Renewable Energy 48 (2012) 565e570 Contents lists available at SciVerse ScienceDirect Renewable Energy journal homepage: www.elsevier.com/locate/renene Selection of working fluids for micro-CHP systems with ORC Guoquan Qiu* 5 Stonebridge Blvd, Scarborough, ON M1W 4A8, Canada a r t i c l e i n f o a b s t r a c t Article history: Received 15 December 2011 Accepted 7 June 2012 Available online 9 July 2012 With depleting fossil fuel reservoirs and the corresponding emissions of air pollutants, we urgently need to look for alternative, renewable and sustainable energy options to counteract our strong dependence on fossil fuels for energy supplies. Solar thermal, geothermal, biomass combustion heat energy and waste heat recovery may be applied to drive combined heat and power (CHP) systems with organic Rankine cycle (ORC). Working fluid is a critical factor to affect the efficiency of the ORC, therefore optimal selection of the working fluid for an ORC-based system needs to be studied. This paper presents the comparison and optimization of 8 mostly-applied working fluids nowadays and gives a preferable ranking by means of spinal point method. The organic working fluids (organic refrigerants) for the medium-to-low temperature ORC have thermodynamic, environmental and economic criteria, such as preferable boiling temperature, high enthalpy drop (i.e., high thermal efficiency), favourable heat transfer characteristics, highly thermal and chemical stability, non-flammability, low toxicity, no ozone depletion and low cost. According to these criteria and spinal point method, the 8 mostly-applied organic working fluids are characterized and listed in order of preferable selection: HFE7000, HFE7100, PF5050, R123, n-pentane, R245fa, R134a and isobutene. An adequate organic fluid should be selected considering these selection criteria and the specific heat source. Ó 2012 Elsevier Ltd. All rights reserved. Keywords: Working fluid Organic Rankine cycle (ORC) Bucket effect Spinal point HFE7000 1. Introduction Electricity is a necessity and a sign of modern life, but currently around 1.5 billion people worldwide have no access to electricity and up to a billion more have access only to unreliable electricity networks [1,2]. Therefore, electricity generation is still in huge demand and traditional steam turbine generators driven by burning fossil fuels are still applied by far. It is widely believed that accelerated consumption of fossil fuels has caused many serious environmental problems such as air pollution, global warming, ozone layer depletion and acid rain. Accordingly, alternative sustainable energy sources, such as solar energy, geothermal energy and biomass energy, have phased in distributed electricity generation in a small-scale mode. However, the sustainable energy conversion technologies for electricity generation must confront the energy “trilemma” of balancing the demand for energy security, affordability and low-carbon generation, which influences their investment decisions and asset portfolio [2,3]. Basically the sustainable energy sources meet the two aspects of the energy “trilemma”, i.e., energy security and low-carbon generation. Price is still a bottleneck for developing sustainable energy products compared with conventional fossil fuel energy sources. In order to * Tel.: þ1 647 996 0886. E-mail address: [email protected]. 0960-1481/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.renene.2012.06.006 solve affordability of sustainable energy, improving working efficiency of sustainable energy products is a dominant approach to reduce product costs. Currently micro-scale combined heat and power (CHP) systems driven by sustainable energy have relatively low working efficiency, resulting from not only the lack of appropriate expanders in the commercial market [3] but also the need of optimal working fluid for ORC. Although less thermodynamically efficient than the idealized Carnot cycle, the Rankine cycle is practical and adaptable. Since a water steam Rankine cycle needs superheating to prevent turbine blade erosion, an organic Rankine cycle has the advantage with the use of organic fluids at lower temperatures and do not require superheating [4]. In order to improve the ORC performance, the selection of working fluids is of considerable importance in the ORCbased micro-CHP systems, since the fluids must have not only thermodynamic properties that match the application but also adequate chemical stability at the desired working temperature. The thermodynamic properties of working fluids will affect the system efficiency, operation and environmental impact. Hundreds of organic fluids are commercially available, but a few of them is fit for the micro-CHP systems with low temperature and low pressure. Accordingly, in the past decade many researchers have investigated the optimal selection of working fluids for ORC. Chen et al. screened 35 potential working fluids for ORC [5] and Saleh et al. gave a thermodynamic screening of 31 pure component working fluids for ORC using BACKONE equation 566 G. Qiu / Renewable Energy 48 (2012) 565e570 of state [6]. Of the 20 fluids investigated, ethanol, R123 and R141b appear as the most suitable for small-scale domestic CHP applications [7]. Considering different heat sources and diverse fluid types, the issue of working fluid comparison and selection for ORC has been widely investigated in the past decades [5e15]. However, these literature usually provide some fluid thermodynamic characteristic comparison and selection criteria for the low-temperature ORC. Some appropriate working fluids for ORC are only suggested but no ranking for the appropriate fluids is presented. In these adequate working fluid candidates, R123 seems to be widely chosen as working fluid by the investigators [11,16e19]. R245fa is also widely concerned as working fluid for ORC [14,20e23]. As a substitute of R12, R134a is applied in ORC by some researchers [14,20]. Some investigators examined PF5050 as working fluid for ORC [19]. Although n-pentane is extremely flammable, n-pentane is also used as working fluid [19,20,24]. The author in the University of Nottingham applied HFE7000 as working fluid for the biomass-fired micro-CHP system [25]. Basically, hydrocarbons such as pentanes or isobutanes, and refrigerants such as R123, R245fa and HFE7000 are good candidates for moderate and low temperatures (typically lower than 200 C) [26]. However, as a matter of fact, these investigators selected their working fluids without full reason provided. This paper presents the ranking for the appropriate working fluids for ORC utilizing bucket effect and spinal point method. 2. Working fluid evaluations 2.1. Working fluid types Although water steam cycles can offer higher pressure ratios and have better heat transfer properties than those of organic fluids, standard pressure and temperature at the inlet of turbine are 100 bars and 450 C [27] which result in safety problem in domestic applications. A large number of organic fluids have lower boiling point than that of water. Such an organic fluid is applied in a Rankine cycle to avoid higher temperature and pressure. Organic Rankine cycles generally refer to moderate-to-low temperature (<200 C) power cycles that operate with common refrigerants in Rankine cycles. The slope (dT/ds) of the saturated vapour curve of organic fluids in a Tes diagram can be negative (e.g. Ammonia), appropriate zero (e.g. R123) or positive (e.g. HFE7000) as shown in Fig. 1 (data from EES [28]), and the fluids are accordingly categorized into three groups: ➢ “Wet” fluids which have negative slope of the saturated vapour curve and are generally of low molecular mass, such as water M ¼ 18, ammonia M ¼ 17. ➢ “Isentropic” fluids which have nearly vertical saturated vapour curves and are commonly of medium molecular mass, such as R134a M ¼ 102, R245fa M ¼ 134 and R123 M ¼ 153. ➢ “Dry” fluids which have positive slope of the saturated vapour curve and are usually of high molecular mass, e.g. HFE7000 M ¼ 200 and HFE7100 M ¼ 250. “Wet” fluids usually need to be superheated prior to entering the expander, while “isentropic” and “dry” fluids do not need superheating, thereby eliminating the concerns of impingement of liquid droplets on the expander blades. Moreover, the superheated apparatus is not needed. Therefore, the working fluids of “dry” or “isentropic” type are more adequate for ORC systems. The organic working fluids for ORC generally refer to common refrigerants with low-temperature boiling points, such as chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs) and hydrofluoroethers (HFEs). The interim replacements for CFCs are HCFCs, which deplete stratospheric ozone, but to a much lesser extent than CFCs. The first C in CFCs and HCFCs represents chlorine that makes an ozone-depleting substance; CFCs and HCFCs are a threat to the ozone layer but HFCs and HFEs are not. Ultimately HFCs will replace HCFCs due to its zero value of ozone depletion potential (ODP). Also perfluorocarbons (PFCs) as high global warming compounds should be replaced with other alternative candidates [29], so PF5050 fluid, a PFC, should be carefully managed so as to minimize emissions. It should be concerned that HFC-134a (R134a) is currently being replaced by R1234yf because of its high Global Warming Potential (GWP). More than 150 fluorinated ethers were examined as alternatives to CFCs, HCFCs and PFCs [29]. HFE7000 is announced as a replacement for R123 due to its non-null Ozone Depletion Potential (ODP) e R123 will be phased out at the latest of 2030 depending on national legislations [26]. 2.2. Criteria for selecting organic working fluids The selection of working fluid for the ORC is critical since the fluid must have not only thermophysical properties that match the application but also meet safety requirement and economic cost. Criteria for selecting organic working fluids for a specific heat Fig. 1. Three types of working fluids: “dry”, isentropic and “wet” (data from EES). G. Qiu / Renewable Energy 48 (2012) 565e570 source (such as solar thermal [10,18,20,30e33], biomass-fired [9,25], geothermal [14,15,19], waste heat recovery [8,26,34e36]) have been presented in numerous studies [5e15]. Some general relevant characteristics may be extracted from those studies and selection criteria of organic working fluids are schematized in Fig. 2. Based on the bucket effect and spinal point evaluation approach to the selection criteria, an optimization fluid ranking for the moderate-to-low temperature ORC may be listed in this paper. In Fig. 2, the criteria are listed in order of importance from the top to the bottom. They are: 567 (1) Currently not phase out: low environmental impact (2) Working fluids should have high enthalpy drop through the expander since the high enthalpy drop means high power output and/or high efficiency. The two main parameters are Low GWP (2) High enthalpy drop Appropriate critical temperature 100-2000C (3) Easy to handle: Boiling temperature: 0-1000C (4) Non-superheating ORC: isentropic or “dry” (5) Large latent heat leads to small equipments Criteria of working fluids for ORC Thermally stable in the working temperature range (6) Preferable thermophysical properties Good heat transfer Low viscosity (1) The selected working fluids are currently not phased out by relevant national regulations. DuPont published its Suva refrigerant phase-out chart e General Replacement Guide: CFC to an HCFC; CFC or HCFC to an HFC, as shown in Fig. 3 [37]. Furthermore details are described by the new regulation EC 2037/2000, effect from 1st October 2000 [38]. Obviously, R11, R12, R13, R500, R502 and R505 in Fig. 3 had been phased out in 1996 and they should not been evaluated as working fluids for ORC [8,13,15,34,39]. Low ODP Low critical pressure Low or non-toxicity (7) High safety Low or non-corrosion Non-flammable (8) Good availability and low cost. Fig. 2. Selection criteria of organic working fluid for ORC. maximum and minimum process temperature (i.e., the given heat source and heat sink temperatures) [9]. With Rankinecycle engines, operating between maximum and minimum temperature limits of 120 C and 40 C, respectively, the conclusions are drawn from investigating the suitabilities of 68 potential working fluids [40]. The larger the temperature Fig. 3. DuPontÒ SuvaÒ refrigerant phase-out chart. 568 G. Qiu / Renewable Energy 48 (2012) 565e570 Fig. 4. Comparison of the working fluids: (a) “wet”, (b) isentropic and (c) “dry”. difference between the max and min temperature, the higher the enthalpy drop through the expander. (3) Working fluids can be easy to handle at ambient environment, so the boiling temperature is expected to be 0e100 C, which the medium-to-low temperature heat sources can provide. Sekiya et al. [29] provided 23 HFEs refrigerants with the boiling temperature in the range of 26e79 C. Only one of them HFE7000 was experimentally examined [25] and others may be expected to replace CFCs, HCFCs and PFCs [29]. Basically, the boiling temperature increases with increasing the critical temperature, as shown in the table by Guo et al. [15], accordingly the working fluid should have a critical temperature lower than 200 C that may make the boiling temperature lower than 100 C. (4) Isentropic or “dry” fluids are more adequate due to the nonsuperheating ORC. The organic Rankine cycle (ORC) is a non-superheating thermodynamic cycle that uses an organic working fluid to rotate an expander. The working fluid is pumped to an evaporator (1e2: pumping process) and heated to boiling (2e3: constant-pressure heat addition process), and the expanding vapour is used to drive an expander (3e4: expansion process). This expander can be used to drive a generator to convert the work into electricity. The working-fluid vapour is condensed back into a liquid (4e1: constant-pressure heat rejection process) and fed back through the system to do work again, as shown in Fig. 4 [11]. From the three ideal cycle comparison in Fig. 4, the expanding vapour at the exit of the expander is below the saturation vapour line in Fig. 4(a) which means impingement of liquid droplets on the expander blades. The expanding vapour at the exit of the expander is on and at the right side of the saturation vapour line in Fig. 4(b, c) (actual expansion process is an entropic increase and the vapour at the exit of the expander is at the upper right of the point 4). Therefore, isentropic or “dry” fluids are appropriate for ORC and a recuperator can be used to increase the ORC efficiency. If the “wet” fluids are applied, the vapour must be superheated at the expander inlet in order to avoid expander blade damages, which decreases cycle performance [16]. Superheating the vapour at the expander inlet will degrade the ORC performance because organic fluids lead to a higher cycle efficiency than water at low temperatures and/or small power plants [9], for instance, R123 cycle has a higher efficiency than water in the case of low temperatures [16]. (5) Large latent heat leads to small equipments at the pump, evaporator and condenser level. (6) Preferable thermophysical properties. Unlike water, organic fluids often suffer from chemical deteriorations and decomposition at high temperatures. The maximum heat source temperature is therefore limited by the chemical stability of the working fluid [41]. Large heat conductivity leads to small equipments at the heat exchanger level (i.e., evaporator, recuperator and condenser). Low viscosity leads to small pump equipment and its power consumption. Low pressures in ORC usually bring lower investment costs and decreasing complexity. (7) High safety requests low or non-toxicity, low or non-corrosion which is compatible with equipment materials and lubricating oil. Non-flammable fluids avoid explosion. (8) Good availability and low cost. Compared with system equipments, working fluid cost is small part of the entire investment [26] because the fluid is enclosed in ORC without the leaking loss. 2.3. Working fluid selection In order to select a suitable organic working fluid for ORC driven by a specific heat source, the bucket effect and spinal point method may be applied to balance the impacts of the above-mentioned criteria. The bucket effect indicates that water content filled in the bucket in Fig. 5 is determined by the shortest board of the bucket but not the any other. The bucket effect may be applied in selecting working fluids for ORC. The shortest board e one of the worst characteristic of a fluid may make the fluid not to be applied for ORC. For instance, some refrigerants, R11, R12, R13, R500 et al, have phased out due to their high ODP and are not permitted to use for ORC. Other fluid characteristics (criteria) have different impacts on fluid selection and spinal point method may be used to obtain total spinal points. 8 organic working fluids, which have been mostly applied and investigated in the past decade, are chosen to compare and evaluated. Their temperatureeentropy curves are put in one chart, as shown in Fig. 6. (Data from EES software [28]. PF5050 data is not available from EES. This chart corrects some errors of the fluid Tes charts in some literature, such as R123 [18], n-pentane [19,42], R134a [14,15]). At last a preferable fluid ranking Fig. 5. Bucket effect. G. Qiu / Renewable Energy 48 (2012) 565e570 569 Fig. 6. Tes curves comparison for mostly applied organic working fluids (data from EES). Table 1 Index indicators of the various working fluids for ORC (min 1 and max 5 spinal points. Spinal points in brackets) [14,15,29,35,43,44]. Working fluids R134a HFC-134a Isobutane R600a HC-600a R245fa HFC-245fa R123 HCFC-123 PF5050 PFC-5050 HFE7000 n-pentane R601 HC-601 HFE7100 Formula Molecular weight Type, x (¼ds/dT) Boiling T, C Critical T, C Critical P, MPa Latent heat, kJ/kg ODPa GWPb Flammable, AIT, Cc Toxicityd Thermal stability Total points Prefer ranking CH2FCF3 102 0.39 isentropic 26.3 (1) 101.5 (2) 4.06 (2) 155.4 (3) 0 (5) 1300 (2) NF 770 (5) A (5) Stable (5) 30 7 CH(CH3)3 58 1.03 “dry” 11.7 (2) 134.7 (3) 3.64 (3) 303.4 (5) 0 (5) 3 (4) Highly 460 (1) B (1) Stable (5) 29 8 CF3CH2CHF2 134 0.19 isentropic 15.3 (4) 157.5 (5) 3.64 (3) 177.1 (3) 0 (5) 1030 (2) NF 412 (5) B (3) Acceptable (4) 34 6 CHCl2CF3 153 0.120 isentropic 27.8 (5) 183.7 (4) 3.66 (3) 168.4 (3) 0.02e0.06 (4) 120 (4) NF 730 (5) B (3) Stable (5) 36 4 (CF3)2(CF2)3 288 >0 “dry” 30 (5) 150 (5) 2.13 (5) 88 (2) 0 (5) High (1) NF n/a (4) Low (5) Stable (5) 37 2 C3F7OCH3 200 >0 “dry” 34 (5) 165 (5) 2.48 (5) 142 (3) 0 (5) 370 (3) Yes 415 (4) Low (5) Stable (5) 40 1 C5H12 72 1.28 “dry” 36.0 (4) 196.5 (4) 3.36 (4) 349 (5) 0 (5) 20 (4) Highly 260 (1) A (3) Stable (5) 35 5 C4F9OCH3 250 1.83 “dry” 61 (4) 195.3 (4) 2.23 (5) 112 (2) 0 (5) 390 (3) Yes 405 (4) Low (5) Stable (5) 37 2 a b c d ODPdozone depletion potential. ODP for R11 ¼ 1.0. GWPdGlobal Warming Potential. GWP for CO2 ¼ 1.0. Preferably non-flammable (NF); slightly flammable (SF) may be acceptable. AITdAuto-ignition temperature. ASHARE standard 34: Adlower toxicity; Bdhigher toxicity. is achieved (preferable fluid first in order): HFE7000, HFE7100, PF5050, R123, n-pentane, R245fa, R134a and isobutene, as shown in Table 1 (boiling temperature increases from left to right). It should be pointed out that the current study of working fluid selection mainly aims at describing a methodology, rather than an accurate fluid selection for an ORC driven by a specific heat source. Therefore, the preferable working fluid ranking may be quite different in a specific case due to the difference of the required criteria. Tchanche et al. regarded R134a as the most suitable working fluid for small-scale solar ORC applications [10]. Hettiarachchi et al. suggested that the preferable working fluids are R123, n-pentane and PF5050 [19]. Quoilin et al. achieved that the thermodynamic optimization leads to the selection of working fluids with overall efficiency (highest efficiency first): n-butane, R245fa, R123, n-pentane, HFE7000, SES36, R134a, R1234yf [26]. Mikielewicz et al. [7] concluded that ethanol, R123 and R141b appear as the most suitable for ORC. 3. Conclusion Currently moderate-to-low temperature heat sources (e.g. solar thermal, geothermal, biomass and waste heat recovery) are used to drive household micro-CHP systems. An organic Rankine cycle (ORC) is commonly used in the micro-CHP systems since the ORC has the advantage with the use of organic working fluids at lower temperatures and low pressures. The working fluid selection is critical because an adequate organic fluid could greatly improve ORC performances. This paper reviews the working fluid selection criteria published in the past decade, and systematically illustrated and described these selection criteria. The selection criteria require the organic fluids to be low environmental impacts (low ODP and low GWP), favourable thermodynamic properties (high enthalpy drop through the expander, favourable boiling temperature, large latent heat, good heat transfer, low viscosity and good thermal stability), high safety (low toxicity, low corrosion and non-flammability) and low costs. The bucket effect is used to eliminate some high ODP and GWP refrigerants which have been phased out by UNFCCC and the Kyoto Protocol. The spinal point method is applied to evaluate each criterion. 8 types of the mostly-applied ORC working fluids nowadays are ranked by spinal point method (optimum first): HFE7000, HFE7100, PF5050, R123, n-pentane, R245fa, R134a and isobutene. It is remarkably noted that the current study presents a methodology of selecting working fluid for ORC. An adequate organic 570 G. 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