Molten salts

7TH
SFERA Networking
SFERA SUMMER SCHOOL
Almería, 9-10 June 2016
Heat Transfer Fluids for Concentrating Solar Systems:
Molten salts
Margarita Rodríguez-García
Thermal Energy Storage (Solar Concentration Systems Unit)
e-mail: [email protected]
Contents
1.
2.
Introduction
Molten salts physical properties
1. Binary salt – Solar salt
2. Ternary salt – Hitec
3. Comparison
3. Molten salt plants operation experience
4. Advances and future works
7th SFERA Summer School
Almería, 9-10 June 2016
Introduction
Beam solar radiation
Optical Concentrator
Concentrated solar radiation
RECEIVER
Thermal Storage
Thermal
Energy
Industrial
Process
Turbine
Power Generation
STE plants
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7th SFERA Summer School
Almería, 9-10 June 2016
Introduction
Sunlight
Power block
Collector field
Molten salt system (Tower)
7th SFERA Summer School
Almería, 9-10 June 2016
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Introduction
• Why using molten salt instead of other fluids?
STORAGE
Heat and electricity power components versus time for a CSP
facility with thermal energy storage. Energy storage can time-shift
supply to meet demand (Philibert, 2011).
7th SFERA Summer School
Almería, 9-10 June 2016
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Introduction
Experience in CSP thermal storage*:
Project
Sponsoring country
Power
output
(MWe)
Heat transfer fluid
Storage medium
Begin of operation
SSPS
Spain
0.5
Liquid sodium
Sodium
1981
EURELIOS
Italy
1.0
Steam
Nitrate Salt/Water
1981
SUNSHINE
Japan
1.0
Steam
Nitrate Salt/Water
1981
Solar One
USA
10.0
Steam
Oil/Rock
1982
CESA-I
Spain
1.0
Steam
Nitrate Salt
1983
MSEE/Cat-B
United States
1.0
Molten Nitrate Salt
Nitrate Salt
1984
THEMIS
France
2.5
Hi-Tec Salt
Hi-Tec Salt
1984
SPP-5
Russia
5.0
Steam
Water/Steam
1986
TSA
Europe
1.0
Air
Ceramic
1993
Solar Two
USA
10.0
Molten Nitrate Salt
Nitrate Salt
1996
* SAND 2001- 3674
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7th SFERA Summer School
Almería, 9-10 June 2016
Contents
1.
2.
Introduction
Molten salts physical properties
1. Binary salt – Solar salt
2. Ternary salt – Hitec
3. Comparison
3. Molten salt plants operation experience
4. Advances and future works
7th SFERA Summer School
Almería, 9-10 June 2016
Molten salt physical properties
• The optimum heat transfer fluid (HTF) to be used as liquid sensible
storage media must present, among others:
– High density, ρ
– High heat capacity, Cp
– Large thermal conductivity, k
– Wide range of thermal stability, ∆T
– Low cost, 
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7th SFERA Summer School
Almería, 9-10 June 2016
Molten salt physical properties
• Stored heat, Q [J], in sensible systems:
𝑇𝑇
– 𝑄𝑄 = 𝑚𝑚 ∗ ∫𝑇𝑇 𝑜𝑜𝑜𝑜𝑜𝑜 𝐶𝐶𝑝𝑝 𝑇𝑇 𝑑𝑑𝑑𝑑 ≅ 𝑚𝑚 ∗ 𝐶𝐶𝑝𝑝 ∗ 𝑇𝑇𝑜𝑜𝑜𝑜𝑜𝑜 − 𝑇𝑇𝑖𝑖𝑖𝑖 = 𝑚𝑚 ∗ 𝐶𝐶𝑝𝑝 ∗ ∆𝑇𝑇
𝑖𝑖𝑖𝑖
• Energy density E [J/m3]
•
Where:
𝐸𝐸 ≅ 𝜌𝜌 ∗ 𝐶𝐶𝑝𝑝 ∗ ∆𝑇𝑇
Tin and Tout: inlet and outlet storage system temperatures [K]
m: mass of storage liquid media [kg]
ρ: density [kg/m3]
Cp: specific heat capacity [J/(kg K)]
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7th SFERA Summer School
Almería, 9-10 June 2016
Molten salt physical properties
•
•
Thermal diffusivity is the material’s ability to change its temperature (thermal inertia)
𝑘𝑘
𝛼𝛼 =
𝜌𝜌𝜌𝜌𝑝𝑝
Where:
k is the thermal conductivity [W/(mK)]
ρ: density [kg/m3]
Cp: specific heat capacity [J/(kg K)]
•
•
𝜌𝜌𝜌𝜌 𝑝𝑝 can be considered the Volumetric heat capacity [J/(m3K)]
From the heat equation and assuming constant properties
𝜕𝜕 2 𝑇𝑇
𝜕𝜕𝑇𝑇
= 𝛼𝛼 2
𝜕𝜕𝑡𝑡
𝜕𝜕𝑥𝑥
7th SFERA Summer School
Almería, 9-10 June 2016
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Molten salt physical properties
Candidate liquid storage media
Temperature
Average density
Average heat
conductivity
Average heat
capacity
Volume specific heat
capacity
Media cost per
kg
Media cost per kWht
Cold
Hot
Hitec salt
Mineral oil
Synthetic oil
Silicone oil
Nitrite salts
Nitrate salts
Carbonate salts
[⁰C]
142
200
250
300
250
240
450
[⁰C]
454
300
350
400
450
565
850
[kg/m3]
1980
770
900
900
1825
1870
2100
[W/mK]
0.48-0.50
0.12
0.11
0.10
0.57
0.52
2.0
[kJ/kgK]
1.30
2.6
2.3
2.1
1.5
1.6
1.8
[kWht/m3]
n.a.
55
57
52
152
250
430
[$/kg]
n.a.
0.30
3.00
5.00
1.00
0.70
2.40
[$/kWht]
n.a.
4.2
43.0
80.0
12.0
5.2
11.0
Liquid sodium
270
530
850
71.0
1.3
80
2.00
21.0
Survey of Thermal Storage for Parabolic Trough Power Plants, NREL/SR-550-27925, 2000
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7th SFERA Summer School
Almería, 9-10 June 2016
Molten salt physical properties
Currently molten salt and thermal oil are both feasible
• Molten salts have a higher melting point
• Parasitic heating required to keep them liquid at night, during low insulation periods, or
during plant shutdowns
• Potential problems with corrosion at high temperatures or at high content of impurities
• Silicone oil is quite expensive, but it is environmental friendly (non-hazardous
material)
• Synthetic oils are hazardous materials
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7th SFERA Summer School
Almería, 9-10 June 2016
Binary salt /Solar salt
NaNO3
KNO3
Melting temperature [ºC]
308
334
pH
6-9
6-9
Thermal decomposition [ºC]
380
400
Water solubility [g/l]
480
320
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7th SFERA Summer School
Almería, 9-10 June 2016
Binary salt /Solar salt
• Fusion diagram for
potassium and sodium
nitrates
Berg, et all. The NaNO3/KNO3 system. The position of
the solidus and sub-solidus, 2004
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7th SFERA Summer School
Almería, 9-10 June 2016
Binary salt /Solar salt
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7th SFERA Summer School
Almería, 9-10 June 2016
Binary salt /Solar salt
Impurities
Typical impurities
Chloride, Cl
Magnesium, Mg
Nitrite, NO2
Sulphate, SO4
Range of maximum concentration (%wt.)
KNO3
0.1-0.2
0.01-0.2
0.02
0.05-0.5
NaNO3
0.1-0.6
0.1-0.6
0.02
0.10-0.50
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7th SFERA Summer School
Almería, 9-10 June 2016
Binary salt /Solar salt
• Corrosion aspects
•
Goods and Bradshaw, 2003. Study of corrosion for KNO3-NaNO3 binary mixture with
different levels of impurities with stainless steel and carbon steel.
•
SANDIA REPORT: SAND 2013-8256, September 2013
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7th SFERA Summer School
Almería, 9-10 June 2016
Binary salt /Solar salt
• Compatibility of molten sodium nitrate and graphite
Bauer et all., SODIUM NITRATE FOR HIGH TEMPERATURE LATENT HEAT
STORAGE.
The 11th International Conference on Thermal Energy Storage – Effstock 14-17 June
2009 in Stockholm, Sweden
7th SFERA Summer School
Almería, 9-10 June 2016
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Ternary Salt/ Hitec® salt
• It is an eutectic mixture of water-soluble,
inorganic salts:
NaNO3 (7%) NaNO2 (40%) KNO3 (53%)
• Freshly prepared is a granular solid; when melted
is pale yellow.
• Freezing point is 142 ºC, and can be melted by
plant steam at a pressure as low as 3 barg (50
psig)
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7th SFERA Summer School
Almería, 9-10 June 2016
Ternary Salt/ Hitec® salt
• Main physical properties of liquid Hitec®
KNO3 – NaNO2- NaNO3
Composition, mole (%)
Molecular weight, approximate
44.2 – 48.9 – 6.9
84
Density [kg/m3] at 538 ⁰C
1681,94
Dynamic viscosity [kg/m s] at 538 ⁰C
1.24x10-3
Specific heat [J/kg K]
1500
Thermal conductivity [W/m K]
0.571
Heat transfer coefficient [W/m2 K]
16500
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7th SFERA Summer School
Almería, 9-10 June 2016
Ternary Salt/ Hitec® salt
• If HITEC® is used in an open system, in contact with air, and at 454-538
ºC, the nitrite is slowly oxidized by atmospheric oxigen:
2 NaNO2 + O2 →2 NaNO3
• Change in Hitec® composition after 6 weeks at 593 ºC in N2 atmosphere
Compound
NaNO3
NaNO2
KNO3
NaO
% Composition
Original
Final
7
18
40
28
53
52
-2
• The melting point changed to 165 ºC from 142 ºC
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7th SFERA Summer School
Almería, 9-10 June 2016
Ternary Salt/ Hitec® salt
• Thermal expansion of solid phase
Solar Salt
Hi tec salt
Iverson, Thermal property testing of nitrate thermal storage salts in the solid-phase, ES2011-54159, Sandia, 2011
7th SFERA Summer School
Almería, 9-10 June 2016
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Binary and Ternary Salt
• Specific heat comparison
Iverson, Thermal property testing of nitrate thermal storage
salts in the solid-phase, ES2011-54159, sandia, 2011
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7th SFERA Summer School
Almería, 9-10 June 2016
Binary and Ternary Salt
• General safety precautions
• Principal hazards are those associated to the
use of liquids at elevated temperatures.
• It can cause dermatitis after prolonged
contact
• It should not be ingested in more than trace
amounts
• It does not liberate toxic vapours
• Salts are nonflammable, but they can support
the combustion of other materials
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7th SFERA Summer School
Almería, 9-10 June 2016
Contents
1.
2.
Introduction
Molten salts physical properties
1. Binary salt – Solar salt
2. Ternary salt – Hitec
3. Comparison
3. Molten salt plants operation experience
4. Advances and future works
7th SFERA Summer School
Almería, 9-10 June 2016
Molten salt plants operation experience
• Two molten salts tanks
• CO2-molten salts heat
exchanger
• Air cooler (for salts)
• Thermal oil loop
• 2 flanged pipe sections
(4” and 20”)
• Electrical heat tracing
• Auxiliary systems:
– Nitrogen loop
– Control and instrumentation
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7th SFERA Summer School
Almería, 9-10 June 2016
Molten salt plants operation experience
Mode 1: Heating salts with CO2
373ºC
344kW
290ºC
Mode 3: Heating salt with hot thermal oil
313ºC
505ºC
380ºC
290ºC
Mode 2: Cooling salt with air cooler
Mode 4: Cooling salt with thermal oil
400ºC
505ºC
380ºC
270ºC
290ºC
270ºC
3900kg/h
290ºC
7th SFERA Summer School
Almería, 9-10 June 2016
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Molten salt plants operation experience
• Avoid installation of small-diameter piping
• Venting
• TES heat exchangers (HX)
• Heat dissipation of immersion heaters
1
2
𝑁𝑁𝑂𝑂3 − = 𝑁𝑁𝑂𝑂2 − + 𝑂𝑂2
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7th SFERA Summer School
Almería, 9-10 June 2016
Molten salt plants operation experience
• Electrical heat tracing and insulation
• All components and pipes must be preheated in order to
avoid the solidification of salt on them.
– Electrical heat tracing
The most used
– Joule effect
• The correct installation and selection of insulation materials is
crucial for avoiding salt solidification
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7th SFERA Summer School
Almería, 9-10 June 2016
Molten salt plants operation experience
• Correct length of the heat tracing elements
• Proper placement along the pipe
• Independent electrical heat tracing in pipes and valves
• Temperature control sensors placement
• EHT at supports
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7th SFERA Summer School
Almería, 9-10 June 2016
Contents
1.
2.
Introduction
Molten salts physical properties
1. Binary salt – Solar salt
2. Ternary salt – Hitec
3. Comparison
3. Molten salt plants operation experience
4. Advances and future works
7th SFERA Summer School
Almería, 9-10 June 2016
Advances and future works
• Reducing the high cost of the operation and maintenance of these
plants.
• It is necessary to improve this fluid to construct less expensive and more
profitable thermosolar plants.
• The potential for improving the salt resides in optimizing its
physicochemical properties, mainly its melting point, thermal stability,
and heat capacity.
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7th SFERA Summer School
Almería, 9-10 June 2016
Advances and future works
Advances in heat
capacity:
Modulated differential
scanning calorimetry
(MDSC) technique
used for its
determination.
Heat capacity of molten salts.
Reprinted from Solar Energy, 79, 3 (2005), Hoshi, A., Mills, D.R., Bittar, A. and Saitoh
T.S., ‘Screening of high melting point phase change materials (PCM) in solar thermal
concentrating technology based on CLFR’, pp. 332–339. Copyright (2013).
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Almería, 9-10 June 2016
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Advances and future works
Heat capacity improvements:
• Different additives have been studied for improving the heat capacity of
storage fluids. Nanoparticles of graphite, Al2O3 and CuO were analyzed
for that purpose by many authors (Wang et al., 2001; Tiznobaik and Shin,
2013), however, the additive with the best results so far has been SiO2.
• Corrosion studies needed: SiO2 particles are abrasive
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7th SFERA Summer School
Almería, 9-10 June 2016
Advances and future works
Advances in melting point:
• Binary mixtures of alkali molten nitrates/nitrites present phase diagrams
with a simple eutectic point.
• By adding one or more components, it is expected that the resulting
mixture will have a lower melting point compared to the initial eutectic
binary mixture
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7th SFERA Summer School
Almería, 9-10 June 2016
Advances and future works
Advances in viscosity:
• High viscosity can cause clogging of pumps and pipes during circulation
of salt in the solar power plant, but in general, the viscosity values of
most molten nitrates at elevated temperatures (100°C or more above the
melting temperature) are similar to water.
• It is observed that the addition of calcium nitrate can significantly
increase the viscosity.
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7th SFERA Summer School
Almería, 9-10 June 2016
Advances and future works
Interested in Thermal Storage Research?
Marie Skłodowska-Curie Actions (H2020-MSCA-IF-2016 )
Deadline: 14 September 2016 17:00:00 (Brussels time)
More General Info:
http://ec.europa.eu/research/participants/portal/desktop/en/opportunities/h2
020/topics/2226-msca-if-2016.html
Contact (before 15th August 2016): [email protected]
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7th SFERA Summer School
Almería, 9-10 June 2016
Thank you for your attention!!
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7th SFERA Summer School
Almería, 9-10 June 2016

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