Stirling Engine System for Solar Thermal Generation and Energy Storage LoCal Retreat, June

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 ChallengesSolar Thermal Advantages

Intermittency and Energy Storage 1.5 MW Wind Turbine 4.6 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$/W Collector0.34 Engine0.5 Balance of System3.6 Total4.44 Energy Storage$/kWh Thermal20 Component$/W PV Module4.70 Inverter0.72 Balance of System3.6 Total9.02 Energy Storage$/kWh Batteries2030 Solar ThermalPhotovoltaic 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

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: T h =175 o C, T k =25 o C Working fluid: 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 model254 W (thermal input) - 24 W (mechanical output) DisplacerPower piston Gamma-Type Free-Piston Stirling

Prototype Operation Power Breakdown (W) Indicated power 26.9 Gas spring hysteresis 10.5 Expansion space enthalpy loss 0.5 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

Piston Systems

Prototype 2: Multi-phase

Components

ParameterValue Working fluidAmbient air Frequency~30 Hz Hot side temperature147 o C Cold side temperature27 o C Power (per phase)12.7 W Calculated 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 Design Design CharacteristicsValue Nominal Power Output2.525 kW Thermal-Electric Efficiency21.5% Fraction of Carnot Efficiency65% Hot Side Temperature180 o C Cold Side Temperature30 o C Pressure25 bar Engine Frequency20 Hz Regenerator Effectiveness Total Heat Exchanger Flow Loss54.5 W Regenerator Flow Loss166.6 W Compression Loss66.8 W Hot Side Heat Exchanger Temp Drop2.74 o C Cold Side Heat Exchanger Temp Drop3.01 o C

Energy Flows and Losses Ideal Stirling Cycle Heat InHeaterCooler 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 Losses Electrical 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

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  sqm collector => 3-5 kWe peak at 10%eff  Reject 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/m 2 (PV standard) Schott ETC-16 collector Engine: 2/3 of Carnot eff. Collector and Engine Efficiency

Energy Storage Comparison Storage TechnologyEnergy DensityCostSelf-dischargeRound Trip EfficiencyLifetime Thermal (various media)20-80 kWh/m3$40-65/ kWh1-2% per day70-80%Unlimited Flywheel0.2 Wh/kg$300/kWhMinimal80-90%~20 years Compressed Air2 kWh/m3$1-5/kWh (storage only) None80%Unlimited Superconducting Magnetic Energy Storage 1-10 Wh/m3$54,000/kWhNone with cooling90-95%Unlimited Pumped Hydro m$10-45/kWhNone75%Unlimited NiMH Battery30–80 Wh/kg$364/kWh30%/month66% cycles NiCad Battery40-60 Wh/kg$400/kWh20%/month70-90%1500 cycles Lithium Ion Battery160 Wh/kg$300/kWh5%/month99.90%1200 cycles Lithum Polymer Battery Wh/kg$500/kWh10%/month99.50%1000 cycles Lead Acid Battery30-40 Wh/kg$ /kWh3%-4%/month70%-92% cycles