric and exergetic analysis of waste heat recovery system based on thermoelectric generator and organic
rankine cycle utilizing R123. Energy 2012;45(1):806e16. [12] Long R, Bao YJ, Huang XM, Liu W. Exergy analysis and working fluid selection of organic Rankine cycle for low grade waste heat recovery. Energy 2014;73: 475e83. lez F, Segovia JJ, Martín MC, Antolín G, Chejne F, Quijano A. A technical, [13] Ve economical and market review of organic Rankine cycles for the conversion of low-grade heat for power generation. Renew Sustain Energy Rev 2012;16(6): 4175e89. [14] Song P, Wei M, Shi L, Danish SN, Ma C. A review of scroll expanders for organic Rankine cycle systems. Appl Therm Eng 2015;75:54e64. [15] Hun KS. Design and experimental study of ORC (organic Rankine cycle) and radial turbine using R245fa working fluid. Energy 2012;41(1):514e24. [16] Costall AW, Gonzalez Hernandez A, Newton PJ, Martinez-Botas RF. Design methodology for radial turbo expanders in mobile organic Rankine cycle applications. Appl Energy 2015;157:729e43. [17] Zheng N, Zhao L, Wang XD, Tan YT. Experimental verification of a rollingpiston expander that applied for low-temperature organic Rankine cycle. Appl Energy 2013;112:1265e74. [18] Clemente S, Micheli D, Reini M, Taccani R. Energy efficiency analysis of Organic Rankine Cycles with scroll expanders for cogenerative applications. Appl Energy 2012;97:792e801. [19] Chang J, Hung T, He Y, Zhang W. Experimental study on low-temperature organic Rankine cycle utilizing scroll type expander. Appl Energy 2015;155: 150e9. [20] Wang W, Wu Y, Ma C, Xia G, Wang J. Experimental study on the performance of single screw expanders by gap adjustment. Energy 2013;62:379e84. [21] Wang W, Wu Y, Ma C, Liu L-D, Yu J. Preliminary experimental study of single screw expander prototype. Appl Therm Eng 2011;31(17e18):3684e8. [22] Declaye S, Quoilin S, Guillaume L, Lemort V. Experimental study on an opendrive scroll expander integrated into an ORC (Organic Rankine Cycle) system with R245fa as working fluid. Energy 2013;55:173e83. [23] Lemort V, Quoilin S, Cuevas C, Lebrun J. Testing and modeling a scroll expander integrated into an Organic Rankine Cycle. Appl Therm Eng 2009;29(14e15):3094e102. [24] Lemort V, Declaye S, Quoilin S. Experimental characterization of a hermetic scroll expander for use in a micro-scale Rankine cycle. Proc Inst Mech Eng Part A J Power Energy 2011;226(1):126e36. [25] Chang J, Chang C, Hung T, Lin J, Huang K-C. Experimental study and CFD approach for scroll type expander used in low-temperature organic Rankine cycle. Appl Therm Eng 2014;73(2):1444e52. [26] He W, Wu Y, Peng Y, Zhang Y, Ma C, Ma G-Y. Influence of intake pressure on the performance of single screw expander working with compressed air. Appl Therm Eng 2013;51(1e2):662e9. [27] Ziviani D, Suman A, Lecompte S, De Paepe M, van den Broek M, Spina PR, et al. Comparison of a single-screw and a scroll expander under part-load conditions for low-grade heat recovery ORC systems. Energy Procedia 2014;61: 117e20. [28] Ziviani D, van den Broek M, De Paepe M. Geometry-based modeling of single screw expander for organic rankine cycle systems in low-grade heat recovery. Energy Procedia 2014;61:100e3. [29] Zhang Y, Wu Y, Xia G, Ma C-F, Ji W-N, Liu S-W, et al. Development and experimental study on organic Rankine cycle system with single-screw expander for waste heat recovery from exhaust of diesel engine. Energy 2014;77:499e508. [30] Quoilin S, Broek MVD, Declaye S, Dewallef P, Lemort V. Techno-economic survey of organic rankine cycle (ORC) systems. Renew Sustain Energy Rev 2013;22:168e86. [31] Quoilin S, Lemort V, Lebrun J. Experimental study and modeling of an Organic Rankine Cycle using scroll expander. Appl Energy 2010;87(4):1260e8. [32] Miao Z, Xu J, Yang X, Zou J. Operation and performance of a low temperature organic Rankine cycle. Appl Therm Eng 2015;75:1065e75.