Scholarly article on topic 'Energetic and Exergetic Investigation of a N 2 O Ejector Expansion Transcritical Refrigeration Cycle'

Energetic and Exergetic Investigation of a N 2 O Ejector Expansion Transcritical Refrigeration Cycle Academic research paper on "Materials engineering"

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Abstract of research paper on Materials engineering, author of scientific article — Kapil Dev Choudhary, M.S. Dasgupta, Shyam sunder

Abstract Thermodynamic analysis of a transcritical N2O ejector expansion refrigeration cycle is analysed. A two phase ejector, as an expansion device, is used in place of a conventional expansion valve. Effect of three performance parameters for cycle, namely COP, refrigerating effect, compressor work and two performance parameters of ejectors namely entrainment ratio and pressure recovery ratio are evaluated for various motive nozzle inlet conditions and evaporator temperatures. Further, both energetic and exergetic comparison of the proposed cycle is presented with respect to conventional CO2 ejector expansion refrigeration cycle. The N2O ejector expansion system is found to have higher COP, lower compressor discharge pressure and higher entrainment ratio but have disadvantage of lower volumetric cooling capacity. Maximum COP of N2O ejector cycle is found to be about 10% higher than maximum COP of CO2 ejector cycle. Exergetic output of N2O ejector cycle is higher whereas losses occurred due to irreversibility during expansion is lower.

Academic research paper on topic "Energetic and Exergetic Investigation of a N 2 O Ejector Expansion Transcritical Refrigeration Cycle"

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Energy Procedía 109 (2017) 122 - 129

International Conference on Recent Advancement in Air Conditioning and Refrigeration, RAAR 2016, 10-12 November 2016, Bhubaneswar, India

Energetic and Exergetic Investigation of a N2O Ejector Expansion

Transcritical Refrigeration Cycle

Kapil Dev Choudhary*, M.S. Dasgupta, Shyam sunder

Birla Institute of Technology & Science, Pilani, 333031, India

Abstract

Thermodynamic analysis of a transcritical N2O ejector expansion refrigeration cycle is analysed. A two phase ejector, as an expansion device, is used in place of a conventional expansion valve. Effect of three performance parameters for cycle, namely COP, refrigerating effect, compressor work and two performance parameters of ejectors namely entrainment ratio and pressure recovery ratio are evaluated for various motive nozzle inlet conditions and evaporator temperatures. Further, both energetic and exergetic comparison of the proposed cycle is presented with respect to conventional CO2 ejector expansion refrigeration cycle. The N2O ejector expansion system is found to have higher COP, lower compressor discharge pressure and higher entrainment ratio but have disadvantage of lower volumetric cooling capacity. Maximum COP of N2O ejector cycle is found to be about 10 % higher than maximum COP of CO2 ejector cycle. Exergetic output of N2O ejector cycle is higher whereas losses occurred due to irreversibility during expansion is lower.

© 2017 The Authors. Publishedby ElsevierLtd. 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 organizing committee of RAAR 2016. Keywords:COP; ejector; transcritical; refrigeration; N2O; CO2

* Corresponding author. Tel.: +919461338404 E-mail address:kapilkaswan1987@gmail.com

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 organizing committee of RAAR 2016. doi:10.1016/j.egypro.2017.03.073

Nomenclature

amb = ambient c = compressor

COP = coefficient of performance d = diffuser

h = specific enthalpy(kJ/kg) eje = ejector

I = specific exergy (kJ/kg) evap = evaporator

P = pressure exp = control valve

Q = specific heat transfer rate (kJ/kg) gc = gas cooler

S = specific entropy (kJ/kgK) gco = gas cooler outlet

T = temperature gen = generation

U = entrainment ratio m = motive nozzle

x = vapour quality mix = mixing chamber

n = efficiency (%) ref = refrigerated object

V = velocity (m/sec) rec = recovery

VCC = volumetric cooling capacity (kJ/m3) rev = reversible

W = specific work (kJ/kg) s = isentropic

Subscripts suc = suction

0 = reference environment t = total

1, 2, 3... = cycle locations

1. Introduction

A number of natural refrigerants are available for use in refrigeration and air conditioning applications. Advantage of natural refrigerants such as air, water, ammonia, carbon dioxide, nitrous oxide, isobutene, propane etc. are their zero ODP and low GWP and cost. CO2 based systems have already gained reasonably good compliance in refrigeration applications in colder countries. Potentials of N2O is yet to be fully explored. N2O has similar properties as CO2 in terms of molecular weight and critical temperature & pressure [1]. However, it is less favourable in terms of GWP compared to CO2. Table 1 shows comparison of important properties of N2O and CO2 and inference about N2O.

Table 1 Properties of Carbon dioxide and nitrous oxide and comparative inferences.

Properties CÖ2 N2O Inference about N2O

Critical pressure (MPa) 7.377 7.245 Lower heat rejection pressure operation

Critical temperature (°C) 31.1 36.4 Moderately warm weather operation

Boiling point (°C) -78.4 -88.47 Ensures single phase at evaporator exit.

Triple point temperature (°C) -56.55 -90.82 Lower evaporator temperature operation

Toxicity (ppm) 5000 1000 Low toxicity

Molecular weight (kg/kmol) 44.01 44.013 -

GWP 1 240 Higher GWP

ODP 0 0 -

Latent heat of vaporization (kJ/kg) 574 374.28 Higher mass of refrigerant in evaporator

Sarkar and bhattacharya [2] presented a comparative study of transcritical N2O refrigeration/heat pump cycle with transcritical CO2 cycle. They concluded that N2O cycles perform better and have lower high side pressure compared to CO2 cycle. Further, Sarkar [3] proposed a transcritical N2O refrigeration cycle with an internal heat exchanger and compared it with CO2 transcritical cycle having same configuration. It was reported that use of IHX is comparatively less effective in N2O cycle. The paper also attempted to optimize the high side pressure for N2O transcritical cycle having IHX. Agrawal et. al. [4], introduced a novel two stage transcritical N2O cycle and compared it with similar cycle configuration of CO2. It was demonstrated that a two stage transcritical N2O cycle exhibits higher performance and lower optimum compressor discharge pressure compared to a two stage CO2

transcritical cycle. They also reported that COP of the N2O cycle was higher compared to single stage N2O cycle.

Critical temperature of N2O and CO2 are 36.4°C & 31.1°C respectively, they require higher heat rejection pressure for maximizing COP, compared to conventional refrigerants in warm weather condition. Because of high operating pressure, large expansion losses typically occur during expansion from supercritical region to subcritical region through the throttling valve. Many researchers [5-8] explored two phase ejector as an expansion device mainly for CO2 transcritical cycle and successfully demonstrated that ejector expansion have higher cycle performance due to expansion work recovery. To the best of knowledge of the authors, use of ejector has not been reported as an expansion device for N2O transcritical system. In this paper, a novel cycle configuration having ejector as an expansion device for N2O based transcritical refrigeration system is analysed. Further, the performance is investigated at various operating conditions and the results are compared with similar cycle configuration of CO2.

2. Ejector expansion cycle

An ejector expansion transcritical refrigeration cycle (EETRC) is conceived having a compressor, a gas cooler, a two phase ejector, a liquid vapour separator, a control valve and an evaporator as shown in Fig. 1(a). The ejector expansion device comprises of a motive nozzle, a mixing chamber, a constant area throat and a diffuser. Fig. 1(b) shows the schematic and P-h chart of ejector expansion transcritical refrigeration cycle.

Fig. 1 Schematic of ejector expansion transcritical refrigeration cycle and P-h chart.

3. Assumptions

EETRC for the two refrigerants (N2O and CO2) are analysed based on the following assumptions:

• Steady state operation.

• Kinetic energies of refrigerant neglected at ejector inlet and outlet.

• Approach temperature (Tgco-T0) of gas cooler is taken as zero.

• Unless stated otherwise, refrigerant temperature at the gas cooler outlet (Tgco) is 45°C and the evaporator temperature (Tevap) is 5°C.

• The refrigerant condition is saturated vapour at the exit of the evaporator.

• Constant pressure mixing at mixing section of ejector.

With an assumption of unit mixture mass flow rate at ejector outlet, computer model of ejector expansion transcritical refrigeration cycle is developed. The ejector entrainment ratio (U) is defined as ratio of suction mass flow rate to motive mass flow rate. Therefore, for 1 kg of refrigerant mixture at ejector exit, the motive mass flow rate is 1/(1+U) kg and suction mass flow rate is U/(1+U). Pressure recovery ratio is defined as the ratio of pressure at the exit of ejector to evaporator pressure.

4. Thermodynamic modelling

Thermodynamic model of ejector expansion transcritical refrigeration cycle is developed using basic energy and exergy relations as tabulated in Table 2.

Table 2 Energy and exergy relation for ejector expansion transcritical refrigeration cycle.

Subsystems Energy relations_Exergy relations_

Compress°r Wc = (ft2 _ 1/(1 + tf)), Ic = Tq(S2 _ 51)/(1 + U) = 1.003 - 0.121 (Pgc/Psuc),Vc

= (h2s - h1)/{h2 - h1)

Gas cooler Qgc = (h2 - h3)/(l + U) Igc = (h2 - h3-T0(S2 - 53))/(l + U) Ejector _ _ h3-h4 , _ Yi ( S3 STU\

Vm - ^ J* 2 , Ieje = To - — - —]

= v4/(i + U),h6 = h3/(1 + U) + hsU/(i + U), he ~h5 = VÏ/2, Vd = x6 = 1/(1 + U)

omn^ hl = h6l 6 5 icv = T0(S7 -S6l)tf/(l + V)

Evap°rator Qevap = (h8 _ h7)U/(l + tf), 7CC = (ft8 -h7)*D7 J = T (s _s , ^zz^i^

erap 0^8 7 r,.^/ 1+U

The relationship between refrigerant vapour mixture quality at ejector outlet and ejector entrainment ratio is given by equation (1)

x6 = 1/(1 + U) (1) Pressure recovery ratio is defined as

P-RR~Pejector/Pevap (2)

Cycle performance is computed as

cop = -h7))/Wc (3)

The total exergy destruction of the cycle is sum of individual component exergy destruction rates:

h = ¡C + ¡CV + Igc + ¡eje + 4vap (4)

An exergy balance for the cycle is:

We = Wrev + /t (5)

The Second law efficiency for each cycle is computed using following equation:

W'rev t r\

Vund = — (6)

5. Results and discussion

Simulation for EETRC is developed in MATLAB. State properties are of refrigerants computed using NIST REFPROP version 9.0.

Fig. 2 shows variation of COP for various gas cooler exit pressure for N2O and CO2 at three different gas cooler exit temperatures keeping evaporator temperature constant as 5°C. COP for both N2O and CO2 cycle is observed to reach a maximum value at lower gas cooler exit pressure and then decreases with increase in pressure. This can be explained as at higher gas cooler exit pressure and temperature, vapour quality at ejector exit is also higher which in turn lead to higher mass flow rate handled by compressor. It is also concluded that N2O have lower optimum gas cooler exit pressure compared to CO2. However, it is observed that N2O ejector cycle gives comparatively better performance at gas cooler pressure below 8.5 MPa, 9.5 MPa and 10.5 MPa for gas cooler exit temperature 35°C, 40°C and 45°C respectively. The maximum COP obtainable of N2O EETRC is 10.13 % higher than maximum COP of CO2 EETRC.

Fig. 3 exhibits effect of gas cooler exit pressure on cycle performance parameters (COP & VCC) at fixed gas cooler exit temperature and evaporator temperature.

1Evap. — 5°C n2o tgco co2

P- - - - a- - - 45° c —

-ex _ ' " " " ' ' - -A ' - - i— - - v— 40° c ■ 35 "c ♦

- - -Q - - - Q. . B/ — -A " :" - "a

7 8 9 10 11 12 13

Gas cooler exit pressure MPa

Fig. 2 Effect of gas cooler exit temperature on cycle performance

O 1.5 d

.. ~—■ ' tu- - . - n

/1 f 1Evap. = 5 °C

1 / TQCO = 45° C

n2o co2

c.o.p. --0— -—

v.c.c. ^

9 10 11 12

Gas cooler exit pressure MPa

Fig. 3 Effect of gas cooler exit pressure on cycle performance

It is noted that COP of N2O EETRC is higher than CO2 EETRC at gas cooler exit pressure below 10.5 MPa. A slight increment in volumetric cooling capacity (VCC) is observed at lower gas cooler exit pressure for N2O. VCC of N2O is found higher than CO2 at gas cooler exit pressure below about 9.3 MPa which in turn helps to have higher COP at lower gas cooler exit pressure. At higher gas cooler exit pressure however, N2O ejector cycle will require more mass of refrigerant in evaporator compared to CO2 ejector cycle. Maximum cooling capacity of CO2 EETRC and N20 EETRC are 22.26 MJ/m3( 12.5 MPa) and 20.07 MJ/m3( 11 MPa) respectively.

£ 1.8

^evap, = 5 °c -A- -< - &

Tgco = 45°C A - A. ~ ~

n2o co2

1 ¡A p.r.r. - - c— ■

' v' I ent.

>' & J

'Q /-0--.-E, Tg____B

0.2 c HI

9 10 11 12

Gas cooler exit pressure MPa

Fig. 4 Effect of gas cooler exit pressure on ejector performance

Variation in ejector performance with respect to gas cooler exit pressure is expressed in in terms of pressure recovery ratio (PRR) and entrainment ratio (ER) as shown in Fig. 4. It is observed that with increase in gas cooler exit pressure, PRR decreases whereas ER increases and beyond about 11 MPa pressure, there is very less change with further increase in gas cooler exit pressure. Possible explanation is that, at higher gas cooler pressure, vapour quality produced at ejector exit is also higher which in turn lead to higher entrainment ratio as per relation given in equation (1). N2O EETRC shows higher ER while CO2 EETRC shows higher PRR for entire range of operation.

From the above discussion, it is concluded that the volumetric cooling capacity for both the refrigerants do not vary much with increase in gas cooler pressure but the entrainment ratio varies significantly. This means, although the mass flow rate in the evaporator remains almost constant with increase in gas cooler exit pressure, yet there is decrease in motive nozzle mass flow rate, which reflects decrease in the total system mass flow rate. In order to compare the performance of two refrigerants, the system mass flow rate is an important criterion. Table 3 shows the computed system mass flow rate for various operating conditions for a 10 kW refrigeration system.

Table 3 System mass flowrate required in a N2O and a CO2 EETRC system.

Pgco Tgco = 35 C Tgco = 40 C Tgco = 45 C

N2O (kg/s) CO2 (kg/s) N2O (kg/s) CO2 (kg/s) N2O (kg/s) CO2 (kg/s)

7.5 0.120957* 0.744079 0.348826 - 0.613258 -

8.0 0.118198 0.188299 0.144636* - 0.489854 -

8.5 0.116435 0.135647* 0.131148 0.328854 0.20928 -

9.0 0.115302 0.128887 0.126591 0.180172 0.156592* 0.659488

9.5 0.114425 0.125473 0.124118 0.150918* 0.142776 0.27299

10.0 0.11368 0.123358 0.122329 0.14131 0.136808 0.195024

10.5 0.113156 0.121854 0.121005 0.136325 0.133197 0.156685*

11.0 0.112646 0.120657 0.119978 0.13312 0.130663 0.149877

11.5 0.112244 0.119735 0.119221 0.130905 0.128882 0.145487

12.0 0.111946 0.119065 0.118602 0.129249 0.127478 0.142237

12.5 0.111654 0.118416 0.118 0.127949 0.12641 0.659488

* System mass flow rates for maximum COP (optimum operating condition)

It can be concluded that at maximum COP condition, N2O EETRC requires less overall mass flow rate compared to

C02 EETRC.

Fig. 5 Effect of evaporator temperature on cycle performance

Fig. 5 shows the effect of evaporator temperature on the cycle performance parameters at fixed inlet condition of motive nozzle (Pgco=10.5 MPa and Tgco=45°C). It is observed that COP for both cycles increases with increase in evaporator temperature. Although there is decrement in RE and Wc with increment in evaporator temperature yet the

decrement in compressor work is more prominent. COP of N2O EETRC is lesser compared to CO2 EETRC at lower evaporator temperature. Both cycles approach towards equal COP at higher evaporator temperature. It can be

ascribed to the fact that at lower evaporator temperature the difference is higher.

■S i.¡

(1) DC

Pgco Tgco = 10.5 MPa - 45° c .A--------* "

n2o co-, '>-—p.r.r. -0--- —•—

^ "- - " " * - □--- Mi. --A--" ■ " - - £J---. . 1 " ......-[

0.2 £= LD

-20 -15 -10 -5 0 5

Evaporator temperature

Fig. 6 Effect of evaporator temperature on ejector performance

Fig. 6 shows the effect of evaporator temperature on ejector performance for both the refrigerants keeping motive nozzle inlet condition (Pgco=10.5 MPa and Tgco=45°C) constant. It is observed that PRR decreases while ER increases as evaporator temperature increases. At lower evaporator temperature & pressure, the difference in pressure of motive nozzle inlet and outlet is much higher, this leads ejector to gain higher pressure recovery and lower ER. Overall, it can be concluded that ER is higher whereas PRR is lower for N2O EETRC compared to CO2 EETRC for entire range of operation.

Exergy Output

f/21.0% 20.7%

Exergy Output

j> 19.2%

K!> 23'3%

¡> 20.7%

(a) Ejector expansion N2O transcritical cycle (b) Ejector expansion CO2 transcritical cycle

Fig. 7 Component-wise exergy destruction (Grassman chart)

Comparison of component wise exergy destruction for both cycles are shown in Fig. 7 for maximum COP condition. Exergy output of N2O EETRC is found higher compared to CO2 EETRC which implies that N2O cycle utilizes the available energy more effectively at the specified condition. Further, the most irreversible components identified are the gas cooler, the compressor and the ejector for both the cycles. Exergy destruction of ejector in N2O EETRC is lesser compared to CO2 cycle which shows that ejector is quite effective in N2O transcritical refrigeration cycle.

Fig. 8 shows the variation of Second law efficiency with change in gas cooler outlet pressure and temperature. It can be seen that the 2nd law efficiency of N2O ejector cycle is substantially higher compared to CO2 EETRC at lower gas cooler exit pressure whereas the same steadily decreases with increase in gas cooler pressure for both the refrigerants.

TEvap. = 5°C N20 Tgco C02

3* 0-25

9 10 11

Gas cooler exit pressure MPa

Fig. 8 Effect of gas cooler exit pressure and temperature on II law efficiency

6. Conclusion

• Maximum COP of N2O EETRC is 10.13% higher than maximum COP of CO2 EETRC for the fixed evaporator temperature analysed.

• N2O EETRC shows better cycle performance at lower gas cooler exit pressure while CO2 EETRC have better cycle performance at higher gas cooler exit pressure.

• Volumetric cooling capacity of N2O EETRC is lower than CO2 EETRC.

• Pressure recovery ratio for N2O EETRC is lower while entrainment ratio is higher compared to CO2 EETRC for full range of gas cooler exit pressure.

• Pressure recovery ratio decreases whereas entrainment ratio increases with increase in both gas cooler exit pressure and evaporator temperature.

• Exergy output for N2O EETRC is 29.9 % whereas for CO2 EETRC, it is 27.2 % at maximum COP condition, implying N2O EETRC uses available energy more effectively.

• Ejector have less irreversibility in case of N2O EETRC compared to CO2 EETRC.

• 2nd law efficiency for N2O EETRC is higher at lower gas cooler exit pressure compared to CO2 EETRC.

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Reference