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