Microscale Power Generation: Challenges and Opportunities Yiguang Ju Princeton University ARPA-E kickoff meeting Chicago, 2015.10.20 AFOSR: University Capstone Project (b) Hot diffusion flame (a) Cool flame
Microscale Power Generation: Challenges and Opportunities
Yiguang Ju
Princeton University
ARPA-E kickoff meeting
Chicago, 2015.10.20
AFOSR: University Capstone Project
(b) Hot diffusion
flame
(a) Cool flame
Contents
• Development and Challenges of Mesoscale Power
• Flame Quenching Limit and Heat Recirculation
• Engine Lean Burn limit:
Ignition energy/ignition kernel size
• Plasma Assisted Combustion:
A new opportunity for ignition/emission control
of small scale engines?
• Summary
Small Engines (kW) in UAVs and
Distributed power generation
3
• Small UAVs for military
and civilian operations.
• Hybrid power system for
remote areas.
https://en.wikipedia.org/wiki/Stand-alone_power_system
Challenges:
Efficiency, efficiency, efficiency…
Emissions..
Wind, solar…
Energy storage Smart grids
Biofuels
Sustainable energy triangles: ideally
SynfuelsBatteries
Fuel cells
Sustainable electricity Sustainable transportation
Boeing 787
Gasoline: 40 MJ/kg,
8 gal/min : ~16 MW
Lithium
Aviation?Battery: < 1 MJ/kg
Tesla charger 120 KW
Advanced
Turbines
(NG, IGCC)
Advanced
engines
(New Fuel, Effic.)
In Reality
In Reality
(3% world, 10% US)
In Reality
Slow progress
Energ
y d
ensity (
kW
hr/
kg)
Lithiu
m ion
Batt
ery
Meth
ane
Eth
anol
Die
sel
Bio
die
sel
Hydra
zin
e
Die
sel
engin
e
Mic
ro-
engin
e
SS
ME
0
5
10
15
Mic
ro-
Fuel c
ell
Comparison of specific energy/power densities
The Ragone plot
Combustion engines have high power density and energy density!
Ju Y. and Maruta K.,
Progress in Energy and Combustion
Science, 37(6), 669-715.
2. Development and Challenges of
Microscale Power Generators
Micro turbine(MIT )
12
.5 m
m
Intake
PortOutput Shaft
Exhaust
Port
Housing &
Rotor
12
.5 m
m
Intake
PortOutput Shaft
Exhaust
Port
Housing &
Rotor
UCB rotary engine
Combustion chamber
Piston
Piston
Combustion chamber
Piston
Piston
free-piston ‘‘knock’’ engine
Minimization with moving parts
• Sealing and leakage are big problems!
• Large surface area and heat loss
Combustion limits of small scale combustion
Nud
Hf
2
24
releaseheat chemical total
wall the tolossheat Scaling of heat loss:
a d2 law
0.0 0.1 0.2 0.3 0.4 0.50.0
0.2
0.4
0.6
0.8
1.0
Extinction
e-1
e-1/2
No
rma
lize
d f
lam
e s
pe
ed
Normalized heat loss, H
Flame speed vs. heat loss
Quenching limit
Quenching distance: d0
Quenching distance
Min
imum
ignitio
n e
nerg
y
Heat recirculation to reuse heat loss
a
2fx1fx
xU
UQ
Q
H
Channel 2
Channel 1
L
W
S
R
U
d Opposite propagating flamesc U shaped flame
b
H
a
2fx1fx
xU
UQ
Q
H
Channel 2
Channel 1
L
W
S
R
U
d Opposite propagating flamesc U shaped flame
b
H
Tad
T
Flow direction
Tad
T
Flow direction
No heat recirculation W. heat recirculation
Te
mpera
ture
0.00 0.02 0.04 0.060.0
0.5
1.0
1.5
2.0
Slow mode
Fast mode
extinction
c
0.0
0.00010.0005
=0.002
Fla
me s
peed, U
Heat loss,
1E-3 0.01 0.1 1
0.1
1
0.00.1
0.2
0.5
1.0
u=2.0
C=0.01, w/
g=0.1
No
rma
lize
d f
lam
e s
pe
ed
, m
Normalized heat loss, H
E
Heat recirculation allows lean burn and breaks quenching limit…
releaseheat chemical total
wall the tolossheat H
• Combustion can happen beyond
critical heat loss
• Flame speed can be higher than
adiabatic flame speed
• Combustion limits depends the flow
speed
Penn-state vortex engine: 49.1 mm3
Heat recirculation and burning w.o. moving parts
Disk-shaped Swiss-roll combustors
Tohoku University
Efficient burning, but only as a heating source
Scheme of heat recirculating burners and burner prototypes fabricated at USC
combustion physics laboratory. (a) concept of heat recirculation in U-shaped
channel, (b) 2-D Swiss-roll burner, (c) Macro- and (d) meso-scale, toroidal 3-D
burners. Both macro- and meso-scale versions attained self-stabilized combustion.
Swissroll combustor(PU, USC,TU)
Thermoelectric? but temperature gradient is two small, efficiency is low
Mesoscale catalytic two-stage burner: Princeton
Fuji Imvac BF-34EI
13
• Weight: 5.89 lbs (2.674 kg)
• Displacement: 2.1 cu in (34cc)
• Bore/Stroke: 1.54x 1.10 in (39x28 mm)
• Practical RPM: 1000-7500
• Horsepower: 2 hp (1491 W) @ 7500 RPM
• Peak Torque: 1.446 lb ft (0.20kgm) @ 5,000 RPM
Small internal combustion engines: 1-10 HP
• 1 = steady combustion
• 0 = no combustion
Poor engine stability in lean burn
Fig.1 Dependence of engine steady state running on air/fuel ratio
0
1
5 15 25 35
No MW
MW
Only stable
at fuel rich cond.
Why???
Air/fuel ratio
Fuji Imvac BF-34EI
Minimum Ignition Energy?
f
f
R
RR
R
R
fT
TZ
de
eReR
de
eRT
)1(
1
2exp
Le
1Q
ULe2
ULe2U2
U2
U2
fuel ofy diffusivit Mass
ydiffusivit ThermalLe
Q ?Assumptions and simplification:
• 1D quasi-steady state, Constant properties
• One-step chemistry
• Center energy deposition
• Adiabatic
Flame radius, R
Fla
me
pro
pag
atin
gsp
eed
,U
10-1
100
101
102
0.0
0.4
0.8
1.2
1.6
Le=0.5
0.8
1.01.2
2.0
OO O O O
adiabatic(h=0.0)
O
1.4
Flame
ball
Extinction
limit
The critical ignition size and energy is
governed by two different length scales:
•Flame ball size (small Le)
•Extinction diameter (large Le)
Source: Chen & Ju, Comb. Theo. Modeling (2007)
Increasing Molecule Size
15
Rc: critical radius
Cube of critical flame radius, RC
3
Min
imu
nig
nitio
np
ow
er,
Qm
in
0 500 1000 1500 2000 25000
0.5
1
1.5
2
Z = 10
Cube of critical flame radius, RC
3
Min
imu
nig
nitio
np
ow
er,
Qm
in
0 500 1000 1500 2000 25000
0.5
1
1.5
2
Z = 13
2.0
1.4
1.6
1.7
1.8 = Le
1.9
1.51.4
2.5
2.0
1.9
1.8
1.7
1.6
1.5
Le = 2.1
2.3
2.4
2.2
Source: Chen, Burke, Ju, Proc. Comb. Inst. Vol.33 (2010)
Activation energy
Molecule size
of fuelMass Diffusivity Le
2
flame
activation
T
TZ
• Smaller molecules lead to lesser ignition energy
• Plasma can dissociate fuel into smaller molecules
• Lesser activation energy leads to lesser ignition energy.
• Plasma can create species which will have small
activation energy when reacting with the fuel, i.e. radicals
and electronically excited molecules.
16
Activation temperature: The temperature
needed to initiate a particular chemical
reaction
ydiffusivit Mass
ydiffusivit ThermalLe
Minimum Ignition Energy depending on fuel
molecule size
How do we improve
engine ignition reliability?
Make the spark > Rc !
Non-equilibrium plasma: large ignition kernel
MGA
Microwave
Microwave
MGA
t1 t2
Lessheatloss
Nanosec pulses
Less
heat
Loss
Large
volume
t1 t2
O, OH, NO, C2H4… production
Microwave
Spark Large
heat
Loss,
Small
volume
t1 t2
Fig.1 Current spark ignition plug: large heat loss, small volume
Fig.2a New microwave gliding ignition
Increased volume, less heat loss
Fig.2b New microwave repetitive nanosecond
ignition with radical production, increased
volume, less heat loss
Comparison of ignition using microwave, gliding arc and conventional
discharge at 10 atm (Knite Inc. and Imagineering Inc.)
Fig.7 Comparison of measured OH
intensity as a function of time in a
gasoline engine by using a conventional
spark plug and a conventional spark
with microwave
Enlarge ignition kernel size by plasma
Test Facilities
Pressure SensorThermocouple
Air Flow Sensor
Carburetor
Throttle Rod Choke Rod
Exhaust
Small engine test facility at Princeton University
Lefkowitz et al. 2012, Ikeda et al. 2009
Success of plasma assisted combustion in engine
30% extension of lean burn!
It is not the fuel is too lean, it is the ignition spark size too small!
COV: Coefficient of Variation
Lean limit of a Fuji (34 cc) engine was extended dramatically
(30%) by a microwave igniter, at 2000 RPM
• 1 = steady combustion
• 0 = no combustion
Microwave enhancement
Fig.1 Dependence of engine steady state running on air/fuel ratio for igniters
with and without microwave discharge.
0
1
5 15 25 35
No MW
MW
Air/fuel ratio
P-V Plots
22
Left, 2000 RPM, A/F = 23.5
Right, 5000 RPM, A/F = 22.2
-500
0
500
1000
1500
2000
2500
0 10 20 30 40
Pre
ss
ure
, k
Pa
Volume, cm3
No MW
MW
-500
0
500
1000
1500
2000
2500
3000
3500
0 10 20 30 40
Pre
ss
ure
, k
Pa
Volume, cm3
No MW
MW
• Regular spark plugs
Equi. Ratio < 1.4
• Regular spark plugs with
thin (Iridium/Platinum) electrodes
Equi. Ratio < 1.6
• RF, “plasma”, etc. plugs
Equi. Ratio < 1.8
• Distributed NS Sparks
Equi. Ratio > 3
Flammability Limit
Existing Plug-based Ignition Systems
Andrey Y. Starikovskiy
More examples of success of plasma assisted combustion
Pulse detonation engine, nanosecond
Lefkowitz and Ombrello et al. 2013
0.30.20.150.125
Fuel mole fraction, C7H16-air
Pla
sm
a
on
off
Low
Ignition/extinction S-curve Flame speed and propagation
(Flammability limit)
High
temperature
flame
Fla
me
te
mp
era
ture
, K
Equivalence ratioΦ0
1200
Φ0,r
Flow residence time
Te
mp
era
ture
, K
Ignition
Extinction
1500
Ignition to flame transition
(critical radius, Rc)
Flame Radius, R
Fla
me
Pro
pa
ga
ting
Sp
ee
d,U
10-1
100
101
102
10-2
10-1
100
101
Q=0.0
Q=0.1
Q=0.15
Q=0.6
(a), Le=1.2, h=0.0
a
b
cd
e
f
g
h
i
j
Critical ignition radius
Rc
Q?
Opportunity: Plasma change combustion limits?
How plasma can assist combustion?
• Shorten ignition time
• Extend extinction limit
Plasma
Plasma
• Increase flame speed
• Extend flammability limit• Make ignition kernel > Rc
• Accelerate ignition to flame transition
A schematic of the key reaction pathways for high pressure fuel
oxidation of at different temperatures
(blue arrow: Below 700K; yellow arrow: 700-1050 K;
red arrow: above 1050K).
Fuel(RH)
+OH
R+O2RO2
QOOH
O2QOOH
HO2
H2O2
2OH
Small
alkeneC2H3/CH2O
H/HCO+O2+(M)
Plasma
e, R*, N2*, O2*
R(*), R(v), N2(v), O2(v)
+O2
+O2
CO/CO2
Plasma assisted fuel oxidation: chemical pathways
e +O2=O+O(1D) +e
H+O2(1Δg) =O+OH
O(1D)+RH =OH+R
N2(A,B,C)+O2=O+O+N2
N2(v)+HO2 =OH+O+N2
R(v,*)+O2=RO+OH
=???
O3+M =O+O2+M
Slow
Ju and Sun, Plasma assisted combustion, PECS. 2015.
(b) Hot diffusion flame
(a) Cool diffusion flame
Fig.18b Direct photos of n-heptane/oxygen cool diffusion
flame (a) and hot diffusion flame (b) flames, observed at the
identical flow condition, fuel mole fraction of 0.07 and
strain rate of 100 s-1 (Won et al. 2014).
Residence time
Tem
pera
ture Plasma
generated
LTC
Ignition
Extinction
HTC
LTC
t2 t1
t2<< t1
What if a fuel has low temperature chemistry
Plasma assisted cool diffusion flames
Cool flames has no soot and NOx emissions
70
75
80
85
90
0.04 0.08 0.12 0.16 0.2 0.24
Glo
ba
l s
tra
in r
ate
[1
/s]
Equivalence ratio
No flame
Cool premixed flame
Hot premixed flame
DME/O2/O3 premixed flame
~ 3% Xozone in O2/O3 mixtureTop: heated N2 at 600 K
Bottom: DME/O2/O3 mixture at 300K
Regime diagram of cool and hot premixed flames for DME/O2/O3 mixture in
global strain rate and equivalence ratio
Plasma assisted premixed cool flames
Cool flame burns leaner with no soot emissions
Conclusions
• Heat loss, sealing, moving parts are problems for small scale power
generation.
• Heat recirculation and thermal management can enable higher flame
speed and sublimit combustion.
• Ignition in a small scale engine can be problematic at fuel lean
condition due to the critical ignition radius.
• Plasma can enlarge the ignition kernel size, and produce chemistry
effect to enhance ignition and combustion.
• Plasma produced cool flames provide a new way to control
combustion and emissions in small scale engines.
Question?
3 references
1. Ju, Y., & Maruta, K. (2011). Microscale combustion: Technology development
and fundamental research. Progress in Energy and Combustion Science, 37(6),
669-715.
2. Ju, Y., & Sun, W. (2015). Plasma assisted combustion: Dynamics and
chemistry. Progress in Energy and Combustion Science, 48, 21-83.
3. Ju,Y., Cadou, C., Maruta, K., Microscale Combustion and Power Generation,
Momentum Press (January 22, 2015), ISBN-13: 978-1606503065