Converting Low-Grade Heat into Electrical Power

 

 

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Introduction

 

Low-Grade Heat Sources

        Solar Thermal
        Geothermal
        Industrial Waste Heat
        Cogeneration

 Organic Rankine Cycle

    The organic Rankine cycle (ORC) applies the principle of the steam Rankine cycle, but uses organic working fluids with low boiling points, instead of steam, to recover heat from a lower temperature heat source. Fig. 1 below shows a schematic of  an ORC and its process plotted in a T-s diagram in Fig.2. The cycle consists of an expansion turbine, a condenser, a pump, a boiler, and a superheater (provided that superheat is needed).

 

Fig.1 A schematic of an organic Rankine cycle 

  

Fig.2 The process of a organic Rankine using R11 as the working fluid

    The working fluid of an organic Rankine cycle is very importmant. Pure working fluids such as HCFC123 (CHCl2CF3), PF5050 (CF3(CF2)3CF3), HFC-245fa (CH3CH2CHF2), HFC-245ca (CF3CHFCH2F), isobutene ((CH3)2C=CH2), n-pentane and aromatic hydrocarbons, have been studied for organic Rankine cycles. Fluid mixtures were also proposed for organic Rankine cycles [1-8]. The organic working fluids have many different characteristics than water [9]. The slope of the saturation curve of a working fluid in a T-S diagram can be positive (e.g. isopentane), negative (e.g. R22) or vertical (e.g. R11), and the fluids are accordingly called “wet”, “dry” or “isentropic”, respectively. Wet fluids, like water, usually need to be superheated, while many organic fluids, which may be dry or isentropic, don’t need superheating. Another advantage of organic working fluids is that the turbine built for ORCs typically requires only a single-stage expander, resulting in a simpler, more economical system in terms of capital costs and maintenance [10].

Examples--Organic Rankine cycle power plant

    Among all these thermodynamic cycles for low-grade heat-to-power conversion, organic Rankine cycle is so far the most commercially developed one. Both large scales and small scales power plants and units can be found in operation.

    Arizona Public Service Company (APS) completed construction of a solar trough organic Rankine cycle power plant in the United Stats in 2007, which is the first new organic Rankine cycle power plant built in the past two decades, and the first power plant that combines solar though technology with an organic Rankine cycle power block (See Fig.3).

Fig. 3 Organic Rankine cycle power plant in Saguaro, Arizona

Figure source: www.altenerg.com/.../index.php?content_id=51

    Turbine is the most important part in a organic Rankine cycle system. Ormat and Infinity are among the leading companies that specialize in turbine design and manufacture for organic Rankine cycles. The turbine used in the above mentioned organic Rankine cycle power plant in Saguaro, Arizona is from Ormat International.  Beside the large scale systems, portable system for decentralized users are also available. Below is a10 kilowatt organic Rankine cycle power generation unit. A unit like this could be very useful for remote areas.

Fig. 4 A portable organic Rankine cycle power generation system

 

References

[1]  V. Maizza and A. Maizza, “Working fluids in non-steady flows for waste energy recovery systems,” Applied Thermal Engineering,  vol. 16, 1996, pp. 579-590.

[2]  K. Gawlik and V. Hassani, “Advanced binary cycles: optimum working fluids,” Energy Conversion Engineering Conference, 1997. IECEC-97., Proceedings of the 32nd Intersociety, 1997, pp. 1809-1814 vol.3.

[3]  V. Maizza and A. Maizza, “Unconventional working fluids in organic Rankine-cycles for waste energy recovery systems,” Applied Thermal Engineering,  vol. 21, 2001, pp. 381-390.

[4]  G. Angelino and P. Colonna di Paliano, “Multicomponent Working Fluids For Organic Rankine Cycles (ORCs),” Energy,  vol. 23, 1998, pp. 449-463.

[5]  C.J. Bliem and G. Mines, “Supercritical binary geothermal cycle experiments with mixed-hydrocarbon working fluids and a near-horizontal in-tube condenser ,” Report, 1989.

[6]  X. Wang and L. Zhao, “Analysis of zeotropic mixtures used in low-temperature solar Rankine cycles for power generation,” Solar Energy,  vol. 83, May. 2009, pp. 605-613.

[7]  A. Borsukiewicz-Gozdur and W. Nowak, “Comparative analysis of natural and synthetic refrigerants in application to low temperature Clausius-Rankine cycle,” Energy,  vol. 32, Apr. 2007, pp. 344-352.

[8]  R. Radermacher, “Thermodynamic and heat transfer implications of working fluid mixtures in Rankine cycles,” International Journal of Heat and Fluid Flow,  vol. 10, Jun. 1989, pp. 90-102.

[9]  W.B. Stine and R.W. Harrigan, Solar Energy Fundamentals and Design, Wiley, 1985.

[10]  W.C. Andersen and T.J. Bruno, “Rapid screening of fluids for chemical stability in organic rankine cycle applications,” Ind. Eng. Chem. Res,  vol. 44, 2005, pp. 5560-5566.

 

 

 

Thermodynamic Cycles for the Conversion

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