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g Heat storage for solar heating systems

Now and in the future

Simon FurboDepartment of Civil EngineeringTechnical University of Denmark

Brovej – building 118DK-2800 Kgs. Lyngby, Denmark

Tel.: +45 45 25 18 57, Fax: +45 45 93 17 55E-mail: sf@byg.dtu.dk

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Solar tanks for• Solar Domestic Hot water Systems

• Solar Combi Systems

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Small Solar Domestic Hot Water Systems

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China

Thermo syphon systems

Southern Europe

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g Central & northern Europe

Pumped systems

Solar collector

Auxiliary energysupply

Mantle tank

Hot waterCold water

Low flowMantle tank

Solar collector

Auxiliary energysupply

Spiral tank

Hot waterCold water

High flowSpiral tank

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g Advantages by low flow mantle tank systems compared to high flow spiral tank systems

• Extra thermal performance: 10-25%

• Improved performance/cost ratio: 39%

• Reduced lime deposits: 2.5 timesLime in tank after 3 years of operation

Spiral tank Mantle tank

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g Marketed mantle tanks can be strongly improved by simple design changes

500

600

700

800

900

1000

1100

1200

Dan

lage

r 100

0

Larg

e H

/D-r

atio

Larg

e H

/D-r

atio

, Sm

alle

rm

antle

Larg

e H

/D-r

atio

, Sm

alle

rm

antle

, Mor

e in

sula

tion

Larg

e H

/D-r

atio

, Sm

alle

rm

antle

, Mor

e in

sula

tion,

Sta

inle

ss s

teel

(3m

m)

Larg

e H

/D-r

atio

, Sm

alle

rm

antle

, Mor

e in

sula

tion,

Sta

inle

ss s

teel

(2m

m)N

et u

tilis

ed s

olar

ene

rgy

[kW

h/ye

ar]

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

Sola

r fra

ctio

n [-]

Net utilised solar energy

Solar fraction

20% extra thermal performance by:• Increased height/diameter ratio• Reduced mantle height• Lowering the position of the mantle inlet• Increased insulation thickness on the sides of the tankFurther improvements by use of tank material with low thermal conductivity

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Control

Improved cold water inlet designStandard cold water inlet

Cold water inlet with reducedmixing during draw-offs

Extra thermal performance: 5%

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g Hot water tank with hot water draw-off from two levels

Extra thermal performance: 5%

RAVI valve

Electric heating element

PEX pipe

PEX pipe

Mantle

Three way valve

Electric heating element

Marketed tank Tank with two draw-off levels

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Potential advantages:• Increased efficiency of natural gas

boiler/oil boiler• Increased thermal performance of

solar heating system• Increased energy savings• Reduced space demand• Easy to install• Easy to sell

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g Smart hot water tankMarketed solar tank Smart solar tank

Tank heated from the topIndividual variable auxiliary volumeAuxiliary energy supply fitted to heat demand

Extra thermal performance: Up to 35%Cost/performance improvement: Up to 25%

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Large Solar Domestic Hot water Systems

Collector area: 336 m², partly facing east, partly facing westCollector tilt: 15°Tank volume: 10000 lMeasured net utilized solar energy (2001-2002): 455 kWh/m² year; solar fraction: 21% (10%)Utilization of solar radiation: 46%

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Solar combi systemsCONTROL

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g Solar tanks for solar combi systemsA huge variety of designs exist:

and so on …………………………………………………

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Important for a well performing system• Good interplay between solar collector and

auxiliary energy supply system• Small auxiliary volume in top of tank• Low temperature level of auxiliary volume• Low heat loss from heat storage

• Good thermal stratification in heat storage• Independent of operation/installation conditions

Vacuum panel

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Increased thermal performance due to increased thermal stratification. Why?

• Low temperature in lower part of tank results in longer operation periods for the solar collector and thus increased collector performance

• High temperature in upper part of tank will meet theheat demand and/or turn off the auxiliary energy supply system

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SH

Solar DHW

Solar

Solar

DHW

Solar

Solar

DHW SH

DHW = Domestic Hot Water SH = Space Heating

Inlet stratifiers for thermal stratification

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Solvis inlet stratifier

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New fabric inlet stratifier – stratifier of the future?

Status• Excellent thermal performance• Water inlet in all levels• Inexpensive• Easy to transport and store• Optimal diameter influenced by

flow rate

Future investigations• Long time durability• Experience from practice

Stratifier with twofabric layers

70 mm

40 mm

Fabric

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SH: Space heating

DHW: Domestic hot water

AUX: Auxiliary energy

SOLAR: Solar energy

AUX

SOLAR

SH

DHW

Theoretical investigations of solar combi system with different solar tanks

Collector area: 20 m²

Solar tank volumne: 1000 l

Danish weather data

1.1

1.2

1.3

1.4

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Reference system

2000220024002600280030003200340036003800400042004400

0 5000 10000 15000 20000Space heating consumption [kWh/year]

Ann

ual n

et u

tiliz

ed s

olar

ene

rgy

[kW

h/ye

ar]

spiralHX/fixSH strsolar/fixSH spiralHX/strSH strsolar/strSH

DHW 200 l/day

DHW 100 l/day

1.1

1.2

1.3

1.4

1

1.02

1.04

1.06

1.08

1.1

1.12

1.14

0 5000 10000 15000 20000Space heating consumption [kWh/year]

Perfo

rman

ce ra

tio [-

]

strsolar/fixSH-DHW100 strsolar/fixSH-DHW200spiralHX/strSH-DHW100 spiralHX/strSH-DHW200strsolar/strSH-DHW100 strsolar/strSH-DHW200

1.1

1.21.3

1.4

1.1

1.21.3

1.4

1.21.3

1.4

Step by step improvement

Optimal position

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1.1

1.2

1.3

1.4

3.2

3.3

3.4

2.2

2.1

3.1

Reference systems

Stratifier in solar collectorloop

Stratifier in space heatingloop

Stratifier in solar collectorloop and in space heatingloop

Theoretical investigations Heat storage with side-arm Hot water tank with heat exchangerspiral for space heating

AUX AUX AUX

SOLAR SOLAR SOLARDHW

DHWDHW

SH SH SH

Tank in tank

Best tank ☺

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General:• Tank/auxiliary energy supply system unit

• Smart tanks/smart control systems• Solar/electric heating systems based on heat from wind turbines in windy

periods; advanced control based on weather forecast

Tank types:• Tank in tank storage with inlet stratifiers• Bikini mantle tanks

• Seasonal PCM heat storage based on material with stable super cooling

Future heat stores for solar combi systems

Condensing natural gas boiler

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g Conclusions• Possiblities for huge improvements of marketed solar tanks!

Future solar tanksGeneral• Solar tank/auxiliary energy supply system units• Low tank heat loss• No thermal bridges/pipe connections in upper part of tanks• Variable auxiliary volume fitted to heating demand• Smart control of auxiliary energy supply system• Highly stratified solar tank: Limited mixing, low downwards heat conduction,

stratified charge and dischargeSDHW systems• Mantle tanks• Hot water tanks with inlet stratifier(s)Solar combi systems• Tank in tank heat storage with inlet stratifiers• Bikini tanks• Seasonal PCM stores

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Ph.D. CourseThermal stratification in solar storage tanks

September 27 – October 17, 2007

Kgs. Lyngby, Denmark: October 10-17, 2007

Organized by:Department of Civil EngineeringTechnical University of Denmark, DTU

SUPPORTED BY SOLNET

Thermal stratification in solar storage tanks

Department of Civil Engineering at the Technical University of Denmark, DTU, is hosting this Ph.D. course on Thermal stratification in solar storage tanks.

For small solar heating systems the heat storage is the most important component, both from a thermal and economical point of view.

The thermal performance of solar heating systems is strongly influenced by the thermal stratification in the heat storage. The better thermal stratification in the solar tank is built up and maintained the higher the thermal performance of the solar heating system. Consequently, it is very important that solar tanks are designed in such a way that thermal stratification is built up in the best possible way.

Further, in order to develop optimum designed solar tanks and evaluate differently designed solar storage tanks it is important to be able to model, measure and characterize thermal stratification in solar storage tanks.

The course is focused on all aspects of thermal stratification in solar storage tanks