Expansion devices for organic Rankine cycle (ORC) using in low temperature heat recovery: A review

Expansion devices for organic Rankine cycle (ORC) using in low temperature heat recovery: A review

Energy Conversion and Management 199 (2019) 111944 Contents lists available at ScienceDirect Energy Conversion and Management journal homepage: www...

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Energy Conversion and Management 199 (2019) 111944

Contents lists available at ScienceDirect

Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman

Expansion devices for organic Rankine cycle (ORC) using in low temperature heat recovery: A review


Yuanyang Zhao , Guangbin Liu, Liansheng Li , Qichao Yang, Bin Tang, Yunxia Liu ⁎

College of Electromechanical Engineering, Qingdao University of Science and Technology, Qingdao 266061, PR China



Keywords: Organic Rankine cycle (ORC) Expansion device Expander Turbine Ejector

Studies and applications of organic Rankine cycle (ORC) systems are increasing in recent years because of the advantages of them on the recovery of low temperature heat sources. The choice of working fluids, system design methods, equipment, and applications of ORC systems were researched by many researchers. Expansion devices are the key equipment for ORC systems. The review of expansion devices for ORC systems was presented in this paper. The devices of scroll, screw, piston, vane, turbine, and ejector for ORC systems are involved. It is expected that the review presented here can summarize what was done before and will provide the investigator with the knowledge and ideas about how to choose and improve the performance of expansion devices for ORC systems.

1. Introduction The problems of the energy shortage and environmental pollution bring a significant challenge to the sustainable development of human society. The improvement of energy efficient utilization has become one of the important research directions. In the energy utilization process, a large amount of low temperature heat was released into the atmosphere directly. For example, in an internal combustion engine (ICE), about 60–70% of fuel energy is lost as waste heat through the coolant or exhaust [1]. There is also a large amount of low temperature heat emission during the industrial production processes, such as the oil refining, steel making, and furnace heating. Waste heat recovery technologies are the key technologies to realize the comprehensive and efficient utilization of energy. The ORC system, which can convert heat at low and medium temperature into mechanical energy or electricity, is researched and used widely due to its simplicity in the design and operation [2–3]. In recent years, the ORC system has been researched in many aspects. Many researchers have carried out extensive and in-depth studies of the thermodynamic characteristics of ORC cycles [4–6], working medium selection [7–9,10], heat transfer [11–13], expansion devices [14,15] and applications [16]. Some researchers have reviewed the research of the ORC systems. And most of them focus on the whole systems layout and working fluids [1–3,16–18]. The scroll expander was reviewed in 2015 by Song et al. [15]. Rahbar [2] and Bao [19] just reviewed the expansion devices of ORC systems briefly in their papers. However, few studies have been

done on the detailed review of all kinds of expansion devices in recent ten years, and their technique characters and trends have not been expressed very well. Moreover, the detailed method of how to choose the structure types of expansion devices for different ORC systems is very necessary for people who design and applicate the ORC systems. In this study, it was aimed to make a comprehensive review of expansion devices using in ORC systems in recent ten years. According to the mechanism and characteristic, the structure and parameters selection of devices during the design process is suggested. The main technology trends for these expansion devices are presented in this paper. 2. ORC system and applications 2.1. ORC system The ORC system is a Rankin power cycle using organics as the working fluid. It is a typical vapor power cycle. The basic layout and T-s diagram are shown in Fig. 1. The ORC system is mainly composed of a pump, an evaporator, an expansion device, and a condenser. The ideal thermodynamic processes in these four parts are as follows (shown in Fig. 1(b)):

• Pump (1-2): isentropic process • Evaporator (2-3-4): isobaric process • Expansion device (4-5): isentropic process • Condenser (5-6-1): isobaric process

Corresponding authors. E-mail addresses: [email protected] (Y. Zhao), [email protected] (L. Li).

https://doi.org/10.1016/j.enconman.2019.111944 Received 25 April 2019; Received in revised form 12 August 2019; Accepted 15 August 2019 0196-8904/ © 2019 Elsevier Ltd. All rights reserved.

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exergy recovery is proposed by Xue et al. [22]. The optimized two-stage ORC system can output 1776.44 kW with the thermal efficiency of 25.64% and exergy efficiency of 31.02%. Kim et al. [23] analyzed a combined ORC and vapor compression cycle (VCC) system for power and refrigeration cogeneration. The results exhibit the potential of the combined ORC-VCC system to utilize low-grade thermal sources efficiently. Chen et al. [24] researched a confluent cascade expansion ORC (CCE-ORC) system using Cyclohexane and R245fa as working fluids, which can generate 8% more net power and the total volume of heat exchangers is 18% less. Wang et al. [25] proposed an ORC system with two-stage evaporation to enhance the performance of the ORC system. The results show that the two-stage evaporation enhances the evaporating temperature of the high stage. Hu et al. [26] found that the superheat at the evaporator outlet is the main parameter correlated with the operation stability of the system. The working fluid is another important factor affecting the performance of ORC systems. Many researchers researched the working fluids for ORC systems [7–8,10,17,19,27–34]. Zhou et al. [35] analyzed a partial evaporating organic Rankine cycle (PEORC) using zeotropic mixtures. The optimized PEORC with R245fa/R227ea is able to generate about 24.7% more power than the traditional subcritical organic Rankine cycle (SCORC) with R227ea. Li et al. [36] compared the thermodynamic performance of the CO2 transcritical (T-CO2) power cycle and R245fa ORC systems. Their research shows that the thermal and exergy efficiencies of R245fa ORCs are both slightly higher than those of T-CO2 power cycles with the same operating conditions and heat transfer assumptions. Li et al. [37] researched an ORC system using R600/R601a mixtures with liquid-separated condensation. The liquid-separated condensation can increase the average condensation heat transfer coefficient by 23.8% and reduce the condenser heat transfer area by 44.1%. 2.2. Applications

Fig. 1. (a) ORC system layout [20] (b) T-s diagram [21].

The ORC system can recover many kinds of low temperature heat. Many studies of ORC systems for the recovery of the ICE exhaust heat have been done [38–41]. Fig. 2 shows the typical ORC system for ICE. The ORC system is also used in the solar energy utilization area [42–45]. Fig. 3 shows an ORC system for a solar application. Zhang

Many thermodynamic improvements of ORC systems are proposed and researched on the foundation of the basic ORC system. A two-stage ORC system using R227ea and R116 for liquefied natural gas cryogenic

Fig. 2. Schematic diagram of ORC in ICE [39]. 2

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fluid is confined in a given space and subsequently expanded by increasing this confined space or volume. At last, the low-pressure working fluid gas is expelled into the discharge piping or vessel system. The positive displacement expanders include a wide range of configurations and geometries, and the main types are the piston, screw, scroll, and vane expanders. Finally, ejectors are the expansion devices without rotors, which means that there is no shaft power output in ejectors. The energy of expansion processes can be only used within the ORC system. 3.1. Turbines 3.1.1. Radial turbine The working principle and basic structure of radial turbines are similar to centrifugal compressors. There is a continuous fluid flow in its impeller, in which energy is converted to power. The main flow direction in radial turbines is radial, that is why we call this type of turbines as a radial turbine. The radial turbine is composed of an outer casing which contains a stator part and a rotor (impeller and shaft) (shown in Fig. 5). Many researchers just simulated the ORC system with simplified turbine models [58–60]. And some other researchers presented the details of design, simulation, and experiment of turbines in ORC systems [61–67]. Costall et al. [68] presented a design method of radial turbines in mobile ORC applications. Turbines with 15.5, 34.1, and 45.6 kW were designed. Rahbar et al. [65] researched the preliminary mean-line design and optimization of a radial turbine. Their work shows that the maximum difference is 7.3% between the turbine efficiencies of npentane and R245fa. The parametric analysis and optimization of a small-scale radial turbine for different working fluids (R245fa, R134a, R123, R236ea, R152a, R236fa, n-pentane, and isobutane) were presented and the achieved efficiencies vary from 82.9% to 84% [69]. The results showed that isobutane exhibited the most favorable characteristics in terms of efficiency (83.82%), rotor size (66.3 mm) and inlet temperature (89.2 °C) if the superheating is to be avoided. A high expansion ratio (120.5) turbine using toluene as the working fluid was investigated by CFD simulation and the results show that FLUENT is by far the most dissipative flow solver [70]. The performance of a small radial turbine and pre-design maps for an ORC system was researched by Mounier et al. [71]. Arifin et al. [72] researched the process manufacture of the radial turbine for ORC systems using the selective laser melting machine. Their work shows that the rotor with material Aluminum Silicon Powder (AlSi) can be made within 5 h. Durá et al. [73] researched the trailing-edge losses in ORC transonic turbines. The results indicate that the balance between fluid viscous and shock loss is a strong function of fundamental derivative, which is mostly dependent on the choice of working fluids and turbine conditions. Moreover, some novel types of turbines were proposed and researched in recent years. Pini et al. [64,74] proposed a centrifugal turbine for ORC system (shown in Fig. 6). The expansion ratio of the designed centrifugal turbine is up to 45. Manfrida et al. [75] and Song et al. [76] researched the Tesla turbines for ORC systems (shown in Fig. 7). Table 1 shows the main parameters of radial turbines in recent years. R245fa and R123 are the high-frequency working fluids used in ORC turbines. Rotation speeds of most turbines in references are higher than 10000 rpm. The maximum inlet temperature and the output power are 314.5 °C and 1500 kW.

Fig. 3. Schematic diagram of ORC in solar energy utilization [43].

et al. [33] presented a 200 kW solar power plant based on ORC systems. The ORC can also be used for the cryogenic exergy recovery of liquefied natural gas[17,34], fuel cell systems [46–49], CHP systems [50–52], and industrial processes[53–55]. 3. Expansion devices There are many similarities between refrigeration compressors and ORC expansion devices. In refrigeration systems, different structure compressors are suitable for different kinds of refrigeration systems. As same as the refrigeration system, the different structure expansion devices could be suitable for different ORC systems. Based on their working principles, there are three kinds of expansion devices, i.e., turbines (dynamic expander), positive displacement expanders, and ejectors. Fig. 4 shows all types of expansion devices used in ORC systems. Firstly, the turbine is a kind of dynamic machines. In the flow path of turbines, the flow speed of high-pressure working fluids is increasing firstly. The pressure energy of working fluids is transformed into velocity energy (kinetic energy). And then, the energy of high-speed working fluids is transferred to impellers of turbines. There are two types of trubines, radial and axial turbines. Usually, the capacity of axial turbines is bigger than that of radial turbines. Secondly, the working principle of positive displacement expanders is different from turbines. The energy transformation is based on the change of the closed volume in expanders. During the working process of positive displacement expanders, a specific inlet volume of working

3.1.2. Axial turbine The basic working principle of axial turbines is similar to radial ones. However, the flow direction of working fluids is axial, which means that the flow direction is in parallel with the rotor of turbines. The flow rate of axial turbines is usually bigger than that of radial turbines. But the expansion ratio of radial turbines is generally bigger

Fig. 4. Expansion devices for ORC system. 3

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Fig. 5. 3D drawing of Radial turbine [56,57].

than axial ones (for one stage). Fig. 8 shows the main structure of axial turbines and the difference between these two types of turbines [85]. Some studies have been done on the design, simulation, and test of axial turbines. Jubori et al. [85–88] researched a small scale axial turbine for OCR system using CFD methods. The optimization technique of multi-objective genetic algorithm (MOGA) is used to enhance the performance of turbines using high-density organic working fluids (R123, R134a, R141b, R152a, R245fa, and isobutane) [88]. The results show that R123 has the best performance with the predicted cycle thermal efficiency of 10.5% and achieve the maximum power output of 6.3 kW with the isentropic efficiency of 88%. Cho et al. [89] tested an axial turbine used in an ORC system and the test system efficiency of 2% was obtained at the turbine inlet temperature of 100 °C. Considering the real fluid properties and loss models, Luca et al. [90] developed a new efficiency chart for the optimum design of axial turbines. Manente et al. [91] investigated the design performance of ORCs equipped with a single-stage axial turbine. And the results show that supercritical ORCs outperform subcritical ORCs even taking into account the detrimental effect of high expansion ratios on turbine efficiency. Weiß et al. [92] tested the axial turbine in an ORC system. The maximum isentropic efficiency of 73.4% has been reached and the maximum isentropic efficiency of the cantilever turbine reaches 76.8%. Pu et al. [93] experimental studied a single-stage axial turbine in R245fa and HFE7100 ORC systems. The isentropic efficiency of the turbine was 59.7% and the overall system efficiency is 4.01%. Sun et al. [94] simulated the performance of an axial turbine using CFD method. The results show that the turbine has the maximum real power output and efficiency of 8.1336 kW and 55.3% with R245fa in the design conditions.

Fig. 6. Structure of Centrifugal Turbines for ORC [74].

Fig. 7. Structure of tesla turbine [75]. Table 1 Main parameters of radial turbines from references. Reference

Working fluid

Output Power / kW

Mass flow rate kg/s

Inlet temperature / ℃

Inlet pressure / MPa

Expansion ratio

Rotation speed / rpm

Efficiency /%


[56] [57] [66] [70] [74] [77] [78] [79] [80] [81] [82] [83] [84]

R245fa R134a R245fa Toluene Siloxane D4 R123 Fluids# Cyclopentane R143a R245fa R123 R245fa R123

~35 641.2 ~1000 – 10.6 6.07 ~140 50 ~ 1500 400 177.4 534 250 8.33

1.4–1.5 41.1 56.07 1.24 0.266 0.467 – – 16.90 7 21.98 11.85 3.3

~115 ~140 110 314.5 242.5 120 – 182 140 140 120 ~120 100

1.72 5 0.514 3.19 0.392 ~0.9 ~0.16–0.45 2 5 2.09 1.187 1.265 0.786

~10–11 2.7 4.2 120.5 45 ~5.0 20.3 – 2.7 2.8, 3.12 7.95 5.2 7.14

21,200 8000 2830

~65 79.9 ~85

– ~3000 30,000 – 25,160 14,000 8000 12,386 60,000

– 58.53 85 – 82 68.1 84.3 63.7 48–63



: R245fa, R245ca, R236fa, R123, Isobutane. * S-Simulation, E-Experiment. 4

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Fig. 8. Structure of turbines.

3.2. Positive displacement expander

main structure of open-drive scroll expander was mentioned. The test isentropic efficiency is almost 40% and volumetric efficiency is about 30%. The geometric design of scroll expanders for small ORCs was researched by Orosz et al [100]. A generalized 8-dimensional planar curve framework is applied to scroll expanders. During the simulation of ORC systems, the model of scroll expanders is significant to get the reasonable performance of ORC systems. The detail models were investigated by many researchers using traditional thermodynamic methods [20,98,101–105]. Moreover, some researchers presented the flow performance in scroll expanders using CFD methods [106–108]. A bilateral symmetric discharge structure on the performance of a scroll expander was researched by ANSYS FLUENT [108]. The results show that the pulsation of the discharge mass flow rate in the expander with bilateral discharge is related to both the eccentric rotation of the orbiting scroll and the decompression drainage of the second discharge port. The deformation distributions of scroll parts were researched by FEM method [109] and the results show that the deformations by pressure were less significant than that by the thermal loads, and thus the coupling deformation was dominated by the thermal loads. Most literature on scroll expanders for ORC systems presented experimental results. The efficiency of scroll expanders is the most important parameter. Hence many references presented the test isentropic

Based on the structural features of positive displacement expanders, there are many kinds of expanders. During the following section, the main structures of positive displacement expanders for ORC systems are reviewed. 3.2.1. Scroll expander The principles of scroll expanders and compressors are similar. Hence, many prototypes of scroll expanders in literature are directly changed from scroll refrigeration compressors [95,96]. The numbers of references on the ORC system experimental research using scroll expanders is more than other kinds of expanders. The main reasons may be that scroll refrigeration compressors are easy to obtain and the capacity of ORCs using scroll machines is not very big or small, which means that it is very suitable for the experimental study in laboratories. Fig. 9 shows the working principle and structure of scroll expanders. Design is the basic work to develop ORC scroll expanders. Some researchers presented the design methods of scroll expander. Garg et al. [98] investigated a generic tool to design scroll expanders for ORC applications. It is found that there is a unique scroll height (or aspect ratio) that leads to the maximum isentropic efficiency. George et al. [99] presented a design procedure of ORC scroll expanders, and the

Fig. 9. (a) Working principle [97] (b) Structure of scroll expander [20]. 5

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Table 2 Main parameters of scroll expander from references. Reference

Working fluid

Output Power /kW

Expansion ratio

Rotation speed /rpm

Efficiency /%

[97] [102] [110] [111] [112] [113] [114] [115] [116] [117]

R123 R123 R245fa R245fa R123 R245fa R245fa R123 R134a R245fa

1.54 0.75 2.3 1.8 2.78 – 1.016 – 0.557 3.75

3.84 – 5.2 2–7 – 4.58 10.68 – 3.6 –

2165 2100 2970 3500 – 3000 3496 3000 3450 3500

86 38 73 75.7 85.17 84.9 77.74 – 78 58

efficiency of scroll expanders [106,110], which is from 38% to 86%. Table 2 shows the main parameters of scroll expanders from experimental results. It can be seen that R123 and R245fa are also the highfrequency working fluids used in ORC systems with scroll expanders. The rotation speeds of scroll expanders are around 3000 rpm, and the maximum output power is 3.75 kW, which is much smaller than that of turbines.

effect on expander performances than the meshing clearance height. Semi-empirical modeling of a single-screw expander for small ORC was researched by Giuffrida [120]. Lei et al. [121] presented the development and experimental study of a single screw expander. 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. Ziviani et al. [122–124] researched the single screw expander by simulation and experimental methods using R245fa and R1233zd(E) as working fluids. Their work shows that while R245fa allowed approximately a 10% higher power output, the single-screw expander was performing at higher isentropic efficiency with SES36. And the friction losses played a major role in the total loss. Xing et al. [125] presented the experimental research of a twin screw expander for the ORC system. And the p-V (pressure–volume)

3.2.2. Screw expander There are two kinds of screw expanders, i.e., single screw expanders and twin screw expanders. The main working principle and structures are shown in Fig. 10. Shen et al. [118] presented a detail simulation model of single screw expanders for ORC systems including the leakage, friction loss, and heat transfer. The results show that the fitting clearance has a significant

Fig. 10. Working principle and structure of screw expander, (a) Single screw expander [118] (b) Twin screw expander [119]. 6

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Table 3 Main parameters of screw expander from references. Reference

Working fluid

Output power /kW

Expansion ratio

Rotation speed /rpm

Efficiency /%

[83] [120] [121] [122] [125] [126] [128] [129] [130] [131]

R218 R245fa R123 R245fa R245fa R123 TY-1 R134a R123 R123

20 ~8.5 8.35 ~7 – 100–300 50 193.75 10.38 6.58

2.64 3–8 4–9 4.5 2.3 – 3 1.8 4.6 6.38

– 3000 3000 3000 1500 6000 1500 3600 3000 3000

60 60 73 ~60 ~79 60–88 67.5 70.82 73.25 42.53

indicator diagrams which indicate the performance of the expander was tested by installing the pressure sensors in the expander casing. The results show that the high rotating speed leads to large suction pressure loss, low volumetric and indicated efficiencies. Xing et al. [126] also investigated a detail simulation model for a twin screw expander and they found that the speed and suction pressure have large influences on expander performances, while the inlet superheat has a relatively small effect. Transient 3D CFD calculations of the twin screw expander were researched by Papes et al. [119]. They found that the biggest pressure drop is caused by a throttling loss at the inlet port and therefore an optimized design of the inlet port is necessary. Astolfi [127] presented a simple model to calculate the screw expander efficiency, which can be used in the simulation model of ORC systems (shown as Eq. (1)).

analysis shows that the output power is dependent on the location of the inlet port to the working chamber, dimensions, and working fluids. Jakub et al. [134] presented the experimental results of 1–10 kW ORC systems using vane expanders. The isentropic efficiency of rotary vane expanders ranges from 25 to 58%. The working fluid is hexamethyldisiloxane. And the vanes were made of the high-temperature plastic PEEK. Vaclav et al. [135] proposed a modified semi-empirical model of a rotary vane expander used for ORC with two different leakages: (1) lumped leakage area between the inlet and outlet; (2) leakage caused by the loss of contact between the vanes and stator. A CFD simulation of a vane expander for ORC system was presented in reference [136]. The leakage occurring at the vane tips has an influence on both the output power and the back pressure at the inlet.


3.2.4. Piston expander The piston expanders have many kinds of structures, which includes the traditional piston, free piston, swash-plate, rolling-piston, and so on. Fig. 12 shows some typical structures of piston expanders. Bianchi et al. [137] experimental researched a micro-ORC driven by a piston expander. The results show that the gross output power varied between 250 W and 1150 W. The total efficiency of the expander showed a barely constant trend of 40%. Dolz et al. [140–142] researched a swash-plate expander for the ORC system. The maximum power of expander is 1.83 kW when the rotation speed is 3354 rpm. The peak isentropic efficiency is 38.3%. Zhang et al. [143–146] investigated a free piston expander for the ORC system. The peak output power is 676 W when the intake pressure, external load resistance, and operating frequency are 10 bar, 9 U, and 15 Hz, respectively. The maximum indicated efficiency is 81%. Zheng et al. [139] experimental verified a rolling-piston expander applied for a low-temperature ORC system using R245fa as the working fluid. The experimental results show that the speed of the expander is between 350 and 800 rpm with a maximum output power of 0.35 kW and the maximum isentropic efficiency of 43.3%.

= c [0.9403305 + 0.0293295ln(Vout)

0.0266298] Vr

where Vr is external volume ratio, Vout is outlet volumetric flow rate, coefficient c is as follows: c= 1 0. 264 ln(Vr /7) for Vr > 7 Table 3 shows the main parameters of screw expanders from references. Similar to scroll expanders, the rotation speed of screw expanders is around 3000 rpm and the fluids of R123 and R245fa are used frequently. The maximum output power is 300 kW, which is bigger than that of scroll expanders but smaller than that of turbines. 3.2.3. Rotary vane expander The main structure of rotary vane expanders is shown in Fig. 11. When the rotor is rotating, the expansion process occurs in the closed volume composed of the rotor, vane, and stator. Fatigati et al. [132] researched a novel strategy to enhance the performance of expanders by an auxiliary injection of fluids. This optimal configuration led to an average mechanical power gain with respect to the experimental performance of 50.6%. Gnutek et al. [133] researched a 1.2 kW vane expander using R123. The theoretical

Fig. 11. Structure of vane expander [134]. 7

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Fig. 12. Structures of piston expanders, (a) Traditional piston [137] (b) Free piston [138] (c) Rolling-piston [139].

3.3. Ejector

ejectors cannot be used for electricity generation directly. The configuration and processes of an organic Rankine cycle with ejector (EORC) are shown in Fig. 13. The structure of ejectors is shown in Fig. 14.

The ejector is another type of expansion device. There are no moving parts in ejectors, which means that the output power from 8

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Like refrigeration compressors, the most important parameters of ORC expansion devices are power, volume flow rate (capacity), expansion ratio, and inlet (or outlet) pressure. Most of these parameters are related to the thermophysical properties of working fluids. Fig. 15 shows the refrigeration capacity of different structure compressors [150]. The input power of the refrigeration compressor can be gotten from Eq. (2). We can choose expansion devices based on Fig. 15 (Assume that the output power of the expansion device is equal to the input power of the compressor). (2)

W = Q/ COP

where W is input power, Q is refrigeration capacity, and COP is coefficient of performance (Value of COP is about 3.5). Based on the data from the references which are cited in this paper, we drew Fig. 16 and it shows the output power and rotation speed of all kinds of expansion devices from the references mentioned in this paper. The application scope of expansion devices is different obviously. The output power range of turbines is the largest, which means that turbines are more suitable for large-scale ORC systems. The rotation speed of turbines can be very high because they have an excellent character on rotor balance. Screw expanders have balanced rotors too. Hence, the rotation speed of screw expander can be a little higher than 3000 rpm. And the screw expander is suitable for medium-scale ORC systems. The balances of piston and scroll expanders are not very well, that is why the rotation speeds of them are mostly less than 3000 rpm. Hence, they are suitable for small and micro ORC systems. Although the balance of vane expander is good, there are frictions between vanes and cylinder. Therefore, the rotation speed and capacity of vane expander are limited also.

Fig. 13. Configuration and processes of ORC system with ejector [147].

Fig. 14. Structure of ejector [148].

The first and second thermodynamics law analyses on the ORC system with an ejector are presented by Li et al [147]. The results show that the cycle exergy efficiency of EORCs is bigger than that of ORCs. Zhang et al. [149] considered the detailed flow and transport processes in an ejector with a CFD method. The performances of ejector in an ORC and ejector refrigeration cycle were evaluated. They found that some operating conditions lead to higher energy losses inside the ejector and limit the performance of the entire system.

4.2. Technology trends Expansion devices are the key power machines of ORC systems. The technical maturity of them affects the applications of the ORC system. Fig. 17 shows the efficiency of expansion devices based on the data from the references which are cited in this paper. We can see that the efficiencies of some expanders are very low, which affect the efficiency of ORC systems directly. The reasons caused these low efficiencies may be the factors of design, manufacture, assemble, and so on. Hence, the key techniques to improve the performance of expansion devices are the most important things in the next few years. The statistical analysis of paper numbers has been done for different ORC expanders during the last decade on the website of ScienceDirect. Based on our statistic data, there are 752 research papers with the words “ORC” and “expander/turbine” in their abstracts or titles. The detail statistical analysis of all kinds of expanders is shown in Fig. 18. From this figure, we can see that the proportion of published papers on turbines is 69.95%, which is the biggest one. That means the application areas and parameter ranges of turbines are the most widespread among all kinds of expanders. The proportions of scroll, screw, vane, and piston expanders are 14.23%, 6.12%, 5.32%, 4.39%, respectively.

4. Discussions 4.1. Choose of structures There are many kinds of expansion devices. And each of them can be used as the power machine in ORC systems. Therefore, the structures choice of expansion devices becomes a significant issue when we design ORC systems. Every expansion device has its characters, which have some advantages and some disadvantages. Table 4 shows a comparison of different expansion devices. When we design an ORC system, we can choose expansion devices by considering these characters. Table 4 Comparison of different expansion devices [133]. Expander type






Power Working fluid flow velocity Technical complication Rotational speed Noise Operation in wet vapor conditions Difficulty of air-tight sealing Cost Service Durability Internal mechanical friction

high/medium very high very high very high high no high high very high high low

medium/low low high low low yes medium medium medium medium medium

medium medium high low medium yes high high high high medium

low low low low low yes low very low very low medium medium

medium medium high medium high no high medium medium medium high


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Fig. 15. Refrigeration compressors capacity.

Fig. 18. Numbers of research papers for different expanders.

of expansion devices should be decided in this period. Structure parameters optimization. Once the main thermodynamic parameters of expansion devices are fixed, the optimization of the structure parameters is the most important task to get the high performance of expansion devices. Considering the working fluid, rotation speed, heat transfer, leakage, flow distribution, and structure parameters, the integrated simulation models should be established and solved. For example, the optimal rotation speed is several thousand for positive displacement expanders and it is dozens of thousands for turbines. Moreover, the leakage character is more important to positive displacement expanders than to turbines. But the flow distribution is more important to turbines than to positive displacement expanders. Flow simulation and analysis. As fluid machines, the flow in expansion devices has a significant effect on the performance of them. Therefore, the flow simulation based on the CFD method is very useful to improve the performance of these devices. For positive displacement expanders, the leakage flow, suction flow, and discharge flow (lead to flow losses, pressure pulsation, and noise) are the main focuses. For turbines, the velocity and pressure distributions in impellers are the most important factor to affect the performance of turbines. Structure design and machining process. Finally, the operation performance of these devices depends on the detail structure design and manufacturing. For positive displacement expanders, the minimal leakage paths are expected to decrease the leakage capacity, and thus the expander performance can be improved. At the same time, the frictions between the moving parts and fixed parts should be taken into account to guarantee the performance of expanders. For turbines, the structural strength of impellers should be concerned during the structural design period because of the high rotation speed of the impeller, which leads to high structural stress in impellers.

Fig. 16. Output power and rotation speed of expanders.

Fig. 17. Efficiency of expanders.

4.2.1. Improvement of expansion devices There are many aspects that can be done to improve the performance of expanders. And the main aspects are shown as follows: Thermodynamic parameters optimization. Many parameters have influences on the performance of expansion devices and ORC systems, such as the inlet temperature and pressure, superheated temperature, expansion ratio, and working fluid. During the design period of the ORC system, these key parameters should be determined carefully using the method of thermodynamic optimizations. The optimal parameters are related to the structure types of expansion devices. Therefore, the type

4.2.2. Technological breakthroughs in other areas Technological breakthroughs in other areas can also push the technical progress of expansion devices. Bearing and electric generator techniques. As power machines, the bearing is one of the key parts for expansion devices. The parameters of 10

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bearings have significant effects on the performance, size, and reliability of these devices. In recent years, the magnet and gas bearings have been already used in refrigeration compressors to improve the compressor performance and to avoid the usage of oil in systems. The bearings lubricated by working fluids have also been developed. These bearing techniques can be used in expansion devices. The high-performance electric generators are needed to enhance the performance of expansion devices. The DC permanent magnet generator with high efficiency should be developed for expansion devices in the near future. Moreover, the ORC system with both characters of small scale and high output power is needed in mobile devices, such as ships and vehicles. And the super-high rotation speed turbine is suitable for these applications. The techniques of high rotation speed should be developed, such as the aerodynamic optimization of the impeller, the control of high-speed turbine, the design and manufacture of high-speed bearings, and the research of high-speed rotor dynamics. Control technique. When the expansion devices are operated, the running control is another key technique. The rotation speed is the main control parameter for expansion devices. The change of speed leads to the change of the output power. Besides the speed control, there are some other methods to change the output power, such as the control of valves for piston expanders, the control of slide valves for screw expanders, and the control of the position of the fixed plate for scroll expanders. All these control techniques would improve the performance of expansion devices significantly.

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5. Conclusion In this paper, the detailed review of all kinds of expansion devices for ORC systems has been done, and the types of expansion devices involve the turbine, scroll expander, screw expander, rotation vane expander, piston expander, and ejector. The working principles and main parameters of these devices have been presented, and their technical features have been compared. Moreover, their technique trends have been expressed. Based on the literature survey and the feature analysis of expansion devices, it was found that turbines are suitable for large-scale ORC systems, screw expanders are suitable for medium-scale ORC systems, and the piston and scroll expanders are suitable for small and micro ORC systems. Furthermore, the scroll expander is more suitable for the ORC experimental systems in laboratories. With reference to the refrigeration compressors, the selection method of expansion devices has been present in this paper. The main technique trends for these expansion devices have been presented. These key techniques involve the thermodynamic parameters optimization, structure parameters optimization, flow simulation and analysis, structure design and machining process, bearing, control, and electric generator. Acknowledgments This work was supported by the Taishan Scholar Program of Shandong (No. tsqn201812073). References [1] Sprouse III C, Depcik C. Review of organic Rankine cycles for internal combustion engine exhaust waste heat recovery. Appl Therm Eng 2013;51:711–22. https://doi. org/10.1016/j.applthermaleng.2012.10.017. [2] Rahbar K, Mahmoud S, Al-dadah RK, Moazami N, Mirhadizadeh SA. Review of organic Rankine cycle for small-scale applications. Energy Convers Manag 2017;134:135–55. https://doi.org/10.1016/j.enconman.2016.12.023. [3] Lecompte S, Huisseune H, Van Den Broek M, Vanslambrouck B, De Paepe M. Review of organic Rankine cycle (ORC) architectures for waste heat recovery. Renew Sustain Energy Rev 2015;47:448–61. https://doi.org/10.1016/j.rser.2015. 03.089. [4] Javanshir A, Sarunac N. Thermodynamic analysis of a simple Organic Rankine


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