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Analysis of a solar Rankine cycle powered refrigerator with zeotropic mixtures
Solar Energy 162 (2018) 57–66
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Solar Energy journal homepage: www.elsevier.com/locate/solener
Analysis of a solar Rankine cycle powered refrigerator with zeotropic mixtures ⁎
T
⁎
Nan Zhenga, , Jinjia Weia,b, , Li Zhaoc a
School of Chemical Engineering and Technology, Xi’an Jiaotong University, Xi’an 710049, China State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an 710049, China c Key Laboratory of Efficient Utilization of Low and Medium Grade Energy (Tianjin University), MOE, Tianjin 300072, China b
A R T I C L E I N F O
A B S T R A C T
Keywords: Organic Rankine cycle Solar thermal energy Zeotropic mixture Refrigeration
A solar organic Rankine cycle (ORC) powered vapor compression cycle (VCC) for refrigeration is under investigation in this paper. To improve the overall system performance, zeotropic mixtures are proposed to be used in the integrated ORC-VCC system for the first time. A thermodynamic model is developed, and a total of eight pure fluids and five zeotropic mixtures with various compositions are evaluated and compared to identify the best combinations of fluids for yielding high system efficiencies. Besides, the influences of generating temperature, refrigerating temperature, superheating and internal heat exchanger (IHE) in ORC on the system performance are analyzed. For the ORC-VCC operating between −5 and 80 °C, dry fluid R600a shows the highest system efficiency (0.2212) among the pure fluids. For zeotropic mixtures, there exists a composition range within which binary mixtures always show higher system efficiency than the component pure fluids. Mixture R161/R600a with an R161 mass fraction of 0.25 shows the highest system efficiency (0.3089) among all fluids, which is increased by 39.6% and 54.7% comparing with R600a and R161, respectively. Adding IHE in ORC benefits the system efficiency, and the benefit is much more evident for dry fluids. Superheating makes wet fluids become applicable in ORC-VCC, but it becomes ineffective in improving the system efficiency when there is no IHE in system.
1. Introduction With the development of social economy, the need for cooling and refrigeration shoots up over the years, which contributes to the dramatic increase in the energy consumption worldwide. The application of solar thermal energy to refrigerating process has a great potential in reducing the fossil fuels consumption and alleviating environmental issues. Solar assisted air-conditioning/refrigerating system is particularly attractive to regions where the insolation supply and the need for refrigeration reach to maximum levels at the same period. Solar thermal energy could be converted into cooling process by use of either the absorption/adsorption refrigeration cycle or the thermomechanical cooling system (Zeyghami et al., 2015). Although the single effect absorption chillers are still the dominant technology in solar cooling, interest in thermo-mechanical cooling systems has been revived recently due to the appearance of new efficient and environmentally friendly refrigerants (Park et al., 2015) and advancements in organic Rankine cycle equipment (Quoilin et al., 2011), as well as the slow development in breaking the operating limitations on absorption chillers (Xu et al., 2013). ⁎
Organic Rankine cycle powered vapor compression cycle (ORCVCC) is one of the most common way to fulfill the thermo-mechanically activated cooling production. Comparing with absorption refrigeration cycle and ejector cooling cycle, the ORC-VCC system has advantages such as: ability to convert heat into electricity when cooling is not needed by coupling the expander with an electric generator, and ability to supply cooling in remote area where the grid cannot reach by coupling the expander and the compressor directly. Great efforts have been devoted to the development of the ORC-VCC system since its concept was proposed. Prigmore and Barber (1975) established and tested the first prototype which used separate power and cooling cycles (termed as separate configuration). The dual fluid system was powered by solar and occupied R113 and R12 as the working fluid in ORC and VCC, respectively. A maximum system COP of 0.5 was obtained with the water temperature at the solar collector outlet being 102 °C. Bu et al. (2013) conducted a performance analysis and working fluids selection of a solar powered ice maker, with the power cycle and cooling cycle operated with the same working fluid (termed as single fluid ORC-VCC). In terms of overall efficiency and ice production capacity, R123 was regarded as the most suitable working
Corresponding authors at: School of Chemical Engineering and Technology, Xi’an Jiaotong University, Xi’an 710049, China. E-mail addresses: [email protected] (N. Zheng), [email protected] (J. Wei).
https://doi.org/10.1016/j.solener.2018.01.011 Received 4 July 2017; Received in revised form 24 November 2017; Accepted 6 January 2018 0038-092X/ © 2018 Elsevier Ltd. All rights reserved.
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Nomenclature COPVCC CPRm CFCs HCs HCFCs HFCs HFOs IHE NBP WRm SHD SCD cP h I m q Qrefrig r
s T ΔTg W x ηETC ηORC ηoval ηs ζ
refrigeration cycle coefficient of performance (–) refrigeration capacity per unit mass flow rate (kW s kg−1) chlorofluorocarbons hydrocarbons hydrochlorofluorocarbons hydrofluorocarbons hydrofluoroolefins internal heat exchanger normal boiling point (°C) output net power per unit mass flow rate in ORC (kW s kg−1) superheating degree (K) subcooling degree (K) specific heat at constant pressure (kJ kg−1 K−1) specific enthalpy (kJ kg−1) solar radiation intensity (kW m−2) mass flow rate (kg s−1) vapor quality (–) refrigerating capacity (kW) latent heat of vaporization (kJ kg−1)
specific entropy (kJ kg−1 K−1) temperature (°C) temperature glide (K) power (kW) mass fraction (–) collector efficiency (–) thermal efficiency of ORC (–) overall system efficiency (–) isentropic efficiency superheating parameter (–)
Subscripts 0 crit cond evap exp in m out
ambient critical condenser, condensing evaporator, evaporating expander inlet mean outlet
Generally, the dual fluid ORC-VCC with separate configuration has higher operational flexibility than single fluid system, but it requires better shaft seals to prevent leakages and suction problems between the expansion and compression sections. The choices of the appropriate working fluid is the most preliminary step of the system design for ORC-VCC. The ideal fluids should meet several basic criteria such as environmental friendliness, safety, stability and energy efficiency. According to the literature review, CFCs (e.g. R113), HCFCs (e.g. R123) and HFCs (e.g. R134a) are the most frequently used working fluids in the previous studies. However, these working fluids either has been, or will be, phased out due to their negative impact on environment. In recent researches, HCs (e.g. R290) and HFOs (e.g. R1234ze) are proposed and investigated as possible alternatives to the CFCs, HCFCs and HFCs. Although HCs and HFOs could be regarded as eco-friendly fluids, flammability and chemical instability become another two arguments against their application in ORC-VCC, respectively. All of the existing studies are focused on pure component working fluids for ORC-VCC system, but it should be pointed out that there are no pure fluids available at present that completely meet all of the criteria.
fluid for their system. Chang et al. (2017) proposed a hybrid residential micro-CCHP system based on proton exchange membrane fuel cell and solar energy. A dual fluid ORC-VCC with dimethylpentane and R290 being used as the working fluid in ORC and VCC, respectively, was adopted to generate power and cooling/heating simultaneously. To reduce the system complexity, Aphornratana and Sriveerakul (2010) proposed a novel configuration of single fluid ORC-VCC system (termed as integrated cycle) in which the power cycle and cooling cycle shared a common condenser. Three different refrigerants, including R123, R134a and R245ca, were evaluated to find the best candidate for the novel cycle. Wang et al. (2011) attempted to couple the ORC and VCC with a free-piston expander-compressor unit to minimize the power transmission loss and simplify the mechanical design further. The proposed system was able to produce cooling temperature of 0 °C with a low grade thermal energy as low as 60 °C. In addition to the above studies, several other researchers also reported the performance of the ORC-VCC with separate power and cooling cycles either using single fluid (Karellas and Braimakis, 2016; Kim and Perez-Blanco, 2015; Li et al., 2013; Wu et al., 2017; Yılmaz, 2015; Yue et al., 2016) or dual fluid (Molés et al., 2015; Nasir and Kim, 2016; Wang et al., 2011).
Fig. 1. Schematic diagram of the solar powered ORC-VCC refrigerator.
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fluids for an ORC-VCC system could be relatively restricted. Using refrigerant mixtures, however, provides a practical way to greatly broaden the options (Wang and Zhao, 2009). Following this introduction, a total of eight common refrigerants, including two HCs (R290 and R600a), four HFCs (R161, R152a, R134a and R227ea) and two HFOs (R1234yf and R1234ze), are taken as the original components. All of the selected pure refrigerants satisfy the hard environmental standard on ODP. Table 1 lists some of the basic thermodynamic information of each refrigerant. The information on GWP as well as safety group of each refrigerant is obtained from Calm and Hourahan (2001), and other properties are derived from NIST REFPEOP (Lemmon et al., 2010). Depending on whether the superheating parameter ζ (a dimensionless number representing the shape of temperature-entropy characteristics) is greater than unit value or not (Zheng et al., 2016), the component refrigerant is distinguished as wet fluid or dry fluid, as shown in the table. When the eight components pair together, a total of 28 binary fluid combinations could be obtained. Since the application of zeotropic binary mixtures in ORC-VCC is proposed herein, only the combinations showing obvious temperature glide (> 5 K) within a broad composition range are considered. As a result, five combinations are finally screened out, as shown in Table 2. In this paper, the thermodynamic properties of the mixtures are calculated based on NIST REFPROP (Lemmon et al., 2010), which provides relatively accurate mixture models for R290/R600a, R152a/R600a, R227ea/R600a and R1234yf/R600a. Due to the lack of test data, the parameters associated with the mixing rule for R161/R600a are actually estimated values, which may lead to relatively large calculation uncertainties on the fluid’s properties.
Using fluid mixtures provides a potential solution to the issues mentioned above. For example, using low volatile HCs as an additive with HCFCs is expected to reduce the flammability of HC mixture, and the short atmospheric lifetime of HCs helps to reduce the mixtures’ GWP (Harby, 2017) at the same time. Another advantage of using fluid mixtures is that the cycle performance could be improved by matching the temperature profiles between the heat transfer medium and zeotropic mixture that presents non-isothermal phase transition process (Heberle et al., 2012). However, none of the previous publications report the performance of ORC-VCC operating with fluid mixtures, especially zeotropic mixtures. To fill up this research gap, in this paper, we concentrate on verifying the potential benefits of zeotropic mixtures on the ORC-VCC system. A steady state model of a solar powered ORCVCC refrigerator is developed, based on which a total of eight pure fluids, including HCs (R290 and R600a), HFCs (R161, R152a, R134a and R227ea) and HFOs (R1234yf and R1234ze) are evaluated as component fluids. Afterwards, five zeotropic mixtures containing R600a are screened out and compared to identify the suitable combinations of fluids which may yield high system efficiencies under a variety of working conditions and cycle configurations. 2. System description Fig. 1 shows the schematic diagram of the solar driven ORC-VCC refrigerator, which mainly consists of solar collector, expander, IHE, feeding pump for power cycle, throttling valve, evaporator, compressor for refrigeration cycle, and a shared condenser for both ORC and VCC. This system has an integrated configuration and uses the same working fluid in both cycles to minimize the problem associated with leaking of the working fluids. The working principle of this system is described briefly below. A portion of liquid working fluid coming from the condenser is pumped to the solar collector, where the liquid absorbs heat and completely vaporizes. Thus, the solar collectors could be regarded as a vapor generator. Superheating of the vapor at the outlet of solar collector is necessary to avoid harmful liquid slugging in the downstream expander. The superheated vapor expands in the expander and generates shaft work to drive the compressor. IHE is usually installed between the expander and condenser in an ORC to reclaim the heat possessed by the expander exhaust. The exhaust cools down in the IHE by transferring the heat to the pressurized liquid from the feeding pump. Meanwhile, the other part of the liquid, after being depressurized in the throttling valve, is directed to the evaporator to vaporize. Then the low temperature vapor is pressurized in the compressor driven by the ORC expander. In order to improve the drive efficiency, the shafts of the expander and the compressor are directly coupled. Thereafter, the expander exhaust mixes with the compressor exhaust, and the combined vapor flow is condensed to subcooled liquid in the condenser to complete the cycle. The T-s diagram of the solar powered ORC-VCC refrigerator with IHE is illustrated in Fig. 2. For the system without IHE, the states points 2′ and 4′ will not appear. It should be noted that the state points 4 and 4′ might be in either the two-phase region or the vapor region depending on the fluid’s nature and the superheating degree (SHD) at the outlet of the solar collectors.
4. Thermodynamic analysis 4.1. Cycle modeling To evaluate the performance of the solar driven ORC-VCC refrigerator, a steady state thermodynamic model is developed, considering the following assumptions: (1) Solar collectors are working in steady state and have uniform radial temperature distribution. (2) The heat and friction losses in heat exchanger and piping are neglected. (3) All heat exchangers are countercurrent type. (4) No leakage loss in system and zeotropic mixtures have constant circulating compositions.
3. Working fluids for ORC-VCC One of the main thermodynamic properties should be considered in selecting working fluid for an integrated ORC-VCC system is the shape of the vapor-liquid phase envelope. According to the different slopes of the saturation vapor curve in the T-s diagram, working fluids are categorized as dry (ds/dT > 0), wet (ds/dT < 0) and isentropic (ds/ dT = 0), from ORC’s point of view (Hung et al., 1997). To avoid formation of droplets in expander and compressor, dry and isentropic fluids are well adapted to ORC, while wet and isentropic fluids are more favorable for VCC. Considering the great differences in working conditions for power and refrigeration, the range of available working
Fig. 2. T-s diagram of the solar powered ORC-VCC refrigerator.
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Table 1 Candidate fluids and their basic properties. Fluid
Molecular mass [g mol−1]
Tcrit [°C]
Pcrit [MPa]
NBP [°C]
GWP [100 yr]
Safety group
ζ [–]
R290 R161 R152a R134a R600a R227ea R1234yf R1234ze
44.096 48.06 66.051 102.03 58.122 170.03 114.04 114.04
96.74 102.1 113.26 101.06 134.66 101.75 94.7 109.36
4.2512 5.01 4.5168 4.0593 3.629 2.925 3.3822 3.6349
−42.114 −37.55 −24.023 −26.074 −11.749 −16.34 −29.45 −18.973
3 12 124 1430 20 3220 4 6
A3 A3 A2 A1 A3 A1 A2L A2L
1.1098 1.3925 1.2658 1.1006 0.8338 0.7395 0.9222 0.9368
ζ is calculated at the same reduced temperature of 0.8 for each fluid. Table 2 Screened out binary fluid combinations.
ηcomp,s =
Combination
Fluid type
Chemical composition
Mixing rule
R290/R600a R152a/R600a R161/R600a R227ea/R600a R1234yf/R600a
Wet/Dry Wet/Dry Wet/Dry Dry/Dry Dry/Dry
HC/HC HC/HFC HC/HFC HC/HFC HFO/HC
Validated Validated Non-validated Validated Validated
Tm−T0 (T −T )2 −0.0199 × m 0 I I
(mORC + mVCC )·h8 = mORC ·h4 + mVCC ·h7
(mORC + mVCC )·h8 = mORC ·h 4′ + mVCC ·h7
(5) (6)
(7)
(8)
Wexp−Wpump mORC
Parameter
Typical value
Range
Parameter
Typical value
Range
mORC [kg s−1] Texp,in [°C] Tevap,in [°C] Tcond,sv [°C] SHDevap,out [K] T0 [°C]
1 80 −5 40 5 35
– 60–120 −20–10 – – –
SHDexp,in [K] SCDcond,out [K] ηexp,s [–] ηpump,s [–] ηcomp,s [–] I [W m−2]
5 5 0.85 0.9 0.8 1000
0, 5 – 0.85–1 – – –
Wexp−Wpump Qgen
(10)
For the VCC:
Wcomp = Wexp
(11)
Wcomp = mVCC ·(h6−h7)
(12)
(19)
Table 3 Input parameters and boundary conditions.
(9)
The thermal efficiency of the ORC is defined as:
ηORC =
(18)
Based on the above assumptions and equations, a computer program in Matlab 2016.b is developed to simulate the thermodynamic performance of the ORC-VCC with various working fluids. The input values of several design parameters and boundary conditions are listed in Table 3. The mass flow rate of working fluid in ORC is set to unit value for all calculations, and thus the mass flow rate in VCC can be determined from Eq. (12). The lowest temperature (Tevap,in) and the highest temperature (Texp,in) of the system is set to be the same for various pure fluids and mixtures. Considering the temperature glide of zeotropic mixtures, the saturated vapor temperature is chosen as a basis and keeps invariable at 40 °C, which corresponds to the typical summer outdoor temperature about 35 °C. For the system with an IHE, the low temperature liquid from the feeding pump is supposed to be heated to the same temperature of the expander exhaust. To avoid the cavitation in the ORC feed pump, the degree of subcooling (SCD) of 5 K at condenser outlet is set. Under the standard operation condition, the degree of SHD at the outlet of evaporator and solar collector is set to 5 K,
The net power output per unit mass flow rate of working fluid of the ORC:
WRm =
Qrefrig mORC + mVCC
ηoval = ηETC ·ηORC ·COPVCC
Solar energy input with IHE:
Qgen = mORC ·(h3−h2′)
(17)
The refrigerating capacity per unit mass flow rate of working fluid of the ORC-VCC:
Solar energy input without IHE:
Qgen = mORC ·(h3−h2)
(16)
Energy balance in mixer (with IHE):
(2)
(4)
h4−h 4′ = h2′−h2
(15)
The overall efficiency of the ORC-VCC refrigerator is defined as:
Wpump = mORC ·(h2−h1) h2−h1 h2,s−h1
Wcomp
For the ORC-VCC: Energy balance in mixer (without IHE):
(1)
(3)
ηpump,s =
Qrefrig
COPVCC =
CPRm =
h3−h4 h3−h4,s
ηexp,s =
(14)
The COP of the VCC is defined as:
For the ORC:
Wexp = mORC ·(h3−h4 )
(13)
Qrefrig = mVCC ·(h6−h5)
The basic equations describing the ORC-VCC system are given below. For solar collectors: Evacuated tube collectors (ETC) are used for solar heat collecting, considering their relatively higher performance than flat plate collectors (FPC) under low and medium temperatures (Sokhansefat et al., 2018). The models of heat-collecting efficiency of ETC is described by Eq. (1) (Li et al., 2016).
ηETC = 0.721−0.89 ×
h 7,s−h6 h 7−h6
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the most suitable working fluid for the solar powered ORC-VCC refrigerator under the given working conditions. However, the relatively higher NBP of R600a would become a restriction for its application in low temperature refrigeration (Tevap < −10 °C). On this condition, R600a should be mixed with the low boiling component, such as R290 and R1234yf, to assure a sufficiently large positive pressure in the evaporator.
respectively, to prevent liquid slugging in the downstream machines, respectively. According to the literature (Bu et al., 2013), a constant isentropic efficiency of 0.85, 0.8 and 0.9 is assumed for expander, compressor and feed pump, respectively. 4.2. Model validation Due to the lack of studies on ORC-VCC running with zeotropic mixtures, the validation of the present calculating program is conducted based on the results of pure refrigerants. The simulation results from Li et al. (2013) are used as the standard for comparisons because of the similar integrated cycle configuration. Since the ORC is driven by a boiler instead of solar collectors in the reference paper, the product of COPVCC and ηORC is used as the index parameter for comparison. It should be noted that the temperature conditions and various isentropic efficiencies are set to the same values as in the reference paper, respectively, when calculating cycle efficiencies using the present model. Fig. 3 shows the comparison in the product of COPVCC and ηORC between the present work and the reference, based on various generation temperatures. The comparison results indicate that the deviation between the calculated efficiencies and the reference values are small, with the mean absolute deviation being 2.72% and 0.30% for R600a and R290, respectively.
5.2. Analysis on zeotropic mixtures The overall system efficiency as a function of mixture composition is plotted for each of the screened out binary mixtures, as shown in Fig. 5. The composition herein is represented by the mass fraction of the first component (x1) in each mixture. It should be noted that all of the first component fluid in each mixture has lower NBP than R600a. It is apparent from Fig. 5 that for all the mixtures, ηoval lies greatly on their compositions, and it firstly increases and then decreases with the increase of mass fraction of the low boiling point component in the mixtures. Mixture R161/R600a (x1 = 0.25) comes top in overall system efficiency with a maximum ηoval of 0.3089, followed by R152a/R600a (x1 = 0.25), R290/R600a (x1 = 0.45), R227ea/R600a (x1 = 0.45) and R1234yf/R600a (x1 = 0.45), respectively. Comparing with pure R161 and R600a, the maximum ηoval of mixture R161/R600a is increased by 54.6% and 39.6%, respectively. For any binary fluid combinations, there exists a composition range within which the mixtures always show higher ηoval than their component fluids. For example, ηoval of the R161/R600a remains higher than that of R600a until xR161 exceeds 0.75. We should also take notice of the rebound and fluctuation in ηoval as x1 approaches to 1 for the mixture R152a/R600a and R227ea/ R600a. The key factors determining the variation trend of ηoval versus the mixture composition will be analyzed in the following part. Fig. 6 shows the change of temperature glide (ΔTg) with respect to the mixture composition for each binary mixture. For any mixtures, system pressures has little influence on the variation trend of ΔTg, which could be reflected by the data shown in Table 4; hence, the temperature glides under condensing pressure (ΔTgcond) are taken as the representative and illustrated in Fig. 6. According to Figs. 5 and 6, the variation tendency of ΔTg is remarkable similar to that of ηoval: the overall system efficiency rises with an increasing ΔTg, and the two parameters reach to the maximum value at the same composition. One of the benefits of temperature glide lies in that it helps to reduce the temperature lift between heat source and sink, when the highest and lowest temperature in the cycle are specified, thereby improving the cycle performance. The average temperature lift between the heat source and sink of the ORC changes little with the mixture composition,
5. Results and discussion 5.1. Analysis on pure fluid The comparison of cycle performance among the eight candidate refrigerants is illustrated in Fig. 4. Fig. 4(a) shows the calculated ηORC and COPVCC for different component refrigerants under the typical working condition, based on the system with IHE. It is obvious from the figure that both ηORC and COPVCC depend largely on the nature of the working fluid. In the case of ηORC, the dry refrigerants present, on the whole, higher performance than wet refrigerants. Dry refrigerant R600a, with the value of ηORC being 0.0816, ranks first among the eight candidates, followed by dry refrigerant R227ea, R1234ze and R1234yf. The value of ηORC is comparable to each other for the four wet refrigerants, with the maximum and minimum value being 0.0744 (R152a) and 0.0723 (R290), respectively. In the case of COP, however, we cannot easily generalize a conclusion that the wet fluids are better than dry fluids, or vice versa. As shown in Fig. 4, wet refrigerant R152a shows the highest refrigerating performance with a COPVCC of 4.067, followed by dry fluid R600a (COPVCC = 4.037) and wet fluid R161 (COPVCC = 3.989). Dry fluid R1234yf and R227ea, with COPVCC being 3.801 and 3.710, respectively, are in the bottom two places. In addition to the temperature-entropy characteristic, the performance of VCC can be affected by other properties, such as specific heat (cp) and latent heat of vaporization (r). When the evaporating temperature is given, the refrigerating COP usually decreases with increasing cp/r (Ma et al., 2017), which could explain why dry fluid R600a shows higher COPVCC than some wet fluids. Fig. 4(b) illustrates the calculated ηETC and ηoval for the eight candidate refrigerants. Like the ηORC and COPVCC, the collector efficiency is also affected, to a certain degree, by the refrigerants’ nature. Generally, wet refrigerants show higher collector efficiency than dry refrigerants, with the maximum and minimum ηETC being 0.6860 (for R152a) and 0.6707 (for R227ea), respectively. According to Eq. (1), the low value of ηETC could be caused by the high mean temperature of the working fluid inside the collectors. This result indicates that using dry fluids helps to increase the mean generation temperature for the solar powered ORC-VCC when an IHE is coupled in the system, which will be further explained in Section 5.4. As to the overall system efficiency, dry refrigerant R600a shows the highest value of 0.2212, followed by R152a (ηoval = 0.2076) and R1234ze (ηoval = 0.2072), respectively. Hence, from the ηoval point of view, dry fluid R600a could be regard as
Fig. 3. Calculation deviation in product of COPVCC and ηORC.
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Fig. 6. Variation of ΔTg versus mixture composition.
slight positive pressure means a high risk of air suction by the compressor. The application of R290, however, will lead to a high pressure (2849.31 kPa) in the solar collector and expander, which inevitably increases the operation risk and manufacturing cost of the ORC-VCC system. Comparing with the two component fluids, the R290/R600a mixtures show much more appropriate pressure levels. According to the calculated ζ, the mixture R290/R600a will changes gradually from dry fluid to wet fluid, and becomes isentropic fluid at certain composition, as xR290 in the mixture increases from 0 to 1. It is interesting to note that for the combination of R290 and R600a, the formed isentropic mixture also possesses the largest ΔTg. This isentropic zeotropic mixture (xR290 = 0.45) shows the highest output net power, refrigerating capacity and overall system efficiency among all of the R290/R600a mixtures. Fig. 7 shows the variation trend of WRm versus the mixture composition for each mixture. WRm refers to the ratio of Wnet to mORC, which reflects the power capability of working fluid per unit mass flow rate. As shown in Fig. 7, R600a shows the largest WRm (about 26.9 kW s kg−1) among the six component fluids, under the typical working condition. However, the change trends of WRm with x1 vary greatly among different fluid combinations. Overall, for the dry/dry combinations, WRm keeps going down as the R600a mass fraction decreases, while for the wet/dry combinations, non-monotonic curves of WRm are observed. Specifically, WRm increases firstly and then decreases with an increasing xR290 for R290/R600a mixtures, reaching to the maximum value of 28.04 kW s kg−1 at xR290 of 0.4, while for R152a/R600a mixtures, an opposite trend is exhibited. For the R161/ R600a mixture, the maximum and minimum value of WRm appears successively under certain compositions as the mass fraction of R161 increases. The variation tendency of WRm is determined by two factors, the expander output power and the power consumption of pump, both of which are closely related to the nature of working fluid. Taking R152a/R600a as an example, the output power decreases steadily while the power consumption first increases and then decreases with an increasing xR152a, thereby resulting in a trend from decline to rise for WRm. For the dry/dry combinations, however, the influence of power consumption on WRm could be neglected due to the significant reduction in output power as xR600a drops from 1 to 0. Fig. 8 shows the variation of CPRm, the index reflecting the refrigerating capacity of unit mass working fluid in ORC-VCC, versus x1 for each mixture. On the whole, the trends of CPRm are similar to that of WRm, excepting that a maximum CPRm appears as xR152a increases to 0.2 for R152a/R600a mixtures. Mixture R161/R600a (xR161 = 0.25) and R290/R600a (xR290 = 0.5) have the highest and second highest CPRm, being 103.33 kW s kg−1 and 99.91 kW s kg−1, respectively. The CPRm of R161/R600a is greater than that of R290/R600a within the
Fig. 4. Candidate refrigerants performance: (a) ηORC and COPVCC; (b) ηETC and ηoval.
Fig. 5. Variation of ηoval versus mixture composition.
in the case where the approximate maximum values of temperatures in solar collector and condenser are determined. As shown in Table 4, ηORC decreases steadily with an increasing xR290, while COPVCC presents nonmonotonic dependence on the mixture composition, within the range of xR290 from 0.3 to 0.6. This indicates that the variation trend of ηORC shown in Fig. 6 also reflects the variation of COPVCC, which is greatly affected by the temperature glide. It can be easily found in Table 4 that the system pressures decrease gradually with the increase of xR600a. Under the typical working conditions, the evaporating pressure is about 130.98 kPa for R600a. Such a 62
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Table 4 Simulation results for R290/R600a. R290/R600a
xR290 = 0
xR290 = 0.3
xR290 = 0.45
xR290 = 0.6
xR290 = 1
Pgen [kPa] Pcond [kPa] Pevap [kPa] T1 [°C] T4 [°C] q4 [–] q4′ [–] q5 [–] T6 [°C] ΔTggen [K] ΔTgcond [K] ΔTgevap [K] mvcc/(morc + mvcc) [–] ζ [–] Wpump [kW] Wexp [kW] Qgen [kW] Qevap [kW] ηorc [–] COPvcc [–] ηETC [–] ηoval [–]
1210.68 531.21 130.98 35.0 54.0 1.0868 0.9343 0.2648 0 0 0 0 0.2964 0.8487 1.40 28.37 330.6 114.53 0.0816 4.04 0.6718 0.2212
1551.13 688.83 210.52 28.6 51.1 1.0695 0.8895 0.2185 5.3 5.2 6.4 7.3 0.3226 0.9526 1.81 29.76 336.05 143.33 0.0832 4.82 0.6749 0.2703
1758.15 788.03 252.67 28.0 49.6 1.0605 0.8881 0.2151 6.2 5.7 7.0 8.0 0.3257 1.0009 2.07 30.09 338.09 148.07 0.0829 4.92 0.6765 0.2759
1996.99 905.86 294.78 28.6 47.9 1.0508 0.8959 0.2205 6.1 5.2 6.4 7.3 0.3213 1.0533 2.37 30.11 339.25 145.54 0.0818 4.83 0.6783 0.2681
2849.31 1369.42 406.34 35.0 42.8 1.0208 0.9610 0.2756 0 0 0 0 0.2735 1.2270 3.44 27.56 333.57 107.31 0.0723 3.89 0.6833 0.1924
Fig. 9. Variation of ηoval and CPRm versus Tgen,out.
Fig. 7. Variation of WRm versus mixture composition.
5.3. Effect of generating and evaporating temperatures In this section, R290/R600a is used as a representative to illustrate the effect of Tgen,out and Tevpa,in on system performance. Fig. 9 reveals the influence of Tgen,out on ηoval and CPRm, under the condition that Tevpa,in and Tcond,sv is fixed at −5 °C and 40 °C, respectively. As expected, ηoval is improved with increasing the generating temperature in solar collector, due to the increase of ηORC. According to Fig. 9, ηoval increases from 0.1380 to 0.4128, as Tgen,out varies from 60 to 120 °C. Also, it can be found that the CPRm keeps going up with the increase of Tgen,out, and its value is increased by nearly 156.9% as the Tgen,out changes from 60 to 120 °C. The increase of CPRm versus Tgen,out could be caused by the increase in the mass flow rate of working fluid in VCC. Due to the increase of Tgen,out, the specific work output of expander improves accordingly, which means more working fluids could be saved from the power cycle and used for refrigerating. With the increase of Tgen,out, the mixture composition corresponding to the maximum ηoval and CPRm presents a trend of decrease in xR290, respectively, as shown in Fig. 9. When the evaporating and condensing temperatures of ORC are specified, the working fluids with higher Tcrit usually shows higher thermal efficiency (Liu et al., 2004). For mixture R290/R600a, Tcrit is expected to increase with an increasing xR600a, but on the other hand,
Fig. 8. Variation of CPRm versus mixture composition.
range of x1 from 0 to 0.4, due to that in the R161/R600a system, the proportion of working fluid used for refrigeration is higher than that in the R290/R600a system, before x1 reaching to 0.4. 63
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(ζ = 1.091), with an ηexp,s of 0.85. In the case of isentropic expansion, the mixtures with xR290 greater than 0.5 (ζ > 1.02) cannot be used for the sake of safety. Evidently, adding the superheating in the ORC helps to make more zeotropic mixtures become applicable in the ORC-VCC system. Fig. 14 illustrates the variation tendency of ηoval versus mixture composition under various IHE and superheating conditions, on the basis of R290/R600a. It can be seen that, the value of ηoval first increases and then decreases with an increasing xR290, and reaches the maximum value at xR290 = 0.45, for all of the four cases. According to the figure, when there is an IHE in system, the ηoval could be enhanced greatly by superheating under arbitrary mixture composition. The relative increment in ηoval firstly increases from 2.3% to 4.1% (xR290 = 0.75) and then decreases to 3.0%, as xR290 increases from 0 to 1. The benefit of superheating, however, is not evident for the case without IHE. For the cycle configuration without IHE, superheating the working fluid in the solar collectors could lead to a slight decrease in ηoval, as the mass fraction of R290 falls in the range of 0 to 0.5. Then a tiny growth from about 0 to 1.1% is observed, with increasing xR290 from 0.5 to unit value. In terms of the four cases shown in Fig. 14, superheating without IHE leads to the lowest ηoval (0.2441) and superheating with IHE results in the highest ηoval (0.2759) for the mixture R290/R600a (xR290 = 0.45) . It can also be inferred from the figure that the benefit of superheating in improving the ηoval is more likely to be highlighted when IHE is used together.
the benefit brought by temperature glide decreases as xR600a increases from 0.45 to 1. As a result, the xR290 corresponding to the maximum ηoval (or CPRm) will not decrease to 0 with the increasing Tgen,out. Fig. 10 illustrates the effect of Tevap,in on ηoval and CPRm, under the condition that Tgen,out and Tcond,sv is fixed at 80 °C and 40 °C, respectively. It shows that ηoval increases from 0.1739 to 0.5150 as Tevap,in varies from −20 to 10 °C. The mixture composition corresponding to the maximum ηoval at different Tevap,in remains unchanged. This is because that the mixture R290/R600a (xR290 = 0.45) always shows the largest temperature glide no matter how the Tevap,in changes. The CPRm also keeps going up with the increasing Tevap,in, and only a slight change in the mixture composition corresponding to the maximum CPRm appears as Tevap,in exceeds 0 °C. 5.4. Effect of IHE and superheating in ORC In this section, cycle performances of the ORC-VCC system with and without IHE are compared based on the same working conditions. The expander exhaust flows directly to the condenser and the state points of 4′ and 2′ will not appear, in the case where no IHE is added in the system. Fig. 11 compares ηoval of the ORC-VCC working between −5 and 80 °C with and without IHE for the pure refrigerants. It can be seen from Fig. 11 that by introducing the IHE, an increase in ηoval can be achieved for all pure fluids. Also, the increments in ηoval for dry fluids are apparently larger than that for wet fluids. The relative increment in ηoval is 19.0%, 11.3%, 10.2% and 9.5% for R227ea, R600a, R1234yf and R1234ze, respectively, while none of the wet fluids shows a relative increment exceeding 5.0%. Using IHE could benefit the system thermal efficiency by reclaiming heat inside system. Comparing with wet fluids, the use of dry fluids usually results in that the expander exhaust possesses a higher temperature and higher dryness, as shown in Table 4, which means more heat could be recovered by IHE. By giving the variation trend of relative increment in ηoval versus the mixture composition, the effect of IHE on system thermal efficiency for refrigerant mixtures is investigated, as shown in Fig. 12. The mixture composition corresponding to the maximum ηoval for each combination is also displayed in the figure using different symbols. As shown in Fig. 12, although the variation trends of the relative increment in ηoval vary among different fluid combinations, all of the mixtures show positive increments within the entire range of composition, which demonstrates that for refrigerant mixtures, introducing an IHE to the ORC-VCC can also improve the system efficiency. And the benefit can be more evident than that of pure fluids, because the curve of relative increment presents a trend from increase to decrease with increasing x1 for all combinations, regardless the fact that a slight rebound in the increment occurs as x1 exceeding 0.8 for R227ea/R600a. Also, dry mixtures show much greater potential to enhance the system performance with IHE than wet fluids. Taking R161/R600a as an example, the relative increment of dry mixture (xR161 = 0.15, ζ = 0.96) is nearly five times more than that of wet mixture (xR161 = 0.8, ζ = 1.29). The mechanism of efficiency improvement for mixtures is related to their temperature glides during condensing. An obvious temperature glides benefits the heat transfer inside IHE by decreasing the vapor quality at the hot side outlet (q4′), as reflected in Table 4. As a result, more latent heat during condensing process can be reclaimed by using zeotropic mixtures, and the overall efficiency of ORC-VCC is further improved. A SHD of 5 K is set at the outlet of solar collector in the typical working condition to prevent liquid slugging in the downstream expander, as described in Section 4.1. Here, the effect of superheating on expansion process is discussed by comparing qexp,out with and without superheating, based on mixture R290/R600a with various compositions, considering the change in ηexp,s. As shown in Fig. 13, a SHD of 5 K can effectively avoid liquid formation inside expander, with qexp,out being above the unit value even for the wet mixtures, no matter how the ηexp,s changes. When there is no superheating, the expander exhaust drops into the vapor-liquid two-phase region as xR290 surpasses 0.7
6. Conclusions In the present paper, a solar powered ORC-VCC refrigerator is investigated theoretically. Zeotropic mixtures are proposed as working fluids for the ORC-VCC system for the first time. Based on a developed mathematical model, the performance of eight pure refrigerants as well as 5 zeotropic mixtures with various compositions are evaluated and compared. The effects of various operating parameters as well as the introduction of IHE in ORC on the overall system performance are also analyzed. Several conclusions can be drawn from the analyses, as followings: (1) R600a is the most suitable pure working fluid for ORC-VCC working between −5 and 80 °C, with the largest ηoval of 0.2212. Dry fluids show more advantage than wet fluids in ηORC, while not all of the wet fluids show superiority in the refrigerating cycle, because of their higher value of cp/r. (2) Zeotropic mixtures show higher ηoval than the component fluids, by taking the advantage of temperature glide in reducing the cycle temperature lift. Among all of the mixtures, R161/R600a
Fig. 10. Variation of ηoval and CPRm versus Tevap,in.
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Fig. 14. Trend of ηoval under various IHE and superheating conditions.
Fig. 11. ηoval with and without IHE for pure refrigerants.
(3) Tgen,out affects the overall system performance of ORC-VCC greatly. Taking R290/R600a as an example, the maximum value of ηoval and CPRm is improved by 199.1% and 156.9%, respectively, with increasing Tgen,out from 60 to 120 °C. Slight decreases in xR290 in mixture composition corresponding to the maximum values appear, due to the increase in Tcrit of the mixture at the same time. (4) The increases in ηoval by IHE for zeotropic mixtures are larger than that for pure fluids, within a certain composition range, since more latent heat during condensing can be reclaimed by taking the advantage of temperature glide. (5) Superheating is necessary for the application of wet fluids in ORCVCC. For mixture R290/R600a, the upper limit in xR290 expands from 0.7 to 1 as the superheating increases from 0 to 5 K. Without IHE, superheating becomes ineffective in improving ηoval for either pure or mixture fluids. Acknowledgments Fig. 12. Variation of improvement in ηoval versus mixture composition.
This work is sponsored by the Research Foundation for Young Teachers of Xi’an Jiaotong University (HG1K023). Reference Aphornratana, S., Sriveerakul, T., 2010. Analysis of a combined Rankine–vapour–compression refrigeration cycle. Energy Convers. Manage. 51 (12), 2557–2564. Bu, X.B., Li, H.S., Wang, L.B., 2013. Performance analysis and working fluids selection of solar powered organic Rankine-vapor compression ice maker. Sol. Energy 95, 271–278. Calm, J.M., Hourahan, G.C., 2001. Refrigerant data summary. Eng. Syst. 18 (11), 74–88. Chang, H., Wan, Z., Zheng, Y., Chen, X., Shu, S., Tu, Z., Chan, S.H., 2017. Energy analysis of a hybrid PEMFC–solar energy residential micro-CCHP system combined with an organic Rankine cycle and vapor compression cycle. Energy Convers. Manage. 142, 374–384. Harby, K., 2017. Hydrocarbons and their mixtures as alternatives to environmental unfriendly halogenated refrigerants: an updated overview. Renew. Sustain. Energy Rev. 73, 1247–1264. Heberle, F., Preißinger, M., Brüggemann, D., 2012. Zeotropic mixtures as working fluids in Organic Rankine Cycles for low-enthalpy geothermal resources. Renewable Energy 37 (1), 364–370. Hung, T.C., Shai, T.Y., Wang, S.K., 1997. A review of organic rankine cycles (ORCs) for the recovery of low-grade waste heat. Energy 22 (7), 661–667. Karellas, S., Braimakis, K., 2016. Energy–exergy analysis and economic investigation of a cogeneration and trigeneration ORC–VCC hybrid system utilizing biomass fuel and solar power. Energy Convers. Manage. 107, 103–113. Kim, K.H., Perez-Blanco, H., 2015. Performance analysis of a combined organic Rankine cycle and vapor compression cycle for power and refrigeration cogeneration. Appl. Therm. Eng. 91, 964–974. Lemmon, E., Huber, M., McLinden, M., 2010. Reference Fluid Thermodynamic and Transport Properties - REFPROP. NIST Standard Reference Database 23 (Version 9.0). Li, H., Bu, X., Wang, L., Long, Z., Lian, Y., 2013. Hydrocarbon working fluids for a Rankine cycle powered vapor compression refrigeration system using low-grade
Fig. 13. Variation trend of qexp,out under different SHD and ηexp,s.
(xR161 = 0.25) comes to the top in ηoval, with a maximum value of 0.3089, followed by R152a/R600a (xR152a = 0.25) and R290/ R600a (xR290 = 0.45). In terms of WRm and CPRm, R290/R600a (xR290 = 0.4) and R161/R600a (xR161 = 0.25) ranks first, respectively. R290/R600a mixtures are the most preferred, considering the non-validated mixing rule for R161/R600a. 65
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Solar Energy journal homepage: www.elsevier.com/locate/solener
Analysis of a solar Rankine cycle powered refrigerator with zeotropic mixtures ⁎
T
⁎
Nan Zhenga, , Jinjia Weia,b, , Li Zhaoc a
School of Chemical Engineering and Technology, Xi’an Jiaotong University, Xi’an 710049, China State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an 710049, China c Key Laboratory of Efficient Utilization of Low and Medium Grade Energy (Tianjin University), MOE, Tianjin 300072, China b
A R T I C L E I N F O
A B S T R A C T
Keywords: Organic Rankine cycle Solar thermal energy Zeotropic mixture Refrigeration
A solar organic Rankine cycle (ORC) powered vapor compression cycle (VCC) for refrigeration is under investigation in this paper. To improve the overall system performance, zeotropic mixtures are proposed to be used in the integrated ORC-VCC system for the first time. A thermodynamic model is developed, and a total of eight pure fluids and five zeotropic mixtures with various compositions are evaluated and compared to identify the best combinations of fluids for yielding high system efficiencies. Besides, the influences of generating temperature, refrigerating temperature, superheating and internal heat exchanger (IHE) in ORC on the system performance are analyzed. For the ORC-VCC operating between −5 and 80 °C, dry fluid R600a shows the highest system efficiency (0.2212) among the pure fluids. For zeotropic mixtures, there exists a composition range within which binary mixtures always show higher system efficiency than the component pure fluids. Mixture R161/R600a with an R161 mass fraction of 0.25 shows the highest system efficiency (0.3089) among all fluids, which is increased by 39.6% and 54.7% comparing with R600a and R161, respectively. Adding IHE in ORC benefits the system efficiency, and the benefit is much more evident for dry fluids. Superheating makes wet fluids become applicable in ORC-VCC, but it becomes ineffective in improving the system efficiency when there is no IHE in system.
1. Introduction With the development of social economy, the need for cooling and refrigeration shoots up over the years, which contributes to the dramatic increase in the energy consumption worldwide. The application of solar thermal energy to refrigerating process has a great potential in reducing the fossil fuels consumption and alleviating environmental issues. Solar assisted air-conditioning/refrigerating system is particularly attractive to regions where the insolation supply and the need for refrigeration reach to maximum levels at the same period. Solar thermal energy could be converted into cooling process by use of either the absorption/adsorption refrigeration cycle or the thermomechanical cooling system (Zeyghami et al., 2015). Although the single effect absorption chillers are still the dominant technology in solar cooling, interest in thermo-mechanical cooling systems has been revived recently due to the appearance of new efficient and environmentally friendly refrigerants (Park et al., 2015) and advancements in organic Rankine cycle equipment (Quoilin et al., 2011), as well as the slow development in breaking the operating limitations on absorption chillers (Xu et al., 2013). ⁎
Organic Rankine cycle powered vapor compression cycle (ORCVCC) is one of the most common way to fulfill the thermo-mechanically activated cooling production. Comparing with absorption refrigeration cycle and ejector cooling cycle, the ORC-VCC system has advantages such as: ability to convert heat into electricity when cooling is not needed by coupling the expander with an electric generator, and ability to supply cooling in remote area where the grid cannot reach by coupling the expander and the compressor directly. Great efforts have been devoted to the development of the ORC-VCC system since its concept was proposed. Prigmore and Barber (1975) established and tested the first prototype which used separate power and cooling cycles (termed as separate configuration). The dual fluid system was powered by solar and occupied R113 and R12 as the working fluid in ORC and VCC, respectively. A maximum system COP of 0.5 was obtained with the water temperature at the solar collector outlet being 102 °C. Bu et al. (2013) conducted a performance analysis and working fluids selection of a solar powered ice maker, with the power cycle and cooling cycle operated with the same working fluid (termed as single fluid ORC-VCC). In terms of overall efficiency and ice production capacity, R123 was regarded as the most suitable working
Corresponding authors at: School of Chemical Engineering and Technology, Xi’an Jiaotong University, Xi’an 710049, China. E-mail addresses: [email protected] (N. Zheng), [email protected] (J. Wei).
https://doi.org/10.1016/j.solener.2018.01.011 Received 4 July 2017; Received in revised form 24 November 2017; Accepted 6 January 2018 0038-092X/ © 2018 Elsevier Ltd. All rights reserved.
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Nomenclature COPVCC CPRm CFCs HCs HCFCs HFCs HFOs IHE NBP WRm SHD SCD cP h I m q Qrefrig r
s T ΔTg W x ηETC ηORC ηoval ηs ζ
refrigeration cycle coefficient of performance (–) refrigeration capacity per unit mass flow rate (kW s kg−1) chlorofluorocarbons hydrocarbons hydrochlorofluorocarbons hydrofluorocarbons hydrofluoroolefins internal heat exchanger normal boiling point (°C) output net power per unit mass flow rate in ORC (kW s kg−1) superheating degree (K) subcooling degree (K) specific heat at constant pressure (kJ kg−1 K−1) specific enthalpy (kJ kg−1) solar radiation intensity (kW m−2) mass flow rate (kg s−1) vapor quality (–) refrigerating capacity (kW) latent heat of vaporization (kJ kg−1)
specific entropy (kJ kg−1 K−1) temperature (°C) temperature glide (K) power (kW) mass fraction (–) collector efficiency (–) thermal efficiency of ORC (–) overall system efficiency (–) isentropic efficiency superheating parameter (–)
Subscripts 0 crit cond evap exp in m out
ambient critical condenser, condensing evaporator, evaporating expander inlet mean outlet
Generally, the dual fluid ORC-VCC with separate configuration has higher operational flexibility than single fluid system, but it requires better shaft seals to prevent leakages and suction problems between the expansion and compression sections. The choices of the appropriate working fluid is the most preliminary step of the system design for ORC-VCC. The ideal fluids should meet several basic criteria such as environmental friendliness, safety, stability and energy efficiency. According to the literature review, CFCs (e.g. R113), HCFCs (e.g. R123) and HFCs (e.g. R134a) are the most frequently used working fluids in the previous studies. However, these working fluids either has been, or will be, phased out due to their negative impact on environment. In recent researches, HCs (e.g. R290) and HFOs (e.g. R1234ze) are proposed and investigated as possible alternatives to the CFCs, HCFCs and HFCs. Although HCs and HFOs could be regarded as eco-friendly fluids, flammability and chemical instability become another two arguments against their application in ORC-VCC, respectively. All of the existing studies are focused on pure component working fluids for ORC-VCC system, but it should be pointed out that there are no pure fluids available at present that completely meet all of the criteria.
fluid for their system. Chang et al. (2017) proposed a hybrid residential micro-CCHP system based on proton exchange membrane fuel cell and solar energy. A dual fluid ORC-VCC with dimethylpentane and R290 being used as the working fluid in ORC and VCC, respectively, was adopted to generate power and cooling/heating simultaneously. To reduce the system complexity, Aphornratana and Sriveerakul (2010) proposed a novel configuration of single fluid ORC-VCC system (termed as integrated cycle) in which the power cycle and cooling cycle shared a common condenser. Three different refrigerants, including R123, R134a and R245ca, were evaluated to find the best candidate for the novel cycle. Wang et al. (2011) attempted to couple the ORC and VCC with a free-piston expander-compressor unit to minimize the power transmission loss and simplify the mechanical design further. The proposed system was able to produce cooling temperature of 0 °C with a low grade thermal energy as low as 60 °C. In addition to the above studies, several other researchers also reported the performance of the ORC-VCC with separate power and cooling cycles either using single fluid (Karellas and Braimakis, 2016; Kim and Perez-Blanco, 2015; Li et al., 2013; Wu et al., 2017; Yılmaz, 2015; Yue et al., 2016) or dual fluid (Molés et al., 2015; Nasir and Kim, 2016; Wang et al., 2011).
Fig. 1. Schematic diagram of the solar powered ORC-VCC refrigerator.
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fluids for an ORC-VCC system could be relatively restricted. Using refrigerant mixtures, however, provides a practical way to greatly broaden the options (Wang and Zhao, 2009). Following this introduction, a total of eight common refrigerants, including two HCs (R290 and R600a), four HFCs (R161, R152a, R134a and R227ea) and two HFOs (R1234yf and R1234ze), are taken as the original components. All of the selected pure refrigerants satisfy the hard environmental standard on ODP. Table 1 lists some of the basic thermodynamic information of each refrigerant. The information on GWP as well as safety group of each refrigerant is obtained from Calm and Hourahan (2001), and other properties are derived from NIST REFPEOP (Lemmon et al., 2010). Depending on whether the superheating parameter ζ (a dimensionless number representing the shape of temperature-entropy characteristics) is greater than unit value or not (Zheng et al., 2016), the component refrigerant is distinguished as wet fluid or dry fluid, as shown in the table. When the eight components pair together, a total of 28 binary fluid combinations could be obtained. Since the application of zeotropic binary mixtures in ORC-VCC is proposed herein, only the combinations showing obvious temperature glide (> 5 K) within a broad composition range are considered. As a result, five combinations are finally screened out, as shown in Table 2. In this paper, the thermodynamic properties of the mixtures are calculated based on NIST REFPROP (Lemmon et al., 2010), which provides relatively accurate mixture models for R290/R600a, R152a/R600a, R227ea/R600a and R1234yf/R600a. Due to the lack of test data, the parameters associated with the mixing rule for R161/R600a are actually estimated values, which may lead to relatively large calculation uncertainties on the fluid’s properties.
Using fluid mixtures provides a potential solution to the issues mentioned above. For example, using low volatile HCs as an additive with HCFCs is expected to reduce the flammability of HC mixture, and the short atmospheric lifetime of HCs helps to reduce the mixtures’ GWP (Harby, 2017) at the same time. Another advantage of using fluid mixtures is that the cycle performance could be improved by matching the temperature profiles between the heat transfer medium and zeotropic mixture that presents non-isothermal phase transition process (Heberle et al., 2012). However, none of the previous publications report the performance of ORC-VCC operating with fluid mixtures, especially zeotropic mixtures. To fill up this research gap, in this paper, we concentrate on verifying the potential benefits of zeotropic mixtures on the ORC-VCC system. A steady state model of a solar powered ORCVCC refrigerator is developed, based on which a total of eight pure fluids, including HCs (R290 and R600a), HFCs (R161, R152a, R134a and R227ea) and HFOs (R1234yf and R1234ze) are evaluated as component fluids. Afterwards, five zeotropic mixtures containing R600a are screened out and compared to identify the suitable combinations of fluids which may yield high system efficiencies under a variety of working conditions and cycle configurations. 2. System description Fig. 1 shows the schematic diagram of the solar driven ORC-VCC refrigerator, which mainly consists of solar collector, expander, IHE, feeding pump for power cycle, throttling valve, evaporator, compressor for refrigeration cycle, and a shared condenser for both ORC and VCC. This system has an integrated configuration and uses the same working fluid in both cycles to minimize the problem associated with leaking of the working fluids. The working principle of this system is described briefly below. A portion of liquid working fluid coming from the condenser is pumped to the solar collector, where the liquid absorbs heat and completely vaporizes. Thus, the solar collectors could be regarded as a vapor generator. Superheating of the vapor at the outlet of solar collector is necessary to avoid harmful liquid slugging in the downstream expander. The superheated vapor expands in the expander and generates shaft work to drive the compressor. IHE is usually installed between the expander and condenser in an ORC to reclaim the heat possessed by the expander exhaust. The exhaust cools down in the IHE by transferring the heat to the pressurized liquid from the feeding pump. Meanwhile, the other part of the liquid, after being depressurized in the throttling valve, is directed to the evaporator to vaporize. Then the low temperature vapor is pressurized in the compressor driven by the ORC expander. In order to improve the drive efficiency, the shafts of the expander and the compressor are directly coupled. Thereafter, the expander exhaust mixes with the compressor exhaust, and the combined vapor flow is condensed to subcooled liquid in the condenser to complete the cycle. The T-s diagram of the solar powered ORC-VCC refrigerator with IHE is illustrated in Fig. 2. For the system without IHE, the states points 2′ and 4′ will not appear. It should be noted that the state points 4 and 4′ might be in either the two-phase region or the vapor region depending on the fluid’s nature and the superheating degree (SHD) at the outlet of the solar collectors.
4. Thermodynamic analysis 4.1. Cycle modeling To evaluate the performance of the solar driven ORC-VCC refrigerator, a steady state thermodynamic model is developed, considering the following assumptions: (1) Solar collectors are working in steady state and have uniform radial temperature distribution. (2) The heat and friction losses in heat exchanger and piping are neglected. (3) All heat exchangers are countercurrent type. (4) No leakage loss in system and zeotropic mixtures have constant circulating compositions.
3. Working fluids for ORC-VCC One of the main thermodynamic properties should be considered in selecting working fluid for an integrated ORC-VCC system is the shape of the vapor-liquid phase envelope. According to the different slopes of the saturation vapor curve in the T-s diagram, working fluids are categorized as dry (ds/dT > 0), wet (ds/dT < 0) and isentropic (ds/ dT = 0), from ORC’s point of view (Hung et al., 1997). To avoid formation of droplets in expander and compressor, dry and isentropic fluids are well adapted to ORC, while wet and isentropic fluids are more favorable for VCC. Considering the great differences in working conditions for power and refrigeration, the range of available working
Fig. 2. T-s diagram of the solar powered ORC-VCC refrigerator.
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Table 1 Candidate fluids and their basic properties. Fluid
Molecular mass [g mol−1]
Tcrit [°C]
Pcrit [MPa]
NBP [°C]
GWP [100 yr]
Safety group
ζ [–]
R290 R161 R152a R134a R600a R227ea R1234yf R1234ze
44.096 48.06 66.051 102.03 58.122 170.03 114.04 114.04
96.74 102.1 113.26 101.06 134.66 101.75 94.7 109.36
4.2512 5.01 4.5168 4.0593 3.629 2.925 3.3822 3.6349
−42.114 −37.55 −24.023 −26.074 −11.749 −16.34 −29.45 −18.973
3 12 124 1430 20 3220 4 6
A3 A3 A2 A1 A3 A1 A2L A2L
1.1098 1.3925 1.2658 1.1006 0.8338 0.7395 0.9222 0.9368
ζ is calculated at the same reduced temperature of 0.8 for each fluid. Table 2 Screened out binary fluid combinations.
ηcomp,s =
Combination
Fluid type
Chemical composition
Mixing rule
R290/R600a R152a/R600a R161/R600a R227ea/R600a R1234yf/R600a
Wet/Dry Wet/Dry Wet/Dry Dry/Dry Dry/Dry
HC/HC HC/HFC HC/HFC HC/HFC HFO/HC
Validated Validated Non-validated Validated Validated
Tm−T0 (T −T )2 −0.0199 × m 0 I I
(mORC + mVCC )·h8 = mORC ·h4 + mVCC ·h7
(mORC + mVCC )·h8 = mORC ·h 4′ + mVCC ·h7
(5) (6)
(7)
(8)
Wexp−Wpump mORC
Parameter
Typical value
Range
Parameter
Typical value
Range
mORC [kg s−1] Texp,in [°C] Tevap,in [°C] Tcond,sv [°C] SHDevap,out [K] T0 [°C]
1 80 −5 40 5 35
– 60–120 −20–10 – – –
SHDexp,in [K] SCDcond,out [K] ηexp,s [–] ηpump,s [–] ηcomp,s [–] I [W m−2]
5 5 0.85 0.9 0.8 1000
0, 5 – 0.85–1 – – –
Wexp−Wpump Qgen
(10)
For the VCC:
Wcomp = Wexp
(11)
Wcomp = mVCC ·(h6−h7)
(12)
(19)
Table 3 Input parameters and boundary conditions.
(9)
The thermal efficiency of the ORC is defined as:
ηORC =
(18)
Based on the above assumptions and equations, a computer program in Matlab 2016.b is developed to simulate the thermodynamic performance of the ORC-VCC with various working fluids. The input values of several design parameters and boundary conditions are listed in Table 3. The mass flow rate of working fluid in ORC is set to unit value for all calculations, and thus the mass flow rate in VCC can be determined from Eq. (12). The lowest temperature (Tevap,in) and the highest temperature (Texp,in) of the system is set to be the same for various pure fluids and mixtures. Considering the temperature glide of zeotropic mixtures, the saturated vapor temperature is chosen as a basis and keeps invariable at 40 °C, which corresponds to the typical summer outdoor temperature about 35 °C. For the system with an IHE, the low temperature liquid from the feeding pump is supposed to be heated to the same temperature of the expander exhaust. To avoid the cavitation in the ORC feed pump, the degree of subcooling (SCD) of 5 K at condenser outlet is set. Under the standard operation condition, the degree of SHD at the outlet of evaporator and solar collector is set to 5 K,
The net power output per unit mass flow rate of working fluid of the ORC:
WRm =
Qrefrig mORC + mVCC
ηoval = ηETC ·ηORC ·COPVCC
Solar energy input with IHE:
Qgen = mORC ·(h3−h2′)
(17)
The refrigerating capacity per unit mass flow rate of working fluid of the ORC-VCC:
Solar energy input without IHE:
Qgen = mORC ·(h3−h2)
(16)
Energy balance in mixer (with IHE):
(2)
(4)
h4−h 4′ = h2′−h2
(15)
The overall efficiency of the ORC-VCC refrigerator is defined as:
Wpump = mORC ·(h2−h1) h2−h1 h2,s−h1
Wcomp
For the ORC-VCC: Energy balance in mixer (without IHE):
(1)
(3)
ηpump,s =
Qrefrig
COPVCC =
CPRm =
h3−h4 h3−h4,s
ηexp,s =
(14)
The COP of the VCC is defined as:
For the ORC:
Wexp = mORC ·(h3−h4 )
(13)
Qrefrig = mVCC ·(h6−h5)
The basic equations describing the ORC-VCC system are given below. For solar collectors: Evacuated tube collectors (ETC) are used for solar heat collecting, considering their relatively higher performance than flat plate collectors (FPC) under low and medium temperatures (Sokhansefat et al., 2018). The models of heat-collecting efficiency of ETC is described by Eq. (1) (Li et al., 2016).
ηETC = 0.721−0.89 ×
h 7,s−h6 h 7−h6
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the most suitable working fluid for the solar powered ORC-VCC refrigerator under the given working conditions. However, the relatively higher NBP of R600a would become a restriction for its application in low temperature refrigeration (Tevap < −10 °C). On this condition, R600a should be mixed with the low boiling component, such as R290 and R1234yf, to assure a sufficiently large positive pressure in the evaporator.
respectively, to prevent liquid slugging in the downstream machines, respectively. According to the literature (Bu et al., 2013), a constant isentropic efficiency of 0.85, 0.8 and 0.9 is assumed for expander, compressor and feed pump, respectively. 4.2. Model validation Due to the lack of studies on ORC-VCC running with zeotropic mixtures, the validation of the present calculating program is conducted based on the results of pure refrigerants. The simulation results from Li et al. (2013) are used as the standard for comparisons because of the similar integrated cycle configuration. Since the ORC is driven by a boiler instead of solar collectors in the reference paper, the product of COPVCC and ηORC is used as the index parameter for comparison. It should be noted that the temperature conditions and various isentropic efficiencies are set to the same values as in the reference paper, respectively, when calculating cycle efficiencies using the present model. Fig. 3 shows the comparison in the product of COPVCC and ηORC between the present work and the reference, based on various generation temperatures. The comparison results indicate that the deviation between the calculated efficiencies and the reference values are small, with the mean absolute deviation being 2.72% and 0.30% for R600a and R290, respectively.
5.2. Analysis on zeotropic mixtures The overall system efficiency as a function of mixture composition is plotted for each of the screened out binary mixtures, as shown in Fig. 5. The composition herein is represented by the mass fraction of the first component (x1) in each mixture. It should be noted that all of the first component fluid in each mixture has lower NBP than R600a. It is apparent from Fig. 5 that for all the mixtures, ηoval lies greatly on their compositions, and it firstly increases and then decreases with the increase of mass fraction of the low boiling point component in the mixtures. Mixture R161/R600a (x1 = 0.25) comes top in overall system efficiency with a maximum ηoval of 0.3089, followed by R152a/R600a (x1 = 0.25), R290/R600a (x1 = 0.45), R227ea/R600a (x1 = 0.45) and R1234yf/R600a (x1 = 0.45), respectively. Comparing with pure R161 and R600a, the maximum ηoval of mixture R161/R600a is increased by 54.6% and 39.6%, respectively. For any binary fluid combinations, there exists a composition range within which the mixtures always show higher ηoval than their component fluids. For example, ηoval of the R161/R600a remains higher than that of R600a until xR161 exceeds 0.75. We should also take notice of the rebound and fluctuation in ηoval as x1 approaches to 1 for the mixture R152a/R600a and R227ea/ R600a. The key factors determining the variation trend of ηoval versus the mixture composition will be analyzed in the following part. Fig. 6 shows the change of temperature glide (ΔTg) with respect to the mixture composition for each binary mixture. For any mixtures, system pressures has little influence on the variation trend of ΔTg, which could be reflected by the data shown in Table 4; hence, the temperature glides under condensing pressure (ΔTgcond) are taken as the representative and illustrated in Fig. 6. According to Figs. 5 and 6, the variation tendency of ΔTg is remarkable similar to that of ηoval: the overall system efficiency rises with an increasing ΔTg, and the two parameters reach to the maximum value at the same composition. One of the benefits of temperature glide lies in that it helps to reduce the temperature lift between heat source and sink, when the highest and lowest temperature in the cycle are specified, thereby improving the cycle performance. The average temperature lift between the heat source and sink of the ORC changes little with the mixture composition,
5. Results and discussion 5.1. Analysis on pure fluid The comparison of cycle performance among the eight candidate refrigerants is illustrated in Fig. 4. Fig. 4(a) shows the calculated ηORC and COPVCC for different component refrigerants under the typical working condition, based on the system with IHE. It is obvious from the figure that both ηORC and COPVCC depend largely on the nature of the working fluid. In the case of ηORC, the dry refrigerants present, on the whole, higher performance than wet refrigerants. Dry refrigerant R600a, with the value of ηORC being 0.0816, ranks first among the eight candidates, followed by dry refrigerant R227ea, R1234ze and R1234yf. The value of ηORC is comparable to each other for the four wet refrigerants, with the maximum and minimum value being 0.0744 (R152a) and 0.0723 (R290), respectively. In the case of COP, however, we cannot easily generalize a conclusion that the wet fluids are better than dry fluids, or vice versa. As shown in Fig. 4, wet refrigerant R152a shows the highest refrigerating performance with a COPVCC of 4.067, followed by dry fluid R600a (COPVCC = 4.037) and wet fluid R161 (COPVCC = 3.989). Dry fluid R1234yf and R227ea, with COPVCC being 3.801 and 3.710, respectively, are in the bottom two places. In addition to the temperature-entropy characteristic, the performance of VCC can be affected by other properties, such as specific heat (cp) and latent heat of vaporization (r). When the evaporating temperature is given, the refrigerating COP usually decreases with increasing cp/r (Ma et al., 2017), which could explain why dry fluid R600a shows higher COPVCC than some wet fluids. Fig. 4(b) illustrates the calculated ηETC and ηoval for the eight candidate refrigerants. Like the ηORC and COPVCC, the collector efficiency is also affected, to a certain degree, by the refrigerants’ nature. Generally, wet refrigerants show higher collector efficiency than dry refrigerants, with the maximum and minimum ηETC being 0.6860 (for R152a) and 0.6707 (for R227ea), respectively. According to Eq. (1), the low value of ηETC could be caused by the high mean temperature of the working fluid inside the collectors. This result indicates that using dry fluids helps to increase the mean generation temperature for the solar powered ORC-VCC when an IHE is coupled in the system, which will be further explained in Section 5.4. As to the overall system efficiency, dry refrigerant R600a shows the highest value of 0.2212, followed by R152a (ηoval = 0.2076) and R1234ze (ηoval = 0.2072), respectively. Hence, from the ηoval point of view, dry fluid R600a could be regard as
Fig. 3. Calculation deviation in product of COPVCC and ηORC.
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Fig. 6. Variation of ΔTg versus mixture composition.
slight positive pressure means a high risk of air suction by the compressor. The application of R290, however, will lead to a high pressure (2849.31 kPa) in the solar collector and expander, which inevitably increases the operation risk and manufacturing cost of the ORC-VCC system. Comparing with the two component fluids, the R290/R600a mixtures show much more appropriate pressure levels. According to the calculated ζ, the mixture R290/R600a will changes gradually from dry fluid to wet fluid, and becomes isentropic fluid at certain composition, as xR290 in the mixture increases from 0 to 1. It is interesting to note that for the combination of R290 and R600a, the formed isentropic mixture also possesses the largest ΔTg. This isentropic zeotropic mixture (xR290 = 0.45) shows the highest output net power, refrigerating capacity and overall system efficiency among all of the R290/R600a mixtures. Fig. 7 shows the variation trend of WRm versus the mixture composition for each mixture. WRm refers to the ratio of Wnet to mORC, which reflects the power capability of working fluid per unit mass flow rate. As shown in Fig. 7, R600a shows the largest WRm (about 26.9 kW s kg−1) among the six component fluids, under the typical working condition. However, the change trends of WRm with x1 vary greatly among different fluid combinations. Overall, for the dry/dry combinations, WRm keeps going down as the R600a mass fraction decreases, while for the wet/dry combinations, non-monotonic curves of WRm are observed. Specifically, WRm increases firstly and then decreases with an increasing xR290 for R290/R600a mixtures, reaching to the maximum value of 28.04 kW s kg−1 at xR290 of 0.4, while for R152a/R600a mixtures, an opposite trend is exhibited. For the R161/ R600a mixture, the maximum and minimum value of WRm appears successively under certain compositions as the mass fraction of R161 increases. The variation tendency of WRm is determined by two factors, the expander output power and the power consumption of pump, both of which are closely related to the nature of working fluid. Taking R152a/R600a as an example, the output power decreases steadily while the power consumption first increases and then decreases with an increasing xR152a, thereby resulting in a trend from decline to rise for WRm. For the dry/dry combinations, however, the influence of power consumption on WRm could be neglected due to the significant reduction in output power as xR600a drops from 1 to 0. Fig. 8 shows the variation of CPRm, the index reflecting the refrigerating capacity of unit mass working fluid in ORC-VCC, versus x1 for each mixture. On the whole, the trends of CPRm are similar to that of WRm, excepting that a maximum CPRm appears as xR152a increases to 0.2 for R152a/R600a mixtures. Mixture R161/R600a (xR161 = 0.25) and R290/R600a (xR290 = 0.5) have the highest and second highest CPRm, being 103.33 kW s kg−1 and 99.91 kW s kg−1, respectively. The CPRm of R161/R600a is greater than that of R290/R600a within the
Fig. 4. Candidate refrigerants performance: (a) ηORC and COPVCC; (b) ηETC and ηoval.
Fig. 5. Variation of ηoval versus mixture composition.
in the case where the approximate maximum values of temperatures in solar collector and condenser are determined. As shown in Table 4, ηORC decreases steadily with an increasing xR290, while COPVCC presents nonmonotonic dependence on the mixture composition, within the range of xR290 from 0.3 to 0.6. This indicates that the variation trend of ηORC shown in Fig. 6 also reflects the variation of COPVCC, which is greatly affected by the temperature glide. It can be easily found in Table 4 that the system pressures decrease gradually with the increase of xR600a. Under the typical working conditions, the evaporating pressure is about 130.98 kPa for R600a. Such a 62
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Table 4 Simulation results for R290/R600a. R290/R600a
xR290 = 0
xR290 = 0.3
xR290 = 0.45
xR290 = 0.6
xR290 = 1
Pgen [kPa] Pcond [kPa] Pevap [kPa] T1 [°C] T4 [°C] q4 [–] q4′ [–] q5 [–] T6 [°C] ΔTggen [K] ΔTgcond [K] ΔTgevap [K] mvcc/(morc + mvcc) [–] ζ [–] Wpump [kW] Wexp [kW] Qgen [kW] Qevap [kW] ηorc [–] COPvcc [–] ηETC [–] ηoval [–]
1210.68 531.21 130.98 35.0 54.0 1.0868 0.9343 0.2648 0 0 0 0 0.2964 0.8487 1.40 28.37 330.6 114.53 0.0816 4.04 0.6718 0.2212
1551.13 688.83 210.52 28.6 51.1 1.0695 0.8895 0.2185 5.3 5.2 6.4 7.3 0.3226 0.9526 1.81 29.76 336.05 143.33 0.0832 4.82 0.6749 0.2703
1758.15 788.03 252.67 28.0 49.6 1.0605 0.8881 0.2151 6.2 5.7 7.0 8.0 0.3257 1.0009 2.07 30.09 338.09 148.07 0.0829 4.92 0.6765 0.2759
1996.99 905.86 294.78 28.6 47.9 1.0508 0.8959 0.2205 6.1 5.2 6.4 7.3 0.3213 1.0533 2.37 30.11 339.25 145.54 0.0818 4.83 0.6783 0.2681
2849.31 1369.42 406.34 35.0 42.8 1.0208 0.9610 0.2756 0 0 0 0 0.2735 1.2270 3.44 27.56 333.57 107.31 0.0723 3.89 0.6833 0.1924
Fig. 9. Variation of ηoval and CPRm versus Tgen,out.
Fig. 7. Variation of WRm versus mixture composition.
5.3. Effect of generating and evaporating temperatures In this section, R290/R600a is used as a representative to illustrate the effect of Tgen,out and Tevpa,in on system performance. Fig. 9 reveals the influence of Tgen,out on ηoval and CPRm, under the condition that Tevpa,in and Tcond,sv is fixed at −5 °C and 40 °C, respectively. As expected, ηoval is improved with increasing the generating temperature in solar collector, due to the increase of ηORC. According to Fig. 9, ηoval increases from 0.1380 to 0.4128, as Tgen,out varies from 60 to 120 °C. Also, it can be found that the CPRm keeps going up with the increase of Tgen,out, and its value is increased by nearly 156.9% as the Tgen,out changes from 60 to 120 °C. The increase of CPRm versus Tgen,out could be caused by the increase in the mass flow rate of working fluid in VCC. Due to the increase of Tgen,out, the specific work output of expander improves accordingly, which means more working fluids could be saved from the power cycle and used for refrigerating. With the increase of Tgen,out, the mixture composition corresponding to the maximum ηoval and CPRm presents a trend of decrease in xR290, respectively, as shown in Fig. 9. When the evaporating and condensing temperatures of ORC are specified, the working fluids with higher Tcrit usually shows higher thermal efficiency (Liu et al., 2004). For mixture R290/R600a, Tcrit is expected to increase with an increasing xR600a, but on the other hand,
Fig. 8. Variation of CPRm versus mixture composition.
range of x1 from 0 to 0.4, due to that in the R161/R600a system, the proportion of working fluid used for refrigeration is higher than that in the R290/R600a system, before x1 reaching to 0.4. 63
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(ζ = 1.091), with an ηexp,s of 0.85. In the case of isentropic expansion, the mixtures with xR290 greater than 0.5 (ζ > 1.02) cannot be used for the sake of safety. Evidently, adding the superheating in the ORC helps to make more zeotropic mixtures become applicable in the ORC-VCC system. Fig. 14 illustrates the variation tendency of ηoval versus mixture composition under various IHE and superheating conditions, on the basis of R290/R600a. It can be seen that, the value of ηoval first increases and then decreases with an increasing xR290, and reaches the maximum value at xR290 = 0.45, for all of the four cases. According to the figure, when there is an IHE in system, the ηoval could be enhanced greatly by superheating under arbitrary mixture composition. The relative increment in ηoval firstly increases from 2.3% to 4.1% (xR290 = 0.75) and then decreases to 3.0%, as xR290 increases from 0 to 1. The benefit of superheating, however, is not evident for the case without IHE. For the cycle configuration without IHE, superheating the working fluid in the solar collectors could lead to a slight decrease in ηoval, as the mass fraction of R290 falls in the range of 0 to 0.5. Then a tiny growth from about 0 to 1.1% is observed, with increasing xR290 from 0.5 to unit value. In terms of the four cases shown in Fig. 14, superheating without IHE leads to the lowest ηoval (0.2441) and superheating with IHE results in the highest ηoval (0.2759) for the mixture R290/R600a (xR290 = 0.45) . It can also be inferred from the figure that the benefit of superheating in improving the ηoval is more likely to be highlighted when IHE is used together.
the benefit brought by temperature glide decreases as xR600a increases from 0.45 to 1. As a result, the xR290 corresponding to the maximum ηoval (or CPRm) will not decrease to 0 with the increasing Tgen,out. Fig. 10 illustrates the effect of Tevap,in on ηoval and CPRm, under the condition that Tgen,out and Tcond,sv is fixed at 80 °C and 40 °C, respectively. It shows that ηoval increases from 0.1739 to 0.5150 as Tevap,in varies from −20 to 10 °C. The mixture composition corresponding to the maximum ηoval at different Tevap,in remains unchanged. This is because that the mixture R290/R600a (xR290 = 0.45) always shows the largest temperature glide no matter how the Tevap,in changes. The CPRm also keeps going up with the increasing Tevap,in, and only a slight change in the mixture composition corresponding to the maximum CPRm appears as Tevap,in exceeds 0 °C. 5.4. Effect of IHE and superheating in ORC In this section, cycle performances of the ORC-VCC system with and without IHE are compared based on the same working conditions. The expander exhaust flows directly to the condenser and the state points of 4′ and 2′ will not appear, in the case where no IHE is added in the system. Fig. 11 compares ηoval of the ORC-VCC working between −5 and 80 °C with and without IHE for the pure refrigerants. It can be seen from Fig. 11 that by introducing the IHE, an increase in ηoval can be achieved for all pure fluids. Also, the increments in ηoval for dry fluids are apparently larger than that for wet fluids. The relative increment in ηoval is 19.0%, 11.3%, 10.2% and 9.5% for R227ea, R600a, R1234yf and R1234ze, respectively, while none of the wet fluids shows a relative increment exceeding 5.0%. Using IHE could benefit the system thermal efficiency by reclaiming heat inside system. Comparing with wet fluids, the use of dry fluids usually results in that the expander exhaust possesses a higher temperature and higher dryness, as shown in Table 4, which means more heat could be recovered by IHE. By giving the variation trend of relative increment in ηoval versus the mixture composition, the effect of IHE on system thermal efficiency for refrigerant mixtures is investigated, as shown in Fig. 12. The mixture composition corresponding to the maximum ηoval for each combination is also displayed in the figure using different symbols. As shown in Fig. 12, although the variation trends of the relative increment in ηoval vary among different fluid combinations, all of the mixtures show positive increments within the entire range of composition, which demonstrates that for refrigerant mixtures, introducing an IHE to the ORC-VCC can also improve the system efficiency. And the benefit can be more evident than that of pure fluids, because the curve of relative increment presents a trend from increase to decrease with increasing x1 for all combinations, regardless the fact that a slight rebound in the increment occurs as x1 exceeding 0.8 for R227ea/R600a. Also, dry mixtures show much greater potential to enhance the system performance with IHE than wet fluids. Taking R161/R600a as an example, the relative increment of dry mixture (xR161 = 0.15, ζ = 0.96) is nearly five times more than that of wet mixture (xR161 = 0.8, ζ = 1.29). The mechanism of efficiency improvement for mixtures is related to their temperature glides during condensing. An obvious temperature glides benefits the heat transfer inside IHE by decreasing the vapor quality at the hot side outlet (q4′), as reflected in Table 4. As a result, more latent heat during condensing process can be reclaimed by using zeotropic mixtures, and the overall efficiency of ORC-VCC is further improved. A SHD of 5 K is set at the outlet of solar collector in the typical working condition to prevent liquid slugging in the downstream expander, as described in Section 4.1. Here, the effect of superheating on expansion process is discussed by comparing qexp,out with and without superheating, based on mixture R290/R600a with various compositions, considering the change in ηexp,s. As shown in Fig. 13, a SHD of 5 K can effectively avoid liquid formation inside expander, with qexp,out being above the unit value even for the wet mixtures, no matter how the ηexp,s changes. When there is no superheating, the expander exhaust drops into the vapor-liquid two-phase region as xR290 surpasses 0.7
6. Conclusions In the present paper, a solar powered ORC-VCC refrigerator is investigated theoretically. Zeotropic mixtures are proposed as working fluids for the ORC-VCC system for the first time. Based on a developed mathematical model, the performance of eight pure refrigerants as well as 5 zeotropic mixtures with various compositions are evaluated and compared. The effects of various operating parameters as well as the introduction of IHE in ORC on the overall system performance are also analyzed. Several conclusions can be drawn from the analyses, as followings: (1) R600a is the most suitable pure working fluid for ORC-VCC working between −5 and 80 °C, with the largest ηoval of 0.2212. Dry fluids show more advantage than wet fluids in ηORC, while not all of the wet fluids show superiority in the refrigerating cycle, because of their higher value of cp/r. (2) Zeotropic mixtures show higher ηoval than the component fluids, by taking the advantage of temperature glide in reducing the cycle temperature lift. Among all of the mixtures, R161/R600a
Fig. 10. Variation of ηoval and CPRm versus Tevap,in.
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Fig. 14. Trend of ηoval under various IHE and superheating conditions.
Fig. 11. ηoval with and without IHE for pure refrigerants.
(3) Tgen,out affects the overall system performance of ORC-VCC greatly. Taking R290/R600a as an example, the maximum value of ηoval and CPRm is improved by 199.1% and 156.9%, respectively, with increasing Tgen,out from 60 to 120 °C. Slight decreases in xR290 in mixture composition corresponding to the maximum values appear, due to the increase in Tcrit of the mixture at the same time. (4) The increases in ηoval by IHE for zeotropic mixtures are larger than that for pure fluids, within a certain composition range, since more latent heat during condensing can be reclaimed by taking the advantage of temperature glide. (5) Superheating is necessary for the application of wet fluids in ORCVCC. For mixture R290/R600a, the upper limit in xR290 expands from 0.7 to 1 as the superheating increases from 0 to 5 K. Without IHE, superheating becomes ineffective in improving ηoval for either pure or mixture fluids. Acknowledgments Fig. 12. Variation of improvement in ηoval versus mixture composition.
This work is sponsored by the Research Foundation for Young Teachers of Xi’an Jiaotong University (HG1K023). Reference Aphornratana, S., Sriveerakul, T., 2010. Analysis of a combined Rankine–vapour–compression refrigeration cycle. Energy Convers. Manage. 51 (12), 2557–2564. Bu, X.B., Li, H.S., Wang, L.B., 2013. Performance analysis and working fluids selection of solar powered organic Rankine-vapor compression ice maker. Sol. Energy 95, 271–278. Calm, J.M., Hourahan, G.C., 2001. Refrigerant data summary. Eng. Syst. 18 (11), 74–88. Chang, H., Wan, Z., Zheng, Y., Chen, X., Shu, S., Tu, Z., Chan, S.H., 2017. Energy analysis of a hybrid PEMFC–solar energy residential micro-CCHP system combined with an organic Rankine cycle and vapor compression cycle. Energy Convers. Manage. 142, 374–384. Harby, K., 2017. Hydrocarbons and their mixtures as alternatives to environmental unfriendly halogenated refrigerants: an updated overview. Renew. Sustain. Energy Rev. 73, 1247–1264. Heberle, F., Preißinger, M., Brüggemann, D., 2012. Zeotropic mixtures as working fluids in Organic Rankine Cycles for low-enthalpy geothermal resources. Renewable Energy 37 (1), 364–370. Hung, T.C., Shai, T.Y., Wang, S.K., 1997. A review of organic rankine cycles (ORCs) for the recovery of low-grade waste heat. Energy 22 (7), 661–667. Karellas, S., Braimakis, K., 2016. Energy–exergy analysis and economic investigation of a cogeneration and trigeneration ORC–VCC hybrid system utilizing biomass fuel and solar power. Energy Convers. Manage. 107, 103–113. Kim, K.H., Perez-Blanco, H., 2015. Performance analysis of a combined organic Rankine cycle and vapor compression cycle for power and refrigeration cogeneration. Appl. Therm. Eng. 91, 964–974. Lemmon, E., Huber, M., McLinden, M., 2010. Reference Fluid Thermodynamic and Transport Properties - REFPROP. NIST Standard Reference Database 23 (Version 9.0). Li, H., Bu, X., Wang, L., Long, Z., Lian, Y., 2013. Hydrocarbon working fluids for a Rankine cycle powered vapor compression refrigeration system using low-grade
Fig. 13. Variation trend of qexp,out under different SHD and ηexp,s.
(xR161 = 0.25) comes to the top in ηoval, with a maximum value of 0.3089, followed by R152a/R600a (xR152a = 0.25) and R290/ R600a (xR290 = 0.45). In terms of WRm and CPRm, R290/R600a (xR290 = 0.4) and R161/R600a (xR161 = 0.25) ranks first, respectively. R290/R600a mixtures are the most preferred, considering the non-validated mixing rule for R161/R600a. 65
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