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A ZERO-ODP, LOW GWP WORKING FLUID FOR HIGH TEMPERATURE HEATING
AND POWER GENERATION FROM LOW TEMPERATURE HEAT: DR-2
Konstantinos (Kostas) KONTOMARIS
DuPont Fluoroproducts--Wilmington, Delaware, USA
Konstantinos.Kontomaris@DuPont.com
ABSTRACT
This paper introduces a new developmental refrigerant, DR-2, as a potential working fluid for high temperature heat
pumps and Rankine cycles. DR-2 is a hydrofluoro-olefin-based fluid with an ozone depletion potential of zero and a
global warming potential of less than 10. It is non-flammable and has a favorable toxicity profile based on testing to
date. DR-2 remained chemically stable in the presence of metals up to the maximum temperature tested of 250 oC.
DR-2 thermodynamic performance under cycle conditions representative of potential applications was evaluated
through computational modeling. DR-2 has a relatively high critical temperature, generates relatively low vapor
pressures and enables high cycle energy efficiencies. It could enable more environmentally sustainable heat pump and
Rankine cycle platforms for the utilization of abundantly available low temperature heat to generate mechanical or
electrical power or meet heating duties at higher temperatures and with higher energy efficiencies than incumbent
working fluids.
Keywords: heat recovery, low GWP, global warming, Hydro-Fluoro-Olefin (HFO), refrigerants, working fluids,
climate change, energy efficiency, sustainability
INTRODUCTION
Current trends shaping the global energy landscape
suggest an expanding utilization of low temperature heat
(i.e. heat at temperatures lower than about 300 oC) in the
near future. Such heat may be recovered from various
commercial or industrial operations, can be extracted
from geothermal or hydrothermal reservoirs or can be
generated through solar collectors. Elevation of the
temperature of available heat through high temperature
mechanical compression heat pumps (HTHPs) to meet
heating requirements and conversion of the available
heat to mechanical or electrical power through Organic
Rankine Cycles (ORCs) are two promising approaches
for the use of low temperature heat. (Cooling, heating
or power generation through absorption cycles is beyond
the scope of this paper.) Heat at temperatures below
about 75 oC would probably be best used for heating (i.e.
as input to a HTHP), since conversion to power with
commonly available sinks for heat rejection would be
uneconomical in most cases due to poor energy
efficiency (except in special circumstances where the
power generated is highly valued).
Heat at
temperatures higher than about 125 oC would probably
be best converted to power in most cases given the high
versatility in power use.
Motivation for low temperature heat utilization is
provided by: i) current and projected energy supply
shortages (e.g. electricity in Japan and China; peak
electricity and petroleum globally); ii) increasing energy
prices resulting from supply/demand imbalances in
conventional forms of energy or the adoption of
generally more expensive alternative (e.g. renewable)
forms of energy; iii) growing awareness of the
environmental impacts, in general, and the threat to the
earth’s climate, in particular, from the use of fossil fuels
and resulting intensifying efforts to reduce fossil energy
use and associated greenhouse gas emissions; iv)
increasing recognition of the economic risks from
dependence on and the geopolitical risks in securing
access to unreliable energy supply sources; v) heightened
awareness by militaries around the world of the risks to
national security and troop safety posed by the logistics
of supplying large amounts of energy to remote and/or
hostile theaters of operation; vi) proliferation of
emerging technologies that could expand the easily
accessible low temperature heat resource (e.g. local,
stand-alone, remote, supplemental or backup distributed
power generation and cogeneration of heat and power for
commercial, industrial and military end use from a
variety of primary energy sources including biomass,
municipal and other waste, natural gas and bio-gas,
bio-diesel, and solar or geothermal heat). It seems
likely that the economic attractiveness of low
temperature heat utilization will be enhanced by
optimum integration within broader energy systems that
will allow highest value use of the available heat. System
optimization at the scale of individual or multiple
buildings is already underway. System optimization at
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the scale of whole cities is emerging on the horizon and
the necessary infrastructure (monitoring, data handling,
computing, electric power grid systems, etc.) is already
under development by credible organizations (e.g. [1]).
Table 1:
Heat pumps operating according to a reverse Rankine
cycle and ORCs require the use of working fluids.
Working fluids currently in common use for HTHPs and
ORCs either are controlled as ozone depleting substances
under the Montreal Protocol (e.g. CFC-114) or are
coming under increasing scrutiny because of their high
GWPs (e.g. HFC-245fa). Clearly, there is an increasing
need for more environmentally sustainable working
fluids for HTHPs and ORCs, especially given that
environmental sustainability is a primary motivation for
low temperature heat utilization. A new generation of
refrigerants
with
low
GWPs,
known
as
Hydro-Fluoro-Olefins (HFOs), may enable the design
and operation of energy efficient HTHPs and ORCs with
reduced environmental impact. DR-2 is a relatively low
pressure working fluid, based on HFO technology, that
was recently evaluated as a replacement for HCFC-123
in centrifugal chillers [2, 3] and as a working fluid for
HTHPs [4, 5]. Preliminary assessments of DR-2 as an
ORC fluid have also been carried out [6, 7].
T b, oC
Tcr, oC
Pcr, MPa
LFL, vol%
OEL, ppmv
Safety
Class(4)
ODP
ALT, yrs
GWP
The main objective of this paper was to evaluate the
potential of DR-2 as a working fluid for HTHPs and
ORCs. A related objective was to assess the chemical
stability of DR-2 in the presence of metals at
temperatures up to 250 oC.
METHODS
The thermo-physical properties of DR-2 were
estimated using group contribution methods and
equations of state from measurements of the liquid
density and vapor pressure over a wide temperature range
and of the DR-2 critical properties. The thermodynamic
performance of DR-2 in illustrative idealized cycles
representative of potential applications was computed
and compared to the performance of HFC-245fa, a
familiar working fluid used in HTHPs and ORCs. The
thermal stability of DR-2 in the presence of aluminum,
steel and copper was assessed according to the familiar
sealed glass tube methodology of ASHRAE-ANSI
Standard 97.
DR-2
33.4
171.3
2.90
None(2)
500(3)
A1(5)
HCFC
-123(1)
27.8
183.7
3.66
None
50
B1
HFC245fa(1)
15.1
154
3.65
None
300
B1
CFC114(1)
3.6
145.7
3.26
None
1,000
A1
0(6)
0.0658(7)
9.4(7)
0.020
1.3
77
0
7.6
1,030
1.000
300
10,040
(1) From [8]
(2) At 60 oC and 100 oC according to ASTM E681-2001
(3) DuPont Allowable Exposure Limit (AEL)
(4) ASHRAE Standard 34
(5) Not established, but meets criteria of A1
(6) No halogen atoms in DR-2 other than fluorine
(7) National Oceanic and Atmospheric Admin., 2010
RESULTS
(a) Thermodynamic Properties
Figure 1 compares the vapor pressure of DR-2 to
HFC-245fa. DR-2 generates vapor pressures at high
temperatures substantially lower than HFC-245fa. As a
result, use of DR-2 could allow higher condensing
temperatures for HTHPs and higher evaporating
temperatures for ORCs than HFC-245fa without
exceeding the maximum design working pressure of
commonly available low cost equipment. For example,
the maximum permissible working pressure for some
commonly available large centrifugal heat pumps may be
limited to values below 2.18 MPa.
DR-2 and
HFC-245fa would allow condensing temperatures of up
to about 155 oC and 126.2 oC, respectively, without
exceeding a vapor pressure of 2.18 MPa. Furthermore,
the higher critical temperature of DR-2 (Tcr=171.3 oC)
would also be advantageous for HTHPs and ORCs
because it would allow conventional subcritical operation
at higher condensing and evaporating temperatures,
respectively, than HFC-245fa (Tcr=154 oC).
4.00
3.50
HFC-245fa
3.00
Pressure (MPa)
Table 1 summarizes key thermodynamic, safety,
health and environmental characteristics of DR-2. It
compares DR-2 to a reference fluid with similar
thermodynamic properties, HCFC-123, and two
reference fluids that have been widely used for HTHPs
and ORCs, CFC-114 and HFC-245fa. HCFC-123 has
not been widely used for HTHPs or ORCs, despite its
attractive thermodynamic properties, apparently because
of its limited thermal stability. DR-2 miscibility with
polyol ester (POE) lubricants at temperatures up to 85 oC,
chemical stability in the presence of metals with and
without lubricant at temperatures up to 175 oC, and
compatibility with common plastics and elastomers at
room temperature have been reported [2].
Key properties of DR-2 compared to
incumbent working fluids.
2.50
2.00
DR-2
1.50
1.00
0.50
0.00
-40
Fig. 1.
-20
0
20
40
60
80
100
Temperature (C)
120
140
160
180
DR-2 vapor pressure compared to HFC-245fa.
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o
Figure 2 compares the computed temperature-entropy
diagram of DR-2 to HFC-245fa.
Similarly to
HFC-245fa, DR-2 exhibits a saturated vapor curve with a
small positive [dT/ds] slope. As a result, superheat at
the inlet of the expander of an ORC cycle would not be
required to ensure dry expansion.
pressure with HFC-245fa at 126.2 C is approaching the
design working pressure limit of commonly available
large centrifugal heat pumps.
Table 2: Predicted thermodynamic performance of
HTHP cycles: Tcond = 100 oC.
HFC-245fa
DR-2
C
100
100
C
60
60
∆Tsuperh
o
C
20
20
∆Tsubc
EFFCcompr
Pevap
Pcond
o
C
10
0.8
0.46
1.26
10
0.8
0.24
0.70
200
Tcond
DR-2
180
o
Temperature [ C]
160
Tevap
140
120
100
80
60
40
20
0
HFC-245fa
-20
-40
0.7
Fig. 2.
(b)
0.8
0.9
1
1.1
1.2 1.3 1.4 1.5
Entropy (kJ/kg.K)
1.6
1.7
1.8
1.9
DR-2 temperature-entropy diagram compared to
HFC-245fa.
o
o
MPa
MPa
PR
Tcompr_disch
COPh
CAPh
2.74
2.92
C
113.4
6.883
109.3
7.043
kJ/m3
3,927
2,248
o
Thermodynamic Cycle Performance: HTHPs
Heating at temperatures 85-100 oC is often needed for
various commercial and industrial applications (e.g.
district heating, hydronic space heating, equipment
cleaning, boiler water preheating, drying and other
process heating). Heating using a HTHP may be
attractive relative to fossil fuel heating, when heat is
available at temperatures in the range of 35-75 oC (e.g.
condenser water, process water, solar or low grade
geothermal heat).
Table 2 compares the thermodynamic performance of
DR-2 and HFC-245fa in a hypothetical heat pump
application meeting a heating duty requiring a
condensing temperature of 100 oC using available heat
supplied to the evaporator operating at 60 oC. DR-2
could enable such an application with a very attractive
coefficient of performance for heating, COPh, higher
than 7. The COPh with DR-2 would be 2.3% higher
than HFC-245fa. The volumetric heating capacity,
CAPh, with DR-2 would be about 42.7% lower than
HFC-245fa, as expected given the lower vapor pressure
of DR-2.
Table 3 exemplifies the use of a HTHP to meet a
heating duty requiring a condensing temperature of 126.2
o
C (e.g. steam generation). Available heat supplied to
the evaporator allows an evaporating temperature of 75
o
C. The resulting COPh of over 5 suggests that heat
pump heating for this application could be quite
attractive relative to heating with a fossil fuel heater (e.g.
steam boiler).
The efficiency/capacity trade-off
expected between fluids of different vapor pressures is
observed: DR-2 has a 4.7% higher COPh and a 39.8%
lower CAPh relative to HFC-245fa. The condensing
Table 3: Predicted thermodynamic performance of
HTHP cycles: Tcond = 126.2 oC.
HFC-245fa
DR-2
C
126.2
126.2
C
75
75
∆Tsuperh
o
C
20
20
∆Tsubc
EFFCcompr
Pevap
Pcond
o
C
10
0.8
0.69
2.18
10
0.8
0.37
1.25
Tcond
Tevap
o
o
MPa
MPa
PR
Tcompr_disch
COPh
CAPh
3.16
3.38
C
138.3
5.125
132.4
5.368
kJ/m3
4,833
2,909
o
Table 4 assesses the possibility of using a heat pump
operating with DR-2 to deliver a condensing temperature
of 155 oC to meet some industrial process heating duty.
Attractive COPh values are enabled when input heat
allows evaporating temperatures higher than 80 oC.
The DR-2 condensing pressure at 155 oC remains within
the capabilities of most commonly available large
centrifugal heat pumps. However, some equipment
modifications would likely be required to mitigate the
effects of the relatively high compressor discharge
temperatures.
Moreover, multistage compression
would probably be required at high temperature lifts.
HFC-245fa could not be used for this application with
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conventional subcritical heat rejection through
condensation because the required condensing
temperature would exceed the critical temperature of
HFC-245fa.
Table 4: Predicted thermodynamic performance of
HTHP cycles: Tcond = 155 oC.
DR-2
DR-2
Table 5: Predicted thermodynamic performance of
ORCs: Tevap = 135 oC; 160 oC.
o
C
155
155
155
o
C
80
100
120
1
2
3
4
∆Tsuperh
o
C
20
20
20
∆Tsubc
EFFCcompr
Pevap
Pcond
o
C
10
0.8
0.43
2.18
10
0.8
0.70
2.18
10
0.8
1.10
2.18
HFC245fa
DR-2
DR-2
DR-2
Tevap, °C
135
135
160
160
Tcond, °C
55
55
55
35
5.07
3.11
1.98
C
158.7
3.075
163.3
4.696
168.0
8.215
∆Tsuph, °C
5
5
5
5
kJ/m3
2,438
4,281
7,221
∆Tsubc, °C
10
10
10
10
EFFCexpn
0.80
0.80
0.80
0.80
EFFCpump
0.60
0.60
0.60
0.60
Pevap, MPa
2.58
1.49
2.38
2.38
Pcond, MPa
0.40
0.21
0.21
0.11
EFFcycle, %
10.30
10.59
12.00
14.36
CAPe, kJ/m3
488
289
335
229
Tcond
Tevap
MPa
MPa
PR
Tcompr_disch
COPh
CAPh
(c)
DR-2
As expected, cycle energy efficiency increases as the
difference between evaporating and condensing
temperature increases. Even at the lowest condensing
temperature considered in Table 5, (35 oC, column 4),
the condenser pressure with DR-2 remains slightly above
atmospheric level, thus minimizing the risk of air
infiltration.
o
Thermodynamic Cycle Performance: ORCs
The performance of DR-2 in ORCs at prescribed
conditions was evaluated and compared to several fluids
by Datla and Brasz (2012) [7]. The heat source
temperature varied from 100 to 80 oC.
The
performance of DR-2 was found to be similar to
HFC-245fa.
Table 5 summarizes the performance of DR-2 in
ORCs driven by heat sources allowing higher evaporator
temperatures than those considered by Datla and Brasz
(2012) [7]. It also compares the performance of ORCs
operating with DR-2 and HFC-245fa. The prescribed
cycles generate power from available heat allowing
evaporator operation at 135 oC or 160 oC. They reject
heat under two scenarios: at high condensing
temperatures (Tcond=55 oC; e.g. air-cooled condensers,
hot climates) or low condensing temperatures (Tcond=35
o
C). The estimated ideal cycle thermal efficiency at
Tevap=135 oC with DR-2, 10.59% (column 2), is 2.8%
higher than with HFC-245fa (column 1). DR-2 would
generate a substantially lower evaporator pressure than
HFC-245fa, which could be advantageous in reducing
equipment costs and fluid leakage. However, DR-2 is
estimated to have a 40.8% lower volumetric capacity for
power generation, CAPe, which would require a larger
turbine impeller (or larger expander, in general) than
HFC-245fa to deliver the same work rate.
Columns 3 and 4 in Table 5 summarize the predicted
performance of DR-2 in ORCs operated at evaporating
temperatures exceeding the critical temperature of
HFC-245fa and condensing temperatures corresponding
to air-cooled and water-cooled condensers, respectively.
(d)
DR-2 Thermal Stability
Sealed glass tubes each containing a carbon steel, a
copper and an aluminum coupon (with the copper
coupon placed between the other two coupons) immersed
in DR-2 were prepared. The stock of DR-2 used in the
sealed tube tests was 99.9864+wt% pure (136 ppmw of
impurities) and was dried and de-aerated before aging.
The tubes were aged in a heated oven for two weeks at
temperatures up to 250 oC. Visual inspection of the
tubes after thermal aging indicated clear liquids with no
discoloration, residues or other visible deterioration of
the refrigerant. Moreover, there was no change in the
appearance of the metal coupons indicating corrosion,
insoluble residues or other degradation.
Chemical analyses of the refrigerant liquids after
aging are summarized in Table 6. They indicated
negligible concentrations of fluoride even at the highest
temperature tested. The fluoride ion concentration can
be interpreted as an indicator of the degree of DR-2
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degradation. Table 6 suggests that DR-2 degradation
was minimal even at the highest temperature tested
(250ºC).
Table 6: Measured fluoride ion concentration in DR-2
samples after aging at various temperatures for two
weeks.
Aging Temperature
F-ion
[ºC]
[ppmm]
175
<0.15(*)
200
0.18
225
0.23
250
1.50
(*) No detectable fluoride
(within the method detection limit of 0.15 ppm)
cooling or heating. However, the combined COP for
both cooling and heating could still be attractive. For
example, a heat pump with DR-2 operated at an
evaporator temperature of 5 oC and a condenser
temperature of 65 oC (with 10 oC of vapor superheat at
the evaporator exit, 5 oC of liquid sub-cooling at the
condenser exit, and a compressor efficiency of 0.80)
would achieve an ideal cycle COP for cooling of 2.854
and a COP for heating of 3.854, for a total COP for both
cooling and heating of 6.708.
For some heat sources, maximum net power
generation (achieved by maximization of the product of
the fraction of the available heat extracted and the
fraction of the extracted heat converted to power) could
require heat extraction at pressures above the critical
pressure of the working fluid. The thermal stability of
DR-2 at supercritical conditions would allow
trans-critical operation of ORC cycles when required to
maximize net power generation from a given heat source.
SUMMARY-DISCUSSION
DR-2 is a relatively low pressure developmental
refrigerant based on HFO technology. In addition to its
attractive safety, health and environmental characteristics,
DR-2 provides an uncommon combination of properties
particularly advantageous, relative to other organic fluids
currently in common use, for high temperature operation
of heat pumps and Rankine cycles: high critical
temperature, low vapor pressure and high thermal
stability. It could enable HTHPs and ORCs with high
energy efficiencies and contribute to meeting
sustainability objectives (e.g. reducing demand for
non-renewable primary energy sources and reducing
greenhouse gas emissions) with attractive economics.
It seems promising that DR-2 could enable large
centrifugal HTHPs, largely consisting of commonly
available chiller components, delivering condensing
temperatures approaching or exceeding 155 oC. The
critical temperature of DR-2 is sufficiently higher than
155 oC that condenser design would probably pose no
insurmountable challenges.
Moreover, the vapor
pressure of DR-2 at 155oC is sufficiently low that it
could be accommodated without major mechanical
component modifications.
However, the high
compressor discharge temperatures could pose
challenges in the compressor heat management and the
selection of lubricants and polymeric materials of
equipment construction.
A HTHP could, in principle, be operated to
simultaneously provide heating (e.g. hot water for
domestic service) and cooling (e.g. chilled water for air
conditioning). Such a mode of operation would impose
a high required temperature lift and, therefore, it would
require a large amount of compression work to lift a unit
of mass of the working fluid from the thermodynamic
state of the evaporator to that of the condenser. The
resulting COP for cooling and COP for heating would,
generally, be lower than the values expected from
machines specifically operated to provide solely either
Work is continuing to assess the chemical stability of
DR-2 at temperatures higher than 250 oC and in the
presence of controlled concentrations of air and moisture
that could occasionally infiltrate ORC equipment. The
design of an accelerated thermal stability test that
accounts for the intermittent nature of fluid exposure to
high temperatures in an ORC would be of value for the
rapid screening of candidate fluids. Finally, evaluation
of candidate working fluids for heat recovery
applications, in general, and ORCs, in particular, often
requires estimation of fluid thermodynamic properties in
the vicinity of the critical point. It could be benefited
by practical methodologies for the accurate estimation of
near-critical thermodynamic properties of HFO-based
fluids.
REFERENCES
[1] Saito, Yutaka: “Building Livable Cities”, Hitachi
Technology
Report,
2012:
http://www.hitachi.com/rev/archive/2012/__icsFiles/af
ieldfile/2012/08/08/r2012_technology02.pdf
[2] Kontomaris, K.: "A Low GWP Replacement for
HCFC-123 in Centrifugal Chillers: DR-2", ASHRAE
& UNEP Conference “Road to Climate Friendly
Chillers: Moving Beyond CFCs and HCFCs”, Cairo,
Egypt, Sep. 30-Oct 1, 2010
[3] Kontomaris, K.: "Global Warming Impact of Low
GWP Refrigerants" 23rd IIR International Congress
of Refrigeration, Prague, Czech Republic, Aug.
21-26, 2011a
[4] Kontomaris, K.: “A Low GWP Working Fluid for
High Temperature Heat Pumps: DR-2”, Chillventa,
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Sep. 28-29, 2011b
[5] Kontomaris, K. and T. J. Leck: “Low GWP Working
JRAIA INTERNATIONAL SYMPOSIUM 2012
Copyright © JRAIA
Fluids for Domestic, Commercial and Industrial Heat
Pumps: DR-5 and DR-2”, The IEA Heat Pump
Center Newsletter, Vol. 29, No 4, pg. 30-33, 2011
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NOMENCLATURE
ALT: Atmospheric Life Time
CAPe: Volumetric capacity for power generation; ORC
expander power divided by the fluid volumetric flow
rate at the expander outlet
CAPh: Volumetric heating capacity; the amount of
heat delivered at the condenser (including the
compressed vapor superheat and liquid
sub-cooling) per unit volume of the working fluid
entering the compressor
COPh: Coefficient of Performance for Heating; ratio of
the heat delivered at the condenser (including the
compressed vapor superheat and the liquid
sub-cooling) and the work of compression
EFFCcompr: Compressor efficiency
EFFCexpn: Expander efficiency
EFFCpump: Liquid pump efficiency
EFFcycle: ORC cycle thermal efficiency (% of heat input
converted to mechanical energy)
GWP: Global Warming Potential (one hundred year
integrated time horizon)
HFO: Hydro-Fluoro-Olefin
HTHP: High Temperature Heat Pump (mechanical
compression)
LFL: Lower Flammability Limit
OEL: Occupational Exposure Limit
ORC: Organic Rankine Cycle
Pcond: Condenser pressure
Pcr: Critical pressure
Pevap: Evaporator pressure
PR: Pressure Ratio (condenser pressure over
evaporator pressure)
Tb: Boiling point at 1 atm
Tcompr_disch: Compressor discharge temperature
Tcond: Condenser temperature
Tcr: Critical temperature
Tevap: Evaporator temperature
∆Tsubc: Liquid sub-cooling at the condenser exit
∆Tsuperh: Vapor superheat at the evaporator exit