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Renewable Energy 48 (2012) 565e570
Contents lists available at SciVerse ScienceDirect
Renewable Energy
journal homepage: www.elsevier.com/locate/renene
Selection of working fluids for micro-CHP systems with ORC
Guoquan Qiu*
5 Stonebridge Blvd, Scarborough, ON M1W 4A8, Canada
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 15 December 2011
Accepted 7 June 2012
Available online 9 July 2012
With depleting fossil fuel reservoirs and the corresponding emissions of air pollutants, we urgently need
to look for alternative, renewable and sustainable energy options to counteract our strong dependence
on fossil fuels for energy supplies. Solar thermal, geothermal, biomass combustion heat energy and waste
heat recovery may be applied to drive combined heat and power (CHP) systems with organic Rankine
cycle (ORC). Working fluid is a critical factor to affect the efficiency of the ORC, therefore optimal
selection of the working fluid for an ORC-based system needs to be studied.
This paper presents the comparison and optimization of 8 mostly-applied working fluids nowadays
and gives a preferable ranking by means of spinal point method. The organic working fluids (organic
refrigerants) for the medium-to-low temperature ORC have thermodynamic, environmental and
economic criteria, such as preferable boiling temperature, high enthalpy drop (i.e., high thermal efficiency), favourable heat transfer characteristics, highly thermal and chemical stability, non-flammability,
low toxicity, no ozone depletion and low cost. According to these criteria and spinal point method, the 8
mostly-applied organic working fluids are characterized and listed in order of preferable selection:
HFE7000, HFE7100, PF5050, R123, n-pentane, R245fa, R134a and isobutene. An adequate organic fluid
should be selected considering these selection criteria and the specific heat source.
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Working fluid
Organic Rankine cycle (ORC)
Bucket effect
Spinal point
HFE7000
1. Introduction
Electricity is a necessity and a sign of modern life, but currently
around 1.5 billion people worldwide have no access to electricity
and up to a billion more have access only to unreliable electricity
networks [1,2]. Therefore, electricity generation is still in huge
demand and traditional steam turbine generators driven by
burning fossil fuels are still applied by far. It is widely believed that
accelerated consumption of fossil fuels has caused many serious
environmental problems such as air pollution, global warming,
ozone layer depletion and acid rain. Accordingly, alternative
sustainable energy sources, such as solar energy, geothermal
energy and biomass energy, have phased in distributed electricity
generation in a small-scale mode. However, the sustainable energy
conversion technologies for electricity generation must confront
the energy “trilemma” of balancing the demand for energy security,
affordability and low-carbon generation, which influences their
investment decisions and asset portfolio [2,3]. Basically the
sustainable energy sources meet the two aspects of the energy
“trilemma”, i.e., energy security and low-carbon generation. Price is
still a bottleneck for developing sustainable energy products
compared with conventional fossil fuel energy sources. In order to
* Tel.: þ1 647 996 0886.
E-mail address: [email protected]
0960-1481/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.renene.2012.06.006
solve affordability of sustainable energy, improving working efficiency of sustainable energy products is a dominant approach to
reduce product costs. Currently micro-scale combined heat and
power (CHP) systems driven by sustainable energy have relatively
low working efficiency, resulting from not only the lack of appropriate expanders in the commercial market [3] but also the need of
optimal working fluid for ORC.
Although less thermodynamically efficient than the idealized
Carnot cycle, the Rankine cycle is practical and adaptable. Since
a water steam Rankine cycle needs superheating to prevent turbine
blade erosion, an organic Rankine cycle has the advantage with the
use of organic fluids at lower temperatures and do not require
superheating [4]. In order to improve the ORC performance, the
selection of working fluids is of considerable importance in the ORCbased micro-CHP systems, since the fluids must have not only thermodynamic properties that match the application but also adequate
chemical stability at the desired working temperature. The thermodynamic properties of working fluids will affect the system efficiency,
operation and environmental impact. Hundreds of organic fluids are
commercially available, but a few of them is fit for the micro-CHP
systems with low temperature and low pressure. Accordingly, in the
past decade many researchers have investigated the optimal selection
of working fluids for ORC. Chen et al. screened 35 potential working
fluids for ORC [5] and Saleh et al. gave a thermodynamic screening of
31 pure component working fluids for ORC using BACKONE equation
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G. Qiu / Renewable Energy 48 (2012) 565e570
of state [6]. Of the 20 fluids investigated, ethanol, R123 and R141b
appear as the most suitable for small-scale domestic CHP applications
[7]. Considering different heat sources and diverse fluid types, the
issue of working fluid comparison and selection for ORC has been
widely investigated in the past decades [5e15]. However, these
literature usually provide some fluid thermodynamic characteristic
comparison and selection criteria for the low-temperature ORC. Some
appropriate working fluids for ORC are only suggested but no ranking
for the appropriate fluids is presented.
In these adequate working fluid candidates, R123 seems to be
widely chosen as working fluid by the investigators [11,16e19].
R245fa is also widely concerned as working fluid for ORC [14,20e23].
As a substitute of R12, R134a is applied in ORC by some researchers
[14,20]. Some investigators examined PF5050 as working fluid for
ORC [19]. Although n-pentane is extremely flammable, n-pentane is
also used as working fluid [19,20,24]. The author in the University of
Nottingham applied HFE7000 as working fluid for the biomass-fired
micro-CHP system [25]. Basically, hydrocarbons such as pentanes or
isobutanes, and refrigerants such as R123, R245fa and HFE7000 are
good candidates for moderate and low temperatures (typically
lower than 200 C) [26]. However, as a matter of fact, these investigators selected their working fluids without full reason provided.
This paper presents the ranking for the appropriate working fluids
for ORC utilizing bucket effect and spinal point method.
2. Working fluid evaluations
2.1. Working fluid types
Although water steam cycles can offer higher pressure ratios and
have better heat transfer properties than those of organic fluids,
standard pressure and temperature at the inlet of turbine are 100 bars
and 450 C [27] which result in safety problem in domestic applications. A large number of organic fluids have lower boiling point than
that of water. Such an organic fluid is applied in a Rankine cycle to
avoid higher temperature and pressure. Organic Rankine cycles
generally refer to moderate-to-low temperature (<200 C) power
cycles that operate with common refrigerants in Rankine cycles. The
slope (dT/ds) of the saturated vapour curve of organic fluids in a Tes
diagram can be negative (e.g. Ammonia), appropriate zero (e.g.
R123) or positive (e.g. HFE7000) as shown in Fig. 1 (data from EES
[28]), and the fluids are accordingly categorized into three groups:
➢ “Wet” fluids which have negative slope of the saturated
vapour curve and are generally of low molecular mass, such
as water M ¼ 18, ammonia M ¼ 17.
➢ “Isentropic” fluids which have nearly vertical saturated
vapour curves and are commonly of medium molecular mass,
such as R134a M ¼ 102, R245fa M ¼ 134 and R123 M ¼ 153.
➢ “Dry” fluids which have positive slope of the saturated vapour
curve and are usually of high molecular mass, e.g. HFE7000
M ¼ 200 and HFE7100 M ¼ 250.
“Wet” fluids usually need to be superheated prior to entering
the expander, while “isentropic” and “dry” fluids do not need
superheating, thereby eliminating the concerns of impingement of
liquid droplets on the expander blades. Moreover, the superheated
apparatus is not needed. Therefore, the working fluids of “dry” or
“isentropic” type are more adequate for ORC systems.
The organic working fluids for ORC generally refer to common
refrigerants with low-temperature boiling points, such as chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs) and hydrofluoroethers (HFEs). The interim
replacements for CFCs are HCFCs, which deplete stratospheric ozone,
but to a much lesser extent than CFCs. The first C in CFCs and HCFCs
represents chlorine that makes an ozone-depleting substance; CFCs
and HCFCs are a threat to the ozone layer but HFCs and HFEs are not.
Ultimately HFCs will replace HCFCs due to its zero value of ozone
depletion potential (ODP). Also perfluorocarbons (PFCs) as high global
warming compounds should be replaced with other alternative
candidates [29], so PF5050 fluid, a PFC, should be carefully managed
so as to minimize emissions. It should be concerned that HFC-134a
(R134a) is currently being replaced by R1234yf because of its high
Global Warming Potential (GWP). More than 150 fluorinated ethers
were examined as alternatives to CFCs, HCFCs and PFCs [29]. HFE7000
is announced as a replacement for R123 due to its non-null Ozone
Depletion Potential (ODP) e R123 will be phased out at the latest of
2030 depending on national legislations [26].
2.2. Criteria for selecting organic working fluids
The selection of working fluid for the ORC is critical since the
fluid must have not only thermophysical properties that match the
application but also meet safety requirement and economic cost.
Criteria for selecting organic working fluids for a specific heat
Fig. 1. Three types of working fluids: “dry”, isentropic and “wet” (data from EES).
G. Qiu / Renewable Energy 48 (2012) 565e570
source (such as solar thermal [10,18,20,30e33], biomass-fired
[9,25], geothermal [14,15,19], waste heat recovery [8,26,34e36])
have been presented in numerous studies [5e15]. Some general
relevant characteristics may be extracted from those studies and
selection criteria of organic working fluids are schematized in Fig. 2.
Based on the bucket effect and spinal point evaluation approach to
the selection criteria, an optimization fluid ranking for the
moderate-to-low temperature ORC may be listed in this paper.
In Fig. 2, the criteria are listed in order of importance from the
top to the bottom. They are:
567
(1) Currently not phase out:
low environmental impact
(2) Working fluids should have high enthalpy drop through the
expander since the high enthalpy drop means high power
output and/or high efficiency. The two main parameters are
Low GWP
(2) High enthalpy drop Appropriate critical temperature 100-2000C
(3) Easy to handle: Boiling temperature: 0-1000C
(4) Non-superheating ORC: isentropic or “dry”
(5) Large latent heat leads to small equipments
Criteria of
working fluids
for ORC
Thermally stable in the
working temperature range
(6) Preferable thermophysical properties Good heat transfer
Low viscosity
(1) The selected working fluids are currently not phased out by
relevant national regulations.
DuPont published its Suva refrigerant phase-out chart e General
Replacement Guide: CFC to an HCFC; CFC or HCFC to an HFC, as
shown in Fig. 3 [37]. Furthermore details are described by the new
regulation EC 2037/2000, effect from 1st October 2000 [38].
Obviously, R11, R12, R13, R500, R502 and R505 in Fig. 3 had been
phased out in 1996 and they should not been evaluated as working
fluids for ORC [8,13,15,34,39].
Low ODP
Low critical pressure
Low or non-toxicity
(7) High safety
Low or non-corrosion
Non-flammable
(8) Good availability and low cost.
Fig. 2. Selection criteria of organic working fluid for ORC.
maximum and minimum process temperature (i.e., the given
heat source and heat sink temperatures) [9]. With Rankinecycle engines, operating between maximum and minimum
temperature limits of 120 C and 40 C, respectively, the
conclusions are drawn from investigating the suitabilities of 68
potential working fluids [40]. The larger the temperature
Fig. 3. DuPontÒ SuvaÒ refrigerant phase-out chart.
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G. Qiu / Renewable Energy 48 (2012) 565e570
Fig. 4. Comparison of the working fluids: (a) “wet”, (b) isentropic and (c) “dry”.
difference between the max and min temperature, the higher
the enthalpy drop through the expander.
(3) Working fluids can be easy to handle at ambient environment,
so the boiling temperature is expected to be 0e100 C, which
the medium-to-low temperature heat sources can provide.
Sekiya et al. [29] provided 23 HFEs refrigerants with the boiling
temperature in the range of 26e79 C. Only one of them
HFE7000 was experimentally examined [25] and others may be
expected to replace CFCs, HCFCs and PFCs [29]. Basically, the
boiling temperature increases with increasing the critical
temperature, as shown in the table by Guo et al. [15], accordingly the working fluid should have a critical temperature
lower than 200 C that may make the boiling temperature
lower than 100 C.
(4) Isentropic or “dry” fluids are more adequate due to the nonsuperheating ORC.
The organic Rankine cycle (ORC) is a non-superheating thermodynamic cycle that uses an organic working fluid to rotate an
expander. The working fluid is pumped to an evaporator (1e2:
pumping process) and heated to boiling (2e3: constant-pressure
heat addition process), and the expanding vapour is used to drive
an expander (3e4: expansion process). This expander can be used
to drive a generator to convert the work into electricity. The
working-fluid vapour is condensed back into a liquid (4e1:
constant-pressure heat rejection process) and fed back through
the system to do work again, as shown in Fig. 4 [11]. From the three
ideal cycle comparison in Fig. 4, the expanding vapour at the exit of
the expander is below the saturation vapour line in Fig. 4(a) which
means impingement of liquid droplets on the expander blades. The
expanding vapour at the exit of the expander is on and at the right
side of the saturation vapour line in Fig. 4(b, c) (actual expansion
process is an entropic increase and the vapour at the exit of the
expander is at the upper right of the point 4). Therefore, isentropic
or “dry” fluids are appropriate for ORC and a recuperator can be
used to increase the ORC efficiency.
If the “wet” fluids are applied, the vapour must be superheated at
the expander inlet in order to avoid expander blade damages, which
decreases cycle performance [16]. Superheating the vapour at the
expander inlet will degrade the ORC performance because organic
fluids lead to a higher cycle efficiency than water at low temperatures and/or small power plants [9], for instance, R123 cycle has
a higher efficiency than water in the case of low temperatures [16].
(5) Large latent heat leads to small equipments at the pump,
evaporator and condenser level.
(6) Preferable thermophysical properties. Unlike water, organic
fluids often suffer from chemical deteriorations and decomposition at high temperatures. The maximum heat source
temperature is therefore limited by the chemical stability of the
working fluid [41]. Large heat conductivity leads to small
equipments at the heat exchanger level (i.e., evaporator, recuperator and condenser). Low viscosity leads to small pump
equipment and its power consumption. Low pressures in ORC
usually bring lower investment costs and decreasing
complexity.
(7) High safety requests low or non-toxicity, low or non-corrosion
which is compatible with equipment materials and lubricating
oil. Non-flammable fluids avoid explosion.
(8) Good availability and low cost. Compared with system equipments, working fluid cost is small part of the entire investment
[26] because the fluid is enclosed in ORC without the leaking loss.
2.3. Working fluid selection
In order to select a suitable organic working fluid for ORC driven
by a specific heat source, the bucket effect and spinal point method
may be applied to balance the impacts of the above-mentioned
criteria. The bucket effect indicates that water content filled in
the bucket in Fig. 5 is determined by the shortest board of the
bucket but not the any other. The bucket effect may be applied in
selecting working fluids for ORC. The shortest board e one of the
worst characteristic of a fluid may make the fluid not to be applied
for ORC. For instance, some refrigerants, R11, R12, R13, R500 et al,
have phased out due to their high ODP and are not permitted to use
for ORC. Other fluid characteristics (criteria) have different impacts
on fluid selection and spinal point method may be used to obtain
total spinal points. 8 organic working fluids, which have been
mostly applied and investigated in the past decade, are chosen to
compare and evaluated. Their temperatureeentropy curves are put
in one chart, as shown in Fig. 6. (Data from EES software [28].
PF5050 data is not available from EES. This chart corrects some
errors of the fluid Tes charts in some literature, such as R123 [18],
n-pentane [19,42], R134a [14,15]). At last a preferable fluid ranking
Fig. 5. Bucket effect.
G. Qiu / Renewable Energy 48 (2012) 565e570
569
Fig. 6. Tes curves comparison for mostly applied organic working fluids (data from EES).
Table 1
Index indicators of the various working fluids for ORC (min 1 and max 5 spinal points. Spinal points in brackets) [14,15,29,35,43,44].
Working fluids
R134a
HFC-134a
Isobutane
R600a HC-600a
R245fa
HFC-245fa
R123
HCFC-123
PF5050
PFC-5050
HFE7000
n-pentane
R601 HC-601
HFE7100
Formula
Molecular weight
Type, x (¼ds/dT)
Boiling T, C
Critical T, C
Critical P, MPa
Latent heat, kJ/kg
ODPa
GWPb
Flammable, AIT, Cc
Toxicityd
Thermal stability
Total points
Prefer ranking
CH2FCF3
102
0.39 isentropic
26.3 (1)
101.5 (2)
4.06 (2)
155.4 (3)
0 (5)
1300 (2)
NF 770 (5)
A (5)
Stable (5)
30
7
CH(CH3)3
58
1.03 “dry”
11.7 (2)
134.7 (3)
3.64 (3)
303.4 (5)
0 (5)
3 (4)
Highly 460 (1)
B (1)
Stable (5)
29
8
CF3CH2CHF2
134
0.19 isentropic
15.3 (4)
157.5 (5)
3.64 (3)
177.1 (3)
0 (5)
1030 (2)
NF 412 (5)
B (3)
Acceptable (4)
34
6
CHCl2CF3
153
0.120 isentropic
27.8 (5)
183.7 (4)
3.66 (3)
168.4 (3)
0.02e0.06 (4)
120 (4)
NF 730 (5)
B (3)
Stable (5)
36
4
(CF3)2(CF2)3
288
>0 “dry”
30 (5)
150 (5)
2.13 (5)
88 (2)
0 (5)
High (1)
NF n/a (4)
Low (5)
Stable (5)
37
2
C3F7OCH3
200
>0 “dry”
34 (5)
165 (5)
2.48 (5)
142 (3)
0 (5)
370 (3)
Yes 415 (4)
Low (5)
Stable (5)
40
1
C5H12
72
1.28 “dry”
36.0 (4)
196.5 (4)
3.36 (4)
349 (5)
0 (5)
20 (4)
Highly 260 (1)
A (3)
Stable (5)
35
5
C4F9OCH3
250
1.83 “dry”
61 (4)
195.3 (4)
2.23 (5)
112 (2)
0 (5)
390 (3)
Yes 405 (4)
Low (5)
Stable (5)
37
2
a
b
c
d
ODPdozone depletion potential. ODP for R11 ¼ 1.0.
GWPdGlobal Warming Potential. GWP for CO2 ¼ 1.0.
Preferably non-flammable (NF); slightly flammable (SF) may be acceptable. AITdAuto-ignition temperature.
ASHARE standard 34: Adlower toxicity; Bdhigher toxicity.
is achieved (preferable fluid first in order): HFE7000, HFE7100,
PF5050, R123, n-pentane, R245fa, R134a and isobutene, as shown in
Table 1 (boiling temperature increases from left to right).
It should be pointed out that the current study of working fluid
selection mainly aims at describing a methodology, rather than an
accurate fluid selection for an ORC driven by a specific heat source.
Therefore, the preferable working fluid ranking may be quite different
in a specific case due to the difference of the required criteria.
Tchanche et al. regarded R134a as the most suitable working fluid for
small-scale solar ORC applications [10]. Hettiarachchi et al. suggested
that the preferable working fluids are R123, n-pentane and PF5050
[19]. Quoilin et al. achieved that the thermodynamic optimization
leads to the selection of working fluids with overall efficiency (highest
efficiency first): n-butane, R245fa, R123, n-pentane, HFE7000, SES36,
R134a, R1234yf [26]. Mikielewicz et al. [7] concluded that ethanol,
R123 and R141b appear as the most suitable for ORC.
3. Conclusion
Currently moderate-to-low temperature heat sources (e.g. solar
thermal, geothermal, biomass and waste heat recovery) are used to
drive household micro-CHP systems. An organic Rankine cycle (ORC)
is commonly used in the micro-CHP systems since the ORC has the
advantage with the use of organic working fluids at lower temperatures and low pressures. The working fluid selection is critical because
an adequate organic fluid could greatly improve ORC performances.
This paper reviews the working fluid selection criteria published in
the past decade, and systematically illustrated and described these
selection criteria. The selection criteria require the organic fluids to be
low environmental impacts (low ODP and low GWP), favourable
thermodynamic properties (high enthalpy drop through the
expander, favourable boiling temperature, large latent heat, good heat
transfer, low viscosity and good thermal stability), high safety (low
toxicity, low corrosion and non-flammability) and low costs.
The bucket effect is used to eliminate some high ODP and GWP
refrigerants which have been phased out by UNFCCC and the Kyoto
Protocol. The spinal point method is applied to evaluate each
criterion. 8 types of the mostly-applied ORC working fluids nowadays are ranked by spinal point method (optimum first): HFE7000,
HFE7100, PF5050, R123, n-pentane, R245fa, R134a and isobutene.
It is remarkably noted that the current study presents a methodology of selecting working fluid for ORC. An adequate organic
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G. Qiu / Renewable Energy 48 (2012) 565e570
fluid should be selected considering these selection criteria and the
specific heat source. The above-mentioned fluid ranking is only
a reference to select an adequate working fluid for a specific case.
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