Penzel Valier is developing an integrated building for ETH Zurich’s digital infrastructure based on the complex requirements of a data center. The Hönggerberg Data Center combines high operational reliability, technical performance, and long-term adaptability with an architectural framework in which the structural system, building services, and design were conceived together from the very beginning.
Technical Infrastructure as Part of the Campus Ensemble
The completed ETH Hönggerberg Data Center (HRZ) houses the server, storage, and communication systems of the IT Services as well as various departments at ETH Zurich. Together with the existing data centers on the Central Campus, it forms an important component of the long-term strategy for a high-performance and adaptable digital infrastructure. The new data center replaces several previously dispersed and outdated infrastructures and was designed to adapt to future technological developments and be expanded as needed.
The building, designed by Penzel Valier, forms the western boundary of the Hönggerberg Campus and complements the existing group of central utility buildings. The HEZ building complex—comprising the boiler house, pump house, recooling plant, and auxiliary operations building—is expanded by the HRZ into an ensemble of technical facilities. The building follows the principle of an infrastructure island embedded in the landscape: the volumes were developed in a staggered arrangement and integrated into the park-like surroundings. Its unique location at the intersection of the campus and a residential area required careful coordination between urban integration, architectural expression, and the complex functional requirements of a data center.
Construction and Materials as Design Elements
The facade of the data center is characterized by exposed concrete containing a high proportion of recycled concrete. From a distance, the building envelope appears understated; upon closer inspection, the carefully executed details of the surface and its construction become visible, resulting from precise planning of formwork, joint patterns, and execution.
The horizontal joint pattern breaks up the large facade surfaces and lends the four interlocking building wings an appropriate sense of scale. The joints were deliberately not joined at the ends of the individual building volumes, thereby preserving the sculptural articulation of the structures.
Since no concrete slabs needed to be connected to the rising concrete walls, climbing formwork could be used. A specially developed system consisting of connection points and spacers made of die-cast zinc makes the construction process of the walls visible in the finished structure. Structural details are not concealed but are carried forward as part of the architectural language.
The load-bearing exterior shell made of concrete enables a material-optimized, thin, and weather-resistant building envelope. At the same time, the surrounding concrete structure stabilizes the building against horizontal forces and, together with the internal timber structure, forms the hybrid load-bearing framework. The dimensions of the building envelope and the insulation standard were specifically tailored to the building physics requirements, particularly with regard to preventing condensation and heat gain. The goal was to keep heat gain—and thus the cooling capacity required for the servers—as low as possible, as well as to prevent uncontrolled heat loss through the building envelope.
Inside the building, too, the structural shell is treated as a finished surface. The concrete surfaces remain exposed, while technical installations are routed openly. Adhesive-bonded or permanently joined finishing and cladding layers were largely avoided. The building thus follows a principle of separability: building components can be more easily dismantled and sorted by material type during later modifications, demolition, or disposal.
Wooden Structural System and Long-Term Flexibility
After completion of the concrete shell, the internal load-bearing structure was added using wood. Together with the vertical concrete walls, it forms the building’s load-bearing system. The horizontal structure, consisting of spruce cross-laminated timber (CLT) floors and laminated timber beams made from BauBuche, was specifically tailored to meet the respective structural requirements. This hybrid system combines the specific properties of both materials: The load-bearing concrete facade ensures stability, protection, and durability, while the timber structure enables a lightweight and resource-efficient load-bearing system.
The wood components were prefabricated in the largest possible formats and then installed in the building as individual elements. The clear separation of the construction processes—first the concrete shell, then the wood construction—enabled efficient implementation.
Depending on the respective loads, the ceiling structures were designed as simple beams or trusses. Spans of up to 18.70 m allow for a column-free layout of the server and utility rooms, thereby creating the conditions for floor plans that can be adapted over the long term. This enables the technical infrastructure to respond to future requirements.
The internal organization of the structural framework remains visible from the outside: The heads of the wooden beams penetrate the concrete walls and trace the structure of the building on the facade. Functional elements such as the entrance gate, smoke exhaust dampers, and the large louvered grilles in front of the ventilation openings were also designed as independent structural components. This ensures the construction remains transparent and highlights the building’s infrastructural character.
Energy-Efficient Building Services
Building services were not treated as an afterthought but were developed as an integral part of the architectural concept. The particular challenge of the data center lies in combining high ICT performance, continuous availability, and the lowest possible energy consumption. The servers’ high heat loads are reliably dissipated without relying on energy-intensive mechanical cooling to cool the system rooms on a permanent basis.
To achieve this, the HRZ primarily uses free cooling via adiabatic air-to-air heat exchangers. In this process, outside air and indoor air remain separate: Heat from the system rooms is dissipated via the heat exchangers, while the cooled air is recirculated back to the server areas. Free cooling covers approximately 92 to 95 percent of operating time. Only when outdoor temperatures rise is it supplemented by adiabatic cooling. Rainwater is designated as the cooling water for this purpose; treated drinking water or municipal water serves as a redundant supply. Mechanical aftercooling is practically unnecessary during regular operation.
The system’s high efficiency is also evident in the calculated pPUE values for the air-to-air heat exchanger cooling: In N+1 operation, these range between 1.05 and 1.09. In relation to the cooling function under consideration, this corresponds to only about 5 to 9 percent additional energy. Combined with the high proportion of free cooling operation and the very low need for mechanical aftercooling, the cooling concept thus ranks among the international leaders in energy-efficient data center cooling. The pPUE value refers exclusively to cooling and should not be equated with the data center’s overall PUE.
The waste heat generated during operation is not simply discharged unused. Part of it is used to heat the data center, while the rest is fed into ETH’s district heating network and made available for external use. In this way, the generated heat is treated as a resource and integrated into ETH Zurich’s energy strategy.
Resource-Conserving Material Selection
The load-bearing outer shell consists of circular exposed concrete with a high proportion of recycled material. The use of recycled concrete has conserved primary resources and reduced the demand for newly extracted raw materials. A complementary CO₂ storage technology helps to further reduce the concrete’s carbon footprint.
The choice of materials was also evaluated as part of a life-cycle assessment. This involved examining the embodied energy and greenhouse gas emissions of the materials used to assess the impact of material decisions over the entire life cycle.
Energy and Ecology
The rooftop photovoltaic system enables on-site electricity generation and complements the energy-efficient building services concept. At the same time, the roof was designed as an ecological habitat. A substrate layer and structural elements such as deadwood, sand, and gravel pockets promote ecological diversity. The beam heads of the structure were designed to serve as nesting sites for birds, creating additional habitats for local wildlife. The surrounding area was also designed with ecological considerations in mind. The landscaping concept extends the existing landscape structure and embeds the new building within groups of oak and Scots pine trees.
Integrated Planning as the Foundation for Realizing Complex Infrastructure
The completed Hönggerberg data center exemplifies Penzel Valier’s integrated planning approach. Architecture, structural engineering, building services, and construction management were not considered in isolation but were developed as an integrated planning task.
The specific requirements of critical infrastructure—high operational reliability, technical complexity, and long-term adaptability—were integrated into the spatial and structural design at an early stage. The possibility of future expansion was already taken into account during the planning phase: Conditions were established for the planned adjacent HRZ+ to enable the infrastructure to be further developed as requirements change. An analysis of operational requirements led to the creation of a robust building structure that integrates technical systems, structural engineering, and architectural expression.
