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Experimental study on an absorption refrigeration system driven by temperature-distributed heat sources
Energy 170 (2019) 471e479
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Experimental study on an absorption refrigeration system driven by temperature-distributed heat sources Qingyu Xu a, b, Ding Lu a, b, Gaofei Chen a, Hao Guo a, Xueqiang Dong a, Yanxing Zhao a, Jun Shen a, b, **, Maoqiong Gong a, b, * a b
Key Laboratory of Cryogenics, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China University of Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 July 2018 Received in revised form 17 November 2018 Accepted 21 December 2018
The absorption refrigeration system driven by low grade heat sources, especially the waste heat sources, becomes more and more attractive in recent decades. However, most traditional absorption systems cannot achieve a high utilization rate of the waste heat with limited heat capacity. These systems are usually designed to obtain heat in the generator, which means that the waste heat sources cannot be utilized to the temperature lower than the generator temperature. This paper proposed a new structure heated by heat conduction oil in the generator and electric heating rings around the stripping section. This structure can simulate the temperature-distributed heat sources when the electric heating rings work. It can also simulate a traditional generator when the electric heating rings do not work. Influences of different heat distributions are analyzed in detail in this paper. The results show that the heat sources utilization rate will increase with the increase of the heat in the stripping section, while the coefficient of performance will be negatively affected by the increasing heat in the stripping section. By optimizing the heating structure, the coefficient of performance can be similar to that of a traditional system when the heat is just added in the middle and lower part of stripping section. The optimum utilization rate of heat sources in this test model can reach 1.8 times to that of a traditional system. Under this heating model, the lowest temperature required in the heating section is 82 C when the heat conduction oil inlet temperature is 169 C. It is much lower than the temperature inside the generator, which is 137.3 C. © 2018 Published by Elsevier Ltd.
Keywords: Absorption refrigeration Ammonia-water Generator Temperature-distributed heat load Heat sources utilization
1. Introduction Absorption refrigeration system is thermally driven system that can meet cooling demands of various application. It is also an efficient means of utilizing middle-to low-grade temperature waste heat [1e3]. The ammonia-water absorption refrigeration system is widely used owing to its no crystallization and freezing concerns [4,5] and outstanding environmental compatibility [6,7]. However, there are still several drawbacks of the ammonia absorption refrigeration system including low utilization rate of low-grade waste heat, low coefficient of performance (COP) and large system required. Adewusi et al. [8] analyzed a basic cycle and the
* Corresponding author. Key Laboratory of Cryogenics, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China. ** Corresponding author. Key Laboratory of Cryogenics, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China. E-mail address: [email protected] (M. Gong). https://doi.org/10.1016/j.energy.2018.12.159 0360-5442/© 2018 Published by Elsevier Ltd.
result showed that a COP of 0.6 could be achieved while the evaporation temperature is in the range of 10 to 15 C in ideal simulations. To further improve the COP of the absorption system, the low-grade heat produced by the absorber and the condenser should be recycled, which can be achieved by the multi-effect cycles. Ma et al. [9] compared the power output, energy and exergy efficiencies of double-effect, half-effect and ejector-combined cycles. The double-effect cycle could effectively improve the energy and exergy efficiencies under high generator temperatures while the half-effect cycle exhibited better in power output capacity. To increase the performance of the system at a low-grade temperature waste heat, the multi-stage absorption refrigeration cycles were proposed. Domingue-Inzunza and Ventas et al. [10,11] analyzed several two-stage absorption cycles, and all the cycles performed well under low generator temperature and large temperature lift applications. The generatoreabsorber heat-exchange (GAX) cycle is another way to improve the system COP through generatoreabsorber heat exchange process which can recover the absorption heat and
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Nomenclature Cp COP GAX k Q T0 E S V TV
specific heat capacity (J/(kg∙K)) coefficient of performance generatoreabsorber heat exchange ratio of final-stage heating to entire heating heat flow rate (kW) ambient temperature ( C) exergy (kW) specific entropy (kJ/(kg∙K)) valve throttle valve
Greek symbols utilization rate
h
Subscripts C cooling capacity output ele electric heating ring H heat source
reduce the heat input. Based on the GAX cycles, many works were carried out to improve the system performance through internal heat recovery. Gomez [12] conducted an experimental and thermodynamic investigation of an indirect-fired GAX prototype cooling system. The experimental results showed that a COP of 0.53 could be achieved using heat conduction oil at 180e195 C. Du et al. [13] used pinch technology to analyze the maximum internal heat recovery of a mass-coupled two-stage cycle and the COP was improved by 14.5%. Jawahar et al. [14] analyzed both simple and GAX-based combined power and cooling cycles. The results showed that about 20% of internal heat was recovered within the GAX cycle. Sun et al. [15] proposed and investigated an integrated power and cooling cogeneration system that can use mid/low-temperature heat sources efficiently. All the studies mentioned above just focused on improving the system COP and only used heat sources in a certain temperature range, while little researches focused on decreasing the outlet temperature of the heat source. The waste heat sources of ARS are usually heat conduction oil and high temperature exhaust gas, which have finite heat capacity, and the heat source temperature declines gradually during the process of generation. Reducing the outlet temperature of the heat source can improve the waste heat utilization ratio. Introducing multi-effect systems, adding assisted compressor and improving the structure of the generator are the main methods to enhance waste heat utilization and decrease the outlet temperature of heat sources Ratlamwala et al. [16] carried out a comprehensive thermodynamic analysis of a triple-effect absorption system which had three generators working at three different temperatures. An exergetic COP of 0.964 could be achieved under the ideal condition. Yang et al. [17] studied a cycle consisting of a LiBreH2O sub-cycle and a NH3eH2O sub-cycle. It could enlarge the utilization of the waste heat while the LiBreH2O sub-cycle works at higher temperature and the NH3eH2O cycle works at lower temperature. Chen et al. [18,19] proposed a new “thermal compressor” model to describe the absorption refrigeration system. Based on this model, they put forward a new absorption-compression refrigeration system that consists of a conventional single-effect absorption sub-cycle and an absorptioncompression refrigeration sub-cycle. This system could improve the performance of the “thermal compressor”. The waste gas outlet temperature can be 35 C lower than that of a traditional system.
in out oil wh abs pump C D dis eva ex gen nw ref sep ss sys wh ws
inlet outlet heat conduction oil waste heat absorber pump cold source destruction distillation evaporator heat exchanger generator net work refrigerant separator strong solution system waste heat weak solution
Wu et al. [20] built an experimental prototype of an NH3eH2O compression-assisted absorption heat pump (CAHP). The results showed that this system gained an improvement under a lower driving temperature. However, these designs greatly increase the complexity of the system and the cost of manufacturing and maintenance [21]. Compared with the multi-effect and the compressor-assisted systems multi-effect system and adding an assisted compressor, optimizing the generator structure will not significantly increase the complexity of the system. Zavaleta-Aguilar et al. [22] carried out an experimental study on a falling-film ammonia-water distiller. The results showed that the distilled ammonia mass flow had an increase under a higher average generator temperature while the ammonia concentration was decreased. Aprile et al. [23] presented a model to simulate a gas-fired generator of an ammoniaewater absorption heat pump. They focused on the heat and mass transfer in the generator but ignored the ability of the stripping section to utilize low-grade heat. Determan et al. [24,25] developed a microchannel device and studied the heat and mass transfer in it. The results showed that microchannel technology can be successfully used both in desorption and absorption processes. Du and Wang et al. [26,27] emphasized the benefits of heat recovery in the rectification and stripping process, but further studies are needed to clarify the influence of different amount and distributions of the recovered heat. The above studies on the performance of the system mostly focused on internal heat recovery and system optimization. The generator processes were considered as having a small temperature span or even as constant-temperature processes. This leads to inferior utilization of waste heat sources and the systems cannot use low-grade temperature waste heat effectively. Thus, this paper proposed a heating structure which is extended the heating area to the stripping section of the distillation column to study the influences of different heat distributions. The heating structure is heated by heat conduction oil in the generator and electric heating rings around the stripping section. The whole system is based on a single-effect ammonia-water absorption refrigeration system. By testing and comparing the coefficient of performance and the heat source utilization rate under different working conditions, the effects of different heat distributions are studied in detail in this paper. A reasonable heating condition was given in this paper,
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which can reduce the impact on COP as much as possible while it improves the utilization rate of waste heat.
2. Experimental system description 2.1. System design and construction A simplified scheme diagram of the experimental setup with the components is given in Fig. 1. The experimental setup is heated by the heat conduction oil in the generator and electric heating rings around the stripping section as shown in Fig. 2. Each electric heating ring can be controlled independently. Compared with the heating method in which heat conducting oil flows through the stripping section which is not conducive to the adjustment of the heat distribution, the heating structure proposed in this paper can easily and effectively adjust the heat distribution. This structure is conductive to the study of the influence of different distributions of the heat sources. Three electric heating rings are evenly distributed along the column body in the longitudinal direction. The electric heating rings are enwrapped in thermal insulation material to reduce the heat leak to the low temperature environment. This heating structure can be regarded as a traditional generator when the electric heating rings do not work. The dimensions of the absorption refrigeration test cycle illustrated in Fig. 3 - (a) are (L W H) 2.5 1 3. The parameters of main components in the experimental system are listed in the
Fig. 3. Photos of the experimental system. (a - whole system, b - distillation column, c - absorber, d - evaporator, e - solution pump).
Table 1. The evaporator is a thermostat which is full of glycol. The refrigerant evaporates in the evaporator coil inside the thermostat. The cooling power is balanced by the heating power of two electric rods. When the temperature inside the thermostat is stable, the electric heating power can be regarded as the cooling power of the system. The absorber is a flooded absorber which has a strong adaptability to variable operation conditions. The solution heat exchanger is a plate heat exchanger. The condenser is a shell tube heat exchanger. An additional electric heating oil furnace is used to generate high temperature heat conduction oil to drive the generator and simulate the change of heating oil flow under the variable operation conditions. The photos of the experimental system and main components are shown in Fig. 3.
2.2. System operation method This experimental test cycle has a high degree of freedom. Most of the parameters that affect the performance of the system can be adjusted in a certain range. a) Heating condition The heating condition of the generator is controlled by the electric heating oil furnace which can provide heat conducting oil up to 300 C. The flow rate of the heat conducting oil can also be adjusted by the valve on the entrance of the generator ranging from 0 to 1 m3/h. The heat produced by electric heating rings around the stripping section which simulate the temperature-distributed heat source is adjusted by the transformer. Each electric heat ring can provide added heat up to 500 W. Fig. 1. Scheme of the chiller prototype.
b) Control of the distillation column
Table 1 Parameters of main components in the experimental system. Components
Descriptions
Remarks
Generator Distillation column
Effective volume: 0.02m3 Height: 750 mm*2 Diameter: 159 mm Heat transfer area: 2 m2
Jacketed heat exchanger Packing: CY 700
Condenser Evaporator
Solution heat exchange Absorber Fig. 2. (a) Distribution of the inlets of the feeding and oil (b) distribution of the electric heating rings (c) photo of the electric heating ring.
Pump
Height: 630 mm Diameter: 430 mm Heat transfer area: 1.2 m2 Number of plates: 24 Heat transfer area: 2 m2 Length: 990 mm Heat transfer area: 4.8 m2 Swept volume: 11 cm3 Motor: 500W
Shell-and-tube heat exchanger Coil type thermostat
Plate heat exchanger Full liquid shell tube heat exchanger Hydraulic-driven diaphragm metering pump
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Four feed inlets are distributed evenly along the column body in the longitudinal direction as shown in Fig. 2. Each inlet is equipped with an independent valve to control the switch respectively. The inlet position of the rich solution can be changed by adjusting the switches of these valves. The distillation column below the feeding stage is usually called stripping section. Therefore, the length of stripping section can be changed by the state of these valves. The reflux ratio is controlled by a time relay switch on the top of the column. The reflux ratio can be adjusted ranging from 0.01 to 100 by changing the ratio of the opening and closing time.
the definition of heat source utilization rate is:
hH ¼
Toil;in Toil;out Toil;in T0
As mentioned in the Chapter 2.1, the test cycle is heated by the heat conduction oil in the generator and electric heating ring around the stripping section to simulation the temperaturedistributed heat sources. The Toil,out need to be redefined as follows: 0
Toil;out ¼ T oil:out
c) Control of the condenser and the absorber Both the condenser and the absorber are cooled by cooling water which is supplied by an additional water chilling unit. This water chilling unit can provide cooling water and keep the fluctuation range of the water temperature less than 2 C. The cooling water flows can be adjusted independently by two valves. d) Solution pump The solution pump is a hydraulic-driven diaphragm metering pump, which has a good sealing performance and strong corrosion resistance at the same time. The flow can be adjusted stepless by regulating the stroke length ranging from 0 to 63 L/h.
The main objective of this test is to study the influence of the different heat source distributions on system performance. Change the operation conditions of the system, especially the different heating distributions. The parameters are recorded by data acquisition system and calculated through the computer. For better quantitative analysis of the results by the different
COPwh ¼
QH;ele 0 $ Toil;in T oil:out : QH;oil
Q C ¼ m$Cp $ Toil;in T0
(3)
Toil,out represents the outlet temperature of the heating oil if the 0
stripping section is also heated by the heat conduction oil. T oil:out represents the actual outlet oil temperature from the generator. c) Coefficient of waste heat utilization performance (COPwh) Considering that the absorption refrigeration system is driven by waste heat flow, it is necessary to calculate the conversion efficiency from the waste heat flow to the heat input. The COPwh is a new criterion proposed in this paper to evaluation the actual cooling coefficient for waste heat sources, which is defined as the ratio of cooling capacity to the heat obtained from the heating oil in the ideal case:
COPwh ¼ QC QH;ALL :
2.3. Test method
(2)
(4)
In the ideal case, the heat that can be obtained from the heat source is given by:
QH;ALL ¼ m$Cp $ Toil;in T0 :
(5)
thus, COPwh can be represented as:
QC : Toil;in T0 m$Cp $ Toil;in Toil;out $ Toil;in Toil;out
(6)
T Toil;out QC $ oil;in ¼ Toil;in T0 m$Cp $ Toil;in Toil;out
operation conditions, three main performance indexes were put out.
COPwh ¼ COP
a) Coefficient of performance As the power of the solution pump is far smaller than the heating power, the coefficient of performance of the system can be simplified as follow:
COP ¼ QC
QH;oil þ QH;ele
Therefore, COPwh can be expressed as:
(1)
b) Heat source utilization rate The heat source utilization rate can be equally regarded as the reduction rate of the heat conduction oil temperature. Therefore,
Toil;in Toil;out ¼ COP hH Toil;in T0
(7)
2.4. Testing facilities The key system operating temperatures and pressures are measured by PT100 type platinum resistance thermometers and pressure transmitter, respectively, which are recorded by a data acquisition system. The power of the heating oil furnace is measured by a three-phase electric power meter. The power of two electric rods in the thermostat are measured by two two-phase electric power meters. The flow rates of the heating oil and the cooling water are measured by flowmeters. Details of testing
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Table 2 Specifications of the test facilities in the experimental system. Instrument
Manufacture
Thermocouple PT100 platinum resistance thermometers Pressure transducers 3351 DP, TOP Wave Instrument Technology (Shanghai) Co., Ltd, China Thermal resistance measurement module COM4015, Beijing Rtech Automation Technology Co., Ltd, China Analog input module COM4017I, Beijing Rtech Automation Technology Co., Ltd, China Industrial personal computer MCGS, TPC7062K, Kunluntongtai Automation Technology Co. LTD, China Electric parameter meter PM9805, Dongguannapu Electronic Technology Co., Ltd, China Three-Phase electrical parameter comprehensive meter AN7931X, Ainuo Instrument Co., Ltd, China Electromagnetic flowmeter LDEe25, Shanghai Sanbin Instrument Technology Co., Ltd, China Vortex flowmeter LUVe15, Shanghai Sanbin Instrument Technology Co., Ltd, China
facilities are illustrated in Table 2.
Range
Accuracy
73e473 K 2e2.5 MPa 200 - 200 K 4e20 mA
±0.1 K ±0.2% ±0.1% ±0.1%
0.1W - 3 kW 2.0W 999.9W - 24.00 kW 0.5e20 m3/h 0.4e4 m3/h, <523 K
±0.5% ±0.5% ±0.5% ±0.1%
0.40 0.39
3. Experimental results
0.38
Q Qoil k ¼ Plast ¼ P Qj Qele þ Qoil
0.37 0.36
COP
The experimental test of this cycle is conducted in several procedures. Firstly, the performance of the system in the traditional generating condition is tested. Then the temperature-distributed heat sources are provided to the stripping section, and the performance of the system is tested under the different distribution heat sources. All the experiments are carried out at the evaporation temperature of 10 C. Considering that the traditional generator can be regarded as the last stage of the column, k is used to represent the ratio of the heat obtained from the last stage to the total heat, as given by follow:
0.35 0.34 0.33 0.32 0.31
Heating at whole section Heating at middle and lower section Heating at lower section
0.30 0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95 1.00
k
(8)
j
Fig. 4. COP - k relations with different distribution heat sources.
3.1. The influence of different heat distributions on COP The length of the stripping section is fixed while the feed inlet valve 2 is open and others are closed. The different distributions of heat sources are achieved by adjusting the flow rate of the heat conduction oil and the power of the electric heating rings. Fig. 4 shows the experimental results of the COP - k relations with different distribution heat source under a fixed length of stripping section. The figure shows that the COP decrease with a decrease of k. The COP of 0.38 is achieved while the system is working under traditional generator model. The downward trend of COP will slow down while the number of heating sections decreases. This is because with the heat which is added into stripping section increase, the temperature of the stripping section stage will also increase. The concentration of NH3 leaving the stripping Table 3 The heating models of the system.
section is the concentration at saturation state, which is negatively correlated with temperature. The cooling power in the evaporator is affected by the excessive proportion of water in the distillation product. Fig. 5 shows the temperature at the top of the stripping section. Fernandez-Seara [28] presented the same result, showing that the COP was decreased by lower separation efficiency of the stripping section. This theory can also illustrate the reason why the downward trend of COP will slow down as the number of heating sections decreases. Therefore, a reasonable structure for the system
90 85
Temperature(°C)
This means that the heat added into the whole system is Q, while that added into the generator is kQ (0 < k 1). The system can be regarded as a traditional cycle when k equals 1, which means that all the heat is added into the last stage. Because the heating of each electric heating ring can be controlled independently, the system is tested under four models, as shown in Table 3.
80 75 70 65 Heating at whole section
60
Heating model
Descriptions
Heating at whole section Heating at middle and lower section Heating at lower section Traditional generator model (k ¼ 1)
All the electric heating rings work The electric heating ring 2 and 3 work The electric heating ring 3 works No electric heating rings work
Heating at middle and lower section Heating at lower section
55 0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95 1.00
k Fig. 5. Temperature at the top of the stripping section.
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is to have one or more stages without heat added into stripping section below the feeding stage.
As the heat added into the generator decreases, the heat conduction oil flow through the generator decreases correspondingly, as shown in Fig. 6. The rest of the heat required by the system can be obtained from in the stripping section at lower temperatures. The temperature distribution in the stripping section when the stripping section is heated by electric heat rings are given in Fig. 7.
Temperature(°C)
3.2. The influence of different heat distributions on COPwh
140
(a) heating at whole section (b) heating at middle and lower section
Heat conduction oil flow (m3/h)
80
1 2 3 4 The stage number of the stripping section (a)
140 120 100
k=1 k=0.95 k=0.91 k=0.89 k=0.88 k=0.81 k=0.77
80 60 1 2 3 4 The stage number of the stripping section (b) Fig. 7. The temperature distribution of the stripping section.
rings can make the heat in the stripping section more even than the model which is just heated by the third electric heating ring. The length of the stripping section is changed while the valve 2 is closed and valve 3 is opened. Fig. 9 shows the COPwh - k relations with different length of the stripping section and different distribution heat sources. It can be found that the change trends of COPwh from different lengths of stripping section are similar. But with the length of the stripping section decrease, separation efficiency of the stripping section will also decrease which leads a lower COPwh under a shorter length of stripping section. In order to ensure that the system has large cooling capacity output and a large COPwh, the stripping section needs enough stages. In this experiment, the length of the stripping section is corresponding to the 3 theoretical stages which is 0.75 m.
0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1
100
60
Temperature(°C)
The temperature distribution showed that with the heat at stripping section increase, the stage temperature will also increase. Compared with the condition which the rest heat added at whole stripping section, heating at middle and lower section can have a lower temperature in the stripping section, even the temperature at the bottom of the tower is almost the same. This result shows that keep one stage unheated at the top of stripping section can effectively reduce the temperature at the top of the stripping section, which ensures the concentration of the rising gas. According to equations (2), (3) and (5), hH and COPwh can be calculated by the following: Qoil, Qele, Toil,in, Toil,out, T0 and COP. Fig. 8 shows the experimental results of hH and COPwh. The heat transfer area in this system is small, as the heat is transfer through the outer wall both in generator and stripping section. This leads to the low utilization (hH) of the heat sources. But the advantages of temperature-distributed heat sources can be clearly found under comparing the experimental results horizontally. Performance test results of several main operation conditions are listed in Table 4. Tlowest represents the internal temperature at the top of the entire heating structure, which means the lowest temperature that the heat conducting oil can reach in the ideal case. Considering the result from Table 4 and the implementation form of temperature-distributed heat sources, it is recommended to turn on the second and third electric heating rings. In this heating model, the system can have a higher hH and COPwh than the traditional system. At the same time, this model can produce a larger cooling capacity output than the model heated by three electric heating rings. Considering the perspective of the reasonableness of the heating structure, heated by two electric heating
120
k=1 k=0.89 k=0.80 k=0.74 k=0.70 k=0.66
Heating at whole section Heating at middle and lower section
0.0 0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95 1.00
k Fig. 6. Flow rate of the heat conducting oil.
4. Exergy analysis The researches on exergy analysis and numerical simulation are proposed in another paper which is focused on the simulation and analysis [29]. The exergy analysis based on the experimental cycle was carried out as follows. The exergy analysis of this part is based
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H
0.12
Heating at whole section Heating at middle and lower section Heating at lower section
0.10 0.08 COPwh Heating at whole section Heating at middle and lower section Heating at lower section
0.06 0.04 0.02
0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95 1.00
k Fig. 8. Influence of the different distribution heat sources on hH and COPwh.
Table 4 Performance test results of several main operation conditions. Heating condition
k
COP
hH
COPwh
Tlowesta ( C)
Heating Heating Heating Heating Heating Heating
1.00 0.66 1.00 0.78 1.00 0.86
0.39 0.32 0.38 0.37 0.39 0.38
0.06 0.13 0.06 0.11 0.06 0.08
0.023 0.042 0.023 0.040 0.022 0.029
138.7 88.6 137.3 82 133.7 80.9
a
at at at at at at
whole section whole section middle and lower section middle and lower section lower section lower section
ED;abs ¼ mref heva;out T0 Seva;out þ mws htv2;out T0 Stv2;out mss habs;out T0 Sabs;out ; (11) ED;vt ¼ mref href T0 Sref mref htv;out T0 Stv;out ;
Tlowest equals to the temperature in the generator when k ¼ 1.
on simulation through ASPEN PLUS. The key parameters of the simulation system, such as temperature of the heat flow, temperature of the last stage, working pressure, evaporation pressure and reflux ratio are adjusted to conform to the actual working conditions. The exergy destruction in each component of the proposed cycle was given as follows:
(12)
ED;pump ¼ mss habs;out T0 Sabs;out þ Wpump mss hpump;out T0 Spump;out ; (13) ED;ex ¼ mws ðhws T0 Sws Þ þ mss hpimp;out T0 Spump;out : mws hex;out T0 Sex;out mfeed hfeed T0 Sfeed (14) The separation (exergy) efficiency of distillation column was
ED;dis ¼ mfeed hfeed T0 Sfeed þ moil hoil;in T0 Soil;in moil hoil;out T0 Soil;out ; mws ðhws T0 Sws Þ mref href T0 Sref
ED;eva ¼ mref htv1;out T0 Stv1;out mref heva;out T0 Seva;out QC ð1 T0 =Teva Þ; (10)
defined as follows:
(9)
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0.045
Inlet 2 Heating at whole section
0.040
Heating at middle and lower section Heating at lower section
COPwh
0.035 0.030 0.025 Inlet 3
0.020
Heating at middle and lower section Heating at lower section
0.015 0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95 1.00
k Fig. 9. COPwh - k relations with different length of the stripping section.
0.95
hsep¼
DEsep
Wnw
mws ðhws T0 Sws Þþmref href T0 Sref mfeed hfeed T0 Sfeed P ¼ Qj 1T0 Tj QC ð1T0 =TC Þ
0.90 0.85
j
Fig. 10 shows the exergy destruction of each component. The difference between the effects of the generators has the greatest impact on distillation exergy destruction. This also explains why changing the distribution of heat will greatly affect the utilization rate of waste heat. Therefore, the exergy analysis focuses on the distillation tower. Figs. 11 and 12 show the separation (exergy) efficiency of distillation column under different heating conditions and different stage number in stripping section, respectively. As illustrated in Fig. 11, the heating condition will affect the separation efficiency of the distillation column. More heating stages will
Exergy Destruction (kW)
2.5 2.0 DIS ABS SHX VT1 PUMP EVA VT2
1.5 1.0 0.5 0.0
0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 k Fig. 10. Exergy losses of each component.
sep
(15)
0.80 0.75
Heating at whole section Heating at middle and lower section Heating at lower section
0.70 0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
k Fig. 11. Separation (exergy) efficiency of distillation column under different heating conditions.
increase the separation efficiency. But a more uniform heat distribution will improve the separation efficiency more effectively. This is reason that the distributed heat sources system can have a higher waste heat utilization ratio than a traditional system. Fig. 12 shows that the more stages in the stripping section will also lead a higher separation efficiency, especially in a lower k value. 5. Conclusion A heating structure driven by temperature-distributed heat sources is illustrated in this paper, which is tested in a single-effect absorption refrigeration system. The heating structure could be heated at four places: generator and three vertical segments on the stripping section. This system is tested under different lengths of stripping section and different heat distributions. The main results are as follows:
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References
0.95 0.90
sep
0.85 0.80 3 stage in stripping section 4 stage in stripping section 5 stage in stripping section
0.75 0.70 0.3
479
0.4
0.5
0.6
0.7
0.8
0.9
1.0
k Fig. 12. Separation (exergy) efficiency of distillation column under different stage number in stripping section.
1) With the increase of the heat added in the stripping section, the utilization of the heat sources has a large increase while the COP of the system has a slightly decreasing. Retaining an unheated stage below the feed inlet can effectively slow down the reduction of the COP. 2) In the operation model which the middle and lower part of the stripping section is heated, the COP of 0.37 is achieved when heat at the stripping section reaches 23% of the total heat. The result is 97% of the data from the traditional system while the heat sources utilization rate can reach 180% of the data from the traditional system. Under this heating model, the lowest temperature required in the heating section is 82 C when the heat conduction oil inlet temperature is 169 C. It is much lower than the temperature inside the generator, which is 137.3 C. 3) In this paper, COPwh is used to consider COP and hH comprehensively. When the system is operated under the condition which the middle and lower part of the stripping section is heated, the optimal value of COPwh can reach 175% of the that from the traditional system. The heat exchange model is a simple external wall heat conduction in both generator and stripping section, which leads to a poor heat transfer effect and a low hH. The improvement of followup experimental system will use internal coil or winding pipe structure to enhance heat exchange and further improve the hH. Nevertheless, the novel design of the absorption refrigeration system is proved to be valid and its concept can be extended to other applications.
Acknowledgement This work was supported by the National Natural Science Foundation of China (Fund No. 51625603 and 61733017) and the International Partnership Program of Chinese Academy of Sciences (Grant No. GJHZ1876).
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Experimental study on an absorption refrigeration system driven by temperature-distributed heat sources Qingyu Xu a, b, Ding Lu a, b, Gaofei Chen a, Hao Guo a, Xueqiang Dong a, Yanxing Zhao a, Jun Shen a, b, **, Maoqiong Gong a, b, * a b
Key Laboratory of Cryogenics, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China University of Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 July 2018 Received in revised form 17 November 2018 Accepted 21 December 2018
The absorption refrigeration system driven by low grade heat sources, especially the waste heat sources, becomes more and more attractive in recent decades. However, most traditional absorption systems cannot achieve a high utilization rate of the waste heat with limited heat capacity. These systems are usually designed to obtain heat in the generator, which means that the waste heat sources cannot be utilized to the temperature lower than the generator temperature. This paper proposed a new structure heated by heat conduction oil in the generator and electric heating rings around the stripping section. This structure can simulate the temperature-distributed heat sources when the electric heating rings work. It can also simulate a traditional generator when the electric heating rings do not work. Influences of different heat distributions are analyzed in detail in this paper. The results show that the heat sources utilization rate will increase with the increase of the heat in the stripping section, while the coefficient of performance will be negatively affected by the increasing heat in the stripping section. By optimizing the heating structure, the coefficient of performance can be similar to that of a traditional system when the heat is just added in the middle and lower part of stripping section. The optimum utilization rate of heat sources in this test model can reach 1.8 times to that of a traditional system. Under this heating model, the lowest temperature required in the heating section is 82 C when the heat conduction oil inlet temperature is 169 C. It is much lower than the temperature inside the generator, which is 137.3 C. © 2018 Published by Elsevier Ltd.
Keywords: Absorption refrigeration Ammonia-water Generator Temperature-distributed heat load Heat sources utilization
1. Introduction Absorption refrigeration system is thermally driven system that can meet cooling demands of various application. It is also an efficient means of utilizing middle-to low-grade temperature waste heat [1e3]. The ammonia-water absorption refrigeration system is widely used owing to its no crystallization and freezing concerns [4,5] and outstanding environmental compatibility [6,7]. However, there are still several drawbacks of the ammonia absorption refrigeration system including low utilization rate of low-grade waste heat, low coefficient of performance (COP) and large system required. Adewusi et al. [8] analyzed a basic cycle and the
* Corresponding author. Key Laboratory of Cryogenics, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China. ** Corresponding author. Key Laboratory of Cryogenics, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China. E-mail address: [email protected] (M. Gong). https://doi.org/10.1016/j.energy.2018.12.159 0360-5442/© 2018 Published by Elsevier Ltd.
result showed that a COP of 0.6 could be achieved while the evaporation temperature is in the range of 10 to 15 C in ideal simulations. To further improve the COP of the absorption system, the low-grade heat produced by the absorber and the condenser should be recycled, which can be achieved by the multi-effect cycles. Ma et al. [9] compared the power output, energy and exergy efficiencies of double-effect, half-effect and ejector-combined cycles. The double-effect cycle could effectively improve the energy and exergy efficiencies under high generator temperatures while the half-effect cycle exhibited better in power output capacity. To increase the performance of the system at a low-grade temperature waste heat, the multi-stage absorption refrigeration cycles were proposed. Domingue-Inzunza and Ventas et al. [10,11] analyzed several two-stage absorption cycles, and all the cycles performed well under low generator temperature and large temperature lift applications. The generatoreabsorber heat-exchange (GAX) cycle is another way to improve the system COP through generatoreabsorber heat exchange process which can recover the absorption heat and
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Nomenclature Cp COP GAX k Q T0 E S V TV
specific heat capacity (J/(kg∙K)) coefficient of performance generatoreabsorber heat exchange ratio of final-stage heating to entire heating heat flow rate (kW) ambient temperature ( C) exergy (kW) specific entropy (kJ/(kg∙K)) valve throttle valve
Greek symbols utilization rate
h
Subscripts C cooling capacity output ele electric heating ring H heat source
reduce the heat input. Based on the GAX cycles, many works were carried out to improve the system performance through internal heat recovery. Gomez [12] conducted an experimental and thermodynamic investigation of an indirect-fired GAX prototype cooling system. The experimental results showed that a COP of 0.53 could be achieved using heat conduction oil at 180e195 C. Du et al. [13] used pinch technology to analyze the maximum internal heat recovery of a mass-coupled two-stage cycle and the COP was improved by 14.5%. Jawahar et al. [14] analyzed both simple and GAX-based combined power and cooling cycles. The results showed that about 20% of internal heat was recovered within the GAX cycle. Sun et al. [15] proposed and investigated an integrated power and cooling cogeneration system that can use mid/low-temperature heat sources efficiently. All the studies mentioned above just focused on improving the system COP and only used heat sources in a certain temperature range, while little researches focused on decreasing the outlet temperature of the heat source. The waste heat sources of ARS are usually heat conduction oil and high temperature exhaust gas, which have finite heat capacity, and the heat source temperature declines gradually during the process of generation. Reducing the outlet temperature of the heat source can improve the waste heat utilization ratio. Introducing multi-effect systems, adding assisted compressor and improving the structure of the generator are the main methods to enhance waste heat utilization and decrease the outlet temperature of heat sources Ratlamwala et al. [16] carried out a comprehensive thermodynamic analysis of a triple-effect absorption system which had three generators working at three different temperatures. An exergetic COP of 0.964 could be achieved under the ideal condition. Yang et al. [17] studied a cycle consisting of a LiBreH2O sub-cycle and a NH3eH2O sub-cycle. It could enlarge the utilization of the waste heat while the LiBreH2O sub-cycle works at higher temperature and the NH3eH2O cycle works at lower temperature. Chen et al. [18,19] proposed a new “thermal compressor” model to describe the absorption refrigeration system. Based on this model, they put forward a new absorption-compression refrigeration system that consists of a conventional single-effect absorption sub-cycle and an absorptioncompression refrigeration sub-cycle. This system could improve the performance of the “thermal compressor”. The waste gas outlet temperature can be 35 C lower than that of a traditional system.
in out oil wh abs pump C D dis eva ex gen nw ref sep ss sys wh ws
inlet outlet heat conduction oil waste heat absorber pump cold source destruction distillation evaporator heat exchanger generator net work refrigerant separator strong solution system waste heat weak solution
Wu et al. [20] built an experimental prototype of an NH3eH2O compression-assisted absorption heat pump (CAHP). The results showed that this system gained an improvement under a lower driving temperature. However, these designs greatly increase the complexity of the system and the cost of manufacturing and maintenance [21]. Compared with the multi-effect and the compressor-assisted systems multi-effect system and adding an assisted compressor, optimizing the generator structure will not significantly increase the complexity of the system. Zavaleta-Aguilar et al. [22] carried out an experimental study on a falling-film ammonia-water distiller. The results showed that the distilled ammonia mass flow had an increase under a higher average generator temperature while the ammonia concentration was decreased. Aprile et al. [23] presented a model to simulate a gas-fired generator of an ammoniaewater absorption heat pump. They focused on the heat and mass transfer in the generator but ignored the ability of the stripping section to utilize low-grade heat. Determan et al. [24,25] developed a microchannel device and studied the heat and mass transfer in it. The results showed that microchannel technology can be successfully used both in desorption and absorption processes. Du and Wang et al. [26,27] emphasized the benefits of heat recovery in the rectification and stripping process, but further studies are needed to clarify the influence of different amount and distributions of the recovered heat. The above studies on the performance of the system mostly focused on internal heat recovery and system optimization. The generator processes were considered as having a small temperature span or even as constant-temperature processes. This leads to inferior utilization of waste heat sources and the systems cannot use low-grade temperature waste heat effectively. Thus, this paper proposed a heating structure which is extended the heating area to the stripping section of the distillation column to study the influences of different heat distributions. The heating structure is heated by heat conduction oil in the generator and electric heating rings around the stripping section. The whole system is based on a single-effect ammonia-water absorption refrigeration system. By testing and comparing the coefficient of performance and the heat source utilization rate under different working conditions, the effects of different heat distributions are studied in detail in this paper. A reasonable heating condition was given in this paper,
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which can reduce the impact on COP as much as possible while it improves the utilization rate of waste heat.
2. Experimental system description 2.1. System design and construction A simplified scheme diagram of the experimental setup with the components is given in Fig. 1. The experimental setup is heated by the heat conduction oil in the generator and electric heating rings around the stripping section as shown in Fig. 2. Each electric heating ring can be controlled independently. Compared with the heating method in which heat conducting oil flows through the stripping section which is not conducive to the adjustment of the heat distribution, the heating structure proposed in this paper can easily and effectively adjust the heat distribution. This structure is conductive to the study of the influence of different distributions of the heat sources. Three electric heating rings are evenly distributed along the column body in the longitudinal direction. The electric heating rings are enwrapped in thermal insulation material to reduce the heat leak to the low temperature environment. This heating structure can be regarded as a traditional generator when the electric heating rings do not work. The dimensions of the absorption refrigeration test cycle illustrated in Fig. 3 - (a) are (L W H) 2.5 1 3. The parameters of main components in the experimental system are listed in the
Fig. 3. Photos of the experimental system. (a - whole system, b - distillation column, c - absorber, d - evaporator, e - solution pump).
Table 1. The evaporator is a thermostat which is full of glycol. The refrigerant evaporates in the evaporator coil inside the thermostat. The cooling power is balanced by the heating power of two electric rods. When the temperature inside the thermostat is stable, the electric heating power can be regarded as the cooling power of the system. The absorber is a flooded absorber which has a strong adaptability to variable operation conditions. The solution heat exchanger is a plate heat exchanger. The condenser is a shell tube heat exchanger. An additional electric heating oil furnace is used to generate high temperature heat conduction oil to drive the generator and simulate the change of heating oil flow under the variable operation conditions. The photos of the experimental system and main components are shown in Fig. 3.
2.2. System operation method This experimental test cycle has a high degree of freedom. Most of the parameters that affect the performance of the system can be adjusted in a certain range. a) Heating condition The heating condition of the generator is controlled by the electric heating oil furnace which can provide heat conducting oil up to 300 C. The flow rate of the heat conducting oil can also be adjusted by the valve on the entrance of the generator ranging from 0 to 1 m3/h. The heat produced by electric heating rings around the stripping section which simulate the temperature-distributed heat source is adjusted by the transformer. Each electric heat ring can provide added heat up to 500 W. Fig. 1. Scheme of the chiller prototype.
b) Control of the distillation column
Table 1 Parameters of main components in the experimental system. Components
Descriptions
Remarks
Generator Distillation column
Effective volume: 0.02m3 Height: 750 mm*2 Diameter: 159 mm Heat transfer area: 2 m2
Jacketed heat exchanger Packing: CY 700
Condenser Evaporator
Solution heat exchange Absorber Fig. 2. (a) Distribution of the inlets of the feeding and oil (b) distribution of the electric heating rings (c) photo of the electric heating ring.
Pump
Height: 630 mm Diameter: 430 mm Heat transfer area: 1.2 m2 Number of plates: 24 Heat transfer area: 2 m2 Length: 990 mm Heat transfer area: 4.8 m2 Swept volume: 11 cm3 Motor: 500W
Shell-and-tube heat exchanger Coil type thermostat
Plate heat exchanger Full liquid shell tube heat exchanger Hydraulic-driven diaphragm metering pump
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Four feed inlets are distributed evenly along the column body in the longitudinal direction as shown in Fig. 2. Each inlet is equipped with an independent valve to control the switch respectively. The inlet position of the rich solution can be changed by adjusting the switches of these valves. The distillation column below the feeding stage is usually called stripping section. Therefore, the length of stripping section can be changed by the state of these valves. The reflux ratio is controlled by a time relay switch on the top of the column. The reflux ratio can be adjusted ranging from 0.01 to 100 by changing the ratio of the opening and closing time.
the definition of heat source utilization rate is:
hH ¼
Toil;in Toil;out Toil;in T0
As mentioned in the Chapter 2.1, the test cycle is heated by the heat conduction oil in the generator and electric heating ring around the stripping section to simulation the temperaturedistributed heat sources. The Toil,out need to be redefined as follows: 0
Toil;out ¼ T oil:out
c) Control of the condenser and the absorber Both the condenser and the absorber are cooled by cooling water which is supplied by an additional water chilling unit. This water chilling unit can provide cooling water and keep the fluctuation range of the water temperature less than 2 C. The cooling water flows can be adjusted independently by two valves. d) Solution pump The solution pump is a hydraulic-driven diaphragm metering pump, which has a good sealing performance and strong corrosion resistance at the same time. The flow can be adjusted stepless by regulating the stroke length ranging from 0 to 63 L/h.
The main objective of this test is to study the influence of the different heat source distributions on system performance. Change the operation conditions of the system, especially the different heating distributions. The parameters are recorded by data acquisition system and calculated through the computer. For better quantitative analysis of the results by the different
COPwh ¼
QH;ele 0 $ Toil;in T oil:out : QH;oil
Q C ¼ m$Cp $ Toil;in T0
(3)
Toil,out represents the outlet temperature of the heating oil if the 0
stripping section is also heated by the heat conduction oil. T oil:out represents the actual outlet oil temperature from the generator. c) Coefficient of waste heat utilization performance (COPwh) Considering that the absorption refrigeration system is driven by waste heat flow, it is necessary to calculate the conversion efficiency from the waste heat flow to the heat input. The COPwh is a new criterion proposed in this paper to evaluation the actual cooling coefficient for waste heat sources, which is defined as the ratio of cooling capacity to the heat obtained from the heating oil in the ideal case:
COPwh ¼ QC QH;ALL :
2.3. Test method
(2)
(4)
In the ideal case, the heat that can be obtained from the heat source is given by:
QH;ALL ¼ m$Cp $ Toil;in T0 :
(5)
thus, COPwh can be represented as:
QC : Toil;in T0 m$Cp $ Toil;in Toil;out $ Toil;in Toil;out
(6)
T Toil;out QC $ oil;in ¼ Toil;in T0 m$Cp $ Toil;in Toil;out
operation conditions, three main performance indexes were put out.
COPwh ¼ COP
a) Coefficient of performance As the power of the solution pump is far smaller than the heating power, the coefficient of performance of the system can be simplified as follow:
COP ¼ QC
QH;oil þ QH;ele
Therefore, COPwh can be expressed as:
(1)
b) Heat source utilization rate The heat source utilization rate can be equally regarded as the reduction rate of the heat conduction oil temperature. Therefore,
Toil;in Toil;out ¼ COP hH Toil;in T0
(7)
2.4. Testing facilities The key system operating temperatures and pressures are measured by PT100 type platinum resistance thermometers and pressure transmitter, respectively, which are recorded by a data acquisition system. The power of the heating oil furnace is measured by a three-phase electric power meter. The power of two electric rods in the thermostat are measured by two two-phase electric power meters. The flow rates of the heating oil and the cooling water are measured by flowmeters. Details of testing
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Table 2 Specifications of the test facilities in the experimental system. Instrument
Manufacture
Thermocouple PT100 platinum resistance thermometers Pressure transducers 3351 DP, TOP Wave Instrument Technology (Shanghai) Co., Ltd, China Thermal resistance measurement module COM4015, Beijing Rtech Automation Technology Co., Ltd, China Analog input module COM4017I, Beijing Rtech Automation Technology Co., Ltd, China Industrial personal computer MCGS, TPC7062K, Kunluntongtai Automation Technology Co. LTD, China Electric parameter meter PM9805, Dongguannapu Electronic Technology Co., Ltd, China Three-Phase electrical parameter comprehensive meter AN7931X, Ainuo Instrument Co., Ltd, China Electromagnetic flowmeter LDEe25, Shanghai Sanbin Instrument Technology Co., Ltd, China Vortex flowmeter LUVe15, Shanghai Sanbin Instrument Technology Co., Ltd, China
facilities are illustrated in Table 2.
Range
Accuracy
73e473 K 2e2.5 MPa 200 - 200 K 4e20 mA
±0.1 K ±0.2% ±0.1% ±0.1%
0.1W - 3 kW 2.0W 999.9W - 24.00 kW 0.5e20 m3/h 0.4e4 m3/h, <523 K
±0.5% ±0.5% ±0.5% ±0.1%
0.40 0.39
3. Experimental results
0.38
Q Qoil k ¼ Plast ¼ P Qj Qele þ Qoil
0.37 0.36
COP
The experimental test of this cycle is conducted in several procedures. Firstly, the performance of the system in the traditional generating condition is tested. Then the temperature-distributed heat sources are provided to the stripping section, and the performance of the system is tested under the different distribution heat sources. All the experiments are carried out at the evaporation temperature of 10 C. Considering that the traditional generator can be regarded as the last stage of the column, k is used to represent the ratio of the heat obtained from the last stage to the total heat, as given by follow:
0.35 0.34 0.33 0.32 0.31
Heating at whole section Heating at middle and lower section Heating at lower section
0.30 0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95 1.00
k
(8)
j
Fig. 4. COP - k relations with different distribution heat sources.
3.1. The influence of different heat distributions on COP The length of the stripping section is fixed while the feed inlet valve 2 is open and others are closed. The different distributions of heat sources are achieved by adjusting the flow rate of the heat conduction oil and the power of the electric heating rings. Fig. 4 shows the experimental results of the COP - k relations with different distribution heat source under a fixed length of stripping section. The figure shows that the COP decrease with a decrease of k. The COP of 0.38 is achieved while the system is working under traditional generator model. The downward trend of COP will slow down while the number of heating sections decreases. This is because with the heat which is added into stripping section increase, the temperature of the stripping section stage will also increase. The concentration of NH3 leaving the stripping Table 3 The heating models of the system.
section is the concentration at saturation state, which is negatively correlated with temperature. The cooling power in the evaporator is affected by the excessive proportion of water in the distillation product. Fig. 5 shows the temperature at the top of the stripping section. Fernandez-Seara [28] presented the same result, showing that the COP was decreased by lower separation efficiency of the stripping section. This theory can also illustrate the reason why the downward trend of COP will slow down as the number of heating sections decreases. Therefore, a reasonable structure for the system
90 85
Temperature(°C)
This means that the heat added into the whole system is Q, while that added into the generator is kQ (0 < k 1). The system can be regarded as a traditional cycle when k equals 1, which means that all the heat is added into the last stage. Because the heating of each electric heating ring can be controlled independently, the system is tested under four models, as shown in Table 3.
80 75 70 65 Heating at whole section
60
Heating model
Descriptions
Heating at whole section Heating at middle and lower section Heating at lower section Traditional generator model (k ¼ 1)
All the electric heating rings work The electric heating ring 2 and 3 work The electric heating ring 3 works No electric heating rings work
Heating at middle and lower section Heating at lower section
55 0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95 1.00
k Fig. 5. Temperature at the top of the stripping section.
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is to have one or more stages without heat added into stripping section below the feeding stage.
As the heat added into the generator decreases, the heat conduction oil flow through the generator decreases correspondingly, as shown in Fig. 6. The rest of the heat required by the system can be obtained from in the stripping section at lower temperatures. The temperature distribution in the stripping section when the stripping section is heated by electric heat rings are given in Fig. 7.
Temperature(°C)
3.2. The influence of different heat distributions on COPwh
140
(a) heating at whole section (b) heating at middle and lower section
Heat conduction oil flow (m3/h)
80
1 2 3 4 The stage number of the stripping section (a)
140 120 100
k=1 k=0.95 k=0.91 k=0.89 k=0.88 k=0.81 k=0.77
80 60 1 2 3 4 The stage number of the stripping section (b) Fig. 7. The temperature distribution of the stripping section.
rings can make the heat in the stripping section more even than the model which is just heated by the third electric heating ring. The length of the stripping section is changed while the valve 2 is closed and valve 3 is opened. Fig. 9 shows the COPwh - k relations with different length of the stripping section and different distribution heat sources. It can be found that the change trends of COPwh from different lengths of stripping section are similar. But with the length of the stripping section decrease, separation efficiency of the stripping section will also decrease which leads a lower COPwh under a shorter length of stripping section. In order to ensure that the system has large cooling capacity output and a large COPwh, the stripping section needs enough stages. In this experiment, the length of the stripping section is corresponding to the 3 theoretical stages which is 0.75 m.
0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1
100
60
Temperature(°C)
The temperature distribution showed that with the heat at stripping section increase, the stage temperature will also increase. Compared with the condition which the rest heat added at whole stripping section, heating at middle and lower section can have a lower temperature in the stripping section, even the temperature at the bottom of the tower is almost the same. This result shows that keep one stage unheated at the top of stripping section can effectively reduce the temperature at the top of the stripping section, which ensures the concentration of the rising gas. According to equations (2), (3) and (5), hH and COPwh can be calculated by the following: Qoil, Qele, Toil,in, Toil,out, T0 and COP. Fig. 8 shows the experimental results of hH and COPwh. The heat transfer area in this system is small, as the heat is transfer through the outer wall both in generator and stripping section. This leads to the low utilization (hH) of the heat sources. But the advantages of temperature-distributed heat sources can be clearly found under comparing the experimental results horizontally. Performance test results of several main operation conditions are listed in Table 4. Tlowest represents the internal temperature at the top of the entire heating structure, which means the lowest temperature that the heat conducting oil can reach in the ideal case. Considering the result from Table 4 and the implementation form of temperature-distributed heat sources, it is recommended to turn on the second and third electric heating rings. In this heating model, the system can have a higher hH and COPwh than the traditional system. At the same time, this model can produce a larger cooling capacity output than the model heated by three electric heating rings. Considering the perspective of the reasonableness of the heating structure, heated by two electric heating
120
k=1 k=0.89 k=0.80 k=0.74 k=0.70 k=0.66
Heating at whole section Heating at middle and lower section
0.0 0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95 1.00
k Fig. 6. Flow rate of the heat conducting oil.
4. Exergy analysis The researches on exergy analysis and numerical simulation are proposed in another paper which is focused on the simulation and analysis [29]. The exergy analysis based on the experimental cycle was carried out as follows. The exergy analysis of this part is based
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477
H
0.12
Heating at whole section Heating at middle and lower section Heating at lower section
0.10 0.08 COPwh Heating at whole section Heating at middle and lower section Heating at lower section
0.06 0.04 0.02
0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95 1.00
k Fig. 8. Influence of the different distribution heat sources on hH and COPwh.
Table 4 Performance test results of several main operation conditions. Heating condition
k
COP
hH
COPwh
Tlowesta ( C)
Heating Heating Heating Heating Heating Heating
1.00 0.66 1.00 0.78 1.00 0.86
0.39 0.32 0.38 0.37 0.39 0.38
0.06 0.13 0.06 0.11 0.06 0.08
0.023 0.042 0.023 0.040 0.022 0.029
138.7 88.6 137.3 82 133.7 80.9
a
at at at at at at
whole section whole section middle and lower section middle and lower section lower section lower section
ED;abs ¼ mref heva;out T0 Seva;out þ mws htv2;out T0 Stv2;out mss habs;out T0 Sabs;out ; (11) ED;vt ¼ mref href T0 Sref mref htv;out T0 Stv;out ;
Tlowest equals to the temperature in the generator when k ¼ 1.
on simulation through ASPEN PLUS. The key parameters of the simulation system, such as temperature of the heat flow, temperature of the last stage, working pressure, evaporation pressure and reflux ratio are adjusted to conform to the actual working conditions. The exergy destruction in each component of the proposed cycle was given as follows:
(12)
ED;pump ¼ mss habs;out T0 Sabs;out þ Wpump mss hpump;out T0 Spump;out ; (13) ED;ex ¼ mws ðhws T0 Sws Þ þ mss hpimp;out T0 Spump;out : mws hex;out T0 Sex;out mfeed hfeed T0 Sfeed (14) The separation (exergy) efficiency of distillation column was
ED;dis ¼ mfeed hfeed T0 Sfeed þ moil hoil;in T0 Soil;in moil hoil;out T0 Soil;out ; mws ðhws T0 Sws Þ mref href T0 Sref
ED;eva ¼ mref htv1;out T0 Stv1;out mref heva;out T0 Seva;out QC ð1 T0 =Teva Þ; (10)
defined as follows:
(9)
478
Q. Xu et al. / Energy 170 (2019) 471e479
0.045
Inlet 2 Heating at whole section
0.040
Heating at middle and lower section Heating at lower section
COPwh
0.035 0.030 0.025 Inlet 3
0.020
Heating at middle and lower section Heating at lower section
0.015 0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95 1.00
k Fig. 9. COPwh - k relations with different length of the stripping section.
0.95
hsep¼
DEsep
Wnw
mws ðhws T0 Sws Þþmref href T0 Sref mfeed hfeed T0 Sfeed P ¼ Qj 1T0 Tj QC ð1T0 =TC Þ
0.90 0.85
j
Fig. 10 shows the exergy destruction of each component. The difference between the effects of the generators has the greatest impact on distillation exergy destruction. This also explains why changing the distribution of heat will greatly affect the utilization rate of waste heat. Therefore, the exergy analysis focuses on the distillation tower. Figs. 11 and 12 show the separation (exergy) efficiency of distillation column under different heating conditions and different stage number in stripping section, respectively. As illustrated in Fig. 11, the heating condition will affect the separation efficiency of the distillation column. More heating stages will
Exergy Destruction (kW)
2.5 2.0 DIS ABS SHX VT1 PUMP EVA VT2
1.5 1.0 0.5 0.0
0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 k Fig. 10. Exergy losses of each component.
sep
(15)
0.80 0.75
Heating at whole section Heating at middle and lower section Heating at lower section
0.70 0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
k Fig. 11. Separation (exergy) efficiency of distillation column under different heating conditions.
increase the separation efficiency. But a more uniform heat distribution will improve the separation efficiency more effectively. This is reason that the distributed heat sources system can have a higher waste heat utilization ratio than a traditional system. Fig. 12 shows that the more stages in the stripping section will also lead a higher separation efficiency, especially in a lower k value. 5. Conclusion A heating structure driven by temperature-distributed heat sources is illustrated in this paper, which is tested in a single-effect absorption refrigeration system. The heating structure could be heated at four places: generator and three vertical segments on the stripping section. This system is tested under different lengths of stripping section and different heat distributions. The main results are as follows:
Q. Xu et al. / Energy 170 (2019) 471e479
References
0.95 0.90
sep
0.85 0.80 3 stage in stripping section 4 stage in stripping section 5 stage in stripping section
0.75 0.70 0.3
479
0.4
0.5
0.6
0.7
0.8
0.9
1.0
k Fig. 12. Separation (exergy) efficiency of distillation column under different stage number in stripping section.
1) With the increase of the heat added in the stripping section, the utilization of the heat sources has a large increase while the COP of the system has a slightly decreasing. Retaining an unheated stage below the feed inlet can effectively slow down the reduction of the COP. 2) In the operation model which the middle and lower part of the stripping section is heated, the COP of 0.37 is achieved when heat at the stripping section reaches 23% of the total heat. The result is 97% of the data from the traditional system while the heat sources utilization rate can reach 180% of the data from the traditional system. Under this heating model, the lowest temperature required in the heating section is 82 C when the heat conduction oil inlet temperature is 169 C. It is much lower than the temperature inside the generator, which is 137.3 C. 3) In this paper, COPwh is used to consider COP and hH comprehensively. When the system is operated under the condition which the middle and lower part of the stripping section is heated, the optimal value of COPwh can reach 175% of the that from the traditional system. The heat exchange model is a simple external wall heat conduction in both generator and stripping section, which leads to a poor heat transfer effect and a low hH. The improvement of followup experimental system will use internal coil or winding pipe structure to enhance heat exchange and further improve the hH. Nevertheless, the novel design of the absorption refrigeration system is proved to be valid and its concept can be extended to other applications.
Acknowledgement This work was supported by the National Natural Science Foundation of China (Fund No. 51625603 and 61733017) and the International Partnership Program of Chinese Academy of Sciences (Grant No. GJHZ1876).
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