Presentation by Cem Akcaoglu for OERC Symposium 2017

1. Thermochemical materials for
thermochemical heat storage
Cem Akcaoglu1, Zhifa Sun2, Steve Moratti3
University of Otago
(1) e-mail: s.akcaoglu@postgrad.otago.ac.nz Phone: 034794006, Department of Physics, UO.
(2) e-mail: zhifa.sun@otago.ac.nz; Phone: 034797812, Department of Physics, UO.
(3) e-mail: smoratti@chemistry.otago.ac.nz Phone: 034797829, Department of Chemistry, UO.
2017 OERC Symposium & Ag@Otago Colloquium,
24th November 2017

2. Contents
Motivation and Background of the Thermochemical Heat Energy Storage
Systems (THSS) Research
Comparison of Heat Storage Systems
Principle of THSS
Selection of THSS Materials
Characteristics of SrBr2 and MgSO4
Characteristics of THSS Pellets
Challenges of This Research
Conclusion
2

3. Motivation and Background of THSS Research
The average household in New Zealand utilises 3,820 kWh energy, of which 34%
is for space heating with a cost of $1,070 per year.
Heating cost is subjected to increase with an increase in energy price (e.g.
electricity price increased by ~76% in the last 13 years.
Seasonal THSS are capable of multi-cyclic operations for more than 10 years and
are able to significantly reduce GHG emissions and energy consumption of
residential space heating.
THSS can utilize low-grade waste industrial heat and abundant solar heat to
heat homes during winter.
The performance of pure thermochemical heat storage materials need to be
improved.
3

4. Comparison of Different Heat Storage Systems
Seasonal heat storage systems should have, Low Heat Losses and High Energy Density
Sensible Heat Storage (SHS), 𝑄 𝑆𝑒𝑛𝑠𝑖𝑏𝑙𝑒 = (ρ 𝑝 𝑉𝑝).C. ΔT
Heat is stored or released respective to temperature changes between system and
the environment.
Latent Heat Storage (LHS) 𝑄 𝐿𝑎𝑡𝑒𝑛𝑡 = m CpdT (s) + m L + m Cp dT(l)
Heat is released or absorbed according to phase changing.
Thermochemical Heat Storage (THSS)
Heat is released or absorbed according to exothermic and endothermic
chemical reactions.
Approximately 8–10 times higher storage density over SHS, and two times
higher over LHS materials Low storage losses, energy stored in chemical bonds.
Solid Phase Sensible Heat Liquid Phase Sensible Heat
Latent Heat Fusion
Temperature°C
Stored Heat
Temperature°C
Stored Heat
SHS
LHS
Figure-1 SHS and LHS heat storage.
4

5. X • nH2O
Principle of Thermochemical Energy Storage (THSS)
X • (m+n)H2O
m H2O(v)
m H2O(v)
Heat
Heat
Discharging
Charging
Low Pressure
High Pressure
Figure-2 Principle of Thermochemical Energy Storage
5

6. Selection of THSS Materials
Figure-3 Equilibrium Charts For Selected Salt Hydrates
Low charging temperatures
(below 100 °C).
Environmentally friendly
(noncorrosive, nontoxicity
non-flammable).
High energy density.
Affordable price and
reachable supply.
No chemical reaction with
ambient gasses .
THSS material should have:
6

7. Selection of THSS Materials
E (GJ/m3) Tch
(°C)
Cost (€/t) Main issues Advantages
MgSO4 1.8–2.2 100 77 Heat release above 50 °C not possible; low
temperature lift, lower energy density
Low cost, energy stored at
< 90 °C
SrBr2 2.02 80 2400 High cost High stability and energy
density
MgSO4 and SrBr2 have highest potential for long term thermochemical heat
storage.
MgSO4could be considered as a better option due to its lower cost.
Both of the salt hydrates have similar energy density and desorption
temperatures.
Table-1 Prosperities of MgSO4 and SrBr2
7

8. Selection of THSS Materials
BET surface
area (m2
/g)
Void fraction
(%)
Thermal
conductivity
(W·m−1
K−1
)
ENG 38 20-60 3–10
AC 1,025 20-50 0.15–0.5
Thermal conductivity of THSS materials is important to achieve efficient heat
transfer and thermal performance. Composites of THSS materials and expanded
natural graphite (ENG) have potential to enhance the thermal conductivity of the
materials.
To enhance chemical reaction between the sorption gas and THSS materials,
activated carbon (AC) can be utilized to form composites to increase Brunauer–
Emmett–Teller (BET) surface area of the materials.
Table-2 Prosperities of ENG and AC
8
Figure-4 Images of AC (A), ENG (B) and manufactured composite (C)
A B C

9. Figure-5 TGA And DSC Results Of SrBr2 6H2O (Top) and
MgSO4 7H2O (Bottom).
Thermogravimetric analysis (TGA) and
differential spectrometric calorimetry (DSC)
measurements of pure hydrous
SrBr2 and MgSO4 have been conducted.
SrBr2.6H2 O (Strontium Bromide
Hexahydrate)
1,059 J/g (0.25 𝐺𝑗𝑚−3) Heat Capacity.
30 % weight loss, conversion towards to
anhydrous form.
MgSO4.7H2 O (Magnesium Sulphate
Heptahydrate)
1371 J/g (0.52 𝐺𝑗𝑚−3) Heat Capacity.
50 % weight loss, conversion towards to
anhydrous form.
MgSO47H2 𝑂 shows higher volumetric
energy density.
Characteristics of SrBr2 and MgSO4
9

10. Characteristics of SrBr2 and MgSO4
Both SrBr26H2 𝑂 and
MgSO47H2 𝑂 are hydrated at room
temperature (~20°C) and
dehydrated at 90 °C in the
experiments.
The two hydrates show the similar
dehydration and hydration trends,
making them suitable for multi-
cycle operations.
Figure-6 Hydration and Dehydration Curves Of SrBr2 6H2O (right)
and MgSO4 7H2O (left).
70
80
90
100
0 1 2 3 4 5 6 7 8 9 1011121314
SampleMass(%)
Duration (Hours)
Strontium Bromide Dehydration Cycles
1st Dehydration Cycle
2nd Dehydration Cycle
70
80
90
100
0 24 48 72 96
SampleMass(%)
Duration (Hours)
Strontium Bromide Hydration Cycles
1st Hydration Cycle
2nd Hydration Cycle
50
60
70
80
90
100
0 200 400 600 800 1000 1200
SampleMass(%)
Duration (hours)
Magnesium Sulphate Hydration Cycles
1st Hydration Cycle
2nd Hydration Cycle
50
60
70
80
90
100
0 1 2 3 4 5
SampleMass(%)
Duration (Hours)
Magnesium Sulphate Dehydration Cycles
1st Dehydration Cycle
2nd Dehydration Cycle
10

11. Characteristics of THHS Pellets (AC-SrBr2)
Figure-8 AS-55 (55% AC, 45% SrBr2) DSC-TGA Results.
More than 40 composite samples utilizing
SrBr2, MgSO4, AC, ENG have been
characterised by thermogravimetric analysis
(TGA) and differential calorimetry
spectrometry (DSC) techniques.
After cyclic test, the composite of 55% AC
and 45% SrBr2 showed:
Geometrical distortion.
Reduction in heat release.
Figure-7 AS-55 (55% AC, 45% SrBr2) before (left)
and after (right) cyclic test. 11

12. Characteristics of THS Pellets (AC- MgSO4)
Figure-10 AM-42 (42% AC, 58% MgSO4) DSC-TGA results.
Unlike the AC-SrBr2 composite, the
composite of 42% AC and 58% MgSO4
did not illustrate geometrical distortion, and
heat capacity was almost similar throughout the
cyclic tests.
MgSO4 illustrated a better performance
when the composite is supported with AC as a
THS supportive medium.
Figure-9 AM-52 (42% AC, 58% MgSO4) composite
before (left) and after (right) cyclic test.
12

13. Characteristics of THS Pellets
Figure-11 Hydration and Dehydration Curves Of AC-SrBr2 And
AC- MgSO4composites (Bottom).
Both AC-SrBr2 and AC-MgSO4
composite pellets are hydrated at
room temperature (~20°C) and
dehydrated at 90 °C.
AC-SrBr2 composite pellets
absorbed excess water corresponding
to 40% of its initial mass.
Absorbing more water resulted with
distortion on pellet’s geometry
AS-55 Hydration and Dehydration Charts
AM-42 Hydration and Dehydration Charts
65
70
75
80
85
90
95
100
105
0 24 48 72 96 120
Mass(%)
Time (Hours)
1st Hydration Cycle
2nd Hydration Cycle65
70
75
80
85
90
95
100
0 2 4 6
Mass(%)
Time (Hours)
1st Dehydration Cycle
2nd Dehydration Cycle
80
90
100
110
120
130
140
0 24 48 72 96 120 144 168 192
Mass(%)
Time (Hours)
1st Hydration Cycle
2nd Hydration Cycle
80
90
100
110
120
130
140
0 1 2
Mass(%)
Time (Hours)
1st Dehydration Cycle
2nd Dehydration Cycle
13

14. Characteristics of THS Pellets (ENG)
Results of ENG-SrBr2 (50% ENG, 50%
SrBr2) composites show:
ENG is not dense enough to host SrBr2.
Therefore, during the dehydration cycle the
salt hydrate crystals leaked from the pellet
composites at 90°C.
A similar amount of heat after cyclic test was
released, indicating that the leaked crystals do
not affect the overall heat release.
Solely ENG is not suitable to support
composite pellets.
Figure-13 ES-50 (50% ENG, 50% SrBr2) DSC-TGA Results.
C
Figure-12 ENG-SrBr2Composite Before Cyclic Test (A), After
Cyclic Test (B,C). 14

15. Characteristics of THS Pellets (ENG-AC)
EAM-60 (15% ENG, 25% AC, 60% MgSO4)
Pellets gained structural strength by utilizing
ENG and AC. Geometrical deformation was
not observed.
Utilization of AC prevented the leakage of salt
crystals from pellets.
Composites have showed similar DSC and TGA
characteristics during the cyclic test. The pellets
are suitable for multi cyclic operations.
Figure-15 EAM-60 (15% ENG, 25% AC, 60% MgSO4) DSC-
TGA results.
Figure-14 EAM-60 (15% ENG, 25% AC, 60% MgSO4) Composite
Before Cyclic Test (Left), After Cyclic Test (right). 15

16. Challenges of This Research
SrBr2is highly hydrophilic, it can be over hydrated and form gel like
structures easily. MgSO4 is less hydrophilic but shows similar problem
during a long operation term.
Storages need to be designed to solve this problem.
ENG is expanding even at low temperatures such as 90°C, damaging the
pellet’s geometric shape.
ENG has a higher thermal conductivity, but it should be used together with AC.
15% ENG and 25%AC in the composites show an optimum performance.
16

17. Conclusion
Composite THSS materials with high energy intensity up to 3,855 J/g and stable
thermal performance have produced in our lab.
11 times more energy dense than SHS (SHS stores nearly 250 J/g by utilizing water).
By utilizing AC-ENG mixture, composite pellets gained geometrical strength in all
the cyclic tests and performed better than pure salts in the TGA-DSC tests.
The composite materials we investigated are cheap enough to be realistically
utilized in a seasonal storage system.
Composite pellets will be implemented into an open-system energy storage
prototype which can be employed as a dehumidifier heating system to solve
damp problem in New Zealand homes.
Figure-16 Open System Operation.
Dry Warm AirCold Wet Air Hydration Cycle
17

18. Thank You
2017 OERC Symposium & Ag@Otago Colloquium,
24th November 2017