1. Stirling engine and high efficiency collectors for solar thermal
Mike He, Achintya Madduri, Seth Sanders

2. Motivation Thermal storage is highly dense, cost-effective
Flexible input – can use gas, solar, or electricity
Storage medium is cheap
Contributes to building slack
Predictable, controllable generation
Reversible process allows off-peak storage
Can reduce fossil fuel footprint
Can use solar input
Waste heat can be utilized

3. System Schematic Non-tracking collector
Low cost Thermal energy storage
Stirling engine generates electricity, waste heat

4. Project Goals
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

5. Novel Design Challenges
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

6. Stirling Cycle Overview4 1 2 3 6

7. Heat Exchanger Design Component Temperature Drop (C)
Hot-side Liquid to Metal
1.79
Hot-side Metal to Air
1.26
Cold-side Liquid to Metal
2.42
Cold-side Metal to Air
1.09

8. Design characteristics
Value
Nominal Power Output
2.525 kW
Thermal-Electric Efficiency
21.5%
Fraction of Carnot Efficiency
65%
Hot Side Temperature
180 oC
Cold Side Temperature
30 oC
Working Gas (Air) Pressure
25 bar
Engine Frequency
20 Hz
Electrical Output
60Hz, 3φ
Regenerator Effectiveness
0.9967
Piston Swept Volume
2.2 L

9. Design and Fabrication

10. Prototype Pictures

11. Collector and Engine Efficiency
Collector with concentration
G = 1000 W/m2 (PV standard)
Schott ETC-16 collector
Engine: 2/3 of Carnot eff.
No Concentration 11 11

12. Concentrator for Evacuated Tube Absorber
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

13. Evacuated Tube Absorber
The operation of the solar collector is very simple. 1. Solar Absorption: Solar radiation is absorbed by the evacuated tubes and converted into heat. 2. Solar Heat Transfer: Heat pipes conduct the heat from within the solar tube up to the header. 3. Solar Energy Storage: Water is circulated through the header, via intermittent pump cycling. Each time the water circulates through the header the temperatures is raised by 5-10oC / 9-18oF. Throughout the day, the water in the storage tank is gradually heated. 
Introduction, How Evacuated tubes work, rated temperature and water tank.

14. Collector testing system

15. Questions

16. Cost Comparison – no concentration
Solar Thermal
Photovoltaic
Component
$/W
Collector
0.95
Engine
0.5
Installation
-Hardware
0.75
-Labor
1.25
Total
$3.45
Component
$/W
PV Module
4.84
Inverter
0.72
Installation
-Hardware
0.75
-Labor
1.25
Total
$7.56
With concentrator: expect substantial cost and area reduction due to
efficiency increase
Source: PV data from Solarbuzz

18. Electrical/Thermal Conversion and Storage Technology and Opportunities
Electricity Arbitrage – diurnal and faster time scales
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

19. Comparison of Water Heating Options
“Consumer Guide to Home Energy Savings: Condensed Online Version” American Council for an Energy-Efficient Economy. August < >.

20. Ex. 3: Waste heat recovery + thermal storage
Waste heat stream
C or higher
Thermal Reservoir
Electric generation
on demand
Heat Engine Converter
Domestic Hot Water ?
Huge opportunity in waste heat

21. Thermal System Diagram

22. Photo courtesy of Stirling Energy Systems.
Solar Dish: 2-axis track, focus directly on receiver (engine heat exchanger)
Photo courtesy of Stirling Energy Systems.

23. Stirling Cycle Overview4 1 2 3 23

24. Residential Example
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.

25. Gamma-Type Free-Piston Stirling
Displacer
Power piston
Temperatures: Th=175 oC, Tk=25 oC
Working fluid: ambient pressure
Frequency: 3 Hz
Pistons
Stroke: 15 cm
Diameter: 10 cm
Indicated power:
Schmidt analysis 75 W (thermal input) - 25 W (mechanical output)
Adiabatic model 254 W (thermal input) - 24 W (mechanical output) 25

26. Prototype 1: free-piston Gamma
Displacer and power piston can independently be driven. 26 26

27. Prototype 2 – Multi-Phase “Alpha”

28. Prototype Operation Power Breakdown (W) 26.9 10.5 0.5 15.9 1.4 5.2 9.3
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 28 28

29. Collector Cost – no concentration
Cost per tube [1] < $3
Input aperture per tube m2
Solar power intensity G W/m2
Solar-electric efficiency 10%
Tube cost $0.34/W
Manifold, insulation, bracket, etc. [2] $0.61/W
Total $0.95/W
Solar-Thermal Collector
Up to 250 oC without tracking [1]
Low cost: glass tube, sheet metal, plumbing
Simple fabrication (e.g., fluorescent light bulbs)
~$3 per tube, 1.5 m x 47 mm[1]
No/minimal maintenance (round shape sheds water)
Estimated lifespan of years, 10 yrs warranty [2]
Easy installation – hr per module [2]
[1] Prof. Roland Winston, also direct discussion with manufacturer
[2] communications with manufacturer/installer 29 29

30. Related apps for eff. thermal conv
Heat Pump
Chiller
Refrigeration
Benign working fluids in Stirling cycle – air, helium, hydrogen