BIOSKIN: A Façade System for Cooling City Heat Islands

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The phrase “extremely hot day” denotes a day on which the temperature is 35°C or higher. The phrase has become standard in summer, but it actually came into use in 2007, when the Japan Meteorological Agency defined it. Although the temperature has risen about 0.6°C globally in the past hundred years due to global warming, in Tokyo it has risen 3°C, with the heat island phenomenon believed to be the major cause.

As a countermeasure to heat islands, the greening of parts of construction areas, such as rooftops and walls, is being promoted, primarily by the Ministry of Land, Infrastructure, Transport and Tourism. Despite being a good measure for producing cool spots, it will prove difficult in terms of using cubic capacity to the maximum.

In keeping, effective countermeasures to the heat island phenomenon are being explored, centering on the construction industry. With countermeasures being hastened, Nikken Sekkei is experimenting with progressive approaches to designing buildings that cool cities, with functions akin to those of trees and forests. Instead of cities becoming hotter the more that construction increases, what if they became cooler? Presented here, together with cases, is BIOSKIN: a technology that puts the brakes on the heat island phenomenon and which was inspired by the natural phenomenon of uchimizu [sprinkling water outside for cooling].


Construction: One Cause of the Heat Island Phenomenon

With the heat island phenomenon, temperatures rise more in cities than in suburbs. The main cause is that the construction and roads that comprise cities collect heat during the day that radiates at night. Because building materials that store a great amount of heat are frequently used, the amount of heat radiated increases in kind. Construction is also a cause of rising temperatures in urban areas because structures cover ground surfaces, decreasing soil and plant transpiration and obstructing cold-air inflow from coastal areas and rivers and such. 
The more construction increases, the more the heat island phenomenon is stimulated. For this reason, the construction side must establish countermeasures to mitigate the heat island phenomenon.

The Concept for BIOSKIN: Rooted in a Seasonal Tradition of Japan

BIOSKIN is a technology expected to stop the heat island phenomenon. What led to the creation of BIOSKIN was the Japanese summertime tradition of uchimizu. An ancient tradition for cooling off in summer, uchimizu has attracted attention in recent years for its effectiveness in curbing temperature rises. In Tokyo, uchimizu is done with rainwater or secondary-use water and is being promoted to mitigate summer heat.
A key to the mechanism by which uchimizu reduces the temperature of the surroundings is the heat of vaporization phenomenon. The heat of vaporization refers to the heat absorbed by the surroundings when water evaporates. With uchimizu, the evaporation of water sprinkled on the ground drains the temperature of the surroundings, thus lowering the temperature. This phenomenon is similar to human perspiration, where the heat of vaporization curbs increases in body temperature.
Focusing on the heat of vaporization generated from the effects of uchimizu, Nikken Sekkei arrived at the idea of covering construction surfaces with devices that use said heat for cooling.

Case Study: The Use of BIOSKIN for the NBF Osaki Building (formerly named Sony City Osaki)

In this way, the world’s first exterior evaporative cooling system, the BIOSKIN System, was created from the concept of uchimizu and was used for the NBF Osaki Building (formerly named Sony City Osaki).

Unglazed porous pipes through which water flows easily and that also serve as balcony hand railings were used on the exterior, inspired by sudare [traditional bamboo screens]. Electric power obtained from sunlight was used to draw up rainwater that accumulated underground, which then evaporates in the unglazed pipes. The heat of vaporization cools the building, as well as the entire surrounding area.
Partial greening of buildings is also effective against the heat island phenomenon. Because wall surface greening involves the spattering of water and dirt and the withering of plants, as well as maintenance and such, it is not suitable for many high-rise buildings. In addition, areas for rooftop greening are limited. The BIOSKIN developed this time requires no maintenance, owing to the effects of the titanium oxide photocatalyst. Moreover, it can make use of the large wall surface area of high-rise buildings.
Other systems that use the heat of vaporization for cooling involve using dry mist or running water directly over glass surfaces, but water spattering is an issue with these systems. With BIOSKIN, water spattering is not a concern as water passes through pipes before seeping out. In addition, the resources for operating BIOSKIN are rainwater and solar energy. BIOSKIN is a mechanism that allows energy saving to be considered while nature’s bounty is received.

Effectiveness of BIOSKIN Proved in Experiments for Extremely Hot Days

Simulations were run on scenarios that involved extremely hot days and the use of the BIOSKIN System: specifically, cladding the twenty-fifth floor of a high-rise in unglazed pipes. Results showed that the surface temperature became 10°C lower than the outdoor temperature, just from using uchimizu on the entire exterior wall surface.
What was surprising was the fact that bringing cool air in on the breeze lowered the ambient temperature by 2°C. It was confirmed that while suppressing the heat island phenomenon, BIOSKIN simultaneously leads to heat load reduction in indoor spaces. The BIOSKIN System started as an attempt to contribute to curbing both temperature rises in buildings themselves and the city heat island phenomenon; these experiments proved that it can also serve to create cool spots that have a positive effect on the surroundings.

Transforming Factors in Heat Islands into Cool Spots

Simulations yielded still other surprising discoveries. The BIOSKIN System recorded transpiration equivalent to that of 24,000 square meters of forest, and it was confirmed to have a nitrogen oxide (NOx) removal effect equivalent to that of 1,400 poplars. It can thus be said that BIOSKIN has imbued construction with abilities similar to those of trees or forests.
Although it can be said that the perpetrators of the heat island phenomenon are the people who build cities, the victims of the phenomenon are the people who live in them. The heat island phenomenon is causing cities to steadily become hotter. Because of this, every year 4,000 to 5,000 people are transported by ambulance and over 1,000 people die -- a grave situation.
Until now, greening has been used to mitigate the effects of construction on the heat island phenomenon. However, the advent of BIOSKIN presents a solution to the construction that is itself a factor in said phenomenon.
Buildings have been cited as a factor in the heat island phenomenon. By erecting them as if planting plants and equipping them with a system that works similarly to trees and forests, environments will improve. This paradigm shift is one of the great values of the BIOSKIN System.

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