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Procedia Engineering 00 (2017) 000–000 ScienceDirect www.elsevier.com/locate/procedia Procedia Engineering 205 (2017) 241–247
10th International Symposium on Heating, Ventilation and Air Conditioning, ISHVAC2017, 19- 22 October 2017, Jinan, China Performance Comparison between an Absorption-compression Hybrid Refrigeration System and a Double-effect Absorption Refrigeration Sys-tem
Jian Wang, Xianting Li, Baolong Wang, Wei Wu, Pengyuan Song, Wenxing Shi*
Beijing Key Laboratory of Indoor Air Quality Evaluation and Control, Department of Building Science, Tsinghua University, Haidian District, Beijing 100084, China
Abstract
Conventional absorption refrigeration systems (ARSs), which are mainly driven by heat, have a competitive primary energy efficiency (PEE) compared with chillers driven by electricity. In the typical ARS, a large quantity of high-temperature heat is supplied into the generator, while a substantial amount of low-temperature heat is rejected to the environment from the condenser, it’s a huge waste. In order to decrease the input heat for generator and enhance the performance of conventional ARS, a kind of absorption-compression hybrid refrigeration system recovering condensation heat for generation (RCHG-ARS) was ever proposed. In the present study, the models of both RCHG-ARS and double-effect absorption refrigeration system (DEARS) are established, and the effects of different parameters on them are simulated and compared with each other. As a conclusion, the PEE of RCHG-ARS can be 29.0% higher than that of DEARS, and RCHG-ARS has a wider working conditions than DEARS due to the existence of the compressor. © 2017 The Authors. Published by Elsevier Ltd. © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the 10th International Symposium on Heating, Ventilation and Air Peer-review under responsibility of the scientific committee of the 10th International Symposium on Heating, Ventilation and Conditioning. Conditioning.Air Conditioning.
Keywords: Absorption chiller; Absorption-compression cycle; Condensing heat recovery; Ammonia-water; Simulation.
1. Introduction
Air-conditioning consumes a large quantity of energy in China [1]. And due to the merits of energy saving and environmental protection, absorption refrigeration systems (ARSs) are widely used both in commercial and industrial projects. However, in conventional ARS, much low-temperature heat is exhausted to the environment
* Corresponding author. Tel.: +86-10-62796114; fax: +86-10-62796114. E-mail address: [email protected]
1877-7058 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the 10th International Symposium on Heating, Ventilation and Air Conditioning.
1877-7058 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the 10th International Symposium on Heating, Ventilation and Air Conditioning. 10.1016/j.proeng.2017.09.959
10.1016/j.proeng.2017.09.959 1877-7058 242 Jian Wang et al. / Procedia Engineering 205 (2017) 241–247 2 Jian Wang et al. / Procedia Engineering 00 (2017) 000–000 Jian Wang et al./ Procedia Engineering 00 (2017) 000–000 3 through the condenser, while the generator requires a large quantity of high-temperature heat at the same time. Thus, ηh energy transformation efficiency from natural gas to heat recovering the condensation heat is an important method to enhance the performance of the absorption refrigerator. ηisen isentropic efficiency of compressor Multi-effect system is such one kind common method, including double-effect ARS (DEARS) shown in Fig. 1(a). In this system, part of the condensation heat originally rejected from the system is recovered for the generation of Abbreviations refrigerant in low-temperature cycles, be-cause the refrigerant vapor from the high-temperature generator condenses in the lower-temperature generator. Therefore, the generation heat inputted from outside is reduced, which leads to a ARS absorption refrigeration system higher COP than single-effect ARS [2]. Kaushik and Arora [3] studied single-effect and DEARSs by simulation, and COP coefficient of performance their results showed that the COP of the double-effect system was nearly 60-70% higher than that of the single- DEARS double-effect absorption refrigeration system effect sys-tem. Kim et al. [4] simulated a single-effect ARS, a double-effect ARS and a triple-effect ARS, finding PEE primary energy efficiency that the COPs of them can be 0.75, 1.24 and 1.54, respectively. However, the lowest driving temperatures of RCHG-ARS absorption-compression hybrid refrigeration system recovering condensation heat for generation DEARS and triple-effect system separately are as high as 140 oC and 150 oC [5]. What’s more, part of the condensation heat in DEARS is still exhausted to the environment directly. Subscripts To fully recover condensation heat, an absorption-compression hybrid refrigeration system recovering condensation heat for generation (RCHG-ARS) was studied and compared with single-effect ARS [6]. Just as Fig. c compressor 1(b) shows, the refrigerant vapor from the generator was compressed by the compressor, and then it condensed and de double-effect absorption refrigeration system released heat in the generator, providing heat for the generation of refrigerant together with/without assistance of a EV evaporator driving heat source from outside. So all condensation heat was recovered for the generation process by improving GE generator the grade of condensation heat through the compressor. Its principle is similar with that of DEARS. Therefore, the in inlet objective of this work is to compare these two systems. new new ARS, RCHG-ARS old conventional ARS out outlet
2. Modeling
o In the systems, the evaporation temperature can reach -30 C, so the working fluids are NH3-H2O instead of H2O- LiBr. And the analyses are completed with the help of the Engineering Equation Solver (EES) software. The models are built up according to mass and energy conservation [6].
mm= (1) in out
mx··= m x (2) in in out out
Q+= mh·· m h (3) in in out out
where m is the mass flow rate, kg/s; x is the mass concentration of NH3, %; Q is the heat ex-change rate, kW; h is the specific enthalpy, kJ/kg. The electricity consumed at every stage in the compressor is calculated by [7]: (a) DEARS (b) RCHG-ARS hhout,, i− in i Fig. 1. Principles of DEARS and RCHG-ARS. Wmci = 3· (4) ηisen Nomenclature where i=1 or 2, means the first or second stage compressor, respectively; hout,i is the specific enthalpy of refrigerant at the exit of each stage compressor when the process is isentropic; hin,i is the specific enthalpy of h specific enthalpy, kJ/kg refrigerant at the inlet of each stage compressor; and ηisen is the is-entropic efficiency of each stage compressor, m mass flow rate, kg/s equal to 0.6 [7]. Q heat exchange rate, kW And primary energy efficiency (PEE) is selected to compare system efficiency. For RCHG-ARS: x mass concentration of NH3, % W compressor power consumption, kW ηe energy transformation efficiency from natural gas to electricity Jian Wang et al. / Procedia Engineering 205 (2017) 241–247 243 2 Jian Wang et al. / Procedia Engineering 00 (2017) 000–000 Jian Wang et al./ Procedia Engineering 00 (2017) 000–000 3 through the condenser, while the generator requires a large quantity of high-temperature heat at the same time. Thus, ηh energy transformation efficiency from natural gas to heat recovering the condensation heat is an important method to enhance the performance of the absorption refrigerator. ηisen isentropic efficiency of compressor Multi-effect system is such one kind common method, including double-effect ARS (DEARS) shown in Fig. 1(a). In this system, part of the condensation heat originally rejected from the system is recovered for the generation of Abbreviations refrigerant in low-temperature cycles, be-cause the refrigerant vapor from the high-temperature generator condenses in the lower-temperature generator. Therefore, the generation heat inputted from outside is reduced, which leads to a ARS absorption refrigeration system higher COP than single-effect ARS [2]. Kaushik and Arora [3] studied single-effect and DEARSs by simulation, and COP coefficient of performance their results showed that the COP of the double-effect system was nearly 60-70% higher than that of the single- DEARS double-effect absorption refrigeration system effect sys-tem. Kim et al. [4] simulated a single-effect ARS, a double-effect ARS and a triple-effect ARS, finding PEE primary energy efficiency that the COPs of them can be 0.75, 1.24 and 1.54, respectively. However, the lowest driving temperatures of RCHG-ARS absorption-compression hybrid refrigeration system recovering condensation heat for generation DEARS and triple-effect system separately are as high as 140 oC and 150 oC [5]. What’s more, part of the condensation heat in DEARS is still exhausted to the environment directly. Subscripts To fully recover condensation heat, an absorption-compression hybrid refrigeration system recovering condensation heat for generation (RCHG-ARS) was studied and compared with single-effect ARS [6]. Just as Fig. c compressor 1(b) shows, the refrigerant vapor from the generator was compressed by the compressor, and then it condensed and de double-effect absorption refrigeration system released heat in the generator, providing heat for the generation of refrigerant together with/without assistance of a EV evaporator driving heat source from outside. So all condensation heat was recovered for the generation process by improving GE generator the grade of condensation heat through the compressor. Its principle is similar with that of DEARS. Therefore, the in inlet objective of this work is to compare these two systems. new new ARS, RCHG-ARS old conventional ARS out outlet
2. Modeling
o In the systems, the evaporation temperature can reach -30 C, so the working fluids are NH3-H2O instead of H2O- LiBr. And the analyses are completed with the help of the Engineering Equation Solver (EES) software. The models are built up according to mass and energy conservation [6].
mm= (1) in out
mx··= m x (2) in in out out
Q+= mh·· m h (3) in in out out
where m is the mass flow rate, kg/s; x is the mass concentration of NH3, %; Q is the heat ex-change rate, kW; h is the specific enthalpy, kJ/kg. The electricity consumed at every stage in the compressor is calculated by [7]: (a) DEARS (b) RCHG-ARS hhout,, i− in i Fig. 1. Principles of DEARS and RCHG-ARS. Wmci = 3· (4) ηisen Nomenclature where i=1 or 2, means the first or second stage compressor, respectively; hout,i is the specific enthalpy of refrigerant at the exit of each stage compressor when the process is isentropic; hin,i is the specific enthalpy of h specific enthalpy, kJ/kg refrigerant at the inlet of each stage compressor; and ηisen is the is-entropic efficiency of each stage compressor, m mass flow rate, kg/s equal to 0.6 [7]. Q heat exchange rate, kW And primary energy efficiency (PEE) is selected to compare system efficiency. For RCHG-ARS: x mass concentration of NH3, % W compressor power consumption, kW ηe energy transformation efficiency from natural gas to electricity 4244 Jian WangJian etWang al. / Procediaet al. / Procedia Engineering Engineering 00 (2017) 205 000–000 (2017) 241 –247 Jian Wang et al./ Procedia Engineering 00 (2017) 000–000 5
QEV, new PEEnew = (5) QGE, new/η h++ ( WW c 12 c )/η e
where QEV,new is the cooling capacity in RCHG-ARS, kW; QGE,new is the generation heat in RCHG-ARS, kW; Wc1 and Wc2 are electricity consumed by the first-stage and second-stage compressor separately, kW; ηh is the energy transformation efficiency from natural gas to heat, equal to 0.9; ηe is the energy transformation efficiency from natural gas to electricity, with a value of 0.5 [8]. For DEARS:
QEV, de PEE = (6) de Q /η GE,de h where QEV,de is the cooling capacity in DEARS, kW; QGE,de is the generation heat in DEARS, kW.
3. Results
To compare RCHG-ARS and DEARS, the effects of generation temperature and evaporation temperature are analysed.
Fig. 2. Effects of TGE on PEE for RCHG-ARS and DEARS. 3.1. Effects of generation temperature (TGE)
o o o o And in Figure. 2, the lowest TGE for RCHG-ARS is 35 C, but DEARS only can work when TGE is higher than 99 Fig. 2 illustrates the effects of TGE on PEE for these two systems when TCO=30 C, TEV=5 C and TAB=30 C. In oC. The reason for this phenomenon is that the generation pressure in RCHG-ARS may be lower than that of this part, the generation pressure in DEARS is 11.67 bar, and several different generation pressures are chosen in DEARS due to the existence of the compressor, therefore, the solution concentration difference in RCHG-ARS is RCHG-ARS, because they can be lower than due to the existence of the compressor. sufficiently large to make the system work effectively when TGE is low, while DEARS might stop under the same It can be seen from Figure. 2 that the PEE of RCHG-ARS increases rapidly and then de-creases gradually, and its conditions. variation trend is similar with that of DEARS. But, the highest PEE of RCHG-ARS is 1.245, which is 29.0% higher than 0.9653 of DEARS. RCHG-ARS im-proves the grade of all condensation heat to replace part of generation heat inputted from out-side at the cost of a little electricity, but DEARS only can recover part of the condensation heat. Thus the generation heat needed in RCHG-ARS should be smaller, just as Figure. 3 shows, the generation heat inputted from outside of RCHG-ARS is 23.3-69.5% less than that of DEARS when the refrigerant mass flows in two systems are same.
Fig. 3. Effects of TGE on QGE and mr for RCHG-ARS and DEARS. 4 Jian Wang et al. / Procedia Engineering 00 (2017) 000–000 Jian JianWang Wang et al./ et Procedia al. / Procedia Engineering Engineering 00 (2017) 205 (2017) 000–000 241 –247 2455
QEV, new PEEnew = (5) QGE, new/η h++ ( WW c 12 c )/η e where QEV,new is the cooling capacity in RCHG-ARS, kW; QGE,new is the generation heat in RCHG-ARS, kW; Wc1 and Wc2 are electricity consumed by the first-stage and second-stage compressor separately, kW; ηh is the energy transformation efficiency from natural gas to heat, equal to 0.9; ηe is the energy transformation efficiency from natural gas to electricity, with a value of 0.5 [8]. For DEARS:
QEV, de PEE = (6) de Q /η GE,de h where QEV,de is the cooling capacity in DEARS, kW; QGE,de is the generation heat in DEARS, kW.
3. Results
To compare RCHG-ARS and DEARS, the effects of generation temperature and evaporation temperature are analysed.
Fig. 2. Effects of TGE on PEE for RCHG-ARS and DEARS. 3.1. Effects of generation temperature (TGE)
o o o o And in Figure. 2, the lowest TGE for RCHG-ARS is 35 C, but DEARS only can work when TGE is higher than 99 Fig. 2 illustrates the effects of TGE on PEE for these two systems when TCO=30 C, TEV=5 C and TAB=30 C. In oC. The reason for this phenomenon is that the generation pressure in RCHG-ARS may be lower than that of this part, the generation pressure in DEARS is 11.67 bar, and several different generation pressures are chosen in DEARS due to the existence of the compressor, therefore, the solution concentration difference in RCHG-ARS is RCHG-ARS, because they can be lower than due to the existence of the compressor. sufficiently large to make the system work effectively when TGE is low, while DEARS might stop under the same It can be seen from Figure. 2 that the PEE of RCHG-ARS increases rapidly and then de-creases gradually, and its conditions. variation trend is similar with that of DEARS. But, the highest PEE of RCHG-ARS is 1.245, which is 29.0% higher than 0.9653 of DEARS. RCHG-ARS im-proves the grade of all condensation heat to replace part of generation heat inputted from out-side at the cost of a little electricity, but DEARS only can recover part of the condensation heat. Thus the generation heat needed in RCHG-ARS should be smaller, just as Figure. 3 shows, the generation heat inputted from outside of RCHG-ARS is 23.3-69.5% less than that of DEARS when the refrigerant mass flows in two systems are same.
Fig. 3. Effects of TGE on QGE and mr for RCHG-ARS and DEARS. 246 Jian Wang et al. / Procedia Engineering 205 (2017) 241–247 6 Jian Wang et al. / Procedia Engineering 00 (2017) 000–000 Jian Wang et al./ Procedia Engineering 00 (2017) 000–000 7
3.2. Effects of evaporation temperature (TEV) References
o o o Fig. 4 shows the effects of TEV on RCHG-ARS and DEARS when TGE,new=75 C, TGE,de=120 C, TCO=30 C and [1] Tsinghua University Building Energy Saving Research Center. 2009 Annual Report on China Building Energy Efficiency. Beijing: China o TAB=30 C. For both RCHG-ARS and DEARS, PEE increases with the increase of TEV, because higher TEV means Architecture and Building Press, 2009. higher absorption pressure, which is beneficial to the absorption process and the improvement of PEE. [2] R. Lizarte, J.D. Marcos. COP optimization of a triple-effect H2O/LiBr absorption cycle under off-design conditions, Applied Thermal Engineering 99 (2016) 195-205. [3] S.C. Kaushik, A. Arora. Energy and exergy analysis of single effect and series flow double effect water-lithium bromide absorption refrigeration systems, International journal of Refrigeration 32 (2009) 1247-1258. [4] J.S. Kim, F. Ziegler, H. Lee. Simulation of the compressor-assisted triple-effect H2O/LiBr ab-sorption cooling cycles, Applied Thermal Engineering 22 (2002) 295-308. [5] L.A. Domínguez-Inzunza, J.A. Hernández-Magallanes, M. Sandoval-Reyes, W. Rivera. Comparison of the performance of single-effect, half- effect, double-effect in series and in-verse and triple-effect absorption cooling systems operating with the NH3-LiNO3 mixture, Applied Thermal Engineering 66 (2014) 612-620. [6] J. Wang, B. Wang, W. Wu, X. Li, W. Shi. Performance analysis of an absorption-compression hybrid refrigeration system recovering condensation heat for generation, Applied Thermal Engineering, 2016, 108: 54-65. [7] R. Ventas, A. Lecuona, A. Zacarías, M. Venegas. Ammonia-lithium nitrate absorption chiller with an integrated low-pressure compression booster cycle for low driving temperatures, Applied Thermal Engineering 30 (2010) 1351-1359. [8] W. Wu, X. Li, T. You, B. Wang, W. Shi. Combining ground source absorption heat pump with ground source electrical heat pump for thermal balance, higher efficiency and better economy in cold regions, Renewable Energy 84 (2015) 74-88.
Fig. 4. Effects of TEV on PEE for RCHG-ARS and DEARS.
Furthermore, Figure. 4 displays that the lowest evaporation temperatures for RCHG-ARS and DEARS are -26 oC and -2 oC, respectively, meaning that RCHG-ARS is capable of achieving cooling capacity at lower temperature. With the help of the compressor, the generation pressure of RCHG-ARS might be much lower than that of DEARS, therefore, even when TEV is low, the solution concentration difference is still large enough to maintain the operation of RCHG-ARS, while DEARS cannot operate.
4. Conclusions
In the present study, the performance of absorption-compression hybrid refrigeration system recovering condensation heat for generation (RCHG-ARS) is compared with double-effect absorption refrigeration system (DEARS), including the effects of generation temperature and evaporation temperature. Accordingly, some conclusions are drawn: (1) The PEE of RCHG-ARS can reach 29.0% higher than that of DEARS, and RCHG-ARS can decrease generation heat inputted from outside by 23.3-69.5% compared with that in DEARS. (2) Compared with DEARS, the lowest generation temperatures drops from 99 oC to 35 oC, and the lowest evaporation temperatures decreases from -2 oC to -26 oC by RCHG-ARS.
Acknowledgements
The authors gratefully acknowledge the support of the National Key Research and Development Program of China (No. 2016YFB0901405).
Jian Wang et al. / Procedia Engineering 205 (2017) 241–247 247 6 Jian Wang et al. / Procedia Engineering 00 (2017) 000–000 Jian Wang et al./ Procedia Engineering 00 (2017) 000–000 7
3.2. Effects of evaporation temperature (TEV) References o o o Fig. 4 shows the effects of TEV on RCHG-ARS and DEARS when TGE,new=75 C, TGE,de=120 C, TCO=30 C and [1] Tsinghua University Building Energy Saving Research Center. 2009 Annual Report on China Building Energy Efficiency. Beijing: China o TAB=30 C. For both RCHG-ARS and DEARS, PEE increases with the increase of TEV, because higher TEV means Architecture and Building Press, 2009. higher absorption pressure, which is beneficial to the absorption process and the improvement of PEE. [2] R. Lizarte, J.D. Marcos. COP optimization of a triple-effect H2O/LiBr absorption cycle under off-design conditions, Applied Thermal Engineering 99 (2016) 195-205. [3] S.C. Kaushik, A. Arora. Energy and exergy analysis of single effect and series flow double effect water-lithium bromide absorption refrigeration systems, International journal of Refrigeration 32 (2009) 1247-1258. [4] J.S. Kim, F. Ziegler, H. Lee. Simulation of the compressor-assisted triple-effect H2O/LiBr ab-sorption cooling cycles, Applied Thermal Engineering 22 (2002) 295-308. [5] L.A. Domínguez-Inzunza, J.A. Hernández-Magallanes, M. Sandoval-Reyes, W. Rivera. Comparison of the performance of single-effect, half- effect, double-effect in series and in-verse and triple-effect absorption cooling systems operating with the NH3-LiNO3 mixture, Applied Thermal Engineering 66 (2014) 612-620. [6] J. Wang, B. Wang, W. Wu, X. Li, W. Shi. Performance analysis of an absorption-compression hybrid refrigeration system recovering condensation heat for generation, Applied Thermal Engineering, 2016, 108: 54-65. [7] R. Ventas, A. Lecuona, A. Zacarías, M. Venegas. Ammonia-lithium nitrate absorption chiller with an integrated low-pressure compression booster cycle for low driving temperatures, Applied Thermal Engineering 30 (2010) 1351-1359. [8] W. Wu, X. Li, T. You, B. Wang, W. Shi. Combining ground source absorption heat pump with ground source electrical heat pump for thermal balance, higher efficiency and better economy in cold regions, Renewable Energy 84 (2015) 74-88.
Fig. 4. Effects of TEV on PEE for RCHG-ARS and DEARS.
Furthermore, Figure. 4 displays that the lowest evaporation temperatures for RCHG-ARS and DEARS are -26 oC and -2 oC, respectively, meaning that RCHG-ARS is capable of achieving cooling capacity at lower temperature. With the help of the compressor, the generation pressure of RCHG-ARS might be much lower than that of DEARS, therefore, even when TEV is low, the solution concentration difference is still large enough to maintain the operation of RCHG-ARS, while DEARS cannot operate.
4. Conclusions
In the present study, the performance of absorption-compression hybrid refrigeration system recovering condensation heat for generation (RCHG-ARS) is compared with double-effect absorption refrigeration system (DEARS), including the effects of generation temperature and evaporation temperature. Accordingly, some conclusions are drawn: (1) The PEE of RCHG-ARS can reach 29.0% higher than that of DEARS, and RCHG-ARS can decrease generation heat inputted from outside by 23.3-69.5% compared with that in DEARS. (2) Compared with DEARS, the lowest generation temperatures drops from 99 oC to 35 oC, and the lowest evaporation temperatures decreases from -2 oC to -26 oC by RCHG-ARS.
Acknowledgements
The authors gratefully acknowledge the support of the National Key Research and Development Program of China (No. 2016YFB0901405).