Light-weight Panels Realize Japan's Largest On-roof Solar Plant
Mountable area expanded on roof of glass substrate plant
Following the implementation of the feed-in tariff (FIT) scheme for reusable energy, a number of on-land-type mega solar (large-scale solar) power plants have been constructed one after another. And areas where they can be constructed relatively easily have been developed. As a result, it is expected to gradually become more difficult to cultivate suitable new areas.
Based on these circumstances, from now, the large-sized roofs of plants, warehouses and public facilities are likely to be focused upon as locations to install solar panels. As of today, the largest on-roof-type mega solar power plant in Japan is Asahi Glass Co Ltd's 5MW Kansai Plant Takasago Factory (Takasago City, Hyogo Prefecture).
Asahi Glass' Kansai Plant Takasago Factory is located in the Harima Coast Area in south-central Hyogo Prefecture, where heavy industry is thriving. In the 2000s, the plant supported the company's mainstay business as a core factory of large glass substrates for LCD panels. After installing solar panels with about 5MW output on this factory's roofs, Asahi Glass started selling electricity in March 2013.
Light-weight panels expand mountable area by 20%, output by 1MW
Regarding the power generation business, Asahi Glass said, "We are approaching the business because it's one of our duties as a material manufacturer. We are focusing on the business but do not give priority to the revenue from power sales." For instance, the company said, the factory is aimed at utilizing and verifying materials and technologies intended for Asahi Glass' proprietary solar panels and power generation systems.
The company chose the Takasago Factory's roofs because the largest area of roof could be secured there. The factory's overall site area is approximately 380,000m2. The solar panels were mounted on about 70,000m2 of some buildings' roofs in the factory. As the glass factory already receives power from the extra-high voltage transmission wire, it can easily have a grid connection even with an output of 2MW or larger.
The mega solar power plant has a notable feature: the weight of its solar panel is less than half the weight of other crystalline silicon solar panels generally used at mega solar power plants. Thanks to this, solar panels could be mounted on the roofs, on which panels could not have been previously mounted because of the roofs' insufficient load capacity (Fig. 1).
As a result, the area where solar panels can be mounted was increased by 20%, compared with the system consisting solely of the existing panels. Specifically, the area and output were increased approximately by 13,000m2 and 1MW, respectively. This is reportedly the first case in the world where light-weight panels with an output of about 1MW were installed on roofs.
For installing solar panels on roofs with an insufficient load capacity, two methods have been conventionally used. One is reinforcing the structure to improve the load capacity of the roof. This method, however, increases the installation cost. The other is a reduction in the number of panels to be installed to meet the load capacity. In this case, the power generation amount decreases due to the reduction in the number of installed solar panels.
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Cover glass thickness reduced by 75%
If the weight of a solar panel were reduced, the load capacity required to install solar panels could be lowered, and the number of roofs that can be equipped with panels would greatly increase. There would be the possibility of introducing an on-roof-type solar power generation system to buildings and houses on which the owners have, thus far, not been able to mount solar panels because of their poor load capacity. In addition, the density of installation can be raised on the roofs on which the density of solar panels was restricted.
Specifically, many roofs on which the solar panels were installed this time had a load capacity of 20kg/m2 or more. On these roofs, Mitsubishi Electric Corp's existing solar panel, which weighs 21kg per panel or 12.5kg/m2, was installed (Fig. 2). However, some roofs only had a load capacity of 10kg/m2. Due to the load capacity, the existing solar panel weighing 12.5kg/m2 could not be set up on these roofs. Accordingly, Fujipream Corp's solar panel, whose weight was cut by about half to 9.5kg per panel or 6.4kg/m2, was installed on these roofs.
Of the 70,000m2 roofs mounted with solar panels, light-weighted roofs account for about 13,000m2. The 20,906 crystalline silicon solar panels installed consist of 16,226 existing panels by Mitsubishi Electric (about 4MW) and 4,680 light-weight panels by Fujipream (about 1MW).
According to Asahi Glass, some roofs' load capacity was low depending on the purpose of the building, not when the building was constructed. The company had some reasons not to give priority to the load capacity, such as there being no need to attach a crane on the roof and the necessity to quickly complete the building.
The light-weight solar panel uses Asahi Glass' chemically enhanced glass "Leoflex" as cover glass. This product achieved a thickness of 0.8mm, a sharp reduction from the existing model's 3.2mm. The weight was 75% less than that of the existing model. But its features such as the cover glass strength were maintained.
The weight of the cover glass accounts for 70% or more of the overall weight of a crystalline silicon solar panel and, therefore, is a major factor when cutting the weight of a solar panel. Asahi Glass not only supplied glass but also offered other information including that on its optimum usage.
On the other hand, there is the problem that this new glass costs more than the existing cover glass. But the cost is expected to decline as its deployment increases because Leoflex is manufactured by the float process just like glass substrates for LCD panels and it can easily generate a volume-production effect.
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Installation time cut by half
There is another benefit of reducing the weight of a solar panel: lower installation cost. The panel is easy to set up because it is light, and the easy installation brings a reduction in the installation time.
Compared with the on-land type, work efficiency usually worsens when setting up solar panels on roofs because they are more difficult to walk on than on the ground. The lighter the panel is, the easier the workers can carry it and the more panels can be brought up at one time and kept on the roof. Furthermore, the load on the workers decreases thanks to the safety of installation work being enhanced by the panel's lightness and handiness.
Asahi Glass said it could cut the installation time almost by half when installing this light-weight model, compared with the time required to install the same number of existing solar panels. While a pair of workers carry and install the existing model one by one, three panels can be carried at one time with the light-weight model. Therefore, it cut the installation period by reducing the time to install solar panels.
When lifting the panels 200kg at a time onto the roof, the number of lifts was cut by half because 20 panels can be lifted at a time with the light-weight model, whereas only 10 panels can be lifted at a time with the existing model.
However, Asahi Glass said the benefit of time reduction has not yet been fully enjoyed because the time required for connection is longer compared with the existing model due to the different method to connect panels with the wiring.
To ensure safety, safety ropes were set on the roof first, and, then, a safety net was set up around the roof before starting to install the panels on the roof. The roof was a metal building material called folded-plate roof, in which V-shapes run crossways. The material is widely used in plants and other large facilities. The solar panels were set up using clasps and bolts strong enough to keep the panels from being blown away by the wind without making a hole in the folded-plate roof (Fig. 3).
The solar panels were tilted at about 2° against the horizon when installed. Asahi Glass said this is the angle at which dust and other particles accumulated on the panel can be easily washed off by rainwater. The panels were tilted at 2° by arranging the clasps, regardless of the roof angle on which the panel is installed.
As the connecting boxes are also placed on the roof, a stainless chassis was adopted as a measure against salt damage and strong wind.
Upon consideration of workers' safety, winter and summer were avoided when installing the panels because strong wind and rain occurs on many days in winter while it is hot on the roof in summer. For these reasons, winter and summer are both dangerous for the installation workers.
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Roof's heat insulating effect unexpectedly improved
At this mega solar power plant, not only the state of power generation but also the amount of solar irradiation, wind speed and the temperature of the roofs and solar panels are measured and verified. The verification tests include how much the crystalline silicon solar panel output declines in the high-temperature environment.
Since the operation started, the amount of power generation has outperformed the initial estimates. As factors behind this, Asahi Glass cited the solar panels that have generated more power than the manufacturers' estimates and the amount of solar irradiation that exceeded the average, primarily in spring.
Asahi Glass also revealed that the heat insulating effect of the roofs had unexpectedly improved. Given many high-temperature processes at the glass factory, the company cools down the roofs by setting up sprinklers, watering the roofs and lowering the temperature inside the factory using air-conditioning systems mainly in the hot summer months.
Although the sprinklers were removed from the roofs where the solar power generation system was installed, the solar panels have worked as a heat insulating material, making the temperature inside the factory lower than that before the panel installation. This effect is expected to cut the cost for air-conditioning inside the factory.