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Thermische Speicherung von Solarenergie Dr. Thomas Bauer Institut für Technische Thermodynamik Stuttgart, Köln 15. Kölner Sonnenkolloquium, 12.6.2012
www.DLR.de/TT • Slide 2 > 15. Kölner Sonnenkolloquium > T. Bauer • Thermische Speicherung von Solarenergie > 12.6.2012
Thermal Energy Storage (TES) Activities at the Institute of Technical Thermodynamics High temperature TES for CSP plants, CC plants, Adiabatic CAES, industrial process heat and CHP
- Sensible heat storage - Solids: Concrete, packed bed, regenerator - Liquids: Molten salt
- Latent heat storage (PCM-salt) - Thermo-chemical storage
www.DLR.de/TT • Slide 3 > 15. Kölner Sonnenkolloquium > T. Bauer • Thermische Speicherung von Solarenergie > 12.6.2012
CeraStorE (Competence Center for Ceramic Materials and Thermal Storage Technologies in Energy Research)
Institute of Technical Thermodynamics (TT) Thermochemical and
molten salt heat storage
Institute of Solar Research (SF) Solar-powered
thermochemical cylces
Thermochemical and
molten salt aspects
Institute of Materials Research (WF) WHIPOX (wound highly porous oxide composite)
Redox material development
Building 26 (opened 8.12.2011) 2000 m² Total Area for Research
Material aspects
www.DLR.de/TT • Slide 4 > 15. Kölner Sonnenkolloquium > T. Bauer • Thermische Speicherung von Solarenergie > 12.6.2012
Material screening
Heat exchanger / heat transfer enhancement
Stability, compatibility, corrosion
Thermal properties of storage material and heat transfer fluids
Thermophysical properties
Technical material quality
Thermo-mechanical design
System integration
Importance of Material Research Development Steps for Storage Systems
Lab-scale tests
Containment, Insulation
www.DLR.de/TT • Slide 5 > 15. Kölner Sonnenkolloquium > T. Bauer • Thermische Speicherung von Solarenergie > 12.6.2012
Materials and Sub-Components for Heat Storage
Take away: TES requires more than storage material research
Storage material Container Heat exchanger Heat transfer fluid
Additional components: pumps, valves, connection pipes, insolation, foundation, instrumentation and control devices
Water, natural rocks, ceramics, concrete salts, metal oxides
Pressurized vessels, packed bed designs,
corrosive media
Finned tubes, shell-and-tube heat
exchanger
Air, flue gas, water/steam, molten
salt, thermal oil
www.DLR.de/TT • Slide 6 > 15. Kölner Sonnenkolloquium > T. Bauer • Thermische Speicherung von Solarenergie > 12.6.2012
Impact of Specific Material Categories Example: PCM storage with Nitrate Salts
18%
27% 18%
37%
Heat exchanger, Tube register
Heat transfer enhancement (fins)
Isolation, Foundation Header, Containment
TES Material (Salt)
Operated with water/steam HTF
www.DLR.de/TT • Slide 7 > 15. Kölner Sonnenkolloquium > T. Bauer • Thermische Speicherung von Solarenergie > 12.6.2012
Solids Concrete Ceramics and rocks
Liquids Molten salt Pressurized water
Mixture of solids and liquids
High-Temperature TES materials and technologies
Sensible heat Latent heat Chemical heat
Source: ACS Cobra
Reactions Gas-gas Liquid-gas (Absorption) Solid-gas Ca(OH)2/CaO/H2O CaCl2/H2O Mn2O3/Mn3O4/O2
-sym
met
ric a
xis
-9 m -5 m
-1 m
Phase change Solid-solid Solid-liquid Alkali nitrate/ nitrite salts
www.DLR.de/TT • Slide 8 > 15. Kölner Sonnenkolloquium > T. Bauer • Thermische Speicherung von Solarenergie > 12.6.2012
Molten Salt Storage - Market status: Commercial CSP experience
- 2-tank indirect concept, max. 390 °C (Andasol, 2009) - 2-tank direct concept, max. 560 °C (Gemasolar, 2011)
- Technology: - Decoupled thermal power and capacity; constant power - Boundaries set by current storage medium (temperature levels, costs)
- Research focus/challenges: - Development of alternative salt systems
- Lower melting temperature to avoid freezing (< 140 °C)
- Stability at higher temperatures (up to 700 °C)
- Cost reduction – alternative tank design (e.g. Thermocline – single tank with filler)
- Components and materials for higher temperatures
Source: Ferrostaal Andasol 3
www.DLR.de/TT • Slide 9 > 15. Kölner Sonnenkolloquium > T. Bauer • Thermische Speicherung von Solarenergie > 12.6.2012
Molten Salt Storage – Phase diagrams 3 different Ions, Binary system with common ion 4 Ions, Additive Ternary
4 Ions, Reciprocal Ternary
5 Ions, Additive Quaternary (e.g. KNO3-NaNO3-LiNO3-Ca(NO3)2)
5 Ions, Reciprocal Quaternary (e.g. Li, Na, K // NO2, NO3, Tmin ≈ 80 °C)
200
250
300
350
0 10 20 30 40 50 60 70 80 90 100NaNO3 [wt%]
Tem
pera
ture
[°C
]
Melting range
Phase diagram KNO3-NaNO3
Solidus
Liquidus
0
10
20
30
40
50
60
70
80
90
100
0 10 20 30 40 50 60 70 80 90 100Mol %
Mol
%
150°C160°C180°C200°C220°C240°C260°C280°C320°C360°C400°C
NO3
Na
NO2
K
www.DLR.de/TT • Slide 10 > 15. Kölner Sonnenkolloquium > T. Bauer • Thermische Speicherung von Solarenergie > 12.6.2012
Molten Salt Storage – Novel Salt Mixtures
-1
0
1
2
3
4
5
6
7
-50 0 50 100 150 200Temperature [°C]
DS
C s
igna
l [μV
/mg] 5 K/min
2 K/min1 K/min 81
82838485
0 1 2 3 4 5 6Heating rate [K/min]
Liqu
idus
te
mpe
ratu
re [°
C]
Heating rate
Take away: Low melting temperature alkali nitrate/nitrite mixtures are feasible
www.DLR.de/TT • Slide 11 > 15. Kölner Sonnenkolloquium > T. Bauer • Thermische Speicherung von Solarenergie > 12.6.2012
Molten Salt Storage – Solar Salt Stability
Take away: Stability of nitrate salts depends on several parameters
First decomposition reaction:
NO3- → NO2
- + ½ O2(gas) (reversible) Second decomposition reaction:
NO3- or NO2
- → O2- + NOx(gas) or N2(gas)
Other Side reactions with CO2 and H2O
Na NO3
Cation Anion
Air
www.DLR.de/TT • Slide 12 > 15. Kölner Sonnenkolloquium > T. Bauer • Thermische Speicherung von Solarenergie > 12.6.2012
Molten Salt Storage – Steel Corrosion with Nitrates
- Major structural alloys: - Low alloyed carbon steel (≤ 400°C) - Cr-Mo steel (≤ 500°C) (Cr-content up to about 9 wt%) - Stainless Cr-Ni steel (≤ 570°C) (with/without Mo, Nb, Ti alloying) - Ni-alloys (≤ 650°C) (i.e. Alloy 800)
- Corrosion aspects: - Nitrate salt impurities (e.g., chlorides) - Nitrate salt decomposition products - Solubility of chromium from oxide layer - Stress corrosion cracking (SCC)
Source: Materials Testing Institute (MPA), University of Stuttgart
www.DLR.de/TT • Slide 13 > 15. Kölner Sonnenkolloquium > T. Bauer • Thermische Speicherung von Solarenergie > 12.6.2012
Molten Salt Test Facility “Tesis”
- Technical figures: - 20 m³ Solar Salt
(60 wt% NaNO3 and 40 wt% KNO3) - Temperature range 250 to 560 °C - Mass flow rate up to 8 kg/s
- Research focus/challenges: - Molten salt storage concepts
(e.g. single tank / thermocline) - Component tests
(e.g. pumps, valves, receiver tubes, measurement & control) industrial partners
- Operational molten salt aspects
www.DLR.de/TT • Slide 14 > 15. Kölner Sonnenkolloquium > T. Bauer • Thermische Speicherung von Solarenergie > 12.6.2012
Thermo-chemical Heat Storage - DLR Development status: Lab-test rigs
- Ca(OH)2 /CaO (300 – 700 °C) - Salt hydrates, reference CaCl2 (up to 200 °C) - Metal oxides (up to 1000 °C)
- Target applications: CSP, industrial heat recovery - Technology:
- Reversible Gas-Solid Reactions - High storage density - Loss-free and long-term storage feasible - Heat transformation possible
- Research focus/challenges: - Characterization of suitable material system - Complexity of heat and mass transfer
with chemical reaction - Reactor designs - Separation of thermal power and capacity
(movement of the reaction bed) Source: DLR
www.DLR.de/TT • Slide 15 > 15. Kölner Sonnenkolloquium > T. Bauer • Thermische Speicherung von Solarenergie > 12.6.2012
Thermochemical Ca(OH)2/CaO storage
- Target applications: - CSP, industrial process heat - Temperatures 300 – 700 °C
- Technology - Ca(OH)2 + ∆H ↔ CaO + H2O - Direct/indirect contact concepts
- Start of larger test rig in progress - Research focus/challenges
- Up-scaling of reactor design - Movement of the
reaction bed - System modeling and
process integration
www.DLR.de/TT • Slide 16 > 15. Kölner Sonnenkolloquium > T. Bauer • Thermische Speicherung von Solarenergie > 12.6.2012
Summary and Conclusions
- Molten salt TES technology - Several parameters affect nitrate salt stability no single limit - Tower systems require novel salts with stabilities up to 700 °C - Parabolic through systems require salts as fluids with a liquidus
temperature as low as possible to avoid freezing
- Thermochemical TES technology offers unique advantages Storage of educts/products at room temperature, heat transformation
- Diversity of fluids and storage materials in the high temperature range Molten salt is one TES option among several others
- TES material development not only refers to improved storage materials, but also to material aspects in other components (e.g., containment)
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