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LiBr
Applied Thermal Engineering 132 (2018) 432–440
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Research Paper
Experimental assessment of double-absorption heat transformer operating with H2O/LiBr W. Rivera a,⇑, A. Huicochea b, R.J. Romero b, A. Lozano a a b
Instituto de Energías Renovables, Universidad Nacional Autónoma de México, Temixco, Morelos, Mexico Centro de Ingeniería y Ciencias Aplicadas, Universidad Autónoma del Estado de Morelos, Av. Universidad 1001, C.P. 62209 Cuernavaca, Morelos, Mexico
h i g h l i g h t s A double-absorption heat transformer was developed operating with H2O/LiBr. Gross temperature lifts as high as 74 °C were achieved with the system. The gross temperature lifts are the highest reported in the literature using H2O/LiBr. Up to 37% of the energy supplied to the DAHT could be retrieved at higher temperature levels.
a r t i c l e
i n f o
Article history: Received 28 July 2017 Revised 28 December 2017 Accepted 29 December 2017 Available online 30 December 2017 Keywords: Heat transformers Double absorption Water/lithium bromide
a b s t r a c t This paper reports on the experimental results of a double-absorption heat transformer operating with a H2O/LiBr mixture. The generator and evaporator are pool boiling type heat exchangers, while the remaining components are coils inside shells. Plots of gross temperature lifts, economizer efficiency, and internal and external performance coefficients are reported as functions of diverse operating parameters of the heat transformer. The results indicate that the system can achieve gross temperature lifts (GTLs) of between 48 °C and 74 °C, with internal performance coefficients varying from 0.12 to 0.37. The GTLs achieved are up to 30 °C higher than those reported in the literature, using a single-stage heat transformer operating with the same working mixture. Furthermore, the system exhibits effective stability and repeatability. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction At present, numerous industries worldwide deliver waste heat to the atmosphere at temperatures below 100 °C. Absorption heat transformers (AHTs) constitute one of the most attractive thermal systems for thermal energy savings in the industry, consuming negligible amounts of primary energy. These systems can upgrade the waste heat temperature to a higher level, in order to be reused during the industrial process. Typically, with single-stage systems, up to half of the heat supply could be increased in temperature, while the remainder is discharged to the atmosphere at lower temperatures. In addition to single-stage heat transformers, advancedabsorption heat transformers are available, which use more components than single-stage systems in order to achieve greater gross temperature lifts or higher performance coefficients.
⇑ Corresponding author. E-mail address: [email protected] (W. Rivera). https://doi.org/10.1016/j.applthermaleng.2017.12.117 1359-4311/Ó 2017 Elsevier Ltd. All rights reserved.
Rivera et al. [1] carried out a bibliographic review related to the development of absorption heat transformers. The review indicated that there have been a considerable number of theoretical studies on advanced-absorption heat transformers, such as double-absorption and two-stage systems operating with diverse working mixtures [2–13]. Regarding experimental studies, there have been certain investigations into developed systems operating with a H2O/LiBr mixture [14–20], as well as with alternative mixtures [21–40]. Moreover, there have been studies related to heat transformers for different applications, such as heat recovery in the industrial process [41–45] or distillation process [46–53], but single-stage absorption heat transformers were utilized in all cases. Furthermore, studies have been carried out on advanced absorption systems operating with a H2O/LiBr mixture, by Chen et al. [54] and Xu and Wang [55], but these were related to the development of advanced absorption cooling systems and not the development of absorption heat transformers, which differ significantly not only in their applications but also their configurations.
W. Rivera et al. / Applied Thermal Engineering 132 (2018) 432–440
Regarding experimental studies related to advanced AHTs, Scott et al. [56] developed a novel multi-compartment absorption heat transformer for different steam temperatures. Both the absorber and generator were partitioned in a number of compartments, depending on the number of high-quality steam demands and waste vapor sources, respectively. The results indicated that the flow rate of the steam fed to the absorber is the most important process variable affecting the U-value and consequently dominating the design process of the absorber heat transfer area. Sekhar and Muthukumar [57] reported the results of a developed prototype of a two-stage heat transformer for upgrading the waste heat available from 393 to 413 K to approximately 463 K. The coefficient of performance (COP) was 0.2 and the maximum gross temperature lift achieved was approximately 80.5 K. Silva-Sotelo et al. [58] reported the results of a two-stage heat transformer controlled by a flow ratio operating with a water/Carrol mixture. The waste heat energy was added to the system at 70 °C, resulting in a temperature of 128 °C in the second absorber. Romero et al. [59] reported the results of a two-stage heat transformer operating with a H2O/Carrol mixture by means of object-oriented programming, and the authors reported that the most effective heat recovery was 47% at 100 °C. Isselhorst and Groll [60] developed a twostage metal hydride prototype heat transformer. The developed system demonstrated the feasibility of upgrading heat of approximately 130–140 °C up to 200 °C with a COP of around 0.27. Willers and Groll [61] reported the results of a prototype two-stage metal hydride heat transformer. The results demonstrated that the system provided 6 kW of useful heat at a temperature of approximately 190–200 °C, with a driving heat of 130–135 °C, achieving a COP close to 0.1. A two-stage absorption heat transformer was designed and constructed by Currie and Pritchard [62] in order to investigate the potential for dehumidifying and reheating a simulated dryer exhaust stream, to make it suitable for recycling to the dryer inlet. The heat transformer performance data indicated that an airstream could be reheated to 160 °C using a LiBr solution of 68% w/w with a circulation ratio (LiBr to steam flow) of 14.8. Temperature lifts of between 50 and 70 °C were possible in the reheat column when using a low circulation ratio and high LiBr concentration. The results indicated that a humid air stream could be dehumidified to a level suitable for recycling by direct contact with a concentrated LiBr stream. It can be seen from the literature review that although there have been numerous studies related to absorption heat transformers in the past decades, none of these has included the experimental study of double-absorption heat transformers (DAHTs). This fact is significant, as it will be seen in this study that DAHTs use only one additional component to single-stage systems, but can achieve considerably higher gross temperature lifts. In fact, the achieved gross temperature lifts are as high as those obtained with twostage heat transformers, but these use a greater number of components, with double the number of components compared to singlestage systems.
2. Description of single and double-absorption heat transformers 2.1. Single-stage heat transformer A single-stage heat transformer operating with a H2O/LiBr mixture basically consists of an evaporator, condenser, generator, absorber, and economizer, as shown in Fig. 1. Waste heat is supplied at an intermediate temperature to the generator in order to vaporize the water from the weak solution (low salt concentration). The vaporized water enters the condenser, where it is condensed, delivering a certain amount of heat at a low ambient
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temperature. The water leaving the condenser is pumped to the evaporator, where it is evaporated by a quantity of waste heat supplied at intermediate temperature. Following this, the water vapor enters the absorber, where it is absorbed by the solution with a high absorbent concentration from the generator, delivering useful heat at the highest system temperature. Finally, the solution with a low absorbent concentration returns to the generator, preheating the solution into the economizer and thereby completing the cycle. 2.2. Double-absorption heat transformer Double-absorption heat transformers (DAHTs) basically consist of a generator, condenser, evaporator, absorber, absorberevaporator, and economizer, as illustrated in Fig. 2. A heat source is supplied in order to separate the working fluid in the generator at an intermediate temperature TG. The vaporized working fluid is condensed in the condenser at a lower pressure PC and temperature TC. Thereafter, the condensed working fluid is split into two streams: one is pumped to the evaporator, where it is vaporized at an intermediate temperature TE and pressure PE, and the other is pumped at a higher pressure PA and vaporized in the absorberevaporator by a certain amount of available heat QAE. The vaporized working fluid is absorbed into the absorber by the solution from the generator, producing useful heat at the highest system temperature TA. The diluted solution at an intermediate concentration XA enters the absorber-evaporator, absorbing the vaporized working fluid from the evaporator and delivering an amount of heat QAE, which is used to vaporize the working fluid in the absorber-evaporator. Then, the diluted solution at a low concentration XAE, leaving the absorber-evaporator, moves to the generator preheating the solution from the generator to the absorber into the economizer, starting the cycle again. Thus, by using only one additional component compared to single-stage heat transformers, it is possible to obtain two temperature lifts: the first in the absorber-evaporator and the second in the absorber, thereby achieving higher useful heat temperatures than those obtained with single-stage systems. This effect is similar to that obtained with two-stage heat transformers (TSHTs), but these systems require more components than DAHTs in order to achieve the same effect. 2.3. Experimental facility The experimental DAHT with approximately 1 kW of power is operated with a H2O/LiBr mixture. The system consists mainly of a generator, absorber, absorber-evaporator, condenser, evaporator, economizer, pump, and expansion valve, as can be seen in Fig. 3. The system was constructed entirely from stainless steel 316 in order to avoid corrosion problems. The generator and evaporator are of the stagnant pool type, whereby heat is supplied by means of electrical heaters immersed in the solution and water, respectively. The condenser is a tank with a coil inside that condenses the water vapor arising from the generator. The absorber is of a vertical falling film type, where oil circulates within the tubes to remove the heat at the higher temperature produced by the water vapor absorption into the strong solution. A round distributor was placed above the coil in the absorber in order to distribute the solution. The economizer is a concentric tube heat exchanger and the absorber-evaporator is similar to the absorber, but inside the coil, the water from the condenser is evaporated by means of the heat delivered by the absorption process. A gear pump with a velocity controller Cole Parmer model 75,211-20 was used to pump the working fluid in the system. The expansion valve is a Swagelok model SS-4MG2-NEMH. All of the components were interconnected with stainless steel tubing and connectors, and the entire system was properly insulated. The heat input was controlled by
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W. Rivera et al. / Applied Thermal Engineering 132 (2018) 432–440
Fig. 1. Pressure versus temperature diagram for a single-stage heat transformer.
Fig. 2. Pressure versus temperature diagram for DAHT.
W. Rivera et al. / Applied Thermal Engineering 132 (2018) 432–440
Fig. 3. Double absorption heat transformer installed in the Instituto de Energías Renovables of the Universidad Nacional Autónoma de México.
two variable transformers, each with a maximum power of 2 kW. Copper piping was used in the auxiliary cooling system and vacuum systems, and neoprene hosepipes were used to connect the main components. Three auxiliary systems were connected to the heat transformer: (i) cooling system, (ii) vacuum system, and (iii) heat-recovery system. The auxiliary cooling system was used to condense the refrigerant vapor entering the condenser. The auxiliary vacuum system was used to produce a vacuum in the system for evaporating the water in the generator and evaporator at temperatures lower than 100 °C, while the heat recovery system was used to recover useful heat at the higher temperature produced in the absorber. The heat in this component was removed by means of Mobil Therm oil. 2.4. Instrumentation In order to determine the useful heat delivered in the absorber as well as the performance coefficient of the heat transformer, process parameters such as temperature, pressure, mass flow rate, concentration, and electrical power were measured with the instrumentation, as follows. The temperature was measured at the inlet and outlet of each main component by means of 24 type T thermocouples (copper/constantan) insulated with fiberglass. In order to facilitate heat transfer, the thermowells were filled with vacuum oil. Each thermocouple was calibrated separately using a constant temperature source. The thermocouples were calibrated in a range between 20 °C and 140 °C. Using the least-squares method, polynomial equations were obtained for the thermocouples and subsequently incorporated into a previously written computer program. The correlation coefficient for the equations was higher than 0.996. Temperatures were recorded with an accuracy of 0.1 °C. Two Bourdon gauges were utilized in the equipment in order to obtain qualitative pressure readings during the experimental process. One of these was connected to the condenser, and the other to the absorber. In addition to the gauges, three pressure transducers were used to measure the pressure in the low, intermediate, and high zones. The transducers were diaphragm types with a maximum error of 0.05% and a working range between 0.1 and 22.1 bar. The pressure transducers were previously calibrated, obtaining equations with correlation coefficients
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higher than 0.99. Pressure transducers with a design exactitude of ±0.25% within the total range were used to measure low and high pressures. The power required for the pressure transducers was supplied by a power source (Lambda, model LQ-302). The electrical power supplied by electrical resistance placed in a spiral winding along the test section and second preheater were measured by a voltmeter (Keithley, model 169-DMM) and an ammeter (Amprobe, model ACD-10). The experimental data were gathered by a data logging system, namely the Hewlett Packard model 2310A with a maximum capacity of 29 channels. The solution and refrigerant flows were measured with analogical flowmeters with a reading accuracy of ±2%. The H2O/LiBr concentrations in the generator and absorber were measured using a refractometer with an accuracy of ±0.0002. In order to supply heat to the evaporator and generator, electrical heaters of 1 kW were placed inside these components. The current intensity and voltage readings for determining the power supplied by the heaters were measured with accuracies of 0.01 A and 0.01 V, respectively. The instrumentation was connected to a data logger, namely Agilent model 34970A, with two acquisition cards of 20 modules for voltage and two for current, as well as one card with modules for digital signal and voltage suppliers. The system was controlled through a Hewlett-Packard Visual Engineering Environment (HPVEE) program. 3. Important parameters Several design parameters exist for a heat-driven absorption heat transformer, and in this we took the following into consideration: (i) gross temperature lift, (ii) economizer effectiveness, and (iii) coefficient of performance. 3.1. Gross temperature lift The gross temperature lift (GTL) is defined as the difference between the temperature of the useful heat produced in the absorber and that of the heat supplied to the generator and evaporator, which are the same in this study. Thus, according to Fig. 2, the GTL can be written as:
GTL ¼ T 11 T 7 :
ð1Þ
3.2. Economizer effectiveness The economizer is used to recover heat energy in AHTs. In particular, the economizer preheats the strong salt solution flowing from the generator to the absorber by using the heat supplied from the weak salt solution flowing from the absorber to the generator. The economizer effectiveness (EFEC) can by defined as the actual heat recovered to the maximum possible heat that can be recovered. Therefore, from the energy balance and according to Fig. 2, the economizer effectiveness can be written as:
EF EC ¼
_ 8 ðh10 h9 Þ m : _ 11 ðh13 h9 Þ m
ð2Þ
3.3. Coefficients of performance (COP) The COP is an important parameter, as it represents the efficiency of an AHT. It is defined as the useful heat delivered in the absorber per unit of heat load supplied to the generator and evaporator plus the work done by the pumps.
COP ¼
Q_ AB _ _P Q GE þ Q_ EV þ W
ð3Þ
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Table 1 Temperatures, concentrations, heat loads and coefficients of performance of the double-absorption heat transformer.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
TE (°C)
TC (°C)
TG (°C)
TG, IN (°C)
TE,OUT (°C)
TA (°C)
TA,IN (°C)
TAB, OUT (°C)
TAE (°C)
TAE, IN (°C)
TAE, OUT (°C)
TOIL, (°C)
66.19 62.29 55.41 57.27 42.49 45.78 48.60 46.11 50.54 52.89 47.30 50.48 44.49 50.69 52.74 34.90 39.08 37.32 36.69 37.71 36.92 35.81 35.31 42.34 42.65 44.02 41.82 44.11 47.21 44.79 45.17 49.01 45.68 44.23 42.30 44.79 40.29 46.47 39.69 41.27 44.89 36.61 36.61 41.58 46.58 38.83 43.92 47.30 46.02 50.33 52.92
35.82 35.64 35.72 46.39 35.49 35.60 36.44 35.10 37.48 39.21 34.81 34.96 33.11 34.98 36.13 29.47 32.15 31.73 33.56 31.95 31.31 30.58 29.50 38.72 35.42 37.00 34.65 33.94 32.83 32.74 32.21 32.80 32.55 31.27 30.89 31.90 30.60 31.81 31.85 30.92 31.65 31.89 31.88 32.44 33.67 31.84 32.43 35.19 31.49 33.12 33.64
71.06 71.32 77.02 98.35 75.17 72.70 75.01 72.93 72.82 74.41 71.76 68.91 73.75 72.22 67.20 66.91 72.13 69.79 69.43 71.51 70.95 71.92 73.27 86.77 81.91 84.40 80.99 76.40 76.77 77.26 77.61 77.48 77.76 78.00 77.89 78.65 75.47 76.47 76.91 74.53 75.56 76.78 77.33 79.23 81.25 77.68 78.65 85.08 75.95 79.53 80.75
71.92 72.61 78.82 96.63 74.54 71.18 74.26 72.82 72.21 73.71 71.81 68.97 73.13 71.97 68.27 67.24 72.86 70.56 69.92 72.19 72.02 72.97 74.58 87.00 81.58 84.26 81.60 76.63 77.81 78.27 78.67 78.47 79.14 79.28 78.68 80.87 75.68 76.66 76.73 74.22 76.74 76.80 77.99 78.81 81.97 78.04 79.96 84.88 77.82 81.25 82.77
70.27 70.81 76.12 97.61 74.13 71.11 73.98 71.91 71.86 73.50 70.82 67.92 72.70 71.22 66.80 66.10 66.10 69.52 69.40 71.19 70.45 71.39 72.57 86.00 81.04 83.55 80.21 75.62 75.93 76.46 77.01 77.18 77.36 77.94 77.48 78.32 75.23 75.82 23.89 74.13 74.94 24.97 25.03 78.95 80.60 77.19 77.89 84.36 75.13 78.70 79.86
105.81 108.09 102.42 108.54 105.59 103.26 99.72 103.78 104.15 107.92 103.48 99.11 102.16 105.69 102.77 95.50 99.23 96.87 93.93 100.63 100.97 104.65 106.77 114.94 108.06 110.87 109.73 105.32 109.49 109.01 111.05 110.21 111.29 110.85 111.58 112.12 109.74 118.64 120.27 108.50 111.58 115.40 116.43 110.80 118.57 116.61 120.02 121.82 118.07 122.42 128.08
83.07 83.26 86.08 97.36 94.14 91.83 91.47 91.64 91.01 93.86 90.94 87.25 91.41 91.87 88.99 86.70 89.28 86.90 84.65 89.48 89.41 91.84 94.35 106.97 99.78 102.48 99.76 94.92 98.27 98.01 99.40 99.42 99.78 99.26 100.29 100.94 97.24 101.90 101.59 97.39 99.64 102.18 102.45 101.85 107.78 103.63 107.11 109.65 104.92 109.22 113.42
97.15 98.93 93.38 100.52 95.64 91.13 91.56 93.85 94.00 97.13 93.95 90.55 92.43 95.76 93.25 88.77 89.71 87.55 85.12 91.23 91.05 94.23 96.83 106.37 99.40 102.06 100.10 96.30 101.00 100.24 102.09 101.44 103.02 101.01 103.36 102.56 99.60 105.04 108.39 99.49 104.19 104.97 107.09 102.63 107.42 108.08 107.43 113.67 108.22 112.32 115.53
74.98 74.55 74.69 84.30 73.88 74.31 78.78 75.54 78.36 78.89 77.11 79.28 77.58 79.87 75.84 64.71 69.58 67.65 66.58 69.03 68.62 69.21 68.43 75.91 72.61 74.74 71.35 68.46 70.75 69.72 71.20 70.74 71.66 70.48 71.43 72.28 70.45 75.55 77.50 70.20 71.88 75.23 76.19 70.93 74.67 74.15 77.48 77.33 78.13 80.97 83.72
83.5 84.4 81.7 77.9 82.6 80.2 81.2 81.9 81.3 83.8 81.2 78.1 81.2 81.7 79.5 76.4 80.2 78.1 76.1 79.7 79.9 80.7 82.0 88.3 88.4 88.2 86.5 84.1 87.6 86.4 87.8 87.1 87.9 87.1 88.0 88.6 85.6 89.4 88.4 85.8 87.5 87.8 88.3 89.6 94.2 89.0 91.5 94.8 91.8 96.2 99.4
70.4 71.7 69.0 74.4 69.5 69.1 73.6 72.0 71.5 72.9 70.4 67.6 72.5 70.4 68.2 61.7 67.1 65.4 64.5 66.3 66.3 65.2 65.4 72.8 69.2 71.9 68.4 66.1 67.3 67.1 68.6 67.6 67.9 66.9 67.1 67.6 66.6 71.8 73.5 66.7 68.6 71.7 72.6 66.8 71.7 70.8 73.9 74.1 74.6 77.9 79.0
71.1 71.5 71.9 69.4 65.5 65.7 69.9 67.4 69.3 67.2 66.0 61.9 68.0 67.4 69.8 69.6 67.1 67.2 66.0 73.0 77.8 78.2 82.0 90.0 76.6 79.1 82.2 81.0 88.4 85.2 91.7 92.4 93.5 92.3 91.5 92.3 93.8 99.4 103.5 93.2 97.8 101.8 106.7 94.2 108.1 104.2 108.3 109.4 108.2 113.9 117.2
IN
TOil, OUt (°C)
TWA, (°C)
92.8 94.0 90.0 95.5 90.0 95.5 91.1 92.8 94.5 97.0 94.8 91.7 90.7 96.2 87.1 85.3 85.5 82.4 81.4 88.0 86.8 91.7 94.7 103.5 95.2 98.9 96.9 93.0 97.8 96.7 99.5 99.0 99.1 98.0 96.2 99.9 98.4 105.3 107.8 97.5 101.0 106.0 108.0 101.0 109.6 106.1 108.9 112.2 109.2 113.9 118.2
27.2 27.5 28.2 29.0 28.1 28.1 28.2 26.6 26.8 27.0 26.8 25.8 26.5 26.8 26.6 26.0 26.7 26.7 27.0 27.0 26.9 26.8 25.7 30.0 29.0 29.2 28.4 28.9 29.0 28.9 28.6 28.4 28.7 28.1 27.4 27.4 26.8 27.4 27.4 27.4 27.8 28.2 28.0 28.3 28.8 28.0 28.0 28.6 27.0 28.5 29.0
IN
TWA, (°C)
OUT
36.2 36.4 36.4 35.1 36.0 35.4 36.5 35.0 35.5 35.3 35.7 34.9 33.7 35.0 32.0 29.8 32.3 31.6 33.9 30.8 31.1 30.8 29.7 33.2 32.0 32.0 31.4 32.1 32.6 32.3 32.2 32.0 32.3 31.5 30.9 32.2 30.0 31.6 31.5 31.0 31.9 32.1 32.2 31.2 32.3 31.8 32.4 31.7 32.2 33.8 34.4
XG (%)
XAE (%)
XA (%)
QC,IN (W)
QE,IN (W)
QG,IN (W)
QAE,IN (W)
QA,IN (W)
QE,EX (W)
QG,EX (W)
QAE,EX (W)
QA,EX (W)
EFE (-)
GTL (°C)
DT (°C)
COPIN (-)
COPEX (-)
53.6 55.2 57.3 53.7 55.9 54.6 55.0 54.8 52.9 52.4 55.1 53.1 57.4 54.6 54.4 56.5 56.8 55.8 55.8
52.1 52.9 51.9 51.5 53.3 51.4 52.1 52.0 50.5 49.1 51.8 50.1 54.3 51.4 52.3 56.3 56.4 55.6 55.7
52.6 53.7 53.8 52.2 54.9 52.8 53.4 52.8 51.9 52.1 54.0 52.6 56.0 54.4 53.0 55.7 56.5 55.2 54.5
332.1 331.4 331.7 386.6 330.8 398.5 536.2 531.1 405.6 344.9 395.5 409.5 321.9 328.9 467.9 321.5 318.2 316.6 323.6
136.1 131.9 131.5 169.9 265.0 266.1 337.4 332.1 205.8 144.8 195.5 196.5 120.9 128.6 269.2 106.2 116.7 115.0 121.9
289.7 312.5 298.7 687.2 433.3 487.6 618.7 579.7 402.3 376.0 426.9 382.5 363.8 341.0 442.9 429.9 418.0 370.0 437.3
439.2 457.5 211.4 224.1 567.8 550.3 570.8 611.8 431.3 368.7 481.2 420.4 250.9 416.9 554.3 369.5 328.2 354.4 354.1
101.5 96.5 67.8 283.0 113.8 290.7 386.1 325.8 206.9 221.4 255.8 221.9 184.9 189.0 244.9 158.6 237.9 206.9 228.7
207.0 148.8 147.0 210.6 142.8 192.9 260.7 293.9 292.6 293.9 294.3 351.4 289.1 347.0 305.3 308.7 303.7 293.3 288.5
727.3 727.3 726.0 726.0 733.9 733.9 733.9 726.0 728.6 730.6 730.4 728.6 731.9 728.6 699.6 743.8 1045.2 891.6 754.4
236.5 281.2 280.0 226.2 285.1 226.3 169.5 203.3 154.9 102.5 203.0 155.2 203.3 154.7 204.6 203.9 206.6 211.7 215.7
178.6 189.1 141.8 223.3 262.4 243.8 169.6 209.3 212.8 260.3 244.2 251.8 181.1 248.6 130.6 115.3 135.6 107.0 106.3
0.39 0.40 0.39 0.31 0.37 0.35 0.34 0.38 0.35 0.37 0.38 0.38 0.35 0.37 0.43 0.43 0.36 0.37 0.30
39.6 45.8 47.0 51.3 63.1 57.5 51.1 57.7 53.6 55.0 56.2 48.6 57.7 55.0 50.0 60.6 60.2 59.5 57.2
21.71 22.58 18.12 26.09 30.44 28.86 21.15 25.44 25.22 29.77 28.81 29.84 22.72 28.76 17.29 15.69 18.40 15.25 15.40
0.18 0.16 0.12 0.28 0.14 0.32 0.35 0.31 0.28 0.33 0.33 0.31 0.30 0.31 0.29 0.23 0.35 0.33 0.33
0.19 0.22 0.16 0.24 0.30 0.26 0.17 0.21 0.21 0.25 0.24 0.23 0.18 0.23 0.13 0.11 0.10 0.09 0.10
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W. Rivera et al. / Applied Thermal Engineering 132 (2018) 432–440 Table 1 (continued)
20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51
XG (%)
XAE (%)
XA (%)
QC,IN (W)
QE,IN (W)
QG,IN (W)
QAE,IN (W)
QA,IN (W)
QE,EX (W)
QG,EX (W)
QAE,EX (W)
QA,EX (W)
EFE (-)
GTL (°C)
DT (°C)
COPIN (-)
COPEX (-)
56.8 57.2 59.1 60.1 60.7 59.0 59.3 59.5 58.3 58.7 59.1 58.9 58.2 58.7 59.6 60.6 60.4 58.7 59.0 59.3 58.5 58.9 59.4 59.5 58.9 60.2 58.9 59.3 60.3 60.2 60.0 60.5
56.5 57.1 58.5 59.9 60.3 58.8 58.9 59.1 58.1 58.5 58.7 58.5 57.8 58.1 58.9 60.3 60.0 58.2 58.4 58.5 58.0 58.3 59.1 59.1 57.9 59.3 58.0 58.8 59.8 59.0 59.0 59.8
56.4 56.6 58.0 59.0 59.5 57.9 58.1 58.3 57.7 58.0 57.9 57.9 57.4 57.8 58.6 59.5 59.3 57.7 57.9 57.8 57.1 57.8 58.4 58.5 57.7 58.6 57.6 58.3 58.9 57.9 58.7 59.6
317.5 315.0 312.3 308.2 342.9 330.6 336.5 327.7 325.0 320.8 320.5 318.5 320.7 319.8 314.9 380.8 330.8 325.8 343.9 344.1 327.1 343.2 344.2 344.2 319.4 350.9 330.5 346.3 343.2 329.2 348.9 364.3
115.9 113.4 110.5 106.3 141.9 129.5 135.6 126.5 124.1 120.1 119.5 117.5 120.1 118.9 113.9 112.2 116.3 110.9 116.1 115.7 112.2 115.5 115.8 115.8 118.1 123.2 115.6 118.3 129.1 114.8 121.4 123.7
421.5 409.4 470.0 450.8 616.0 604.5 609.0 586.8 524.6 488.0 512.5 515.4 532.7 528.8 558.0 618.5 533.5 550.7 502.8 514.3 552.3 500.1 517.4 513.7 639.6 610.6 543.7 534.7 600.2 470.1 420.8 429.5
334.3 326.5 361.0 353.3 414.9 443.2 409.0 418.2 412.6 446.8 429.6 434.6 447.6 454.4 438.3 418.2 428.2 425.3 416.7 375.2 429.2 414.8 378.0 374.9 508.8 494.9 431.7 416.9 457.6 407.3 577.6 438.8
193.6 195.5 221.4 214.7 328.9 289.5 302.1 264.6 239.5 181.9 253.9 195.0 215.0 183.8 213.6 251.1 203.0 204.8 210.1 111.7 188.5 195.7 141.3 131.7 243.7 328.8 100.7 300.9 195.3 196.2 229.2 279.9
289.8 287.7 271.6 270.9 266.7 292.6 281.0 268.0 265.4 265.4 260.0 261.2 255.4 249.9 256.6 265.6 318.2 272.9 272.7 268.8 222.3 222.3 216.3 216.3 225.5 207.9 211.7 204.8 181.9 267.5 255.8 245.3
740.3 743.6 739.9 743.8 749.5 744.5 742.5 750.4 738.5 735.9 745.8 747.1 751.0 739.6 759.8 751.5 875.7 740.8 754.5 749.0 750.3 750.7 744.9 747.6 737.0 739.9 742.6 750.5 740.6 742.6 745.8 748.1
213.7 216.7 211.0 216.7 211.1 211.7 212.7 215.4 216.4 217.7 216.4 216.7 216.4 215.4 213.7 214.0 257.8 217.1 210.3 210.7 217.1 213.9 216.0 216.4 215.7 215.0 216.3 217.7 217.1 210.7 211.0 209.0
114.5 67.8 109.1 106.9 133.9 158.8 177.5 127.8 98.5 83.9 100.3 70.1 59.5 50.2 50.9 40.0 69.1 40.7 60.1 47.1 37.6 29.9 44.6 15.0 60.0 14.5 20.0 7.2 33.6 10.9 1.4 12.2
0.36 0.38 0.39 0.39 0.35 0.34 0.33 0.36 0.35 0.39 0.38 0.38 0.38 0.39 0.37 0.38 0.38 0.38 0.43 0.38 0.42 0.41 0.44 0.43 0.37 0.46 0.41 0.51 0.37 0.48 0.49 0.52
62.9 64.1 68.8 71.5 72.6 65.4 66.8 67.9 61.2 62.3 64.2 65.9 61.2 65.6 66.6 69.3 67.3 69.4 72.2 80.6 67.2 66.7 78.8 79.8 69.2 72.0 77.8 76.1 74.5 72.1 72.1 75.2
14.96 9.02 13.48 12.63 13.50 18.63 19.77 14.63 11.94 9.48 11.52 7.74 6.61 5.58 5.75 4.63 7.60 4.56 5.82 4.30 4.27 3.20 4.25 1.37 6.81 1.55 1.90 0.64 2.89 0.97 1.03 1.01
0.29 0.29 0.31 0.31 0.36 0.33 0.34 0.31 0.30 0.24 0.33 0.25 0.27 0.23 0.26 0.29 0.26 0.25 0.28 0.14 0.23 0.26 0.18 0.17 0.27 0.38 0.13 0.38 0.22 0.27 0.34 0.40
0.11 0.07 0.11 0.11 0.13 0.15 0.17 0.13 0.10 0.08 0.10 0.07 0.06 0.05 0.05 0.04 0.06 0.04 0.06 0.05 0.04 0.03 0.05 0.02 0.06 0.02 0.02 0.01 0.04 0.01 0.01 0.01
The COP can be internal, when is calculated based on the internal streams, or external, when is estimated based on external streams or heat loads. From the mass and energy balances and based on Fig. 2, the internal COP be written as:
COP INT ¼
_ 5 h5 þ m _ 10 h10 m11 h11 m _P _ 8 h8 m _ 15 h15 þ m _ 6 ðh7 h6 Þ þ W _ 1 h1 þ m m
ð4Þ
and the external COP can be calculated as:
COP EXT ¼
_ 16 C Poil ðT 17 T 16 Þ m
_P V GE IGE þ V EV IEV þ W
;
ð5Þ
where CPoil is the specific heat of the oil circulating in the heat recovery system, VGE and IGE are the voltage and electric current supplied to the electrical heater in the generator, and VGE and IGE are the voltage and electric current supplied to the electrical heater in the evaporator. The physical and thermodynamic properties for the H2O/LiBr mixture were taken from McNeely [63]. 4. Results In order to evaluate the DAHT, more than 70 test runs were carried out at different temperatures and solution concentrations; however, only 51 were considered in this study, as they were the only ones to reach the steady state conditions. The steady state was considered when the system temperatures varied by less than 1 °C over a period of one hour. Table 1 shows the temperatures, concentrations, heat loads and coefficients of performance for the 51 experimental test runs. An uncertainty evaluation was carried out based on the National Institute of Standards of Technology (NIST) uncertainty propagation, with the actual instrumentation accuracy from Sec-
tion 2.3. The GTL uncertainty value was approximately 0.1414 °C for all operating conditions. For the economizer effectiveness, the uncertainty was approximately 0.06325, with a significant dependency on the mass flow values. The COPINT uncertainty values were approximately 0.066 for TAB from 90 to 130 °C, while the COPEXT uncertainty values were approximately 0.0100, depending mainly on the mass flow values. Fig. 4 illustrates the GTL versus generation solution concentration. It can be seen that the GTL increases linearly with the increment in generator solution concentration. This behavior was expected, as a higher concentration of LiNiO3 (in the solution leaving the generator) means that a greater amount of water was generated in the generator, which will be absorbed into the absorber posteriorly. On the other hand, it can be seen that the GTLs vary between 48 °C and 74 °C. According to Eq. (1), this means that the heat transformer can increase the absorber temperature between 48 °C and 74 °C over the temperature of the heat supplied to the evaporator. Based on this figure, it appears to be desirable to increase the LiNiO3 concentration in the solution even further; however, it is important to remember that crystallization problems may occur at higher concentration values. On the other hand, it is important to note that, from the bibliographic review, it is clear that with single-stage heat transformers, it is possible to achieve GTLs of up to 44 °C, while it is possible to achieve up to 74 °C using the developed system. Fig. 5 illustrates the GTL versus absorber solution concentration, and a similar trend can be observed to that in Fig. 4. Once again, this behavior was expected, because at a higher LiNiO3 concentration in the solution inside the absorber, its capacity is increased to absorb greater amounts of water, thus increasing its temperature due to the exothermic reaction during the absorption process. The GTL values were similar to those reported in the previous figure. Fig. 6 illustrates the internal COP versus internal useful heat produced in the absorber. It can be seen that the internal COP
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increases with an increment in the absorber heat load. This occurs because, according to Eqs. (3) and (4), the internal COP is directly
Fig. 4. GTL versus generator solution concentration.
Fig. 5. GTL versus absorber solution concentration.
proportional to the absorption heat load. Furthermore, it can be observed that the inlet COPs vary between 0.12 and 0.37, meaning that up to 37% of the energy supplied to the generator and evaporator could be reused as useful heat at higher temperature levels. According to the literature review, it is possible to achieve internal COPs close to 0.5 with single-stage heat transformers, while in this work we achieve COPs no higher than 0.37. This is the price paid by double-absorption systems in order to produce useful heat at higher temperature levels. Fig. 7 illustrates the external COPs against the external absorber heat load. It can once again be observed that the external COP increases with an increment in the external absorption heat load, for the same reasons as those explained in Fig. 6. However, it can be seen in Fig. 7 that the external COPs are lower than the internal coefficient reported in Fig. 6. This occurs because the internal values do not consider the system heat losses to the atmosphere, while the external values do. Comparing Figs. 6 and 7, it can be observed that higher data dispersion of the COP values exists in Fig. 6 than in Fig. 7. This occurs because the internal COP depends on a greater number of variables (see Eq. (4)), while the external values depend only on several variables, which are also easy to control (see Eq. (5)). Fig. 8 illustrates the external COP against the absorber temperature. It can be observed that the external COP decreases drastically with an increase in absorber temperature. This occurs because when the absorber temperature increases, the concentration differences between the strong and weak solutions decrease considerably, tending to zero at high absorber temperatures, which means that the refrigerant production decreases considerably; thus, the COP decreases abruptly. Fig. 9 illustrates the economizer efficiency versus absorber temperature. It can be seen that EFEC increases with an increment in the absorber temperature. This occurs because, as the absorber temperature increases, the heat gained by the solution moving from the generator to the absorber increases, thereby increasing economizer efficiency.
Fig. 6. Internal COP as function of internal absorber heat load.
W. Rivera et al. / Applied Thermal Engineering 132 (2018) 432–440
Fig. 7. Internal coefficient of performance as function of internal absorber heat load.
439
Fig. 9. Economizer efficiency as function of absorber temperature.
5. Conclusions This paper has reported on the experimental results of a DAHT operating with a H2O/LiBr mixture. The results indicated that the system can achieve GTLs of between 48 °C and 74 °C, with internal COPs varying from 0.12 to 0.37, and external COPs of up to 0.3. From the bibliographic review, it was clear that it is possible to achieve GTLs of up to 44 °C with single-stage heat transformers operating with the same working mixture; however, with the actual system, it was possible to achieve GTLs of up to 74 °C, which is 30 °C more than with single-stage systems. Acknowledgements The authors thank the projects SENER-CONACyT 117914 and CONACYT 154301 for the economic support provided for the development of this study. References
Fig. 8. External COP as function of absorber temperature.
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Research Paper
Experimental assessment of double-absorption heat transformer operating with H2O/LiBr W. Rivera a,⇑, A. Huicochea b, R.J. Romero b, A. Lozano a a b
Instituto de Energías Renovables, Universidad Nacional Autónoma de México, Temixco, Morelos, Mexico Centro de Ingeniería y Ciencias Aplicadas, Universidad Autónoma del Estado de Morelos, Av. Universidad 1001, C.P. 62209 Cuernavaca, Morelos, Mexico
h i g h l i g h t s A double-absorption heat transformer was developed operating with H2O/LiBr. Gross temperature lifts as high as 74 °C were achieved with the system. The gross temperature lifts are the highest reported in the literature using H2O/LiBr. Up to 37% of the energy supplied to the DAHT could be retrieved at higher temperature levels.
a r t i c l e
i n f o
Article history: Received 28 July 2017 Revised 28 December 2017 Accepted 29 December 2017 Available online 30 December 2017 Keywords: Heat transformers Double absorption Water/lithium bromide
a b s t r a c t This paper reports on the experimental results of a double-absorption heat transformer operating with a H2O/LiBr mixture. The generator and evaporator are pool boiling type heat exchangers, while the remaining components are coils inside shells. Plots of gross temperature lifts, economizer efficiency, and internal and external performance coefficients are reported as functions of diverse operating parameters of the heat transformer. The results indicate that the system can achieve gross temperature lifts (GTLs) of between 48 °C and 74 °C, with internal performance coefficients varying from 0.12 to 0.37. The GTLs achieved are up to 30 °C higher than those reported in the literature, using a single-stage heat transformer operating with the same working mixture. Furthermore, the system exhibits effective stability and repeatability. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction At present, numerous industries worldwide deliver waste heat to the atmosphere at temperatures below 100 °C. Absorption heat transformers (AHTs) constitute one of the most attractive thermal systems for thermal energy savings in the industry, consuming negligible amounts of primary energy. These systems can upgrade the waste heat temperature to a higher level, in order to be reused during the industrial process. Typically, with single-stage systems, up to half of the heat supply could be increased in temperature, while the remainder is discharged to the atmosphere at lower temperatures. In addition to single-stage heat transformers, advancedabsorption heat transformers are available, which use more components than single-stage systems in order to achieve greater gross temperature lifts or higher performance coefficients.
⇑ Corresponding author. E-mail address: [email protected] (W. Rivera). https://doi.org/10.1016/j.applthermaleng.2017.12.117 1359-4311/Ó 2017 Elsevier Ltd. All rights reserved.
Rivera et al. [1] carried out a bibliographic review related to the development of absorption heat transformers. The review indicated that there have been a considerable number of theoretical studies on advanced-absorption heat transformers, such as double-absorption and two-stage systems operating with diverse working mixtures [2–13]. Regarding experimental studies, there have been certain investigations into developed systems operating with a H2O/LiBr mixture [14–20], as well as with alternative mixtures [21–40]. Moreover, there have been studies related to heat transformers for different applications, such as heat recovery in the industrial process [41–45] or distillation process [46–53], but single-stage absorption heat transformers were utilized in all cases. Furthermore, studies have been carried out on advanced absorption systems operating with a H2O/LiBr mixture, by Chen et al. [54] and Xu and Wang [55], but these were related to the development of advanced absorption cooling systems and not the development of absorption heat transformers, which differ significantly not only in their applications but also their configurations.
W. Rivera et al. / Applied Thermal Engineering 132 (2018) 432–440
Regarding experimental studies related to advanced AHTs, Scott et al. [56] developed a novel multi-compartment absorption heat transformer for different steam temperatures. Both the absorber and generator were partitioned in a number of compartments, depending on the number of high-quality steam demands and waste vapor sources, respectively. The results indicated that the flow rate of the steam fed to the absorber is the most important process variable affecting the U-value and consequently dominating the design process of the absorber heat transfer area. Sekhar and Muthukumar [57] reported the results of a developed prototype of a two-stage heat transformer for upgrading the waste heat available from 393 to 413 K to approximately 463 K. The coefficient of performance (COP) was 0.2 and the maximum gross temperature lift achieved was approximately 80.5 K. Silva-Sotelo et al. [58] reported the results of a two-stage heat transformer controlled by a flow ratio operating with a water/Carrol mixture. The waste heat energy was added to the system at 70 °C, resulting in a temperature of 128 °C in the second absorber. Romero et al. [59] reported the results of a two-stage heat transformer operating with a H2O/Carrol mixture by means of object-oriented programming, and the authors reported that the most effective heat recovery was 47% at 100 °C. Isselhorst and Groll [60] developed a twostage metal hydride prototype heat transformer. The developed system demonstrated the feasibility of upgrading heat of approximately 130–140 °C up to 200 °C with a COP of around 0.27. Willers and Groll [61] reported the results of a prototype two-stage metal hydride heat transformer. The results demonstrated that the system provided 6 kW of useful heat at a temperature of approximately 190–200 °C, with a driving heat of 130–135 °C, achieving a COP close to 0.1. A two-stage absorption heat transformer was designed and constructed by Currie and Pritchard [62] in order to investigate the potential for dehumidifying and reheating a simulated dryer exhaust stream, to make it suitable for recycling to the dryer inlet. The heat transformer performance data indicated that an airstream could be reheated to 160 °C using a LiBr solution of 68% w/w with a circulation ratio (LiBr to steam flow) of 14.8. Temperature lifts of between 50 and 70 °C were possible in the reheat column when using a low circulation ratio and high LiBr concentration. The results indicated that a humid air stream could be dehumidified to a level suitable for recycling by direct contact with a concentrated LiBr stream. It can be seen from the literature review that although there have been numerous studies related to absorption heat transformers in the past decades, none of these has included the experimental study of double-absorption heat transformers (DAHTs). This fact is significant, as it will be seen in this study that DAHTs use only one additional component to single-stage systems, but can achieve considerably higher gross temperature lifts. In fact, the achieved gross temperature lifts are as high as those obtained with twostage heat transformers, but these use a greater number of components, with double the number of components compared to singlestage systems.
2. Description of single and double-absorption heat transformers 2.1. Single-stage heat transformer A single-stage heat transformer operating with a H2O/LiBr mixture basically consists of an evaporator, condenser, generator, absorber, and economizer, as shown in Fig. 1. Waste heat is supplied at an intermediate temperature to the generator in order to vaporize the water from the weak solution (low salt concentration). The vaporized water enters the condenser, where it is condensed, delivering a certain amount of heat at a low ambient
433
temperature. The water leaving the condenser is pumped to the evaporator, where it is evaporated by a quantity of waste heat supplied at intermediate temperature. Following this, the water vapor enters the absorber, where it is absorbed by the solution with a high absorbent concentration from the generator, delivering useful heat at the highest system temperature. Finally, the solution with a low absorbent concentration returns to the generator, preheating the solution into the economizer and thereby completing the cycle. 2.2. Double-absorption heat transformer Double-absorption heat transformers (DAHTs) basically consist of a generator, condenser, evaporator, absorber, absorberevaporator, and economizer, as illustrated in Fig. 2. A heat source is supplied in order to separate the working fluid in the generator at an intermediate temperature TG. The vaporized working fluid is condensed in the condenser at a lower pressure PC and temperature TC. Thereafter, the condensed working fluid is split into two streams: one is pumped to the evaporator, where it is vaporized at an intermediate temperature TE and pressure PE, and the other is pumped at a higher pressure PA and vaporized in the absorberevaporator by a certain amount of available heat QAE. The vaporized working fluid is absorbed into the absorber by the solution from the generator, producing useful heat at the highest system temperature TA. The diluted solution at an intermediate concentration XA enters the absorber-evaporator, absorbing the vaporized working fluid from the evaporator and delivering an amount of heat QAE, which is used to vaporize the working fluid in the absorber-evaporator. Then, the diluted solution at a low concentration XAE, leaving the absorber-evaporator, moves to the generator preheating the solution from the generator to the absorber into the economizer, starting the cycle again. Thus, by using only one additional component compared to single-stage heat transformers, it is possible to obtain two temperature lifts: the first in the absorber-evaporator and the second in the absorber, thereby achieving higher useful heat temperatures than those obtained with single-stage systems. This effect is similar to that obtained with two-stage heat transformers (TSHTs), but these systems require more components than DAHTs in order to achieve the same effect. 2.3. Experimental facility The experimental DAHT with approximately 1 kW of power is operated with a H2O/LiBr mixture. The system consists mainly of a generator, absorber, absorber-evaporator, condenser, evaporator, economizer, pump, and expansion valve, as can be seen in Fig. 3. The system was constructed entirely from stainless steel 316 in order to avoid corrosion problems. The generator and evaporator are of the stagnant pool type, whereby heat is supplied by means of electrical heaters immersed in the solution and water, respectively. The condenser is a tank with a coil inside that condenses the water vapor arising from the generator. The absorber is of a vertical falling film type, where oil circulates within the tubes to remove the heat at the higher temperature produced by the water vapor absorption into the strong solution. A round distributor was placed above the coil in the absorber in order to distribute the solution. The economizer is a concentric tube heat exchanger and the absorber-evaporator is similar to the absorber, but inside the coil, the water from the condenser is evaporated by means of the heat delivered by the absorption process. A gear pump with a velocity controller Cole Parmer model 75,211-20 was used to pump the working fluid in the system. The expansion valve is a Swagelok model SS-4MG2-NEMH. All of the components were interconnected with stainless steel tubing and connectors, and the entire system was properly insulated. The heat input was controlled by
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Fig. 1. Pressure versus temperature diagram for a single-stage heat transformer.
Fig. 2. Pressure versus temperature diagram for DAHT.
W. Rivera et al. / Applied Thermal Engineering 132 (2018) 432–440
Fig. 3. Double absorption heat transformer installed in the Instituto de Energías Renovables of the Universidad Nacional Autónoma de México.
two variable transformers, each with a maximum power of 2 kW. Copper piping was used in the auxiliary cooling system and vacuum systems, and neoprene hosepipes were used to connect the main components. Three auxiliary systems were connected to the heat transformer: (i) cooling system, (ii) vacuum system, and (iii) heat-recovery system. The auxiliary cooling system was used to condense the refrigerant vapor entering the condenser. The auxiliary vacuum system was used to produce a vacuum in the system for evaporating the water in the generator and evaporator at temperatures lower than 100 °C, while the heat recovery system was used to recover useful heat at the higher temperature produced in the absorber. The heat in this component was removed by means of Mobil Therm oil. 2.4. Instrumentation In order to determine the useful heat delivered in the absorber as well as the performance coefficient of the heat transformer, process parameters such as temperature, pressure, mass flow rate, concentration, and electrical power were measured with the instrumentation, as follows. The temperature was measured at the inlet and outlet of each main component by means of 24 type T thermocouples (copper/constantan) insulated with fiberglass. In order to facilitate heat transfer, the thermowells were filled with vacuum oil. Each thermocouple was calibrated separately using a constant temperature source. The thermocouples were calibrated in a range between 20 °C and 140 °C. Using the least-squares method, polynomial equations were obtained for the thermocouples and subsequently incorporated into a previously written computer program. The correlation coefficient for the equations was higher than 0.996. Temperatures were recorded with an accuracy of 0.1 °C. Two Bourdon gauges were utilized in the equipment in order to obtain qualitative pressure readings during the experimental process. One of these was connected to the condenser, and the other to the absorber. In addition to the gauges, three pressure transducers were used to measure the pressure in the low, intermediate, and high zones. The transducers were diaphragm types with a maximum error of 0.05% and a working range between 0.1 and 22.1 bar. The pressure transducers were previously calibrated, obtaining equations with correlation coefficients
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higher than 0.99. Pressure transducers with a design exactitude of ±0.25% within the total range were used to measure low and high pressures. The power required for the pressure transducers was supplied by a power source (Lambda, model LQ-302). The electrical power supplied by electrical resistance placed in a spiral winding along the test section and second preheater were measured by a voltmeter (Keithley, model 169-DMM) and an ammeter (Amprobe, model ACD-10). The experimental data were gathered by a data logging system, namely the Hewlett Packard model 2310A with a maximum capacity of 29 channels. The solution and refrigerant flows were measured with analogical flowmeters with a reading accuracy of ±2%. The H2O/LiBr concentrations in the generator and absorber were measured using a refractometer with an accuracy of ±0.0002. In order to supply heat to the evaporator and generator, electrical heaters of 1 kW were placed inside these components. The current intensity and voltage readings for determining the power supplied by the heaters were measured with accuracies of 0.01 A and 0.01 V, respectively. The instrumentation was connected to a data logger, namely Agilent model 34970A, with two acquisition cards of 20 modules for voltage and two for current, as well as one card with modules for digital signal and voltage suppliers. The system was controlled through a Hewlett-Packard Visual Engineering Environment (HPVEE) program. 3. Important parameters Several design parameters exist for a heat-driven absorption heat transformer, and in this we took the following into consideration: (i) gross temperature lift, (ii) economizer effectiveness, and (iii) coefficient of performance. 3.1. Gross temperature lift The gross temperature lift (GTL) is defined as the difference between the temperature of the useful heat produced in the absorber and that of the heat supplied to the generator and evaporator, which are the same in this study. Thus, according to Fig. 2, the GTL can be written as:
GTL ¼ T 11 T 7 :
ð1Þ
3.2. Economizer effectiveness The economizer is used to recover heat energy in AHTs. In particular, the economizer preheats the strong salt solution flowing from the generator to the absorber by using the heat supplied from the weak salt solution flowing from the absorber to the generator. The economizer effectiveness (EFEC) can by defined as the actual heat recovered to the maximum possible heat that can be recovered. Therefore, from the energy balance and according to Fig. 2, the economizer effectiveness can be written as:
EF EC ¼
_ 8 ðh10 h9 Þ m : _ 11 ðh13 h9 Þ m
ð2Þ
3.3. Coefficients of performance (COP) The COP is an important parameter, as it represents the efficiency of an AHT. It is defined as the useful heat delivered in the absorber per unit of heat load supplied to the generator and evaporator plus the work done by the pumps.
COP ¼
Q_ AB _ _P Q GE þ Q_ EV þ W
ð3Þ
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Table 1 Temperatures, concentrations, heat loads and coefficients of performance of the double-absorption heat transformer.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
TE (°C)
TC (°C)
TG (°C)
TG, IN (°C)
TE,OUT (°C)
TA (°C)
TA,IN (°C)
TAB, OUT (°C)
TAE (°C)
TAE, IN (°C)
TAE, OUT (°C)
TOIL, (°C)
66.19 62.29 55.41 57.27 42.49 45.78 48.60 46.11 50.54 52.89 47.30 50.48 44.49 50.69 52.74 34.90 39.08 37.32 36.69 37.71 36.92 35.81 35.31 42.34 42.65 44.02 41.82 44.11 47.21 44.79 45.17 49.01 45.68 44.23 42.30 44.79 40.29 46.47 39.69 41.27 44.89 36.61 36.61 41.58 46.58 38.83 43.92 47.30 46.02 50.33 52.92
35.82 35.64 35.72 46.39 35.49 35.60 36.44 35.10 37.48 39.21 34.81 34.96 33.11 34.98 36.13 29.47 32.15 31.73 33.56 31.95 31.31 30.58 29.50 38.72 35.42 37.00 34.65 33.94 32.83 32.74 32.21 32.80 32.55 31.27 30.89 31.90 30.60 31.81 31.85 30.92 31.65 31.89 31.88 32.44 33.67 31.84 32.43 35.19 31.49 33.12 33.64
71.06 71.32 77.02 98.35 75.17 72.70 75.01 72.93 72.82 74.41 71.76 68.91 73.75 72.22 67.20 66.91 72.13 69.79 69.43 71.51 70.95 71.92 73.27 86.77 81.91 84.40 80.99 76.40 76.77 77.26 77.61 77.48 77.76 78.00 77.89 78.65 75.47 76.47 76.91 74.53 75.56 76.78 77.33 79.23 81.25 77.68 78.65 85.08 75.95 79.53 80.75
71.92 72.61 78.82 96.63 74.54 71.18 74.26 72.82 72.21 73.71 71.81 68.97 73.13 71.97 68.27 67.24 72.86 70.56 69.92 72.19 72.02 72.97 74.58 87.00 81.58 84.26 81.60 76.63 77.81 78.27 78.67 78.47 79.14 79.28 78.68 80.87 75.68 76.66 76.73 74.22 76.74 76.80 77.99 78.81 81.97 78.04 79.96 84.88 77.82 81.25 82.77
70.27 70.81 76.12 97.61 74.13 71.11 73.98 71.91 71.86 73.50 70.82 67.92 72.70 71.22 66.80 66.10 66.10 69.52 69.40 71.19 70.45 71.39 72.57 86.00 81.04 83.55 80.21 75.62 75.93 76.46 77.01 77.18 77.36 77.94 77.48 78.32 75.23 75.82 23.89 74.13 74.94 24.97 25.03 78.95 80.60 77.19 77.89 84.36 75.13 78.70 79.86
105.81 108.09 102.42 108.54 105.59 103.26 99.72 103.78 104.15 107.92 103.48 99.11 102.16 105.69 102.77 95.50 99.23 96.87 93.93 100.63 100.97 104.65 106.77 114.94 108.06 110.87 109.73 105.32 109.49 109.01 111.05 110.21 111.29 110.85 111.58 112.12 109.74 118.64 120.27 108.50 111.58 115.40 116.43 110.80 118.57 116.61 120.02 121.82 118.07 122.42 128.08
83.07 83.26 86.08 97.36 94.14 91.83 91.47 91.64 91.01 93.86 90.94 87.25 91.41 91.87 88.99 86.70 89.28 86.90 84.65 89.48 89.41 91.84 94.35 106.97 99.78 102.48 99.76 94.92 98.27 98.01 99.40 99.42 99.78 99.26 100.29 100.94 97.24 101.90 101.59 97.39 99.64 102.18 102.45 101.85 107.78 103.63 107.11 109.65 104.92 109.22 113.42
97.15 98.93 93.38 100.52 95.64 91.13 91.56 93.85 94.00 97.13 93.95 90.55 92.43 95.76 93.25 88.77 89.71 87.55 85.12 91.23 91.05 94.23 96.83 106.37 99.40 102.06 100.10 96.30 101.00 100.24 102.09 101.44 103.02 101.01 103.36 102.56 99.60 105.04 108.39 99.49 104.19 104.97 107.09 102.63 107.42 108.08 107.43 113.67 108.22 112.32 115.53
74.98 74.55 74.69 84.30 73.88 74.31 78.78 75.54 78.36 78.89 77.11 79.28 77.58 79.87 75.84 64.71 69.58 67.65 66.58 69.03 68.62 69.21 68.43 75.91 72.61 74.74 71.35 68.46 70.75 69.72 71.20 70.74 71.66 70.48 71.43 72.28 70.45 75.55 77.50 70.20 71.88 75.23 76.19 70.93 74.67 74.15 77.48 77.33 78.13 80.97 83.72
83.5 84.4 81.7 77.9 82.6 80.2 81.2 81.9 81.3 83.8 81.2 78.1 81.2 81.7 79.5 76.4 80.2 78.1 76.1 79.7 79.9 80.7 82.0 88.3 88.4 88.2 86.5 84.1 87.6 86.4 87.8 87.1 87.9 87.1 88.0 88.6 85.6 89.4 88.4 85.8 87.5 87.8 88.3 89.6 94.2 89.0 91.5 94.8 91.8 96.2 99.4
70.4 71.7 69.0 74.4 69.5 69.1 73.6 72.0 71.5 72.9 70.4 67.6 72.5 70.4 68.2 61.7 67.1 65.4 64.5 66.3 66.3 65.2 65.4 72.8 69.2 71.9 68.4 66.1 67.3 67.1 68.6 67.6 67.9 66.9 67.1 67.6 66.6 71.8 73.5 66.7 68.6 71.7 72.6 66.8 71.7 70.8 73.9 74.1 74.6 77.9 79.0
71.1 71.5 71.9 69.4 65.5 65.7 69.9 67.4 69.3 67.2 66.0 61.9 68.0 67.4 69.8 69.6 67.1 67.2 66.0 73.0 77.8 78.2 82.0 90.0 76.6 79.1 82.2 81.0 88.4 85.2 91.7 92.4 93.5 92.3 91.5 92.3 93.8 99.4 103.5 93.2 97.8 101.8 106.7 94.2 108.1 104.2 108.3 109.4 108.2 113.9 117.2
IN
TOil, OUt (°C)
TWA, (°C)
92.8 94.0 90.0 95.5 90.0 95.5 91.1 92.8 94.5 97.0 94.8 91.7 90.7 96.2 87.1 85.3 85.5 82.4 81.4 88.0 86.8 91.7 94.7 103.5 95.2 98.9 96.9 93.0 97.8 96.7 99.5 99.0 99.1 98.0 96.2 99.9 98.4 105.3 107.8 97.5 101.0 106.0 108.0 101.0 109.6 106.1 108.9 112.2 109.2 113.9 118.2
27.2 27.5 28.2 29.0 28.1 28.1 28.2 26.6 26.8 27.0 26.8 25.8 26.5 26.8 26.6 26.0 26.7 26.7 27.0 27.0 26.9 26.8 25.7 30.0 29.0 29.2 28.4 28.9 29.0 28.9 28.6 28.4 28.7 28.1 27.4 27.4 26.8 27.4 27.4 27.4 27.8 28.2 28.0 28.3 28.8 28.0 28.0 28.6 27.0 28.5 29.0
IN
TWA, (°C)
OUT
36.2 36.4 36.4 35.1 36.0 35.4 36.5 35.0 35.5 35.3 35.7 34.9 33.7 35.0 32.0 29.8 32.3 31.6 33.9 30.8 31.1 30.8 29.7 33.2 32.0 32.0 31.4 32.1 32.6 32.3 32.2 32.0 32.3 31.5 30.9 32.2 30.0 31.6 31.5 31.0 31.9 32.1 32.2 31.2 32.3 31.8 32.4 31.7 32.2 33.8 34.4
XG (%)
XAE (%)
XA (%)
QC,IN (W)
QE,IN (W)
QG,IN (W)
QAE,IN (W)
QA,IN (W)
QE,EX (W)
QG,EX (W)
QAE,EX (W)
QA,EX (W)
EFE (-)
GTL (°C)
DT (°C)
COPIN (-)
COPEX (-)
53.6 55.2 57.3 53.7 55.9 54.6 55.0 54.8 52.9 52.4 55.1 53.1 57.4 54.6 54.4 56.5 56.8 55.8 55.8
52.1 52.9 51.9 51.5 53.3 51.4 52.1 52.0 50.5 49.1 51.8 50.1 54.3 51.4 52.3 56.3 56.4 55.6 55.7
52.6 53.7 53.8 52.2 54.9 52.8 53.4 52.8 51.9 52.1 54.0 52.6 56.0 54.4 53.0 55.7 56.5 55.2 54.5
332.1 331.4 331.7 386.6 330.8 398.5 536.2 531.1 405.6 344.9 395.5 409.5 321.9 328.9 467.9 321.5 318.2 316.6 323.6
136.1 131.9 131.5 169.9 265.0 266.1 337.4 332.1 205.8 144.8 195.5 196.5 120.9 128.6 269.2 106.2 116.7 115.0 121.9
289.7 312.5 298.7 687.2 433.3 487.6 618.7 579.7 402.3 376.0 426.9 382.5 363.8 341.0 442.9 429.9 418.0 370.0 437.3
439.2 457.5 211.4 224.1 567.8 550.3 570.8 611.8 431.3 368.7 481.2 420.4 250.9 416.9 554.3 369.5 328.2 354.4 354.1
101.5 96.5 67.8 283.0 113.8 290.7 386.1 325.8 206.9 221.4 255.8 221.9 184.9 189.0 244.9 158.6 237.9 206.9 228.7
207.0 148.8 147.0 210.6 142.8 192.9 260.7 293.9 292.6 293.9 294.3 351.4 289.1 347.0 305.3 308.7 303.7 293.3 288.5
727.3 727.3 726.0 726.0 733.9 733.9 733.9 726.0 728.6 730.6 730.4 728.6 731.9 728.6 699.6 743.8 1045.2 891.6 754.4
236.5 281.2 280.0 226.2 285.1 226.3 169.5 203.3 154.9 102.5 203.0 155.2 203.3 154.7 204.6 203.9 206.6 211.7 215.7
178.6 189.1 141.8 223.3 262.4 243.8 169.6 209.3 212.8 260.3 244.2 251.8 181.1 248.6 130.6 115.3 135.6 107.0 106.3
0.39 0.40 0.39 0.31 0.37 0.35 0.34 0.38 0.35 0.37 0.38 0.38 0.35 0.37 0.43 0.43 0.36 0.37 0.30
39.6 45.8 47.0 51.3 63.1 57.5 51.1 57.7 53.6 55.0 56.2 48.6 57.7 55.0 50.0 60.6 60.2 59.5 57.2
21.71 22.58 18.12 26.09 30.44 28.86 21.15 25.44 25.22 29.77 28.81 29.84 22.72 28.76 17.29 15.69 18.40 15.25 15.40
0.18 0.16 0.12 0.28 0.14 0.32 0.35 0.31 0.28 0.33 0.33 0.31 0.30 0.31 0.29 0.23 0.35 0.33 0.33
0.19 0.22 0.16 0.24 0.30 0.26 0.17 0.21 0.21 0.25 0.24 0.23 0.18 0.23 0.13 0.11 0.10 0.09 0.10
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W. Rivera et al. / Applied Thermal Engineering 132 (2018) 432–440 Table 1 (continued)
20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51
XG (%)
XAE (%)
XA (%)
QC,IN (W)
QE,IN (W)
QG,IN (W)
QAE,IN (W)
QA,IN (W)
QE,EX (W)
QG,EX (W)
QAE,EX (W)
QA,EX (W)
EFE (-)
GTL (°C)
DT (°C)
COPIN (-)
COPEX (-)
56.8 57.2 59.1 60.1 60.7 59.0 59.3 59.5 58.3 58.7 59.1 58.9 58.2 58.7 59.6 60.6 60.4 58.7 59.0 59.3 58.5 58.9 59.4 59.5 58.9 60.2 58.9 59.3 60.3 60.2 60.0 60.5
56.5 57.1 58.5 59.9 60.3 58.8 58.9 59.1 58.1 58.5 58.7 58.5 57.8 58.1 58.9 60.3 60.0 58.2 58.4 58.5 58.0 58.3 59.1 59.1 57.9 59.3 58.0 58.8 59.8 59.0 59.0 59.8
56.4 56.6 58.0 59.0 59.5 57.9 58.1 58.3 57.7 58.0 57.9 57.9 57.4 57.8 58.6 59.5 59.3 57.7 57.9 57.8 57.1 57.8 58.4 58.5 57.7 58.6 57.6 58.3 58.9 57.9 58.7 59.6
317.5 315.0 312.3 308.2 342.9 330.6 336.5 327.7 325.0 320.8 320.5 318.5 320.7 319.8 314.9 380.8 330.8 325.8 343.9 344.1 327.1 343.2 344.2 344.2 319.4 350.9 330.5 346.3 343.2 329.2 348.9 364.3
115.9 113.4 110.5 106.3 141.9 129.5 135.6 126.5 124.1 120.1 119.5 117.5 120.1 118.9 113.9 112.2 116.3 110.9 116.1 115.7 112.2 115.5 115.8 115.8 118.1 123.2 115.6 118.3 129.1 114.8 121.4 123.7
421.5 409.4 470.0 450.8 616.0 604.5 609.0 586.8 524.6 488.0 512.5 515.4 532.7 528.8 558.0 618.5 533.5 550.7 502.8 514.3 552.3 500.1 517.4 513.7 639.6 610.6 543.7 534.7 600.2 470.1 420.8 429.5
334.3 326.5 361.0 353.3 414.9 443.2 409.0 418.2 412.6 446.8 429.6 434.6 447.6 454.4 438.3 418.2 428.2 425.3 416.7 375.2 429.2 414.8 378.0 374.9 508.8 494.9 431.7 416.9 457.6 407.3 577.6 438.8
193.6 195.5 221.4 214.7 328.9 289.5 302.1 264.6 239.5 181.9 253.9 195.0 215.0 183.8 213.6 251.1 203.0 204.8 210.1 111.7 188.5 195.7 141.3 131.7 243.7 328.8 100.7 300.9 195.3 196.2 229.2 279.9
289.8 287.7 271.6 270.9 266.7 292.6 281.0 268.0 265.4 265.4 260.0 261.2 255.4 249.9 256.6 265.6 318.2 272.9 272.7 268.8 222.3 222.3 216.3 216.3 225.5 207.9 211.7 204.8 181.9 267.5 255.8 245.3
740.3 743.6 739.9 743.8 749.5 744.5 742.5 750.4 738.5 735.9 745.8 747.1 751.0 739.6 759.8 751.5 875.7 740.8 754.5 749.0 750.3 750.7 744.9 747.6 737.0 739.9 742.6 750.5 740.6 742.6 745.8 748.1
213.7 216.7 211.0 216.7 211.1 211.7 212.7 215.4 216.4 217.7 216.4 216.7 216.4 215.4 213.7 214.0 257.8 217.1 210.3 210.7 217.1 213.9 216.0 216.4 215.7 215.0 216.3 217.7 217.1 210.7 211.0 209.0
114.5 67.8 109.1 106.9 133.9 158.8 177.5 127.8 98.5 83.9 100.3 70.1 59.5 50.2 50.9 40.0 69.1 40.7 60.1 47.1 37.6 29.9 44.6 15.0 60.0 14.5 20.0 7.2 33.6 10.9 1.4 12.2
0.36 0.38 0.39 0.39 0.35 0.34 0.33 0.36 0.35 0.39 0.38 0.38 0.38 0.39 0.37 0.38 0.38 0.38 0.43 0.38 0.42 0.41 0.44 0.43 0.37 0.46 0.41 0.51 0.37 0.48 0.49 0.52
62.9 64.1 68.8 71.5 72.6 65.4 66.8 67.9 61.2 62.3 64.2 65.9 61.2 65.6 66.6 69.3 67.3 69.4 72.2 80.6 67.2 66.7 78.8 79.8 69.2 72.0 77.8 76.1 74.5 72.1 72.1 75.2
14.96 9.02 13.48 12.63 13.50 18.63 19.77 14.63 11.94 9.48 11.52 7.74 6.61 5.58 5.75 4.63 7.60 4.56 5.82 4.30 4.27 3.20 4.25 1.37 6.81 1.55 1.90 0.64 2.89 0.97 1.03 1.01
0.29 0.29 0.31 0.31 0.36 0.33 0.34 0.31 0.30 0.24 0.33 0.25 0.27 0.23 0.26 0.29 0.26 0.25 0.28 0.14 0.23 0.26 0.18 0.17 0.27 0.38 0.13 0.38 0.22 0.27 0.34 0.40
0.11 0.07 0.11 0.11 0.13 0.15 0.17 0.13 0.10 0.08 0.10 0.07 0.06 0.05 0.05 0.04 0.06 0.04 0.06 0.05 0.04 0.03 0.05 0.02 0.06 0.02 0.02 0.01 0.04 0.01 0.01 0.01
The COP can be internal, when is calculated based on the internal streams, or external, when is estimated based on external streams or heat loads. From the mass and energy balances and based on Fig. 2, the internal COP be written as:
COP INT ¼
_ 5 h5 þ m _ 10 h10 m11 h11 m _P _ 8 h8 m _ 15 h15 þ m _ 6 ðh7 h6 Þ þ W _ 1 h1 þ m m
ð4Þ
and the external COP can be calculated as:
COP EXT ¼
_ 16 C Poil ðT 17 T 16 Þ m
_P V GE IGE þ V EV IEV þ W
;
ð5Þ
where CPoil is the specific heat of the oil circulating in the heat recovery system, VGE and IGE are the voltage and electric current supplied to the electrical heater in the generator, and VGE and IGE are the voltage and electric current supplied to the electrical heater in the evaporator. The physical and thermodynamic properties for the H2O/LiBr mixture were taken from McNeely [63]. 4. Results In order to evaluate the DAHT, more than 70 test runs were carried out at different temperatures and solution concentrations; however, only 51 were considered in this study, as they were the only ones to reach the steady state conditions. The steady state was considered when the system temperatures varied by less than 1 °C over a period of one hour. Table 1 shows the temperatures, concentrations, heat loads and coefficients of performance for the 51 experimental test runs. An uncertainty evaluation was carried out based on the National Institute of Standards of Technology (NIST) uncertainty propagation, with the actual instrumentation accuracy from Sec-
tion 2.3. The GTL uncertainty value was approximately 0.1414 °C for all operating conditions. For the economizer effectiveness, the uncertainty was approximately 0.06325, with a significant dependency on the mass flow values. The COPINT uncertainty values were approximately 0.066 for TAB from 90 to 130 °C, while the COPEXT uncertainty values were approximately 0.0100, depending mainly on the mass flow values. Fig. 4 illustrates the GTL versus generation solution concentration. It can be seen that the GTL increases linearly with the increment in generator solution concentration. This behavior was expected, as a higher concentration of LiNiO3 (in the solution leaving the generator) means that a greater amount of water was generated in the generator, which will be absorbed into the absorber posteriorly. On the other hand, it can be seen that the GTLs vary between 48 °C and 74 °C. According to Eq. (1), this means that the heat transformer can increase the absorber temperature between 48 °C and 74 °C over the temperature of the heat supplied to the evaporator. Based on this figure, it appears to be desirable to increase the LiNiO3 concentration in the solution even further; however, it is important to remember that crystallization problems may occur at higher concentration values. On the other hand, it is important to note that, from the bibliographic review, it is clear that with single-stage heat transformers, it is possible to achieve GTLs of up to 44 °C, while it is possible to achieve up to 74 °C using the developed system. Fig. 5 illustrates the GTL versus absorber solution concentration, and a similar trend can be observed to that in Fig. 4. Once again, this behavior was expected, because at a higher LiNiO3 concentration in the solution inside the absorber, its capacity is increased to absorb greater amounts of water, thus increasing its temperature due to the exothermic reaction during the absorption process. The GTL values were similar to those reported in the previous figure. Fig. 6 illustrates the internal COP versus internal useful heat produced in the absorber. It can be seen that the internal COP
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increases with an increment in the absorber heat load. This occurs because, according to Eqs. (3) and (4), the internal COP is directly
Fig. 4. GTL versus generator solution concentration.
Fig. 5. GTL versus absorber solution concentration.
proportional to the absorption heat load. Furthermore, it can be observed that the inlet COPs vary between 0.12 and 0.37, meaning that up to 37% of the energy supplied to the generator and evaporator could be reused as useful heat at higher temperature levels. According to the literature review, it is possible to achieve internal COPs close to 0.5 with single-stage heat transformers, while in this work we achieve COPs no higher than 0.37. This is the price paid by double-absorption systems in order to produce useful heat at higher temperature levels. Fig. 7 illustrates the external COPs against the external absorber heat load. It can once again be observed that the external COP increases with an increment in the external absorption heat load, for the same reasons as those explained in Fig. 6. However, it can be seen in Fig. 7 that the external COPs are lower than the internal coefficient reported in Fig. 6. This occurs because the internal values do not consider the system heat losses to the atmosphere, while the external values do. Comparing Figs. 6 and 7, it can be observed that higher data dispersion of the COP values exists in Fig. 6 than in Fig. 7. This occurs because the internal COP depends on a greater number of variables (see Eq. (4)), while the external values depend only on several variables, which are also easy to control (see Eq. (5)). Fig. 8 illustrates the external COP against the absorber temperature. It can be observed that the external COP decreases drastically with an increase in absorber temperature. This occurs because when the absorber temperature increases, the concentration differences between the strong and weak solutions decrease considerably, tending to zero at high absorber temperatures, which means that the refrigerant production decreases considerably; thus, the COP decreases abruptly. Fig. 9 illustrates the economizer efficiency versus absorber temperature. It can be seen that EFEC increases with an increment in the absorber temperature. This occurs because, as the absorber temperature increases, the heat gained by the solution moving from the generator to the absorber increases, thereby increasing economizer efficiency.
Fig. 6. Internal COP as function of internal absorber heat load.
W. Rivera et al. / Applied Thermal Engineering 132 (2018) 432–440
Fig. 7. Internal coefficient of performance as function of internal absorber heat load.
439
Fig. 9. Economizer efficiency as function of absorber temperature.
5. Conclusions This paper has reported on the experimental results of a DAHT operating with a H2O/LiBr mixture. The results indicated that the system can achieve GTLs of between 48 °C and 74 °C, with internal COPs varying from 0.12 to 0.37, and external COPs of up to 0.3. From the bibliographic review, it was clear that it is possible to achieve GTLs of up to 44 °C with single-stage heat transformers operating with the same working mixture; however, with the actual system, it was possible to achieve GTLs of up to 74 °C, which is 30 °C more than with single-stage systems. Acknowledgements The authors thank the projects SENER-CONACyT 117914 and CONACYT 154301 for the economic support provided for the development of this study. References
Fig. 8. External COP as function of absorber temperature.
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