Solar Power is Hot
With today’s cost of energy, it’s no wonder there is a renewed interest in generating electricity by means of the sun’s energy. This is not a new concept. The development of solar energy goes back more than 100 years. Early solar power plants produced steam from the heat of the sun, and the steam was used to drive machinery. In the same time frame, Henri Becquerel discovered the photovoltaic (PV) effect, which is the production of electricity directly from the sun. This early research was further enhanced by the work of Werner Siemens.
The key to PV technology is making it cost effective as a means to generate energy. This can be done by improving the efficiency of the PV cells to generate electricity from the sun as well as by decreasing the cost to manufacture the cell itself. As technology improves, the efficiency of PV technology has benefitted. Years ago, solar panels were about 5% efficient. Today, commercial modules using single-crystal silicon cells exceed 15% efficiency. Physics limits this efficiency to about 26%. Higher efficiencies to 40+% have been achieved in research applications using a high-concentration device requiring sophisticated tracking optics with a concentrating lens the size of a table and about one foot thick.
PV cells work by creating a semiconductor (typically silicon), which absorbs a certain amount of energy from light. This energy frees electrons, allowing them to flow. Electric fields in the cells act to force the flow of electrons in a particular direction. This flow is a current, which can be drawn from the cell by placing metal contacts on the top and bottom.
But how are the silicon semiconductors made in the first place? The raw silicon must be melted and solidified several times in an environment that removes impurities and prevents recontamination. Silicon comprises 25.7% of the Earth’s crust and, after oxygen, is the second most abundant element on earth. Although widely available, silica (SiO2) sand contains too many impurities and is not used for semiconductor production. Pure quartz lumps are the typical raw material.
In order to make solar panels, the silicon must be refined to 99.9999% (six nines) purity to create solar-grade silicon (SG-Si). It is first melted in an electric-arc furnace to create metallurgical-grade silicon. At temperatures over 3450°F (1900°C), the silica is chemically reduced by carbon to create elemental silicon. Only 5% of the metallurgical-grade silicon (MG-Si) produced in 2005 was used for the production of solar cells and semiconductor devices. In order to be used in the solar industry, this MG-Si (99% pure) must be further refined, and SG-Si should have a monocrystalline structure for highest efficiency.
With an increased demand for SG-Si and the need to improve its affordability, new melting methods are showing promise. Early semiconductor technology was designed for small volume. Furnaces used resistive radiant-heating elements and, combined with the furnace design limitations, could deliver a power density of about 5 W/cm2. Induction furnaces are improving on this with power densities up to 30 W/cm2, which improves the speed and volume of silicon processing, thereby decreasing the manufacturing cost.
The further processing and refining of silicon to produce SG-Si is complex, and the detail is beyond the scope of this column. One of the refining steps is directional solidification, which results when a bath of molten silicon is slowly solidifed from the bottom to the top. The impurities are rejected to the solid/liquid interface and move to the surface during the solidification process.
Another process manufactures the single-crystal ingots. As discussed previously, monocrystalline silicon results in the highest efficiency of conversion of solar energy into electricity. Solar wafers – 156 mm x 156 mm – are sliced from large ingots to manufacture the solar panels with which we are familiar.
Why is it important that we continue to reduce the costs of this technology? Only then will it be viable for solar technology to become widely used. To equip a modest-sized house to obtain about half its total electricity needs from solar power (without batteries), the installed system would cost around $32,000. Needless to say, the payback on that system would be quite lengthy.
Solar panels continue to find a variety of applications, however. Toyota just recently announced that it plans to install solar panels on some Prius hybrids for the next remodel. The panels will be used to power part of the air-conditioning system on high-end versions of the gasoline-electric Prius.
Whatever the application of solar panels made from SG-Si, it’s clear that thermal processes will continue to make this material better and cheaper. Who knows what we may be powering using the sun’s energy in the coming years. IH