Stirling Engine System for Solar Thermal Generation and Energy Storage

LoCal Retreat, June 8-9 2009

Outline Overview/Motivation System Description Early Prototypes Higher Power Engine Design

Thermal Energy Applications Solar Thermal

Dispatchable Generation Low cost, simple manufacturing

Thermal Storage Dispatchable Resource Low capital cost

Waste Heat Recovery Free energy source – Industrial Processes,

Combined Cycle Low Temperature

Renewable Energy Challenges

Cost Intermittency Production

bottlenecks

Lower Cost Inherent Storage Simple

Manufacturing Versatility

Renewable Energy Challenges

Solar Thermal Advantages

Intermittency and Energy Storage

1.5 MW Wind Turbine4.6 MW Solar Installation

Source: J. Apt, A. Curtright, “The Spectrum of Power from Utility-Scale Wind Farms and Solar Photovoltaic Arrays”, CEIC 2008

Cost Comparison

Component $/WCollector 0.34Engine 0.5Balance of System 3.6Total 4.44

Energy Storage $/kWhThermal 20

Component $/WPV Module 4.70Inverter 0.72Balance of System 3.6Total 9.02

Energy Storage $/kWhBatteries 2030

Solar Thermal Photovoltaic

Source: PV data from Solarbuzz

Solar System Schematic

Stirling Engine Can achieve large fraction (60-70%) of

Carnot efficiency Low cost, simple components Fuel Flexible Reversible Scalable engine and storage capacity

1

2

3

4

Stirling Cycle Overview

Research Designed, built, tested two low power

prototypes Single phase and multiphase machines Low power Verified engineering models

Design of high power prototype Improved simulation and design Heat exchanger design Optimization of geometry, parameters

Prototype 1: Single Phase

Temperatures: Th=175 oC, Tk=25 oC Working fluid: Air @ ambient pressure Frequency: 3 Hz Pistons

– Stroke: 15 cm– Diameter: 10 cm

Indicated power:– Schmidt analysis 75 W (thermal input) - 25 W (mechanical output)– Adiabatic model 254 W (thermal input) - 24 W (mechanical output)

Displacer Power piston

Gamma-Type Free-Piston Stirling

Prototype Operation

Power Breakdown (W)Indicated power 26.9 Gas spring hysteresis 10.5 Expansion space enthalpy loss 0.5Cycle output pV work 15.9 Bearing friction and eddy loss 1.4 Coil resistive loss 5.2Power delivered to electric load 9.3

Piston Systems

Prototype 2: Multi-phaseNylon flexure

(cantilever spring)

HeaterCooler

Cold side piston plate

Actuator mounting jaw

Axis of rotation

Diaphragm

Sealed clearance

Components

Parameter ValueWorking fluid Ambient airFrequency ~30 HzHot side temperature 147 oCCold side temperature 27 oCPower (per phase) 12.7 WCalculated damping (per phase)

19.5 W

Dominant damping (per phase) Gas hysteresis (10.8 W)

Experimental Data

Gas Compression Loss

More Phases => Less Compression

Reverser

High Power DesignDesign Characteristics Value Nominal Power Output 2.525 kW Thermal-Electric Efficiency 21.5% Fraction of Carnot Efficiency 65% Hot Side Temperature 180 oC Cold Side Temperature 30 oC Pressure 25 bar Engine Frequency 20 Hz Regenerator Effectiveness 0.9967 Total Heat Exchanger Flow Loss 54.5 WRegenerator Flow Loss 166.6 WCompression Loss 66.8 WHot Side Heat Exchanger Temp Drop 2.74 oCCold Side Heat Exchanger Temp Drop 3.01 oC

Energy Flows and Losses

Ideal Stirling Cycle

Heat In Heater Cooler Rejected Heat

Heat Transfer Leakages

Regenerator Ineffectiveness

PV Work Out

Heater and ½ Regenerator Flow Loss

Cooler and ½ Regenerator Flow Loss

Gas Hysteresis Loss

Alternator Inefficiency, Bearing LossesElectric

al Output

Internal Bearing & Motion Losses

Differences from prototypes Design Improvements

Improved heat exchanger design Refined simulation and models Extensive optimization

Scaling Increased pressure Increased frequency Increased volume Relatively smaller losses

Efficiency and Power Output Contour Plot

20Hz, 25bar Air

0.190.1950.2

0.2

0.205

0.205

0.205

0.21

0.21

0.21

0.21

0.21

0.215

0.215

0.215

0.215

1000 1500

1500

1500

1500

2000

20002000

2000

2500

2500

2500

30003000

3500

Displacer Stroke (m)

Pow

er P

isto

n S

troke

(m)

Efficiency and Power (W)

0.008 0.009 0.01 0.011 0.012 0.013 0.014 0.015 0.016 0.017 0.0180.03

0.035

0.04

0.045

0.05

0.055

0.06

0.065

0.07

What’s Next? Finalize designs Fabrication and testing of high power

prototype Design/experimental work with thermal

storage Explore waste heat electric generation Economic analysis of cogen, energy

storage opportunities

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

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 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.

Collector and Engine Efficiency

Energy Storage Comparison

Storage Technology Energy Density Cost Self-discharge Round Trip Efficiency Lifetime

Thermal (various media) 20-80 kWh/m3 $40-65/ kWh 1-2% per day 70-80% Unlimited

Flywheel 0.2 Wh/kg $300/kWh Minimal 80-90% ~20 yearsCompressed Air 2 kWh/m3 $1-5/kWh

(storage only)None 80% Unlimited

Superconducting Magnetic Energy Storage

1-10 Wh/m3 $54,000/kWh None with cooling 90-95% Unlimited

Pumped Hydro 0.3 kWh/m3 @ 100m $10-45/kWh None 75% Unlimited

NiMH Battery 30–80 Wh/kg $364/kWh 30%/month 66% 500-1000 cyclesNiCad Battery 40-60 Wh/kg $400/kWh 20%/month 70-90% 1500 cyclesLithium Ion Battery 160 Wh/kg $300/kWh 5%/month 99.90% 1200 cycles

Lithum Polymer Battery 130-200 Wh/kg $500/kWh 10%/month 99.50% 1000 cycles

Lead Acid Battery 30-40 Wh/kg $100-200/kWh 3%-4%/month 70%-92% 500-800 cycles