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• At present, electricity storage with advanced adiabatic compressed air energy storage (AA-CAES) is considered to be the only large-scale alternative to pumped hydro storage, offering high cycle efficiency (70-75%) thanks to incorporation of a thermal energy storage (TES) system

• Thermocline storage has gained increasing interest as solution for TES with potentially high efficiency and low costs.

• The outflow temperature of a thermocline TES system with only sensible heat storage material drops during discharge if the tank is not large enough and not sufficiently pre-charged

• For several applications, this is unfavorable (thermodynamic power cycles, chemical reactions)

• Measure PCM properties for more accurate simulations

• Simulation-based determination of heat-transfer coefficients for different encapsulation configurations

• Numerical optimization of TES considering efficiency and material costs

• Simulation of TES for AA-CAES in tunnel

• Experiments with TES for AA-CAES in tunnel

• Sensible • Combined

High-Temperature Combined Sensible/Latent-Heat Storage for AA-CAES Experiments and Simulations Lukas Geissbühler1, Michael Kolman1, Dr. Giw Zanganeh2, Dr. Andreas Haselbacher1, Prof. Dr. Aldo Steinfeld1 1Professorship of Renewable Energy Carriers, ETH Zurich, 2Airlight Energy Manufacturing SA, Biasca

• Phase change materials (PCMs) can deliver heat at constant temperature

• PCMs have high energy densities • However, they are expensive and not well suited

to large temperature ranges • Combined sensible/latent heat TES avoids

disadvantages Approach: • The system of combined sensible/latent heat

TES is studied using an experimental-numerical approach

• An experimental setup was built consisting of a packed bed of rocks (sensible heat section) and steel encapsulated AlSi12 PCM tubes (latent heat section) on top (Etot = 42.3 kWhth). Air at ambient pressure was used as heat transfer fluid.

• An unsteady one-dimensional heat transfer model was developed. The validated model is used to predict the dynamic behavior of large-scale TES systems and compare the combined storage with the sensible only storage considering exergy efficiency and material costs for a given maximum temperature drop during discharging ( ).

Experimental tests of a combined sensible/latent-heat TES were performed at Airlight Energy SA in Biasca. The model was compared to measurements for various operating conditions and multiple cycles.

3 Experimental Results and Model Validation - Labscale TES

5 Outlook

6 References

1. Geissbühler L., Kolman M., Zanganeh G., Haselbacher A., Steinfeld A., Analysis of industrial-scale high-temperature combined sensible/latent thermal energy storage, to be presented at the ASME-ATI-UIT conference on thermal energy systems, May 2015

2. Zanganeh G., Khanna R., Walser C.,Pedretti A., Haselbacher A., Steinfeld A., Experimental and numerical investigation of combined sensible-latent heat for thermal energy storage at 575 °C and above, Sol. Energy, 114:77-90, 2015

3. Zanganeh G., Pedretti A., Zavattoni S., Barbato M., Steinfeld A., Packed-bed thermal storage for concentrated solar power – pilot-scale demonstration and industrial-scale design, Sol. Energy, 86:3084-3098, 2012

4 Simulation Results - Large-Scale TES

Comparison of material costs and exergy efficiency between sensible only and combined storages (with AlSi12) for given maximum temperature drops during discharging at steady cycling.

1 Background

2 Concept

Schematic of combined TES

Comparison of experimental (dots) an numerical (lines) results Thermocouple positions

Acknowledgment Funding by the Commission for Technology and Innovation through the SCCER is gratefully acknowledged

0.0 0.2 0.4 0.6 0.8 1.0

0.0

0.2

0.4

0.6

0.8

1.0

t/Dt

c

= 0

t/Dt

c

= 1

PCM

x/L

[�

]

Simulation

Experiment, centerline

Experiment, wall

0.0 0.2 0.4 0.6 0.8 1.0

0.0

0.2

0.4

0.6

0.8

1.0

t/Dt

c

= 2

t/Dt

c

= 1

PCM

(T �T

d

)/(Tc

�T

d

) [�]

x/L

[�

]

Simulation

Experiment, centerline

Experiment, wall

0 1 2 3 40.00

0.20

0.40

0.60

0.80

1.00

(t �Dtpc)/Dtcycle [�]

(T�

T d)/(T

c�

T d)

[�]

0

1 2

3

0.85

0.90

0.95

1.00

(T�

T d)/(T

c

�T d)

[�

]

Simulation

Experiment

Rocks

Insulation

PCMPlate

Plate

x

394

1270

90120

1680

200

200

xx

x

x

x

xx x

xx x

x

xx

x

x

x

xx

xxx

x41

083

610

8312

3713

0313

48

4 6 8 10 12 14 160

1

2

3

4

5

⌘ex

> 98.5 %

�Td,max

[%]

Mat.

Costs/Net

EnergyOutput[$/kW

h]

Sensible, referenceCombined

4 6 8 10 12 140

5

10

15

20

25

30

35

⌘ex

> 95 %

�Td,max

[%]

Mat.Costs

/Net

Energy

Output[$/k

Wh]

Sensible, referenceCombinedSensible, double insulation

0

20

40

60

80

100

S2-6S2-7S2-13C5C6C7S8S10S14

4.3 6.211.34.7

6.8

12.3

7.6 9.2 11.4

6.86.5

5.6

5.3 7.3 8.6

43 42.5 41.939.1

37.5

32.9

60.2 59.5 58.9

49.4 48.3 46.6 45 43.137.8 34.5 33.2 32.4

Percentageoftotalmaterialcosts

Concrete Insulation Rocks

PCM Encapsulation

0

20

40

60

80

100

C6C8C14S6S8S16

6.7 9.4 13.97.3

10.3

15.2

20 23.6 26.3

17.115.9

1437.1

35.6 34.631.9

29.8

26.4

43 40.7 39.1 37 34.5 30.6

Percentageoftotalmaterialcosts

Concrete Insulation Rocks

PCM Encapsulation

Eout,cycle = 23 MWhth Eout,cycle = 1000 MWhth

�Td,max