Japan’s First 100% Self-Sufficient Office Building in Energy Usage

The Office in the Forest（in Hokuto City, Yamanashi Prefecture; ordered by Seicho-No-Ie）was contracted design and build by Shimizu Corporation（President, Kazuyuki Inoue）. This midsize office building was completed in May 2013. It has been operating off-grid and has continued to achieve a balance power supply and demand since March, 2020 without requiring any commercial electric power. Nearly eight months have passed since the building’s power source was disconnected from the commercial electric power grid, and the building is balancing electric power supply and demand in real time through mutual supplementation by solar power, biomass power, and storage batteries, as planned by Shimizu. This is the first attempt in Japan to operate an office building off-grid that supplies 100％ of building energy usage on its own.

The Office in the Forest is a midsize office that consists of six 2-floor buildings built of wood with a total floor area of 8,154 m²（including parking space）. At the time it was completed, Office in the Forest had net energy consumption is 45％ of the energy used by a normal office building of the same size. It achieved this through technologies that reduced the heating, cooling, and lighting load. Shimizu worked to achieve energy savings of roughly 475 MWh/year, and aimed to make it a zero-energy building（ZEB）by supplementing commercial electric power with solar power（470 kW）, biomass power（175 kW）, and storage batteries（408 kWh）to supply the electricity used.

The office was functioned as a positive energy buildings（PEB）, which produced 200-300 MWh/year than exceeding their energy usage for seven years, from the time it began operating in July 2013 to February 2020. This causes from the high level of energy-saving consciousness among office users. The client requested an even greater reduction in environmental impact and Shimizu responded by upgrading the storage batteries, taking the building off-grid, and making it 100％ self-sufficient in electric power to reach the substantial RE100 goal（100％ renewable energy）.

The main challenges in taking the building off-grid were establishing the optimal storage battery capacity, prioritizing operation of solar power to further reduce the environmental impact, achieving real-time balance in electric power supply and demand, and protecting the storage batteries from rapid over-charge and over-discharge.

For the storage battery capacity, Shimizu decided to procure two sets of storage batteries with more than two times the capacity to accommodate maximum power usage of around 1,500 kWh per day from November through April, when heating needs are high, and to enable off-grid operation and account for the risk of power generation equipment and storage battery malfunction and maintenance. More specifically, this involved incorporating a mechanism for charging the batteries to the targeted capacity while operating two sets of 1,824 kWh storage batteries（manufacturer product specifications）in parallel and repeatedly charging and discharging them. However, the storage batteries will automatically shut down when over-charged or discharged in a low-charge state, so solar and biomass power are automatically controlled to prevent such states. This achieves real-time balance in electric power supply and demand by controlling storage battery charging and discharging to absorb rapid fluctuations in electric power from prioritized operation of solar power. Real-time control of the series of processes involved in power generation, storage, and discharge is achieved through the Shimizu Smart BEMS system, which was developed by Shimizu.

The major advanced economies have set ambitious greenhouse gas reduction targets for 2050, and Japan is targeting 80％ reduction compared to 2013. Shimizu intends to focus on developing energy-saving technologies, renovating building to energy-efficiency, proposing ZEB, and RE100 power business to contribute to controlling greenhouse gas emissions.