Scholarly article on topic 'Off-Design Analysis of Hydrocarbon-based Ejector-Expansion Refrigeration Cycle'

Off-Design Analysis of Hydrocarbon-based Ejector-Expansion Refrigeration Cycle Academic research paper on "Materials engineering"

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{"Vapor compression refrigeration cycle" / "Ejector expansion" / "Hydrocarbon refrigerant" / "Thermodynamic analysis"}

Abstract of research paper on Materials engineering, author of scientific article — Zhitong Ma, Xi Liu, Hanzhi Wang, Huashan Li, Xianlong Wang

Abstract In this paper, the off-design performance of an ejector-expansion refrigeration cycle (EERC) using several hydrocarbon refrigerants including propane, butane, isobutane and propylene as working fluids is analyzed based on energy and exergy concepts. A constant-pressure ejector flow model is employed in the simulation. The results indicate that, the hydrocarbon-based EERCs have higher COP, volumetric cooling capacity and exergy efficiency, as well as lower exergy destruction compared with the standard refrigeration cycle. In addition, as the difference between the condensing and evaporation temperatures increases, the advantages of the hydrocarbon-based EERCs become more prominent under off-design conditions. Propane and propylene used in the EERC in general have better performance than those of isobutene and butane.

Academic research paper on topic "Off-Design Analysis of Hydrocarbon-based Ejector-Expansion Refrigeration Cycle"

Procedia

The 8th International Conference on Applied Energy - ICAE2016

Off-Design Analysis of Hydrocarbon-based Ejector-Expansion Refrigeration Cycle

Zhitong Maa,b,c,d, Xi Liua,b,c, Hanzhi Wanga,b,c, Huashan Lia,b, Xianlong Wanga'b'*

a Key Laboratory of Renewable Energy, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou

510640, China

bGuangdong Key Laboratory of New and Renewable Energy Research and Development, Guangzhou 510640, China c University of Chinese Academy of Sciences, Beijing 100049, China _dShenzhen Academy of Metrology & Quality Inspection, Shenzhen, 518055, China_

Abstract

In this paper, the off-design performance of an ejector-expansion refrigeration cycle (EERC) using several hydrocarbon refrigerants including propane, butane, isobutane and propylene as working fluids is analyzed based on energy and exergy concepts. A constant-pressure ejector flow model is employed in the simulation. The results indicate that, the hydrocarbon-based EERCs have higher COP, volumetric cooling capacity and exergy efficiency, as well as lower exergy destruction compared with the standard refrigeration cycle. In addition, as the difference between the condensing and evaporation temperatures increases, the advantages of the hydrocarbon-based EERCs become more prominent under off-design conditions. Propane and propylene used in the EERC in general have better performance than those of isobutene and butane.

© 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the scientific committee of the 8th International Conference on Applied Energy. Keywords: Vapor compression refrigeration cycle; Ejector expansion; Hydrocarbon refrigerant; Thermodynamic analysis

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Available online at www.sciencedirect.com

ScienceDirect

Energy Procedia 105 (2017) 4685 - 4690

1. Introduction

In the past, several measures to improve the performance of vapour compression refrigeration cycle (VCRC) have been proposed. With this regard, the recovery of the work that is otherwise lost by the throttling process in the standard VCRC is one of the remarkable alternatives [1]. This improvement can be realized by using an expander, ejector or vortex tube instead of the throttle valve. Among them, the ejector is interesting thanks to the advantages of low cost, no moving parts and ability to handle two-phase flow without damage [2]. The modified VCRC that uses an ejector as an expansion device is denoted as the ejector-expansion refrigeration cycle (EERC) in this paper hereafter.

* Corresponding author. Tel.: +86 20 87057792; fax: +86 20 87057791. E-mail address: wangxl@ms.giec.ac.cn

1876-6102 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the scientific committee of the 8th International Conference on Applied Energy. doi:10.1016/j.egypro.2017.03.1015

Many researchers have focused on the EERC since 1990s [3]. All of the theoretical and experimental studies involved shows that the EERC has better energy performance than the standard VCRC. It is also found from the open literature that the previous EERC works usually relate to chlorofluorocarbons (CFCs [1,4]), hydrochlorofluorocarbons (HCFCs [4]), hydrofluorocarbons (HFCs [4-6]) and especially to CO2 [7-9]. It should be mentioned that, due to the increased environmental awareness, the CFCs, HCFCs and HFCs are now being regulated [10]. In addition, although CO2 is environment friendly, it suffers from high operation pressure, which will result in a heavy and costly machine. On the other hand, there is a renewed interest in the use of hydrocarbon (HC) refrigerant in the refrigeration industry recently [11]; however, the EERC studies on HC refrigerants are very limited.

The present paper aims to analyze and evaluate the EERC using HC refrigerants as working fluids based on energy and exergy concepts. The major HCs currently under consideration as refrigerants such as propane (R290), butane (R600), isobutane (R600a) and propylene (R1270) are considered. The cycle performance characteristics for each HC refrigerant are investigated. Also, the COP, volumetric cooling capacity, exergetic efficiency and exergy destruction of the cycle as well as their improvements over the standard VCRC are presented.

2. Cycle description

The schematic and corresponding p-h diagrams of the EERC are shown in Fig. 1. The main difference between the EERC and the standard VCRC is that, in the EERC the ejector served as an expansion device instead of the throttle valve in the standard VCRC. In the ejector, a high-pressure motive stream, i.e. the condensed refrigerant from the condenser, is expanded through the motive nozzle to a low pressure and high velocity. Meanwhile, a low-pressure suction stream, i.e. the vapor refrigerant from the evaporator, is entrained by the motive stream and enters the ejector through the suction nozzle. The two streams mix in the ejector mixing chamber, and then the mixed flow enter the diffuser, where its velocity drops and pressure increases to a value that is higher than the initial pressure of the suction stream. So, the ejector elevates the pressure at the compressor inlet compared with that of in the standard VCRC.

3. Thermodynamic analysis

The following assumptions are used in the analysis: (1) the system is operated under steady-state conditions; (2) the pressure drop in the heat exchangers and connection pipes are neglected; (3) the refrigerant leaving the condenser and evaporator outlets is saturated; (4) a constant-pressure mixing ejector is used; (5) one-dimensional homogeneous equilibrium flow in the ejector is considered; (6) the velocities at the inlet and outlet of the ejector are neglected.

Based on these assumptions, with one unit of the refrigerant mass at the compressor inlet, the cooling capacity and compressor work of the EERC are defined as [1]

where ^ is the ejector entrainment ratio; ^comp is the compressor isentropic efficiency and can be determined by the empirical relation proposed by Brunin et al [12]. Then the COP and volumetric cooling capacity (VCC) of the EERC can be obtained

öevap=^(h9 " )

(1) (2)

COP=u(h, -h8)/(h2 -hi) VCC=^(h9 -h8)/Vl

Fig. 1 Schematic and correspondingp-h diagrams of ejector-expansion refrigeration cycle

An exergy analysis can reflect the magnitude and location of irreversibility in the system [9], and in turn help to optimize the system. Assuming the reference state (0) at 300 K, the equations of exergy destruction for the compressor, condenser, evaporator, ejector and throttle valve are expressed as follows

EDcomp=T0 (S2 - * )

EDcond = (h2 - h3 )- T0 (S 2 - S 3 )

EDevap=^0 [(s9 -hg) + (hg -s9)/Tr ]

where 7=7^+5.

EDejet =T0 [(! + M)>S6 - S3 - MS9 ] EDthvv=^T0 (Sg " S7 )

The total exergy destruction (EDtot) of the EERC can be obtained EDtt=ED + ED d + ED + ED . t + EDth

tot comp cond evap ejet thvv

Then the exergy efficiency (nexgy) of the EERC is found by

^eXgy=1-EDtot/(h2 -\)

(10) (11)

Thus the COP, VCC, EDtot and nexgy improvements of the EERC over the standard VCRC are evaluated by Eqs. (12)-(15), respectively

A COP=(COP - COPsd)/COPsd (12)

A VCC=(VCC - VCCsd)/VCCsd (13)

AEDtot =(EDt0tsd "EDt0t )/EDt0t,Sd (I4)

^^exgy (^exgy ^exgy,sd ,sd (15)

where sd denotes a standard VCRC at the same condensing and evaporation temperatures.

For a given condensing temperature, evaporation temperature, ejector suction nozzle pressure drop, and specific ^mn, nsn and the algorithm presented by Sarkar [13] is used in the present study to calculate the ejector entrainment ratio. And then the other parameters can be found in ordinary fashion.

4. Results and discussion

A computer code in MATLAB is developed to model the HC-based EERCs, and the fluid thermophysical properties are obtained through a link with REFPROP 8.0 from NIST [14]. In the

following section, with ^mn=0.85, nsn=0.85 and nd=0.85 [1,13], the performance of the HC-based EERCs is investigated for the condensing temperature, Tcond, between 35 and 55 oC and the evaporation temperature, Tevap, varying from -10 to 15 oC.

The previous researches such as Bilir and Ersoy [5] and Sarkar [13] reported that there is a pressure drop, hp, in the suction nozzle, that allows the refrigerant to be entrained from the evaporator, makes the ejector area ratio optimum and the cycle performance best. Here, for example, the variations of COP, VCC, nexgy and EDtot of the R290 EERC at the typical air-conditioning conditions of TCond=40 oC and Tevap=5 oC with the hp are presented in Fig. 2. It shows that when the hp changes from 0.5 to 50 kPa, the COP, VCC and nexgy increase first to a peak valve and then decrease, while the EDtot deops first to a minimum valve and then increases. With the optimum hp of about 21.3 kPa for the case of R290, the COP, VCC, nexgy and EDtot read 6.060, 3594.684 kJ/m3, 29.159 kJ/kg and 0.361, respectively. The corresponding optimum ejector area ratio is 6.960. It should be mentioned that the COP and area ratio here agree well with those of reported by Sarkar [13] for R290 at the same operation conditions, which confirms the validity of the model developed in this paper. Also, it is found for the typical air-conditioning applications, the optimum ejector area ratios for the cases of R600, R600a and R1270 are 8.222, 7.552 and 7.078, respectively.

However, it is well known that a system may not always work at its design conditions [5]. Therefore, it is of great significance to analyze the performance of the HC-based EERCs under off-design operating conditions. The EERC with the ejector designed at Tcond=40 oC and Tevap=5 oC is considered in the offdesign analysis, which means the ejector area ratios for the cases of R290, R600, R600a and R1270 are optimum and constant at 6.960, 8.222, 7.552 and 7.078, respectively. The COP, VCC, nexgy and EDtot improvements of the HC-based EERCs over the corresponding standard VCRCs under the off-design operating conditions versus Tcond (Tevap=5 oC) and Tevap (Tcond=40 oC) are depicted in Fig. 3. The notation of dp in these graphs indicates the design points of the EERC.

With the figure, it can be found that even with the operation temperature deviates by at least ±10 oC from the design point, in terms of the COP, VCC, nexgy and EDtot, the HC-based EERCs with constant ejector area ratio still have improved performance compared with that of the standard VCRC. As Tcond increases from 30 to 55 oC with Tevap=5 oC, the cases of R290 and R1270 have comparable higher COP, VCC, nexgy and EDtot improvements than that of R600 and R600a. This finds are also valid for the varied Tevap considered. Moreover, the improvement ratios increase with the increased Tcond and the decreased Tevap too for the off-design operation. This means that the HC-based EERC is preferred to the standard VCRC for low-temperature applications and air-cooled systems.

¿ 6.00 o

o.at.i

gj 0.357 t?

0.354 0.351

3.55 <

3.56 i

3.54 5

3.52 30,4

30.0 „

29.6 £ 2

29.2 £

10 15 20 25 3» 35 40 45 50 Af>(M'a)

Fig. 2 COP, VCC, nexgy and EDtot of the R290 EERC versus pressure drop in suction nozzle with rcond=40 oC and

T =5 oC

1 evap ^ ^

'criint vs 'ova]] I— R290

I- -O- R600 I- -A- RMOa r- —V— R1270

'■.•ilnil vs 'evap I- —□- K2W -O- R600 I- Rfi№]

r- —V— R1270

CIINll '

Fig. 3 Performance improvement of HC-based EERCs over standard VCRCs versus 7cond (revap=5 oC) and revap (TcOnd=40 oC) under off-design conditions, (a) for COP, (b) for VCC, (c) for nexgy and (d) for EDtot

5. Conclusions

In this paper, with a developed mathematical model based on a constant-pressure ejector flow model, the off-design performance of the EERC using R290, R600, R600a and R1270 as refrigerants is analyzed and evaluated as the condensing temperature is between 30 and 55 oC and the evaporation temperature varies from -10 to 15 oC. The calculated parameters include COP, volumetric cooling capacity, exergy efficiency and exergy destruction.

The results indicate that, the HC-based EERC has higher COP, volumetric cooling capacity and exergy efficiency, as well as lower exergy destruction compared with the standard cycle, especially with the difference between condensing and evaporation temperatures increasing. At the typical air-conditioning conditions of Tcond=40 oC and Tevap=5 oC, the optimum performance of the EERC using R290, R600, R600a and R1270 can be found when the ejector area ratios reach 6.960, 8.222, 7.552 and 7.078, respectively. Besides, the HC-based EERC is strongly dependent on the refrigerant properties, and under the off-design conditions considered, the maximum improvement over the standard cycle is found in the cases of propane and propylene, in general, followed by isobutene, and butane is the worst one.

Acknowledgments

This research was supported by the Natural Science Foundation of Guangdong Province, China (Grant Nos. 2016A030313174 and 2015A030313714), the Science and Technology Program of Guangzhou, China (Grant Nos. 201607010106 and 2014J2200079) and the Key Laboratory of Renewable Energy, Chinese Academy of Sciences (Grant Nos. Y607j11001 and y507j71001).

References

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[2] Li HS, Cao F, Bu XB, Wang LB, Wang XL. Performance characteristics of R1234yf ejector-expansion refrigeration cycle. Appl Energy 2014; 121: 96-103.

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[6] Chaiwongsa P, Wongwises S. Experimental study on R-134a refrigeration system using a two-phase ejector as an expansion device. Appl Therm Eng 2008; 28: 467-77.

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[8] Elbel S, Hrnjak P, Experimental validation of a prototype ejector designed to reduce throttling losses encountered in transcritical R744 system operation. Int J Refrig 2008; 31:411-22.

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[10] Jung D. Editorial: Energy and environmental crisis: let's solve it naturally in refrigeration and air conditioning!. HVAC&R Res 2008; 14:631-4.

[11] Li HS, Bu XB, Wang LB, Long Z, Lian YW. Hydrocarbon working fluids for a Rankine cycle powered vapor compression refrigeration system using low-grade thermal energy. Energy Buildings 2013; 65: 167-72.

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Xianlong Wang, born in 1979, is now an assistant research fellow in Guangzhou Institute of Energy Conversion at Chinese Academy of Sciences. His research interests mainly include solar energy, seawater desalination and low-temperature thermal power generation technologies.