Sunshine Power

	Module Selection Technologies Efficiencies Other important factors
	In selecting the right modules the following factors need to be considered: 1. Technologies 2. Efficiencies 3. Other important factors 1. Technologies There are two basic technical alternatives: Crystalline Silicon based Modules Multi Chrystalline Silicon (c-Si) Monocristaline Silicon Thin Film Modules Cadmium Teluride (CdTe)) Amorphouse Silicon (a-Si) Crystalline Silicon based Modules Crystalline silicon, or c-Si, is currently the most commonly used PV technology in the world, owing to its abundance in nature and a mature process technology that leverages the accumulated knowledge base of the semiconductor industry. Essentially, a c-Si cell consists of one or more silicon wafers of a few hundred microns thickness, which are placed on a glass substrate. Crystalline Si PV can further be divided into two broad categories — monocrystalline and multicrystalline silicon. Monocrystalline Silicon Monocrystalline silicon, as the name implies, consists of silicon that exists as a single crystal; the internal structure is completely homogenous, and can be recognized by an even external coloring. Monocrystalline ingots are most often produced by the so-called Czochralski (CZ) process, where a single pure “seed crystal” is dipped into a crucible of molten silicon and then pulled slowly, growing a cylindrical single crystal as the silicon crystallizes on the seed. The cylindrical ingots are then sliced into six or eight inch wafers which are made into circular or square solar cells and mounted and wired together in a weather-resistant package, creating a module. The package is usually the same as for multicrystalline silicon. Typical modules efficiencies are currently 14% - 16 %. As the CZ process is slower and uses more energy the costs per watt yielded are about 20% higher than with multicrystalline silicon. Multicrystalline Silicon Multicrystalline silicon consists of many small silicon crystals that form in a mold. In order to form a multicrystalline silicon ingot, solar grade polysilicon is placed in a rectangular parallelepiped crucible, usually of pure quartz. The large casting is then sawed into bricks, which are sliced into 15 cm2 wafers, which are made into cells. The cells are then mounted and wired together in a weather-resistant package to form modules. The package is normally made from a sandwich of tempered glass, ethyl-vinyl acetate (EVA), cells wired together, EVA, and a back cover. Typical modules efficiencies are currently 12 % - 14 %. Thin Films Thin-film technology utilizes layers only a few microns thick as the light-absorbing material, deposited onto a substrate using a film manufacturing process such as sputtering, deposition, or printing. Unlike crystalline Si, where the substrate must be glass, thin-film PV can use a range of substrates, both rigid and flexible: examples include glass, metal (such as steel or aluminum foil), and plastic. Turning the deposited PV material and substrate into a module requires additional processing to define the individual cells, create the electrical contacts and encapsulate the surface to protect it from natural agents. Unlike c-Si, which requires a multistep, batch manufacturing process, thin-film manufacturing processes can produce modules continuously and at a high speed. Amorphous Silicon (a-Si) Unlike in crystalline Si based cells, the silicon used in a-Si cells has no defined shape. PV modules based on a-Si were the first thin films manufactured and commercialized in large volumes; research into a-Si PV began as early as 1975, when Sanyo produced the first hybrid a-Si on a crystalline silicon cell. Amorphous Silicon can be deposited in layers less than 1 micron thick from gaseous compounds: silane (SiH4), a compound of silicon and hydrogen, is the most common a-Si feedstock. The cells are produced using a variety of deposition techniques at relatively low temperatures (less than 300 degrees C) on a flexible (glass) or rigid (steel, plastic) substrate. Typical or “single-junction” a-Si cells have rather low efficiency (currently about 6.5 percent on average); because of this, the technology has evolved over the years into so-called multijunction cells (discussed subsequently in greater detail). As shown in Figure 2-8, multiple layers of PV material tuned to specific spectral bands are deposited on a substrate and between two layers of electrical contacts to form the front and back contacts. A major drawback of a-Si cells is the tendency of cell efficiency to degrade upon initial exposure to sunlight, a phenomenon termed the Staebler-Wronski effect. An important new technology developed to target this problem combines an amorphous-Si layer with a microcrystalline silicon layer, to form a tandem cell structure called a micromorph cell (a- Si/µSi). The microcrystalline layer exhibits enhanced optical absorption and stability under extended light-soaking conditions, resulting in cells that are both higher efficiency and more stable than traditional a-Si. Cadmium Telluride (CdTe) A compound of the elements cadmium and tellurium, CdTe is another mature and well understood PV material. It has been around almost as long as amorphous silicon, with research beginning in the early 1970s, and is the technology employed by the largest thinfilm manufacturer in the world today, U.S.-based First Solar. Commercial module conversion effi ciencies have been increasing steadily over time, and are currently around 10.4 percent, with highest demonstrated laboratory efficiencies reaching 15.8 percent. CdTe-based cells are produced by depositing four layers of material: a transparent conductive oxide (TCO) layer used as the front electrical contact, a layer of n-type cadmium sulphide (CdS), a layer of p-type CdTe, and a layer of conducting material as the back contact. Glass is the only substrate that can be used for CdTe modules, as the Closed Space Sublimation (CSS) process used to deposit CdTe takes place at extremely high temperature. Given cadmium’s well-established toxic nature, some have voiced concerns regarding the safety of CdTe modules. However, a U.S. Department of Energy study put these fears to rest, concluding that CdTe is more stable than elemental cadmium, and that large-scale use of CdTe PV modules did not pose any significant health or environmental risks given the implementation of module recycling programs. Copper Indium (Gallium) DiSelenide (CIS/CIGS) Copper Indium Gallium DiSelenide (CIGS) and Copper Indium DiSelenide (CIS) are semiconducting composites with high optical absorption coefficients, which leads to high conversion efficiencies relative to other thin-fi lm products. CIGS cells also exhibit excellent stability over an extended period of time; additionally, the CIGS material itself is robust and defect-tolerant, and can be deposited on both rigid and fl exible substrates. Most importantly, however, CIGS cells have the potential to be produced with high-speed manufacturing technologies (some companies have claimed 100 to 1,000 feet/minute), which would mean signifi cantly lower capex per watt than the competition. Because of these characteristics, CIGS has been hailed a potential “game-changer” and companies that have adopted CIGS have been the subject of considerable media and investor attention over the past 18 months. Progress, however, has been rocky; technology setbacks and longer-than-expected ramp times have plagued many companies in the space, and only in the last half of 2008 did some manufacturers succeed in ramping up production at scale. Presently, production efficiency for CIGS modules is around 10 percent. 2. Efficiencies / Returns The different module types described above have the following efficiencies: Crystalline Silicon (c-Si) Multichrsitalline Silicon: 12 – 14% Monochristalline Silicon: 14 – 16% Thin Film Cadmium Teluride (CdTe): approximately 10,4% Amorphous Silicon (a-Si): up to 8,3% Copper Indium (Gallium) DiSelenide (CIS / CIGS): 10% (19,3% record cell efficiency) In general, since crystalline silicon modules are on average more efficient than thin film modules, it is possible to install more kwp capacity on a specific space with crystalline modules. However, given the often higher kwh / kwp production of thin film modules vs. c-Si modules, in many cases the total production of thin film modules is not substantially lower than the production of c-Si modules. Taking into account the lower cost of thin film modules vs. c-Si modules (-15 – 25%), projects using thin film modules often time have a higher IRRs. Note, however, each project is different based on the exact location, inclination of the roof and orientation. Hence, only after an exact analysis of the situation it is possible to make a good final assessment. 3. Other important factors Besides the described technical and financial factors, the following important questions have to be asked when selecting a module: 1. Information on the module company What is the general reputation of the company producing the module? How long is the producing company active? What is the production capacity of the company? What reference projects does the company have in my region? What is the financial strength of the company? Will the company still exist in 20 years from now? Does the company have a subsidy in my country? Is the place of jurisdiction in my country? 2. Guarantees and Warranties How long do guaranties and warranties last? 3. Product Performance What is the yearly efficiency loss over 20 years? What is the statistical deviation from the projected module performance? Does the company offer a “plus tolerance”, i.e. is there a guarantee that over 90% of the modules perform above a defined level? 4. Damages What happens in the case of damages? Who is responsible? How much time does it take until the damaged modules are exchanged by new ones? Sunshine Power helps you assess and answer all the questions raised above and further questions you might have. Given the high investments that flow into solar projects and the long duration of such projects it is crucial to carefully consider and study all the factors mentioned above.