Long term heat storage based on
sodium acetate with stable supercooling
Simon Furbo
Department of Civil Engineering
Technical University of Denmark
Brovej - building 118
DK-2800 Kgs. Lyngby
Denmark
Email: [email protected]
Partners in COMTES line C
2
Phase Change Material (PCM) with supercooling
Heat storage capacity of sodium acetate tri-hydrate
Stored energy [kJ/litre ]
800
700
Sodium acetate
600
Supercooling
500
400
Water
300
Activation of
solidification
200
Melting point = 58 °C
100
0
20
30
40
50
60
70
Temperature [°C]
3
80
90
100
Sodium acetate trihydrate
NaCH3COO · 3H2O
60% sodium acetate + 40% H2O
Incongruently melting - phase
separation
Supercooled SAT
Supercooled SAT + H2O
Extra water principle
55% sodium acetate + 45% H20
Additives
- Carboxy Methyl Cellulose, CMC
- Xanthan rubber, X-Gum
- Ethylene Diamine Tetraacetic, EDTA
4
Supercooled SAT + CMC
Small scale tests
- Heat content summary
230
220
Heat content [kJ/kg]
210
200
190
180
170
160
150
* Some mixtures include extra water
5
Flat heat storage module with
sodium acetate trihydrate
Start of solidification by cooling the outside of a module
or use of needle
Advantages:
• High heat transfer areas
• Reduced risk of phase separation
• Equipment inside module not needed
• Only sodium acetate trihydrate inside module
• Reduced risk of remaining salt hydrate crystals after charge periods
• Reduced risk of supercooling failure
• Relatively simple and cheap module design
6
Storage module design
- sandwich
Closed PCM box
Simple PCM box - no cracks
Internal and external expansion volume for PCM - no pressure build-up
Heat exchanger on top and bottom surface
•
•
•
•
Dimension: 240 x 120 cm
Box height: 5 cm
Heat exchanger height: 4 mm
Material thickness: 2mm
Steel mass: 236 kg
Sodium acetate water mixture: 200 kg
Sodium acetate trihydrate CMC mixture: 220 kg
7
Storage module design
- sandwich
Closed PCM box
Simple PCM box - no cracks
Internal and external expansion volume for PCM - no pressure build-up
Heat exchanger on top and bottom surface
•
•
•
•
TOP MANIFOLD
14 PARALLEL CHANNELS
TOP IN
8
TOP MANIFOLD
TOP OUT
Storage module design
- sandwich
Closed PCM box
Simple PCM box - no cracks
Internal and external expansion volume for PCM - no pressure build-up
Heat exchanger on top and bottom surface
•
•
•
•
BOTTOM IN
MANIFOLD
16 PARALLEL CHANNELS
MANIFOLD
9
BOTTOM OUT
Challenges
• Sufficiently high heat exchange capacity rate
• Heat content of module high enough
• Long term stability of heat storage with high heat content
• Stable supercooling
• Reliable activation of the solidification
10
Solutions
•Increase heat transfer during discharge by adding oil to the PCM box. Oil will fill
the holes in the solidified PCM
•Increase thermal conductivity of PCM by adding graphite powder
•Avoid phase separation with thickening agent
•Increase heat content and active temperature level by use of thickening agent
instead of extra water
•Decrease evaporation from the PCM box to the expansion tube by adding oil to
the PCM box
•Avoid high pressures in the PCM box by use of expansion vessel
•Use needle for start of solidification
11
Density of sodium acetate
trihydrate
12
Effective heat transfer
in PCM
Problem: Air gaps between PCM and heat exchangers
Oil
Air/vacuum
Solution? Increase the effective heat transfer between PCM and heat exchanger
by adding an oil layer to be sucked into gaps when sample crystallises and
contracts
13
Start of solidification by
needle with crystals
Needle control unit for triggering devices
14
Gained experience
Supercooling
 By time salt gets in the tube between PCM box and expansion bag resulting in
failure of supercooling
 Stable supercooling can be achieved both in black steel and stainless steel modules
 Stable supercooling can be achieved if all parts of the PCM volume is heated to
temperatures higher than 77°C
 Stable supercooling can be achieved if an expansion volume is connected to the
PCM box
 Decreased evaporation from PCM box to expansion tube by adding oil in the PCM
box
Activation of solidification
 CO2 activation principle used for cooling reliable
 Peltier element principle used for cooling reliable
 Use of needle reliable
15
Gained experience
Heat exchange capacity rate
 Low heat exchange capacity rates for charge and discharge
 Measured heat exchange capacity rates lower than calculated
 Oil can increase heat exchange capacity rate during discharge. Oil will fill the
holes in the solidified PCM
 Graphite powder can increase thermal conductivity
Heat content
 Heat content decreased over time for sodium acetate water mixture (45%
water)
 Heat content constant over time for sodium acetate trihydrate with additives
 Phase separation avoided, even in high heat stores
 Measured heat content close to theoretical calculated heat content
 Increased heat content and active temperature level by use of additive instead
of extra water
Long term stability
 Module not damaged if the pressure in the heat exchangers is low enough
16
Demonstration system
Location:
Solar heating test facility of Technical University of Denmark
17
Demonstration system
•
22.4 m² evacuated tubular solar collectors
•
4 PCM modules
with 800 kg
sodium acetate
trihydrate
•
735 l tank in tank buffer storage
•
Simulated heat demand: Danish passive house
•
Expected yearly solar fraction: 80%
18
Demonstration system
- schematic sketch
19
Simulated heat demand
Domestic hot water consumption: 100 l/day, corresponding to 1700 kWh/year
Space heating demand: About 2000 kWh/year
20
Demonstration system
21
Single module charge
Charge of module no. 1 on the 8th of September 2015:
•
Melted within 4.5 hours on a sunny afternoon
•
Stored heat: 27.4 kWh (sensible and latent heat)  state of charge=100%
22
Parallel module operation
•
Parallel charge at sunny days is necessary (limited heat exchange capacity rates)
 No overheating was occurring (1-3 modules in parallel)
•
2 sunny days in October allowed to recharge 3 modules from supercooled state
•
No solidification happened during recharge
23
Experience
 System working
 Monitoring system working
 Solar collectors can charge PCM modules
 Proof of supercooling concept for 3 modules
 Heat of fusion used to cover heat demand
Further:
 High PCM modules stable by use of additive
Challenge
There is a long way to optimized/attractive compact long term heat
storages for the market
24
Recommendations
for further activities
• Development of inexpensive seasonal heat storage based on
cylindrical tanks with sodium acetate trihydrate + additives
• Optimization of solar heating systems inclusive seasonal heat of fusion
storage and buffer tank for different houses
• Development of (solar) heating systems with long term heat storage
and auxiliary energy system based on heat pump/electric heating
elements utilizing low cost electricity for different houses
• Integration of the above systems in the future energy system
25
Recent publications
Long term thermal energy storage with stable supercooled sodium acetate trihydrate, Mark Dannemand,
Jørgen M. Schultz, Jakob Berg Johansen, Simon Furbo. Applied Thermal Engineering (91), 2015
Solidification behaviour and thermal conductivity of bulk sodium acetatet trihydrate mixtures with thickening
agents and graphite powder, Mark Dannemand, Jakob Berg Johansen, Simon Furbo. Solar Energy Materials
and Solar cells, Vol 145, part 3, pp. 287-295, 2016
Thermal conductivity enhancement of sodium acetate trihydrate by adding graphite powder and the effect on
stability of supercooling, Jakob Berg Johansen, Mark dannemand, Weiqiang Kong, Jianhua Fan, Janne
Dragsted,
Simon
Furbo.
Energy
Procedia
(ISSN:
1876-6102)
(DOI:
http://dx.doi.org/10.1016/j.egypro.2015.02.121), vol: 70, pages: 249-256, 2015
Laboratory test of a prototype heat storage module based on stable supercooling of sodium acetate trihydrate,
Mark Dannemand, Weiqiang Kong, Jianhua Fan, Jakob Berg Johansen, Simon Furbo. Energy Procedia (ISSN:
1876-6102) (DOI: http://dx.doi.org/10.1016/j.egypro.2015.02.113), vol: 70, pages: 172-181, 2015
Testing of PCM heat storage modules with solar collectors as heat source, Gerald Englmair, Mark Dannemand,
Jakob Berg Johansen, Weiqiang Kong, Janne Dragsted, Simon Furbo, Jianhua Fan. International Conference on
Solar Heating and Cooling for Buildings and Industry, SHC 2015
Laboratory test of a cylindrical heat storage module with water and sodium acetate trihydrate. Mark
Dannemand, Weiqiang Kong, Jakob Berg Johansen, Simon FurboInternational Conference on Solar Heating
and Cooling for Buildings and Industry, SHC 2015
Validation of a CFD model simulating charge and discharge of a small heat storage test module based on a
sodium acetate water mixture, Mark Dannemand, Jianhua Fan, Simon Furbo, Janko Reddi. Energy Procedia
(ISSN: 1876-6102) (DOI: http://dx.doi.org/10.1016/j.egypro.2014.10.254), vol: 57, pages: 2451–2460,
2014
26
Acknowledgement
The COMTES project has received funding from the European Union Seventh
Framework Program under grant agreement no. 295568
27