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

• 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

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]

[1] Prof. Roland Winston, CITRIS Research Exchange, UC Berkeley, Spring 2007, also Apricus and Schott[2] SunMaxxSolar (SolarHotWater.SiliconSolar.com), confirmed by manufacturer

Stirling Engine• Can achieve large fraction (70%) of Carnot efficiency• Low cost: bulk metal and plastics• Simple components • Possible direct AC generation (eliminates inverter)

System Components

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

G = 1000 W/m2 (PV standard)

Schott ETC-16 collector

Engine: 2/3 of Carnot eff.

Electrical Efficiency

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

Stirling Engine (alpha)

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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 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

2nd Prototype: 3-Phase Free-Piston

Nylon flexure (cantilever spring)

HeaterCooler

Cold side piston plate

Actuator mounting jaw

Axis of rotation

Diaphragm

Sealed clearance

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

0.220.225

0.225

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Displacer Stroke

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Mech Work vs Strokes

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7000

0.008 0.009 0.01 0.011 0.012 0.013 0.014 0.0150.015

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60Hz, 10bar Air

Displacer stroke

Power piston stroke

Displacer Subsystem

Sm-Co magnet

Linear ball bearing

PEEK body

System Schematic