Chemical Energy Storage
Anker Degn Jensen &
Jakob Munkholt Christensen
Contributions from:
Peter Vang Hendriksen, DTU
Jan-Dierk Grunwaldt, KIT
Introduction
• Renewable energy will to a large extent be based on
energy harvested as electricity (e.g. wind, solar cells)
• The fluctuating nature of these energy sources makes
energy storage a requirement of such an energy system
• Energy is most effectively stored in chemical bonds:
Pumped
hydro
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Compressed
air
Electrical
capacitor
Li-ion
battery
Hydrogen
Source: Schüth (2011), Chem. Ing. Tech., 1984-1993
14.11.2013
Requirements to storage molecules
• High volumetric storage density
• Handling should be as easy as possible (liquids or gas !)
– preferably compatible with our current infra structure
• Dangers in production, storage, and conversion should
be as low as possible
• The cycle efficiency:
cycle
Energy recovered by conversion of the molecule

Energy used for production of the molecule
should be as high as possible
•
Storage in chemical compounds generally has lower cycle
efficiency than e.g. hydropower and batteries, but still
seems the only option for large scale storage
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Energy density of some possible
storage chemicals and batteries
40
Energy density [MJ/L]
35
30
25
Energy on
lower heating value basis
20
15
10
5
0
Source: DTU International Energy Report 2013
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Requirements to storage molecules
• Requirements to storage molecules will depend the use
• For generation of electrical energy the electrical cycle
efficiency should be as high as possible
• In a situation with no fossil fuels - or the wish not to
use them - storage molecules for transport (trucks and
planes) is also needed
• Additionally, local conditions may play an important role
as to which storage molecule makes sense
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Interesting storage molecules
• Hydrogen, H2
• Methane, CH4
• Methanol (CH3OH)
• Liquid hydrocarbons
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Hydrogen, H2
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Hydrogen
• High gravimetric energy density of 120 MJ/kg
• The only combustion product is water: H2 + ½O2  H2O
• May be produced using renewable electricity by SOEC
• Converted back to electricity in SOFC or gas turbine
• Cycle efficiency about 50 % in SOC, but only around 30 %
including compression for storage
0.8 V
1.4 V
Chemical energy
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Electricity + Heat
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Hydrogen
•
Low volumetric energy density of 8,5 MJ/L (for liquid
hydrogen at 20 K)
•
Storage facilities must be available/established
•
Transport of hydrogen in pipelines requires special
consideration due to materials issues
•
Efficient (SOC) electrolyzers not yet commercial in
grid scale, and are expensive
•
Not generally applicable for transportation
•
Leaves limited biomass for other applications
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Methane, CH4
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Methane – synthetic natural gas
• An interesting storage molecule:
 Pressurized methane 3 times higher storage density
than gaseous hydrogen, but still lower than liquid
hydrocarbons
 Widespread infrastructure (transport, storage,
handling) available – also in Denmark
 Can be used as a fuel in internal combustion engines
and gas stations selling NG is (re)appearing
 May be produced from synthesis gas (CO/CO2/H2O) by
a catalytic proces : CO + 3H  CH + H O
2
4
2
CO2 + 4H2  CH4 + 2H2O
 Syngas methanization is commercially available
 Could also be produced by fermentation
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Methane – synthetic natural gas
CO2 from combustion of
SNG/biomass/fossil fuel:
Power-to-gas/
carbon capture and recycling (CCR)
Electrolysis cell
2H2O  2H2 + O2
Storage of
CO2 needed !
CH4
Metanisation
Ni based
catalyst
Renewable electricity
CO2  CO + 1/2O2
Concentrated CO2
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H 2O
Coupling of heat consuming electrolysis
with heat releasing chemical synthesis
14.11.2013
Methane – synthetic natural gas
Gasfication technology developed at DTU:
TwoStage downdraft wood gasification for CHP production: 2MW unit built in Hadsund/Hillerød, Denmark by Weiss A/S
Very high
efficiency >93 %
Courtesy Weiss A/S
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Methane – synthetic natural gas
Case of surplus of renewable electricity:
From Ahrenfeldt (2013)
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Methane – synthetic natural gas
Case of demand for electricity:
From Ahrenfeldt (2013)
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Methanol, CH3OH
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Methanol
• A liquid fuel at room conditions with fairly high energy
density – easily stored
• Efficient fuel in internal combustion engines – although
miscibility issues with gasoline in presence of water
• Could be used as fuel in a direct methanol fuel cell
• Can be produced in an efficient catalytic proces from
biomass via syngas – close to 60 % efficiency including
gasification step:
CO + 2H2  CH3OH + H2O
CO2 + 3H2  CH3OH + H2O
• Methanol could be used also to make the diesel substitute
fuel (DME) and gasoline as well as a range of other
important chemicals : formaldehyde, olefins, acetic acid
• ’Methanol Economy’ strongly promoted by nobel prize winner
G. A. Olah: ’Beyond Oil and Gas: The Methanol Economy’, 2009
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Liquid hydrocarbons
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Liquid hydrocarbons
• The fuels we know
• Completely compatiple with the existing infra structure
• The highest energy density
• Synthesized from syngas by Fischer-Tropsch (FT) catalytic
process (40 % efficiency):
n(CO + 2H2 )  [CH2 ]n + nH2O
• Synthesized via catalytic methanol conversion (MTG
process, 50-60 % efficiency):
nCH3OH  [CH2 ]n + nH2O
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Liquid hydrocarbons
Haldor Topsøe methanol-to gasoline (TIGAS) process:
High power need
From Joensen et al., Biomass Conv. Bioref., 2011, 85-90
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Liquid hydrocarbons
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Concluding remarks
• A future energy and transport sector relying on renewable
wind, solar and biomass energy will require energy storage in
chemicals
• Tight relations between energy and chemicals
• Several of the discussed energy carriers may be relevant:
 Methane is consistent with existing storage and transport
facilities (Energinet.dk)
 Methanol and products thereof (DME, gasoline etc) and
upgraded pyrolysis oil to fulfill transport needs
•
•
For electricity direct biomass combustion most efficient
Biomass resources are limited (20% of the need)
H2 and CCR still attractive
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
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Concluding remarks
• Some technology is available now – other requires further
development. In all cases further development is needed to
lower costs
• Gasification and pyrolysis processes likely will play a role
• Catalysis and electrolysis will play pivotal roles in this
development – no use of noble metals !!
• The Catalysis for Sustainable Energy (CASE) initiative at DTU
has led to significant advances in many areas of relevance
for our future energy system
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Overview of future energy system
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Thank you for your attention !
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14.11.2013