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

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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

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• 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

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Residential Example

• 30-50 sqm collector gt 3-5 kWe peak at 10eff
• 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

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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 mm1
• 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.Silicon
Solar.com), confirmed by manufacturer
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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

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

• Cost per tube 1 lt 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
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Stirling Engine (alpha)
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1
2
3
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Prototype 1
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Prototype Operation

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

All units are in Watts All units are in Watts 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
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2nd Prototype 3-Phase Free-Piston
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Whats 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 p10 bar, f60 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

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Efficiency and Power Output Contour Plot
60Hz, 10bar Air
Power piston stroke
Displacer stroke
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Displacer Subsystem
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System Schematic