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Photovoltaic (PV) systems are usually composed of numerous solar arrays, which in turn, are composed of numerous PV cells. The performance of the system is therefore dependent on the performance of its components.
The reliability of PV arrays is an important factor in the cost of PV systems and in consumer acceptance. However, the building blocks of arrays, PV cells, are considered "solid-state" devices with no moving parts and, therefore, are highly reliable and long-lived. Therefore, reliability measurements of PV systems are usually focused not on cells but on modules and whole systems.
Reliability can be improved through fault-tolerant circuit design, which involves using various redundant features in the circuit to control the effect of partial failure on overall module yield and array power degradation. Degradation can be controlled by dividing the modules into a number of parallel solar cell networks called branch circuits. This type of design can also improve module losses caused by broken cells and other circuit failures. Bypass diodes or other corrective measures can mitigate the effects of local cell hot-spots. However, today's component failure rates are low enough that, with multiple-cell interconnects, series/paralleling, and bypass diodes, it is possible to achieve high levels of reliability.
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Module Performance Measurements
PV module performance is measured with peak watt ratings. The peak watt (Wp) rating is determined by measuring the maximum power of a PV module under laboratory conditions of relatively high light, favorable air mass, and low cell temperature. But these conditions are not typical in the real world. Therefore, researchers may use a different procedure, known as the NOCT—or normal operating cell temperature—rating. In this procedure, the module first equilibrates with a specified ambient temperature so that maximum power is measured at a nominal operating cell temperature. This NOCT rating results in a lower watt value than the peak-watt rating, but it is probably more realistic.
However, neither of these methods is designed to indicate the performance of a solar module under realistic operating conditions. Another technique, the AMPM Standard, involves considering the whole day rather than "peak" sunshine hours. This standard, which is intended to address the practical user's needs, is based on the description of a standard solar global-average day (or a practical global average) in terms of light levels, ambient temperature, and air mass.
Solar arrays are designed to provide specified amounts of electricity under certain conditions. The following factors are usually considered when determining array performance: characterization of solar cell electrical performance, determination of degradation factors related to array design and assembly, conversion of environmental considerations into solar cell operating temperatures, and calculation of array power output capability.
The amount of electricity required may be defined by any one, or a combination, of the following performance criteria:
• Power output
Power output is the power (in watts) available at the power regulator, specified either as peak power or average power produced during one day.
• Energy output
The energy (watt-hour or Wh) output. This indicates the amount of energy produced during a certain period of time. The parameters are output per unit of array area (Wh/m2), output per unit of array mass (Wh/kg), and output per unit of array cost (Wh/$).
• Conversion efficiency
This parameter is defined as: energy output from array / energy input from sun x 100%. It is often given as a power efficiency, equal to: power output from array / power input from sun x 100%.
Power is typically given in units of watts (W), and energy is typical in units of watt-hours (Wh).
To ensure the consistency and quality of photovoltaic systems and increase consumer confidence in system performance, groups such as the Institute of Electrical and Electronics Engineers and the American Society for Testing and Materials are working on standards and performance criteria for PV systems.
Semiconductors and the Built-In Electric Field for Crystalline Silicon
To separate electrical charges, crystalline silicon cells must have a built-in electric field. Light shining on crystalline silicon may free electrons within the crystal lattice, but for these electrons to do useful work—such as provide electricity to a light bulb—they must be separated and directed into an electrical circuit.
Although both materials are electrically neutral, n-type silicon has excess electrons and p-type silicon has excess holes. Sandwiching these together creates a p/n junction at their interface, thereby creating an electric field.
Substituting a phosphorus atom (with five valence electrons) for a silicon atom in a silicon crystal leaves an extra, unbonded electron that is relatively free to move around the crystal.
To create an electric field within a crystalline silicon photovoltaic (PV) cell, two silicon semiconductor layers are sandwiched together. P-type (or positive) semiconductors have an abundance of positively charged holes, and n-type (or negative) semiconductors have an abundance of negatively charged electrons. When n- and p-type silicon layers contact, excess electrons move from the n-type side to the p-type side. The result is a buildup of positive charge along the n-type side of the interface and a buildup of negative charge along the p-type side.
Because of the flow of electrons and holes, the two semiconductors behave like a battery, creating an electric field at the surface where they meet. This area where they meet is called the p/n junction. The electrical field causes the electrons to move from the semiconductor toward the negative surface, making them available for the electrical circuit. At the same time, the holes move in the opposite direction, toward the positive surface, where they await incoming electrons.
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