Upload
lammien
View
214
Download
0
Embed Size (px)
Citation preview
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
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
11
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
13
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
19
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
21
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
22
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
24
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