International Science Index


Cascaded Transcritical/Supercritical CO2 Cycles and Organic Rankine Cycles to Recover Low-Temperature Waste Heat and LNG Cold Energy Simultaneously


Low-temperature waste heat is abundant in the process industries, and large amounts of Liquefied Natural Gas (LNG) cold energy are discarded without being recovered properly in LNG terminals. Power generation is an effective way to utilize low-temperature waste heat and LNG cold energy simultaneously. Organic Rankine Cycles (ORCs) and CO2 power cycles are promising technologies to convert low-temperature waste heat and LNG cold energy into electricity. If waste heat and LNG cold energy are utilized simultaneously in one system, the performance may outperform separate systems utilizing low-temperature waste heat and LNG cold energy, respectively. Low-temperature waste heat acts as the heat source and LNG regasification acts as the heat sink in the combined system. Due to the large temperature difference between the heat source and the heat sink, cascaded power cycle configurations are proposed in this paper. Cascaded power cycles can improve the energy efficiency of the system considerably. The cycle operating at a higher temperature to recover waste heat is called top cycle and the cycle operating at a lower temperature to utilize LNG cold energy is called bottom cycle in this study. The top cycle condensation heat is used as the heat source in the bottom cycle. The top cycle can be an ORC, transcritical CO2 (tCO2) cycle or supercritical CO2 (sCO2) cycle, while the bottom cycle only can be an ORC due to the low-temperature range of the bottom cycle. However, the thermodynamic path of the tCO2 cycle and sCO2 cycle are different from that of an ORC. The tCO2 cycle and the sCO2 cycle perform better than an ORC for sensible waste heat recovery due to a better temperature match with the waste heat source. Different combinations of the tCO2 cycle, sCO2 cycle and ORC are compared to screen the best configurations of the cascaded power cycles. The influence of the working fluid and the operating conditions are also investigated in this study. Each configuration is modeled and optimized in Aspen HYSYS. The results show that cascaded tCO2/ORC performs better compared with cascaded ORC/ORC and cascaded sCO2/ORC for the case study.

[1] H. Yu, X. Feng, Y. Wang, L.T. Biegler, J. Eason, A systematic method to customize an efficient organic Rankine cycle (ORC) to recover waste heat in refineries, Applied Energy 179 (2016) 302-315.
[2] H. Yu, X. Feng, Y. Wang, A new pinch based method for simultaneous selection of working fluid and operating conditions in an ORC (Organic Rankine Cycle) recovering waste heat, Energy 90 (2015) 36-46.
[3] J. Wang, J. Wang, Y. Dai, P. Zhao, Thermodynamic analysis and optimization of a transcritical CO2 geothermal power generation system based on the cold energy utilization of LNG, Applied Thermal Engineering 70 (2014) 531-540.
[4] X. Shi, D. Che, A combined power cycle utilizing low-temperature waste heat and LNG cold energy, Energy conversion and management 50 (2009) 567-575.
[5] H. Liu, L. You, Characteristics and applications of the cold heat exergy of liquefied natural gas, Energy Conversion and Management 40 (1999) 1515-1525.
[6] W. Lin, M. Huang, H. He, A. Gu, A transcritical CO2 Rankine cycle with LNG cold energy utilization and liquefaction of CO2 in gas turbine exhaust, Journal of Energy Resources Technology 131 (2009) 042201.
[7] U. Lee, K. Park, Y.S. Jeong, S. Lee, C. Han, Design and analysis of a combined Rankine cycle for waste heat recovery of a coal power plant using LNG cryogenic exergy, Industrial & Engineering Chemistry Research 53 (2014) 9812-9824.
[8] Y. Song, J. Wang, Y. Dai, E. Zhou, Thermodynamic analysis of a transcritical CO2 power cycle driven by solar energy with liquified natural gas as its heat sink, Applied Energy 92 (2012) 194-203.
[9] J. Sarkar, Review and future trends of supercritical CO2 Rankine cycle for low-grade heat conversion, Renewable and Sustainable Energy Reviews 48 (2015) 434-451.
[10] G. Bisio, L. Tagliafico, On the recovery of LNG physical exergy by means of a simple cycle or a complex system, Exergy, an International Journal 2 (2002) 34-50.
[11] H. Yu, X. Feng, Y. Wang, Working fluid selection for organic Rankine cycle (ORC) considering the characteristics of waste heat sources, Industrial & Engineering Chemistry Research 55 (2016) 1309-1321.
[12] J. Szargut, I. Szczygiel, Utilization of the cryogenic exergy of liquid natural gas (LNG) for the production of electricity, Energy 34 (2009) 827-837.
[13] I.-H. Choi, S. Lee, Y. Seo, D. Chang, Analysis and optimization of cascade Rankine cycle for liquefied natural gas cold energy recovery, Energy 61 (2013) 179-195.
[14] Aspen Technology Inc., Aspen HYSYS V9.0, Aspen Technology Inc.; Bedford, MA, 2016.
[15] Y. Chen, P. Lundqvist, A. Johansson, P. Platell, A comparative study of the carbon dioxide transcritical power cycle compared with an organic Rankine cycle with R123 as working fluid in waste heat recovery, Applied Thermal Engineering 26 (2006) 2142-2147.