sfera networking 7th sfera summer school school... · receiver. power generationprocess. industrial...

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

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


Introduction

3

Optical Concentrator

Beam solar radiation

Concentrated solar radiation

RECEIVER

Power Generation Industrial Process Turbine

STE plants

Thermal Energy

Thermal Storage


Introduction

4

Collector field

Molten salt system (Tower)

Power block Sunlight


Introduction

5

• 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


Introduction

6

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


Contents





Molten salt physical properties

8

• 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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• 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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• 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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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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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


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

13

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



14

• 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.



15



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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



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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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• 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!!!


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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• 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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• 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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• 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


Binary and Ternary Salt

23

• 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


Binary and Ternary Salt

24

• 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


Contents





Molten salt plants operation experience

26

• 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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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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• 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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• 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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• 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


Contents





Advances and future works

32

• 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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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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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



35

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



36

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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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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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

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