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Margarita Rodríguez-García Thermal Energy Storage (Solar Concentration Systems Unit) e-mail: [email protected]
SFERA Networking 7TH SFERA SUMMER SCHOOL
Almería, 9-10 June 2016
Heat Transfer Fluids for Concentrating Solar Systems:
Molten salts
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7th SFERA Summer School Almería, 9-10 June 2016
Contents
1. Introduction 2. 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
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7th SFERA Summer School Almería, 9-10 June 2016
Introduction
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Optical Concentrator
Beam solar radiation
Concentrated solar radiation
RECEIVER
Power Generation Industrial Process Turbine
STE plants
Thermal Energy
Thermal Storage
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7th SFERA Summer School Almería, 9-10 June 2016
Introduction
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Collector field
Molten salt system (Tower)
Power block Sunlight
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7th SFERA Summer School Almería, 9-10 June 2016
Introduction
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• 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).
ModeradorNotas de la presentaciónThermal-energy storage in CSP allows electricity to be dispatched to the grid when the power demand is the highest, thus increasing the monetary value of the electricity. Power output from the turbine remains constant through fluctuations in solar radiation and until all of the energy stored in the hot tank is depleted.The operating hours per year is increased reducing the LEC (levelized electricity costs).Energy storage and dispatchability are very important for the success of solar power tower technology.
In a typical installation, solar energy collection occurs at a rate that exceeds the maximum required to provide steam to the turbine.Thermal storage system can be charged at the same time that the plant is producing power at full capacity. SOLAR MULTIPLE: ratio of the thermal power provided by the collector system (the heliostat field and receiver) to the peak thermal power required by the turbine generator.With a solar multiple of approximately 2.7, a molten-salt power tower can be designed for an annual capacity factor of about 65%. A power tower could potentially operate for 65% of the year without the need for a back-up fuel source. Without energy storage, solar technologies are limited to annual capacity factors near 25%.
http://journal.frontiersin.org/article/10.3389/fenrg.2013.00003/full%23B9
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7th SFERA Summer School Almería, 9-10 June 2016
Introduction
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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. Introduction 2. 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
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7th SFERA Summer School Almería, 9-10 June 2016
Molten salt physical properties
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• 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
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• Stored heat, Q [J], in sensible systems: – 𝑄𝑄 = 𝑚𝑚 ∗ ∫ 𝐶𝐶𝑝𝑝 𝑇𝑇 𝑑𝑑𝑇𝑇 ≅
𝑇𝑇𝑜𝑜𝑜𝑜𝑜𝑜𝑇𝑇𝑖𝑖𝑖𝑖
𝑚𝑚 ∗ 𝐶𝐶𝑝𝑝 ∗ 𝑇𝑇𝑜𝑜𝑜𝑜𝑜𝑜 − 𝑇𝑇𝑖𝑖𝑖𝑖 = 𝑚𝑚 ∗ 𝐶𝐶𝑝𝑝 ∗ ∆𝑇𝑇
• Energy density E [J/m3] 𝐸𝐸 ≅ 𝜌𝜌 ∗ 𝐶𝐶𝑝𝑝∗ ∆𝑇𝑇
• Where: 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
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• Thermal diffusivity is the material’s ability to change its temperature (thermal inertia)
𝛼𝛼 =𝑘𝑘𝜌𝜌𝐶𝐶𝑝𝑝
• Where: k is the thermal conductivity [W/(mK)]
ρ: density [kg/m3]
Cp: specific heat capacity [J/(kg K)]
• 𝜌𝜌𝐶𝐶𝑝𝑝 can be considered the Volumetric heat capacity [J/(m3K)] • From the heat equation and assuming constant properties
𝜕𝜕𝑇𝑇𝜕𝜕𝑡𝑡
= 𝛼𝛼𝜕𝜕2𝑇𝑇𝜕𝜕𝑥𝑥2
ModeradorNotas de la presentaciónIn a substance with high thermal diffusivity, heat moves rapidly through it because the substance conducts heat quickly relative to its volumetric heat capacity
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7th SFERA Summer School Almería, 9-10 June 2016
Molten salt physical properties
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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
[⁰C] [⁰C] [kg/m3] [W/mK] [kJ/kgK] [kWht/m3] [$/kg] [$/kWht] Hitec salt 142 454 1980 0.48-0.50 1.30 n.a. n.a. n.a. Mineral oil 200 300 770 0.12 2.6 55 0.30 4.2 Synthetic oil 250 350 900 0.11 2.3 57 3.00 43.0 Silicone oil 300 400 900 0.10 2.1 52 5.00 80.0 Nitrite salts 250 450 1825 0.57 1.5 152 1.00 12.0 Nitrate salts 240 565 1870 0.52 1.6 250 0.70 5.2 Carbonate salts 450 850 2100 2.0 1.8 430 2.40 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
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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
NaNO3 KNO3
Melting temperature [ºC] 308 334
pH 6-9 6-9
Thermal decomposition [ºC] 380 400
Water solubility [g/l] 480 320
Binary salt /Solar salt
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ModeradorNotas de la presentaciónSodium nitrate NaNO3 and potassium nitrate (saltpeter) KNO3 are found in naturally occurring deposits.They require processing to obtain pure salt compound.Colorless or white cristalline salt. Very soluble in water Hygroscopic, but it does not form hydrated solid phases.Used as fertilizer and other industrial processes
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7th SFERA Summer School Almería, 9-10 June 2016
Binary salt /Solar salt
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• Fusion diagram for potassium and sodium nitrates
Berg, et all. The NaNO3/KNO3 system. The position of
the solidus and sub-solidus, 2004
ModeradorNotas de la presentaciónThe equimolar molten salt mixture NaNO3-KNO3 was proposed as heat transfer fluid and as thermal storage medium for solar applications. Maximum operation temperature is 565 ºCBecause of the high KNO3 price, the optimized NaNO3-KNO3 (60%wt, 40%wt) mixture is chosen.
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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
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Impurities
Typical impurities Range of maximum concentration (%wt.)
KNO3 NaNO3 Chloride, Cl 0.1-0.2 0.1-0.6 Magnesium, Mg 0.01-0.2 0.1-0.6 Nitrite, NO2 0.02 0.02 Sulphate, SO4 0.05-0.5 0.10-0.50
ModeradorNotas de la presentaciónThe requirements of the chemical quality of the salts in the mixture are very high.Impurities -
7th SFERA Summer School Almería, 9-10 June 2016
Binary salt /Solar salt
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• 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
ModeradorNotas de la presentaciónRates of metal loss found 6-15 pm/year for stainless steel at 570 ºC.For the typical range of impurities in commercially nitrate salts, corrosion rates remained acceptable for all of the alloys examined.
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7th SFERA Summer School Almería, 9-10 June 2016
Binary salt /Solar salt
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• 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
ModeradorNotas de la presentaciónNitrate salt oxidize graphite at temperatures above 350 ºC. Below 300 ºC graphite seals show a good compatibility with molten KNO3-NaNO3Graphite is the most used packing material for components at high temperature!!!
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7th SFERA Summer School Almería, 9-10 June 2016
Ternary Salt/ Hitec® salt
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• 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
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• Main physical properties of liquid Hitec®
KNO3 – NaNO2- NaNO3
Composition, mole (%) 44.2 – 48.9 – 6.9
Molecular weight, approximate 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
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• 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
• The melting point changed to 165 ºC from 142 ºC
Compound % Composition Original Final
NaNO3 7 18 NaNO2 40 28 KNO3 53 52 NaO -- 2
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7th SFERA Summer School Almería, 9-10 June 2016
Ternary Salt/ Hitec® salt
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• Thermal expansion of solid phase
Iverson, Thermal property testing of nitrate thermal storage salts in the solid-phase, ES2011-54159, Sandia, 2011
Solar Salt Hi tec salt
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7th SFERA Summer School Almería, 9-10 June 2016
Binary and Ternary Salt
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• Specific heat comparison
Iverson, Thermal property testing of nitrate thermal storage salts in the solid-phase, ES2011-54159, sandia, 2011
ModeradorNotas de la presentaciónLarge initial spike for solar salt and HITEC salt is indicative of solid–solid phase transitions (non eutectic mixtures)
Secondary spike for these salts indicates the solid–liquid phase change at the melting point
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7th SFERA Summer School Almería, 9-10 June 2016
Binary and Ternary Salt
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• 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. Introduction 2. 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
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7th SFERA Summer School Almería, 9-10 June 2016
Molten salt plants operation experience
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• 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
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Mode 1: Heating salts with CO2
Mode 2: Cooling salt with air cooler
Mode 3: Heating salt with hot thermal oil
Mode 4: Cooling salt with thermal oil
290ºC 505ºC
344kW
505ºC
290ºC
3900kg/h
290ºC
373ºC
380ºC
313ºC
290ºC
400ºC
270ºC
380ºC
270ºC
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7th SFERA Summer School Almería, 9-10 June 2016
Molten salt plants operation experience
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• Avoid installation of small-diameter piping
• Venting
• TES heat exchangers (HX)
• Heat dissipation of immersion heaters
𝑁𝑁𝑂𝑂3− = 𝑁𝑁𝑂𝑂2− +12𝑂𝑂2
ModeradorNotas de la presentación- Wall curvature makes difficult a good contact between pipe wall and electrical heat tracing.The bigger the diameter, the higher the mass per unit of transfer, and the thermal inertia to avoid crystallization- Salts are usually delivered with a high water content because they are very hygroscopic
- Heat exchangers required to transfer heat to other fluid. Drainable to avoid plugs during non operation periods must be considered. If it is tilted or have draining channels the HX, the heat transfer is jeopardized. Must be taken into account during the design.- Used to compensate thermal losses.Salt is still in the tank, and thus the energy must dissipate fast enough to avoid the salt from reaching temperatures near the 620 ºC where degradation occurs:Surface load concentration of electrical heaters must be carefully calculated
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7th SFERA Summer School Almería, 9-10 June 2016
Molten salt plants operation experience
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• 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
– Joule effect
• The correct installation and selection of insulation materials is crucial for avoiding salt solidification
The most used
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7th SFERA Summer School Almería, 9-10 June 2016
Molten salt plants operation experience
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• 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
ModeradorNotas de la presentación- Redundant heat tracingThe cable has to be carefully installed:Running parallelWell-attached to the pipeMetallic mesh is used to help attaching the heat cable to the pipeStainless steel foil avoid the direct contact between cable and insulation and improve the heat transfer- Thermal inertia in valves is higher than in pipesIndependent heat tracing and control in valves and pipesUse different temperature control sensors in different parts
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7th SFERA Summer School Almería, 9-10 June 2016
Contents
1. Introduction 2. 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
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7th SFERA Summer School Almería, 9-10 June 2016
Advances and future works
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• 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
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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).
ModeradorNotas de la presentaciónThe heat capacity is a function of the melting point for the components of molten salt mixtures used in sensible heat storage.Materials should increase of both parameters: heat capacity and low melting temperature
Few and low reproducibility results have been reported from measurements because this parameter is very sensitive to the Modulated differential scanning calorimetry (MDSC) technique used for its determination.
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7th SFERA Summer School Almería, 9-10 June 2016
Advances and future works
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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
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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
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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.
ModeradorNotas de la presentaciónThe viscosity is the other important parameter for the new formulations of molten salts with potential to be used in solar technology.
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7th SFERA Summer School Almería, 9-10 June 2016
Advances and future works
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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/h2020/topics/2226-msca-if-2016.html Contact (before 15th August 2016): [email protected]
http://ec.europa.eu/research/participants/portal/desktop/en/opportunities/h2020/topics/2226-msca-if-2016.htmlhttp://ec.europa.eu/research/participants/portal/desktop/en/opportunities/h2020/topics/2226-msca-if-2016.htmlhttp://ec.europa.eu/research/participants/portal/desktop/en/opportunities/h2020/topics/2226-msca-if-2016.htmlhttp://ec.europa.eu/research/participants/portal/desktop/en/opportunities/h2020/topics/2226-msca-if-2016.htmlhttp://ec.europa.eu/research/participants/portal/desktop/en/opportunities/h2020/topics/2226-msca-if-2016.htmlhttp://ec.europa.eu/research/participants/portal/desktop/en/opportunities/h2020/topics/2226-msca-if-2016.htmlhttp://ec.europa.eu/research/participants/portal/desktop/en/opportunities/h2020/topics/2226-msca-if-2016.htmlhttp://ec.europa.eu/research/participants/portal/desktop/en/opportunities/h2020/topics/2226-msca-if-2016.htmlmailto:[email protected]
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7th SFERA Summer School Almería, 9-10 June 2016
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Thank you for your attention!!
Heat Transfer Fluids for Concentrating Solar Systems:��Molten saltsContentsIntroductionIntroductionIntroductionIntroductionContentsMolten salt physical propertiesMolten salt physical propertiesMolten salt physical propertiesMolten salt physical propertiesMolten salt physical propertiesBinary salt /Solar saltBinary salt /Solar saltBinary salt /Solar saltBinary salt /Solar saltBinary salt /Solar saltBinary salt /Solar saltTernary Salt/ Hitec® saltTernary Salt/ Hitec® saltTernary Salt/ Hitec® saltTernary Salt/ Hitec® saltBinary and Ternary SaltBinary and Ternary SaltContentsMolten salt plants operation experienceMolten salt plants operation experienceMolten salt plants operation experienceMolten salt plants operation experienceMolten salt plants operation experienceContentsAdvances and future worksAdvances and future worksAdvances and future worksAdvances and future worksAdvances and future worksAdvances and future worksNúmero de diapositiva 38