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STIRLING ENGINE AND HIGH EFFICIENCY COLLECTORS FOR SOLAR THERMAL Mike He, Achintya Madduri, Seth Sanders

Motivation

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 Thermal storage is highly dense, cost-effective  Flexible input – can use gas, solar, or electricity  Storage medium is cheap  Contributes to building slack  Predictable, controllable generation  Reversible process allows off-peak storage  Can reduce fossil fuel footprint  Can use solar input  Waste heat can be utilized

System Schematic

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 Non-tracking collector  Low cost Thermal energy storage  Stirling engine generates electricity, waste heat

Project Goals

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 Design, Build, and Test Stirling engine prototype to demonstrate efficiency and low cost  Design and test passive concentrator design for higher efficiency  Evaluate commercialization potential

Novel Design Challenges

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 Designing for high efficiency, given low temperatures from distributed solar  High importance of low cost and long lifetime design  Improve commercially available collectors with passive concentrators

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Stirling Cycle Overview

2 3 4 1

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Heat Exchanger Design

Component Hot-side Liquid to Metal Hot-side Metal to Air Cold-side Liquid to Metal Cold-side Metal to Air Temperature Drop (C) 1.79

1.26

2.42

1.09

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Design characteristics

Design Characteristics Nominal Power Output Thermal-Electric Efficiency Fraction of Carnot Efficiency Hot Side Temperature Cold Side Temperature Working Gas (Air) Pressure Engine Frequency Electrical Output Regenerator Effectiveness Piston Swept Volume Value 2.525 kW 21.5% 65% 180 30 o C o C 25 bar 20 Hz 60Hz, 3 φ 0.9967

2.2 L

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Design and Fabrication

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Prototype Pictures

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

No Concentration

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Concentrator for Evacuated Tube Absorber  Passive involute-shaped concentrator  Produces concentration ratio ~pi in ideal case  Can reduce # tubes by concentration ratio  Lowers losses and/or increases operating temperature, improving efficiency

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Evacuated Tube Absorber

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Collector testing system

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Questions

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Cost Comparison – no concentration

Solar Thermal

Component Collector Engine Installation -Hardware -Labor Total

Photovoltaic

$/W 0.95

0.5

0.75

1.25

$3.45

Component PV Module Inverter Installation -Hardware -Labor Total $/W 4.84

0.72

0.75

1.25

$7.56

With concentrator: expect substantial cost and area reduction due to efficiency increase

Source: PV data from Solarbuzz

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Electrical/Thermal Conversion and Storage Technology and Opportunities       Electricity Arbitrage – diurnal and faster time scales  LoCal market structure provides framework for valuation  Demand Charges avoided Co-location with variable loads/sources relieves congestion Avoided costs of transmission/distribution upgrades and losses in distribution/transmission Power Quality – aids availability, reliability, reactive power Islanding potential – controlling frequency, clearing faults Ancilliary services – stability enhancement, spinning reserve

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Comparison of Water Heating Options “Consumer Guide to Home Energy Savings: Condensed Online Version” American Council for an Energy-Efficient Economy. August 2007. < http://www.aceee.org/Consumerguide/waterheating.htm >.

Ex. 3: Waste heat recovery + thermal storage

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Waste heat stream 100-250 C or higher Thermal Reservoir Electric generation on demand Heat Engine Converter Domestic Hot Water ?

•Huge opportunity in waste heat

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Thermal System Diagram

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Solar Dish: 2-axis track, focus directly on receiver (engine heat exchanger) Photo courtesy of Stirling Energy Systems.

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Stirling Cycle Overview

2 3 4 1

Residential Example

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    30 sqm collector => 3 kWe at 10% electrical system eff.

15 kW thermal input. Reject 12 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. Potential for very high cyclability.

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Displacer Power piston      Temperatures: Working fluid: Frequency: Pistons – Stroke: – Diameter: Indicated power: – Schmidt analysis – Adiabatic model T h =175 o C, T k =25 o C Air @ ambient pressure 3 Hz 15 cm 10 cm 75 W (thermal input) - 25 W (mechanical output) 254 W (thermal input) - 24 W (mechanical output)

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Prototype 1: free-piston Gamma

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Prototype 2 – Multi-Phase “Alpha”

Actuator mounting jaw Nylon flexure (cantilever spring) Axis of rotation Sealed clearance Cooler Heater Diaphragm Cold side piston plate

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Prototype Operation

Power Breakdown (W)

Indicated power 26.9

Gas spring hysteresis 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

Collector Cost – no concentration

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

Related apps for eff. thermal conv

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 Heat Pump  Chiller  Refrigeration  Benign working fluids in Stirling cycle – air, helium, hydrogen