1. Distributed Solar-Thermal-Electric Generation and Storage Seth R
Distributed Solar-Thermal-Electric Generation and Storage Seth R. Sanders, Artin Der Minassians, Mike He EECS Department, UC Berkeley
Technology:
rooftop solar thermal collector +
thermal energy storage +
Low/medium temperature Stirling engine +
hot water cogen with rejected heat

2. Present status: prototype Stirling machines prove concept
Economic Analysis:
Estimate installed cost at about $3/W for solar-thermal electric generation only system, substantially lower than present day installed PV
Present status: prototype Stirling machines prove concept
Future Opportunity:
Multi-thermal source heat conversion – waste, solar, cogen, storage (bidirectional)
Scalable thermal-electric energy storage – capacity (kw-hr, kw) separately scalable
Co-locate with other intermittent sources/loads – key component of microgrid type system
Other apps: heat pump, refrigeration,..
Research needs:
Economic opportunity assessment of thermal cogen and thermal electric storage
Component work on:
low temp Stirling engine
High performance (eg. concentrating cpc) evacuated tube collectors
Thermal energy storage subsystem

3. Residential Example 30-50 sqm collector => 3-5 kWe peak at 10%eff
Reject 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

4. System Components Solar-Thermal Collector Stirling Engine
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]
Stirling Engine
Can achieve large fraction (70%) of Carnot efficiency
Low cost: bulk metal and plastics
Simple components
Possible direct AC generation (eliminates inverter)
[1] Prof. Roland Winston, CITRIS Research Exchange, UC Berkeley, Spring 2007, also Apricus and Schott
[2] SunMaxxSolar (SolarHotWater.SiliconSolar.com), confirmed by manufacturer 4

5. 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 – orders of magnitude higher than pumped storage
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

6. Electrical Efficiency
G = 1000 W/m2 (PV standard)
Schott ETC-16 collector
Engine: 2/3 of Carnot eff. 6

7. Collector Cost 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
[1] Prof. Roland Winston, CITRIS Research Exchange, UC Berkeley, Spring 2007, also direct discussion with manufacturer
[2] communications with manufacturer/installer 7

8. Stirling Engine (alpha)4 1 2 3

9. Prototype #1
Displacer and power piston can independently be driven. 9

10. Prototype Operation
PhD dissertation of Artin Der Minassians for complete details:
All units are in Watts
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
*Experimentally measured values 10

11. 2nd Prototype: 3-Phase Free-Piston11

12. What’s Next?
Experimental work so far uses ambient pressure air, low frequency, resulting in low power density and low efficiency
Scaling: P = k * p * f * V_sw
Similar design with p=10 bar, f=60 Hz yields ~5 kW at very high efficiency, the promised 75% of Carnot
Design/experimental work with thermal storage
Economic analysis of cogen, energy storage opportunities

13. Efficiency and Power Output Contour Plot
60Hz, 10bar Air
Power piston stroke
Displacer stroke

14. Displacer Subsystem

15. System Schematic