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.

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.