Electrical/Thermal Storage 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, transmission losses

• Avoided costs of transmission/distribution upgrades • Power Quality – get .999+ availability with less

pressure on grid + distribution, and reject/compensate dips, spikes, v. short outages, harmonics, reactive power

Electrical/Thermal Storage Opportunities (cont’d)

• Islanding potential enhanced: back-up energy and low-impedance inverter convenient for controlling frequency, clearing faults

• Ancilliary services to grid: stability enhancement, spinning reserve function

“Pure” Electrical Storage Technologies – Analysis of 10

kWh scale devices (May 2009 EECS290N Energy Storage Group Class Report)

Cost effective electrical energy storage remains holy grail !

Flow and NaS technologies also of interest:•MWh scale•Costs still too high for general arbitrage appl

“Pure” Thermal Storage • Pre-heating/cooling of working space• Water/ice very dense storage media

– Water has ~60 W-hr/kg for 50 degC swing

– Ice-water heat of fusion ~ 0.1 kWhr/kg• Established useful applications in pre-chilling

for cooling and refrigeration• Established applications in storing heat for

space heating, hot water

Combined Opportunities:•Main ideas:

•Water, mineral oil, and some salts (KNO3+NaNO3) are very low cost liquid media that can be directly interfaced with heat exchanger(s)

•Heat engine (eg. Stirling) provides high efficiency, eg. better than ~ 2/3 of reversible limit

•Stirling converter enables excellent durability, cycle-ability (contrast with IC engine)Ex.1: Solar Thermal Electric System

Combined opportunities (cont’d)

Ex. 2: Co-generation with thermal storage:

Combustion-to meet electric demand (300 C ?)

Thermal-Electric Conversion

Thermal Reservoir(s)60 -100 C

Electrical output On Demand

Thermal output on demand

One tank system:• cycle avg temp, or thermoclineTwo tank system

Thermal-Electric conversion eff ~ >28% withhigh performance, longlife Stirling Converter

Combined Opportunities (cont’d)

Thermal Reservoir

Waste heat stream100-250 C or higher

Ex. 3: Waste heat recovery + thermal storage

Heat Engine Converter

Domestic Hot Water ?

Electric generationon demand

•Huge opportunity in waste heat

Related apps for effic. thermal conv

• Heat Pump

• Chiller

• Refrigeration– Benign working fluids in Stirling cycle – air,

helium, hydrogen

Thermal System Diagram

Solar Dish: 2-axis track, focus directly

on receiver (engine heat exchanger)

Photo courtesy of Stirling Energy Systems.

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

Cost Comparison

Component $/WCollector 0.95Engine 0.5Installation -Hardware 0.75 -Labor 1.25Total $3.45

Component $/WPV Module 4.84Inverter 0.72Installation -Hardware 0.75 -Labor 1.25Total $7.56

“Rooftop” Solar Thermal Photovoltaic

Source: PV data from Solarbuzz

Residential Example

• 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.

1

2

3

4

Stirling Cycle Overview

Stirling Engine

• Can achieve large fraction (70%) of Carnot efficiency• Low cost possible for low temp design:

– bulk metal and plastics• Simple components • Fuel (heat source !) Flexible• Reversible• Independent scalable engine and storage capacity• 25 kW systems (SES), MW scale designs proposed

by Infinia

Free-Piston “Gamma” Engine (Infinia)

• Designed for > 600 C operation, deep space missions with radioisotope thermal source• Two moving parts – displacer and power piston, each supported by flexures, clearance seals• Fully sealed enclosure, He working fluid, > 17 year life•Sunpower (Ohio) has designs with non-contacting gas bearings

Prototype 1: free-piston Gamma

Displacer Power piston

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)

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 5.2Power delivered to electric load

9.3

Prototype 2 – Multi-Phase “Alpha”

Nylon flexure (cantilever spring)

HeaterCooler

Cold side piston plate

Actuator mounting jaw

Axis of rotation

Diaphragm

Sealed clearance

Efficiency and Power Output Contour Plot

60Hz, 10bar Air

0.220.225

0.225

0.23

0.23

0.23

0.23

0.23

5

0.235 0.235

0.235

0.235

0.24

0.24 0.24

0.24

0.24

0.24Displacer Stroke

Pow

er P

isto

n S

trok

eMech Work vs Strokes

2000 3000

3000

300040

00

4000

4000

40005000

5000

5000

6000

6000

7000

0.007 0.008 0.009 0.01 0.011 0.012 0.013 0.014 0.0150.015

0.02

0.025

0.03

0.035

0.04