Research Paper on "Grid Connected Photovoltaic PV Systems"

Research Paper 8 pages (2702 words) Sources: 8

[EXCERPT] . . . .

Grid-Connected Photovoltaic (PV) Systems

Though French physicist Edmond Becquerel discovered the photovoltaic (PV) phenomenon about 1839, the earliest practical application came in the 1950s, when crystalline silicon cells powered United States space satellites (Solar Direct) by generating electrons from sunshine. Despite the requirement for electricity storage at night, photovoltaics are becoming increasingly common, particularly as grid-connected systems (St. John). The four main parts of PV systems (silicon cells, inverters, controllers, and batteries) are becoming ever cheaper despite issues with grid integration and environmental concerns (Solar Direct). Financial incentives make PV particularly attractive (Database of State Incentives for Renewables & Efficiency (DSIRE)).

Gerald Pearson, a physicist, accidentally developed crystalline silicon cells in 1953 at Bell Laboratories (Perlin). Further fine-tuning came from Daryl Chapin and Calvin Fuller at Bell Laboratories. Initial commercialization failed due to the high price of solar electricity (600 times "regular" power plants per watt,) but the federal government used and continues to use PV for space-based applications, such as satellites (Perlin). In the mid-70s, Dr. Elliot Berman started his own company to design solar cells for use on Earth. Exxon financed productive research and the price fell to $20/watt, competitive for non-grid situations like deep-sea oil rigs (Palz 565).

Crystalline silicon cells were the original basis for PV. Five silicon atoms make a crystal lattice with four "spare" valence electrons. The "band gap" is "the amount of energy required to dislodge an electron from its covalent bond and allow it to become part of an electrical circuit" (U.S. Department of Energy). Silicon has a band gap of 1.1 electron-volts (eV), though the band gap increases as temperature rises, which means that solar cells are more efficient in sunny, cold weather. In band gap engineering very pure silicon wafers are "doped" with phosphorus (5 valence electrons) and used to make positively charged silicon, just as silicon wafers "doped" with boron (3 valence electrons) are used to make negatively charged silicon. Various methods are used to dope the silicon wafers, including heating the elements together, spraying the "dopants" on the silicon, and pushing the dopants into the crystal lattice. Layers of positive and negatively charged silicon wafers make up solar cells, so that when light hits the lattice crystal, excess photons force positive valance electrons to jump the band gap to the negatively charged wafers, with a little push from the doping for extra efficiency. A spectrum divides light, which is why not all photons vibrate on the same frequency. By design, solar cells absorb specific frequencies of light. Rather than leaving the electrons bouncing around within the lattice, placement of the positively charged wafers near the junction creates a continuous flow of electrons (U.S. Department of Energy). When done well, monocrystalline solar cells can achieve 19% efficiency (BrighterEnergy.org).

Solar cells are variable in their efficiency. Different manufacturing processes create assorted levels of efficiency, with distinct characteristics. There are three main types of solar cells: crystalline silicon cells (most expensive), polycrystalline (less expensive), and amorphous (least expensive). Crystalline silicon grows in a continuous lattice, ideal for transferring electrons, usually through the Czochralski process, which dips a seed crystal into pure molten silicon and slowly pulls it out. As it comes out, the silicon solidifies into a single crystal around the original seed. The Czochralski process forms cylinders up to 6 feet long and 2.5 feet in diameter, which slices into the thin wafers that form solar cells (U.S. Department of Energy).

In contrast, polycrystalline silicon is much less expensive to create, though it results in lower PV efficiency, typically 10-12% (Solar Direct). A large mass of silicon is heated to the melting point, or cast, and then cooled and sliced (Foll). There are a couple of ways to do this, including the "edge defined film-fed crystal growth technique," which uses capillary action and pressure to create a long, thin slab of silicon (Foll). Plasma-enhanced chemical vapor deposition (PECVD) is the primary source of thin film silicon, another type of polycrystalline silicon (U.S. Department of Energy). An alternative to PECVD is catalytic chemical vapor deposition (CCVD) (Fuhs 100), though it has similar efficiency rates. On top of the thin film is a window layer that absorbs light energy, often made out of copper indium diselenide (CuInSe2 or CIS), or cadmium telluride (CdTe). Thin film saves material, allows monolithic device design, uses cheaper material, and is easier to manufacture (U.S. Department of Energy).

The last type of solar panel is amorphous silicon, which has no crystals but when hydrogenated permits electron flow. It is the least efficient solar cell, and thus more common in wristwatches and calculators, with less demanding energy usage. However, amorphous silicon absorbs light 40 times more efficiently than monocrystalline silicon, so a very thin film can absorb most of the light energy on it. Unlike monocrystalline silicon, amorphous silicon is made at low temperatures on cheap substrates like plastic, glass, and metal, which makes solar shingles practical. Unlike monocrystalline and polycrystalline solar cells, which are still at maximum efficiency after 45 years, amorphous silicon degrades about 20% of efficiency over time (U.S. Department of Energy).

PV modules are composed of front surface materials, encapsulant, rear surface, and frame. Front surface materials are usually low-iron glass, which is cheap, stable, and easy to clean. The encapsulant cradles the silicon, which makes ethyl vinyl acetate (EVA) a good choice, because it is soft, stable, and bonds to the other components. Frames are commonly made of aluminum. The rear surface should let heat out, but prevent water from getting in. Polyvinyl fluoride (PVF) is a common choice (U.S. Department of Energy).

Because solar cells are solid state, they last indefinitely, making them ideal for remote locations, "where the cost and trouble of bringing in utility power outweighs the higher initial expense of PV, and where mobile generator sets present more fueling and maintenance trouble" (Solar Direct). As the use of consumer electronics grew over the last few decades, consumers grew tired of always supplying batteries, and so a market for smaller PVs was born (Florida Solar Energy Center). As the cost of oil has risen, interest has grown in utility "intertie" systems that generate household electricity backed up by the power grid (Perlin).

HOW A GRID CONNECTED PV SYSTEM WORKS

However, solar cells are not used alone. Cells, which produce 1 or 2 watts, combine into modules, usually in groups of 36. In turn, groups of modules create arrays. Ideally, modules have devices that move them to the best angle for electricity production, called solar trackers. Charge controllers regulate the current so as not to "fry" delicate electronics downstream of the solar cells. Inverters convert direct current (DC) from the solar cells to alternating current (AC) to match most household appliances and the grid. Grid tie inverters allow the system to connect with the utility grid by matching phase with a utility-supplied sine wave (St. John). Batteries may be included for further back up. Everything but the solar panels makes up the balance of system (BOS) (U.S. Department of Energy). Everything together makes a PV system.

Arrays are composed of PV modules in circuits, usually 12 volts. Circuits will eventually fail, so designs that accommodate the partial failure to retain as much power as possible are popular. For this reason, branch circuits and bypass diodes are often used. Performance is measured in peak watt ratings (Wp), which are like car mileage ratings -- real world results are often significantly less than ideal laboratory conditions. So, other criteria are often used as measurements, including average daily power produced, watt-hours (often as a ratio of area, mass, or cost), and conversion efficiency (U.S. Department of Energy).

Grid connected, or intertie, PV systems work beside and with the power company's electrical grid. An inverter changes the 12V DC electricity produced by the PV array into phase matched 110V AC by synchronizing with a utility-supplied sine wave (St. John). Then, the electricity can supply either the household, or the grid. As a safety measure, the inverter automatically stops supplying the grid with power when the grid electricity is isn't present. At night, or during periods of high demand, the system works like "normal," and uses electricity supplied by the power company (Solar Direct). Utility scale PV systems are built to work with the "regular" power company, complete with distribution feeders, interconnection transformers, and inverters with safety overrides. Smaller scale (household size) PV systems use "normal" transformers, with intertie inverters (Florida Solar Energy Center).

GRID ISSUES FROM PV SYSTEMS

Issues arise when grids designed to "deliver power one way at constant voltages and frequencies have trouble accommodating that two-way, intermittent flow," according to greentechgrid (St. John). Both an excess of power (creating higher voltage) and a shortage (high demand) can negatively influence grid reliability in unforeseen ways. Some known issues are issues with electricity quality, equipment and component overload, overuse of voltage -- control and regulators, economic impacts (local power purchase prices) and islanding operations (Srisaen and Sangswang 852), (Katiraei and Romero Aguero 62-71).

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