Distributed Solar-Thermal-Electric Generation and Storage Seth R. Sanders, Artin Der Minassians, Mike He EECS Department, UC Berkeley

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1 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 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 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 Up to 250 o Cwithout 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

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/m 2 (PV standard) Schott ETC 16 collector Engine: 2/3 of Carnot eff.

7 Collector Cost Cost per tube [1] < $3 Input aperture per tube m 2 Solar power intensity G 1000 W/m 2 Solar-electric efficiency 10% Tube cost Manifold, insulation, bracket, etc. [2] Total $0.34/W $0.61/W $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)

9 Prototype #1

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

11 2 nd Prototype: 3-Phase Free-Piston Actuator mounting jaw Nylon flexure (cantilever spring) Axis of rotation Sealed clearance Cooler Heater Diaphragm Cold side piston plate

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 0.24 Efficiency and Power Output Contour Plot 60Hz, 10bar Air Mech Work vs Strokes Power piston stroke Power Piston Stroke Displacer Stroke Displacer stroke

14 Displacer Subsystem Linear ball bearing Sm-Co magnet PEEK body

15 System Schematic