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In 1996 Canon Inc. Ecology R&D Center In Kyoto Japan had developed amorphous silicon photovoltaic (PV) cell integrated roofing module. This module can work as roof itself and be installed by the same manner as existing metal roof element. This module have already been installed as roofs inover one hundred homes in Japan. The same concept modules have also been installed in other applications. This module has been proved to be high value added advanced metal roof and is expected to help future dissemination of metal roof.
With the assistance of the Japanese Government's subsidy policy, the grid-connected residential PV system is about to take off in Japan. Integration of PV into roof covering materials must be the ideal combination, but actually few products have been successful in the past. Different from a conventional glass covered solar modules, Canon's module has resin film lamination structure which leads to unique features as hybrid roofing material with synergy effect. Canon's triple stacked amorphous silicon(a-Si) solar cell is flexible, since the a-Si layer is formed on the thin stainless steel substrate. (Fig.1 left) Flexible cell is embedded in the thermoplastic adhesiveresin between metal back plate and fluoro resin film. (Fig.1 right) Having this module structure Canon's module can be bent into the same designs as existing metal roof element for example. The advantage in this case is that basic roof performance such as water tightness and wind resistance is assured at the same level as existing metal roof, secondly the PV roof module can be sheathed mixed together with normal metal roof element without PV side to side and thirdly a-Si solar cell is expected to perform better under higher temperature like rooftop. This paper reports the primary features of Canon's PV roof module and its installation examples.
1. Primary Features
1. Photovoltaic (PV) Cell
Triple Stacked Amorphous SIlicon - Conversion Efficiency = 8% - Flexible and not fragile
2. PV Cells Integrated Roofing Module
Series connected PV cells were embedded in adhesive layer between steel backing plate and fluoro resin cover film.
Can be bent into desired roof panel shape by a roller forming machine or a bending machine.
4. Excellent Weatherability
Accelaration tests show an excellent weatherability of the module over 20 years.
5. Compatibility with Conventional Method
Design Compatibility - Installation Practices Compatibility
6. No Maintenance
Requires no painting, no cleaning
2-1 Machine Processability
2. Major Special Features of PV Roofing Module
The marginal parts of the module allow bending process by a conventional roller forming machine to get proper roofing panel configuration and also be pressed by press machine to the minimum radius of about 3 mm.
Even solar cell area can permit somewhat weaker bending to get wavy form for example.
Two major materials which constitute this module are the resin lamination and steel back plate. The resin lamination has shown an excellent weather resistance in accelarated weathering tests so far. The laminated test samples were subjected to outdoor exposure test (EMMAQUA) at DSET laboratories Inc. in Arizona. Eight times concentrated solar irradiation has been irradiated to the samples for about a year whcih corresponds to about 10 years stress in Tokyo. Another test is accelarated UV irradiation in the laboratory test chamber which is equivalent to 30 years stress in Tokyo. The samples were evaluated in terms of discoloration, peeling, electrecal insulation and power generation efficiency in both tests. No anomaly results was obtained in any test so far.
Fig 3 shows the result of acid rain corrosion test conducted on steel sheet samples. Laminated steel plate has shown a noticeable improvement of surface corrosion resistance thanks to the fluoro resin film lamination. Commercially availble steel roof plate with and without lamination were subjected to the cycle of acid spray and dry up for 48 days whcih is estimated equivalent to 15 to 30 years of outdoor exposure in Tokyo. Standard galvanized steel sheet without lamination showed heavy white rust and red rust over whole surface, whereas the laminated surface showed no corrosion except edge corrosion. 55% aluminum-zinc alloy plated steel sheet showed less white rust even on inside surface. No corrosion was observed on laminated surface either and edge corrosion was also observed.
Fig 3 Results of Accelarated Acid Rain Corrosion Test of Steel Roof Sheets
It is often said that metal roof is noisy when rain drops. Small stell ball with 8mm in diameter was fallen on to a conventional metal roof and on the PV roof to measure the magnitude of noise level. Result shows that PV roof generated about 93dB whereas metal roof did 103dB in terms of "A characteristics" which corresponds to human ears' sound sensitivity. It is commonly understood that 10dB difference corresponds to two times in magnitude.
2-3 Reduced Noise
3-1 Fire Resistance
3. Major Features as Roof Structure
Flat roof and stepped roof have passed stringent fire tests and obtained the fire resistance certificate issued y the Ministry of Construction. This is the first certificate in PV integrated roof in Japan. Fig. 3 shown the illustration of cross sectional structure of flat roof of this type.
Flat roof with 0.8mm thick steel back plate has more than 1000 Kg/m2 strength against press and pull stress caused by wind, which is thought enough strength in any architechtureal conditions. Over 400 kg/m2 strength was obtained even with 0.4mm thick steel back plate in slide-in type. Once the metal sheet is bent into proper configuration, required wind resistance can be obtained.
3-2 Wind Resistance
Fig. 4 Wind Resistance Test
Fig. 3 Fire Resistant Structure
4. Electrical Safety
4-1 Electrical Insulation
Since the surface of the module is covered by resin film, strength against surface scratch is inevitable concern in terms of electrical insulation. UL (Underwriters Laboratories Inc., USA) 1703 stipulates the test procedure for surface cut susceptivility for solar module. The surface of the module is cut by knife edge and evaluated by measuring leak current under wet condition and high voltage application. The module has been proved to well exceed the criterion.
4-2 Anti-Fire Breakout
In a string of solar modules in which each module is electrically connected in series, if some of the modules or cells are shaded by obstacles, shaded cells generate less current than fully irradiated cells. The surplus current would go through shaded cells forcibly with the result of heat generation at the shaded cells if cells don't hace bypass circuit. Every cell in Canon's PV module has equipped with bypass circuit (bypass diode) which can eliminate the possibility of heating and minimize powergeneration loss due to shading. But assuming an open mode failure of bypass diode, the reverse current fire test has been conducted on the special module without bypass diodes which is also stipulated in UL1703. No fire flame was observed in the special module which means double safety redundancy of our module.