Stirling Engine System for Solar Thermal
Generation and Energy Storage
LoCal Retreat, June 8-9 2009
Outline

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Overview/Motivation
System Description
Early Prototypes
Higher Power Engine Design
Thermal Energy Applications

Solar Thermal
 Dispatchable
Generation
 Low cost, simple manufacturing

Thermal Storage
 Dispatchable
Resource
 Low capital cost

Waste Heat Recovery
 Free
energy source – Industrial Processes, Combined
Cycle
 Low Temperature
Renewable Energy Challenges
Renewable Energy Challenges
Solar Thermal Advantages

Cost

Lower Cost

Intermittency

Inherent Storage

Production bottlenecks

Simple Manufacturing

Versatility
Intermittency and Energy Storage
4.6 MW Solar Installation
1.5 MW Wind Turbine
Source: J. Apt, A. Curtright, “The Spectrum of Power from Utility-Scale Wind Farms and Solar Photovoltaic Arrays”, CEIC 2008
Cost Comparison
Solar Thermal
Component
Collector
Engine
Balance of System
Total
Energy Storage
Thermal
Source: PV data from Solarbuzz
Photovoltaic
$/W
0.34
0.5
3.6
4.44
$/kWh
20
Component
PV Module
Inverter
Balance of System
Total
Energy Storage
Batteries
$/W
4.70
0.72
3.6
9.02
$/kWh
2030
Solar System Schematic
Stirling Engine

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Can achieve large fraction (60-70%) of Carnot
efficiency
Low cost, simple components
Fuel Flexible
Reversible
Scalable engine and storage capacity
Stirling Cycle Overview
4
1
2
3
Research

Designed, built, tested two low power prototypes
 Single
phase and multiphase machines
 Low power
 Verified engineering models

Design of high power prototype
 Improved
simulation and design
 Heat exchanger design
 Optimization of geometry, parameters
Prototype 1: Single Phase
Gamma-Type Free-Piston Stirling
Displacer




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Temperatures:
Working fluid:
Frequency:
Pistons
–
Stroke:
–
Diameter:
Indicated power:
–
Schmidt analysis
–
Adiabatic model
Power piston
Th=175 oC, Tk=25 oC
Air @ ambient pressure
3 Hz
15 cm
10 cm
75 W (thermal input) - 25 W (mechanical output)
254 W (thermal input) - 24 W (mechanical output)
Prototype Operation
Power Breakdown (W)
Indicated power
26.9
Gas spring hysteresis
10.5
Expansion space enthalpy loss
0.5
Cycle output pV work
15.9
Bearing friction and eddy loss
1.4
Coil resistive loss
5.2
Power delivered to electric load
9.3
Piston Systems
Prototype 2: Multi-phase
Nylon flexure
(cantilever spring)
Axis of rotation
Actuator
mounting jaw
Sealed
clearance
Cooler
Heater
Diaphragm
Cold side
piston plate
Components
Experimental Data
Parameter
Value
Working fluid
Ambient air
Frequency
~30 Hz
Hot side temperature
147 oC
Cold side temperature
27 oC
Power (per phase)
12.7 W
Calculated damping (per phase)
19.5 W
Dominant damping (per phase)
Gas hysteresis (10.8 W)
Gas Compression Loss
Reverser
More Phases => Less Compression
High Power Design
Design Characteristics
Nominal Power Output
Thermal-Electric Efficiency
Fraction of Carnot Efficiency
Hot Side Temperature
Cold Side Temperature
Pressure
Engine Frequency
Regenerator Effectiveness
Total Heat Exchanger Flow Loss
Regenerator Flow Loss
Compression Loss
Hot Side Heat Exchanger Temp Drop
Cold Side Heat Exchanger Temp Drop
Value
2.525 kW
21.5%
65%
180 oC
30 oC
25 bar
20 Hz
0.9967
54.5 W
166.6 W
66.8 W
2.74 oC
3.01 oC
Energy Flows and Losses
Regenerator
Ineffectiveness
Heat Transfer
Leakages
Heat In
Heater
Ideal
Stirling
Cycle
Heater and ½
Regenerator
Flow Loss
Cooler
Cooler and ½
Regenerator
Flow Loss
Rejected
Heat
Internal Bearing
& Motion Losses
PV Work
Out
Gas Hysteresis
Loss
Electrical
Output
Alternator
Inefficiency,
Bearing Losses
Differences from prototypes

Design Improvements
 Improved
heat exchanger design
 Refined simulation and models
 Extensive optimization

Scaling
 Increased
pressure
 Increased frequency
 Increased volume
 Relatively smaller losses
Efficiency and Power Output Contour Plot
20Hz, 25bar Air
Efficiency and Power (W)
0.215
30
00
25
00
00
20
15
0.2
0.05
0.045
00
15
Power Piston Stroke (m)
00
30
0
15 0
0.06
0.055
5
0.21
21
0.
5
21
.
0
0.04
0.035
35
00
0.2
1
1
0.2
00
25
0
20 0
0.065
0.
20
5
0.07
15
00
25 0
0
20
00
1
0.2
05
0.2
0.2
20 00
95
0.1
2
10
.
0
05
0.19
0.21
00
0.2
15 00
0.03
0.008 0.009 0.01 0.011 0.012 0.013 0.014 0.015 0.016 0.017 0.018
Displacer Stroke (m)
What’s Next?
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Finalize designs
Fabrication and testing of high power prototype
Design/experimental work with thermal storage
Explore waste heat electric generation
Economic analysis of cogen, energy storage
opportunities
Residential Example
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30-50 sqm collector => 3-5 kWe peak at 10%eff
Reject 12-20 kW thermal power at peak. Much larger
than normal residential hot water systems – would provide
year round hot water, and perhaps space heating
Hot side thermal storage can use insulated (pressurized)
hot water storage tank. Enables 24 hr electric generation
on demand.
Another mode: heat engine is bilateral – can store energy
when low cost electricity is available
Thermal Storage Example
Sealed, insulated water tank
 Cycle between 150 C and 200 C
 Thermal energy density of about 60 W-hr/kg, 60 W-hr/liter
 Considering Carnot (~30%) and non-idealities in conversion
(50-70% eff), remain with
10 W-hr/kg
 Very high cycle capability
 Cost is for container & insulator

Collector and Engine Efficiency
G = 1000 W/m2 (PV standard)
Schott ETC-16 collector
Engine: 2/3 of Carnot eff.
Energy Storage Comparison
Storage Technology
Energy Density
Cost
Self-discharge
Round Trip Efficiency Lifetime
Thermal (various media)
20-80 kWh/m3
$40-65/ kWh
1-2% per day
70-80%
Unlimited
Flywheel
Compressed Air
0.2 Wh/kg
2 kWh/m3
Minimal
None
80-90%
80%
~20 years
Unlimited
Superconducting Magnetic
Energy Storage
1-10 Wh/m3
$300/kWh
$1-5/kWh
(storage only)
$54,000/kWh
None with cooling 90-95%
Unlimited
Pumped Hydro
0.3 kWh/m3 @ 100m $10-45/kWh
None
75%
Unlimited
NiMH Battery
NiCad Battery
Lithium Ion Battery
30–80 Wh/kg
40-60 Wh/kg
160 Wh/kg
$364/kWh
$400/kWh
$300/kWh
30%/month
20%/month
5%/month
66%
70-90%
99.90%
500-1000 cycles
1500 cycles
1200 cycles
Lithum Polymer Battery
130-200 Wh/kg
$500/kWh
10%/month
99.50%
1000 cycles
Lead Acid Battery
30-40 Wh/kg
$100-200/kWh
3%-4%/month
70%-92%
500-800 cycles