property data for low-gwp refrigerants: what do we know...
TRANSCRIPT
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Property Data for Low-GWP Refrigerants:
What Do We Know and
What Don’t We Know?
Mark O. McLinden
Thermophysical Properties Division
National Institute of Standards and Technology
Boulder, Colorado USA
ASHRAE Winter Meeting, Las Vegas, NV
Seminar 6—Removing Barriers for Low-GWP Refrigerants
January 30, 2011
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Removing Barriers to Low-GWP RefrigerantsLearning Objectives
Describe the climate change issue associated with high-GWP
refrigerants and the leading low-GWP options available today
Explain the refrigerant thermophysical property requirements needed
for new low-GWP refrigerants and how property data may be used
Be able to explain the challenges with measuring the flammability
properties of refrigerants that are only marginally flammable and
options to make these measurements
Explain the development history of hydrofluoroolefin low-GWP
refrigerants such as HFO-1234yf and why this new class of
compounds has unique properties
Describe the codes and regulations in the U.S., Europe, and Japan
that govern the use of low-GWP refrigerants, such as CO2, ammonia,
hydrocarbons, and HFOs, and the barriers in current standards for
their potential use
Apply learnings from seminar to begin selecting low-GWP refrigerants
for specific applications and begin designing new HVAC&R systems
with these refrigerants
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Outline
What data do we need?
What are the candidate fluids?
Assessment of the data
How are the new fluids different?
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Safety
toxicity (acute and chronic)
flammability
Environmental
ozone depletion potential (ODP)
green house warming potential (GWP)
atmospheric life (impacts ODP & GWP)
Materials
compatibility with metals, seals, etc.
lubricant
stability (hydrolysis, polymerization, etc.)
Performance
thermodynamic properties
transport properties
What Data Do We Need on a Refrigerant?
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What Counts as “Low-GWP”?
R134a: GWP = 1430
(relative to CO2 w/ 100-year time horizon)
must have GWP << 1430
EU regulation for automotive A/C:
GWP < 150
(takes effect 2011 for new models)
North American Proposal to Montreal Protocol:
85 % phase-down by 2033 (0.15 1430 = 215)
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Low-GWP Options (Current Fluids)
A31–24.8E170Ether
A1<1100.0718 (water)H2O
B2L<1–33.3717 (ammonia)NH3
A11–56.6744CO2
A320–0.5600 (butane)
A320–11.7600a (isobutane)
A320–42.1290 (propane)HC
B17727.8123HCFC
A2124–24.0152a
A11430–26.1134a
N/A (flam)12–37.6161
A2L675–51.732
N/A (flam)92–78.341HFC
ASHRAEclass
(tox/flam)
GWP[CO2 = 1]
NBP(˚C)Fluid
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Hydro-Fluoro-Olefin (HFO)
C=C double bond
fluorine-containing
hydrogen-containing
Low-GWP Refrigerants—New Possibilities
2L(pending)6–19.01234ze(E)
A2L4–29.51234yfHFO
ASHRAEclass
(tox/flam)
GWP[CO2 = 1]
NBP(˚C)Fluid
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This is the major “What We Don’t Know”
Additional HFOs Under Development
Announced in talks and conference papers
but not identified
Many (most?) are in patent or chemical literature
molecules themselves are not patentable
(production processes are patentable)
Blends w/ HFOs also being developed
azeotropic blends are patentable
zeotropic blends may be patentable
?
? ?
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Low-GWP Refrigerants—Additional Possibilities
Interest in fluorinated ethers in 1990s
Examples among specialty solvents and heat-transfer fluids:
–fluorinated ethers
especially “segregated” ethers (all H on a single carbon)
–fluorinated ketones
non-flammable
low(ish) GWP
commercially available
(but boiling points higher than typical refrigerants)
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Property Data and
Equations of State
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Fluid Properties—Why Should ASHRAE Care?
Condenser
Evaporator
CompressorExpansion Valve
1
23
4
S2 = S1P2 = P3
q = 0T3 = TcondP3 = Psat(Tcond)
h4 = h3P4 = P1
q = 1T1 = TevapP1 = Psat(Tevap)
COPR =h1 h4h2 h1
= f Tevap ,Tcond , properties( )
Cycle analysis IS properties
Enthalpy
Pressure
1
23
4
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Equation of State(Thermodynamic Properties)
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Equation of State—Helmholtz Energy Form
A thermodynamically consistent representation of the properties of a fluid
=A
RT=
ideal + Niti dk
i
+ Njt j d j exp aj
l j( )j
+ Nktk dk exp ak k( )
lk
k
exp k k( )mk
where : = crit , = Tcrit T
“Gaussian terms” (critical region)
“traditional terms”
All other properties by differentiation:
p = RT 1 +
r
, CV = R 2
2
2
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Equation of State—Mixture Form
Sum pure component contributions + “excess” (mixing) term
“excess” contribution
ideal solution terms
(ideal gas & “real gas”)
Evaluate at reduced T, (not mixture T, ) given by “reducing
parameters”
mix = xi i0 ,( )+ ln xi( )[ ]
i
+ xi ir ,( )
i
+ xij=i+1i
x jFij Nkik jk
k
“mixing function”
T*= xiTi
crit
i=1
n
+ xij=i+1
n
i
n 1
x j ij 1* =
xi
icrit
i=1
n
+ xij =i +1
n
i
n 1
xi ij
= T* T =*
Adjustable parameters , , Fij + mixing function give great flexibility in
fitting mixtures with limited or extensive data
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Data Needed to Establish Equation of State(Thermodynamic Properties)
Critical parameters (Tcrit, pcrit, crit)
Vapor pressure as f(T)
Density (liquid, vapor, supercritical)
Ideal-gas heat capacity [Cp as p 0]
Caloric data (liquid): Cp or CV or w
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Recent R1234ze(E) Data
225
275
325
375
425
Tem
pera
ture
(K
)
0 200 400 600 800 1000 1200 1400
Density (kg•m–3)
saturation
filling 3
filling 2
filling 1
0.0
1.0
2.0
3.0
4.0
Pre
ssur
e (M
Pa)
260 280 300 320 340 360 380
Temperature (K)
Critical region data, Higashi, et al.,
J. Chem. Eng. Data (2010)
p- -T and vapor pressure data
McLinden, et al., Int. Ref. Conf. Purdue (2010)
Liquid-phase speed of sound
Lago, et al. (2010)
200
300
400
500
600
700
800
w (
m/s
)
250 275 300 325 350 375
Temperature (K)
p = 10 MPa
p = 2 MPa
Liquid-phase heat capacity
Tanaka, et al., J. Chem. Eng. Data (2010)
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Equation of State—Fitting Process(R1234ze(E) as Example)
Non-linear in the parameters
must start with initial guess for EOS
propane equation with 14 terms
Objective function:
minimize sum of squares
Fit numerical coefficients
Fit exponents on temperature terms
(0 ~ 5), max for R1234ze is 2.5
Fixed exponents on density terms
integers 1
Highly iterative process
(1000’s of iterations)
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Equation of State—Fitting Process (Constraints)
Numerous further constraints
e.g., shape of critical isotherm
EOS with correct shape:
small addition to sum of squares
EOS with incorrect shape:
large penalty to sum of squares
for < crit :
p> 0;
2 p2 < 0;
3p3 > 0;
4 p4 < 0;
for > crit :
p> 0;
2 p2 > 0;
3p3 > 0;
4 p4 > 0;
add expp
to sum of squares
0.0
2.0
4.0
6.0
8.0
Pre
ssur
e (M
Pa)
0 200 400 600 800 1000 1200 1400
Density (kg/m3)
T = Tcrit
saturatedliquid
saturatedvapor
criticalpoint
calculatedpoints
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Data Comparisons and
Assessments
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Data Comparisons—EOS vs. p- -T Data, R1234ze(E)
1.31Grebenkov (vapor)
0.52Grebenkov (liquid)
Source (%)
McLinden et al. 0.032
Tanaka + 0.087
Grebenkov (sat’n) 0.22
–0.30
–0.20
–0.10
0.00
0.10
0.20
0.30
100
(e
xp –
E
OS)/
EO
S o
r 10
0 (p
exp
– p
EO
S)/
p EO
S
225 275 325 375 425
Temperature (K)
–0.30
–0.20
–0.10
0.00
0.10
0.20
0.30
0 200 400 600 800 1000 1200 1400
Density (kg m–3)
Tcrit
crit
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Data Comparisons—EOS vs. psat Data, R1234ze(E)
1.78Grebenkov
Source (%)
McLinden et al. 0.028
Tanaka + 0.039
–0.20
–0.10
0.00
0.10
0.20
100
(pe
xp –
pE
OS)/
p EO
S
240 280 320 360 400
Temperature (K)
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Data Comparisons—EOS vs. Cp and Sound Speed Data
R1234ze(E)
0.49Lago (w)
Source (%)
Tanaka (Cp) + 2.16
–8.00
–6.00
–4.00
–2.00
0.00
2.00
4.00
100
(Xe
xp –
XE
OS)/
XE
OS
260 300 340 380
Temperature (K)
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Data Summary
limited datagood (limited data)1234ze(E)
limited datagood (limited data)1234yfHFO
HFC
fair (limited data)fair (limited data)E170DME
excellentexcellent718 (water)H2O
goodfair (!)*717 (ammonia)NH3
very goodexcellent744CO2
very goodvery good600 (butane)
very goodvery good600a (isobutane)
very goodexcellent290 (propane)HC
fair (visc)–good (t.c.)fair (old form)123HCFC
goodfair (old form)152a
goodgood134a
goodgood32
Transport(visc. & t.c.)
Eqn. of State(thermo)Fluid
*new EOS under development
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Thermo Data Summary—Blends
available, generally good,
but dated (2001)
predictive model for
refrigerant blends
proprietary data (azeotropes disclosed)
Arakawa et al. (2010): R32/1234yf*HFO + HFC
very good data and models (natural gas)HC blends
virtually no dataHFC + ethers
limited pairs, generally only VLE dataHFC + HC
numerous mixture pairs, but generallyonly VLE data (i.e., no (T,p,x))
other HFC blends
very good data and modelHFC 32/125/134a
Data AssessmentBlend Class
*additional work in progress in Japan0.12
0.18
Pressure
0 1Composition
azeotrope
0.200
0.400
0.600
0.800
1.000
Pre
ssu
re
0 1
Composition
bubble line (liquid)
dew line (vapor)
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Refrigerant Molecules Are Getting More Complex
Increased complexity:
changes fundamental shape of thermodynamic surface
increased flash losses
The cycle may need to be modified
CFC-12 HFC-134a HFO-1234yfNBP: (–29.8 ˚C) (–26.1 ˚C) (–29.5 ˚C)
Enthalpy (mass basis)
Pre
ssu
re
R12
R134a
R1234yf Te
mp
era
ture
Entropy (mass basis)
R12
R134a
R1234yf
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Conclusions
Adequate data for a range of traditional and new low-GWP fluids
But much work remains:
–Additional candidates are proprietary
–Blend data w/ HFOs are very limited
New fluids may require modified cycles
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Questions?