Converting Low-Grade Heat into
Electrical Power
Trilateral Flash Cycle
The Trilateral Flash Cycle (TFC) is a thermodynamic power cycle whose expansion
starts from the saturated liquid rather than a vapor phase. By avoiding the boiling part,
the heat transfer from a heat source to a liquid working fluid is achieved with almost
perfect temperature matching. Irreversibilities are thereby minimized. According to
Stiedel et. Al. [1], its potential power recovery could be 14 - 85% more than from
ORC or flash steam systems provided that the two-phase expansion process is
efficient. Fig. 1 and Fig.2 are the configuration of a trilateral flash cycle and its process
in a T-s diagram, respectively.
Fig. 1 The configuration of a trilateral flash cycle
Fig. 2 The process of a trilateral flash cycle in a T-s diagram
Although this system has been considered for over 30 years, a lack of suitable twophase expanders with high adiabatic efficiencies is the main obstacle for it to become
reality and only small scale demonstration unit of it is known to have been built. Twophase expanders were studied extensively during the 1970�s, among which a
Lysholm screw expander in a twin screw machine proposed by Sprankle and further
studied by Steudel, et al. [1] was said to have adiabatic efficiencies of the order of
50%. However, studies conducted by Smith, et al. show that it is possible to design
and construct twin screw expenders for trilateral flash cycle application with predicted
adiabatic efficiencies of the order of 80% or more [1]. They have realized the design,
and test results of screw machines showing two-phase fluid expansion with adiabatic
efficiencies of more than 70% [2].
Example--Trilateral Flash Cycle
It has been mentioned that although theoretically trilateral flash cycle has a lot of
advantages in term of the efficiency, the difficulty of developing an efficient expander
for two phase flash has also been the main obstacle. There is no trilateral flash cycle
power plant reportedly in operation. However, some pilot demonstrations have been
conducted by Smith, Stosic and Kovacevic [3]. The following are the setup of the of
expander and its components.
Fig. 3 Screw expander and its main components in a trilateral flash cycle
Figure source: http://www.staff.city.ac.uk/~ra601/grc2005.pdf
References
[1] RF Stiedel, KA Brown, and DH Pankow, �The empirical modeling of a Lysholm screw expander,� Proc.,
Intersoc. Energy Convers. Eng. Conf.; (United States), Orlando, FL, USA: 1983.
[2] N. Stosic and A. Kovacevic, �Power Recovery from Low Cost Two-Phase Expanders,� Expanders GRC
Annual Meeting, San Diego: 2001.
[3] http://www.staff.city.ac.uk/~ra601/grc2005.pdf
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.
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.