|SEMICONDUCTOR NON-TRADITIONAL ENERGY SOURCES|
The present photovoltaic market is dominated by "first generation"
product based in silicon wafers, either single-crystalline as in microelectronics,
or lower grade multicrystalline wafer. The sharp growth of the price of power
resources has forced to reconsider in the field of development of solar elements.
Price became the determining factor at an estimation of prospects of solar converters
Solar cells have a lifetime of approximately 30 yr. They incur no fuel expenses, but they do involve a capital cost. The cost for the electricity produced by the cell is calculated by amortizing the capital cost over the lifetime of the cell and considering the total electrical output energy produced over the cell lifetime. Higher PV efficiency thus directly impacts the overall electricity cost, because higher-efficiency cells will produce more electrical energy per unit of cell area over the cell lifetime. The cost figure of merit for PV cell modules ($/Wp) is determined by the ratio of the module cost per unit of area ($/m2) divided by the maximum amount of electric power delivered per unit of area (module efficiency multiplied by 1,000 W/m2, the peak insolation power). In Figure 2, this cost per peak watt ($/Wp) is indicated by a series of dashed straight lines having different slopes. Any combination of areal cost and efficiency that is on a given dashed line produces the same cost per peak watt indicated by the line labels. Present single-crystalline Si PV cells, with an efficiency of 10% and a cost of $350/m2, thus have a module cost of $3.50/Wp. The area labeled I in Figure 2 represents the first generation (Generation I) of solar cells and covers the range of module costs and efficiencies for these cells.
In addition to module costs, a PV system also has costs associated with the non-photoactive parts of the system. These are called balance of system (BOS) costs, and they are currently in the range of $250/m2 for Generation I cells. Thus, the total cost of present PV systems is about $6/Wp. Taking into account the cost of capital funds, interest rates, depreciation, system lifetime, and the available annual solar irradiance integrated over the year (i.e., considering the diurnal cycle and cloud cover, which produces an average power over a year that is about 1/5 of the peak power rating), the $/Wp cost figure of merit can be converted to $/kWh by the following simple relationship: $1/Wp ~ $0.05/kWh. This calculation leads to a present cost for grid-connected PV electricity of about $0.30/kWh. Areas labeled II and III in Figure 2 present the module costs for Generation II (thin-film PV) and Generation III (advanced future structures) PV cells. Figure 3 presents the historical progress of the best reported solar cell efficiencies to date (Surek 2005). The efficiencies of commercial (or even the best prototype) modules are only about 50Ц 65% of the efficiency of the best research cells. The plot includes the various PV technologies of single-crystal Si, thin films, multiple-junction cells, and emerging technologies Ч such as dyesensitized nanocrystalline TiO2 cells and cells based on organic compounds.
Over the past decades, improvements have also been made in a second important metric, the manufacturing cost of PV modules. The prices of PV modules have followed a historical trend along a so-called У80% learning curve.Ф That is, for every doubling of the total cumulative production of PV modules worldwide, the price has dropped by approximately 20%. This trend is illustrated in Figure 4 (Surek 2005). These data are based on annual surveys conducted by PV News (PV Energy Systems 2004). The final data point for 2003 corresponds to about $3.50/Wp and a cumulative PV capacity of 3 GW. An important issue, in terms of future projections, is how this price-reduction trend will continue in the future. As Figure 4 shows, a major reduction in the projected future cost of PV modules depends upon the introduction of thin films, concentrator systems, and new technologies. The third significant metric for PV cells is module reliability. Today, most crystalline Si module manufacturers offer warranties of 25 years, typically guaranteeing that the power output of the module will not decrease by more than 20% over this period.
The new generation of photo-electric converters should first of all meet the requirements of mass manufacture and low price. It is necessary to hope, that this task will solved in a near future. For the benefit of it s growing assignments for creation of manufactures of the second generation batteries and development of batteries on essentially new principles speak.
Thin film solar cells on the base of amorphous silicon (a-Si:H)
Until now the base materail for production of thin film panels was a-Si:H. Hidrogen is used in order to passivate dangling bonds in amorphous film. A contaminaton of hidrogen influences on the a-Si:H band gap. A-Si:H in solar panels contain 15 - 20 % of hidrogen and band gap aproximately equals 1,7 eV. Since specral curve of optical absorption ia alike the curve of semiconductor with direct optical transitions, tha absorption coefficient in the solar radiation band more than 104cm-1, the a-Si:H film thickness in solar battaries near 1 mkm.