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Thermodynamic analysis of a novel GAX absorption refrigeration cycle
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Thermodynamic analysis of a novel GAX absorption refrigeration cycle Yuqi Shi a,*, Qin Wang a, Daliang Hong b, Guangming Chen a a
Institute of Refrigeration and Cryogenics, Zhejiang University, Key Laboratory of Refrigeration and Cryogenic Technology of Zhejiang Province, Hangzhou, China b China Electronics Technology Group Corporation No. 38 Research Institute, Hefei, Anhui Province, China
article info
abstract
Article history:
A novel GAX absorption refrigeration cycle is proposed in this paper. Part of the solution
Received 18 August 2016
out from water cooled absorber flows to a refrigerant cooled absorber to absorb part of the
Received in revised form
vapor out from evaporator. The refrigerant cooled absorber is cooled by refrigerant which
27 October 2016
evaporates at an intermediate pressure. The vapor at intermediate pressure is absorbed by
Accepted 28 October 2016
the solution out from refrigerant cooled absorber in a high pressure absorber. The proposed
Available online xxx
cycle can make use of the absorption heat that can not be utilized by standard GAX cycle to make additional refrigeration. Therefore, the COP of the proposed cycle is much higher
Keywords:
than that of the standard GAX (SGAX) cycle. Simulation results show that the COP of the
Absorption cycle
proposed cycle is 20% higher than that of SGAX cycle at most simulated conditions.
Refrigeration
© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Simulation Thermodynamic analysis GAX
Introduction Because of the environmental problems caused by conventional vapor compression refrigerators, sorption refrigeration machines have attracted more and more attentions [1e4]. Absorption refrigeration devices are narrowly defined as closed-liquid sorption cycles [5]. Absorption chillers and heat pumps can be integrated with various systems such as hydrogen liquefaction, cooling and power generation [6e9]. Usage of the second thermodynamic law, thermo-economic method, artificial neural network method and commercial software like Aspen-Plus for analyzing and optimizing the performance of the systems has become popular in recent years [10e13]. The main working pairs used in absorption
refrigeration machines are H2O/LiBr and NH3/H2O, which are friendly to environmental [14,15]. The refrigerators using H2O/ LiBr as working pair can not make refrigeration at temperature lower than 0 C due to freezing and crystallization issues [16]. Generally, NH3/H2O absorption refrigeration machines were used to make refrigeration at low temperature [17] and can be driven by waste heat with large temperature glide [18]. As ammonia concentration decreases, the saturation temperature of the solution at a given pressure increases. If the ammonia concentration of the solution out from generator is low enough, there may be a temperature overlap between generator and absorber. Therefore, GAX absorption refrigeration cycle is regarded as a good method to improve the performance of the NH3/H2O absorption refrigeration machine by using part of the absorption heat to drive generator [19e21]
* Corresponding author. E-mail address: [email protected] (Y. Shi). http://dx.doi.org/10.1016/j.ijhydene.2016.10.155 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Shi Y, et al., Thermodynamic analysis of a novel GAX absorption refrigeration cycle, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.10.155
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Nomenclature Symbols h m P Q T W x
specific enthalpy, kJ mol1 molar mass flow rate, mol s1 pressure, kPa heat transfer rate, W temperature, C pump work molar ratio, mol/mol
Greek symbols h relative increasing ratio, % Subscripts A/abs absorber C/con condenser E/evap evaporator G/g generator rec rectifier WCA water cooled absorber h, m, l high, middle, low 1, 2, 3 … state points Abbreviations COP coefficient of performance GAX generation absorption heat exchange GAXAH high temperature GAX absorber GAXAL low temperature GAX absorber GAXGH high temperature GAX generator GAXGL low temperature GAX generator HPWCA high pressure water cooled absorber LGS liquid gas separator PGAX proposed GAX RCA refrigerant cooled absorber SCA solution cooled absorber SGAX standard GAX SHG solution heated generator SHX solution heat exchanger WCA water cooled absorber But SGAX absorption refrigeration cycle can not make use of the absorption heat at lower temperature, which has negative effect on its performance and limits its applications. Although HGAX absorption refrigeration cycle can make use of part of the low grade absorption heat by combing a compressor [22e29], it needs to consume additional electric power. This paper proposed an improved GAX absorption refrigeration cycle. Part of the low grade absorption heat is also used to make additional refrigeration. Then the performance of the proposed cycle can be much better than that of SGAX absorption refrigeration cycle.
Cycle description Fig. 1 shows the diagram of a SGAX absorption refrigeration cycle. Fig. 2 shows the diagram of the proposed cycle. The proposed cycle consists of the following components: generator, SHG(solution heated generator), GAXGH (high
Fig. 1 e The diagram of SGAX absorption refrigeration cycle.
temperature GAX generator), GAXGL (low temperature GAX generator), GAXAH (high temperature GAX absorber), GAXAL (low temperature GAX absorber), SCA (solution cooled absorber), WCA (water cooled absorber), HPWCA (high-pressure water cooled absorber), LGS (liquidegas separator), RCA (refrigerant cooled absorber), SHX (solution heat exchanger), Pre-cooler, Rectifier, evaporator, condenser, Pumps, throttles valves and mixers. The working principle of the proposed cycle using NH3/H2O, as working pair is shown as follow. The vapor separated from generator (1) flows to rectifier through SHG, GAXGH and GAXGL in turn. The purified ammonia vapor from rectifier (5) flows to condenser and is condensed to be saturated liquid there. Then the liquid (6) passes through the TV 1 and is throttled to an intermediate pressure. Part of the liquid ammonia at this pressure evaporates and the cooling capacity is used to balance the heat rejected by the RCA. Then the stream (8) flows to LGS and is separated into liquid phase and vapor phase there. The liquid ammonia (9) is subcooled and then is throttled to evaporating pressure and completely evaporates in the evaporator. The vapor (12) out from evaporator passes through the pre-cooler. Then part of the vapor (43) flows to RCA and is absorbed there; the other part of the vapor (13) is absorbed by WCA, SCA, GAXAL and GAXAH respectively. The weak solution from generator (23) passes through SHG and is cooled there. Then the solution is throttled to evaporating pressure. The depressurized solution (25) passes through GAXAH, GAXAL, SCA, WCA in turn and absorbs vapor there. The heat rejected by GAXAH and GAXAL is used to drive GAXGH and GAXGL respectively. Part of the solution (34) out from WCA flows to RCA through SHX and absorbs vapor there. Then the solution (36) is pumped to intermediate pressure and absorbs vapor from LGS in HPWCA. The solution (39) out from HPWCA is preheated by heat rejected by SCA. Then the solution (40) is pumped to generation pressure and flows to GAXAL after mixing with the solution from rectifier. The other part of the solution out from WCA (30) is pumped to generation pressure and flows to GAXGH after mixing with the solution out from GAXGL. The solution (21) out from GAXGH flows to generator through SHG. The cycle is finished. The term ‘‘weak solution’’ represents a solution that is weak with ammonia.
Please cite this article in press as: Shi Y, et al., Thermodynamic analysis of a novel GAX absorption refrigeration cycle, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.10.155
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e8
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Fig. 2 e The diagram of the proposed cycle.
It can be found that the heat rejected by GAXAL that can not be used by SGAX absorption refrigeration cycle to drive generator is utilized by the proposed cycle. The heat is used to drive GAXGL to separate the additional refrigerant vapor. The additional refrigerant is absorbed by RCA and HPWCA. The additional refrigerant absorbed by RCA provides the additional mass flow rate of the refrigerant in evaporator, which provides additional cooling capacity directly. In addition to cooling the RCA, the cooling capacity supplied the additional refrigerant absorbed by HPWCA is also used to subcool the refrigerant out from condenser, which will improve the cooling capacity per unite mass of refrigerant evaporating in the evaporator. Then the performance of the proposed cycle can be much better than that of SGAX absorption refrigeration cycle.
A Du¨hring diagram of the proposed cycle is illustrated in Fig. 3.
Mathematical model The authors wrote a procedure to evaluate the performance of the proposed cycle. To simplified the model, some assumptions are assumed as follow, (1) The system runs in a steady state. (2) The process of pumps is assumed to be adiabatic processes and isentropic efficiency is assumed to be 75%. (3) Pressure drops along the pipe lines are neglected.
Fig. 3 e Du ¨ hring diagram of the proposed cycle. Please cite this article in press as: Shi Y, et al., Thermodynamic analysis of a novel GAX absorption refrigeration cycle, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.10.155
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(4) The solutions out from generator, WCA, HPWCA, rectifier, GAXGH, GAXGL, SCA, GAXAL, GAXAH are saturated solutions. (5) The outlet state from the condenser is saturated liquid and the outlet state from the evaporator is saturated vapor. (6) The temperature difference between the cold end of GAXGL and that of GAXAL is 0 C; the temperature difference between the cold end of GAXGH and that of GAXAH is 0 C. (7) The temperature difference at the cold end of SHX is 5 C. (8) The temperature difference at the cold end of RCA is 5 C. (9) The effectiveness of Pre-cooler is 0.8. (10) The condensing temperature is 40 C, the temperature of the solutions out from WCA and HPWCA is 40 C. The basic models for all of the components include mass balance equations, energy balance equations and ammonia mass balance equations. Ammonia mass balance equation is not considered for the components such as evaporator, condenser, because the ammonia concentration does not change when the stream passes through these components. The balance equations of each component are shown respectively as follows, Generator m22 h22 þ Q g ¼ m1 h1 þ m23 h23
(1)
m22 x22 ¼ m1 x1 þ m23 x23
(2)
m22 ¼ m1 þ m23
(3)
SHG m22 h22 þ m24 h24 þ m2 h2 ¼ m1 h1 þ m21 h21 þ m23 h23
(4)
m22 x22 þ m24 x24 þ m2 x2 ¼ m1 x1 þ m21 x21 þ m23 x23
(5)
m23 ¼ m24
(6)
x23 ¼ x24
(7)
m22 þ m24 þ m2 ¼ m1 þ m21 þ m23
(8)
m4 ¼ m5 þ m17
(16)
m4 x4 ¼ m5 x5 þ m17 x17
(17)
Condenser m5 h5 ¼ m6 h6 þ Q con
(18)
m5 ¼ m6
(19)
TV 1 m6 h6 ¼ m7 h7
(20)
m6 ¼ m7
(21)
RCA m35 h35 þ m43 h43 þ m7 h7 ¼ m36 h36 þ m8 h8
(22)
m35 x35 þ m43 x43 þ m7 x7 ¼ m36 x36 þ m8 x8
(23)
m35 þ m43 ¼ m36
(24)
m7 ¼ m8
(25)
LGS m8 h8 ¼ m9 h9 þ m44 h44
(26)
m8 ¼ m9 þ m44
(27)
TV 2 m10 h10 ¼ m11 h11
(28)
m10 ¼ m11
(29)
Evaporator
GAXGH m2 h2 þ m20 h20 þ Q GAXGH ¼ m3 h3 þ m21 h21
(9)
m2 x2 þ m20 x20 ¼ m3 x3 þ m21 x21
(10)
m2 þ m20 ¼ m3 þ m21
(11)
GAXGL m3 h3 þ m18 h18 þ Q GAXGL ¼ m4 h4 þm19 h19
(12)
m3 x3 þ m18 x18 ¼ m4 x4 þ m19 x19
(13)
m3 þ m18 ¼ m4 þ m19
(14)
m11 h11 þ Q evap ¼ m12 h12
(30)
m11 ¼ m12
(31)
TV 3 m24 h24 ¼ m25 h25
(32)
m24 ¼ m25
(33)
GAXAH m25 h25 þ m16 h16 ¼ m26 h26 þ Q HPWCA
(34)
m25 x25 þ m16 x16 ¼ m26 x26
(35)
m25 þ m16 ¼ m26
(36)
GAXAL m15 h15 þ m26 h26 þ m32 h32 ¼ m16 h26 þ m27 h27 þ m33 h33 þ Q GAXAL (37)
Rectifier m4 h4 ¼ m5 h5 þ m17 h17 þ Q rec
(15)
m15 x15 þ m26 x26 þ m32 x32 ¼ m16 x26 þ m27 x27 þ m33 x33
(38)
Please cite this article in press as: Shi Y, et al., Thermodynamic analysis of a novel GAX absorption refrigeration cycle, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.10.155
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m15 þ m26 þ m32 ¼ m16 þ m27 þ m33
(39)
SCA m14 h14 þ m27 h27 þ m31 h31 þ m39 h39 ¼ m15 h15 þ m28 h28 þ m32 h32 þ m40 h40
(40)
Pump 3 m40 h40 þ WP3 ¼ m41 h41
(60)
m40 ¼ m41
(61)
Mixer 1
m14 x14 þ m27 x27 ¼ m15 x15 þ m28 x28
(41)
m17 h17 þ m41 h41 ¼ m18 h18
(62)
m14 þ m27 ¼ m15 þ m28
(42)
m17 x17 þ m41 x41 ¼ m18 x18
(63)
m31 ¼ m32
(43)
m17 þ m41 ¼ m18
(64)
x31 ¼ x32
(44)
m39 ¼ m40
(45)
x39 ¼ x40
(46)
WCA m28 h28 þ m13 h13 ¼ m29 h29 þ m14 h14 þ Q WCA
(47)
m28 x28 þ m13 x13 ¼ m29 x29 þ m14 x14
(48)
m28 þ m13 ¼ m29 þ m14
(49)
Mixer 2 m19 h19 þ m33 h33 ¼ m20 h20
(65)
m19 x19 þ m33 x33 ¼ m20 x20
(66)
m19 þ m33 ¼ m20
(67)
Pre-cooler m9 h9 þ m12 h12 ¼ m10 h10 þ m42 h42
Pump 1 m30 h30 þ WP1 ¼ m31 h31
(50)
m30 ¼ m31
(51)
HPWCA
(68)
In addition, it is obvious that Q GAXAH ¼ Q GAXGH
(69)
Q GAXAL ¼ Q GAXGL
(70)
m42 ¼ m13 þ m43
(71)
m29 ¼ m30 þ m34
(72)
m38 h38 þ m44 h44 ¼ m39 h39 þ Q HPWCA
(52)
COP is adopted for the cycle performance evaluation in this paper. It is defined as:
m38 x38 þ m44 x44 ¼ m39 x39
(53)
. COP ¼ Q evap Q g
m38 þ m44 ¼ m39
(54)
SHX m37 h37 þ m34 h34 ¼ m38 h38 þ m35 h35
(55)
m37 x37 þ m34 x34 ¼ m38 x38 þ m35 x35
(56)
m37 þ m34 ¼ m38 þ m35
(57)
Pump 2
(73)
Relative increasing ratio is used to compare the performance of the proposed cycle with that of SGAX cycle, it is given as h ¼ ðCOPPGAX COPSGAX Þ COPSGAX *100%
(74)
In this paper, the thermodynamic properties of the ammoniaewater solutions and pure ammonia are calculated by Refprop [30].
Validation of the model
m36 h36 þ WP2 ¼ m37 h37
(58)
m36 ¼ m37
(59)
To validate the simulation model, the simulation results of SGAX cycle are compared to the simulation results presented in the literatures in Table 1 [3,26,31]. It can be found that the
Table 1 e Validation of the model. TG/ C TC/ C TA/ C TE/ C 163.3 198.7
40 45.3
40 53
5 1
u
COP in this work
COP in literature
Relative deviation
0.80 0.85
1.06 0.86
1.08 (Rameshkumar. A et al., 2007), 1.10 (Herold KE et al., 1995) zquez. N et al., 2010) 0.85 (Vela
<3.8% 1.2%
Please cite this article in press as: Shi Y, et al., Thermodynamic analysis of a novel GAX absorption refrigeration cycle, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.10.155
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results in this work were perfectly consistent with those in the literatures.
Results and discussion Fig. 4 shows the effect of generation temperature values on mab,GAXA when evaporation temperature is 5 C and m12 is 1 mol s1. It can be found that the mass flow rate of the ammonia absorbed by the GAXA of the proposed cycle is much more than that of SGAX cycle. The reason is that the solution out from GAXAH of the proposed cycle continues to absorb the vapor in GAXAL. It also can be found that the limited generation temperature of the proposed cycle needed to make GAXA work is much lower than that of the SGAX cycle. When the generation temperature is lower than 155 C, there is no ammonia absorbed by the GAXA of the SGAX cycle. Then the SGAX cycle will work as a single effect cycle. For proposed cycle, GAXAL can work well even the generation temperature decreases to 140 C. Fig. 5 shows the effect of generation temperature values on m42 when evaporation temperature is 5 C and m12 is 1 mol s1. As generation temperature increases, the ammonia concentration of the solution out from generator decreases, in addition, m12 is 1 mol s1, and then m42 decreases. Fig. 6 shows the effect of generation temperature values on COPs and h when evaporation temperature is 5 C and m12 is 1 mol s1. Because the proposed cycle can make use of the heat rejected by the GAXAL to drive GAXGL to make additional refrigeration, the COP of the proposed cycle is much higher than that of the SGAX cycle. It can be found that relative increasing ratio increases at first and then decreases as generation temperature increases. When generation temperature is lower than 155 C, the absorption heat can not be used to drive GAXGH, the ammonia concentration of the solution out from GAXAL will be higher than that of the solution out from WCA. Then it needs to add outer heat to concentrate the solution. Because only part of the refrigerant separated from solution at this concentration can make refrigeration in evaporator, then this has a significant
Fig. 4 e The effect of generation temperature values on mab,GAXA.
Fig. 5 e The effect of generation temperature values on m42.
Fig. 6 e The effect of generation temperature values on COPs and h.
Fig. 7 e The effect of evaporation temperature values on mab,GAXA.
Please cite this article in press as: Shi Y, et al., Thermodynamic analysis of a novel GAX absorption refrigeration cycle, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.10.155
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Fig. 8 e The effect of evaporation temperature values on m42.
7
SGAX cycle. When the evaporation temperature is lower than 7.5 C, there is no ammonia absorbed by the GAXA of the SGAX cycle. Then the SGAX cycle will work as a single effect cycle. For proposed cycle, GAXAL can work well even the evaporation temperature decreases to 20 C. Fig. 8 shows the effect of evaporation temperature values on m42 when generation temperature is 165 C and m12 is 1 mol s1. As evaporation temperature increases, the ammonia concentration of the solution out from WCA increases, in addition, m12 is 1 mol s1, and then m42 decreases. Fig. 9 shows the effect of evaporation temperature values on COPs and h when generation temperature is 165 C and m12 is 1 mol s1. It can be found that the COP of the proposed cycle is much higher than that of SGAX cycle. In addition, the relative increasing ratio increases at first and then decreases as evaporation temperature increases. When evaporation temperature is lower than 7.5 C, it needs to add outer heat to concentrate the solution out from GAXAL. Then relative increasing ratio increases at first. When evaporation temperature is higher than 7.5 C, the decrease of the m42 makes the relative increasing ratio decrease as evaporation temperature increases. It also can be found that the relative increasing ratio is always higher than 20% at simulated conditions when evaporation temperature is higher than 15 C but lower than 2.5 C.
Summary and conclusions
Fig. 9 e The effect of evaporation temperature values on COPs and h.
negative effect on the performance of the proposed cycle. As generation temperature increases, the ammonia concentration of the solution out from GAXGL decreases, then the quantity of the outer heat used to concentrate the solution decreases. Therefore, relative increasing ratio increases at first. When generation temperature is higher than 155 C, the heat rejected by GAXAH is added to the GAXGH, and the ammonia concentration of the solution out from GAXGL is equal to the ammonia concentration of the solution out from WCA. Then the decrease of the m42 makes the relative increasing ratio decrease as generation temperature increases. It can be found that the relative increasing ratio is always higher than 20% at simulated conditions when generation temperature is higher than 147 C. Fig. 7 shows the effect of evaporation temperature values on mab,GAXA when generation temperature is 165 C and m12 is 1 mol s1. It can be found that the mass flow rate of the ammonia absorbed by the GAXA of the proposed cycle is much more than that of SGAX cycle. It also can be found that the limited evaporation temperature of the proposed cycle needed to make GAXA work is much lower than that of the
This paper proposed an improved GAX absorption refrigeration cycle. The COP of the proposed cycle is 20% higher than that of the SGAX cycle at most simulated conditions by making use of low grade absorption heat that can not be used by SGAX cycle. As generation temperature increases, relative increasing ratio increases at first and then decreases. Compared to SGAX cycle, the GAXA of the proposed cycle can work well at much lower generation temperature. As evaporation temperature increases, relative increasing ratio increases at first and then decreases. Compared to SGAX cycle, the GAXA of the proposed cycle can work well at much lower evaporation temperature. If the liquid out from condenser is subcooled by the vapor out from gaseliquid separator or the vapor flows to WCA, the COP of the proposed cycle can be higher.
Acknowledgements This paper is financially supported by National Key Research and Development Program (No 2016YFB0901404).
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Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Thermodynamic analysis of a novel GAX absorption refrigeration cycle Yuqi Shi a,*, Qin Wang a, Daliang Hong b, Guangming Chen a a
Institute of Refrigeration and Cryogenics, Zhejiang University, Key Laboratory of Refrigeration and Cryogenic Technology of Zhejiang Province, Hangzhou, China b China Electronics Technology Group Corporation No. 38 Research Institute, Hefei, Anhui Province, China
article info
abstract
Article history:
A novel GAX absorption refrigeration cycle is proposed in this paper. Part of the solution
Received 18 August 2016
out from water cooled absorber flows to a refrigerant cooled absorber to absorb part of the
Received in revised form
vapor out from evaporator. The refrigerant cooled absorber is cooled by refrigerant which
27 October 2016
evaporates at an intermediate pressure. The vapor at intermediate pressure is absorbed by
Accepted 28 October 2016
the solution out from refrigerant cooled absorber in a high pressure absorber. The proposed
Available online xxx
cycle can make use of the absorption heat that can not be utilized by standard GAX cycle to make additional refrigeration. Therefore, the COP of the proposed cycle is much higher
Keywords:
than that of the standard GAX (SGAX) cycle. Simulation results show that the COP of the
Absorption cycle
proposed cycle is 20% higher than that of SGAX cycle at most simulated conditions.
Refrigeration
© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Simulation Thermodynamic analysis GAX
Introduction Because of the environmental problems caused by conventional vapor compression refrigerators, sorption refrigeration machines have attracted more and more attentions [1e4]. Absorption refrigeration devices are narrowly defined as closed-liquid sorption cycles [5]. Absorption chillers and heat pumps can be integrated with various systems such as hydrogen liquefaction, cooling and power generation [6e9]. Usage of the second thermodynamic law, thermo-economic method, artificial neural network method and commercial software like Aspen-Plus for analyzing and optimizing the performance of the systems has become popular in recent years [10e13]. The main working pairs used in absorption
refrigeration machines are H2O/LiBr and NH3/H2O, which are friendly to environmental [14,15]. The refrigerators using H2O/ LiBr as working pair can not make refrigeration at temperature lower than 0 C due to freezing and crystallization issues [16]. Generally, NH3/H2O absorption refrigeration machines were used to make refrigeration at low temperature [17] and can be driven by waste heat with large temperature glide [18]. As ammonia concentration decreases, the saturation temperature of the solution at a given pressure increases. If the ammonia concentration of the solution out from generator is low enough, there may be a temperature overlap between generator and absorber. Therefore, GAX absorption refrigeration cycle is regarded as a good method to improve the performance of the NH3/H2O absorption refrigeration machine by using part of the absorption heat to drive generator [19e21]
* Corresponding author. E-mail address: [email protected] (Y. Shi). http://dx.doi.org/10.1016/j.ijhydene.2016.10.155 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Shi Y, et al., Thermodynamic analysis of a novel GAX absorption refrigeration cycle, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.10.155
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Nomenclature Symbols h m P Q T W x
specific enthalpy, kJ mol1 molar mass flow rate, mol s1 pressure, kPa heat transfer rate, W temperature, C pump work molar ratio, mol/mol
Greek symbols h relative increasing ratio, % Subscripts A/abs absorber C/con condenser E/evap evaporator G/g generator rec rectifier WCA water cooled absorber h, m, l high, middle, low 1, 2, 3 … state points Abbreviations COP coefficient of performance GAX generation absorption heat exchange GAXAH high temperature GAX absorber GAXAL low temperature GAX absorber GAXGH high temperature GAX generator GAXGL low temperature GAX generator HPWCA high pressure water cooled absorber LGS liquid gas separator PGAX proposed GAX RCA refrigerant cooled absorber SCA solution cooled absorber SGAX standard GAX SHG solution heated generator SHX solution heat exchanger WCA water cooled absorber But SGAX absorption refrigeration cycle can not make use of the absorption heat at lower temperature, which has negative effect on its performance and limits its applications. Although HGAX absorption refrigeration cycle can make use of part of the low grade absorption heat by combing a compressor [22e29], it needs to consume additional electric power. This paper proposed an improved GAX absorption refrigeration cycle. Part of the low grade absorption heat is also used to make additional refrigeration. Then the performance of the proposed cycle can be much better than that of SGAX absorption refrigeration cycle.
Cycle description Fig. 1 shows the diagram of a SGAX absorption refrigeration cycle. Fig. 2 shows the diagram of the proposed cycle. The proposed cycle consists of the following components: generator, SHG(solution heated generator), GAXGH (high
Fig. 1 e The diagram of SGAX absorption refrigeration cycle.
temperature GAX generator), GAXGL (low temperature GAX generator), GAXAH (high temperature GAX absorber), GAXAL (low temperature GAX absorber), SCA (solution cooled absorber), WCA (water cooled absorber), HPWCA (high-pressure water cooled absorber), LGS (liquidegas separator), RCA (refrigerant cooled absorber), SHX (solution heat exchanger), Pre-cooler, Rectifier, evaporator, condenser, Pumps, throttles valves and mixers. The working principle of the proposed cycle using NH3/H2O, as working pair is shown as follow. The vapor separated from generator (1) flows to rectifier through SHG, GAXGH and GAXGL in turn. The purified ammonia vapor from rectifier (5) flows to condenser and is condensed to be saturated liquid there. Then the liquid (6) passes through the TV 1 and is throttled to an intermediate pressure. Part of the liquid ammonia at this pressure evaporates and the cooling capacity is used to balance the heat rejected by the RCA. Then the stream (8) flows to LGS and is separated into liquid phase and vapor phase there. The liquid ammonia (9) is subcooled and then is throttled to evaporating pressure and completely evaporates in the evaporator. The vapor (12) out from evaporator passes through the pre-cooler. Then part of the vapor (43) flows to RCA and is absorbed there; the other part of the vapor (13) is absorbed by WCA, SCA, GAXAL and GAXAH respectively. The weak solution from generator (23) passes through SHG and is cooled there. Then the solution is throttled to evaporating pressure. The depressurized solution (25) passes through GAXAH, GAXAL, SCA, WCA in turn and absorbs vapor there. The heat rejected by GAXAH and GAXAL is used to drive GAXGH and GAXGL respectively. Part of the solution (34) out from WCA flows to RCA through SHX and absorbs vapor there. Then the solution (36) is pumped to intermediate pressure and absorbs vapor from LGS in HPWCA. The solution (39) out from HPWCA is preheated by heat rejected by SCA. Then the solution (40) is pumped to generation pressure and flows to GAXAL after mixing with the solution from rectifier. The other part of the solution out from WCA (30) is pumped to generation pressure and flows to GAXGH after mixing with the solution out from GAXGL. The solution (21) out from GAXGH flows to generator through SHG. The cycle is finished. The term ‘‘weak solution’’ represents a solution that is weak with ammonia.
Please cite this article in press as: Shi Y, et al., Thermodynamic analysis of a novel GAX absorption refrigeration cycle, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.10.155
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3
Fig. 2 e The diagram of the proposed cycle.
It can be found that the heat rejected by GAXAL that can not be used by SGAX absorption refrigeration cycle to drive generator is utilized by the proposed cycle. The heat is used to drive GAXGL to separate the additional refrigerant vapor. The additional refrigerant is absorbed by RCA and HPWCA. The additional refrigerant absorbed by RCA provides the additional mass flow rate of the refrigerant in evaporator, which provides additional cooling capacity directly. In addition to cooling the RCA, the cooling capacity supplied the additional refrigerant absorbed by HPWCA is also used to subcool the refrigerant out from condenser, which will improve the cooling capacity per unite mass of refrigerant evaporating in the evaporator. Then the performance of the proposed cycle can be much better than that of SGAX absorption refrigeration cycle.
A Du¨hring diagram of the proposed cycle is illustrated in Fig. 3.
Mathematical model The authors wrote a procedure to evaluate the performance of the proposed cycle. To simplified the model, some assumptions are assumed as follow, (1) The system runs in a steady state. (2) The process of pumps is assumed to be adiabatic processes and isentropic efficiency is assumed to be 75%. (3) Pressure drops along the pipe lines are neglected.
Fig. 3 e Du ¨ hring diagram of the proposed cycle. Please cite this article in press as: Shi Y, et al., Thermodynamic analysis of a novel GAX absorption refrigeration cycle, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.10.155
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(4) The solutions out from generator, WCA, HPWCA, rectifier, GAXGH, GAXGL, SCA, GAXAL, GAXAH are saturated solutions. (5) The outlet state from the condenser is saturated liquid and the outlet state from the evaporator is saturated vapor. (6) The temperature difference between the cold end of GAXGL and that of GAXAL is 0 C; the temperature difference between the cold end of GAXGH and that of GAXAH is 0 C. (7) The temperature difference at the cold end of SHX is 5 C. (8) The temperature difference at the cold end of RCA is 5 C. (9) The effectiveness of Pre-cooler is 0.8. (10) The condensing temperature is 40 C, the temperature of the solutions out from WCA and HPWCA is 40 C. The basic models for all of the components include mass balance equations, energy balance equations and ammonia mass balance equations. Ammonia mass balance equation is not considered for the components such as evaporator, condenser, because the ammonia concentration does not change when the stream passes through these components. The balance equations of each component are shown respectively as follows, Generator m22 h22 þ Q g ¼ m1 h1 þ m23 h23
(1)
m22 x22 ¼ m1 x1 þ m23 x23
(2)
m22 ¼ m1 þ m23
(3)
SHG m22 h22 þ m24 h24 þ m2 h2 ¼ m1 h1 þ m21 h21 þ m23 h23
(4)
m22 x22 þ m24 x24 þ m2 x2 ¼ m1 x1 þ m21 x21 þ m23 x23
(5)
m23 ¼ m24
(6)
x23 ¼ x24
(7)
m22 þ m24 þ m2 ¼ m1 þ m21 þ m23
(8)
m4 ¼ m5 þ m17
(16)
m4 x4 ¼ m5 x5 þ m17 x17
(17)
Condenser m5 h5 ¼ m6 h6 þ Q con
(18)
m5 ¼ m6
(19)
TV 1 m6 h6 ¼ m7 h7
(20)
m6 ¼ m7
(21)
RCA m35 h35 þ m43 h43 þ m7 h7 ¼ m36 h36 þ m8 h8
(22)
m35 x35 þ m43 x43 þ m7 x7 ¼ m36 x36 þ m8 x8
(23)
m35 þ m43 ¼ m36
(24)
m7 ¼ m8
(25)
LGS m8 h8 ¼ m9 h9 þ m44 h44
(26)
m8 ¼ m9 þ m44
(27)
TV 2 m10 h10 ¼ m11 h11
(28)
m10 ¼ m11
(29)
Evaporator
GAXGH m2 h2 þ m20 h20 þ Q GAXGH ¼ m3 h3 þ m21 h21
(9)
m2 x2 þ m20 x20 ¼ m3 x3 þ m21 x21
(10)
m2 þ m20 ¼ m3 þ m21
(11)
GAXGL m3 h3 þ m18 h18 þ Q GAXGL ¼ m4 h4 þm19 h19
(12)
m3 x3 þ m18 x18 ¼ m4 x4 þ m19 x19
(13)
m3 þ m18 ¼ m4 þ m19
(14)
m11 h11 þ Q evap ¼ m12 h12
(30)
m11 ¼ m12
(31)
TV 3 m24 h24 ¼ m25 h25
(32)
m24 ¼ m25
(33)
GAXAH m25 h25 þ m16 h16 ¼ m26 h26 þ Q HPWCA
(34)
m25 x25 þ m16 x16 ¼ m26 x26
(35)
m25 þ m16 ¼ m26
(36)
GAXAL m15 h15 þ m26 h26 þ m32 h32 ¼ m16 h26 þ m27 h27 þ m33 h33 þ Q GAXAL (37)
Rectifier m4 h4 ¼ m5 h5 þ m17 h17 þ Q rec
(15)
m15 x15 þ m26 x26 þ m32 x32 ¼ m16 x26 þ m27 x27 þ m33 x33
(38)
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m15 þ m26 þ m32 ¼ m16 þ m27 þ m33
(39)
SCA m14 h14 þ m27 h27 þ m31 h31 þ m39 h39 ¼ m15 h15 þ m28 h28 þ m32 h32 þ m40 h40
(40)
Pump 3 m40 h40 þ WP3 ¼ m41 h41
(60)
m40 ¼ m41
(61)
Mixer 1
m14 x14 þ m27 x27 ¼ m15 x15 þ m28 x28
(41)
m17 h17 þ m41 h41 ¼ m18 h18
(62)
m14 þ m27 ¼ m15 þ m28
(42)
m17 x17 þ m41 x41 ¼ m18 x18
(63)
m31 ¼ m32
(43)
m17 þ m41 ¼ m18
(64)
x31 ¼ x32
(44)
m39 ¼ m40
(45)
x39 ¼ x40
(46)
WCA m28 h28 þ m13 h13 ¼ m29 h29 þ m14 h14 þ Q WCA
(47)
m28 x28 þ m13 x13 ¼ m29 x29 þ m14 x14
(48)
m28 þ m13 ¼ m29 þ m14
(49)
Mixer 2 m19 h19 þ m33 h33 ¼ m20 h20
(65)
m19 x19 þ m33 x33 ¼ m20 x20
(66)
m19 þ m33 ¼ m20
(67)
Pre-cooler m9 h9 þ m12 h12 ¼ m10 h10 þ m42 h42
Pump 1 m30 h30 þ WP1 ¼ m31 h31
(50)
m30 ¼ m31
(51)
HPWCA
(68)
In addition, it is obvious that Q GAXAH ¼ Q GAXGH
(69)
Q GAXAL ¼ Q GAXGL
(70)
m42 ¼ m13 þ m43
(71)
m29 ¼ m30 þ m34
(72)
m38 h38 þ m44 h44 ¼ m39 h39 þ Q HPWCA
(52)
COP is adopted for the cycle performance evaluation in this paper. It is defined as:
m38 x38 þ m44 x44 ¼ m39 x39
(53)
. COP ¼ Q evap Q g
m38 þ m44 ¼ m39
(54)
SHX m37 h37 þ m34 h34 ¼ m38 h38 þ m35 h35
(55)
m37 x37 þ m34 x34 ¼ m38 x38 þ m35 x35
(56)
m37 þ m34 ¼ m38 þ m35
(57)
Pump 2
(73)
Relative increasing ratio is used to compare the performance of the proposed cycle with that of SGAX cycle, it is given as h ¼ ðCOPPGAX COPSGAX Þ COPSGAX *100%
(74)
In this paper, the thermodynamic properties of the ammoniaewater solutions and pure ammonia are calculated by Refprop [30].
Validation of the model
m36 h36 þ WP2 ¼ m37 h37
(58)
m36 ¼ m37
(59)
To validate the simulation model, the simulation results of SGAX cycle are compared to the simulation results presented in the literatures in Table 1 [3,26,31]. It can be found that the
Table 1 e Validation of the model. TG/ C TC/ C TA/ C TE/ C 163.3 198.7
40 45.3
40 53
5 1
u
COP in this work
COP in literature
Relative deviation
0.80 0.85
1.06 0.86
1.08 (Rameshkumar. A et al., 2007), 1.10 (Herold KE et al., 1995) zquez. N et al., 2010) 0.85 (Vela
<3.8% 1.2%
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results in this work were perfectly consistent with those in the literatures.
Results and discussion Fig. 4 shows the effect of generation temperature values on mab,GAXA when evaporation temperature is 5 C and m12 is 1 mol s1. It can be found that the mass flow rate of the ammonia absorbed by the GAXA of the proposed cycle is much more than that of SGAX cycle. The reason is that the solution out from GAXAH of the proposed cycle continues to absorb the vapor in GAXAL. It also can be found that the limited generation temperature of the proposed cycle needed to make GAXA work is much lower than that of the SGAX cycle. When the generation temperature is lower than 155 C, there is no ammonia absorbed by the GAXA of the SGAX cycle. Then the SGAX cycle will work as a single effect cycle. For proposed cycle, GAXAL can work well even the generation temperature decreases to 140 C. Fig. 5 shows the effect of generation temperature values on m42 when evaporation temperature is 5 C and m12 is 1 mol s1. As generation temperature increases, the ammonia concentration of the solution out from generator decreases, in addition, m12 is 1 mol s1, and then m42 decreases. Fig. 6 shows the effect of generation temperature values on COPs and h when evaporation temperature is 5 C and m12 is 1 mol s1. Because the proposed cycle can make use of the heat rejected by the GAXAL to drive GAXGL to make additional refrigeration, the COP of the proposed cycle is much higher than that of the SGAX cycle. It can be found that relative increasing ratio increases at first and then decreases as generation temperature increases. When generation temperature is lower than 155 C, the absorption heat can not be used to drive GAXGH, the ammonia concentration of the solution out from GAXAL will be higher than that of the solution out from WCA. Then it needs to add outer heat to concentrate the solution. Because only part of the refrigerant separated from solution at this concentration can make refrigeration in evaporator, then this has a significant
Fig. 4 e The effect of generation temperature values on mab,GAXA.
Fig. 5 e The effect of generation temperature values on m42.
Fig. 6 e The effect of generation temperature values on COPs and h.
Fig. 7 e The effect of evaporation temperature values on mab,GAXA.
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Fig. 8 e The effect of evaporation temperature values on m42.
7
SGAX cycle. When the evaporation temperature is lower than 7.5 C, there is no ammonia absorbed by the GAXA of the SGAX cycle. Then the SGAX cycle will work as a single effect cycle. For proposed cycle, GAXAL can work well even the evaporation temperature decreases to 20 C. Fig. 8 shows the effect of evaporation temperature values on m42 when generation temperature is 165 C and m12 is 1 mol s1. As evaporation temperature increases, the ammonia concentration of the solution out from WCA increases, in addition, m12 is 1 mol s1, and then m42 decreases. Fig. 9 shows the effect of evaporation temperature values on COPs and h when generation temperature is 165 C and m12 is 1 mol s1. It can be found that the COP of the proposed cycle is much higher than that of SGAX cycle. In addition, the relative increasing ratio increases at first and then decreases as evaporation temperature increases. When evaporation temperature is lower than 7.5 C, it needs to add outer heat to concentrate the solution out from GAXAL. Then relative increasing ratio increases at first. When evaporation temperature is higher than 7.5 C, the decrease of the m42 makes the relative increasing ratio decrease as evaporation temperature increases. It also can be found that the relative increasing ratio is always higher than 20% at simulated conditions when evaporation temperature is higher than 15 C but lower than 2.5 C.
Summary and conclusions
Fig. 9 e The effect of evaporation temperature values on COPs and h.
negative effect on the performance of the proposed cycle. As generation temperature increases, the ammonia concentration of the solution out from GAXGL decreases, then the quantity of the outer heat used to concentrate the solution decreases. Therefore, relative increasing ratio increases at first. When generation temperature is higher than 155 C, the heat rejected by GAXAH is added to the GAXGH, and the ammonia concentration of the solution out from GAXGL is equal to the ammonia concentration of the solution out from WCA. Then the decrease of the m42 makes the relative increasing ratio decrease as generation temperature increases. It can be found that the relative increasing ratio is always higher than 20% at simulated conditions when generation temperature is higher than 147 C. Fig. 7 shows the effect of evaporation temperature values on mab,GAXA when generation temperature is 165 C and m12 is 1 mol s1. It can be found that the mass flow rate of the ammonia absorbed by the GAXA of the proposed cycle is much more than that of SGAX cycle. It also can be found that the limited evaporation temperature of the proposed cycle needed to make GAXA work is much lower than that of the
This paper proposed an improved GAX absorption refrigeration cycle. The COP of the proposed cycle is 20% higher than that of the SGAX cycle at most simulated conditions by making use of low grade absorption heat that can not be used by SGAX cycle. As generation temperature increases, relative increasing ratio increases at first and then decreases. Compared to SGAX cycle, the GAXA of the proposed cycle can work well at much lower generation temperature. As evaporation temperature increases, relative increasing ratio increases at first and then decreases. Compared to SGAX cycle, the GAXA of the proposed cycle can work well at much lower evaporation temperature. If the liquid out from condenser is subcooled by the vapor out from gaseliquid separator or the vapor flows to WCA, the COP of the proposed cycle can be higher.
Acknowledgements This paper is financially supported by National Key Research and Development Program (No 2016YFB0901404).
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