mixed salts in thermochemical heat storage

MIXED SALTS IN THERMOCHEMICAL HEAT STORAGE

MIXED SALTS IN THERMOCHEMICAL HEAT STORAGE BY DIPL. MIN/MET MONA-MARIA DRUSKE

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Agenda • • • • •

Introduction Microscale: Methodology and results (TGA/DSC) Macroscale: Methodology and results (Labscale) Discussion Conclusion

Mixed salts in thermochemical heat storage by Mona-Maria Druske

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Introduction


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Introduction

• consumption of fossil fuels for generating heat • heat demand varies by season • long term storage battery system for excess heat → reliable storage material?


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Introduction • Reversible thermochemical reaction:

AB ∙ nH2O(s) + Heat

AB ∙ (n-x)H2O(s) + xH2O(g)

• Salts: - low cost - availability • Practical application depends on individual material properties • different salts can be mixed to change properties Mixed salts in thermochemical heat storage by Mona-Maria Druske

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Introduction Undesired properties Low heat storage capacity Low cycle stability caused by: • Agglomeration - the material lumps together and will only hydrate/dehydrate at the surface

• High deliquescence - the material will take up water until it dissolves and may solidify in an amorphous state at dehydration

• Low melting temperature - the material will melt during dehydration and may solidify in an amorphous state at dehydration

• Generation of byproducts (eg.: Fe2O3, HCl) High material costs Mixed salts in thermochemical heat storage by Mona-Maria Druske

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K2SO4

x

MgSO4

x

x

ZnSO4

x

x

FeSO4

x

x

Al2(SO4)3

x

KCl

CaBr2

MgBr2

LiBr

KBr

NaBr

• 53 tested mixtures • varying mixing ratios: e.g. 1:1; 1:2

x

x

• grown from liquid solution (H2O dest)

x

x

MgCl2

o

CaCl2

Δ

o

Δ

x

ZnCl2

CaCl2

MgCl2

KCl

FeSO4

ZnSO4

MgSO4

Na2SO4 K2SO4

Material selection

Δ

SrBr2

• mixtures dried and stored in desiccator o x

x

o

Δ

Δ

x low cycle stability and heat storage capacity Δ low heat storage capacity o good heat storage capacity and improved cycle stability (compared to untreated material) Mixed salts in thermochemical heat storage by Mona-Maria Druske

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Methodology 53 mixtures microscale prescreening (TGA/DSC): heat storage capacity & cycle stability

4 mixtures further selection criteria: deliquescence & material costs

2 mixtures microscale 25 cycles screening macraoscale labscale experiments


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Methodology: Hydration 2MgCl2 + CaCl2 • •


msample = 10mg p = atm

1.

pH2O = 10 mbar

2.

pH2O = 17 mbar

3.

pH2O = 20 mbar

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Methodology: dehydration 2MgCl2 + CaCl2

• •


T < 100°C T > 100°C

1.

T = 25°C, t = 20 min

2.

T = 25 to 100°C, ΔT/Δt = 5 K min-1

3.

T = 100°C, t = 30 min

4.

T = 100 to 200°C, ΔT/Δt = 5 K min-1

5.

T = 200°C, t = 30 min

6.

T = 200 to 25°C, ΔT/Δt = -10 K min-1

7.

T = 25°C, t = 30 min

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Results: TGA/DSC Material

1 SrBr2 : 2 LiBr

ΔH

ΔH

H2O/anhydrate

Observed

(anhydrate)

(H2O)

[g g-1]

cycle

[J g-1]

[J g-1]

2140

3298

Comments

Stability 0.65

+

high material costs

1 CaCl2 : 2 ZnCl2

1320

2386

0.55

+

Tm~ 25°C Deliquescence

-2 MgCl2 :1 KCl

2049

3105

0.66

+

Treaction < 120°C

+ 2 MgCl2 : 1 CaCl2

3330

3028

1.10

+

heat storage capacity

+ positive material property – negative material property Mixed salts in thermochemical heat storage by Mona-Maria Druske

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25 cycles of 2MgCl2 + KCl normalized to dry salt

• pH2O = 15mbar

2,5 2,3

• Tmax = 120°C, heating rate

2,1

1K min-1

ΔH [-], Δm [-]

1,9

• Enthalpy stabilizes after

1,7 1,5

5th cycle

1,3 1,1

• low starting enthalpy

Δm [-]

reasoned to be caused by low water vapor pressure during storage

0,9 ΔH [-]

0,7 0,5 0

5

10

15 cycles


20

25

• Decline in mass change 25.04.2016

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25 cycles of 2MgCl2 + CaCl2 normalized to dry salt

• pH2O = 15mbar

3,5

• Tmax = 120°C Heating rate

3

ΔH [-], Δm [-]

1K min-1 2,5

• Enthalpy shows a strong

incline after 15th cycle and a decline after the 22th cycle

2

1,5

• Slow activation of the

Δm [-] 1

material

ΔH [-] 0,5 0

5

10

15 cycles


20

25

• Mass change stabilizes after 5th cycle

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Methodology labscale

• •

• • •


20g samples dried beforehand at Tmax = 120°C and cooled in a desiccator

p = 20 to 30mbar TH2O is the temperature of the water vapor Tsample is the temperature within the salt sample 25.04.2016

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Results labscale

• Samples dried at

100

120°C

90

• Hydration temperatures of

80 70

T [°C]

Tmax =

{2MgCl2 : CaCl2}and {2MgCl2 : KCl} measured and compared to untreated MgCl2 and CaCl2 (anhydrates)

60 50

40 30 20 0

0,5 2MgCl2 + CaCl2 MgCl2

1 time [h]

1,5

2MgCl2 +KCl CaCl2


2

• {2MgCl2 + CaCl2} mixture needs higher reaction temperatures for improved results 25.04.2016

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Discussion At the same mass and under the same conditions the {2MgCl2 : KCl} mixture has a higher energy storage density than all other tested materials with the exception of CaCl2. The {2MgCl2 : CaCl2} mixture has a lower energy storage density than the other tested materials.


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Conclusion As the results of the first labscale experiments are contradictionary to those of the microscale measurements, further tests are necessary.


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Thank you for your attention!


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