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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: [email protected]
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Solar tanks for• Solar Domestic Hot water Systems
• Solar Combi 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
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1000
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1200
Dan
lage
r 100
0
Larg
e H
/D-r
atio
Larg
e H
/D-r
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, Sm
alle
rm
antle
Larg
e H
/D-r
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, Sm
alle
rm
antle
, Mor
e in
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tion
Larg
e H
/D-r
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, Sm
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antle
, Mor
e in
sula
tion,
Sta
inle
ss s
teel
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Larg
e H
/D-r
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, Sm
alle
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antle
, Mor
e in
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tion,
Sta
inle
ss s
teel
(2m
m)N
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olar
ene
rgy
[kW
h/ye
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0
0,1
0,2
0,3
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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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g Boiler/hot water tank unit
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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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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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
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olar
ene
rgy
[kW
h/ye
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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