Distributed Solar-Thermal-Electric Generation and Storage

Seth R. Sanders, Artin Der Minassians, Mike He • Technology: EECS Department, UC Berkeley – rooftop solar thermal collector + – thermal energy storage + – Low/medium temperature Stirling engine + – hot water cogen with rejected heat

• 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

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

System Components

• • • • • • •

Solar-Thermal Collector

Up to 250 o C 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

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

Electrical Efficiency

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

Collector Cost

• Cost per tube [1] • Input aperture per tube • Solar power intensity G • Solar-electric efficiency • Total < $3 0.087 m 2 1000 W/m 10% • Tube cost • Manifold, insulation, bracket, etc. [2] $0.34/W $0.61/W $0.95/W 2 [1] Prof. Roland Winston, CITRIS Research Exchange, UC Berkeley, Spring 2007, also direct discussion with manufacturer [2] communications with manufacturer/installer

Stirling Engine (alpha)

2 3 4 1

Prototype #1

Prototype Operation

• PhD dissertation of Artin Der Minassians for complete details: http://www.eecs.berkeley.edu/Pubs/TechRpts/2007/EECS-2007-172.pdf

All units are in Watts

Indicated power Gas spring hysteresis * 26.9

Expansion space enthalpy loss Cycle output pV work * Bearing friction and eddy loss Coil resistive loss * Power delivered to electric load * 15.9

9.3

10.5

0.5

1.4

5.2

* Experimentally measured values

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

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

Efficiency and Power Output Contour Plot

0.04

Power piston stroke

0.

23 5 0.035

50 00 0.2

4 0.03

40 00

60Hz, 10bar Air

Mech Work vs Strokes 60 00 70 00 0.24

50 00 60 00 0 .2

4 0.025

0.02

30 00 0.24

0.015

0.008

0.235

0.009

500 0 400 0 0.2

4 0.2

35 0.23

3000 0.23

5 0.23

0.01

0.011

0.22

5 0.012

Displacer Stroke 4000 0.23

0.22

0.013

0.22

5 0.014

4000 0.015

Displacer stroke

Displacer Subsystem

Linear ball bearing Sm-Co magnet PEEK body

System Schematic