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SOLAR PHOTOVOLTAIC |
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PHYSICS OF PHOTOVOLTAICS A conventional solar cell consists of a wafer of silicon that is about 1/50th of an inch thick. Typical cells that are four inches in diameter produce about one watt of power, and are grouped into modules of dozens of cells. Modules are further grouped into panels and then arrays, which may produce several kilowatts of power.
Conventional PV cells today convert between 5 and 15% of the energy in sunlight into usable energy. Experimental cells have achieved about double that efficiency, but only under carefully controlled laboratory conditions and with expensive materials and high production cost. Efficiency is constantly increasing, however, as new materials and manufacturing processes are developed. The Effects of the Junction Because of the presence of the dopants, an "electric field" exists across the junction of the two halves of the crystal that sweeps free electrons across the junction in one direction only. It is this property of the junction that causes current flow in a solar cell.
If an electron is freed in the half of the cell that has excess electrons, the junction prevents the electron from drifting into the other half, recombining with a hole, and losing its energy. If an electron is freed in the half of the cell with excess holes, the electric field sweeps the electron into the other half. These effects induce electrons to flow in only one direction across the junction. When Light Strikes Silicon
When light shines on a crystal of pure silicon (A-B), particles called "electrons" are ejected from silicon atoms and move about the crystal somewhat randomly (C). The place the electron came from is called a "hole". It takes energy from the light to eject the electron from its normal resting place, and energy is released when the electron returns to an atom that is missing an electron, and recombines with a hole (D). How a Cell Powers an External LoadA electric "load" is any device that uses electricity such as an electric motor or a lamp. A load is connected to a power supply by two wires called "leads". When a load is connected to silicon cell, one lead is connected to one half of the doped crystal, and one lead to the other half.
When light shines on the crystal and electron-hole pairs are created, the electrons travel through the load to recombine with the holes. The electrons moving through the load are what cause the motor to spin or the lamp to shine. Moving electrons are also known as electric "current". As long as light is shining on the crystal, the process is repeated: (1) energy from the light is absorbed by electrons and they are freed from their resting state, (2) electrons are drawn across the junction in the crystal which only permits movement in one direction, and (3) the electrons move through an externally-connected load to recombine with the holes they left behind, performing an energy service in doing so. How a Silicon Semiconductor is Created
To create a semiconductor, two halves of a crystal of pure silicon are contaminated, or "doped", with two different types of material called "dopants": one that contains excess electrons, and one that is electron deficient. The junction between the halves is critical to the operation of the cell. TRACKERS AND CONCENTRATORS Trackers As discussed in the solar physics section, surfaces that are oriented perpendicular to the sun's rays receive the most light. "Tracking arrays" are flat-plate arrays that follow the sun to maximize insolation. "Single axis" trackers allow the module to follow the sun across the sky. The module rotates around a single axis, fixed at an angle with respect to the ground "Two axis"
tracking systems allow the module to both follow the sun moving from east to west and
track the sun as its altitude changes. These systems are more expensive than single axis
trackers, but receive more sunlight. Up to 50% more energy can be obtained on an annual
basis from a two axis-tracking array. Manually adjusted tracking arrays can be almost as effective as automatic systems at capturing the maximum amount of sunlight. Manual arrays are tilted by hand several times per year to account for the height of the sun in the sky or a few times a day to account for the sun moving across the sky.
A freon tracking system, such as the Track Rack (tm) manufactured by Zomeworks, Inc., uses heat from the sun to shift a liquid between one side of an array and the other. The weight of the liquid tilts the array. Tubes are attached to either side of the array, and are connected by a pipe. The tubes are filled with a substance with a low boiling point such as freon.
When the array is tilted away from the sun, the sun shines only on the tube on the far side of the array. The heat from the sun boils the liquid and some of it moves through the connecting pipe into the other tube. The shifting of weight from the far side of the array to the near side tilts the module toward the sun. Concentrator Photovoltaic Modules
Concentrator PV modules use lenses to focus sunlight onto small solar cells. This approach reduces the number of solar cells required per module, effectively replacing cells with less-expensive lenses. The efficiency of concentrator modules is about 15%. TYPES OF PV CELLSThin Film Photovoltaic CellsThin film photovoltaic cells are made of silicon but use an advanced manufacturing technique. Thin film cells use far less silicon than conventional cells, but are less efficient and cost more to produce.
Spheral cells are a new solar technology being developed by Southern California Edison, an electric utility, and the Texas Instruments Corporation. A spheral cell is operationally the same as a conventional solar cell, but differs in its geometry. A spheral cell consists of many tiny spheres of silicon coated with aluminum foil to provide electrical contacts. The advantages of spheral cells are that the manufacturing process is simple, and that low cost, low purity silicon feedstock material can be used. Ribbon Growth Photovoltaic CellsSome photovoltaic firms are experimenting with a special process that pulls a ribbon of silicon out of molten silicon. The ribbons are then cut and formed into panels as usual. Polycrystalline Photovoltaic Cells In making most solar cells today, silicon is purified and refined into a single, large crystal and then sliced into wafers. "Poly-crystalline" silicon cells are also made of purified silicon, but are not formed of a single crystal. While the cells made in this way are less efficient than single-crystal cells, they are much cheaper to produce. Non-Silicon Photovoltaic CellsA number of other metals can be transformed into semiconductors and used in photovoltaic cells. Some of them show a great deal of promise and are already in production, while others are in the experimental or design phase. The most hopeful metals include copper indium diselinide, cadmium sulfide, cadmium telluride, gallium arsenide, and indium phosphide. While some of these demonstrate high efficiencies, other factors such as durability, cost, and availability of raw materials can limit performance. Further research is helping to solve these problems. Multijunction Photovoltaic Cells Multijunction photovoltaic cells employ multiple layers of semi conducting materials to create two or more junctions. Different layers in the cell absorb different parts of the solar spectrum, so the overall efficiency of the cell can be high Conventional Photovoltaic Cells Most cells in
operation today are single crystal silicon cells. Silicon cells provide a good balance of
cost effectiveness, reliability, and efficiency. Silicon is obtained by purifying
naturally-occuring silicon-oxygen compounds found in many common rocks.
APPLICATIONS OF PHOTOVOLTAICSThe market for photovoltaics is rapidly expanding. Currently, there are a few large utility PV power plants, thousands of residential systems, and tens of thousands of remote power systems in use. As prices fall, the number of installed systems will continue to increase. Photovoltaics
have several characteristics that make them attractive to utilities, and not just those
with interest in demand-side management (DSM).
The timing of the electricity produced by PV cells is often coincidental with the utility's demand for electricity. This is primarily due to the electrical loads of commercial and industrial facilities. Photovoltaic
arrays can be installed in small or large quantities and can closely match the electricity
needs of the user. Furthermore, PV arrays can be installed and being operating in months
instead of years for conventional power plants. This flexibility decreases costs for the
utility and provides superior service to the utility's customers. The cost of
transmitting and distributing power to a utility's customers is often as high or higher
than the cost of producing the power. When a utility has new demand (due to new customers
or increased demand by existing customers) and T&D lines are operating at their
maximum capacity, the utility would normally build additional lines, which can be very
expensive. An alternative is to use PV arrays at the customer end of the system to provide
the additional power, removing the need to upgrade the T&D lines. The cost of transmitting and distributing power to a utility's customers is often as high or higher than the cost of producing the power. When a utility has new demand (due to new customers or increased demand by existing customers) and T&D lines are operating at their maximum capacity, the utility would normally build additional lines, which can be very expensive. An alternative is to use PV arrays at the customer end of the system to provide the additional power, removing the need to upgrade the T&D lines. Residential Homes can use photovoltaic systems to replace or supplement electric power from the utility. A stand-alone residential system consists of solar panels (A), a battery to store power for use at night (B), and a device called an "inverter" to allow conventional appliances to be powered by solar electricity (C). Some systems use appliances specially designed to be powered by solar electricity and do not require an inverter. A.
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B.
C.
