The storage tank is the core of a solar thermal system

SOLAR THERMAL Solar storage tanks a wide variety It is essential to have the right storage tank: There is a multitude of solar thermal systems, and many different reservoir types are available for them. At present, small-scale and combined tanks are in fashion. The storage tank is the core of a solar thermal system because even the best collector is of little use if the reservoir cannot store the heat until the user of the system wants to take a shower. In April, S&WE asked almost 30 European manufacturers of solar thermal storage tanks about the current market trends. Only nine companies replied (see table). Of the big heating companies, only the Vaillant Group and the Baxi Group with its Turkish subsidiary company Baymak are represented, although others, such as Viessmann (see page 70) or the European market leader Bosch Thermotechnik, also produce solar storage tanks. The three companies with the longest experience in the production of solar storage tanks are Elbi from Italy, Ezinç from Turkey and Ratiotherm from Germany. The two latter companies started production in 1991. Cordivari from Italy entered the solar market in 2006 and is thus the newcomer of this overview. With an output of 6,000 storage tanks, the company was a successful market participant in 2007 (see figure on page 52). Baymak produced 40,000 storage tanks last year, thereby leading the field. Only four companies, however, gave any sales figures. For the current year, all the companies expect their production to increase considerably, by more than 30 %. For each application the right storage tank Roughly speaking, three types of solar thermal storage tank can be distinguished. The most common type is the solar thermal hot water storage tank. It is used in solar thermal systems for hot water generation and contains domestic drinking water. Such tanks are available Ezinç produces thermosiphon storage tanks up to a volume of 500 litres. Photos (2): Ezinç Compact storage tanks with integrated solar stations and control units are in high demand, such as this model from Ezinç. for different system types. In thermosiphon systems, either the solar heated water flows directly through the storage tank, or the tank is double-walled, and the antifreeze medium of the solar circuit flows around the outside of the inner tank containing domestic water, and exchanges heat with it. For pumped solar thermal systems two other types are available. If the post-heating is accomplished by means of an electrical heating element, only a single heat exchanger is installed in the tank to transfer the solar energy yield into the reservoir. If there is a boiler for post-heating, a second heat exchanger for the post-heating system needs to be incorporated into the so-called bivalent storage tank. As a further option, many bivalent storage tanks can also be equipped with an electrical heating element. If the owner of the solar thermal system wants to tap the sun s energy for heating as well, combined storage tanks are used. These contain heating circuit water as well as domestic water. The most common model is the tank-in-tank system, in which the domestic water is heated up inside an inner water container. Alternatively, a second heat exchanger inside the storage tank can heat up the domestic water as it flows through the tank. In order to ensure efficient heat exchange, the manufacturers use stainless steel corrugated tube heat exchangers or copper ribbed tube heat exchangers for this purpose. The third type of solar thermal storage tank is the buffer tank. It contains only heating circuit water and serves as heating support. Depending on the size of the system and the hot water requirement, buffer tanks can be combined with domestic water storage tanks or combined tanks, in which the domestic water is heated up. In Germany and other countries, yet another type is gaining ground, which makes use of the fresh water 50

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SOLAR THERMAL The production of storage tanks and especially their warehousing require a lot of space. Photo: Elbi Timeline: start of production of solar storage tanks Elbi 1978 Ezinç 1991 Ratiotherm 1991 Wikora 1995 Consolar 1996 Rotex 1999 Baymak 2000 Vaillant 2004 Cordivari 2006 60,000 50,000 40,000 30,000 20,000 10,000 0 Baymak Sales figures for solar storage tanks in 2007. For this year, all the companies expect sales to increase substantially. Source: information provided by the manufacturers Ezinç Cordivari 2008 2007 Consolar principle. In this type, the domestic water is heated via an external plate heat exchanger. This has two advantages. Firstly: In the reservoir, there is no warm drinking water that is stored over a longer period of time, and the plate heat exchanger circuit contains only minimal amounts. Legionellae have no chance of multiplying. Secondly: The problem of scaling, which occurs when hard (lime-rich) drinking water is heated above 65 C, is reduced, because the temperature in the plate heat exchanger can be limited to 65 C by means of a mixer, even if the buffer temperature approaches 95 C in times of strong solar radiation. Most solar thermal storage tanks are made of enamelled steel. Two-layer enamelling is used by Wikora and Baymak. Consolar and the Italian manufacturer Elbi apply a single protective layer. Ezinç works with both methods. A Cordivari specialty:»we use our patented Polywarm material for coating,«says Jolanda Martella, a member of the Cordivari staff. Elbi uses an epoxy resin coating for large storage tanks with a volume range from 1,500 to 5,000 litres and for thermosiphon storage tanks. Material thickness varies from 2 mm for small tanks to up to 5 mm for the large-scale models with a volume of up to 5,000 litres. Stainless steel tanks do not play an important role. For unpressurised devices, the two German suppliers Rotex and Consolar use polypropylene, primarily for solar thermal drain-back systems. Polyurethane foam is the most commonly used insulation material, either as directly foamed solid foam or in the form of flexible foam covers. Other insulation materials are EPP, EPS and polyester fibre. Insulation thickness ranges from 25 mm for very small tanks to up to 130 mm for large models. With 125 mm of insulation material, Consolar insulates its tanks particularly well, including the bivalent domestic water storage tanks. Smart stratification systems Several storage tank manufacturers apply special stratification systems in their tanks. Good stratification in the tank has the advantage that a large amount of do- 52

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SOLAR THERMAL Manufacturer Type of solar storage unit Tank material Wall thickness [mm] Range of Volume Type of insulation [litre] Baymak A.S. storage tank for thermosiphonic systems enamelled steel 2-2.5 mm (inner shell) 100 / 200 PU 1.8-2 mm (outer shell) domestic water tank with electric heating element enamelled steel depends on tank size 80-1,000 PU combination storage tank-in-tank enamelled steel 2.5 800 (160) / 1,000 (200) special foam Consolar GmbH bivalent domestic water storage tank St 37-2 4 180 / 394 LEEPS combination storage tank with tube heat exchanger PP / St 37-2 approx. 4 / 3 490 /550-2,200 EPP / ALU-EPS buffer storage tanks St 37-2 3 800 / 1,000 EPS Cordivari Srl storage tank for thermosiphonic systems steel with polywarm coating / stainless steel depends on tank size, 2-5 150-300 hard foam bivalent domestic water storage tank steel with polywarm coating / stainless steel depends on tank size, 2-5 150-1,000 hard foam or soft foam domestic water tank with electric heating element steel with polywarm coating / stainless steel depends on tank size, 2-5 200-5,000 soft foam or M1/M0 combination storage tank-in-tank steel with polywarm coating / stainless steel depends on tank size, 2-5 500-2,000 soft foam buffer storage tank n/a depends on tank size, 2-5 200-5,000 soft foam combination storage tank with tube heat exchanger n/a depends on tank size, 2-5 500-2,000 soft foam Elbi S.p.A. storage tank for thermosiphonic systems epoxy coated n/a 80-300 PU hard foam bivalent domestic water storage tank enamelled steel n/a 200-5,000 PU soft foam / PU hard foam combination storage tank with tube heat exchanger enamelled steel / epoxy coated 2 n/a 750-5,000 PU hard foam Ezinç A.S. storage tank for thermosiphonic systems enamelled steel / stainless steel depends on tank size 85-500 direct-injected PU foam bivalent domestic water storage tank enamelled steel depends on tank size 100-1,000 direct-injected PU foam domestic water tank with electric heating element enamelled steel depends on tank size 60 / 85 direct-injected PU foam combination storage tank-in-tank enamelled steel depends on tank size 100-1,000 direct-injected PU foam buffer storage tank enamelled steel depends on tank size 100-1,000 direct-injected PU foam Ratiotherm buffer storage tank St 37-2 2.5-4 400-4,000 polyester-fiber GmbH & Co. KG Rotex GmbH combination storage tanks PP 4 500 n/a Vaillant Group storage tank for thermosiphonic systems enamelled steel n/a 150/200/300 n/a mono- or bivalent domestic water tank with optional or integrated electric heating element enamelled steel n/a 150 /250 PU bivalent domestic water storage tank enamelled steel / stainless steel n/a 300-500 / 200-300 EPS combination storage tank-in-tank steel n/a 700 / 1,000 PU combination storage tank with tube heat exchanger steel n/a 500-1,500 PU buffer storage tanks steel n/a 500 / 750 /1,000 PU Wikora GmbH storage tank for thermosiphonic systems enamelled steel, stainless steel (outer shell) depends on tank size 120-300 PU domestic water storage tanks stainless steel, interior with double-coated enamel depends on tank size 300-2,000 PU with PVC jacket combination storage tank-in-tank steel 1 depends on tank size 750 / 1,000 soft foam with PVC-cover combination storage tank with tube heat exchanger steel 1 depends on tank size 800-2,000 soft foam with PS-cover buffer storage tank steel 1 depends on tank size 500-2,000 soft foam with PVC-cover The flexible stainless steel tube Low-noise Pressure-tested Corrosion resistant Hygienic TÜV approved Diffusion-tight Flexbible Products with Something Extra No welding required. No Pressing needed. Easy connection - no special tools required! 54

Thickness of insulation [mm] Type of solar heat exchanger Type of hot water heat exchanger or of tank-in-tank units Comments on technical features SOLAR THERMAL 38 steel jacket type www.baymak.com.tr 50 enamelled steel 100 enamelled steel steel 125 enamelled steel enamelled steel efficient solar heat exchanger, innovative www.consolar.de insulation concept, electric heating device optionally possible 70-130 / 125 Cu-ripped tube Cu-ripped tube light tank (54 kg), for drain back only 42 kg, patented high efficient stratified charge and discharge 100 Cu-ripped tube plate heat exchanger stainless steel patented high efficient stratified charge, external HX for DHW 50 steel smooth www.cordivari.it 70 / 100 steel tube with external coating 50 Cu-ripped tube 100 steel tube polywarm coating or stainless steel tank 100 steel tube 100 steel smooth stainless steel corrugated tube 30 double wall www.elbi.it 50 / 70 enamelled steel 70 enamelled steel Cu-ripped tube 50-70 steel or stainless steel www.ezinc.com.tr 50-70 enamelled steel 25 steel 50-70 steel steel 50-70 100-120 external plate-type heat exchanger High-grade steel 1.4401 plate heat exchanger, stainless steel integration of all foreseeable technologies into the heating system is possible Website www.ratiotherm.de 80 direct-flow, no heat exchanger stainless steel corrugated tube unpressurised stratified storage tank www.rotex.de n/a double wall www.vaillant-group.com 60 smooth steel coil drainback storage units with integrated pump unit, control and pre-filled drainback heat exchanger 80 smooth steel coil lowest stand-by energy losses 100 smooth steel coil enamelled steel big solar heat exchanger 100 Cu-ripped tube stainless steel corrugated tube multivalent storage, e.g. for solar & heat pump combinations 90 smooth steel coil big solar heat exchanger n/a fitting for heating element included www.wikora.de 100 (tanks up to 500 l) steel smooth fitting for heating element included 100 steel smooth stainless steel with double-coated enamel protection 100 steel smooth stainless steel corrugated tube 100 steel smooth 1 in certified quality steel S235JRG2; 2 from 1,500 up to 5,000 litres epoxy coated Overview of the different types of solar storage tanks Source: information provided by the manufacturers intersolar, Munich hall/booth C1.574 SenCon analog HE compact and completely equipped solar station high-efficiency pump with low energy consumption completely premounted controller Save energy through high-efficiency technology www.paw.eu 55

SOLAR THERMAL Combined storage tank with stainless steel corrugated tube heat exchangers for domestic water heating Photo: Wikora Sweltering heat: In the oven, the enamel is melted to form a glassy protective layer. Photo: Elbi For its unpressurised drain-back system, Rotex uses a plastic storage tank. Photo: Rotex Thermosiphon storage tanks are usually equipped with a double wall. Photo: Wikora mestic water in the top of the tank is available to the user (either directly in the solar domestic water storage tank or via heat exchangers in the combined storage tank). At the bottom, however, the reservoir keeps cool for a long time and can thus take up the heat from the collectors more efficiently, because the efficiency of the collectors drops with an increasing average temperature. Additionally, in combined storage tanks, the heating circuit can additionally be served from the middle zone of the tank, at the ideal temperature level for forward and return flow. In the case of solar domestic water storage tanks, the objective is to stratify the heated water. In its Coax storage tank, Consolar places the heat exchanger close to the tank wall. This creates a chimney effect, which transports the warm water effectively to the top of the tank. In the plastic tank Conus, a stratifier lance is inserted above the solar heat exchanger, which ensures that the heated water is fed to the optimal level of the tank, depending on water temperature.»the internal patented heat exchanger system is driven by thermosiphon power. Via stratification tubes, the mixing of different water temperatures is drastically reduced and higher annual energy savings are the result,«explains Consolar staff member Heiko Deumer. In combined storage tanks, it is also useful to stratify the return flow of the heating circuit. Baffle or guiding plates at the inlet of the heating circuit return flow into the tank ensure stratification with little mixing. Other manufacturers apply stratifier lances for this purpose as well. The stratifier in the Oskar-10 from Ratiotherm contains five chambers: Three forward flow influx chambers and two return flow influx chambers. These allow water to be fed in and taken out without creating turbulence.»the thermohydraulic 5-chamber layer insert of the Oskar-10 consists of a heat-resistant plastic material, however with poor heat conduction,«explains Alexander Weidinger (Ratiotherm). 56

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SOLAR THERMAL In the final inspection, the container is pressurised and tested for leaks. Photo: Huch Storage tank production: The welding robot takes over the task of producing the blank, as here at the company Huch from Germany. Photo: Jens-Peter Meyer Specific products for different solar markets What are the trends in the solar storage tank market?»we think that good quality enamel coated steel tanks will continue to be popular. On the other hand, smaller size storage units would be the new trend,«says Hakan Alas (Ezinç). He assumes that the demand for 50 litre and 70 litre wall-mounted storage units and 100 litre and 160 litre floor standing units will increase. Heiko Deumer (Consolar) sees the most important task in detail optimisation and reduction of heat losses. He sees especially good sales opportunities for storage tanks with copper ribbed tube heat exchangers:»sales of the ribbed copper heat exchanger systems are growing faster than the market.«thomas Becker of Wikora expects a trend toward combined tanks with hygienic hot water generation. Baymak is adjusting increasingly to the requirements of its customers in order to be able to offer devices with suitable specifications:»we are adapting the tanks according to the countries standards. For example, European standards are different from Australian standards,«says Tacnur Yilmaz (Baymak). Also for Elbi, the challenge is to adapt to the different solar thermal markets. Established solar markets such as Germany, Austria, Switzerland and Sweden require different products to those for the new markets such as Spain, Portugal and Italy (except for South Tyrol).»In countries where the use of solar thermal energy has reached an advanced level, there is no demand any more for simple devices,«explains Elbi employee Claudio Cazzola. Jens-Peter Meyer Jens-Peter Meyer is a long term S&WE author and an expert in solar thermal. 58

Anzeige Specialist and German market leader in the production of tube collectors Industrially manufactured top quality products High quality, high performance Distribution exclusively to OEM partners Certificates and authorisations: EN 12975, RAL UZ 73, Solar Keymark / ISO 9001 www.rittersolar.de Ritter Solar GmbH & Co. KG Kuchenäcker 1 72135 Dettenhausen Germany Tel. +49 (0)7157 53 59-0 Fax: +49 (0)7157 53 59-20 info@rittersolar.de