Heat storage for solar heating systems Department of Civil - PowerPoint PPT Presentation

Heat storage for solar heating systems Department of Civil Engineering Now and in the future Simon Furbo Department of Civil Engineering Technical University of Denmark Brovej – building 118 DK-2800 Kgs. Lyngby, Denmark Tel.: +45 45 25 18 57, Fax: +45 45 93 17 55 E-mail: sf@byg.dtu.dk

Department of Civil Engineering BYG DTU BYG DTU Solar tanks for • Solar Domestic Hot water Systems • Solar Combi Systems

Solar Domestic Hot Water Systems Small Department of Civil Engineering BYG DTU BYG DTU

Southern Europe Thermo syphon systems China Department of Civil Engineering BYG DTU BYG DTU

Central & northern Europe Department of Civil Engineering BYG DTU BYG DTU High flow Low flow Spiral tank Mantle tank Solar collector Solar collector Auxiliary energy Auxiliary energy supply supply Spiral tank Mantle tank Hot water Hot water Cold water Cold water Pumped systems

Department of Civil Engineering BYG DTU BYG DTU 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 times Lime in tank after 3 years of operation Spiral tank Mantle tank

Marketed mantle tanks can be strongly improved by simple design changes Department of Civil Engineering BYG DTU BYG DTU Net utilised solar energy [kWh/year] Net utilised solar energy 1200 0,7 Solar fraction 1100 0,6 Solar fraction [-] 1000 0,5 900 0,4 800 0,3 700 0,2 600 0,1 500 0 Large H/D-ratio Large H/D-ratio, Smaller Large H/D-ratio, Smaller Large H/D-ratio, Smaller Large H/D-ratio, Smaller Danlager 1000 mantle, More insulation, mantle, More insulation, mantle, More insulation Stainless steel (3mm) Stainless steel (2mm) mantle 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 tank Further improvements by use of tank material with low thermal conductivity

Department of Civil Engineering BYG DTU BYG DTU Improved cold water inlet design Standard cold water inlet Cold water inlet with reduced mixing during draw-offs Control Extra thermal performance: 5%

Hot water tank with hot water draw-off from two levels Department of Civil Engineering BYG DTU BYG DTU Marketed tank Tank with two draw-off levels Electric heating Electric heating element element PEX pipe PEX pipe Mantle RAVI valve Three way valve Extra thermal performance: 5%

Boiler/hot water tank unit Department of Civil Engineering BYG DTU BYG DTU 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

Smart hot water tank Department of Civil Engineering BYG DTU BYG DTU Marketed solar tank Smart solar tank Tank heated from the top Individual variable auxiliary volume Auxiliary energy supply fitted to heat demand Extra thermal performance: Up to 35% Cost/performance improvement: Up to 25%

Department of Civil Engineering BYG DTU BYG DTU Large Solar Domestic Hot water Systems Collector area: 336 m², partly facing east, partly facing west Collector tilt: 15° Tank volume: 10000 l Measured net utilized solar energy (2001-2002): 455 kWh/m² year; solar fraction: 21% (10%) Utilization of solar radiation: 46%

Solar combi systems CONTROL Department of Civil Engineering BYG DTU BYG DTU

Solar tanks for solar combi systems Department of Civil Engineering BYG DTU BYG DTU A huge variety of designs exist: and so on …………………………………………………

Department of Civil Engineering BYG DTU BYG DTU 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 Vacuum panel • Good thermal stratification in heat storage • Independent of operation/installation conditions

Department of Civil Engineering BYG DTU BYG DTU 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 the heat demand and/or turn off the auxiliary energy supply system

Inlet stratifiers for thermal stratification Department of Civil Engineering BYG DTU BYG DTU DHW Solar SH Solar Solar Solar DHW DHW SH Solar DHW = D omestic H ot W ater SH = S pace H eating

Solvis inlet stratifier Department of Civil Engineering BYG DTU BYG DTU

Department of Civil Engineering BYG DTU BYG DTU New fabric inlet stratifier – stratifier of the future? Stratifier with two fabric layers Fabric Status • Excellent thermal performance • Water inlet in all levels • Inexpensive • Easy to transport and store • Optimal diameter influenced by flow rate 40 mm Future investigations 70 mm • Long time durability • Experience from practice

Theoretical investigations of solar combi system Department of Civil Engineering BYG DTU BYG DTU with different solar tanks 1.1 Collector area: 20 m² Solar tank volumne: 1000 l Danish weather data 1.2 SH AUX DHW SOLAR 1.3 SH: Space heating DHW: Domestic hot water 1.4 AUX: Auxiliary energy SOLAR: Solar energy

spiralHX/fixSH strsolar/fixSH spiralHX/strSH strsolar/strSH Department of Civil Engineering BYG DTU BYG DTU 4400 Annual net utilized solar energy 1.1 4200 1.4 4000 DHW 1.2 3800 200 l/day 1.3 1.2 3600 Step by step 1.1 [kWh/year] 3400 1.4 3200 DHW 1.2 1.3 improvement 3000 100 l/day 1.3 2800 1.1 2600 1.4 2400 2200 2000 0 5000 10000 15000 20000 Space heating consumption [kWh/year] strsolar/fixSH-DHW100 strsolar/fixSH-DHW200 spiralHX/strSH-DHW100 spiralHX/strSH-DHW200 strsolar/strSH-DHW100 strsolar/strSH-DHW200 1.14 1.4 1.12 Performance ratio [-] Reference 1.1 system 1.08 1.2 1.06 1.3 1.04 Optimal position 1.02 1 0 5000 10000 15000 20000 Space heating consumption [kWh/year]

Heat storage with side-arm Hot water tank with heat exchanger Tank in tank Theoretical investigations Department of Civil Engineering BYG DTU BYG DTU spiral for space heating DHW DHW 1.1 3.1 AUX SH AUX AUX SH SH Reference systems DHW SOLAR 2.1 SOLAR SOLAR 1.2 3.2 Stratifier in solar collector loop 2.2 1.3 3.3 Stratifier in space heating loop 1.4 3.4 Best tank ☺ Stratifier in solar collector loop and in space heating loop

Future heat stores for solar combi systems Department of Civil Engineering BYG DTU BYG DTU Condensing natural General: gas boiler • 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

Conclusions Department of Civil Engineering BYG DTU BYG DTU • Possiblities for huge improvements of marketed solar tanks! Future solar tanks General • 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 discharge SDHW 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

Department of Civil Engineering BYG DTU BYG DTU Ph.D. Course Thermal stratification in solar Thermal stratification in solar storage tanks 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. September 27 – October 17, 2007 For small solar heating systems the heat storage is the Kgs. Lyngby, Denmark: October 10-17, 2007 most important component, both from a thermal and economical point of view. Organized by: The thermal performance of solar heating systems is strongly influenced by the thermal stratification in the heat Department of Civil Engineering storage. The better thermal stratification in the solar tank Technical University of Denmark, DTU 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 SUPPORTED BY SOLNET 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