Photovoltaic Systems

Yogi Goswami and Jan F. Kreider

CONTENTS

4.1 Semiconductor Types

4.1.1 p-n Junction

4.1.2 The Photovoltaic Effect

4.1.3 Materials Overview

4.1.4 Manufacture of Solar Cells and Panels

4.1.4.1 Single Crystal and Polycrystalline Silicon

4.1.4.2 Amorphous Silicon

4.2 PV System Efficiency and Design

4.2.1 Efficiency of Solar Cells

4.2.2 Basic Design for Photovoltaic Systems

4.3 Technical Developments and Barriers

4.4 PV System Controls and Diagnostics

4.5 PV System Capacity Credit

4.6 The Utility Interface

4.6.1 Example Interface Standard — State of Texas

4.6.1.1 Safety Standards

4.6.1.2 System Stability Requirements

4.6.1.3 Protection Requirements

4.6.1.4 Switchgear Requirements

4.6.1.5 Metering Requirements

4.6.1.6 Generation Control

4.6.1.7 Testing and Record Keeping

4.6.1.8 Review of Interconnection Proposals

4.7 PV System Costs

4.7.1 System Costs

4.7.2 Cost of PV Power References

Photovoltaic (PV) systems involve the direct conversion of sunlight into electricity with no intervening heat engine. PV devices are solid state; therefore, they are rugged and simple in design and require very little maintenance. A key advantage of PV systems is that they can be constructed as either grid-connected or stand-alone to produce outputs from microwatts to megawatts. They have been used as the power sources for calculators, watches, water pumping, remote buildings, communications, satellites and space vehicles, as well as megawatt-scale power plants. Because they are lightweight, modular, and do not require a gaseous or liquid fuel supply, PVs fit a niche that is unavailable to other DG technologies. For an overview of solar energy resources, technologies, and design approaches, the reader is referred to Goswami et al. (2000). Figure 4.1 shows a residential PV system.

FIGURE 4.1

PV array located adjacent to residential building.

Figure 4.2 shows the historical trend of PV shipments since the early 1970s when the only significant consumer was the U.S. Space Program. By contrast, all shipments in 1998 exceeded 150 MW,* up 21% from 1997. The U.S. is the world's largest producer, followed closely by Japan. Europe ranks third with significant production levels in Australia, India, China, and Taiwan.

For most of the 1990s, PV modules for buildings were used in off-grid modes. However, as Figure 4.3 indicates, grid-connected sales caught up with off-grid sales in 1997 because of various incentive programs that made PV power more competitive with conventional power. Among these were net metering and state payments based on kW installation levels. However, without subsidies, PV power remains two to five times as expensive as grid power, where grid power exists. Where there is no grid, as is the case in most of the world, PV power is the cheapest electricity source when operating and maintenance costs are considered.

Finally, with the drop of PV prices as shown in Figure 4.4, installation volume grew rapidly. In 1998, for the first time, PV prices dropped below $4/watt (not including the balance of system needed to control and convert the DC power into AC power, for example).

Historically, the photoelectric effect was first noted by Becquerel in 1839 when light was incident on an electrode in an electrolyte solution. Adams and

* PV systems are rated by their output under standard sunny conditions. Their average output is considerably less than this peak rating. The average rating is used to determine the value of energy produced, not the peak rating, which is often the basis of costs.

PV Shipments

180.0 160.0 140.0120.0

1968 1973 1978 1983 1988 1993 1998

Year

FIGURE 4.2

World shipments of PV modules 1971 to 1998 (courtesy of Paul Maycock).

PVs in Buildings, Grid-Connected and Off-Grid, 1990-97

Compiled by Worldwatch Institute

FIGURE 4.3

PV applications for buildings, grid-connected (lower line), and off-grid (upper line) for 1990 to 1998 (courtesy of Paul Maycock).

World Price for Photovoltaic Modules

Compiled by Worldwatch Institute

FIGURE 4.4

Average factory prices of PV modules ($1997/watt) for 1975-1998 (courtesy of Paul Maycock).

Compiled by Worldwatch Institute

FIGURE 4.4

Average factory prices of PV modules ($1997/watt) for 1975-1998 (courtesy of Paul Maycock).

Day first observed the effect in solids in 1877 while working with selenium. Early work was done with selenium and copper oxide by pioneers such as Schottkey, Lange, and Grandahl. In 1954, researchers at RCA and Bell Laboratories reported achieving efficiencies of about 6% by using devices made of p and n types of semiconductors. The space race between the U.S. and the Soviet Union resulted in dramatic improvements in the photovoltaic devices.

This chapter discusses features of PV systems, devoting a section to each in the following order:

• Semiconductor types

• System efficiency and design

• Technical barriers

• Conventional generation capacity displacement by PV systems

• Utility interconnection issues

• Economic summary

4.1 Semiconductor Types

A basic understanding of atomic structure is quite helpful in understanding the behavior of semiconductors and their use as PV energy conversion devices. Any fundamental book on physics or chemistry generally gives adequate background for basic understanding. For any atom, the electrons arrange themselves in orbital shells around the nucleus so as to result in the minimum amount of energy. In elements that have electrons in multiple shells, the innermost electrons have the minimum energy and therefore require the maximum amount of externally imparted energy to overcome the attraction of the nucleus and become free. Electrons in the outermost band of subshells are the only ones that participate in the interaction of an atom with its neighboring atoms. If these electrons are very loosely attached to the atom, they may attach themselves to a neighboring atom to give that atom a negative charge, leaving the original atom as a positive charged ion. The positive and negative charged ions become attached by the force of attraction of the charges, thus forming ionic bonds. If the electrons in the outermost band do not fill the band completely but are not loosely attached either, they arrange themselves such that neighboring atoms can share them to make the outermost bands full. The bonds thus formed between the neighboring atoms are called covalent bonds.

Since electrons in the outermost band of an atom determine how an atom will react or join with a neighboring atom, the outermost band is called the valence band. Some electrons in the valence band may be so energetic that they jump into an even higher band and are so far removed from the nucleus that a small amount of impressed force would cause them to move away from the atom. Such electrons are responsible for the conduction of heat and electricity, and that band is called a conduction band. The difference in the energy of an electron in the valence band and the innermost subshell of the conduction band is called the band gap, or the forbidden gap.

Materials whose valence bands are full have very high band gaps (> 3 eV). Such materials are called insulators. Materials that have relatively empty valence bands, on the other hand, and may have some electrons in the conduction band are good conductors. Metals fall in this category. Materials with valence bands partly filled have intermediate band gaps (< 3 eV). Such materials are called semiconductors. Pure semiconductors are called intrinsic semiconductors, while semiconductors doped with very small amounts of impurities are called extrinsic semiconductors. If the dopant material has more electrons in the valence band than the semiconductor, the doped material is called an n-type semiconductor. Such a material seems to have excess electrons available for conduction, even though the material is electronically neutral.

Silicon, for example, has four electrons in the valence band. Atoms of pure silicon arrange themselves in such a way that each atom shares two electrons with each neighboring atom with covalent bands to form a stable structure. If phosphorous, which has five valence electrons (one more than silicon), is introduced as an impurity in silicon, the doped material seems to have excess electrons, even though it is electrically neutral. Such a doped material is called n-type silicon. If, on the other hand, silicon is doped with boron, which has three valence electrons (one less than silicon), there seems to be a positive hole (missing electron) in the structure, even though the doped material is electrically neutral. Such material is called p-type silicon. n- and p-type semiconductors make it easier for the electrons and holes, respectively, to move in the semiconductors.

Solar Stirling Engine Basics Explained

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