Converting Low-Grade Heat into Electrical Power

 

 

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Introduction

 

Low-Grade Heat Sources

        Solar Thermal
        Geothermal
        Industrial Waste Heat
        Cogeneration

 Supercritical Rankine Cycle

    Working fluids with relatively low critical temperature and pressure can be compressed directly to their supercritical pressures and heated to their supercritical state before expansion so as to obtain a better thermal match with the heat source. Fig. 1 and Fig.2 show the configuration and process of a CO2 supercritical Rankine cycle in a T-s diagram, respectively.

Fig. 1 The configuration of a supercritical Rankine cycle

Fig. 2 The process of a supercritcal Rankine cycle using CO2 as the working fluid (a→b→c→d→e→f→g) [1]

    The heating process of a supercritical Rankine cycle does not pass through a distinct two-phase region like a conventional Rankine or organic Rankine cycle thus getting a better thermal match in the boiler with less irreversibility.

     The transformation between liquid CO2 and supercritical CO2 is demonstrated in the following video by British chemist Martyn Poliakoff from University of Nottingham.

    Chen et al. [1-3] did a comparative study of the carbon dioxide supercritical power cycle and compared it with an organic Rankine cycle using R123 as the working fluid in a waste heat recovery application. It shows that a CO2 supercritical power cycle has higher system efficiency than an ORC when taking into account the behavior of the heat transfer between the heat source and the working fluid. The CO2 cycle shows no pinch limitation in the heat exchanger. Zhang et al.  [4-11] has also conducted research on the supercritical CO2 power cycle. Experiments revealed that the CO2 can be heated up to 187℃ and the power generation efficiency was 8.78% to 9.45% [7] and the COP for the overall outputs from the cycle was 0.548 and 0.406, respectively, on a typical summer and winter day in Japan [5].

    Organic fluids like isobutene, propane, propylene, difluoromethane and R-245fa [12] have also been suggested for supercritical Rankine cycle. It was found that supercritical fluids can maximize the efficiency of the system. However, detailed studies on the use of organic working fluids in supercritical Rankine cycles have not been widely published.

    There is no supercritical Rankine cycle in operation up to now. However, it is becoming a new direction due to its advantages in thermal efficiency and simplicity in configuration.

References

[1]  Y. Chen, P. Lundqvist, A. Johansson, and 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,  vol. 26, 2006, pp. 2142-2147.

[2]  Y. Chen, “Novel cycles using carbon dioxide as working fluid: new ways to utilize energy from low-grade heat sources,” Thesis, KTH, 2006.

[3]  Y. Chen, P. Lundqvist, and P. Platell, “Theoretical research of carbon dioxide power cycle application in automobile industry to reduce vehicle's fuel consumption,” Applied Thermal Engineering,  vol. 25, 2005, pp. 2041-2053.

[4]  X. Zhang, H. Yamaguchi, and D. Uneno, “Experimental study on the performance of solar Rankine system using supercritical CO2,” Renewable Energy,  vol. 32, 2007, pp. 2617-2628.

[5]  X. Zhang, H. Yamaguchi, K. Fujima, M. Enomoto, and N. Sawada, “Study of solar energy powered transcritical cycle using supercritical carbon dioxide,” International Journal of Energy Research,  vol. 30, 2006, pp. 1117-1129.

[6]  X. Zhang, H. Yamaguchi, and D. Uneno, “Thermodynamic analysis of the CO2-based Rankine cycle powered by solar energy,” International Journal of Energy Research,  vol. 31, 2007, pp. 1414-1424.

[7]  H. Yamaguchi, X.R. Zhang, K. Fujima, M. Enomoto, and N. Sawada, “Solar energy powered Rankine cycle using supercritical CO2,” Applied Thermal Engineering,  vol. 26, 2006, pp. 2345-2354.

[8]  X.R. Zhang, H. Yamaguchi, D. Uneno, K. Fujima, M. Enomoto, and N. Sawada, “Analysis of a novel solar energy-powered Rankine cycle for combined power and heat generation using supercritical carbon dioxide,” Renewable Energy,  vol. 31, 2006, pp. 1839-1854.

[9]  X.R. Zhang, H. Yamaguchi, K. Fujima, M. Enomoto, and N. Sawada, “Experimental Performance Analysis of Supercritical CO[sub 2] Thermodynamic Cycle Powered by Solar Energy,” AIP Conference Proceedings,  vol. 832, 2006, pp. 419-424.

[10]  X.R. Zhang, H. Yamaguchi, K. Fujima, M. Enomoto, and N. Sawada, “Theoretical analysis of a thermodynamic cycle for power and heat production using supercritical carbon dioxide,” Energy,  vol. 32, 2007, pp. 591-599.

[11]  Xin-rong Zhang, H. Yamaguchi, and K. Fujima, “A feasibility study of CO2-based rankine cycle powered by solar energy,” JSME Int J Ser B (Jpn Soc Mech Eng), 2005, pp. 8-540.

[12]  H.B. Matthews and M. Boylston, “Geothermal energy conversion system,” U.S. Patent 4142108, 1977.

 

 

 

 

 

Thermodynamic Cycles for the Conversion

        Kalina Cycle
        Goswami Cycle
        Trilateral Flash Cycle
        Organic Rankine Cycle
        Supercritical Rankine Cycle