Hotel Achalm, Reutlingen

Refurbishment transforms property from the sixties into a working environment for tomorrow

Façade ventilation units were key to the renovation

Project Hotel Achalm
Country Germany

The challenge with this project was to set up the right air flows for all areas of the hotel so that absolute comfort was possible.

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Hotel Achalm, Reutlingen

The newly opened Hotel Achalm on Reutlingen’s local mountain was designed to operate in harmony with nature. A sustainable building services concept predominantly uses renewable energy and also uses economical and virtually silent induction technology to air-condition the hotel to four-star superior standard.

The new four-star superior Hotel Achalm on Reutlingen’s local mountain has 101 rooms, a 350 m² wellness area with outdoor pool, a 300 m² conference wing and two dining rooms with sun terraces. The hotel replaces a smaller one that stood here in a protected landscape area. The new building was constructed in around 20 months. Its energy and media supply – and therefore the hotel’s air conditioning – is sustainable, as the careful use of resources was a key concern of hotel operator Lothar Dollinger and his investors.

Under the leadership of the engineering firm Wagner from Reutlingen, an HVAC concept was implemented that relies on near-surface geothermal energy, a brine-to-water heat pump, thermal solar collectors and a gas condensing boiler to cover peak loads. For the air conditioning of the hotel rooms, conference and dining rooms and the wellness area, the owners opted for induction units from the Stuttgart-based manufacturer LTG, which were specially developed for hotel applications. The installation and assembly work for the HVAC technology was carried out by the company Niersberger AG from Pforzheim, the building management system was supplied by Kieback & Peter, Reutlingen.

Compared to conventional technology with a chiller and a boiler to provide heating and hot water, the energy-efficient Hotel Achalm saves 430,000 kWh/a of energy or 14,500 euros every year. The amortization period is therefore expected to be between nine and twelve years.

Harmonious in the landscape

The new hotel is partially integrated into the slope below the Achalm ruins. In addition to the first floor and three upper floors, there are two basement floors, which have a larger floor area than the free-standing upper part of the building. The lower levels house the underground garage with 72 parking spaces and technical rooms as well as 26 guest rooms, the fronts of which open out onto the valley. Due to its location, the hotel offers a sublime view that stretches from Stuttgart in the Neckar valley over Pfullingen and Reutlingen to the Swabian Alb.

The Hotel Achalm blends harmoniously into the landscape of orchards, pastures and forest. The elongated building is aligned diagonally to the slope line, so that the hotel appears much smaller from a distance than it actually is. At the same time, this arrangement offers the advantages of a morning and afternoon sun terrace and provides a view from every room. A wooden slat construction made of untreated Black Forest silver fir frames the cuboid contour of the upper storeys in a slightly curved shape, giving the impression of a tree trunk sawn into planks. In addition to the triple-glazed solar control glass, the slats provide shade and also deflect birds away from the large glass surfaces. The building concept was the brainchild of Jochen Schmid (from the architectural firm Hartmaier + Partner in Münsingen and Reutlingen) and Ralph Flum (from Flum Design in Hamburg).

Energy-efficient overall concept

The building envelope consists of 200 to 300 mm thick reinforced concrete plus 180 mm full thermal insulation (ETICS EPS 0.35) and triple thermal and solar control glazing. The roof surfaces are insulated with at least 200 mm thick EPS insulation (WLG 031). When it came to the technical equipment, the building owners went a significant step further in favor of sustainable operation. They adapted many of the heating and air conditioning details of the four-star Parkhotel in Leinfelden-Echterdingen (which belongs to the same owners) to the conditions on Reutlingen’s local mountain. For example, near-surface geothermal energy supplies the heat or cold for air conditioning buildings.

The temperature at a depth of 150 meters is tapped as a heat source in the cooler seasons and as a heat sink on warm days via 18 boreholes arranged along the contour of the building on the southern slope. A brine-to-water heat pump, which is designed for simultaneous heating and cooling operation, provides the maximum flow temperatures of 42 °C heating and 10 °C cooling required in the building. The heating output of the heat pump reaches 145 kW, the cooling output 173 kW; with free cooling, 156 kW is available.

The tempered hot and cold brine is stored in two buffer tanks, each holding 4,000 liters. Heat generated in summer is available in winter via the storage units and cold from winter can be used in summer. On average, one third of the hot water requirement is covered by solar thermal energy.

Primary energy requirement 20 % below ENEV 2009

Part of the electricity to drive the heat pump is generated by the hotel’s own photovoltaic system. During winter heating load peaks, a 200 KW gas condensing boiler system installed in the neighboring building (the so-called sheepfold) supplies the additional heat required. A pipeline was laid from the city to the property to supply gas to the existing boiler. Thanks to the combination of a well-insulated building envelope and sustainable building technology, the primary energy requirement is 335 kWh / (m² x a) and thus undercuts the EnEV 2009 requirement value (455 kWh / (m² x a) by 20 percent.

The two central ventilation units on the roof are supplied with the required flow temperatures via the hot or cold brine, depending on the weather conditions. The ventilation unit for air conditioning the hotel rooms has an output of 6,500 m³/h and ensures the required maximum pressure of 100 Pa at the induction units in the guest rooms.

The HFFsuite induction units from LTG Aktiengesellschaft provide draught-free air conditioning in the hotel rooms, dining and conference rooms of the hotel, which is virtually silent at 24 dB(A) (NR23 / NC20), adjustable by the guest and low-maintenance. In the hotel rooms, the devices are installed on the corridor ceiling in the entrance area next to the bathroom, while in the conference and dining rooms they are mounted above the suspended ceiling. Appliances with up to 1300 W (without condensation) provide uniformly fresh air at a pleasant temperature.

The latest induction technology

The primary air pre-tempered by the central ventilation units flows through aerodynamically optimized nozzles in the clad induction units. It draws in around four times the amount of room air. The room air drawn in first flows through a heat exchanger, which heats or cools it as required, before it is mixed with the primary air flow and blown back into the hotel room at an angle.

A mixing zone is created near the facade and a low-pulse displacement flow is created in the depth of the room. The air speed in the bed area only reaches around 15 cm/s even at maximum cooling capacity. The primary air supply is around 60 m³/h per guest room. Thanks to the induction principle, however, up to 300 m³ of air can be circulated per hour. The air exchange rate in the room, on the other hand, is only one fifth, namely 60 m³/h. This enables a high heating or cooling output with a low and energy-saving supply air volume.

In summer, a maximum cooling capacity of 200 W per appliance is introduced into the room via the primary air. The induction units offer an additional cooling capacity of 780 W at 16 °C in the cold water or cold brine flow. The maximum heating output via the hot brine flow is 1,250 W. In summer, at outside temperatures above 32 °C, the room temperature is regulated to 6 K below the outside temperature in accordance with German building regulations.

Air exchange rates between 1.5 and 3.5

As induction units do not have a fan, they are virtually silent and maintenance-free compared to fan coil units. A normal vacuum cleaner is sufficient for cleaning the heat exchanger. Another plus is the adjustable air guide elements, which can be used to adapt the room air flow to the individual room geometry.
In order to ensure the minimum pressure required for induction at the room units, only around 2 W per room unit is used in the central ventilation system, so that the savings effect achieved in the room is not canceled out by the central ventilation system. For comparison: The fan of a fan coil unit requires around 30 W per room for the same cooling capacity. Induction technology therefore protects the environment and the hotel operator’s wallet.

The control system for the induction units in the Hotel Achalm distinguishes between the base load without people and the full load when hotel guests are present. This information comes from the booking system and is taken into account accordingly by the building management system. The primary air flow in base load operation is 30 m³/h, while it is 60 m³/h when people are present. Depending on the operating mode, this achieves an air exchange rate of between 1.5 and 3.5. In the conference rooms, the air exchange rate is also regulated using the measured values from CO2 sensors. The hotel rooms are heated to 20 °C as standard, but hotel guests also have the option of individually and continuously adjusting their personal comfort temperature between 18 and 24 °C. If the hotel guest switches off “their air conditioning”, the induction unit is used exclusively to supply fresh air or runs in base load mode.

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