Understanding the Path to High- Efficiency Chemical Engines Chris F. Edwards Kwee Yan Teh, Shannon Miller, Matthew Svrcek, Sankaran Ramakrishnan, and Adam Simpson Advanced Energy Systems Laboratory Department of Mechanical Engineering Stanford University
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Understanding the Path to High- Efficiency Chemical EnginesUnderstanding the Path to High-Efficiency Chemical Engines Chris F. Edwards Kwee Yan Teh, Shannon Miller, Matthew Svrcek,
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Understanding the Path to High- Efficiency Chemical Engines
Chris F. Edwards
Kwee
Yan Teh, Shannon Miller, Matthew Svrcek, Sankaran
Ramakrishnan, and Adam Simpson
Advanced Energy Systems Laboratory Department of Mechanical Engineering
Stanford University
40%40%
34%34%
>74% of U.S. CO2
is emitted by engines.
Engines•
All
engines have three essential features:
–
they produce work (by definition)–
they require a resource (1st
Law)
–
they reject energy to surroundings (2nd
Law)
Engine WorkEnergy Resource
Rejected Energy
(surroundings)
Efficiency Limits•
Only
four ways to transfer energy:
–
work (entropy free)–
heat (energy transfer due to ΔT )
–
matter (internal and external)•
External:
K.E., gravitational P.E., electrostatic P.E.
•
Internal: thermal, chemical, nuclear
–
radiation (not considered here)
•
It is the combination of energy resource and surroundings that determines the ultimate efficiency limitation of an engine (exergy).
+All species taken as ideal gases. †Environment taken as: 25°C, 1 bar, 363 ppm CO2, 2% H2O, 20.48% O2, balance N2 .*Reaction with stoichiometric air at 25°C, 1 bar. All products present as ideal gases, including water.
Fuel Conversion Efficiency potential (maximum first-law efficiency based on LHV) of most fuels is ~100%.
Classification & Architecture
Restrained
Reaction
Unrestrained
Reaction
ElectricalWork
(e.g., SOFC)
MechanicalWork
(e.g., None)
ElectricalWork
(e.g., MHD)
MechanicalWork
(e.g., GT)
Chemical
Engines
Architecture: the set of
components & connections, and the corresponding set of thermodynamic idealizations & device limitations that constitute a particular engine.
Classification: (1)
(2)
(3)
Two Approaches to Reaction•
Unrestrained–
Reactants are initially internally restrained, i.e., frozen in chemical non-equilibrium (e.g. combustion, fuel reforming).
–
Internal restraint is released, allowing reaction to proceed.–
Reaction “stops”
when equilibrium is achieved or kinetics are so slow as to be negligible (frozen again).
–
Inherently irreversible.
•
Restrained–
Reactants are initially externally restrained, i.e., in chemical equilibrium (e.g. electrochemistry, solution chemistry).
–
External restraints are changed, allowing reaction to proceed.–
Never stops; always dynamically balanced.–
Reversible only in the limit
of infinitesimal rate and constrained chemical pathway (chemical reversibility).
Restrained vs. Unrestrained Architectures
* After Primus, et al. “Proceedings of International Symposium on Diagnostics and Modeling of Combustion in Reciprocating Engines, (1985) p.529-538.
Restrained (SOFC) Unrestrained (DI Diesel*)
•
Efficiency declines with load•
Irreversibility reduced via facile kinetics (reaction and transport)
•
Efficiency improves with load•
Irreversibility reduced by reaction at extreme states
Entropy Generation with Unrestrained Reaction
Stoichiometric
propane/air mixture modeled as ideal gases. Includes the effects of variable specific heats, reaction, & dissociation.
Four ways to transfer energy…
100
101
1020
20
40
60
80
100
Compression Ratio
First Law (per LHV)70-80% First LawFuel Exergy/LHV
Efficiency Achievable with Simple- Cycle Extreme Compression
CI
SI
Stoichiometric
propane/air
Firs
t-Law
Eff
icie
ncy
(%)
Extreme-Compression Post-Combustion Conditions
Stoichiometric
propane/air mixture modeled as ideal gases. Includes the effects of variable specific heats, reaction, & dissociation.
3300K! 1000 bar!
Must be fast! Must be balanced!
Free-Piston Engines
Example: Junkers Compressor
M. Nakahara and H. Kohama, “Junkers High Pressure Air Compressor-A Case of Technology Transfer through the Imperial Japanese Navy,”
in The 1st international conference on business and technology transfer, 2004.
Van Blarigan/Aichlmayr
Linear Alternator Concept
Experimental Apparatus
16
Operating Space
Combustion VisualizationCR = 30:1 CR = 100:1
1 ms injection duration, finishing at TDC
Combustion Data at CR = 70
10-1100
101
102
Volume (V/V0)
Pres
sure
(bar
)
Air-onlyCombustionIsentrope
φ
= 0.35
19
First-Law Efficiency: Initial Results
10 20 30 40 50 60 70 80 90 100
30
40
50
60
70
80
90
Compression Ratio
Effic
ienc
y (%
)
First law (per LHV), φ = 0.3570-80% first lawCombustion data
20
First-Law Efficiency: Initial Results
10 20 30 40 50 60 70 80 90 100
30
40
50
60
70
80
90
Compression Ratio
Effic
ienc
y (%
)
First law (per LHV), φ = 0.3570-80% first lawCombustion dataTheoretical efficiency with air losses Losses in air
experiments
Additionallosses due tocombustion
21
First-Law Efficiency: Initial Results
10 20 30 40 50 60 70 80 90 100
30
40
50
60
70
80
90
Compression Ratio
Effic
ienc
y (%
)
First law (per LHV), φ = 0.3570-80% first lawCombustion dataTheoretical efficiency with air lossesLow blowby
53%, 20°C walls
Losses in airexperiments
Additionallosses due tocombustion
•
Confident we can demonstrate 60% indicated•
Speculate 70% is achievable regeneratively
Simple-Cycle Steady Flow
What is the optimal action to be taken (transfer or transformation) at each step in order to minimize Sgen
?
Challenges w/Steady Flow•
Irreversibility–
Chemical reaction
–
Reactant mixing–
Rejection of non-equilibrium exhaust
–
Polytropic
compression and expansion (Friction, viscous dissipation)
•
Material Limitations–
Temperature limit
–
Pressure limit
Polytropic
work
-0.5 0 0.5 1 1.5 2-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
s-si(kJ/kgmixK)
h-h i(M
J/kg
mix
)
i
b' c'
b
c
f '
f
Pi
a'a
Reversible WorkCycle
Net WorkOut
Equilibrium Attractor Trajectory
Irreversible Work Cycle
Pi
Premixed Reactants, GRI 3.0Polytropic
efficiency --
0.9
Optimal Pressure Ratio
100 101 102 1030
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
Pressure Ratio
Ent
ropy
Gen
erat
ion
(kJ/
kgm
ixK
)
CombustionFluid FrictionTotal
P*
Nonpremixed
reactantsPolytropic
efficiency --
0.8
Effect of Polytropic
Efficiency
Polytropic Efficiency η
Pres
sure
Lim
it P* (b
ar)
0.6 0.65 0.7 0.75 0.8 0.85100
101
102
103
0.6 0.65 0.7 0.75 0.8 0.851500
1750
2000
2250
2500
2750
3000
3250
Max
imum
Tem
pera
ture
Tm
ax (K
)
Nonpremixed
reactants
In the absence of material limitations, the pressure ratio of today’s engines is well below optimum.
A temperature-limited, extreme-state cycle gives the optimal simple-cycle GT architecture.
Nonpremixed
reactants Polytropic
efficiency --
0.9
T-Limited Simple-Cycle GT
Take-Home Messages (1 of 2)•
Despite three centuries of effort, engine efficiency remains well below theoretical limits
(resource exergy)—often by more than a factor of two.
•
Misconceptions
about what ultimately limits engine efficiency (e.g., Carnot) are sometimes to blame.
•
Working in the space between the exergy limit and real engines, we have found the ideas of classification and
architecture to be useful.
•
Our approach is to use the principles of optimal control
to identify the most efficient architecture
for any given set of allowable devices, resources, and environment.
•
For chemical engines, a key to understanding is whether the architecture uses restrained or
unrestrained reaction.
Take-Home Messages (2 of 2)•
Irreversibility in restrained reaction
engines can be reduced by improving kinetics. To date, the only examples of restrained reaction engines are electrochemical (i.e., fuel cells).
•
Irreversibility in unrestrained reaction
engines can be reduced by reaction at states of high energy density (extreme-states principle).
•
For simple-cycle engines, we believe that architectures capable of delivering 60% first-law efficiency
are possible.
•
For regenerative engines, we believe a systematic approach to identifying optimal architectures can be developed. We speculate that such engines are capable of 70% first-law efficiency.
•
For combined-cycle engines, we speculate that a systematic approach is again possible and can lead to the development of engines with first-law efficiencies in excess of 80%.