Net-zero energy buildings (ZEBs) are currently receiving attention as the ultimate form of environmentally friendly buildings. Constructing a building capable of meeting its own everyday primary energy needs by producing energy on its own is no longer a dream.

Utilizing various technologies, Takenaka has completed a number of ZEBs and nearly net-zero energy buildings* including office building and sports arena building projects. The renovation of its Higashi Kanto Branch Office building, carried out while continuing business activities, was the culmination of Takenaka's proven technologies.

　

* The Agency for Natural Resources and Energy defines a building whose annual primary energy consumption is less than 25 percent of that of a conventional building of similar size as a nearly net-zero energy building ("nearly ZEB") and a less-than-50-percent building as a "ZEB ready" building (i.e. categorized as a net-zero energy building in a broader sense).

The key is to achieve a higher level of energy efficiency in existing buildings

At the 21st Session of the Conference of the Parties to the United Nations Framework Convention on Climate Change (COP 21) held in 2015, Japan announced a goal of reducing greenhouse gas emissions by 26% relative to 2013 by 2030. This means that a reduction of about 40% is required in commercial and other sectors. Reducing emissions in those sectors, therefore, is the key to achieving this goal.

Because buildings are rebuilt only once every several decades, improving energy efficiency in existing buildings is particularly important. For these reasons, further improving energy efficiency has become an urgent priority for numerous office buildings in Japan.

Four key concepts for net-zero energy building renovation

(1) Change the theory of comfort
We believe that maintaining a constant temperature and humidity is not the only requirement for comfort. Various changes in the building, such as breeze that can be felt and changing light conditions, can extend the variety of comfortability for people and at the same time lead to a substantial reduction in thermal loading.

(2) Create super energy-saving
Deploying a complete suite of advanced technologies such as super heat insulation, double skin, automatic blind control, automatic natural ventilation control, dimmable LED lighting, desiccant air-conditioning, geothermal heat utilization, radiant air-conditioning and solar power generation, we renovate buildings so that those features can be controlled in an integrated manner.

(3) Think smart work-style
We help boost productivity by designing working environments optimized to meet task-specific needs and maximize the sharing of office equipment and terminals by creating common spaces and filing spaces that can be shared. By reconsidering the use of computers, air-conditioning systems and lighting systems, which tend to be left on even when not in use, we will provide solutions to make a switchover to a more energy-efficient work style so that they can be used only where needed, only when needed and only in the amount needed, in order to reduce total energy consumption.

(4) Become resistant to disaster
By implementing the measures described in (1), (2) and (3) above, existing buildings can be upgraded so that they can be operated with a smaller amount of energy and, even when lifeline facilities become non-serviceable in the event of a disaster, can stay operational as an office building by utilizing solar power generation systems and electric energy storage systems.

Technologies for making net-zero energy building renovation possible

Net-zero energy building renovation achieved

The office building used as Takenaka's Higashi Kanto Branch Office (completed in 2003) was renovated into a net-zero energy building while it was being used as an office building. The renovated building went into full operation in May 2016, and its first year as a net-zero energy building ended in April 2017. As a result, annual generative energy exceeded annual energy consumption achieving a net positive annual energy balance.

The renovation of our Higashi Kanto Branch Office was the first project in Japan in which an office building was renovated into a net-zero energy building while the building remained in use. The project involved replacing the exterior materials with high-performance heat insulation materials, leveraging passive energy technology to harness natural energy, utilizing carefully designed environmental control technology and reforming the work style. Annual energy consumption resulting from these efforts was 403 MJ/m2∙year, more than a 70% reduction from the pre-renovation level. Solar power generation further increased annual generative energy to 417 MJ/m2∙year.

The one-year operation confirmed the following effects:

1. Boosting the overall heat insulation efficiency of the building significantly reduces exterior loads and makes it possible to keep the indoor environment comfortable throughout the year by reducing temperature changes even in window zones.
2. By maximizing the use of natural lighting and automatically controlling the opening and closing of the external blinds, it is possible to maintain a sense of brightness in the room and significantly reduce the amount of electricity used for lighting.
3. A newly developed compact desiccant air-conditioning system and radiant air-conditioning system that directly harness geothermal and solar energy make it possible to significantly reduce energy used for air-conditioning while keeping indoor spaces comfortable.
4. Personal preferences can be learned by measuring user activity through wearable devices and sensation questionnaire surveys in order to enable wellness control for optimizing temperature and airflow conditions.
5. Because dependence on external energy sources is minimized, the building can stay operational for a longer time—even if the public infrastructure goes down—so that the business continuity plan can be dramatically improved.