Rooftop Solar Plant Prevents Roof Damage From Typhoon

In Japan, a mega- (large-scale) solar power plant with 4,354 solar panels equivalent to roughly 1MW of output is located on the rooftop of a warehouse in Yamanashi Prefecture.

This power plant was constructed and named “Kai-no-kuni Mega Solar Station” by Saito Warehouse Co Ltd (Chofu City, Tokyo), which developed and runs this warehouse.

The company considered its warehouse suitable for the power generation business.

Yamanashi Prefecture boasts the longest hours of sunshine in Japan, and there are no high-rise buildings that would block sunshine around the warehouse.

Saito Warehouse also considered the business risk was minimal as long as a secure power generation system was set up, and it could easily continue the power selling business even after the FIT-based purchase period is over, selling power at a low rate to the tenant companies in the warehouse, for example.

The company says it is more wary than industry peers in its mainstay warehouse business about risks related to long-term reliability and damage. It adopted safer and more reliable facilities as much as possible even though initial investment slightly increased and secured more-than-enough load bearing capacity for the roofs where the solar panels were set up.

Such a stance has also generated unexpected benefits, leading to minimizing problems caused by natural disasters after power selling started.

To boost the load bearing capacity of the roofs, Saito Warehouse invested about three times the cost originally required. These costs came from the measures to meet the required load bearing capacity of about 14kg/m2, which some areas of the roofs could not realize after arraying solar panels equivalent to roughly 1MW on them.

The company secured the required load bearing capacity in these areas by reinforcing the roofs. Saito Warehouse could have realized the roof reinforcement just to meet the original load bearing requirements at about 1.5 million yen. However, considering further safety improvements to cope with unexpected circumstances, it eventually spent about 5 million yen, more than three times the 1.5 million yen, to reinforce the roofs.

Tess Engineering Co Ltd (Osaka City) provided engineering, procurement and construction (EPC) services. Solar panels of Kyocera Corp and PV inverters manufactured by Toshiba Mitsubishi-Electric Industrial Systems Corp (TMEIC) were adopted.

Both products were selected according to Saito Warehouse’s policy to choose products of Japanese manufacturers with excellent long-term reliability, in particular.

Every year since power generation began in June 2013, the solar plant has generated about 10% more power than initially estimated in the business plan, even though it has been struck each year by problems affecting the power generation amount. The power generation amount outperformed the estimate by about 13% in 2017 and about 10% in 2018.

Compared with the annual power generation of about 1,190,000kWh estimated in the initial business plan, the actual amount ranged from 1,270,000 to 1,300,000kWh per year, with the total amount reaching 7,480,000kWh.

Setting up solar panels on warehouse roofs brought about benefits other than power generation. For instance, solar panels boost the heat insulating effect of the roofs.

Solar panels keep the high temperature outside from transferring through the roof to inside the warehouse. This can reduce air conditioning costs in hot summers. On the other hand, solar panels block the sun’s heat in winter, but they are more effective in keeping the temperature inside the warehouse from falling and help the heating system work better.

Such effects could even improve with a completely closed building. However, as warehouses are used with the shutters on the sidewalls kept open, it seems difficult to quantify the degree of these effects.

Even so, Saito Warehouse said it could realize the effects. For example, the room temperature used to rise above the set temperature when the air conditioner was set at 28° on a hot summer day, but it can be maintained at 28° since the solar panels were installed. Of the two buildings at the warehouse in Kofu, the power bills of the tenant companies in the old building have decreased noticeably, according to the company.

When a typhoon passed through and caused serious damage around Kofu in the fall of 2018, Saito Warehouse realized that the rooftop solar panels seemed to have eased the damage from the strong winds on the roofs themselves.

This typhoon damaged building shutters and roof materials in the surrounding area and knocked over hundreds of trees at golf courses. The warehouse in Kofu is located near a river, where winds grow stronger, in particular, in a typhoon. As a result, part of the roof materials called “frieze boards,” which protect the edge of the roof, came off after being bent up from below.

The company believes that if it had not been for the solar panels on the roof, the damage would have extended to a wider range or to other roof materials. Saito Warehouse feels that the solar panels effectively worked like an anchor.

Following the typhoon, the company discovered the cover glass of one solar panel had been broken among the panels on the rooftop This is the only panel that has ever broken at the rooftop mega-solar plant.

Although the cause is not yet known, the company believes that the typhoon’s strong winds caused an object from outside the warehouse to hit the solar panel.

Not only is the cover glass broken, but also some areas are burnt due to cell (power generation element) damage in the broken solar panel The burns appear to have been generated after the cover glass was broken.

The plant has been struck by heavy snow. However, the company’s previously mentioned efforts to boost load bearing costs proved effective.

In February 2014, about six months after the operation began, record-breaking heavy snow fell for two consecutive weeks in the Kanto-Koshin region. The average snow cover in Kofu City reached 1.40m. The second snowfall accumulated on the roofs at the warehouse before the first snow cover completely melted.

After the heavy snow, the tenant companies focused on restoring the warehouse first so it could again operate as usual. What proved effective was the roofs’ load bearing capacity which had been reinforced more than necessary.

The load bearing capacity required to mount solar panels on the roof was about 14kg/m2. This value is equivalent to the weight of water on the rooftop up to 1.4cm in height. However, the actual load of snow is even heavier. If the reinforcement had been limited to the minimum, the reinforced areas might have been damaged, according to the company. Part of the frieze boards on the roof bent as a result, but the roofs themselves remained intact.

About three weeks after the second snowfall, the snow on the panels could finally be removed as a lot of the panel surfaces became visible. The company had refrained from removing the snow on the panels until then, because it could not secure areas to dump the snow removed from the roof, which was about 10,000m2. As a result, the plant could only generate sufficient power for about one week during this month until it was hit by the first snowfall.

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