Over 25,000 OM Solar homes have been built in Japan within the last 20-30 years. This figure would appear to put the system’s popularity on par with Europe’s Passive House (Passivhaus) standard. Yet, the OM Solar method is unique and seems almost unknown outside of Japan.

The acronym ‘OM’ stands for the Japanese ‘Omoshiroi’ — meaning interesting — and ‘Mottainai’ — translated as non-wasteful. While the name seems vague, the technique is quite specific…

OM Solar homes feature a south-facing roof which is hollow, allowing air to circulate inside. As the roof’s exterior warms from sunlight exposure, fresh air is drawn inside from vents within the eaves. As the roof surface heats in the sun, so does the air inside the cavity which rises to the ridge. The hot air then passes through a fan-driven handling unit, forcing it downward through a large vertical duct. The ground floor is typically elevated above the foundation to form a plenum. The hot air enters the interior through floor vents placed throughout the house.

The handling unit also incorporates a built-in heat exchanger to pre-heat the household water supply which is stored inside an insulated tank. The system is controlled through a digital console that constantly reports on system performance and can tell how much CO2 is being saved.

The system has several modes depending on time of day and season. During the winter, the system circulates hot air as described above. At nighttime, air circulation is cut off. Designing the building fabric with thermal mass helps to retain warmth and radiate it throughout the night.

In summer, the hot air can still be used for heating water, but exits the home after it passes through the handling unit. At nighttime, cool air can be channeled downward through the system.

In spring and autumn, the user can balance the distribution of hot air between space and water heating to keep a comfortable internal environment.

Several features optimize the performance of the system. The topmost portion of the roof cavity is glazed to increase solar gain within. A small photovoltaic panel top the ridge powers the handling unit so as not to rely on additional energy inputs. In newer versions, special OM photovoltaic panels can cover the roof which not only convert solar power into electricity, but also transfer heat into the roof cavity in order to warm the air inside. It is also possible to use the system in conjunction with a conventional PV array, but this reduces OM Solar’s heating effectiveness.

Loft and open-plan living arrangements work best, allowing warm air to rise from the floor and freely circulate throughout the interior. Enclosed rooms can still be served by ducts connected to the central hot air supply.

The OM Solar movement embraces a holistic approach to sustainable design. Most interiors appear natural and woody. The use of Japanese timber is promoted in order to reinvigorate a sustainable forestry industry within Japan. However, the system is easily integrated within most conventional construction. From the outside, OM Solar houses look fairly typical, recognizable only by the strip of glazing along the roof ridge.

The OM Solar system is scalable and has been used to build a number of larger public buildings throughout Japan. These include schools, art museums, churches, sports facilities, etc. The OM Solar Association‘s 8.2-acre headquarter complex on a reclaimed eel farm is a notable example. Called ‘Egg of the Earth’ the complex serves as a research hub and educational facility.

Builders train to become OM accredited, learn to use energy assesment tools, and can market themselves as OM contractors. They also gain access to the latest developments and technology. OM Solar appears to be the sole supplier of specialized equipment (air handling unit, console, specialized PV, etc). The Japanese website shows a good number of books have already been written about the system (unfortunately, none in English).

Visit http://omsolar.jp/ for more…