Fire hazard and other safety concerns of photovoltaic systems

Photovoltaic (PV) modules are usually considered safe and reliable. But in case of grid-connected PV systems that are becoming popular, the issue of fire safety of PV modules is becoming increasingly important due to the employed high voltages of 600 to 1000 V. The two main factors, i.e., open circuiting of the dc circuit and of the bypass diodes and ground faults that are responsible for the fire in the PV systems, have been discussed in detail along with numerous real life examples. Recommendations are provided for preventing the fire hazards such as designing the PV array mounting system to minimize the chimney effect, having proper bypass and blocking diodes, and interestingly, having an ungrounded PV system

Computation of photovoltaic parameters under lunar temperature variation

Photovoltaic (PV) arrays with regenerative-fuel-cell energy storage is a prime, power-system candidate for lunar photo-power. The PV module performance decreases at higher temperatures. Surface temperature variations of the moon are extreme, the maximum (noon) temperature being 384 K. The present work utilizes detailed computations of photovoltaic parameters with computer program developed earlier for the computation of optimum bandgaps of single- and two-junction solar cells at different temperatures, and calculates the power output of single and two-junction solar modules under different configurations which constitutes an improvement over the assumption of a linear variation of efficiency with temperature. The program also calculates the necessary PV-array size to satisfy stipulated levels of day- and night-time power consumption

Device for detailed analysis of leakage current paths in photovoltaic modules under high voltage bias

High voltages used in photovoltaic (PV) systems are known to induce long-term power loss in PV modules due to leakage current flowing through the module packaging materials. It has been difficult to identify the specific materials and interfaces responsible for degradation based on an analysis of only the total leakage current. A detailed investigation of the leakage current paths within the PV modules, under high voltage bias, is carried out by utilizing a device that measures the independent contributions of various paths in real-time. Knowledge about dominant leakage current paths can be used to quantify the physical and chemical changes occurring within the module packaging materials

Preparation and characterization of transparent conducting ZnTe : Cu back contact interface layer for CdS/CdTe solar cell for photoelectrochemical application

FSEC Photovoltaic Materials Laboratory has developed a photoelectrochemical (PEC) cell using multiple band gap tandem of thin-film photovoltaic (PV) cells and a photocatalyst for hydrogen production by water splitting. CdS/CdTe solar cell, a promising candidate for low-cost, thin-film PV cell, is used as one of the thin-film solar cells in a PEC cell. The back contact has been developed for a CdS/CdTe solar cell which involves the deposition of a primary p-type ZnTe:Cu back contact interface layer followed by the deposition of transparent and conducting ZnO:AI and Ni-Al outer metallization layer. This article presents preparation and characterization of ZnTe:Cu back contact interface layer deposited by hot wall vacuum evaporation (HWVE) technique. HWVE technique has produced highly stoichiometric ZnTe:Cu thin films with cubic phase having {111} texture orientation and produced better transparency in the near infrared region on a glass substrate. (c) 2006 American Vacuum Society

Long-term performance analysis of copper indium gallium selenide thin-film photovoltaic modules

Current accelerated qualification tests of photovoltaic (PV) modules mostly assist in avoiding premature failures but can neither duplicate changes occurring in the field nor predict useful product lifetime. Therefore, outdoor monitoring of field-deployed thin-film PV modules was undertaken at FSEC with the goal of assessing their performance in hot and humid climate under high system-voltage operation. Significant and comparable degradation rate of -5.13 +/- 1.53% and -4.5 +/- 1.46% per year was found using PVUSA type regression analysis for the positive and negative strings, respectively of 40W glass-to-glass Cu-In-Ga-Se (CIGS) thin-film PV modules in the hot and humid climate of Florida. Using the current-voltage measurements, it was found that the performance degradation within the PV array was mainly due to a few (8% to 12%) modules that had substantially higher degradation. The remaining modules within the array continued to show reasonable performance ( \u3e 96% of the rated power after similar to four years)

Cu(In,Ga)S2, Thin-Film Solar Cells Prepared by H2S Sulfurization of CuGa-In Precursor

Thin-film CuInS2 solar cell is the leading candidate for space power because of bandgap near the optimum value for AM0 solar radiation outside the earth's atmosphere, excellent radiation hardness, and freedom from intrinsic degradation mechanisms unlike a-Si:H cells. Ultra-lightweight thin-film solar cells deposited on flexible polyimide plastic substrates such as Kapton(trademark), Upilex(trademark), and Apical(trademark) have a potential for achieving specific power of 1000 W/kg, while the state-of-art specific power of the present day solar cells is 66 W/kg. This paper describes the preparation of Cu-rich CuIn(sub 1-x)Ga(sub x)S(sub 2) (CIGS2) thin films and solar cells by a process of sulfurization of CuGa-In precursor similar to that being used for preparation of large-compact-grain CuIn(sub 1-x)Ga(sub x)Se2 thin films and efficient solar cells at FSEC PV Materials Lab

Study of molybdenum back contact layer to achieve adherent and efficient CIGS2 absorber thin-film solar cells

Molybdenum is used as back contact layer in I-III-VI2 compound thin-film solar cells. Mo film was sputter deposited on 125-mm-diameter Si wafer having 100 orientation using dc magnetron sputtering. Films with similar parameters were also deposited on 2.5 cm x 10 cm soda-lime glass for studying the adhesion to the substrate and chemical reactivity of molybdenum with H2S gas at 475 degrees C for 20 min. Mo being refractory material develops stresses. It is essential to deposit stress-free and relatively inert Mo films in order to achieve well adherent and highly efficient CuIn1-xGaxS2 absorber thin film solar cells on flexible metallic foil and glass substrates. Earlier have shown that films deposited at sputtering power of 300 W and 0.3 x 10(-3) Torr working argon pressure develop compressive stress, while the films deposited at 200 W and 5 x 10(-3) Torr pressure develop tensile stress. Four sets of experiments were carried out to achieve optimum deposition cycle to deposit stress-free Mo. In the first experiment, Mo thickness of 138 nm was deposited at 300 W power and 0.3 x 10(-3) Torr pressure. In the second experiment Mo thickness of 127 nm was deposited at power of 200 W and pressure of 5 x 10(-3) Torr. Two more experiments were carried out by using alternate layers to reduce the overall stress. In a third experiment, two high power cycles were sandwiched between three low power cycles with total film thickness of 330 nm. In a fourth experiment two low power cycles were sandwiched between three high power cycles resulting in effective thickness of 315 nm. This article describes the wafer bending analysis for stress measurement, x-ray diffraction for crystal quality, scanning electron microscopy for surface morphology and Auger electron spectroscopy for the extent of sulfur diffusion in Mo layer. (c) 2005 American Vacuum Society

Development of CIGS2 Thin Films on Ultralightweight Flexible Large Area Foil Sunstrates

The development of thin film solar cells is aimed at reducing the costs for photovoltaic systems. Use of thin film technology and thin foil substrate such as 5-mil thick stainless steel foil or 1-mil thick Ti would result in considerable costs savings. Another important aspect is manufacturing cost. Current single crystal technology for space power can cost more than $300 per watt at the array level and weigh more than 1 kg/sq m equivalent to specific power of approx. 65 W/kg. Thin film material such as CuIn1-xGaxS2 (CIGS2), CuIn(1-x)Ga(x)Se(2-y)S(y) (CIGSS) or amorphous hydrogenated silicon (a-Si:H) may be able to reduce both the cost and mass per unit area by an order of magnitude. Manufacturing costs for solar arrays are an important consideration for total spacecraft budget. For a medium sized 5kW satellite for example, the array manufacturing cost alone may exceed$ 2 million. Moving to thin film technology could reduce this expense to less than $ 500K. Earlier publications have demonstrated the potential of achieving higher efficiencies from CIGSS thin film solar cells on 5-mil thick stainless steel foil as well as initial stages of facility augmentation for depositing thin film solar cells on larger (6 in x 4 in) substrates. This paper presents the developmental study of achieving stress free Mo coating; uniform coatings of Mo back contact and metallic precursors. The paper also presents the development of sol gel process, refurbishment of selenization/sulfurization furnace, chemical bath deposition (CBD) for n-type CdS and scrubber for detoxification of H2S and H2Se gases

Technical Paper Session I-B - CIGSeS and CIGS2 Thin Film Solar Cells on Flexible Foils for Space Power

The objective of the research is to develop flexible, lightweight, radiationresistant, high-specific-power, highly efficient CuIn1-xGaxSe2-ySy (CIGSeS) and CuIn1- xGaxS2 (CIGS2) thin-film solar cells for space electric power. The near optimum bandgap, potential for higher specific power, and superior radiation resistance make this technology an ideal candidate for space electric power. The superior radiation resistance of CIGSeS thin-film solar cells relative to the conventional silicon and gallium arsenide single-crystal cells in the space radiation environment would extend mission lifetimes substantially. The conventional rigid Si and GaAs cells must be folded in an accordion style for deployment space. This can cause problems of opening up and folding of the solar array as has happened recently with the International Space Station. On the other hand, the flexible solar cells and modules can be packaged and rolled out more easily. The stainless steel and titanium foil substrate materials are capable of withstanding high temperatures required for preparing good quality CIGSeS absorber layer. They also do not sag easily and hence do not require rigidizing as is the case with plastic sheet substrates. The CIGSeS absorber film is prepared by selenization/sulfurization of DC magnetron sputter-deposited CuGa, In metallic precursors on 10 cm x 10 cm metallic foil substrate coated with molybdenum back contact layer. CdS heterojunction partner is deposited by chemical bath deposition. Transparent and conducting bilayer of intrinsic ZnO and aluminum doped ZnO:Al is deposited by RF magnetron sputtering. Cells are completed by depositing Ni/Al front contact fingers by thermal evaporation. The sputtering technique utilized in the preparation of solar cells provides an added advantage of facilitating easy scale-up of the laboratory size cells for economic large-area manufacture by the roll-to-roll process. Chemical composition, crystallographic structure and morphology of CIGSeS thin films are analyzed by energy dispersive spectroscopy, Auger electron spectroscopy, X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The photovoltaic properties of completed cells are studied by measurement of current-voltage characteristics and quantum efficiency. Best efficiencies of 10.4% under AM 1.5 conditions and 8.84% under AM 0 conditions were achieved on small-area CIGS2 thin-film solar cells