thermal energy storage - cyprus university of...

31/08/2014

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European Cooperation in the field of Scientific and Technical Research

COST is supported by

the EU RTD Framework Programme ESF provides the COST Office

through an EC contract

Building Integration of Solar Thermal Systems – TU1205 – BISTS

www.cost.esf.org

Thermal Energy Storage

Prof. Dr. Luisa F. Cabeza

University of Lleida

Spain






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Introduction

• Energy storage (ES) has only recently developed

to a point where it can have a significant impact

on modern technology

• ES is critically important to the success of any

intermittent energy source in meeting demand

– For example, the need for storage for solar energy

applications is severe, especially when the solar

availability is lowest, in winter

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Introduction

• ES systems can contribute significantly to

meeting society’s need for more efficient,

environmentally benign energy use in

building heating and cooling, aerospace

power, and utility applications

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Introduction

• The use of ES systems often result in such

significant benefits as:

– Reduced energy costs

– Reduced energy consumption

– Improved indoor air quality

– Increased flexibility of operation

– Reduced initial and maintenance costs

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Energy storage methods

• A large variety of ES techniques are under

development, which can be groups as follows:

– Mechanical

– Thermal

– Chemical

– Biological

– Magnetic

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• Thermal energy storage

– Thermal energy may be stored by:

• Elevating or lowering the temperature of a substance

(i.e. altering its sensible heat)

• Changing the phase of a substance (i.e. altering its

latent heat)

• A combination of the two

– TES is the temporary storage of high- or low-

temperature energy for later use

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• Thermal energy storage

– TES offers the possibility of storing energy before

its conversion to electricity

– Energy quality as measured by the temperatures

of the materials entering, leaving and stored

within a storage is an important consideration for

TES

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Thermal energy storage methods

• Basic principle of TES

– Energy is supplied to a storage system for removal

and use at a later time

– What mainly varies is the scale of

the storage and the storage method

used

– Seasonal storage requires immense

storage capacity

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– A complete storage process involves at least three

steps: charging, storing and discharging

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– In terms of storage media, a wide variety of choices

exists depending on the temperature range and

application

– For sensible heat storage:

• Water is a common choice because it has one of the highest

specific heats of any liquid at ambient temperatures

• Solids have the advantage of higher specific heat capacities,

which allow for more compact storage units

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Basic thermodynamics

• Heat storage as sensible heat

– solids (stone, brick,…)

– liquids (water,…)

The most common method

for heat storageStored heat

Temperaturesensible

TcmQ ∆⋅⋅=∆

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Stored heat

Temperaturesensible

sensible

latent

sensible

Temperature of phase change

Materials with useful phase change = latent heat storage material, = phase change material (PCM)

Basic thermodynamics• Heat storage as latent

heat

– melting at constant

pressure

⇒ small volume change

⇒ no temperature change

hmQ ∆⋅=∆

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Basic thermodynamics• Heat storage as latent

heat

– PCM can store about 3 to 4

times more heat per

volume than is stored as

sensible heat in solids or

liquids, in a ∆T of 20 ºC

Stored heat

Temperaturesensible

sensible

latent

sensible


hmQ ∆⋅=∆

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Basic thermodynamics

• Potential applications

1. Stabilization of temperatures

⇒ buildings, transport boxes, ...

2. Storage of heat or cold

– at small temperature changes

– with high storage density

⇒ domestic heating, ...

Stored heat

Temperaturesensible

sensible

latent

sensible


Stored heat

Temperaturesensible

sensible

latent

sensible


Stored heat

Temperaturesensible

sensible

latent

sensible


Stored heat

Temperaturesensible

sensible

latent

sensible


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Basic thermodynamicskJ/L kJ/kg comment

sensible heatgranite 50 17 ∆T=20°Cwater 84 84 ∆T=20°Clatent heatice 300 330 0°Cparaffin 180 200 5°C to 130°Csalthydrate 300 200 5°C to 130°Csalt 600 - 1500 300 - 700 300°C to 800°Cchemical energyH gas 11 120000 300K, 1barH gas 2160 120000 300K, 200barH liquid 8400 120000 20K, 1bargas (petroleum) 33000 44600electrical energybattery 200 zinc/manganeseoxide

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sugar alkohols

melting temperature [°C]

mel

ting

enth

alpy

[kJ/

L]

-100 0 +100 +200 +300 +400 +500 +600 +700 +800

900

800

700

600

500

400

300

200

100

0

aqueous salt

solutions

salthydrates and eutectic

mixtureswater

gas hydrates

fatty acidsparaffines

nitrates

hydroxides

carbonates

chlorides

Fluorides

sugar alkohols

melting temperature [°C]

mel

ting

enth

alpy

[kJ/

L]

-100 0 +100 +200 +300 +400 +500 +600 +700 +800

900

800

700

600

500

400

300

200

100

0

aqueous salt

solutions

salthydrates and eutectic

mixtureswater

gas hydrates

fatty acidsparaffines

nitrates

hydroxides

carbonates

chlorides

Fluorides

© ZAE BAYERN

Classes of materials

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Physical and technical

requirementsPhysical:� Suitable phase change temperature� Large ∆H and cp� Large thermal conductivity� Reproducible phase change� Little subcooling Technical:

� Low vapor pressure� Small volume change� Chemical and physical stability� Compatibility with other materials

Economical:� Low price� Non toxic� Recyclable

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Building applications

• Passive systems in the envelope

• Active systems in the envelope (ventilated

façade)

• DHW solar energy with PCM in water tank

• Heat pump with PCM for daily storage

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shape of the buildingreduction of heat loss• small surface to volume ratio• small number of added parts• multiple unit buildings

orientation of the buildingreduction of heat loss• small surface to the wind (convection heat loss)• large area to the south (solar gain)• sleeping rooms in the northern part of the building

outside construction of the buildingreduction of heat gain• white surfaces• roof construction summer

winter

outside plantsreduced solar gain in summer increased solar gain in winter

outdoor design conditions

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Passive building applications

• To increase the thermal inertia of the building

• Practical assessment of the influence of PCM

integrated in building structures and their

influence on thermal stability of indoor

environment

• In order to assess the impact of the solutions

adopted, monitoring is necessary

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• University of Lleida

– Different cubicles were built using different

Mediterranean typical constructive solutions.

Internal dimensions are 2.4x2.4x2.4 meters

– Typical Spanish continental climate, with cold

winters and warm and relatively dry summers

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– The important temperature oscillations during day

and night make it very suitable for the PCM

operation (melting during the day and solidifying

during the night)

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– The PCMs tested were designed for cooling

applications

• Reference cubicle without insulation and without PCM

• Experimental cubicle without insulation and with

microencapsulated PCM

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• University of Lleida (Spain)

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• Instrumentation

– Heat flux sensor

– Temperature sensors

(PT 100)

– Piranometer

– Meteorological station

– Internal ambient temperature and humidity

– Energy consumption

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• Envelope with macroencapsulated PCM

– Experimental set-up:

• Small house-shaped cubicles

• Different constructive solutions:

– Concrete

– Conventional brick

– Alveolar brick

No insulation

Different insulation materials

Insulation and macroencapsulated PCM

No insulation

w/wo microencapsulated PCM

No insulation

w/wo macroencapsulated PCM

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• Concrete envelope with microencapsulated PCM

– PCM MICRONAL®PCM (BASF)

• 26ºC melting temperature

• Phase change enthalpy of 110 kJ/kg

• Each panel incorporates around 5% in weight

– No insulation

– Windows � south, east and west walls

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2.4 m

2.4 m2.4 m

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10

14

18

22

26

30

34

38

42

0:00 3:00 6:00 9:00 12:00 15:00 18:00 21:00 0:00

Hours

Tem

pera

rtur

e (º

C)

WEST T.OUT

10

14

18

22

26

30

34

38

42

0:00 3:00 6:00 9:00 12:00 15:00 18:00 21:00 0:00

Hours

Tem

pera

rtur

e (º

C)

WEST WESTPCM T.OUT

Phase change

Phase

changeWithout PCM

With PCM

2ºC

3ºC

26ºC

Outdoors temperature and temperatures of the west wall, July 2005

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– Brick cubicles

• Reference cubicle (Reference): This cubicle has no

insulation

• Polyurethane cubicle (PU): 5 cm of spray foam

polyurethane as insulation

• PCM cubicle (RT27+PU): 5 cm of spray foam polyurethane

as insulation and an additional layer of PCM

– CSM panels containing RT-27 paraffin are located between the

perforated bricks and the polyurethane

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– Alveolar brick cubicles

• Reference cubicle (Alveolar): The alveolar brick has a

special design which provides both thermal and

acoustic insulation. No additional insulation was used in

this cubicle

• PCM cubicle (SP25+Alveolar): Several CSM panels

containing SP-25 A8 hydrate salt are located inside the

cubicle, between the alveolar brick and the plaster

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Paraffin RT-27 Melting point (ºC) 28 Congealing point (ºC) 26 Heat Storage Capacity (kJ/kg) 179 Heat conductivity (W/m·K) 0.2

• Brick envelope with macroencapsulated PCM

• Conventional brick:

– Reference: No insulation

– Polyurethane: 5 cm of polyurethane

– RT27+PU: CSM panels (RT-27) and 5 cm of polyurethane

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Hydrated salt SP-25 A8 Melting point (ºC) 26 Congealing point (ºC) 25 Heat Storage Capacity (kJ/kg) 180 Heat conductivity (W/m·K) 0.6

• Alveolar brick:

– Alveolar: No insulation

– SP25+Alveolar: CSM panels (SP-25 A8) inside the cubicle

• Brick envelope with macroencapsulated PCM

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• Brick envelope with macroencapsulated PCM.

Free-floating experiments

CONVENTIONAL BRICKSummer period – 04/08/08 to 07/08/08

26

26.527

27.528

28.5

2929.5

3030.5

31

31.532

32.533

04/08/0812:00

05/08/080:00

05/08/0812:00

06/08/080:00

06/08/0812:00

07/08/080:00

07/08/0812:00

Date

Tem

pera

ture

(ºC

)

Inside Reference Inside PU Inside RT27+PU RT-27

Phase Change Range

2.9ºC

1ºC

Outside temperatures swing: 18-36 ºCSolar radiation: 900 W/m2

every day

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Free-floating experiments

ALVEOLAR BRICKSummer period – 02/08/08 to 04/08/08

26

26.5

27

27.5

28

28.5

29

29.5

30

30.5

31

02/08/0812:00

02/08/0818:00

03/08/080:00

03/08/086:00

03/08/0812:00

03/08/0818:00

04/08/080:00

04/08/086:00

04/08/0812:00

Date

Tem

pera

ture

(ºC

)

Inside Alveolar Inside SP25+Alveolar SP-25 A8

Phase Change Range

1.1ºC

0.9ºC

Outside temperatures swing: 18-36 ºCSolar radiation: 900 W/m2

every day

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Controlled temperature experiments

– Different experiments done in controlled

temperature mode

– The weather conditions were ideal for the PCM

operation

Summer period: June - AugustSet point @ 24 ºC

Outside temperatures swing: 12-35 ºCSolar radiation: 800-900 W/m2 during all days

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Controlled temperature experiments


Energy consumptionSet point @ 24 ºC

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Controlled temperature experimentsEnergy Consumption1 (Wh)

Energy Savings2 (Wh)

Energy Savings2 (%)

Improvement3 (%)

Reference 9376 0 0 -

PU 4583 4793 51.12 0

RT27+PU 3907 5469 58.33 14.75

Alveolar 5053 4323 46.11 0

SP25+Alveolar 4188 5188 55.33 17.12

1Set point of 24 ºC during 5 days

2Referred to the Reference cubicle

3Referred to the cubicle with analogue constructive solution and without PCM

Energy consumption: Set point @ 24 ºC

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Conclusions

– Free floating

• Reduction of the maximum and minimum peaks during

the warm season

• Reduction of the temperature oscillation (up to 1 ºC)

• Delay on the heat flux entering through the inside wall

(3-8 hours)

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Conclusions

– Controlled temperature

• Important effect of the set point

• Moderate set point (24 ºC) � Important energy savings

(about 15%)

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• Ventilated Double Skin Facade with PCM

– Reference and ventilated facade cubicles

Active system in the envelop

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• Programmable door openings and fans


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• Operation in free floating conditions

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• Operation with heat pump at 21 ⁰C

20% of net energy savings

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DHW solar energy with PCM

• Solar Thermal Systems: Use of Phase Change

Materials (PCM) for the improvement of

energy storage in solar water heating systems

– To investigate the potential of

hot water storage tank size

reduction by introducing PCM

– To increase of storage capacity

of the system

Increase cost

effectiveness of

the system

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• PCM modules are

introduced in the (top)

hot water storage tank

and the system is tested

under conditions that

mimic real-life operation

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• PCM

– Granular compound of 90 vol.%

of sodium acetate trihydrate and

10 vol.% graphite (commercial)

� heat transfer improvement

– Encapsulation used was

commercial aluminium bottles

8.8 cm of diameter and 31.5

cm height, capacity 1.5 L

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• Description of the installation

– 2 thermal solar collectors

– 2 storage water tanks of 146 L

– Electrical heater outside the tanks

– Installation automatically

controlled

– Different PCM modules located at

the upper part of the storage tank

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• Instrumentation

– Water temperature at 0, 30, 90, 110 and 120 cm

from the bottom of the tank

– Ambient temperature

– Inlet and outlet water temperature

– PCM temperature inside the

modules

– Water mass flow

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– Experiments with 2, 4 and 6 modules

• Cooling down, reheating: water

heated up to 80 ºC

• Solar operation

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Nº modules

PCM mass (kg)

IPF (%)

Energy density increase (%) (∆T = 1 K)

Energy density increase (%) (∆T = 8 K)

2 2.1 2.05 40 6

4 4.2 4.1 57.2 12

6 6.3 6.16 66.7 16.4

IPF: PCM volume/tank volume

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5051525354555657585960

0:00 6:00 12:00 18:00 0:00 6:00 12:00Time (h:min)

Tem

pera

ture

(ºC

)

T-30T-90T-110T-120 (top)PCM1PCM2

30

35

40

45

50

55

60

65

70

75

80

12:00 0:00 12:00 0:00 12:00 0:00 12:00 0:00 12:00 0:00 12:00 0:00

Time (h:min)

Tem

pera

ture

(ºC

)

T-30

T-90

T-110

T-120 (top)

PCM1

PCM2

PCM effect

Top water layer keep the temperature around 54-55 ºC between 7-9 hours

Results of Cooling Down with 2 PCM modules

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– Cooling down experiments with 2 and 6 modules:

• The top water layer in contact with the PCM modules

kept the temperature around 54-55 ºC between 7-9

hours

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26

28

30

32

34

36

38

40

17:45 18:00 18:14 18:28 18:43Time (h:min)

Tem

pera

ture

(ºC

)

T-0 (bottom)T-30T-90T-110T-120 (top)PCM 1PCM 2

15

25

35

45

55

65

75

17:45 17:52 18:00 18:07 18:14 18:21 18:28 18:36 18:43

Time (h:min)

Tem

pera

ture

(ºC

)

T-0 (bottom)

T-30

T-90

T-110

T-120 (top)

PCM 1

PCM 2

Extra time with hot water at 36-38 ºC ≈ 50 min

Theoretical compensation temperature about 10 ºC (from 27 ºC to 37 ºC)

Zoom in

Results of Reheating with 6 PCM modules

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– Reheating experiments also with 2 and 6 modules:

• The time with water at a useful temperature highly

depends on the amount of PCM

• A theoretical compensation temperature of:

– About 5 ºC during 30 minutes (29 ºC to 34 ºC) (2 modules)

– About 10 ºC during 50 minutes (27 ºC to 37 ºC) (6 modules)

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0

10

20

30

40

50

60

70

80

0:00 6:00 12:00 18:00 0:00 6:00 12:00 18:00 0:00Time (h:min)

Tem

pera

ture

(ºC

)

0100200300400500600700800900

Rad

iatio

n (W

/m^2

)

Inlet 1 Outlet 1 Inlet 2Outlet 2 Solarimeter

30

35

40

45

50

55

60

65

70

0:00 6:00 12:00 18:00 0:00 6:00 12:00 18:00 0:00Time (h:min)

Tem

pera

ture

(ºC

)

T-0 (bottom) T-30 T-90T-110 T-120 (top) PCM 1PCM 2

PCM effect

Tank draw-off

Charging in sunny hours and partial draws-off

Reheating capability as well as keeping nearly constant the water temperature next to the PCM modules

Results of Solar Operation with 4 PCM modules

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– Experiments with solar collectors were carried out

for 4 and 6 modules

• No significant differences were observed

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• Development of thermal storage applications for

HVAC solutions based on Phase Change Materials

– TES system is integrated to a HVAC system

– PCM thermal energy storage units are used to reduce the energy consumption

– The objective is to take advantage of the low electricity night tariff:

• The PCM is charged at night and discharged during the day

Heat pump with PCM for daily storage

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• PCM thermal energy storage units

– Two PCM units are coupled to a heat pump and an

AHU

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• Salt hydrate PCM is used

Tank PCMMelting

temperatureMelting enthalpy

Cold TES tank S10 10 ºC 155 kJ/kg

Hot TES tank S46 46 ºC 210 kJ/kg

12 flat slabs in 2 columns

310

Dimensions in mm

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• The prototype will be tested in Spanish

summer and Estonian winter conditions in a

3x3x3 m cubicle

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• Results for space heating experiments

Indoor setpoint: 21 ºC Dead band: ±0.5 ºCSimilar outdoor conditions

From 09:00 h:- Water tank: 247 min- PCM tank: 298 min

+ 51 min

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Thank you for your attention

[email protected]

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