Shaping Tomorrow's Built Environment Today

Transforming an Occupied Office into a Zero Energy Building

By Hiroaki Takai, Associate Member ASHRAE; Koji Tanaka, P.E.Jp, Member ASHRAE; Kazuki Wada, Member ASHRAE; Hiroki Kawakami, Member ASHRAE

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©2019 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 61, no. 5, May 2019.

About the Authors
Hiroaki Takai is a principal engineer of architectural environment, Koji Tanaka, P.E.Jp, is a group leader of mechanical and electrical engineering, Kazuki Wada is a group leader of architectural environmental engineering in the R&D Institute, and Hiroki Kawakami is a mechanical and electrical  engineer with TAKENAKA Corporation in Japan.


In the Paris Agreement, Japan promised to target a 40% reduction in greenhouse gas emissions for business and other sectors by fiscal year 2030 relative to fiscal year 2013. Japan’s national energy saving policies also target new public buildings to be a zero energy building (ZEB) by 2020 and for other new buildings to be ZEB by 2030. Because buildings with a gross floor space of 10,000 m2 (107,639 ft2) or less account for 98% of all small- and medium-sized office buildings in Japan, achieving energy savings in these buildings is an urgent issue to achieve the target reduction.

One existing building to be renovated into a ZEB is TAKENAKA Corporation Higashikanto Branch Office near Tokyo, which was renovated while the offices were in use. This marked the first time a net ZEB building in Japan was achieved while occupied. The renovation achieved both a comfortable office environment and energy savings by introducing technologies including high thermal insulation; natural ventilation and daylighting; radiant cooling and heating and desiccant air conditioning using geothermal heat and solar heat; and wellness control for providing an optimum indoor thermal environment to satisfy personal preferences.

The renovation also diversified the office environment to provide spaces suitable for concentration and communication. In addition, it allowed occupants to share office equipment, terminals, and other facilities, reducing plug loads and the like while enhancing productivity. To cover the remaining energy consumption and to be more resilient building, the renovation achieved energy generation and storage by introducing photovoltaics and lithium ion batteries. Thus, the renovation resulted in a building that exceeds net ZEB performance.


Energy Efficiency

Thermal Load and Energy Consumption

Achieving a drastic reduction in the thermal load on the external skin is an essential part of ZEB renovation.

Additional insulating material was installed in external walls and the roof, and glass was replaced with argon gas-charged low-e glass. Blinds and single glass as outer skin were installed to form a double skin on the outside surface. In addition, vertical louvers that had been used before renovation were reattached to the outside surface. The actual annual peak thermal load is small, only 60 W/m2 (5.6 W/ft2).

The energy produced and energy consumed by air conditioning and ventilation exhibit seasonal changes. The annual total consumption of primary energy including electricity supplied through plug loads is 403 MJ/m2·yr (13.1kBtu/ft2·yr), and the total amount of energy generation is 417 MJ/m2·yr (13.5kBtu/ft2·yr). 

Direct Use of Geothermal and Solar Heat

To minimize operation time of heat sources, a system was employed that cools and heats by using geothermal heat and solar heat. Specifically, water flowing through ground water pipes is cooled between 19°C to 21°C (66°F to 70°F) by using geothermal heat (underground temperature is 17°C [63°F]) and then directly supplied to radiant ceiling panels for cooling. In addition, hot water at 45°C to 60°C (113°F to 140°F) supplied by the solar heat system is used as a heat source for regenerating desiccant.

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