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Appropriate heat load ratio of generator for different types of air cooled lithium bromide–water double effect absorption chiller
Energy Conversion and Management 99 (2015) 264–273
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Appropriate heat load ratio of generator for different types of air cooled lithium bromide–water double effect absorption chiller Zeyu Li ⇑, Jinping Liu School of Electric Power, South China University of Technology, Guangzhou 510640, China Guangdong Province Key Laboratory of High Efficient and Clean Energy Utilization, South China University of Technology, Guangzhou 510640, China
a r t i c l e
i n f o
Article history: Received 15 March 2015 Accepted 20 April 2015 Available online 18 May 2015 Keywords: Heat load ratio of generator Air cooled Double effect Absorption chiller
a b s t r a c t The lower coefficient of performance and higher risk of crystallization in the higher surrounding temperature is the primary disadvantage of air cooled lithium bromide–water double effect absorption chiller. Since the coefficient of performance and risk of crystallization strongly depend on the heat load ratio of generator, the appropriate heat load ratio of generator can improve the performance as the surrounding temperature is higher. The paper mainly deals with the appropriate heat load ratio of generator of air cooled lithium bromide–water double effect absorption chiller. Four type systems named series, pre-parallel, rear parallel and reverse parallel flow configuration were considered. The corresponding parametric model was developed to analyze the comprehensive effect of heat load ratio of generator on the coefficient of performance and risk of crystallization. It was found that the coefficient of performance goes up linearly with the decrease of heat load ratio of generator. Simultaneously, the risk of crystallization also rises slowly at first but increases fast finally. Consequently, the appropriate heat load ratio of generator for the series and pre-parallel flow type systems is suggested to be 0.02 greater than the minimum heat load ratio of generator and that for the rear parallel and reverse parallel flow chillers should be 0.01 higher than the minimum heat load ratio of generator. Besides, the changes of minimum heat load ratio of generator for different type systems with the working condition were analyzed and compared. It was found that the minimum heat load ratio of generator goes up with the increase of temperature of high pressure generator as well as surrounding temperature and it goes down with the rise of evaporator temperature and effectiveness of high temperature heat exchanger. The dependence of minimum heat load ratio of generator for the series and rear parallel flow system on the effectiveness of low temperature heat exchanger is weak. While the minimum heat load ratio of generator of pre-parallel and reverse parallel flow chiller rises with the increase of effectiveness of low temperature heat exchanger. The minimum heat load ratio of generator of reverse parallel flow configuration is independent upon the distribution ratio. The minimum heat load ratio of generator of pre-parallel flow system goes up fast with the rise of distribution ratio when the distribution ratio exceeds to 0.55. But the minimum heat load ratio of generator of rear parallel flow configuration just rises slightly with the increase of distribution ratio as the distribution is greater than 0.5. The paper is helpful to the development and performance improvement of air cooled lithium bromide–water double effect absorption chiller. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Recently, the demand of air conditioning grows significantly because of the effect of global warming. However, the consequent increasing consumption of air conditioning can further worsen global warming, in turn the demand of air conditioning continues to rise inevitably. Therefore, it is urgent to reduce the consumption ⇑ Corresponding author at: School of Electric Power, South China University of Technology, Guangzhou 510640, China. E-mail address: [email protected] (Z. Li). http://dx.doi.org/10.1016/j.enconman.2015.04.055 0196-8904/Ó 2015 Elsevier Ltd. All rights reserved.
of air conditioning. Since the LiBr/H2O solution is nontoxic and the corresponding absorption chiller can be driven by solar energy or waste heat, the application of LiBr/H2O absorption chiller is an effective way that lowers the consumption of air conditioning. Comparing with the single effect or water cooled system, the air cooled LiBr/H2O double effect absorption chiller is better and more flexible due to the high efficient, absence of cooling tower and the independence upon water. Although the simulation showed that the working characteristic of air cooled LiBr/H2O absorption chiller is close to the water cooled one as the solar irradiance is strong [1], the development
Z. Li, J. Liu / Energy Conversion and Management 99 (2015) 264–273
265
Nomenclature COP h m Q T DT x
coefficient of performance enthalpy (kJ/kg) mass flow rate (kg/s) energy (kW) temperature (°C) temperature difference (°C) concentration of solution
Greek symbols a heat load ratio of generator e heat exchanger effectiveness
of air cooled LiBr/H2O absorption chiller is difficult due to the poor heat transfer coefficient of air. A small indirect air cooled LiBr/H2O single effect absorption chiller was firstly developed by Izquierdo et al. [2]. Nevertheless, this prototype is not compact and its performance is lower because of the complicated cooling of absorber and condenser. Hence, the direct air cooled LiBr/H2O single effect absorption chiller was developed subsequently [3]. This prototype works without the crystallization when the surrounding temperature is up to 37.7 °C [4]. The comparative test showed that the absorber temperature and the condenser temperature of direct air cooled prototype are lower than that of indirect one under the same working condition [5]. Based on the above mentioned successful development, the small air cooled LiBr/H2O double effect absorption chiller was developed as well [6]. This prototype is more compact and it can work without the crystallization as the surrounding temperature is up to 45 °C due to the better performance of new flat-sheet adiabatic absorber. Recently, a new solar air cooled LiBr/H2O double effect absorption chiller was developed by Izquierdo et al. [7]. The prototype works as the double effect cycle when the solar energy is sufficient and it can work as the single effect cycle as the solar irradiance is insufficient to drive the double effect operational mode. The double effect LiBr/H2O absorption chiller has many different configurations and the performance of each configuration is different even the working condition is identical. For the series flow type double effect LiBr/H2O absorption chiller, it was gotten that the coefficient of performance (COP) and total heat transfer area goes down slightly with the increase of solution concentration [8]. Gomri and Hakimi [9] pointed out that the exergy loss of high pressure generator (HPG) and absorber is the highest in the series flow type double effect LiBr/H2O absorption chiller. Besides, in the series flow configuration, the pressure drop of absorber and evaporator plays an important role in the COP and exergy efficiency [10] and the relative humidity has less effect on the exergy efficiency [11]. Misra et al. [12] carried out the thermoeconomic analysis on the series flow type double effect LiBr/H2O absorption chiller and it was concluded that the variation of optimal decision variable is not sensitive to the fuel cost. For the parallel flow configuration, it was obtained that the change of distribution ratio varies the COP less [13]. It was found that the coefficient of performance (COP) of rear parallel flow configuration is more sensitive to the change of evaporator temperature [14]. In the rear parallel flow configuration, it was shown that the COP goes up but the exergy efficiency reduces with the rise of load factor [15]. The comparison of COP and exergy efficiency for different type double effect LiBr/H2O absorption chillers showed that the performance of parallel flow configuration is superior to the series flow one [16]. Moreover, it was concluded that the product cost flow rate of reverse flow type chiller is the lowest while that of series flow
Subscripts a absorber c condenser e evaporator hg high pressure generator i inlet lg low pressure generator o outlet s surrounding
one is the highest by the exergoeconomic analysis [17]. For the different type air cooled LiBr/H2O double effect absorption chillers, it was obtained that the COP of pre-parallel flow configuration is the lowest under the same working condition [18]. Because of rise of absorber temperature and condenser temperature with the increase of surrounding temperature, the air cooled LiBr/H2O absorption chiller suffers from crystallization easily especially when the outdoor temperature is higher. Izquierdo et al. [19] pointed out that the double stage air cooled LiBr/H2O absorption chiller is more suitable than the single effect one as the outdoor temperature exceeds to 40 °C. A new air cooled LiBr/H2O absorption cycle which can operate without crystallization as the outdoor temperature is up to 50 °C was proposed by Kim and Infante Ferreira [20]. Han et al. [21] presented that the distribution ratio of pre-parallel flow type double effect LiBr/H2O absorption chiller should vary in term of working condition to prevent the crystallization and the suitable range of distribution ratio becomes narrow with the increase of HPG temperature, evaporator temperature and the effectiveness of solution heat exchanger. The analysis on the risk of crystallization in the double effect LiBr/H2O absorption chiller was carried out by Garousi Farshi et al. [22]. It was concluded that the rear parallel configuration and the reverse parallel configuration are more difficult to suffer from the crystallization than the series system in the same working condition. The lower COP and higher risk of crystallization in the higher surrounding temperature is the primary disadvantage of air cooled LiBr/H2O double effect absorption chiller. The heat load ratio of generator is the important parameter to the design of area of low pressure generator (LPG). And it was found that the variation of heat load ratio of generator can significantly change the system performance [23]. Consequently, the appropriate heat load ratio of generator that maximizes the COP and still prevents the crystallization can make the air cooled LiBr/H2O double effect absorption chiller work efficiently and reliably as the outdoor temperature is higher. However, the influence of heat load ratio of generator on the system performance is not studied adequately. Furthermore, the relationship of appropriate heat load ratio of generator with the working condition is also not clear. Therefore, the objective of paper is to obtain the appropriate heat load ratio of generator of air cooled LiBr/H2O double effect absorption chiller. Four configurations named series, pre-parallel, rear parallel and reverse parallel flow type system were considered. The corresponding parametric model was developed to investigate the effect of heat load ratio of generator on the COP and risk of crystallization. The changes of appropriate heat load ratio of generator for four type chillers were compared and analyzed. The paper is helpful to the development and performance improvement of air cooled LiBr/H2O double effect absorption chiller.
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configuration, the parametric model of pre-parallel, rear parallel as well as reverse parallel flow type system was developed by the suitable modification of previous model [24]. The assumptions of model were shown in Table 1. The working characteristic of different type air cooled double effect LiBr/H2O absorption chillers was described by the mass and energy conservation:
2. Model Four type air cooled LiBr/H2O double effect absorption chillers were considered. The first type is named series flow system. The second type is called pre-parallel flow system: the solution flows into the HPG and the LPG in parallel at the outlet of absorber. The third type is called rear parallel flow system: the solution flows into the HPG and the LPG in parallel at the low concentration solution outlet of low temperature heat exchanger (LTHE). The fourth type is named reverse parallel system: the solution firstly enters the LPG, and then flows into the HPG and LTHE in parallel at the solution outlet of LPG. The schematic of series, pre-parallel, rear parallel and reverse parallel flow configurations are shown from Figs. 1–4. Based on the model of Gomri and Hakimi [9] and Garousi Farshi et al. [22], the parametric model of series flow type air cooled double effect LiBr/H2O absorption chiller was developed in the previous study [24]. Here, according to the corresponding
X X
mi ¼
X
mi xi ¼
Q¼
X
mo
X
ð1Þ
mo xo
mi hi
X
ð2Þ
m o ho
Here, the (+) sign of Q refers to the output energy and the () sign of Q denotes the input energy. In order to be consistent with our previous investigation [23], the heat load ratio of generator was also equals to the ratio of energy inputting into the HPG and energy inputting into the LPG:
14 HPG Hot fluid inlet 24
LPG
12
15 Hot fluid outlet 25
16
Condenser
13 Air inlet 19 10
11
8
9
21 Air outlet
High temperature heat exchanger
17
6
7
4
5
Low temperature heat exchanger 18
Solution pump
Air outlet 3 2 20 Absorber 19 1 Air inlet
Chilled water outlet 23 Evaporator 22 Chilled water inlet
Fig. 1. Schematic of series flow type system.
14 HPG Hot fluid inlet 24
LPG
12
15
16
Condenser
Hot fluid outlet 25 Air inlet 19 10
7
11
6
21 Air outlet
Low temperature heat exchanger
High temperature heat exchanger 8
5
9
17
4
13 18 Solution pump
3 2 Absorber 1
Air inlet 19
ð3Þ
Chilled water outlet 23 22 Evaporator Chilled water inlet
20 Air outlet
Fig. 2. Schematic of pre-parallel flow type system.
Z. Li, J. Liu / Energy Conversion and Management 99 (2015) 264–273
267
14 HPG Hot fluid inlet 24
LPG
12
15 Hot fluid outlet 25 11
10 High temperature heat exchanger 8
26
16
Condenser
27 Air inlet 19
21 Air outlet
9
17
13 6
7
4
5
Low temperature heat exchanger Solution pump
18 Chilled water outlet 23 22 Evaporator Chilled water inlet
3
2
Absorber 1 Air inlet 19
20 Air outlet
Fig. 3. Schematic of rear parallel flow type system.
14 HPG Hot fluid inlet 24
LPG
12
15 Hot fluid outlet 25
16
Condenser
26 Air inlet 19 11
10
9
8
High temperature heat exchanger
21 Air outlet
Solution pump 13
17
6
7
Low temperature heat exchanger 5
4 Solution pump
3 2 Absorber 1 Air inlet 19
18 Chilled water outlet 23 22 Evaporator Chilled water inlet
20 Air outlet
Fig. 4. Schematic of reverse parallel flow type system.
a ¼ Q hg =Q lg Table 1 Assumption of air cooled LiBr/H2O double effect absorption chiller. Number
Assumption
1 2
Steady state The refrigerant (water) at the outlet of condenser is saturated liquid and its temperature is condensation temperature The refrigerant (water) at the outlet of evaporator is saturated vapor and its temperature equals to temperature of evaporator The solution (LiBr) at the outlet of absorber, HPG and LPG is saturated and the corresponding temperature is equal to the temperature of absorber, HPG and LPG, respectively The pressure loss of pipeline and heat exchanger except that between LPG and condenser is ignored The heat exchange between the system and surroundings is not considered The work of solution pump is negligible
3 4
5 6 7
ð4Þ
The distribution ratios of pre-parallel, rear parallel and reverse parallel flow system are expressed as follows:
D ¼ m8 =m2
ð5Þ
D ¼ m8 =m2
ð6Þ
D ¼ m8 =m26
ð7Þ
The simulation condition is shown in Table 2. The inlet and outlet temperature of hot fluid as well as chilled water was set in the identical value of Garousi Farshi et al. [22]: T24 = Thg + 18, T25 = Thg + 10, T22 = Te + 8, T23 = Te + 3. The absorber and condenser temperature are calculated as follows:
T a ¼ T 20 þ DT a
ð8Þ
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Z. Li, J. Liu / Energy Conversion and Management 99 (2015) 264–273
Table 2 Simulated condition.
Input the fixed data: T19, T20, T21, T22, T23, T24, T25, QEV,
Item
Value
Generator temperature Surrounding temperature Evaporator temperature Effectiveness of HTHE Effectiveness of LTHE Distribution ratio (for parallel flow system) Cooling capacity of system
120–170 °C 27–37 °C 4–9 °C 0.5–0.9 0.5–0.9 0.4–0.7 20 kW
T c ¼ T 21 þ DT c
Calculate the following parameter: m1, Thg, Ta, Tc, Te
Assume the solution concentration of HPG xhg
ð9Þ
Here, the DTa and DTc is the temperature difference of heat transfer in absorber and condenser, respectively. From the experiment of Gonzalez-Gil [3], it was gotten that DTa and DTc is 5 °C. The outlet temperature of cooling air for absorber and condenser is:
T 20 ¼ T 19 þ DT s;a
ð10Þ
T 21 ¼ T 19 þ DT s;c
ð11Þ
Assume the solution concentration of LPG xlg
Calculates the thermodynamic properties of each point
Here, the DTs,a and DTs,c is the temperature rise of cooling air in absorber and condenser, respectively. The experimental results showed that the ratio of cooling load in absorber to that in condenser is about 1.5 and the total temperature rise of cooling air is 5 °C [3]. Thereby, the DTs,a is 3 °C and the DTs,c is 2 °C. The parametric model was solved by the FORTRAN program. The thermodynamic property of LiBr/H2O solution was referred by Pa´tek [25]. And the corresponding function of Refprop7 was used in the program to calculate the thermodynamic property of water. The calculation process was shown in Fig. 5. The COP was equal to:
Energy balance in LPG?
ð12Þ
Because it is well known that the crystallization usually happens at the high concentration solution outlet of two solution heat exchangers, the risk of crystallization was evaluated by the difference between the outlet temperature of high concentration solution of solution heat exchanger and the crystallization temperature. The high temperature heat exchanger (HTHE) and the LTHE were all considered. It can be inferred that the closer the temperature difference, the higher the risk of crystallization. The crystallization occurs when the outlet temperature of high concentration solution of solution heat exchanger is less than the crystallization temperature. The data of Garousi Farshi et al. [22] was used to verify the model of paper. As seen in Fig. 6, a good agreement of them was obtained. 3. Results and discussion The performance with energy distribution ratio of generator for the series, pre-parallel, rear parallel and reverse parallel flow type air cooled LiBr/H2O double effect absorption chiller is shown from Figs. 7–10. The corresponding T–s diagram can be seen from Figs. 11–14. The dash line represents the decreased a. The superscript (⁄) means the state of decreased a. The working condition in the simulation is: Thg = 120 °C, Ts = 33 °C, Te = 5 °C, D = 0.5, eHT = 0.7, eLT = 0.7. It is seen that the COP is sensitive to the change of a and it goes up linearly with the decrease of a. It is attributed to the drop of circulation ratio with the decrease of a. Furthermore, the effect of a on the COP of series, pre-parallel, rear parallel and reverse parallel flow configuration becomes stronger in turn. The mean increasing rate of COP with the decrease of a for the series, pre-parallel, rear parallel and reverse parallel flow type chiller is 0.638, 1.137, 1.214 and 1.653, respectively. The less influence of a on the COP of series flow type system is attributed to the increase
converge?
No
Yes
No
Yes
End Fig. 5. Process flowchart of model.
Solution inlet temperature of absorber (°C)
COP ¼ Q e =Q hg
If
Reverse parallel, Garousi Farshi [22] Reverse parallel, model Rear parallel, Garousi Farshi [22] Rear parallel, model
65
60
εLT=0.5 εLT=0.5
εLT=0.5
Tg=147°C Te=4°C
55
Ta=Tc=37°C
εHT=0.7 D for rear parallel 50 flow syste, is 0.42
D for reverse parallel flow system is 0.65
45
εLT=0.9
Series, Garousi Farshi [22] εLT=0.9 Series, model
εLT=0.9
40 0.60
0.61
0.62
0.63
0.64
Concentration at the absorber inlet Fig. 6. Validation of model.
of flow rate of refrigerant vapor in the HPG with the decrease of a. While for the parallel flow configuration, the inlet flow rate of solution and flow rate of refrigerant vapor in the HPG all go down with the drop of a. The greater the decrease of above mentioned flow rate, the stronger the effect of a on the COP.
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Z. Li, J. Liu / Energy Conversion and Management 99 (2015) 264–273 100 1.28
90
Tcry,o,HT
80
To,HT To,LT
60
1.24
50 1.22 40
Temperature (°C)
Tcry,o,LT COP
Tcry,o,HT
80
To,HT
70
Tcry,o,LT
1.26
COP
Temperature (°C)
70
1.4
1.2
To,LT
COP
60
1.3
1.1
50
COP
90
40 1.0 30
1.20 30
20 1.18
20 1.30
1.35
1.40
1.45
α
1.50
Fig. 7. Performance with a for the series flow system.
Tcry,o,HT To,HT
80
T *
xlg
1.26
Tcry,o,LT
1.24
To,LT
1.22
Thg
1.30
1.35
α
1.40
1.45
1.50
xhg* xlg xhg
11*
xa
xhg
xhg* xlg
12
11
xlg*
12*
15*
15
60
Tlg*
1.20 1.18
50
1.14 30
Tlg Ta
1.16
COP
40
COP
Temperature (°C)
70
1.25
Fig. 10. Performance with a for the reverse parallel flow system.
1.28
90
0.9
10 1.20
14* 7* 7
14
2
Tc
17
Te
18
16
16*
1.12
20
1.10 1.30
1.35
1.40
1.45
α
1
1.50
s
Fig. 8. Performance with a for the pre-parallel flow system. Fig. 11. T–s diagram of series flow system for different a.
1.35 90 80
To,HT
70
1.25
Tcry,o,LT
COP
To,LT
60
1.20 50
COP
Temperature (°C)
1.30
Tcry,o,HT
1.15
40 30
1.10
20 1.05 1.25
1.30
1.35
α
1.40
1.45
1.50
Fig. 9. Performance with a for the rear parallel flow system.
From Figs. 7–10, it is also found that the outlet temperature of high concentration solution of HTHE as well as LTHE for four type air cooled LiBr/H2O double effect absorption chillers is nearly fixed with the change of a. It is explained that the outlet temperature of
high concentration solution of two solution heat exchangers is mainly determined by the effectiveness of solution heat exchanger. Moreover, the outlet temperature of high concentration solution of LTHE for four configurations is very approximate since the inlet temperature of low concentration solution of LTHE for every system is equal to the absorber temperature. But the outlet temperature of high concentration solution of HTHE for four systems is different. It is attributed to the different inlet temperature of low concentration solution of HTHE for every type chiller. The higher the inlet temperature of low concentration solution of HTHE, the higher the outlet temperature of high concentration solution of HTHE. Meanwhile, with the decrease of a, the corresponding crystallization temperature for four systems all gradually go up at first but rise quickly finally. Therefore, the risk of crystallization increases with the drop of a. The crystallization happens once the crystallization temperature exceeds to the outlet temperature of high concentration solution of solution heat exchanger. For the series and reverse parallel flow type chiller, the crystallization caused by the decrease of a only occurs at the high concentration solution outlet of LTHE and HTHE, respectively. While for the pre-parallel and rear parallel flow configuration, the high concentration solution outlet of HTHE and LTHE can be subjected to crystallization with the drop of a. Whereas, according to the simulation result, the solution heat exchanger of which has the highest risk of
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Z. Li, J. Liu / Energy Conversion and Management 99 (2015) 264–273
T xhg*
xm* xlg* xhg xm xlg
xa
xlg
xm* xlg*xm xhg
1.4
xhg
*
1.3
Thg
11*
12
11
12*
15*
1.2
15 7*
Tlg
14*
COP
Tlg
*
14 7
1.1
2
Ta
Series Pre-parallel Rear parallel Reverse parallel
1.0
Tc
16
16*
17
0.9
Te
18
1
1.25
1.30
1.35
1.40
1.45
1.50
α s Fig. 15. Comparison of COP for four type systems. Fig. 12. T–s diagram of pre-parallel flow system for different a.
T *
xhg
Thg
11*
xm* xlg* xhg xm xlg
xa
xlg
xm* xlg*xm xhg
12
11
xhg*
12*
15*
15
Tlg*
7*
Tlg
14* 14
7
2
Ta Tc
16
16*
17
Te
18
1
s Fig. 13. T–s diagram of rear parallel flow system for different a.
T *
xhg
Thg
11*
xm* xlg* xhg xm xlg
xa
xlg
xm* xlg*xm xhg
12
11
xhg*
12*
15*
15
Tlg* Tlg Ta Tc Te
14*
7* 14 7
2
16
16*
17
18
1
s Fig. 14. T–s diagram of reverse parallel flow system for different a.
crystallization with the decrease of a for the pre-parallel and rear parallel flow configuration is the HTHE and LTHE, respectively. By taking into account the comprehensive variation of COP and risk of crystallization, it is known that despite the COP goes up with the decrease of a, too low a can lead to the crystallization. Thereby, the minimum heat load ratio of generator (MHLRG) is defined as the corresponding difference between outlet temperature of high concentration solution of crystallized solution heat exchanger and the crystallization temperature is within 2 °C. Nevertheless, in order to improve the COP and prevent the crystallization sufficiently, the appropriate heat load ratio of generator for the series and pre-parallel flow type systems is suggested to be 0.02 greater than the MHLRG and that for the rear parallel and reverse parallel flow chillers should be 0.01 higher than the MHLRG. The comparison of COP for four type chillers is seen in Fig. 15. The a goes down from 1.5 to corresponding appropriate value. It is found that the COP for series, pre-parallel, rear parallel and reverse parallel flow system reduces in turn in the same working condition. It is attributed to that the inlet flow rate of solution and flow rate of refrigerant vapor in the HPG for above mentioned systems rise in turn under identical situation. Since the increasing rate of COP with the drop of a for the pre-parallel flow chiller is stronger than that for the series flow chiller, the COP in the appropriate a for the series and pre-parallel flow chiller is approximate. Furthermore, the COP in the appropriate a for the rear parallel and reverse parallel flow system is close and superior to that of series and pre-parallel flow chillers as the appropriate a of rear parallel and reverse parallel flow systems is less. The following part is to analyze the relationship of MHLRG and working condition. The variation of MHLRG with the HPG temperature is shown in Fig. 16. The MHLRG increases linearly with the rise of HPG temperature. It is attributed to that the rise of HPG temperature makes the crystallization temperature become higher. The increasing rate of MHLRG with the rise of HPG temperature for the series, pre-parallel, rear parallel and reverse parallel flow type air cooled LiBr/H2O double effect absorption chiller is 3.44 103, 0.44 103, 0.7 103 and 0.81 103/°C, respectively. For the series flow configuration, with the increase of generator temperature, the rise of outlet temperature of high concentration solution in the crystallized solution heat exchanger is evident less than the rise of crystallization temperature. Therefore, its increasing rate of MHLRG with the rise of HPG temperature is significant higher than that for other three systems. The change of MHLRG with the evaporator temperature is seen in Fig. 17. The MHLRG goes down linearly with the increase of
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Z. Li, J. Liu / Energy Conversion and Management 99 (2015) 264–273 1.45
1.50
Tg=120°C 1.45
Ts=33°C
Ts=33°C
1.40
Te=5°C
Te=5°C
D=0.5 (for parallel flow chiller)
D=0.5 (for parallel flow chiller)
1.35
εHT=0.7 εLT=0.7
MHLRG
MHLRG
1.40
1.35
Series Pre-parallel Rear parallel Reverse parallel
1.30
1.25
1.20
1.15
120
130
140
150
160
170
Generator temperature (°C)
1.34
1.32
1.30
Tg=120°C Series Ts=33°C Pre-parallel D=0.5 (for parallel flow chiller) Rear parallel Reverse parallel εHT=0.7 εLT=0.7
1.28
1.26
1.24
1.22
1.20 4
5
6
7
8
9
Evaporator temperature (°C) Fig. 17. Minimum heat load ratio of generator with evaporator temperature.
1.38 1.36
Tg=120°C
1.34
Te=5°C
1.32
εHT=0.7
1.30
εLT=0.7
D=0.5 (for parallel flow chiller)
1.28
Series Pre-parallel Rear parallel Reverse parallel
1.26 1.24 1.22 1.20 27
28
29
30
31
32
33
34
0.5
0.6
0.7
0.8
0.9
Effectiveness of HTHE
Fig. 16. Minimum heat load ratio of generator with generator temperature.
MHLRG
1.30
1.25
1.20
MHLRG
Series Pre-parallel Rear parallel Reverse parallel
εLT=0.7
35
36
37
Surrounding temperature (°C) Fig. 18. Minimum heat load ratio of generator with surrounding temperature.
evaporator temperature. It is explained that the rise of evaporator temperature decreases the solution concentration, hence the less MHLRG can be obtained in lower evaporator temperature. The
Fig. 19. Minimum heat load ratio of generator with effectiveness of HTHE.
decreasing rate of MHLRG with the rise of evaporator temperature for the series, pre-parallel, rear parallel and reverse parallel flow type air cooled LiBr/H2O double effect absorption chiller is 9.6 103, 9.8 103, 5.86 103 and 2.76 103/°C, respectively. It is gotten that the dependence of MHLRG for the reverse parallel flow system is weaker than that for other systems. The change of MHLRG with the surrounding temperature can be seen in Fig. 18. The MHLRG goes up linearly with the increase of surrounding temperature. It is explained that the rise of outdoor temperature results in the significant increase of solution concentration, so that the MHLRG becomes higher to avoid the crystallization. The increasing rate of MHLRG with the rise of surrounding temperature for the series, pre-parallel, rear parallel and reverse parallel flow type air cooled LiBr/H2O double effect absorption chiller is 0.57 102, 0.995 102, 0.538 102, 0.16 102/°C. Consequently, the effect of surrounding temperature on the MHLRG for the reverse parallel flow chiller is the least in four configurations. The variation of MHLRG with the effectiveness of HTHE is shown in Fig. 19. The MHLRG reduces with the drop of effectiveness of HTHE. It is attributed to that the drop of eHT decreases the solution concentration when the HPG temperature is fixed. As a result, lower MHLRG can be obtained in the higher effectiveness of HTHE. Besides, it is seen that the MHLRG of series flow air cooled LiBr/H2O double effect absorption chiller is sensitive to the change of eHT. While the influence of eHT on the MHLRG for other three configurations is approximate. The change of MHLRG with the effectiveness of LTHE can be seen in Fig. 20. It is seen that the dependence of MHLEG for series and rear parallel flow air cooled LiBr/H2O double effect absorption chiller on the effectiveness of LTHE is weak. But the MHLRG for pre-parallel and reverse parallel flow configuration goes up evidently with the increase of eLT. Moreover, it should be mentioned that the crystallized solution heat exchanger of pre-parallel with the change of eLT is not HTHE but LTHE. The variation of MHLRG with the distribution ratio is shown in Fig. 21. It can be thought that the MHLRG of reverse parallel flow air cooled LiBr/H2O double effect absorption chiller is independent on the distribution ratio. For the pre-parallel flow system, its MHLRG is nearly constant with the change of distribution ratio as the distribution ratio is less than 0.55. However, the MHLRG rises significantly with the increase of distribution ratio when the distribution ratio is higher than 0.55. Furthermore, the crystallized solution heat exchanger changes from HTHE to LTHE as the distribution ratio exceeds to 0.5. For the rear parallel flow
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Four systems named series, pre-parallel flow, rear parallel and reverse parallel flow configuration were considered. The appropriate heat load ratio for different type system was obtained by the comprehensive analysis of COP and risk of crystallization. The change of appropriate heat load ratio with the working condition was also discussed. The conclusions are summarized as follows:
1.34 1.32
MHLRG
1.30
Series Pre-parallel Rear parallel Reverse parallel
1.28 1.26
Tg=120°C Ts=33°C Te=5°C D=0.5 (for parallel flow chiller) εHT=0.7
1.24 1.22 1.20 0.5
0.6
0.7
0.8
0.9
Effectiveness of LTHE Fig. 20. Minimum heat load ratio of generator with effectiveness of LTHE.
1.40
Tg=120°C Ts=33°C Te=5°C
1.35
εHT=0.7
MHLRG
εLT=0.7
Pre-parallel Rear parallel Reverse parallel
1.30
1.25
1.20
0.40
0.45
0.50
0.55
0.60
0.65
0.70
Distribution ratio Fig. 21. Minimum heat load ratio of generator with distribution ratio.
configuration, the weak dependence of MHLRG on the distribution ratio is also found when the distribution ratio does not exceed to 0.5. But the MHLRG goes up slightly with the rise of distribution ratio when the distribution ratio is greater than 0.5. It should be noted that the crystallized solution heat exchanger changes from LTHE to HTHE as the distribution ratio is less than 0.5. In the paper, the relationship of MHLRG with working condition for different type air cooled LiBr/H2O double effect absorption chiller was obtained. This relationship can be used as the design criterion of heat load ratio of generator. For a real chiller, in order to make it work more efficiently and reliably, the design of heat load ratio of generator should be based on the highest generator temperature, surrounding temperature, effectiveness of LTHE and distribution ratio and the lowest evaporator temperature and effectiveness of HTHE. Since the working heat load ratio of generator is not fixed at the design one and changes with the variation of working condition, the variation of heat load ratio of generator and the corresponding control algorithm which can attain to the appropriate heat load ratio of generator are to be studied in future.
4. Conclusion The influence of heat load ratio of generator on the performance of air cooled LiBr/H2O double effect absorption chiller was studied.
(1) The COP goes up linearly with the decrease of heat load ratio of generator. Simultaneously, the risk of crystallization rises slowly at first but goes up quickly finally. The appropriate heat load ratio of generator for the series and pre-parallel flow type system is suggested to be 0.02 greater than the minimum heat load ratio of generator (MHLRG) and that for the rear parallel and reverse parallel flow chiller should be 0.01 higher than the MHLRG. (2) The MHLRG goes up with the increase of HPG temperature as well as surrounding temperature and it goes down with the rise of evaporator temperature and effectiveness of HTHE. The dependence of MHLRG for the series and rear parallel flow system on the effectiveness of LTHE is weak. While the MHLRG of pre-parallel and reverse parallel flow chiller rises with the increase of effectiveness of LTHE. The MHLRG of reverse parallel flow configuration is independent upon the distribution ratio. The MHLRG of pre-parallel flow system goes up fast with the rise of distribution ratio when the distribution ratio exceeds to 0.55. But the MHLRG of rear parallel flow configuration just rises slightly with the increase of distribution ratio as the distribution is greater than 0.5. (3) By taking into account the variation of working condition, the design of MHLRG should be based on the highest HPG temperature, surrounding temperature, effectiveness of LTHE and distribution ratio and the lowest evaporator temperature, effectiveness of HTHE to improve the COP and avoid the crystallization. (4) In the appropriate heat load ratio of generator, the COP for the series and pre-parallel flow system is approximate. And that for the rear parallel and reverse parallel flow configuration is also close and superior to the one for the series and pre-parallel flow system.
Acknowledgements This work is supported by: (1) National Science Foundation of China under the contract No. 51206056, (2) Fundamental Research Funds for the Central Universities under the contract No. 2014ZZ0016, (3) Key Laboratory of Efficient and Clean Energy Utilization of Guangdong Higher Education Institutes under the contract No. KLB10004. References [1] Alva LH, Gonza´lez JE. Simulation of an air-cooled solar-assisted absorption air conditioning system. J Sol Energy Eng 2002;124:276–82. [2] Izquierdo M, Lizarte R, Marcos JD, Gutierrez G. Air conditioning using an aircooled single effect lithium bromide absorption chiller results of a trial conducted in Madrid in August 2005. Appl Therm Eng 2008;28:1074–81. [3] González-Gil A, Izquierdo M, Marcos JD, Palacios E. Experimental evaluation of a direct air-cooled lithium bromide–water absorption prototype for solar air conditioning. Appl Therm Eng 2011;31:3358–68. [4] Lizarte R, Izquierdo M, Marcos JD, Palacios E. An innovative solar-driven directly air-cooled LiBr–H2O absorption chiller prototype for residential use. Energy Build 2012;47:1–11. [5] Lizarte R, Izquierdo M, Marcos JD, Palacios E. Experimental comparison of two solar-driven air-cooled LiBr–H2O absorption chillers indirect versus direct aircooled system. Energy Build 2013;62:323–34. [6] Izquierdo M, Marcos JD, Palacios ME, González-Gil A. Experimental evaluation of a low-power direct air-cooled double-effect LiBr–H2O absorption prototype. Energy 2012;37:737–48.
Z. Li, J. Liu / Energy Conversion and Management 99 (2015) 264–273 [7] Izquierdo M, González-Gil A, Palacios E. Solar-powered single-and doubleeffect directly air-cooled LiBr–H2O absorption prototype built as a single unit. Appl Energy 2014;130:7–19. [8] Xu GP, Dai YQ, Tou KW, Tso CP. Theoretical analysis and optimization of a double-effect series-flow-type absorption chiller. Appl Therm Eng 1996;16:975–87. [9] Gomri R, Hakimi R. Second law analysis of double effect vapour absorption cooler system. Energy Convers Manage 2008;49:3343–8. [10] Kaushik SC, Arora A. Energy and exergy analysis of single effect and series flow double effect water–lithium bromide absorption refrigeration systems. Int J Refrig 2009;32:1247–58. [11] Avanessian T, Ameri M. Energy, exergy, and economic analysis of single and double effect LiBr–H2O absorption chillers. Energy Build 2014;73:26–36. [12] Misra RD, Sahoo PK, Gupta A. Thermoeconomic analysis of a single and doubleeffect LiBr–H2O absorption refrigeration system. Int J Refrig 2005;28:331–43. [13] Xu GP, Dai YQ. Theoretical analysis and optimization of a double-effect parallel-flow-type absorption chiller. Appl Therm Eng 1997;17:157–70. [14] Arun MB, Maiya MP, Srinivasa Murthy S. Performance comparison of doubleeffect parallel-flow and series flow water–lithium bromide absorption systems. Appl Therm Eng 2001;21:1273–9. [15] Ahmed MSAMS, Gilani SIUH. Exergy analysis of a double-effect parallel-flow commercial steam absorption chiller. J Appl Sci 2012;12:2580–5. [16] Garousi Farshi L, Seyed Mahmoudi SM, Rosen MA, Yari M. A comparative study of the performance characteristics of double-effect absorption refrigeration systems. Int J Energy Res 2012;36:182–92. [17] Garousi Farshi L, Mahmoudi SMS, Rosen MA, Yari M, Amidpour M. Exergoeconomic analysis of double effect absorption refrigeration systems. Energy Convers Manage 2013;65:13–25.
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[18] Chen HX, Shang R, Shu ZH, Xie JL. Effect of different operating conditions on cycle performance of double-effect air-cooled absorption refrigeration. J Chem Ind Eng 2008;59:210–4 [in Chinese]. [19] Izquierdo M, Venegas M, Rodriguez P, Lecuona A. Crystallization as a limit to develop solar air-cooled LiBr–H2O absorption systems using low-grade heat. Sol Energy Mater Sol Cells 2004;81:205–16. [20] Kim DS, Infante Ferreira CA. Air-cooled LiBr–water absorption chillers for solar air conditioning in extremely hot weathers. Energy Convers Manage 2009;50:1018–25. [21] Han ZW, Ji J, He W, Pei G. Theoretical study of dual-heat mode operation of double-effect parallel-flow LiBr–H2O absorption refrigeration system. J Univ Sci Technol 2010;40:24–9 [in Chinese]. [22] Garousi Farshi L, Seyed Mahmoudi SM, Rosen MA. Analysis of crystallization risk in double effect absorption refrigeration systems. Appl Therm Eng 2011;31:1712–7. [23] Li ZY, Ye XY, Liu JP. Optimal temperature of collector for solar double effect LiBr/H2O absorption cooling system in subtropical city based on a year round meteorological data. Appl Therm Eng 2014;69:19–28. [24] Li ZY, Ye XY, Liu JP. Performance analysis of solar air cooled double effect LiBr/ H2O absorption cooling system in subtropical city. Energy Convers Manage 2014;85:302–12. [25] Pa´tek J, Klomfar J. A computationally effective formulation of the thermodynamic properties of LiBr–H2O solutions from 273 to 500 K over full composition range. Int J Refrig 2006;29:566–78.
Contents lists available at ScienceDirect
Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Appropriate heat load ratio of generator for different types of air cooled lithium bromide–water double effect absorption chiller Zeyu Li ⇑, Jinping Liu School of Electric Power, South China University of Technology, Guangzhou 510640, China Guangdong Province Key Laboratory of High Efficient and Clean Energy Utilization, South China University of Technology, Guangzhou 510640, China
a r t i c l e
i n f o
Article history: Received 15 March 2015 Accepted 20 April 2015 Available online 18 May 2015 Keywords: Heat load ratio of generator Air cooled Double effect Absorption chiller
a b s t r a c t The lower coefficient of performance and higher risk of crystallization in the higher surrounding temperature is the primary disadvantage of air cooled lithium bromide–water double effect absorption chiller. Since the coefficient of performance and risk of crystallization strongly depend on the heat load ratio of generator, the appropriate heat load ratio of generator can improve the performance as the surrounding temperature is higher. The paper mainly deals with the appropriate heat load ratio of generator of air cooled lithium bromide–water double effect absorption chiller. Four type systems named series, pre-parallel, rear parallel and reverse parallel flow configuration were considered. The corresponding parametric model was developed to analyze the comprehensive effect of heat load ratio of generator on the coefficient of performance and risk of crystallization. It was found that the coefficient of performance goes up linearly with the decrease of heat load ratio of generator. Simultaneously, the risk of crystallization also rises slowly at first but increases fast finally. Consequently, the appropriate heat load ratio of generator for the series and pre-parallel flow type systems is suggested to be 0.02 greater than the minimum heat load ratio of generator and that for the rear parallel and reverse parallel flow chillers should be 0.01 higher than the minimum heat load ratio of generator. Besides, the changes of minimum heat load ratio of generator for different type systems with the working condition were analyzed and compared. It was found that the minimum heat load ratio of generator goes up with the increase of temperature of high pressure generator as well as surrounding temperature and it goes down with the rise of evaporator temperature and effectiveness of high temperature heat exchanger. The dependence of minimum heat load ratio of generator for the series and rear parallel flow system on the effectiveness of low temperature heat exchanger is weak. While the minimum heat load ratio of generator of pre-parallel and reverse parallel flow chiller rises with the increase of effectiveness of low temperature heat exchanger. The minimum heat load ratio of generator of reverse parallel flow configuration is independent upon the distribution ratio. The minimum heat load ratio of generator of pre-parallel flow system goes up fast with the rise of distribution ratio when the distribution ratio exceeds to 0.55. But the minimum heat load ratio of generator of rear parallel flow configuration just rises slightly with the increase of distribution ratio as the distribution is greater than 0.5. The paper is helpful to the development and performance improvement of air cooled lithium bromide–water double effect absorption chiller. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Recently, the demand of air conditioning grows significantly because of the effect of global warming. However, the consequent increasing consumption of air conditioning can further worsen global warming, in turn the demand of air conditioning continues to rise inevitably. Therefore, it is urgent to reduce the consumption ⇑ Corresponding author at: School of Electric Power, South China University of Technology, Guangzhou 510640, China. E-mail address: [email protected] (Z. Li). http://dx.doi.org/10.1016/j.enconman.2015.04.055 0196-8904/Ó 2015 Elsevier Ltd. All rights reserved.
of air conditioning. Since the LiBr/H2O solution is nontoxic and the corresponding absorption chiller can be driven by solar energy or waste heat, the application of LiBr/H2O absorption chiller is an effective way that lowers the consumption of air conditioning. Comparing with the single effect or water cooled system, the air cooled LiBr/H2O double effect absorption chiller is better and more flexible due to the high efficient, absence of cooling tower and the independence upon water. Although the simulation showed that the working characteristic of air cooled LiBr/H2O absorption chiller is close to the water cooled one as the solar irradiance is strong [1], the development
Z. Li, J. Liu / Energy Conversion and Management 99 (2015) 264–273
265
Nomenclature COP h m Q T DT x
coefficient of performance enthalpy (kJ/kg) mass flow rate (kg/s) energy (kW) temperature (°C) temperature difference (°C) concentration of solution
Greek symbols a heat load ratio of generator e heat exchanger effectiveness
of air cooled LiBr/H2O absorption chiller is difficult due to the poor heat transfer coefficient of air. A small indirect air cooled LiBr/H2O single effect absorption chiller was firstly developed by Izquierdo et al. [2]. Nevertheless, this prototype is not compact and its performance is lower because of the complicated cooling of absorber and condenser. Hence, the direct air cooled LiBr/H2O single effect absorption chiller was developed subsequently [3]. This prototype works without the crystallization when the surrounding temperature is up to 37.7 °C [4]. The comparative test showed that the absorber temperature and the condenser temperature of direct air cooled prototype are lower than that of indirect one under the same working condition [5]. Based on the above mentioned successful development, the small air cooled LiBr/H2O double effect absorption chiller was developed as well [6]. This prototype is more compact and it can work without the crystallization as the surrounding temperature is up to 45 °C due to the better performance of new flat-sheet adiabatic absorber. Recently, a new solar air cooled LiBr/H2O double effect absorption chiller was developed by Izquierdo et al. [7]. The prototype works as the double effect cycle when the solar energy is sufficient and it can work as the single effect cycle as the solar irradiance is insufficient to drive the double effect operational mode. The double effect LiBr/H2O absorption chiller has many different configurations and the performance of each configuration is different even the working condition is identical. For the series flow type double effect LiBr/H2O absorption chiller, it was gotten that the coefficient of performance (COP) and total heat transfer area goes down slightly with the increase of solution concentration [8]. Gomri and Hakimi [9] pointed out that the exergy loss of high pressure generator (HPG) and absorber is the highest in the series flow type double effect LiBr/H2O absorption chiller. Besides, in the series flow configuration, the pressure drop of absorber and evaporator plays an important role in the COP and exergy efficiency [10] and the relative humidity has less effect on the exergy efficiency [11]. Misra et al. [12] carried out the thermoeconomic analysis on the series flow type double effect LiBr/H2O absorption chiller and it was concluded that the variation of optimal decision variable is not sensitive to the fuel cost. For the parallel flow configuration, it was obtained that the change of distribution ratio varies the COP less [13]. It was found that the coefficient of performance (COP) of rear parallel flow configuration is more sensitive to the change of evaporator temperature [14]. In the rear parallel flow configuration, it was shown that the COP goes up but the exergy efficiency reduces with the rise of load factor [15]. The comparison of COP and exergy efficiency for different type double effect LiBr/H2O absorption chillers showed that the performance of parallel flow configuration is superior to the series flow one [16]. Moreover, it was concluded that the product cost flow rate of reverse flow type chiller is the lowest while that of series flow
Subscripts a absorber c condenser e evaporator hg high pressure generator i inlet lg low pressure generator o outlet s surrounding
one is the highest by the exergoeconomic analysis [17]. For the different type air cooled LiBr/H2O double effect absorption chillers, it was obtained that the COP of pre-parallel flow configuration is the lowest under the same working condition [18]. Because of rise of absorber temperature and condenser temperature with the increase of surrounding temperature, the air cooled LiBr/H2O absorption chiller suffers from crystallization easily especially when the outdoor temperature is higher. Izquierdo et al. [19] pointed out that the double stage air cooled LiBr/H2O absorption chiller is more suitable than the single effect one as the outdoor temperature exceeds to 40 °C. A new air cooled LiBr/H2O absorption cycle which can operate without crystallization as the outdoor temperature is up to 50 °C was proposed by Kim and Infante Ferreira [20]. Han et al. [21] presented that the distribution ratio of pre-parallel flow type double effect LiBr/H2O absorption chiller should vary in term of working condition to prevent the crystallization and the suitable range of distribution ratio becomes narrow with the increase of HPG temperature, evaporator temperature and the effectiveness of solution heat exchanger. The analysis on the risk of crystallization in the double effect LiBr/H2O absorption chiller was carried out by Garousi Farshi et al. [22]. It was concluded that the rear parallel configuration and the reverse parallel configuration are more difficult to suffer from the crystallization than the series system in the same working condition. The lower COP and higher risk of crystallization in the higher surrounding temperature is the primary disadvantage of air cooled LiBr/H2O double effect absorption chiller. The heat load ratio of generator is the important parameter to the design of area of low pressure generator (LPG). And it was found that the variation of heat load ratio of generator can significantly change the system performance [23]. Consequently, the appropriate heat load ratio of generator that maximizes the COP and still prevents the crystallization can make the air cooled LiBr/H2O double effect absorption chiller work efficiently and reliably as the outdoor temperature is higher. However, the influence of heat load ratio of generator on the system performance is not studied adequately. Furthermore, the relationship of appropriate heat load ratio of generator with the working condition is also not clear. Therefore, the objective of paper is to obtain the appropriate heat load ratio of generator of air cooled LiBr/H2O double effect absorption chiller. Four configurations named series, pre-parallel, rear parallel and reverse parallel flow type system were considered. The corresponding parametric model was developed to investigate the effect of heat load ratio of generator on the COP and risk of crystallization. The changes of appropriate heat load ratio of generator for four type chillers were compared and analyzed. The paper is helpful to the development and performance improvement of air cooled LiBr/H2O double effect absorption chiller.
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configuration, the parametric model of pre-parallel, rear parallel as well as reverse parallel flow type system was developed by the suitable modification of previous model [24]. The assumptions of model were shown in Table 1. The working characteristic of different type air cooled double effect LiBr/H2O absorption chillers was described by the mass and energy conservation:
2. Model Four type air cooled LiBr/H2O double effect absorption chillers were considered. The first type is named series flow system. The second type is called pre-parallel flow system: the solution flows into the HPG and the LPG in parallel at the outlet of absorber. The third type is called rear parallel flow system: the solution flows into the HPG and the LPG in parallel at the low concentration solution outlet of low temperature heat exchanger (LTHE). The fourth type is named reverse parallel system: the solution firstly enters the LPG, and then flows into the HPG and LTHE in parallel at the solution outlet of LPG. The schematic of series, pre-parallel, rear parallel and reverse parallel flow configurations are shown from Figs. 1–4. Based on the model of Gomri and Hakimi [9] and Garousi Farshi et al. [22], the parametric model of series flow type air cooled double effect LiBr/H2O absorption chiller was developed in the previous study [24]. Here, according to the corresponding
X X
mi ¼
X
mi xi ¼
Q¼
X
mo
X
ð1Þ
mo xo
mi hi
X
ð2Þ
m o ho
Here, the (+) sign of Q refers to the output energy and the () sign of Q denotes the input energy. In order to be consistent with our previous investigation [23], the heat load ratio of generator was also equals to the ratio of energy inputting into the HPG and energy inputting into the LPG:
14 HPG Hot fluid inlet 24
LPG
12
15 Hot fluid outlet 25
16
Condenser
13 Air inlet 19 10
11
8
9
21 Air outlet
High temperature heat exchanger
17
6
7
4
5
Low temperature heat exchanger 18
Solution pump
Air outlet 3 2 20 Absorber 19 1 Air inlet
Chilled water outlet 23 Evaporator 22 Chilled water inlet
Fig. 1. Schematic of series flow type system.
14 HPG Hot fluid inlet 24
LPG
12
15
16
Condenser
Hot fluid outlet 25 Air inlet 19 10
7
11
6
21 Air outlet
Low temperature heat exchanger
High temperature heat exchanger 8
5
9
17
4
13 18 Solution pump
3 2 Absorber 1
Air inlet 19
ð3Þ
Chilled water outlet 23 22 Evaporator Chilled water inlet
20 Air outlet
Fig. 2. Schematic of pre-parallel flow type system.
Z. Li, J. Liu / Energy Conversion and Management 99 (2015) 264–273
267
14 HPG Hot fluid inlet 24
LPG
12
15 Hot fluid outlet 25 11
10 High temperature heat exchanger 8
26
16
Condenser
27 Air inlet 19
21 Air outlet
9
17
13 6
7
4
5
Low temperature heat exchanger Solution pump
18 Chilled water outlet 23 22 Evaporator Chilled water inlet
3
2
Absorber 1 Air inlet 19
20 Air outlet
Fig. 3. Schematic of rear parallel flow type system.
14 HPG Hot fluid inlet 24
LPG
12
15 Hot fluid outlet 25
16
Condenser
26 Air inlet 19 11
10
9
8
High temperature heat exchanger
21 Air outlet
Solution pump 13
17
6
7
Low temperature heat exchanger 5
4 Solution pump
3 2 Absorber 1 Air inlet 19
18 Chilled water outlet 23 22 Evaporator Chilled water inlet
20 Air outlet
Fig. 4. Schematic of reverse parallel flow type system.
a ¼ Q hg =Q lg Table 1 Assumption of air cooled LiBr/H2O double effect absorption chiller. Number
Assumption
1 2
Steady state The refrigerant (water) at the outlet of condenser is saturated liquid and its temperature is condensation temperature The refrigerant (water) at the outlet of evaporator is saturated vapor and its temperature equals to temperature of evaporator The solution (LiBr) at the outlet of absorber, HPG and LPG is saturated and the corresponding temperature is equal to the temperature of absorber, HPG and LPG, respectively The pressure loss of pipeline and heat exchanger except that between LPG and condenser is ignored The heat exchange between the system and surroundings is not considered The work of solution pump is negligible
3 4
5 6 7
ð4Þ
The distribution ratios of pre-parallel, rear parallel and reverse parallel flow system are expressed as follows:
D ¼ m8 =m2
ð5Þ
D ¼ m8 =m2
ð6Þ
D ¼ m8 =m26
ð7Þ
The simulation condition is shown in Table 2. The inlet and outlet temperature of hot fluid as well as chilled water was set in the identical value of Garousi Farshi et al. [22]: T24 = Thg + 18, T25 = Thg + 10, T22 = Te + 8, T23 = Te + 3. The absorber and condenser temperature are calculated as follows:
T a ¼ T 20 þ DT a
ð8Þ
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Z. Li, J. Liu / Energy Conversion and Management 99 (2015) 264–273
Table 2 Simulated condition.
Input the fixed data: T19, T20, T21, T22, T23, T24, T25, QEV,
Item
Value
Generator temperature Surrounding temperature Evaporator temperature Effectiveness of HTHE Effectiveness of LTHE Distribution ratio (for parallel flow system) Cooling capacity of system
120–170 °C 27–37 °C 4–9 °C 0.5–0.9 0.5–0.9 0.4–0.7 20 kW
T c ¼ T 21 þ DT c
Calculate the following parameter: m1, Thg, Ta, Tc, Te
Assume the solution concentration of HPG xhg
ð9Þ
Here, the DTa and DTc is the temperature difference of heat transfer in absorber and condenser, respectively. From the experiment of Gonzalez-Gil [3], it was gotten that DTa and DTc is 5 °C. The outlet temperature of cooling air for absorber and condenser is:
T 20 ¼ T 19 þ DT s;a
ð10Þ
T 21 ¼ T 19 þ DT s;c
ð11Þ
Assume the solution concentration of LPG xlg
Calculates the thermodynamic properties of each point
Here, the DTs,a and DTs,c is the temperature rise of cooling air in absorber and condenser, respectively. The experimental results showed that the ratio of cooling load in absorber to that in condenser is about 1.5 and the total temperature rise of cooling air is 5 °C [3]. Thereby, the DTs,a is 3 °C and the DTs,c is 2 °C. The parametric model was solved by the FORTRAN program. The thermodynamic property of LiBr/H2O solution was referred by Pa´tek [25]. And the corresponding function of Refprop7 was used in the program to calculate the thermodynamic property of water. The calculation process was shown in Fig. 5. The COP was equal to:
Energy balance in LPG?
ð12Þ
Because it is well known that the crystallization usually happens at the high concentration solution outlet of two solution heat exchangers, the risk of crystallization was evaluated by the difference between the outlet temperature of high concentration solution of solution heat exchanger and the crystallization temperature. The high temperature heat exchanger (HTHE) and the LTHE were all considered. It can be inferred that the closer the temperature difference, the higher the risk of crystallization. The crystallization occurs when the outlet temperature of high concentration solution of solution heat exchanger is less than the crystallization temperature. The data of Garousi Farshi et al. [22] was used to verify the model of paper. As seen in Fig. 6, a good agreement of them was obtained. 3. Results and discussion The performance with energy distribution ratio of generator for the series, pre-parallel, rear parallel and reverse parallel flow type air cooled LiBr/H2O double effect absorption chiller is shown from Figs. 7–10. The corresponding T–s diagram can be seen from Figs. 11–14. The dash line represents the decreased a. The superscript (⁄) means the state of decreased a. The working condition in the simulation is: Thg = 120 °C, Ts = 33 °C, Te = 5 °C, D = 0.5, eHT = 0.7, eLT = 0.7. It is seen that the COP is sensitive to the change of a and it goes up linearly with the decrease of a. It is attributed to the drop of circulation ratio with the decrease of a. Furthermore, the effect of a on the COP of series, pre-parallel, rear parallel and reverse parallel flow configuration becomes stronger in turn. The mean increasing rate of COP with the decrease of a for the series, pre-parallel, rear parallel and reverse parallel flow type chiller is 0.638, 1.137, 1.214 and 1.653, respectively. The less influence of a on the COP of series flow type system is attributed to the increase
converge?
No
Yes
No
Yes
End Fig. 5. Process flowchart of model.
Solution inlet temperature of absorber (°C)
COP ¼ Q e =Q hg
If
Reverse parallel, Garousi Farshi [22] Reverse parallel, model Rear parallel, Garousi Farshi [22] Rear parallel, model
65
60
εLT=0.5 εLT=0.5
εLT=0.5
Tg=147°C Te=4°C
55
Ta=Tc=37°C
εHT=0.7 D for rear parallel 50 flow syste, is 0.42
D for reverse parallel flow system is 0.65
45
εLT=0.9
Series, Garousi Farshi [22] εLT=0.9 Series, model
εLT=0.9
40 0.60
0.61
0.62
0.63
0.64
Concentration at the absorber inlet Fig. 6. Validation of model.
of flow rate of refrigerant vapor in the HPG with the decrease of a. While for the parallel flow configuration, the inlet flow rate of solution and flow rate of refrigerant vapor in the HPG all go down with the drop of a. The greater the decrease of above mentioned flow rate, the stronger the effect of a on the COP.
269
Z. Li, J. Liu / Energy Conversion and Management 99 (2015) 264–273 100 1.28
90
Tcry,o,HT
80
To,HT To,LT
60
1.24
50 1.22 40
Temperature (°C)
Tcry,o,LT COP
Tcry,o,HT
80
To,HT
70
Tcry,o,LT
1.26
COP
Temperature (°C)
70
1.4
1.2
To,LT
COP
60
1.3
1.1
50
COP
90
40 1.0 30
1.20 30
20 1.18
20 1.30
1.35
1.40
1.45
α
1.50
Fig. 7. Performance with a for the series flow system.
Tcry,o,HT To,HT
80
T *
xlg
1.26
Tcry,o,LT
1.24
To,LT
1.22
Thg
1.30
1.35
α
1.40
1.45
1.50
xhg* xlg xhg
11*
xa
xhg
xhg* xlg
12
11
xlg*
12*
15*
15
60
Tlg*
1.20 1.18
50
1.14 30
Tlg Ta
1.16
COP
40
COP
Temperature (°C)
70
1.25
Fig. 10. Performance with a for the reverse parallel flow system.
1.28
90
0.9
10 1.20
14* 7* 7
14
2
Tc
17
Te
18
16
16*
1.12
20
1.10 1.30
1.35
1.40
1.45
α
1
1.50
s
Fig. 8. Performance with a for the pre-parallel flow system. Fig. 11. T–s diagram of series flow system for different a.
1.35 90 80
To,HT
70
1.25
Tcry,o,LT
COP
To,LT
60
1.20 50
COP
Temperature (°C)
1.30
Tcry,o,HT
1.15
40 30
1.10
20 1.05 1.25
1.30
1.35
α
1.40
1.45
1.50
Fig. 9. Performance with a for the rear parallel flow system.
From Figs. 7–10, it is also found that the outlet temperature of high concentration solution of HTHE as well as LTHE for four type air cooled LiBr/H2O double effect absorption chillers is nearly fixed with the change of a. It is explained that the outlet temperature of
high concentration solution of two solution heat exchangers is mainly determined by the effectiveness of solution heat exchanger. Moreover, the outlet temperature of high concentration solution of LTHE for four configurations is very approximate since the inlet temperature of low concentration solution of LTHE for every system is equal to the absorber temperature. But the outlet temperature of high concentration solution of HTHE for four systems is different. It is attributed to the different inlet temperature of low concentration solution of HTHE for every type chiller. The higher the inlet temperature of low concentration solution of HTHE, the higher the outlet temperature of high concentration solution of HTHE. Meanwhile, with the decrease of a, the corresponding crystallization temperature for four systems all gradually go up at first but rise quickly finally. Therefore, the risk of crystallization increases with the drop of a. The crystallization happens once the crystallization temperature exceeds to the outlet temperature of high concentration solution of solution heat exchanger. For the series and reverse parallel flow type chiller, the crystallization caused by the decrease of a only occurs at the high concentration solution outlet of LTHE and HTHE, respectively. While for the pre-parallel and rear parallel flow configuration, the high concentration solution outlet of HTHE and LTHE can be subjected to crystallization with the drop of a. Whereas, according to the simulation result, the solution heat exchanger of which has the highest risk of
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T xhg*
xm* xlg* xhg xm xlg
xa
xlg
xm* xlg*xm xhg
1.4
xhg
*
1.3
Thg
11*
12
11
12*
15*
1.2
15 7*
Tlg
14*
COP
Tlg
*
14 7
1.1
2
Ta
Series Pre-parallel Rear parallel Reverse parallel
1.0
Tc
16
16*
17
0.9
Te
18
1
1.25
1.30
1.35
1.40
1.45
1.50
α s Fig. 15. Comparison of COP for four type systems. Fig. 12. T–s diagram of pre-parallel flow system for different a.
T *
xhg
Thg
11*
xm* xlg* xhg xm xlg
xa
xlg
xm* xlg*xm xhg
12
11
xhg*
12*
15*
15
Tlg*
7*
Tlg
14* 14
7
2
Ta Tc
16
16*
17
Te
18
1
s Fig. 13. T–s diagram of rear parallel flow system for different a.
T *
xhg
Thg
11*
xm* xlg* xhg xm xlg
xa
xlg
xm* xlg*xm xhg
12
11
xhg*
12*
15*
15
Tlg* Tlg Ta Tc Te
14*
7* 14 7
2
16
16*
17
18
1
s Fig. 14. T–s diagram of reverse parallel flow system for different a.
crystallization with the decrease of a for the pre-parallel and rear parallel flow configuration is the HTHE and LTHE, respectively. By taking into account the comprehensive variation of COP and risk of crystallization, it is known that despite the COP goes up with the decrease of a, too low a can lead to the crystallization. Thereby, the minimum heat load ratio of generator (MHLRG) is defined as the corresponding difference between outlet temperature of high concentration solution of crystallized solution heat exchanger and the crystallization temperature is within 2 °C. Nevertheless, in order to improve the COP and prevent the crystallization sufficiently, the appropriate heat load ratio of generator for the series and pre-parallel flow type systems is suggested to be 0.02 greater than the MHLRG and that for the rear parallel and reverse parallel flow chillers should be 0.01 higher than the MHLRG. The comparison of COP for four type chillers is seen in Fig. 15. The a goes down from 1.5 to corresponding appropriate value. It is found that the COP for series, pre-parallel, rear parallel and reverse parallel flow system reduces in turn in the same working condition. It is attributed to that the inlet flow rate of solution and flow rate of refrigerant vapor in the HPG for above mentioned systems rise in turn under identical situation. Since the increasing rate of COP with the drop of a for the pre-parallel flow chiller is stronger than that for the series flow chiller, the COP in the appropriate a for the series and pre-parallel flow chiller is approximate. Furthermore, the COP in the appropriate a for the rear parallel and reverse parallel flow system is close and superior to that of series and pre-parallel flow chillers as the appropriate a of rear parallel and reverse parallel flow systems is less. The following part is to analyze the relationship of MHLRG and working condition. The variation of MHLRG with the HPG temperature is shown in Fig. 16. The MHLRG increases linearly with the rise of HPG temperature. It is attributed to that the rise of HPG temperature makes the crystallization temperature become higher. The increasing rate of MHLRG with the rise of HPG temperature for the series, pre-parallel, rear parallel and reverse parallel flow type air cooled LiBr/H2O double effect absorption chiller is 3.44 103, 0.44 103, 0.7 103 and 0.81 103/°C, respectively. For the series flow configuration, with the increase of generator temperature, the rise of outlet temperature of high concentration solution in the crystallized solution heat exchanger is evident less than the rise of crystallization temperature. Therefore, its increasing rate of MHLRG with the rise of HPG temperature is significant higher than that for other three systems. The change of MHLRG with the evaporator temperature is seen in Fig. 17. The MHLRG goes down linearly with the increase of
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Z. Li, J. Liu / Energy Conversion and Management 99 (2015) 264–273 1.45
1.50
Tg=120°C 1.45
Ts=33°C
Ts=33°C
1.40
Te=5°C
Te=5°C
D=0.5 (for parallel flow chiller)
D=0.5 (for parallel flow chiller)
1.35
εHT=0.7 εLT=0.7
MHLRG
MHLRG
1.40
1.35
Series Pre-parallel Rear parallel Reverse parallel
1.30
1.25
1.20
1.15
120
130
140
150
160
170
Generator temperature (°C)
1.34
1.32
1.30
Tg=120°C Series Ts=33°C Pre-parallel D=0.5 (for parallel flow chiller) Rear parallel Reverse parallel εHT=0.7 εLT=0.7
1.28
1.26
1.24
1.22
1.20 4
5
6
7
8
9
Evaporator temperature (°C) Fig. 17. Minimum heat load ratio of generator with evaporator temperature.
1.38 1.36
Tg=120°C
1.34
Te=5°C
1.32
εHT=0.7
1.30
εLT=0.7
D=0.5 (for parallel flow chiller)
1.28
Series Pre-parallel Rear parallel Reverse parallel
1.26 1.24 1.22 1.20 27
28
29
30
31
32
33
34
0.5
0.6
0.7
0.8
0.9
Effectiveness of HTHE
Fig. 16. Minimum heat load ratio of generator with generator temperature.
MHLRG
1.30
1.25
1.20
MHLRG
Series Pre-parallel Rear parallel Reverse parallel
εLT=0.7
35
36
37
Surrounding temperature (°C) Fig. 18. Minimum heat load ratio of generator with surrounding temperature.
evaporator temperature. It is explained that the rise of evaporator temperature decreases the solution concentration, hence the less MHLRG can be obtained in lower evaporator temperature. The
Fig. 19. Minimum heat load ratio of generator with effectiveness of HTHE.
decreasing rate of MHLRG with the rise of evaporator temperature for the series, pre-parallel, rear parallel and reverse parallel flow type air cooled LiBr/H2O double effect absorption chiller is 9.6 103, 9.8 103, 5.86 103 and 2.76 103/°C, respectively. It is gotten that the dependence of MHLRG for the reverse parallel flow system is weaker than that for other systems. The change of MHLRG with the surrounding temperature can be seen in Fig. 18. The MHLRG goes up linearly with the increase of surrounding temperature. It is explained that the rise of outdoor temperature results in the significant increase of solution concentration, so that the MHLRG becomes higher to avoid the crystallization. The increasing rate of MHLRG with the rise of surrounding temperature for the series, pre-parallel, rear parallel and reverse parallel flow type air cooled LiBr/H2O double effect absorption chiller is 0.57 102, 0.995 102, 0.538 102, 0.16 102/°C. Consequently, the effect of surrounding temperature on the MHLRG for the reverse parallel flow chiller is the least in four configurations. The variation of MHLRG with the effectiveness of HTHE is shown in Fig. 19. The MHLRG reduces with the drop of effectiveness of HTHE. It is attributed to that the drop of eHT decreases the solution concentration when the HPG temperature is fixed. As a result, lower MHLRG can be obtained in the higher effectiveness of HTHE. Besides, it is seen that the MHLRG of series flow air cooled LiBr/H2O double effect absorption chiller is sensitive to the change of eHT. While the influence of eHT on the MHLRG for other three configurations is approximate. The change of MHLRG with the effectiveness of LTHE can be seen in Fig. 20. It is seen that the dependence of MHLEG for series and rear parallel flow air cooled LiBr/H2O double effect absorption chiller on the effectiveness of LTHE is weak. But the MHLRG for pre-parallel and reverse parallel flow configuration goes up evidently with the increase of eLT. Moreover, it should be mentioned that the crystallized solution heat exchanger of pre-parallel with the change of eLT is not HTHE but LTHE. The variation of MHLRG with the distribution ratio is shown in Fig. 21. It can be thought that the MHLRG of reverse parallel flow air cooled LiBr/H2O double effect absorption chiller is independent on the distribution ratio. For the pre-parallel flow system, its MHLRG is nearly constant with the change of distribution ratio as the distribution ratio is less than 0.55. However, the MHLRG rises significantly with the increase of distribution ratio when the distribution ratio is higher than 0.55. Furthermore, the crystallized solution heat exchanger changes from HTHE to LTHE as the distribution ratio exceeds to 0.5. For the rear parallel flow
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Four systems named series, pre-parallel flow, rear parallel and reverse parallel flow configuration were considered. The appropriate heat load ratio for different type system was obtained by the comprehensive analysis of COP and risk of crystallization. The change of appropriate heat load ratio with the working condition was also discussed. The conclusions are summarized as follows:
1.34 1.32
MHLRG
1.30
Series Pre-parallel Rear parallel Reverse parallel
1.28 1.26
Tg=120°C Ts=33°C Te=5°C D=0.5 (for parallel flow chiller) εHT=0.7
1.24 1.22 1.20 0.5
0.6
0.7
0.8
0.9
Effectiveness of LTHE Fig. 20. Minimum heat load ratio of generator with effectiveness of LTHE.
1.40
Tg=120°C Ts=33°C Te=5°C
1.35
εHT=0.7
MHLRG
εLT=0.7
Pre-parallel Rear parallel Reverse parallel
1.30
1.25
1.20
0.40
0.45
0.50
0.55
0.60
0.65
0.70
Distribution ratio Fig. 21. Minimum heat load ratio of generator with distribution ratio.
configuration, the weak dependence of MHLRG on the distribution ratio is also found when the distribution ratio does not exceed to 0.5. But the MHLRG goes up slightly with the rise of distribution ratio when the distribution ratio is greater than 0.5. It should be noted that the crystallized solution heat exchanger changes from LTHE to HTHE as the distribution ratio is less than 0.5. In the paper, the relationship of MHLRG with working condition for different type air cooled LiBr/H2O double effect absorption chiller was obtained. This relationship can be used as the design criterion of heat load ratio of generator. For a real chiller, in order to make it work more efficiently and reliably, the design of heat load ratio of generator should be based on the highest generator temperature, surrounding temperature, effectiveness of LTHE and distribution ratio and the lowest evaporator temperature and effectiveness of HTHE. Since the working heat load ratio of generator is not fixed at the design one and changes with the variation of working condition, the variation of heat load ratio of generator and the corresponding control algorithm which can attain to the appropriate heat load ratio of generator are to be studied in future.
4. Conclusion The influence of heat load ratio of generator on the performance of air cooled LiBr/H2O double effect absorption chiller was studied.
(1) The COP goes up linearly with the decrease of heat load ratio of generator. Simultaneously, the risk of crystallization rises slowly at first but goes up quickly finally. The appropriate heat load ratio of generator for the series and pre-parallel flow type system is suggested to be 0.02 greater than the minimum heat load ratio of generator (MHLRG) and that for the rear parallel and reverse parallel flow chiller should be 0.01 higher than the MHLRG. (2) The MHLRG goes up with the increase of HPG temperature as well as surrounding temperature and it goes down with the rise of evaporator temperature and effectiveness of HTHE. The dependence of MHLRG for the series and rear parallel flow system on the effectiveness of LTHE is weak. While the MHLRG of pre-parallel and reverse parallel flow chiller rises with the increase of effectiveness of LTHE. The MHLRG of reverse parallel flow configuration is independent upon the distribution ratio. The MHLRG of pre-parallel flow system goes up fast with the rise of distribution ratio when the distribution ratio exceeds to 0.55. But the MHLRG of rear parallel flow configuration just rises slightly with the increase of distribution ratio as the distribution is greater than 0.5. (3) By taking into account the variation of working condition, the design of MHLRG should be based on the highest HPG temperature, surrounding temperature, effectiveness of LTHE and distribution ratio and the lowest evaporator temperature, effectiveness of HTHE to improve the COP and avoid the crystallization. (4) In the appropriate heat load ratio of generator, the COP for the series and pre-parallel flow system is approximate. And that for the rear parallel and reverse parallel flow configuration is also close and superior to the one for the series and pre-parallel flow system.
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