Double-sided Solar Panels Turn Snow Into Advantage

Power generation efficiency improves even in winter

2014/02/09 15:24
Kenji Kaneko, Nikkei BP CleanTech Institute

Asahikawa City, Hokkaido, is known as one of the coldest cities in Japan. The average depth of snow is about 70cm, which is less than in other heavy snowfall areas, such as the Joetsu region, but the fallen snow freezes and hardly melts in midwinter because the temperature often goes below freezing even in the daytime.

In July 2012, when the feed-in tariff (FIT) program was implemented, leading companies visited Asahikawa in search of a place to build a mega (large-scale) solar power plant. However, no leading company has ever constructed a mega solar power plant there owing to the risk regarding the sustainability of the business.

Snow slips off solar panels

On November 29, 2013, "Asahikawa Hokuto Solar Power Plant," with 1.25MW output, started operation in the region that seemed ill-suited for a mega solar power plant (Fig. 1). Local electric facility company Nishiyama Sakata Denki Co Ltd (Asahikawa City, Hokkaido) has been engaged in the plant's design, construction, maintenance and power generation business. Ahead of Asahikawa Hokuto Solar Power Plant, Nishiyama Sakata Denki also launched "Asahikawa Kuranuma Solar Power Plant" with 250kW output Sept 30, 2013 (Fig. 2). The company runs these two solar power plants in Asahikawa City.

In late January 2014, I visited these two plants to gather information. The weather was fine that day, but in the city there was about 60cm of snow that had fallen three days earlier. Asahikawa Kuranuma Solar Power Plant with 250kW output is a 20-minute drive from the downtown area. Despite the snowy landscape in the surrounding area, almost no snow was found on the solar panels (Fig. 3).

"As soon as the solar panels start generating power under the sun, the snow on the panels naturally starts slipping off, melting as the temperature of the panel surface rises," said Satoshi Nishiyama, director and chief of the Corporate Planning Department, Nishiyama Sakata Denki.

Snow should be removed before it consolidates

"When the snow slips off the panels, the fallen snow forms banks under the panels," Nishiyama said. "And the snow only slips off the panels until the snow banks reach the height of the solar panels. At Asahikawa Kuranuma Solar Power Plant, snow easily fell off the panels because the snow banks under the panels had been removed by heavy machinery. At Asahikawa Hokuto Solar Power Plant, on the other hand, fresh snow fell onto the snow banks before they were removed, and, consequently, some snow remained on the panels without slipping off."

He regretted that the company had delayed snow removal.

I also visited Asahikawa Hokuto Solar Power Plant. The snow banks under the panels had adhered to the snow on the panels, and the snow remained on a large portion of the panels (Fig. 4). The plant is in a suburb a 10-minute drive from the downtown area of Asahikawa City. Using the grounds of a closed high school, the entire mega solar power plant can be looked down upon from the parapet of a nearby bridge, which is positioned a little higher than the surrounding area (Fig. 5). It is an ideal spot for a mega solar power plant close to citizens.

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In these two plants of Nishiyama Sakata Denki, the solar panels are tilted at 40° and set at 1.8m from the ground (Fig. 6). This was for the purpose of making the snow easily slip off the panels and preventing the fallen snow from adhering to the panels. At mega solar power plants in snowy regions, solar panels are generally tilted at 30 to 40° and set at about 2m from the ground.

In Asahikawa City, there are some other solar power plants run by local enterprises. At these plants, the tilted angle and the height of the solar panels are almost the same as those at Nishiyama Sakata Denki's two plants. But on the same day when I made my inspection visit, the snow on their panels remained almost without slipping off at all (Fig. 7).

At Asahikawa Hokuto Solar Power Plant, where snow removal had been delayed, the amount of snow remaining on the panels was less than that at other enterprises' solar power plants though some snow that could not completely slip off was left on the panels.

Double-sided solar panel with top-class efficiency

Nishiyama Sakata Denki revealed it had not expected that the double-sided panel would be so effective in making snow slide off. In fact, it was the first company that ever adopted a double-sided solar panel for a mega-class solar power plant. Double-sided panels use a cell (power generating element) that generates photovoltaic power even on the reverse side.

Normal solar panels have a white plastic component called "back sheet" on the reverse side (Fig. 8). On the other hand, the adopted double-sided solar panel is covered with a transparent sheet on its reverse side so that it can absorb sunlight from both sides and generate power. Compared with normal single-sided panels, double-sided panels can generate more power because they can generate power on the reverse side.

Nishiyama Sakata Denki deployed the double-sided solar cell "EarthON" developed and manufactured by PV cell venture PVG Solutions Inc (Kohoku-ku, Yokohama City). Although the principle of double-sided solar cells has been known for quite a while, few manufacturers have commercialized the technology due to challenges such as the difficulty in boosting the conversion efficiency on the reverse side.

Having established a volume production technology that can boost conversion efficiency on the reverse side, PVG Solutions launched its double-sided solar panel product. The company manufactures the cells at its plant in Saijo City, Ehime Prefecture, and entrusts the panel assembly to manufacturers in China and Taiwan.

Nishiyama Sakata Denki decided to use the double-sided solar panel, considering such a panel that can also generate power on the reverse side to be more advantageous in snowy regions, where the reverse side of solar panels can easily receive sunshine reflected by snow on the ground.

Regarding the conversion efficiency of double-sided solar panels, there is no official figure that reflects the amount of power generation on the reverse side. It is because there is no measurement reference that assumes double-sided models in the current solar panel standards. PVG Solutions conducted verification tests for power generation using double-sided solar panels with the Kitami Institute of Technology and others in 2012.

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Based on the results of such verification tests and other factors, PVG Solutions concluded that the conversion efficiency of the panel using "EarthON" is about 18%, including the reverse side, when the ground surface is covered with snow, and 16 to 17% when the ground surface is soil, according to Director Shinji Horiuchi of PVG Solutions. Should the amount of reflection be increased by coloring the ground surface white, the conversion efficiency would reach the highest level among current mass-produced solar panels.

Nishiyama resolved to use the double-sided solar panel after visiting the verification site in Kitami City. He considered that snow would even reinforce the solar power generation business if the double-sided panel were used. To increase the amount of power generation on the reverse side of the panels during the seasons without snow, Nishiyama Sakata Denki is now considering scattering white materials on the ground.

Overcoming hurdles of securing site, fund-raising

Having entered the first winter since the double-sided panel-based mega-solar power plant started operation, it gradually became apparent that the panels are very effective in making snow slip off. It is already known that the cell temperature rises by 20 to 30° as the solar panel starts generating power, and thus melts any snow accumulated on the panel. However, once a large amount of snow fell at one time and accumulated on the panel, sunlight could barely reach the panel. In this case, the snow-melting effect accompanied with power generation cannot be guaranteed.

With the double-sided solar panel, however, the panel would generate power from the sunlight reflected onto the reverse side of the panel once the sun comes out, even after heavy snow. Then the panel temperature would rise and easily melt the snow on the panel. Although this had been expected, the effect gradually turned out to be larger than expected.

"After the plant started operation, the amount of power generation slightly fell short of the estimate in October," Nishiyama said. "But it has outperformed the estimates since November."

The plan to make snow assist the business is getting on track.

Nishiyama Sakata Denki's mega solar power plant business appears to be progressing smoothly at present. But it faced a number of challenges at the beginning. Besides the double-sided solar panel, the plant chose a reliable PV inverter manufactured by Toshiba Mitsubishi-Electric Industrial Systems Corp (TMEIC) (Fig. 9). However, Nishiyama Sakata Denki initially faced difficulty in finding a site.

Although there are many large pieces of land around Asahikawa City, most of them are designated for agricultural use. The areas that are not specified as farmland are mostly unsunny and hence unsuited for a large-scale solar power plant. Nishiyama Sakata Denki was on the verge of giving up when Asahikawa City adopted its mega solar power plant proposal after a public appeal to use the grounds of a closed municipal high school.

The next challenge was fund-raising. Should Nishiyama Sakata Denki directly borrow funds from financial institutions, its capital adequacy ratio would decrease and its corporate rating would degrade as a result. This would make it difficult for the company to receive orders for public projects. Based on these circumstances, a method was applied in which an anonymous partnership financed a special purpose company (SPC) formed by a joint venture.

Through this method, the company succeeded in taking out ¥600 million (approx US$5.87 million) loans for the SPC from Japan Finance Corp and some regional banks. The challenges concerning land and fund-raising were solved this way. Nishiyama Sakata Denki's mega power plant business is likely to be a good reference for local enterprises in terms of both technology and fund-raising.