Recent Advances in Thermal Energy

Storage Using PCMs

D. Yogi Goswami, Ph.D, PEDistinguished University Professor

Director, Clean Energy Research Center

University of South Florida, Tampa, Florida

Editor-in Chief, Solar Energy Journal

• Can be integrated with fossil fuels• Cost-effective Thermal Energy Storage

Provides buffer storage for

transient weather conditions

Extension or displacement of

delivery period

Increases annual plant

capacity factor

Reduces LCOE

Costs 5 – 10 times less than

battery storage

Cost effective for conventional

thermal power as well as other

applications

Concentrating Solar Field

Power Block

Energy StorageSolar Heat

Fossil Fuel

Electricity

Basic Layout of a CSP Plant

Effect of Storage on Levelized Cost of Energy

Effect of thermal storage hours and solar multiple on Levelized

Cost of Electricity (LCOE, cents/kWh) for a thermal storage

system cost of 10 $/kWht. Location: Daggett, USA

A. Present Plant Costs B. Plant Capital Costs of $2000/kW

Selection Criteria

Cost

Storage Material

Heat Exchanger

Technical Criteria

Storage capacity

Efficiency, thermal losses

Stability, Lifetime

Compatibility/Safety

Design Criteria

Operation Strategy

Maximum Load

Nominal Temperature

Integration into Power Plant

Cost of TES enclosure

Heat Storage Media

Sensible Heat

• Heat storage in

high

temperature

oils or low

melting molten

salt mixtures,

or solids such

as rocks

Latent Heat

• Heat stored

during melting

of a solid,

which can be

retrieved

during

solidification

Chemical Energy

• Heat of reaction

in reversible

chemical

reactions, or heat

of adsorption

/desorption of

gases, hydration

/ dehydration

Two-tank Oil Storage

Two-tank Molten Salt with Nitrates

Commercially Available TES

State of the Art

• Synthetic oil storage has temperature limitation and

high cost

• Molten salt systems have corrosion issues and

require expensive piping, pumps and tanks

• Both have low energy density and high cost

Alternate Concepts

solid media storage (natural rock, sand etc.)

PCM storage

Concrete-PCM storage

Solid media-PCM storage

Very high storage density and

low cost materials

But have problems

Low thermal conductivity

Corrosion problems

Our Recent Developments

10

Encapsulate PCMs for use in packed bedsystems to overcome heat transfer andcorrosion problems

Use low cost materials and industriallyscalable encapsulation methods

Reduce TES system costs from the present ~$40/kWhth to < $15/kWhth

Storage

Container

Encapsulated

PCM

Heat transfer

fluid

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PCM Melting point (0C) Latent Heat (kJ/kg)

NaNO3 308 172*

KCl(22)-50MgCl2-30NaCl

396 291

NaCl(56.2)-43.8MgCl2 442 325

CaCl2(52.8)-47.2NaCl 500 239

KCl(45)-55KF 605 407

NaCl(50)-50KCl 657 338*

K2CO3(51)-49Na2CO3 710 163

NaCl 801 510*

* Experimental measured values.

All salt concentrations are in mole percent.

Our Recent paper gives an in-depth review of TES for CSP

“Thermal Energy Storage Technologies and Systems for Concentrating Solar Power Plants” Progress in Energy and Combustion Science, March 2013.

Temperature range 730oC – 1100oC

Less corrosive than metal chlorides

NameExperimental Literature

Tm (°C) ΔHfus (J/g) Tm (°C) ΔHfus (J/g)

Sodium metasilicate (anhydrous) 1089 370 1088 424

Lithium metasilicate 1201 TBD 1201 311

Sodium metaborate 964 501 966 550

Lithium metaborate 845 665 - -

Sodium-lithium metasilicate

eutectoid 847 200 - -

New PCMs

Low Cost Encapsulation of PCMs

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Requires

• Low cost industrial

fabrication of PCM pellets

• Low cost coating that

becomes hard shell

Encapsulation of PCM pellets

Major issues in encapsulation of PCMs:

Large volumetric expansion of PCM on melting.

Pressure buildup due to expansion on heating

14

An innovative solution to these problems:

A coating which is both flexible and selectively permeable in nature was conceived and

developed

Encapsulation of PCM pellets

15

Encapsulation of PCM pellets

16

Polymer Coated Capsules to 3500C

Second Innovation:

Electroless metalization of polymer coating

Metalized Capsules to 4500C

Cyclic Performance of PCM capsules (300 – 4000C)

Final PCM Capsule for

300 – 4000 C range

Capsule after 2200 Thermal

Cycles ~ >7 years equivalent

Cycling continuing

Enhancement Methods in PCM Research

Longitudinal Fins

Metal Rigs

Circular Fins

Multi tubes and Carbon Brushes

Encapsulation Metal Matrix

Bubble AgitationShell and Tube

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Innovations High temperature PCMs with uniquely tailored heat

transfer characteristics for fast charging and

discharging

Optically active PCMs and shell linings for enhanced

heat transfer

Layer -2: Encapsulating layer

Layer -1: Thin layer with high emittance

PCM pellet with tailored radiative properties

Innovative Heart Transfer EnhancementHigh Temperature PCM Capsules (600 – 10000C)

NaCl melts faster with an absorbing and emitting additive. The total melting time is reduced by 35% when the absorption

coefficient increases from 0 to 100m-1

Heat Transfer Enhancement by Radiation

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Major Challenges

Require ceramics for outer shell

Ceramics are normally porous. To make

them non-porous requires sintering at

temperatures of 1200-16000C, which is

much higher than the melting

temperatures of PCMs

High Temperature Encapsulation (400 – 10000C)

Capsules made of Alumina

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Sealed with Sodium tetraborate

Sealed with Alumina paste

Our Development vs Conventional TES

Current Two-Tank system- Storage cost ~ $40/kWhth)

• Our approach has reduced cost to

$15/ kWhth vs. conventional ~ $40

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Summary & Conclusions

• Solar Power is increasing rapidly worldwide

– costs reducing rapidly

• Could cause grid instability beyond 15-20%

penetration – so energy storage is key

• Thermal Energy Storage (TES) more cost

effective than battery storage ($40 vs $500)

• Our development of PCM capsules brings

down TES cost down even further $15/kWhth

vs present cost ~ $40

Thank You