Proceedings of the ASME th Joint US-European Fluids Engineering Division Summer Meeting FEDSM2014 August 3-7, 2014, Chicago, Illinois, USA

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1 Proceedings of the ASME th Joint US-European Fluids Engineering Division Summer Meeting FEDSM2014 August 3-7, 2014, Chicago, Illinois, USA FEDSM REVIEW OF EXPANDER SELECTION FOR SMALL-SCALE ORGANIC RANKINE CYCLE Saeedeh SAGHLATOUN, Weilin ZHUGE, Yangjun ZHANG* State Key Laboratory of Automotive Safety and Energy Tsinghua University, Beijing , PR China ABSTRACT After more than twenty years working on the selection of an appropriate expander for Organic Rankine cycles and wide research and attentions about its influence on the performance and total cost of waste heat recovery systems, now there is a goodenough background studies and achievement for large scale applications. But small-scale industries is like a art space to modify and revise the previous results. As it is clearly known, in small-scale applications and industries especially in internal combustion engines, besides the investigation of performance, physical properties and final efficiency of expander, other parameters should be analyzed accurately like manufacturing cost, availability, reliability, sensitivity to operating condition fluctuations. Due to a significant role of expander equipment to enhance the efficiency of ORC system in the first step expanders is investigated. In this paper, as per related operating characteristics, a complete comparison of small-scale expanders will be debated to guide designers to select more appropriate and the best efficient expansion machine as per their requirements. According to available literatures there is more need to do research about different types of expanders with various operating conditions in small-scale industries. INTRODUCTION The total energy consumption is around 500EJ annually. Figure 1 represents the contribution of world primary energy consumption of various energy sources in our century. It is an estimation for future energy consumption proportion in case of no attention to energy recovery [4]. As per such references, energy demand will be increase 60% more than that is now. So due to impressive increase in global energy consumption, harmful environmental effects of fossil fuels and its rising prices, fossil sources limitation, 1 increasing transportation needs, the demands of more sophisticated energy supplies options is increased effectively. The 2010 World Energy Council survey insets energy efficiency and other energy forms like renewable types as the highest "need for action" issues in its energy agenda. Fig.1 world primary energy from different energy sources (IEA,2006) [4] The main international environmental concerns is in the form of air emission, water and soil pollution and global warming. In this regard, there are different methods or sources to minimize such harmful environmental impacts such as geothermal reservoirs, solar heat, ocean thermal energy, industrial waste heat recovery and power cogeneration [4]. Using waste heat recovery systems gaits a signification step to derive energy out of sources like low pressure-temperature applications and enhance total efficiency in a very wide range of industries. A key technology to generate power in electrical or mechanical form from heat energy in Rankine cycle systems. Copyright 2014 by ASME

2 Basically such systems used steam to increase efficiency and performance but in low temperature application steam or water cannot play its role as well as when high temperature medium (>340 C) is available. Unfortunately it is rarely happened that energy resources produced high temperature except in power plants, ships and etc and undoubtedly it is estimated between 20 and 50% of total industrial energy inputs are lost in the form of heat and 60% of the this losses are at low temperatures (<230 C) conditions, which are not typically accessible to steam Rankine cycles [3]. As it is mentioned before, for large scale applications the operating condition is in higher range and we observe low temperature conditions for small- or micro-scale applications like in vehicles (see Figure 2). For small-scale industries water should be replaced with other fluids which have lower boiling temperature and work in lower thermodynamically range compared to water. This requirement distinguishes Organic Rankine Cycle (ORC) fluid to use in low temperature conditions. ORC system has ability of using heat sources as low as 65 C [3]. On the other hand, the main sources of inefficiency in vehicles are through the exhaust gases and the engine coolant, which, together, can account up to 60% of the total energy produced by the diesel combustion engines (see Figure 3) [3]. Fig 2: ORC in Vehicle [8] Oil cooler Intercooler Exhaust gases Fuel Energy Jacket water Useful work (Shaft power) Fig.3 General energy stream of internal combustion engine [3] The main benefits of ORC systems is reducing system cost and complexity beside increasing its safety [12] but it must be remembered that ORC systems have a lot of limitations. Besides limitation of working fluids that should be appropriate for ozone layer, compared to power generation components such systems are more expensive. EXPANSION MACHINES The heart of each ORC system is the expander which is a critical component limiting the cycle efficiency. The selection of an appropriate expander will increase micro-chp system efficiency dramatically. Choosing of expander depends on both operating conditions and power magnitude.[10] Generally expanders divided into two groups: one is velocity type like axial turo-expanders (turbine type) and the other is volume type such as screw, scroll and reciprocating expanders. Firstly the advantageous and disadvantageous of various expanders types will be discussed, then according to small-scale application especially for vehicles the selection procedure will be followed up. At the time of a suitable expander selection, several parameters should be considered such as below [4,12]: 1) high isentropic efficiency, 2) pressure ratio, 3) power output, 4) lubrication requirements, 5) complexity, 6) rotational speed, 7) dynamic balance, 8) reliability, availability, 9) Light weight design, 10) Dimensions and size limitations 11) cost TURBINES: Turbines which are used in ORC systems basically are the same with one's which operate in steam systems. Turbo-expanders work based on the high-pressure gas being directed past the turbine blades causing them to rotate as the gas expands. This velocity type of expanders divided into three basically categories: axial, radial and mixed flow. The most critical factor in high performance application is blade speed ratio when it is around 0.7 for maximum possible efficiency. In small and micro scale industries turbo-expanders work at rotational speed as high as rpm which represents cost impacts both for manufacturing and operation. The main losses of turbo-expanders operation is aerodynamically that is roughly 10-30%. Other losses such as tip leakage, bearings, seal leakage are 1-2% for each of them [4]. The maximum isentropic efficiency for turbine expanders is about 78-90% [3,4,12]. As it is mentioned above, there are various mechanically requirements to select suitable expander for waste heat recovery systems. Some merits and demerits are considered below and after that for small scale systems like vehicle the conclusion will be presented finally. 1) In vehicle with very limited packing dimension, high rotational speed is required and a speed reduction gearbox should be applied to couple expander to engine shaft [11]. 2) In Small scale industries turbo-expanders losses are more than large scale ones [4]. 2 Copyright 2014 by ASME

3 3) Higher cost of turbines compared with other types like reciprocating expander [11]. 4) In the case of waste heat energy conversion to electrical type using of turbine expanders is recommended [11]. 5) Turbine expanders are not useful for non-stable or transient operating conditions [11]. 6) Some of organic fluids are inflammable, explosive or expensive so for safety or economy sealing of turbines is so important. For organic fluids, operating medium is gas and it is suggested to use double mechanical seal. Hence special sealing, bearing and lubrication methods are needed for such high speed application. [2,11] 7) Condensate formation from wet expansion in one of the most important concerns of turbine types of expanders. Because appearing of droplets causes erosion effects and reduces life time of system [4]. Fig. 5: Operating map of radial inflow turbine for different mediums [2] A new turbine expander called Euler can be used as well especially for operating range of 250 KW [8]. This expander can work with lower rotational speed about less than half of radial type. In the case of moisture existence in operating medium, there is no harmful effect of Euler turbine performance. In other words, it is erosion resistance. This turbine has radial out flow that can be worked and coupled to the engine with no gear box [8]. Fig. 4: working principles of infinity turbine [1] 8) Due to bigger molar mass of organic fluids, their sound velocity is lower than steam. To reduce shock losses during design process, attention should be taken account to avoid supersonic appearing in nozzle outlet. [2] Also among different types of turbine expanders it should be considered which one is more recommended. In small scale applications more tendency is to use one-stage turbo-expander. Radial one-stage turbines are more appropriate due to some more reasons such as: 1) Through minor modifications standard radial inflow turbines can be optimized for different geothermal sources. [2] 2) Radial turbines are less sensitive to blade profile than other turbine types. In other words, in case of size decreases system can reach high efficiency also. 3) Radial type is more robust in case of load fluctuation especially whenever high density fluids as either subcritical or supercritical are used in system. 4) Manufacturing of this type is easier in contrast with axial or mixed types [2]. 5) The rotor dynamic stability of the system is also improved due to a higher stiffness. Fiaschi presented a reliable preliminary design of ORC radial expanders and the comparison of different working fluid behavior. Results showed that of the considered working fluids, R134a had the highest efficiency of 0.85 [17]. SCREW EXPANDER: Screw expander is a pair of helical rotors, one male and one female, their profile should be so accurate to trap the operating fluid. Such equipment is a positive displacement type. Manufacturing of screw expanders need special tools because allowable clearance or gap between rotors and casing is in the order of 50μm [1]. These expanders cover 20Kw-1Mw with ability to handle fluids from lit/s. Maximum reported rotational speed is about rpm. Today, screw expanders compared to scroll and reciprocating types have much larger technical maturity [6]. Fig.6: Screw expander main component [5] For more efficient operation of screw expanders below conditions are needed. 1) Max. flow rate formed between lobes and casing 2) Min. leakage 3) Correct volume ration of expansion 3 Copyright 2014 by ASME

4 4) Correct tip speed Ian K Smith, Nikola Stosic and Ahmed Kovacevic worked in their literature about details of optimized design [5]. Like turboexpanders, screw expanders have positive and negative specifications: 1) Because much smaller built in volume ratios are needed, the leakage of equipment decreases as well. But like other positive displacement expanders leakage one of the most critical aspects of its mechanical operation. 2) Unlike turbo-expanders, screw equipments can operate in 2-phase condition without efficiency drop and blades erosion [5]. 3) Screw expanders need no control-valves on inlet and outlet connections. 4) Operating of such equipments is presented in Figure 7 and 8 [6] 5) Very large radial and axial forces will apply on the rotor because of pressure difference between entry and exit. 6) Screw expanders can be 40-60% less expensive compared to turbine expanders. 7) Screw expanders are useful with low rotational speed and working with wet fluid. 8) If working medium has lubrication specification, there is no need to lubrication. 9) Due to low rotational speed, expander can be coupled to generators easily without gearbox. 10) Manufacturing of each screw and surface of each rotor hardening and material uniformity should be considered during fabrication processes [9]. 11) In addition to prevent dry working, screw material must be self lubricant type especially for oil-less operations. Or it must be used a fluid with oil characteristic in small scale applications. In other words wet medium is more suggested to use in contrast with dry fluids. Fig.8: Operating maps for screw expanders as per different operating medium [2, 6] For small scale applications there is a test for screw expanders in which maximum adiabatic efficiency specified around 53% and the main key to achieve high adiabatic efficiency with this type of expander is for built in volume ratio of the expander to be substantially less than the actual volumetric expansion ratio of the fluid being expander. So it is possible to enhance adiabatic efficiency of screw expanders up to 75% [9]. SCROLL EXPANDER: Scroll expander was patented in As it is clear in fig.7 this type of expander is composed of two scroll, one is fixed and acts like a casing, the other is orbiting. As it is obviously seen in figure.9 scroll expander has the most complicated geometry and very accurate and tight gap is needed during manufacturing process. With reference to compressors used for mobile air conditioner systems, the maximum rotational speed of scroll expander would be roughly 10000rpm [4,6]. Nowadays due to some positive point of scroll expander such as no need to valve, low parts count and more affordable prices compared to turbo- and screw expanders, small scale industries prefer to use it more and more. With lubrication of scroll wraps and journal bearings, scroll compressor can be adapted instead of scroll expander easily. There is capability to use this expander type in small or even micro scale applications (0.1-1 kw) [1]. Like other positive displacement devices, scroll expanders work with fixed volumetric ratio. This characteristic causes two types of losses which reduce system performance and efficiency. Fig. 7: Measured performance of different screw expanders [6] Based on previous more general studies, two working fluids were considered. R134a was selected as the best working fluid for brines in the 100o-120oC temperature range and Isobutane for brines in the o C temperature range [5]. Fig.9: Scroll expander with both fixed and orbiting scroll 4 Copyright 2014 by ASME

5 6) It is possible to act as an expander under two-phase medium condition. 7) Reciprocating expanders are not so sensitive to non-stable operating situation unlike turbine expanders. 8) Due to several static and dynamic parts not only the noise, vibration and durability of system should be modified but also the weight of expansion machine is heavy. 9) There are valve and torque impulses. 10) Because of a lot of movable parts, the reliability and balancing of this type are problems [2]. 11) Reciprocating expander can coupled to crank shaft directly. Fig.10: operating map of scroll expander as per different mediums [2] Scroll expanders due to a few number of movable parts have low level of noise and vibration, also high durability of unit and it is one the most important positive points. On the other hand manufacturing clearances are a critical parameter in sealing. Leakage is the most critical factor in scroll expander efficiency and operation. In addition friction and wearing should be considered and analyzed during design process. In summarize, scroll expanders have high efficiency and good performance under wide range of operating conditions. This means at the best controlled condition 83% for scroll expander efficiency can be achievable but the efficiency of scroll expanders mentioned 30-50% by Demirkaya. G [13]. Scroll expanders can work with wet fluids without any harmful effects. This type of equipment is not sensitive to variety of operating conditions[4]. Meanwhile compared to others scroll expander is inexpensive and readily due to its widespread use as air conditioner compressors [10]. RECIPROCATING PISTON EXPANDER: Reciprocating expander is a positive displacement equipment with three processes: intake, expansion and exhaust step. In contrast with other above mentioned expanders, reciprocating piston type is widely used in the exhaust pipe of internal combustion engines [2]. The maximum expander efficiency is expected to be 70% [4]. To know more about advantageous and disadvantageous reciprocating piston expanders, it is recommended to pay attention below specified characteristics: 1) This type is complex to design and manufacture. 2) It is expensive especially to manufacturing of various parts. 3) Reciprocating piston expanders have large friction losses because of the large number of interacting surfaces such as between piston rings, piston and cylinder wall. 4) In ORC system, by suing dissolving oil into working medium the impact of losses will be reduced [4]. 5) Achievable efficiency is not as much as turbine expanders. ROTARY VANE EXPANDER: The other type of positive displacement expander is rotary vane expander. The main parts of this expander is casing, rotor, vane slots and vanes (see Figure 11). Badr mentioned some positive characteristics for it like: 1) Its construction is simple to design and manufacture 2) Due to few number of internal part noise level and vibration is low. 3) High volumetric expansion ratio as large as ten is possible for rotary vane expanders. 4) This expansion machine can handle high pressure mediums. 5) Wearing and sealing are the most important factors in operating of rotary vane expanders. 6) Rotary vane expanders work with flat operating efficiency curves over a widespread range of conditions. 7) Due to working with low rotational speed, there is no need to gearbox to couple with engine or generator. 8) There is chance for system to work with two-phase medium. 9) Maintenance requirements of rotary vane expander is minimum compared to others. 10) This expansion machine is well-matched with organic working fluids 11) The main significant losses are because of breathing, internal leakage and friction [10]. 12) It is possible to mixed 5% oil with working fluid to lubricate expander during operating time. 13) With use of rotary vane expanders the durability, reliability would be in safe side. Inlet 1 2 Housing Rotor Vane Outlet 3 4 Fig.11: Rotary vane expander phases: 1-intake 2,3-expansion 4- exhaust [11] 5 Copyright 2014 by ASME

6 The best performance for rotary vane machines is reported by Mikielewic et al. [11] with isentropic efficiency of 84%. GEROTOR EXPANDER: Gerotor expansion consists of an outer stator and inner rotor with eccentric center. Firstly this expansion machine has simple construction rather than others. Beside this positive point, gerotors have the combination of rotary vane expanders and positive displacement advantageous with less friction losses. But there is a few number of document available about the use of gerotor in small scale ORC system [4,12]. Fig.12: Gerotor expander: 1-intake 2-expansion [4] There is an experimental literature about the available space between stator and rotor in gerotor expanders, which represents maximum 66%, 66% and 85% isentropic efficiency for large, insufficient and required space respectively [4]. SELECTION OF EXPANDER FOR VEHICLE: After consideration of several literatures it can be obviously found out there are a lot of applications for micro-scale ORC systems such as domestic, vehicle and institutional buildings application because of safety, useful effect on environment, simple operation and other positive merits. But this point should be considered except cost and efficiency other factors such as below mentioned items play an important role in expansion machine selection process: 1) Working temperature and pressure: the limitation for operating temperature and pressure of some expanders is around 200 C and 10bars respectively. But the limitation for rotary vane type is 7bar and 120 C. 2) Leaking: the loss of organic working fluid due to leaking is not acceptable. 3) Noise and vibration: according to environmental limitation there are some noise level for various areas, such as domestic, industrial, road and etc. so noise level should be our concern as well. 4) High expansion ratio: one of other important factors during selection of appropriate expansion machine should be high expansion ratio. 5) Power output range: Scroll and rotary vane expanders are likely to be suitable choices especially for small-scale application because not only these expanders work with high expansion ratio but also it is a chance to work in various range applications. In addition, from design and economy point of view, these expanders have simple design and are more affordable in power range of 1-10 kw [1,2]. if operating condition is lower than 2kw, rotary vane expander is more recommended. Based on previous experimental papers, mainly R123, R134a, R245fa, CO 2 are used in ORC systems. The isentropic efficiency of turbines are from 10% to 85% applicable. For turbine types expander in micro or small scale applications to reach higher efficiency higher rotational speed is needed from rpm to But there is a new patented gear with total gear ratio of 1:55 which can use so limited input speed around 4000rpm. This gear without any large pulley can achieve high output ratio around 220,000 rpm. It can couple with micro turbines very well because not only it is so compact and flexible to work, but also noise and vibration levels are so low. With the use of such compact and high ratio gear the problem of high rotational speed of turbine types is solved [15]. It seems in the case of turbine reduction cost this type of expansion machine can be the first option whenever an engineer wants to design small or even micro scale ORC system. There is a rough comparison available among different expanders in table 1. Table 1- comparison among various types of expanders [2] Expander Type Capacity Rotate Speed kw rpm Cost Radial inflow turbine High Scroll 1-10 <6000 Low Screw <6000 Medium Reciprocating piston Medium Rotary Vane 1-10 <6000 Low According to table-1, the capacity of scroll and rotary vane expanders are more well-matched with small or micro scale ORC systems. In spite of the fact that turbine expanders need high rotational speeds and are more sensitive to the quality of fluid (to operate under single-phase condition), there is no need to lubrication and there is no risk of wearing and such losses for turbines. It means the future of modified turbines which work in more stable conditions and coupled with new generation of gearboxes are so brilliant. This point should be taken into consideration that to increase the system efficiency, expander performance should be increase as well. All losses must be minimized [2] In this regard, the name of some manufacturer of different types of expanders with normal power ranges are specified in table 2 [3]. Table 2: list of manufacturers of various expanders [3] Company Expander type Power Output Turbo-expanders Ormat Axial, multi-stage 11.3MW Turboden Axial 1-7MW Cryostar Radial Inflow 500kW-12MW Pratt and Whitney Radial Inflow 280kW General Electric Radial Inflow 125 kw Freepower, UK Radial Inflow 2,3 stage 6*, 60*, 85*, 6 Copyright 2014 by ASME

7 120 kw Tri-O-Gen Radial Inflow kW Nelson, Cummins Radial Inflow 60kW Teng, AVL centrifugal 11.6 kw Positive Displacement Expanders Electratherm Twin- screw 65 kw Ener-g-rotors Gerotor kw Infinity Turbine Screw kw Eneftech Scroll kw Freymann et al., BMW Axial Piston 10 kw Endo et al., Honda Swash plate axial piston 2.5 kw Oomori and Ogino,Toyota Scroll 400w Table 4:Summary of operation condition of different expanders [4] In addition to predict expansion machines performance using dimensionless parameters specific speed-specific diameter diagram is so recommended. To compare all different types of expanders' performance especially among turbo-expanders, piston and rotary types N s -D s curves help designers effectively (Fig.13 [16]). Fig.13 : Expander performance map (specific speed-specific diameter operation curves of different expanders [16]) For low power output, positive displacement expanders are better than turbines. For more information about such expanders tables 3 and 4 are available below: Table 3: some example efficiencies of some positive displacement expanders [3] Studied by Expander Type Isentropic Efficiency Tahir Rotary multistage vane % Badr Rotary multistage vane 73% Ng Screw 73-85% Badr Screw 70% Mathias/Johnston Gerotor 45-85% Mathias/Johnston Scroll 50-83% Lemort Scroll 68% CONCLUSION: In spite of the fact that there are a lot of literatures about ORC system and its various applications in large and small scales, there is more need to investigate and do research to optimize available expander machines to reach a final decision. Micro-Scale expanders suitable for ORC systems are not available on the commercial market yet. After mass production and modification of expanders for such application make a decision will be better and more acceptable. REFERENCES: [1]. Qiu. G, Liu. H, Riffat. S (2011). " Expanders for micro-chp systems with organic Rankine cycle." Journal applied thermal engineering. [2]. Bao. J, Zhao. L. (2013). " A review of working fluid and expander selections for organic Rankine cycle." Journal applied thermal engineering. [3]. Cirincione. N (2011). " design, construction and commissioning of an organic Rankine cycle waste heat recovery system with a tesla-hybrid turbine expander." Master thesis of mechanical engineering department of Colorado state university [4]. Oralli. E (2010). " Conversion of a Scroll Compressor to an Expander for Organic Rankine Cycle: Modeling and Analysis" master thesis of the University of Ontario Institute of Technology 7 Copyright 2014 by ASME

8 [5]. Smith. I. K, Stosic. N and Kovacevic. A. " Screw Expanders Increase Output and Decrease the Cost of Geothermal Binary Power Plant Systems." City University of London [6]. Lemorta.V, Guillaumea. L, Legrosa. A, Declayea. S, Quoilina. S (2013). "a comparison of piston, screw and scroll expanders for small scale Ranikne cycle systems." Thermodynamics Laboratory, University of Liège, Campus du Sart Tilman, B49, 4000, Liège, Belgium [7]. Stosic. N, Smith. I.K and Kovacevic. A. " A twin screw combined compressor and expander for CO2 refrigeration systems" Centre for Positive Displacement Compressor Technology City University, London, EC1V 0HB, U.K. [8]. Rowshanzadeh. R. (2011). " Performance and cost evaluation of Organic Rankine Cycle at different technologies." Master thesis of KTH university of Sweden [9]. Stosic. N, Smith. I.K and Kovacevic. A. "Power recovery from low cost two-phase expanders." Centre for Positive Displacement Compressor Technology, City University, London EC1V 0HB, England [10]. Reid. A. D. (2010). "Low temperature power generation using HFE-7000 in a Rankine cycle" master thesis of San Diego State University [11]. Lopes. J, Douglas. R, McCullough. G and Richard O'Shaughnessy (2012). "Review of Rankine Cycle Systems Components for Hybrid Engines Waste Heat Recovery." Queen's Univ of Belfast, SAE international [12]. Weiss. D. K. (2011). "Comparison of the organic Rankine cycle and variable phase cycle for low-grade waste heat recovery." Master thesis of civil, energy and environmental engineering department, University of Calgary, Alberta, Canada [13]. Demirkaya. G. (2011). "Theoretical and Experimental Analysis of Power and Cooling Cogeneration Utilizing Low Temperature Heat Sources." PhD thesis of University of South Florida. USA [14]. Panesar. A. S. (2012). "A study of organic Rankine cycle systems with the expansion process performed by twin screw machines." Master thesis of City University, London, UK [15]. [16]. Kenneth E. Nichols P.E. "How to select turbomachinery for your application", Barber-Nichols Inc. [17]. Fiaschi D. Manfrida G. MaraschielloF. Thermo-fluid dynamics preliminary design of turbo-expanders for ORC cycles. Applied Energy 2012; 97: Copyright 2014 by ASME