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A paradigm for thermal comfort in tropics

Wolfgang Kessling | Principal, Transsolar KlimaEngineering

There is a myth that ‘green’ design is at odds with architectural quality premised on the belief, that building performance can be delivered independently of architectural form. Architectural styles from around the world are replicated in the tropics, following the belief that façade design can be imagined independently from the local climate. It is possible for a building e.g. to be platinum-rated without deep engagement of architecture. Thirty to forty percent reduction in energy – required for top tier certification – can be attained, for now, through procurement of energy efficient systems, chillers, lights, etc. (Kishnani, 2017). So, the responsibility for thermal comfort and indoor air quality is delegated from architecture to so called innovative mechanical systems. This is most noticeable for highly glazed buildings, where the budget for mechanical systems can absorb up to forty percent of the construction cost.

Common practice in many tropical countries is to design air-conditioned buildings that operate at 22°C to 24°C all year. To maintain low indoor temperature using less energy, these buildings are sealed-off from the environment and hence do not take advantage of outdoor conditions when favorable. Façades are sealed and natural ventilation is considered to conflict with air-conditioning.

The way we define comfort plays a significant role in design. What if we challenge this definition? Are the conventional standardized requirements to cool and condition spaces the only way to achieve thermal comfort? And are architects missing the opportunity to take (back) ownership of the design and performance of their buildings to meet the comfort criteria? To raise a strong argument for climate responsive architecture we need to revisit the way thermal comfort is defined and delivered.

In the tropics, people living in tropical climate zone are acclimatized to high humidity and temperatures. Elevated air speed has long been used in practice in this region as a way to achieve comfort by offsetting higher temperatures. With adaptive models, thermal comfort can be reliably evaluated for a wide range of the six environmental and personal parameters: air temperature, mean radiant temperature, relative humidity, air speed, clothing factor and metabolic rate. Here the cooling effect of elevated air speed can be well described.

This approach allows designers to evaluate alternative comfort strategies such as Hybrid System Design where elevated air speed is combined with supply of tempered air. In such a system, buildings are designed with openable facades to take advantage of natural ventilation. Thus the spaces are designed for adaptive comfort rather than relying on a static temperature band throughout the year. This hybrid system combines excellent fresh air supply of tempered air with fans that elevate air speed to satisfy thermal comfort requirements. The fresh air rates are designed for good indoor air quality. In case of high indoor temperatures, the occupants can elevate the air speed at their position serving their personal preferences.

Rethinking how to deliver comfort and to develop buildings and building envelopes which focus on high autonomy for building operation, good conditions for thermal comfort, high daylight availability, excellent natural fresh air supply by passive design is key. Applying these design principles provides architects  with more choices to develop climate responsive building design. The Net-Zero building School of Design at the National University of Singapore perfectly illustrates this approach. The integrative design process executed by the team was key for its success with the electro-mechanical systems well integrated into spatial planning and the envelope design.

What we learned is that challenging the way we define comfort plays a significant role in exploring new solutions in tropical architecture. To design for adaptive environment, developing a holistic view on all the parameters affecting human comfort is key: not focusing on temperature set-points but including physical phenomena such as heat radiation as well as environmental parameters such as wind speed.

In warm climates a Hybrid System Design presents an attractive choice for achieving high quality and comfort with reduced air-conditioning, chiller capacities and low energy demand. This increases the potential to generate energy on site to achieve a net-zero energy target as in the case of the School of Design in Singapore.

Adaptive Comfort and Hybrid Comfort Design are not a poor man’s choice. It is the intentional choice of a climate-responsive and responsible design. Redefining “thermal comfort” can change the fundamental approach towards architecture and mechanical system design resulting in a high-performance. It is a choice for a new breathing architecture, aesthetic functional solutions with openable façades creating semi-outdoor spaces, for context sensitive design that is appropriate to local climate and cultural conditions.

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