Pratik Raval, Transsolar Climate Engineering, New York, explains through a couple of examples the need for a holistic design approach – an approach for generating a design solution in which local climate conditions, architectural vision, and engineering components are all integral part of a well-orchestrated climate responsive system. Such integrated design approach not only provides outstanding energy performance but also ensure maximum occupant comfort. Above all, it addresses the fundamental aspect of design that conventional definition of sustainability, through certifications and labels, fails to address – to strengthen the identity of architecture with a place and culture.
The desire to tightly control indoor environment for maintaining occupant comfort has resulted in extensive use of mechanical conditioning systems in built environment. In fact, one of the most marked trends in architecture over the past century has been that of replacing the primary function of building design: provide healthy, comfortable, and pleasant indoor environment, by such mechanical systems.
Conventionally these mechanical conditioning systems are designed such that they operate independent of local outdoor conditions and separately from building form, rest of the building components, and many a times from each other. Consequently, the buildings themselves have become separated from the outdoor environment they are placed in. This inherently faulty design approach has made the built environment the highest energy consumer. On the other hand, occupant comfort is also compromised due to little or no opportunity for controlling indoor environment and lack to physical connection to outdoors.
Architecturally integrated climate responsive design strives to achieve harmonious balance between local outdoor conditions and building’s architectural vision (form and function); whereas mechanical conditioning systems are supplemental and well integrated with the overall functioning of the building. The goal of this philosophy is to create buildings that can dynamically adjust their physical properties and energetic performance in concert with changing outdoor conditions and indoor demands as well as allow occupants to intervene and maintain comfortable space conditions.
Climate responsive design requires a “smart” building envelope that can facilitate such dynamic connection between indoors and outdoors by admitting, rejecting, storing, tempering, or redirecting energy. Building façade thus becomes very critical component, both at macro level – where it definesthe shape of the built environment and separates indoors from outdoors, as well as at micro level – where it functions as an individual component (e.g. a window or a wall).
Frankfurt am Main, Germany
Architect: Sauerbruch Hutton Architecten
Climate concept: Transsolar Climate Engineering
Completion date: July 2010
Net area: 38,000m2
Principal use: Offices and administrative spaces
In keeping with kfw banking group’s interest in environmental impact and stewardship, the main design goal for the new facility was to set a new standard for energy consumption in high-rise office buildings – use less than 100kWh/m2/year source energy and provide manually operated windows in all the offices for natural ventilation while maintaining space temperature below 260C throughout summer.
To achieve these ambitious energy and comfort targets, the design team looked carefully at the relationship between the new building and its surrounding urban and environmental context. Consequently, building massing, shape, and orientation are designed to respond to local solar, wind, and temperature conditions.
The tower is oriented in the direction of prevailing winds and the shape of the floor plan resembles an airfoil. The façade consists of an inner layer of insulated glazing separated from an outer layer of single pane glazing by a façade cavity that is approximately 70cm at its deepest point. This double envelope façade creates a “pressure ring” that serves to neutralize wind pressure conditions that might otherwise be too turbulent, particularly for such a high-rise office tower.
The airflow and pressure within the “pressure ring” are controlled by dynamically adjusting flap openings on the outer layer in response to ambient conditions, while the inner layer has manually operable windows. The flaps are designed to respond to different combinations of wind speed, wind direction, ambient temperature, and solar radiation, as well as pressure difference between windward and leeward side of the building. This aerodynamically controlled arrangement allows constant low-speed ventilation on both windward and leeward side simultaneously.
The façade is designed to operate in three different modes: (Pressure ring modes) flaps are closed during winter to create thermal buffer zone and wind is directed around the building; flaps are opened, mostly on windward side, in “shoulder seasons” to funnel air into the cavity and naturally ventilate the offices; flaps are open in the summer, air is not admitted into the offices, and pressure differentials pull warm air through and from the cavity space.
This design allows all the offices to be naturally ventilated for eight months without any drafts or undesired heat gains or losses. Air is returned through sound attenuating transfer elements into the corridors, and through a central ventilation shaft it is exhausted naturally due to stratification.
For the remaining period, in mechanical ventilation mode, minimum required outside air for maintaining indoor air quality is supplied to the offices. Fresh air is drawn in from the adjacent botanical garden into a 30m long geothermal earth duct. This air flows through vertical shafts into raised floors and then enters offices through displacement air diffusers along the façade. The air is returned the same way as in natural ventilation mode through a central shaft, where before exhaust, energy is recovered from the outgoing air and is supplied to the incoming fresh air from the earth duct
Exposed concrete slabs provide efficient, quiet, and uniform radiant heating and cooling by running “low temperature” hot water and “high temperature” chilled water through embedded hydronic tubing. Such thermally activated slabs continuously absorb or reject heat during occupied hours to maintain comfortable conditions. The hydronic system receives hot and chilled water from high-efficiency district hot and chilled water loop. The system also takes advantage of “free cooling” by manipulating seasonal shift in ground temperature at the beginning of each cooling season, where as “free heat” is received from the heat rejected from the large data center of the building
The building’s potential source energy use is estimated to be 82 kWh/m2/year, which is well below its original target. A third-party organization is monitoring real time building performance, and when confirmed, kfw westarkade will be the most energy efficient high-rise office tower in the world with source energy demand below 100 kWh/m2/year.