Abundant Harvests on Office Building Rooftops

The Surprising Environmental Effects of Sweet Potatoes

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As global warming continues in central Tokyo, an unlikely plant is demonstrating energy-saving effects. On the rooftop of the Sumitomo Shoji Mitoshiro Building in Kanda, Chiyoda-ku, Tokyo, the seedling of sweet potatoes are grown to cover the outdoor units of air conditioning equipment. Transpiration from the leaves of the sweet potatoes cools the air that the outdoor units take in, succeeding in reducing air conditioner operational energy by as much as 10% during the hottest period in summer, thus demonstrating energy-saving effects. The energy-saving effects of transpiration from the leaves of sweet potatoes have been researched for a while, but this is the first case in which they were incorporated in an actual building.

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Sweet Potatoes and a Happy Byproduct

Although sweet potatoes grow in soil and their leaves run along the ground, in this case, the objective was to cool the air surrounding the outdoor units of air conditioners, rather than produce insulating effects with soil and greenery. In keeping, instead of creating fields, bucket-size fabric sacks containing seedlings and earth were hung from stands as a device to make leaves grow lushly. In terms of harvesting, the sweet potatoes can be easily removed by cutting the bags with scissors on top of plastic sheets; thus, even someone wearing a necktie or pumps can harvest them in comfort. The crop yield, a matter of concern, is an incredible 250 to 350 kilograms per 450 square meters, which is the area of the place where the outdoor units are. A happy byproduct of the sweet potatoes is that they help form community: they can be distributed to the tenants who occupy the building or steamed in the basement cafeteria and enjoyed together.
This system, which uses a familiar plant, the sweet potato, is officially known as the Greening System Using Sweet Potatoes to Cover Outdoor Units of Air Conditioners (Greening System Using Sweet Potatoes, hereinafter) and was jointly developed by Sumitomo Shoji and Nikken Sekkei. The two young facility designers in charge of the project, Satoshi Suzuki and Taro Hongo, were the ones who brought in the idea, and they worked together with Sumitomo Shoji to actualize it.



Various applications are envisioned for the Greening System Using Sweet Potatoes. For green gardens made at home, for example, bitter or sponge gourds are often cited as plants harvested in summer that can be used. With sweet potatoes, however, different uses are conceivable: in early summer, the seedlings grow hanging down from sacks; in midsummer, the leaves trail near the outdoor units of air conditioners, producing energy-saving effects; and in autumn, benefits can be enjoyed at the individual level, as people enjoy harvesting and eating delicious sweet potatoes.

Greening with Sweet Potatoes: Creating Additional Value

As of 2018, the implementation of greening systems using sweet potatoes and outdoor units is in its sixth year, which includes the experimental stage. A sake brewery in Kumamoto is currently brewing sweet potato shochu [distilled liquor] with the sweet potatoes harvested last year. This time, they are making a blend with Kumamoto sweet potatoes, but if the Greening System Using Sweet Potatoes spreads even more, to other office buildings in the area, it will be possible to have 100％office-made shochu. In fact, the day may come when people associate Kanda with shochu.
In this way, the Greening System Using Sweet Potatoes not only saves energy but contains latent possibilities for creating additional value. Nikken Sekkei is thus actively taking on the challenge of unprecedented new attempts, like this one.

  • So Suzuki

    So Suzuki

    MEP Engineer
    General Manager
    Sustainable Design Group
    Engineering Department

    Joined Nikken Sekkei in 2007 after working at a facility design office. His major was machinery and facility design. He manages facility designs for a range of buildings including high-rise offices, high-rise residential complexes, large-scale government buildings, computer centers and others. In recent years he has managed projects including Jinbocho Terrace Square, the Fukuoka High, Regional, Summary and Family Court Building, the Sapporo Sosei Sanku 1.1.1 Type 1 Urban Area Redevelopment Project, and conservation planning for the Tokyo Sumitomo Twin Building. He works hard to provide designs that meet the needs of clients. At present he is affiliated with the LCD Design Department, which was newly established in 2016. In addition to thinking about building life cycle design, he also hopes to work on the popularization of greening systems that make use of sweet potatoes. He is a first-class registered architect, a first-class registered architect for facility design, a building mechanical and electrical engineer, a CASBEE building assessor, and a CASBEE real estate assessor.

  • Taro Hongo

    Taro Hongo

    Project Manager
    Associate
    Global Business Development Group
    Global Business Department

    Joined Nikken Sekkei in 2007 after completing his master’s degree program (Osamu Ishiyama Lab) at Waseda University.
    His major was machine and facility design.
    He strives to propose buildings that are fun to look at, fun to use, and are loved by all.
    He has managed projects including the bicycle parking lot in front of Oshiage Station (2012), the Bank of Chengdu Main Office (currently being planned), the Ebisu Subaru Building (2014), the Sumitomo Corporation next generation office (AC condenser greening through sweet potatoes), and the YKK80 Building (2015). He has also managed facility and other renovations to the National Museum of Western Art and the Chubu Electric Power Chiyoda Building (2014).
    In addition, he pursues the greening of AC condensers through sweet potatoes as his life work.

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