Polycrystalline and Semicrystalline

This is relatively a fast and low cost process to manufacture thick crystalline cells. Instead of drawing single crystals using seeds, the molten silicon is cast into ingots. In the process, it forms multiple crystals. The conversion efficiency is lower, but the cost is much lower, giving a net reduction in cost per watt of power.

Ingot Plant

FIGURE 3-12

Single-crystal ingot-making by Czochralski process. (Source: Cook, G., Photovoltaic Fundamental, DOE/NREL Report DE91015001, February 1995.)

FIGURE 3-12

Single-crystal ingot-making by Czochralski process. (Source: Cook, G., Photovoltaic Fundamental, DOE/NREL Report DE91015001, February 1995.)

3.3.3 Thin Films

These are new types of photovoltaics entering the market. Copper Indium Diselenide, Cadmium Telluride, and Gallium Arsenide are all thin film materials, typically a few pm or less in thickness, directly deposited on glass, stainless steel, ceramic or other compatible substrate materials. This technology uses much less material per square area of the cell, hence, is less expensive per watt of power generated.

3.3.4 Amorphous Silicon

In this technology, amorphous silicon vapor is deposited on a couple of pm-thick amorphous (glassy) films on stainless steel rolls, typically 2,000-feet long and 13-inches wide. Compared to the crystalline silicon, this technology uses only 1 percent of the material. Its efficiency is about one-half of the crystalline silicon at present, but the cost per watt generated is projected to

FIGURE 3-13

Round-shape pv cell reduces material waste typically found in rectangular cell. (Depiction based on cell used by Applied Solar Energy Corporation, City of Industry, California.)

FIGURE 3-13

Round-shape pv cell reduces material waste typically found in rectangular cell. (Depiction based on cell used by Applied Solar Energy Corporation, City of Industry, California.)

be significantly lower. On this premise, two large plants to manufacture amorphous silicon panels started in the U.S.A. in 1996.

3.3.5 Spheral

This is yet another technology that is being explored in the laboratories. The raw material is low-grade silicon crystalline beads, presently costing about $1 per pound. The beads are applied on typically 4-inch squares of thin perforated aluminum foil. In the process, the impurities are pushed to the surface, from where they are etched away. Since each sphere works independently, the individual sphere failure has negligible impact on the average performance of the bulk surface. According to a Southern California Edison Company's estimate, 100 square feet of spheral panels can generate 2,000 kWh per year in an average southern California climate.

Area Primary Lens Active Cell Area

FIGURE 3-14

Lens concentrating the sunlight on small area reduces the need of active cell material. (Source: Photovoltaic Fundamental, DOE/NREL Report DE91015001, February 1995.)

FIGURE 3-14

Lens concentrating the sunlight on small area reduces the need of active cell material. (Source: Photovoltaic Fundamental, DOE/NREL Report DE91015001, February 1995.)

3.3.6 Concentrated Cells

In an attempt to improve the conversion efficiency, the sunlight is concentrated into tens or hundreds of times the normal sun intensity by focusing on a small area using low cost lenses (Figure 3-14). The primary advantage is that such cells require a small fraction of area compared to the standard cells, thus significantly reducing the pv material requirement. However, the total module area remains the same to collect the required sun power. Besides increasing the power and reducing the size or number of cells, such cells have additional advantage that the cell efficiency increases under concentrated light up to a point. Another advantage is that they can use small area cells. It is easier to produce high efficiency cells of small areas than to produce large area cells with comparable efficiency. On the other hand, the major disadvantage of the concentrator cells is that they require focusing optics adding into the cost.

The annual production of various pv cells in 1995 is shown in Table 3-1. Almost all production has been in the crystalline silicon and the amorphous silicon cells, with other types being in the development stage. The present status of the crystalline silicon and the amorphous silicon technologies is shown in Table 3-2. The former is dominant in the market at present and the latter is expected to be dominant in the near future.

TABLE 3-1

Production Capacities of Various pv Technologies in 1995

PV Technology

1995 Production

Crystalline Silicon

55 MW

Amorphous Silicon

9 MW

Ribbon Si, GaAs, CdTe

1 MW

TOTAL

65 MW

(Source: Carlson, D. E., Recent Advances in Photovoltaics, 1995. Proceedings of the Intersociety Engineering Conference on Energy Conversion, 1995.)

(Source: Carlson, D. E., Recent Advances in Photovoltaics, 1995. Proceedings of the Intersociety Engineering Conference on Energy Conversion, 1995.)

TABLE 3-2

Comparison of Crystalline and Amorphous Silicon Technologies

TABLE 3-2

Comparison of Crystalline and Amorphous Silicon Technologies

Crystalline Silicon

Amorphous Silicon

Present Status

Workhorse of terrestrial and

New rapidly developing technology, tens of

space applications

MW yearly production facilities have been built in 1996 to produce low cost cells

Thickness

200-400 pm (.004-.008 inch)

2 pm (less then 1 percent of that in crystalline silicon)

Raw Material Cost

High

About 3 percent of that in crystalline silicon

Conversion

16-18 percent

8-9 percent

Efficiency

Module Costs

$6-8 per watt, expected to fall

$6-8 per watt, expected to fall rapidly to $2 per

(1995)

slowly due to the matured

watt in 2000 due to heavy DOE funding to

nature of this technology

fully develop this new technology

Getting Started With Solar

Getting Started With Solar

Do we really want the one thing that gives us its resources unconditionally to suffer even more than it is suffering now? Nature, is a part of our being from the earliest human days. We respect Nature and it gives us its bounty, but in the recent past greedy money hungry corporations have made us all so destructive, so wasteful.

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