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The Trade Show. Window. Door. Facade.

18 - 21 March 2020 // Nuremberg, Germany

FENSTERBAU FRONTALE Newsroom

From water tower to PlusEnergy hotel – including windows developed in-house

The flap opening cylinder and the lower flaps are clearly visible.
The flap opening cylinder and the lower flaps are clearly visible. // © Jörg Pfäffinger

Flying high in Radolfzell: Along with his two sons, building contractor Jürgen Räffle refurbished a water tower decommissioned in 1979 into a showcase project for the hotel industry. In 2017, the Ministry for the Environment rated the finished building as a "Demonstration Building of the Federal Republic of Germany".

The three-member team already began their planning in 1999 with preliminary sketches by one of the contractor’s sons who was still in training at the time. These sketches were developed further in 2008. In 2011, what was initially conceived of as a high-rise office building became the "Hotel aquaTurm", which generates as much energy as it consumes.

The following specifications had to be met:

- Insulation of the existing and new building structure with passive house technology
- Power generation to guarantee hotel operation
- Reinforcement of structure to handle new loads and safety requirements


For this last item, the foundation had to be reinforced with piles sunk to a depth of up to 15 meters in accordance with the new use and current requirements in 2008. The timing was prescribed because at that time, the city was redesigning the space around the tower and an external deadline for this work would have been too complicated (and even more expensive).

The "power generation" item referred to the technical design of the façade.

CIS/CIGS thin-film PV modules were installed on all sides, a total of 1,000 m² with a 69.6 kWp output, to retain solar gains even in cloudy weather. In addition, a total of 54 m² of full vacuum tube solar collectors are installed in the roof area and in front of the windows on the south façade. This may not sound spectacular, but technically speaking it is complex and quite an accomplishment. According to Norman Räffle, who headed up the planning, "The tower is considered to be a special building because, at over 50 meters, it constitutes a high-rise. Radolfzell is in a Wind Zone 2 area because of the nearness of the lake, plus Earthquake Zone 2. These conditions called for special fastening technologies for the 1,800 m² façade. A façade structural engineer supported me with type statics, which I then used as the basis for creating design plans for the façade."

On the roof, a vertical wind energy plant with a Darrieus H-rotor supplies an additional 5.5 kW of power. The 5 meter pole can be hydraulically lowered for maintenance work and decoupled with two passive systems towards the building. Based on its design, the plant has a rotation limiter and its noise level is below the wind acoustics.

The energy concept also includes power storage with a 60 kW capacity and an emergency power supply. "All the components came from Germany," says Norman Räffle, who supports regional value creation.

Insulating the building shell – windows as an important component

For the façade to achieve the necessary passive house level, the façade required 24 centimetre-thick rockwool insulation on the main tower’s 55 cm brickwork masonry. "For this purpose, we attached 10,000 fasteners to the façade, including thermally decoupled stainless-steel holders and 20 centimetre-long, stainless-steel perforated sleeves to secure the insulation to the old masonry. The newly built access tower is outside the thermal shell but was insulated with 10 centimetres of rockwool to keep condensation out of the interior," says the engineer.

The windows are important not only for the panoramic view of the city and lake, but they also have an important function in the building shell’s insulation system. Because the builders couldn’t find any suitable window systems on the market, they worked with a window manufacturer to develop their own system. Overheating is often a problem with large glass surfaces and can only be remedied with the aid of external sun shading, according to Jürgen Räffle. Because of the frequent high wind velocities by the lake and the building’s height, blinds and curtains must be protected behind a pane of glass. But because the fixed outer pane that this would require couldn’t be cleaned without the use of an expensive crane, a composite window design was selected with blinds enclosed in the space between the panes that can be maintained from inside the building. Tests performed on composite windows showed that when exposed to solar radiation, the temperature between the panes ranged from 70 to 80°C even in February. What would sometimes be desirable in winter as additional radiant heat from the outside would be unacceptable in the summer.

To avoid this temperature increase but still have a high-insulating window, a composite design was used comprising aluminium facing formwork and inside casements made of local spruce with a double-glazed outer pane (Ug 1.1) and triple-glazed inner pane (Ug 0.5). Horizontal slats between the panes provide shading.

Norman Räffle: "Heat absorption in the space between the panes was an important issue, because when the temperature between panes is high, the glass surface heats the interior uncontrollably like panel heaters. Almost all high-rises struggle with the issue of how to shade the glass. At the same time, there’s a desire for daylight, wind stability, and a view, which always involve energy as well. In addition, an increase in temperature between the panes exposes the panes to pressure and pump stress. That’s why we developed a window system with a pressure-relieved, back-ventilated intermediate space, opting for a non-electronic solution which, according to my research, did not yet exist on the market."

The team relied on mechanical thermocylinders that function with wax. The cylinder, the spring and the valve that open the integrated flaps are fastened to a baseplate. The lower flaps open outward over an insect screen. As of 20°C they are fully opened. The upper flaps, using identical technology, are fully opened as of 38°C. The mechanism is simple: In the summer, the hot air rises and escapes via the top flaps. When it gets cooler, the flaps close and store the heated air, similarly to a solar air collector.

This window system is condensation-free and is sealed on three levels: the inner seal, centre seal and outer seal.

The outer pane is ESG-H glass, an ESG glass that has undergone a heat soak test at approx. 290°C to check for nickel sulphide inclusions and which, therefore, runs only a very slight risk of spontaneous breakage.

Most of the windows are 2 meters tall. A casement weighs approx. 100 to 120 kilograms, which places special demands on the hinges.

Light transmission is approx. 51-44 percent, the g-value is approx. 28-19 percent and Uw is 0.52-0.59 W/m²K.

Water tower Radolfzell: a PlusEnergy hotel is being built.

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