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STIRLING ENGINE AND HIGH
EFFICIENCY COLLECTORS FOR SOLAR
THERMAL
Mike He, Achintya Madduri, Seth Sanders
Motivation
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Thermal storage is highly dense, cost-effective
Flexible input – can use gas, solar, or electricity
 Storage medium is cheap
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Contributes to building slack
Predictable, controllable generation
 Reversible process allows off-peak storage
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Can reduce fossil fuel footprint
 Can
use solar input
 Waste heat can be utilized
System Schematic
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Non-tracking collector
Low cost Thermal energy storage
Stirling engine generates electricity, waste heat
Project Goals
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Design, Build, and Test Stirling engine prototype to
demonstrate efficiency and low cost
Design and test passive concentrator design for
higher efficiency
Evaluate commercialization potential
Novel Design Challenges
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Designing for high efficiency, given low
temperatures from distributed solar
High importance of low cost and long lifetime
design
Improve commercially available collectors with
passive concentrators
Stirling Cycle Overview
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4
1
2
3
Heat Exchanger Design
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Component
Hot-side Liquid to Metal
Temperature Drop (C)
1.79
Hot-side Metal to Air
1.26
Cold-side Liquid to Metal
2.42
Cold-side Metal to Air
1.09
Design characteristics
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Design Characteristics
Nominal Power Output
Thermal-Electric Efficiency
Fraction of Carnot Efficiency
Hot Side Temperature
Cold Side Temperature
Working Gas (Air) Pressure
Engine Frequency
Electrical Output
Regenerator Effectiveness
Piston Swept Volume
Value
2.525 kW
21.5%
65%
180 oC
30 oC
25 bar
20 Hz
60Hz, 3φ
0.9967
2.2 L
Design and Fabrication
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Prototype Pictures
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Collector and Engine Efficiency
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Collector with concentration
G = 1000 W/m2 (PV standard)
Schott ETC-16 collector
Engine: 2/3 of Carnot eff.
No Concentration
Concentrator for Evacuated Tube
Absorber
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Passive involute-shaped concentrator
Produces concentration ratio ~pi in
ideal case
Can reduce # tubes by
concentration ratio
Lowers losses and/or increases
operating temperature, improving
efficiency
Evacuated Tube Absorber
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Collector testing system
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Questions
Cost Comparison – no concentration
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Solar Thermal
Component
Collector
Engine
Installation
-Hardware
-Labor
Total
Photovoltaic
$/W
0.95
0.5
0.75
1.25
$3.45
Component
PV Module
Inverter
Installation
-Hardware
-Labor
Total
$/W
4.84
0.72
0.75
1.25
$7.56
With concentrator: expect substantial cost and area reduction due to
efficiency increase
Source: PV data from Solarbuzz
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Electrical/Thermal Conversion and Storage
Technology and Opportunities
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Electricity Arbitrage – diurnal and faster time scales
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LoCal market structure provides framework for valuation
Demand Charges avoided
Co-location with variable loads/sources relieves congestion
Avoided costs of transmission/distribution upgrades and losses
in distribution/transmission
Power Quality – aids availability, reliability, reactive power
Islanding potential – controlling frequency, clearing faults
Ancilliary services – stability enhancement, spinning reserve
Comparison of Water Heating Options
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“Consumer Guide to Home Energy Savings: Condensed Online Version” American Council for an
Energy-Efficient Economy. August 2007. <http://www.aceee.org/Consumerguide/waterheating.htm >.
Ex. 3: Waste heat recovery + thermal storage
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Waste heat stream
100-250 C or higher
Thermal Reservoir
Electric generation
on demand
Heat Engine Converter
Domestic Hot Water ?
•Huge opportunity in waste heat
Thermal System Diagram
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Solar Dish: 2-axis track, focus directly
on receiver (engine heat exchanger)
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Photo courtesy of Stirling Energy Systems.
Stirling Cycle Overview
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4
1
2
3
Residential Example
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30 sqm collector => 3 kWe at 10% electrical
system eff.
15 kW thermal input. Reject 12 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.
Potential for very high cyclability.
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Displacer
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Temperatures:
Working fluid:
Frequency:
Pistons
– Stroke:
– Diameter:
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Power piston
Th=175 oC, Tk=25 oC
Air @ ambient pressure
3 Hz
15 cm
10 cm
Indicated power:
– Schmidt analysis
– Adiabatic model
75 W (thermal input) - 25 W (mechanical output)
254 W (thermal input) - 24 W (mechanical output)
Prototype 1: free-piston Gamma
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Prototype 2 – Multi-Phase “Alpha”
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N ylon flexu re
(can tilever sp rin g )
A xis of rotation
A ctu ator
m ou n tin g jaw
S ealed
clearan ce
C ooler
H eater
D iap h ragm
C old sid e
p iston p late
Prototype Operation
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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
Collector Cost – no concentration
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Cost per tube [1]
Input aperture per tube
Solar power intensity G
Solar-electric efficiency
< $3
0.087 m2
1000 W/m2
10%
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Tube cost
Manifold, insulation, bracket, etc. [2]
$0.34/W
$0.61/W
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Total
$0.95/W
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[1] Prof. Roland Winston, also direct discussion with manufacturer
[2] communications with manufacturer/installer
Related apps for eff. thermal conv
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Heat Pump
Chiller
Refrigeration
 Benign
working fluids in Stirling cycle – air, helium,
hydrogen