Heat Bus System

Few people enjoy the view of air-conditioners hanging off façades, therefore we choose to hide them away, in recessed space, alcoves and behind louvers. The hot air rejected from air-cooled condensers creates a bubble of increasingly hot air that rises up alongside the building. Residents further up now have to close their windows to prevent the hot air from entering their apartments, rendering it impossible to benefit from cross-ventilation. Their own air-conditioners now have to operate in a local heat island of 50 ºC and more, at over 30% reduced energy efficiency, because of this stack effect and because hot air gets trapped behind the louvers!


Air-conditioning in the hot-humid tropics of Singapore accounts for 50% of the electricity demand in buildings. Under the Low Exergy paradigm, our objective is not only to reduce the need for cooling through active and passive means but also to increase the overall performance of the cooling system. By considering the operating temperatures of the chiller, by decreasing the heat rejection temperature for the air-conditioning system, the energy efficiency can be considerably increased. In a climate where everything – air, sea, rivers and ground – is hot, evaporative cooling towers are the best option to get rid of heat, as it is often done in large central systems (Bruelisauer et al. 2013b).


We suggest and evaluate a novel system, the HEAT BUS SYSTEM developed at the LOW EXERGY module at FCL where we replace air-cooled with water-cooled condensers and pump the warm water to a central, evaporative cooling tower where the heat is rejected at much better conditions. The resulting system increases energy efficiency beyond what would ever be possible with current split units and can easily be integrated into the structure of new and existing buildings, as suggested for the 3for2 – BEYOND EFFICIENCY concept. Water has 4000 times the heat capacity of air, resulting in very small pipes necessary for the transport of that heat. The additional infrastucture is simple and cheap because the water is pumped at temperatures close to outdoor temperatures, omitting the necessity for insulation. We have shown that the energy used for pumping is much smaller than the gain in chiller efficiency (Bruelisauer 2013). The heat bus setup facilitates a modular build-up as individual condensers can be plugged into the network as and when the need arises. It also enables to benefit from much higher chiller performance for high-temperature cooling when the provision for cooling and dehumidification are separated (Meggers et al. 2012), as investigated in the BubbleZERO LABORATORY.


The heat bus system is not only limited to high-rise buildings with large cooling demands in themselves but may also be applied to low-rise neighbourhoods in a horizontal setting. The façades of the shophouses in Rochor and Chinatown shine brightly and in full colour, but as soon as we step behind, into the backlanes, we are met by heat and noise from hundreds of split type air-conditioners. Implementing heat bus networks with central cooling towers along these lanes not only reduces energy use by 50% but also ameliorates the thermal microclimate, increasing the thermal comfort for pedestrians. It can be used as a trigger for architectural and urban design interventions that upgrade the backlane qualitatively and transform the backlane into a space that fosters social encounters and new economic activities. The nature of the shared infrastructure can be utilised to re-evaluate the neighbourhood, how to increase its value for occupants and owners (Bruelisauer et al. 2013a). The IMPROVING BACKLANES project is a synergetic, multidisciplinary research project that attempts to consider these different factors and proposes a conceptual design vision for backlanes of shophouse neighbourhoods, based on studies in energy efficiency, pedestrian movement, historic stock analyses and urban diversity studies. The objective of the new project phase – RECLAIMING BACKLANES – is to evaluate the feasibility and develop design visions for specific sites in Singapore.




  • Bruelisauer, Marcel (2013). Heat bus for the tropics – exergy analysis of coupling decentralised chillers with central cooling towers, 11th REHVA World Congress CLIMA 2013, Prague, Czech Republic.
  • Bruelisauer, Marcel, Sonja Berthold, Gideon Aschwanden, Iris Belle, Edda Ostertag and Forrest Meggers (2013a). ‘Reclaiming backlanes – addressing energy efficiency, outdoor comfort and urban space – Accepted Manuscript’, paper presented at SB13 Singapore, Realising Sustainability in the Tropics, Singapore.
  • Bruelisauer, Marcel, Forrest Meggers and Hansjürg Leibundgut (2013b). ‘Choosing heat sinks for cooling in tropical climates’, Frontiers of Architectural Research (In Press).
  • Meggers, Forrest, Luca Baldini, Marcel Bruelisauer and Hansjürg Leibundgut (2012). ‘Air conditioning without so much air – Low exergy decentralized ventilation and radiant cooling systems’, in Proceedings of the 5th IBPC: The Role of Building Physics in Resolving Carbon Reduction Challenge and Promoting Human Health in Buildings, ed. The 5th IBPC organizing committee, 529–536, paper presented at 5th International Building Physics Conference (IBPC), Kyoto, Japan.



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Array of split type condenser units on the back façades of buildings

Evaporative cooling tower

Split type condenser units on high-rise building

Split type condenser units on high-rise building