Technology: rooftop solar thermal collector + thermal energy storage +

1. Distributed Solar-Thermal-Electric Generation and StorageSeth 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. 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 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

4. System Components • 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 25-30 years, 10 yrs warranty [2] • Easy installation – 1.5-2 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

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.

7. Collector Cost Cost per tube [1] < $3 Input aperture per tube 0.087 m2 Solar power intensity G 1000 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

8. Stirling Engine (alpha) 4 1 2 3

9. Prototype #1

10. Prototype Operation • PhD dissertation of Artin Der Minassians for complete details: http://www.eecs.berkeley.edu/Pubs/TechRpts/2007/EECS-2007-172.pdf *Experimentally measured values

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

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