Super-Tall Green Buildings in China

Chris Li
Rapid urbanisation and limited urban land supply have encouraged super-tall buildings to spring up across China.

Architectural expression of super-tall buildings can be influenced by the unique surrounding micro climate. Different expression or building envelope specifications between the top section and the bottom section can be justified to reflect the environment.

Faithful+Gould is working with a top building design team on a 400m high super-tall building in South China, advising on sustainability aspects. Design stage contribution includes a feasibility study on operable window design based on Computational Fluid Dynamic (CFD) simulation, providing the city authority with scientific evidence of the potential issues and benefits. We are also assisting in the design of a prevailing wind ventilation system for the building’s sky hall (high bay lobby area) at the elevation of 330 meters, to avoid overheating of the space while harvesting daylight to create an indoor leisure area with exceptional views.

Faithful+Gould is working with a top building design team on a 400m high super-tall building in South China, advising on sustainability aspects.

Despite a burgeoning green building movement in China, and the development of a green building standard and building energy codes, sustainable design concepts for super-tall buildings are far from sophisticated. Opportunities are frequently overlooked. Natural ventilation (NV), for instance, is an accepted strategy for low rise buildings in temperate climates, but is often excluded from super-tall sustainable design strategies. Most super-tall building envelope designs are well enclosed because the wind magnitudes dramatically increase with the elevation. Opening windows at the higher section of a super-tall building causes safety issues.

In fact NV is a realistic energy saving measure for super-tall buildings, if the segmentation approach is adopted - the stack effects mechanism which has been successfully applied on low rise buildings can also be partially applied to super-tall buildings. Sometimes this energy-efficient design opportunity is ignored by local design institute designers who may adhere to conventional design philosophy. As a result, 80 per cent of the energy efficiency contribution of these super-tall buildings typically relies on costly MEP improvements, to comply with green building schemes like USGBC’s LEED.

Micro climate changes surrounding the super tall building have significant impacts on the cooling and heating building energy consumption.

Architects and engineers are becoming more aware of how the sky brings an additional source of energy efficiency benefits to tall buildings. Micro climate changes surrounding the super-tall building have significant impacts on the cooling and heating building energy consumption. In standard atmosphere, dry bulb temperature decreases linearly with elevation, and this "lapse rate" of outdoor temperature could be 6.5 degrees Celsius per 1000 metres according to the ASHRAE Fundamentals Handbook. This brings three major benefits for tall buildings: lowering cooling energy due to conduction; lower sensible heat gain from infiltration, and the desired ventilation.

Air pressure decreases with elevation, allowing exterior air to become less dense. Subsequently less energy is required to cool or heat the same volume of exterior air. And also it will consume less energy to deliver the same volume of exterior air at upper floor sections of a super-tall building versus the lower floors. Wind speed increases with altitude, which makes operable window design risky and also lead to increased infiltration. However, higher wind speed increases the convection coefficient between building façade and the outside air. As a result, heat gain or loss between the building’s interior and exterior is increased.

Taking account of altitude effects can contribute significantly to the sustainability of super-tall buildings.

Moisture ratio can decrease with altitude depending on the dry bulb temperature, pressure, and the lapse rate. Moisture reduction with altitudes is significant for super-tall energy savings, especially in climates with high humidity like Southern China.

Taking account of altitude effects can contribute significantly to the sustainability of super-tall buildings. However calculating the annual energy consumption of these buildings is challenging, as the commercially available energy programmes are not sufficiently sophisticated to model the micro climate changes with altitude. Using mid-level elevation weather data gives an acceptable approximation but correlation with actual site measured data is preferable and will enhance simulation-based design decisions.

Measurement during the building commissioning phase will verify these design concepts, producing benchmark data to better advise future super-tall building development in the same area. This aligns with the industry-accepted design philosophy of locally feasible green building expertise, providing clients with affordable sustainability services.