Thermal Energy Storage
Sudhakar Neti
Senior Research Scientist
Energy Research Center, Lehigh University
Bethlehem, PA 18015 USA
Project Researchers
• Profs. S. Neti and A. Oztekin, Carlos Romero
Students – James Blaney#, Weihuan Zhao, Bandar Alzahrani*,Abdul Al Kurdi*, Ali El Mozhughi, Laura Solomon, Xing Chao,Josh Charles, He Yun, C J Pan
Department of Mechanical Engineering and Mechanics and ERC
# now with Air Products and Chemicals Inc., in Saudi Arabia
• Profs. J. C. Chen and K. Tuzla
Students – Ying Zheng, John Barton, Arunachalam,
Department of Chemical Engineering
• Prof. W. Z. Misiolek
Students – J. C. Sabol, Mike Kracum
Department of Materials Science and Engineering
2
TES Research Needs
Research needs based on applications HVAC Dr. Carlos RomeroProcess heat use – Temporal, Spatial shiftsPower generation applicationsEnergy capture, regenerative appsAutomotive applications, etc.
Research areas includeMATERIALS, methods, processes, thermodynamics, economic optimization
TES Implementation
TES entails energy capture/reuse TES can be implemented using
the manipulation of .H2O (as in salts, low temp)
Storage of sensible heat (salts, concrete, sand)
Storage using latest heat of phase change Thermo-chemical changes in media
Current CSP systems use two (hot/cold) tank systems using sensible heat storage in KNO3-NaO3 salts between 200oC and 550oC.
• ASME Standards Committee for TES
Thermal Energy Storage Schematic
Thermal Energy can be stored directly in hundreds of MWh quantities
Large scale TES
TES needs for CSP are large scale, ~1,800 MWhth
Smaller and larger prototype studies are needed
Economic studies are needed determining the real impact on LCOE
DOE goal is ~ $15/kWhth . . . . . . . .. …………(and going down)
• Sensible Heat Storage• Solid or liquid •𝐐 = 𝐦𝐜𝐩∆𝐓
• Latent Heat Storage• Solid-Liquid, Liquid-Gas, Solid-Solid•𝐐 = 𝐦𝐋
• Thermochemical Energy Storage• Reversible chemical reactions•𝐐 = 𝐦𝐇𝐫
Thermal Energy Storage - Methods Temperature Ranges of Interest –HIGH 500 to 750C, MEDIUM 300C, LOW 29C
Analysis of Thermal Energy Storage
• Structural Analysis and Corrosion issues of EPCM
• Choice of PCM – Zn, Al, NaNO3, KNO3, MgCl2 -NaCl, MgCl2, NaCl, Metal Oxides
• Calorimetry -- Energy Storage & Retrieval
• Heat Transfer Analysis of PCM
• Thermocline Experiments
• Numerical Predictions – HT & Exergy
• Cost Analysis of Energy Storage Systems
8
Analysis of Thermal Energy Storage
• Structural Analysis and Corrosion issues of EPCM
9
Stress Analysis of Cylindrical Shell
0
50
100
150
200
250
300
350
400
450
70% 72% 74% 76% 78% 80% 82% 84% 86% 88%
Str
ess
[Mp
a]
Initial PCM Content
Corner
Tangential
Point force
Yield Stress
UTS
10
Materials and Structural Analysis of Encapsulation
The EPCM capsule void is filled with an inert gas such as Argon.
Photo of sectioned EPCM capsule with NaCl-
MgCl2 eutectic after significant thermal
cycling for temperatures ~500 °C.
Photo of sectioned MgCl2 EPCM capsule
after significant thermal cycling for
temperatures ~750 °C.
Multiple shapes and materials analyzed including plastic deformation
Inter-metallic Studies Ni/Zn /SS316
Region 1Region 2
Region 3
Region 4
Region 5
Region 6
Region 7Region 8
Region 9Region 10
Ni
Zn
Photomicrograph of the Ni/Zn specimen
Region 5
Region 4
Region 3
Region 2
Region 1
Photomicrograph of the SS316L/Zn specimen
Analysis of Thermal Energy Storage
• Structural Analysis and Corrosion issues of EPCM
• Choice of PCM – Zn, Al, NaNO3, KNO3, MgCl2 -NaCl, MgCl2, NaCl, Metal Oxides
13
MgCl2-NaCl Eutectic Preparation
The melting temperatures
of NaCl or MgCl2 are 804oC
and 714oC respectively.
The eutectic composition of
55 wt% MgCl2 – 45 wt%
NaCl has a melting point
(444oC).
The Eutectic PCM enclosed
in 304 stainless steel and 1018
carbon steel. 14
TES Systems Considered • Zn• Al• NaCl• MgCl2
• NaCl / MgCl2
• KNO3
• NaNO3
• KNO3/ NaNO3
• NaNO2
• NaNO3/NaNO2
• KNO3/NaNO3/NaNO2
• Sensible heat only
• Convection and Void I EPCM
• Charging and DichargingTemps (~551 K, ~660 K)
• 12 hr charging & discharging cycles
• 0.26 kg/m2-s mass flux
Energy and Exergy Analysis
Metal Oxides as PCM -- Comparison
MaterialTm
(°C)
Latent Heat
(kJ/kg)
Solid Cp
(J/kg K)
Liquid Cp
(J/kg K)Material
Tm
(°C)
Latent Heat
(kJ/kg)
NaNO3 308 162.5 1588 1650 K2B4O7 816 446.4
MgCl2 714 454 798 974 KBO2 947 383.4
NaCl 800 481 987 1200 Na4P2O7 970 220.4
Al 660 397.3 903 1177 KPO3 810 74.5
Na2B4O7 742 403.5 1174.3 2213.3 K2SiO3 976 325.4
NaBO2 967 509.1 1349.8 2218.8
Na4B2O5 641 617.3 1166.4 2048.8
Oxides have higher energy densities at comparable melting temperatures
Metal OXIDES
Phase Diagram of Li Na-KNO3 System
Phase Diagram of Li NaNO3 System
Viscocity of Li NaNO3 System
Latent Heat of AB-NaNO3 System
Specific Heat of NaKNO3 System
Specific and Latent Heats of PCMs
Nacl-MgCl2 NaNO3
23
Storage Capacity of EPCM
0 250 3500
200
400
600
800
Temperature (oC)
Sto
rage C
ap
acit
y (
kJ/k
g)
0 Tm0
200
400
600
800
1000
Temperature (oC)
Sto
rage C
ap
acit
y (
kJ/k
g)
Present Work for NaNO3 Bauer,T. et al.2012Jriri,T. et al. 1995 Takahashi,Y. et al. 1988
MgCl2-NaCl (m.p.: 450 °C)
328 kJ/kg
Latent Heat: 54% 370 kJ/kg
Latent Heat: 67%
NaNO3 (m.p.: 308 °C)
NaNO3 and MgCl2-NaCl are promising as storage medium.
Analysis of Thermal Energy Storage
• Structural Analysis and Corrosion issues of EPCM
• Choice of PCM – Zn, Al, NaNO3, KNO3, MgCl2 -NaCl, MgCl2, NaCl, Metal Oxides
• Calorimetry -- Energy Storage & Retrieval
24
EPCM Sample:
TS
TCalo.
Ta
Stirrer
Silicon Oil
EPCM
DAS &
Computer
25
Characterization of EPCM by Calorimetry Calorimeter Design, Operation and Principle
~ 5 minTime
Tem
per
atu
re
EPCM
Calorimeter
Ambient
TCalo.,t
TS,t
t0 te
TS,0
Ta, t
400 °C
40 °C
Q = Q + QEPCM Calo. Loss
Calorimetry and Numerical Simulations
Air
Oil
PCM
Air
Shell
Experimental Setup Computational Domain
Calorimeter Simulations – Chloride Salts
MgCl2
Accurately capture solid-liquid interface and void expansion
SteFo=0.08
30 s 60 s 120 s 200 s
SteFo=0.16 SteFo=0.31 SteFo=0.52
Analysis of Thermal Energy Storage
• Structural Analysis and Corrosion issues of EPCM
• Choice of PCM – Zn, Al, NaNO3, KNO3, MgCl2 -NaCl, MgCl2, NaCl, Metal Oxides
• Calorimetry -- Energy Storage & Retrieval
• Heat Transfer Analysis of PCM
28
• ANSYS-FLUENT is applied to analyze the heat transfer process in EPCM capsules using theenthalpy method
• Equations are solved by iteration method.
• 2,560 nodes in finite volume discretization with time step of 2 seconds are sufficient to achievespatial convergence as well as temporal convergence.
Number of
nodes
Melting times
(Seconds)
170 2748
802 2766
2922 2772
Number of
nodes
Dimensionle
ss time step,
Δτ
Melting
times
(Seconds)
2922
0.0006 2772
0.00024 2772
0.00012 2772
Spatial convergence Temporal convergence
29
29LTES-ELMInterface - Charging Process with Air Isotherms after 3 hrs – HTS is Air Interface - Charging Process with VP-1
(Fig. for 76.2 mm diameter NaCl-MgCl2 eutectic capsule)
ANSYS-FLUENT Results
Assumptions and considerations:
NaNO3 is used as pure PCM material
76.2 mm diameter of 2-D capsule
Air is HTF for Re = 1230.
Various convective heat transfer coefficients as boundary at Re = 1230
The density of solid is constant, but the density of molten salt varies with temperature
Gravity effect is considered.
Boussinesq approximation is applied for buoyancy flow.
No volume changing between solid and molten salt
The effect of buoyancy-driven convection in the molten PCM enhances melting
30
Interface movement and temperature profiles in NaNO3 using air as HTF
Long cylinder 89 minutes during charging process
Effects of • Property
changes• Void• Buoyancy• Solid PCM
sinking
Void Constant Wall Temperature Sphere Melting
22mm Stainless steel sphere capsule with Sodium Nitrate. Liquid Fraction @240s
Spherical EPCM Capsule
SteFo=0.011 (t=5s)12.9% Void
SteFo=0.034 (t=15s)7.9% Void
SteFo=0.057 (t=25s)6.1% Void
SteFo=0.004 (t=5s)6.7% Void
SteFo=0.047 (t=60s)9.9% Void
SteFo=0.19 (t=240s)11.4% Void
Melting is convection-dominated Solidification is conduction-dominated
Solid
Void
Liquid
Effects of:• Shape• Gravity• Properties• Void• Conve-
ction• Solid
sinking• Power of
(J/s) Storage
Analysis of Thermal Energy Storage
• Structural Analysis and Corosion issues of EPCM
• Choice of PCM – Zn, Al, NaNO3, KNO3, MgCl2 -NaCl, MgCl2, NaCl, Metal Oxides
• Calorimetry -- Energy Storage & Retrieval
• Heat Transfer Analysis of PCM
• Thermocline Experiments
34
Thermocline Experimental Systems to TEST EPCM
Thermocline Test Section Copper capsules placed without insulation into the Test Section
EPCM
CapsulesStore Energy
36
Heater
HTF
Thermal Energy Storage System A pilot-scale TES system is designed, built and tested.
EPCM
EPCM
EPCM
EPCM
Insulation
wall
Q =Q (Q +Q +Q )Capsules HTF wall insulation loss
37
Measurements of Thermal Energy – Methodology
0 0.2 0.4 0.6 0.8 1 1.20
2
4
6
8
10
12
14
16
18
20
Time (hr)
En
erg
y S
tore
d i
n C
op
per
Cap
sule
s (M
J)
Meas. of Capsule Temperature
Meas. and Calc. by Energy Balance
38
Verification of Experimental Methodology
(1)
(2)
Q =Q -(Q +Q +Q )Capsules HTF wall insulation loss
Q = m Cp (T -T )Capsules s s s s,0
The instrumentation is qualified (>95%), for measurements of
energy storage and retrieval in thermal cycles
ProcessTemp. Range (°C)
of copper capsule
Difference
(Q2-Q1)/Q1
Heating #1 27-425 1.0%
Cooling #2 425-65 0.8%
Heating #3 65-421 1.8%
Cooling #4 421-148 2.2%
Heating #5 148-421 4.9%
Cooling #6 421-53 1.4%
Heating #7 281-431 -1.6%
Cooling #8 431-47 0.2%
Heating #9 47-426 -0.3%
39
Testing Thermal Energy Storage System Three configurations of thermal energy storage system
Results Analysis1. Temperature trace of outlet HTF in the storage system.
2. Temperature history of PCM in a charging and a discharging process.
3. Energy storage and retrieval of EPCM capsules in a thermal cycle.
4. Rates of energy storage or retrieved in a thermal cycle.
10 NaNO3 -- 308 °C10 MgCl2-NaCl
-- 444 °C
5 MgCl2-NaCl (444°C)
+ 5 NaNO3 (308°C)
1
2
3
4
5
6
7
8
9
10
1
2
3
4
5
6
7
8
9
10
1
2
3
4
5
6
7
8
9
10
40
NaNO3 Tests – Energy Storage and Retrieval
0 0.5 1 1.5 2 2.5 3 3.50
5
10
15
20
Time (hr)
En
erg
y S
tore
d i
n N
aN
O 3 C
ap
sule
s (M
J)
QNaNO
3
0 0.5 1 1.5 2 2.5 3 3.50
100
200
300
400
Time (hr)
NaN
O3 T
em
pera
ture
(oC
)
NaNO3 Temperature
~ 100°C
∆Q ~100°C
Phase change
contributes
37% of ∆ Q ~100°C
41
NaNO3 Tests – Rate of Energy Storage and Retrieval
0 0.5 1 1.5 2 2.5 3 3.5-10
-8
-6
-4
-2
0
2
4
6
8
10
Time (hr)
Rate
of
En
erg
y S
tora
ge o
r R
em
ov
al
(kW
)
HTF - Air
NaNO3 Capsules
Energy stored / retrieved is ~ 6 kW, at its maximum.
dtdQ
Q
TTCpmQ
EPCMEPCM
outfinfffAir
)( ,,
Overall temperature difference is at
its maximum, between HTF and
EPCM capsules
Analysis of Thermal Energy Storage
• Structural Analysis and Corosion issues of EPCM
• Choice of PCM – Zn, Al, NaNO3, KNO3, MgCl2 -NaCl, MgCl2, NaCl, Metal Oxides
• Calorimetry -- Energy Storage & Retrieval
• Heat Transfer Analysis of PCM
• Thermocline Experiments
• Numerical Predictions – HT & Exergy
42
Numerical Prediction of Thermocline Flow and Heat Transfer
Stream lines for Re = 38,000, 0.038 kg/s, Inlet temp of 400 C
Flow is symmetric around the
rear stagnation point in laminar ,
the counter – rotating vortices
become larger and the wake
region extends downstream
several times.
Temperature counter, during
charging process, the melting
interface much faster in
turbulent and also with 𝜻 = 𝟎.9
Laminar at 5400 s Turbulent at 1800 s Laminar at 5400 s Turbulent at 900 s
= 0.7 = 0.9
44LTES-ELM
Instantaneous streamlines and isotherms of HTF for EPCM in channel:
Computational Results and discussion
Fig. streamlines and isotherms of HTF for EPCM in channel
LES – RSM: Numerical Analysis
Fig. vorticity contour – stream function
The Strouhal number (St = 0.36) for = D/Dw = 0.7
The comparison of the heat transfer coefficient predicted between RANS and LES for Re = 38,334 and = 0.7. The heat transfer coefficient predicted by the LES displays small amplitude fluctuation as the heat transfer from HTF to the EPCM is influenced by the vortex shedding. The time averaged value of the heat transfer coefficient predicted by both LES and RANS agree well.
This will ensure that the phase change phenomenon and the heat transfer inside the EPCM will hardly be influenced by the vortex shedding or other smaller scale flow structure.
45LTES-ELMFig. Comparison between the LES and the RANS for the heat transfer coefficient
46
Temperature of Heat Transfer Fluid
Temperature of HTF at outlet are well predicted by the model.
0 0.5 1 1.5 2 2.5 3 3.50
50
100
150
200
250
300
350
400
450
Time (hr)
Air
Tem
pera
ture
(oC
)
Model - Air - Inlet
Model - Air - Outlet
Experiment - Air - Inlet
Experiment - Air - Outlet
47
Energy into PCM from HTF Air – Expt vs. Numerical
Temperature distribution for one charging and discharging cycle of the last capsue in the thermocline 6 mm away from the capsule
Analysis of Thermal Energy Storage
• Structural Analysis and Corosion issues of EPCM
• Choice of PCM – Zn, Al, NaNO3, KNO3, MgCl2 -NaCl, MgCl2, NaCl, Metal Oxides
• Calorimetry -- Energy Storage & Retrieval
• Heat Transfer Analysis of PCM
• Thermocline Experiments
• Numerical Predictions – HT & Exergy
• Cost Analysis of Energy Storage Systems
48
Cost Analysis of Thermal Energy Storage
System
Cost of storage units ($/kWh) as a function of the D, diameter of
the cylindrical EPCM capsule.
49
Cost Analysis of Thermal Energy Storage
System
Cost of storage unit ($/kWh) as a function of length of the
capsule
50
Cost Analysis of Thermal Energy Storage
System
Cost of pumps and balance of plant for storage system based on 2010 NREL
Report ~ $ 6,963,600
51
Cost Analysis of Thermal Energy Storage System
Cost of the storage unit ($/kWh) for different combinations of PCM and
encapsulation materials for D = 75 mm and Length of Capsule 15 D.
52
The best EPCM geometry based on heat transfer considerations, stress analysis, large-scale fabrication (millions of EPCM capsules) and cost analysis is a large aspect ratio cylinder (L/D of cylinder > 5) and diameter > 75 mm containing a salt.
Storage System cost $/kWh for D = 75 mm and H/D = 15
53
Cost of Thermal Energy Storage System
Thermal Storage Technologies for Solar Power KEY CONTRIBUTIONS
• Demonstrated Storage Technology – Sensible + Latent• Choice of PCM & EPCM – Materials for Temperature (29C to 750C)• Testing of EPCM – Calorimetry, Thermocline Flow Experiments• Numerical prediction of Transient Temperature Distributions• Numerical predictions of TES Exergy Analysis• New Metal Oxide PCMs and Corrosion considerations• Cost analysis for high temperature EPCM with continued decrease
of costs (~$15/kWhth).
Summary• Fossil fuels will be in use for decades
• Energy challenges driven by CO2 and global warming
• Nuclear has its own uncertainties
• Abundance of solar energy – PV, CST
• Costs of implementation of solar energy needs to be brought down
• Challenges – Capital, Bankability!
• Thermal Energy Storage has great potential – has broad implications.
55
Thank you.QUESTIONS ?
Gracias.¿Preguntas?
Gràcies. Preguntes?
All our life and energy
come from our Sun
S. Neti 6/28/12