PV refers to the direct conversion of insolation (incident solar radiation) to electricity. A PV cell (also known as a solar cell) is simply a large-area semiconductor pn junction diode with the junction positioned very close to the top surface.
Typically, a metallic grid structure on the top and a sheet structure in the bottom collect the minority carriers crossing the junction and serve as terminals. The minority carriers are generated by the incident photons with energies greater than or equal to the energy gap of the semiconductor material.
Since the output of an individual cell is rather low (1 or 2 W at a fraction of a volt), several (30 to 60) cells are combined to form a module. Typical module ratings range from 40 to 50 W at 15 to 17 V. PV modules are progressively put together to form panels, arrays (strings or trackers), groups, segments (subfields), and ultimately a PV plant consisting of several segments. Plants rated at several MW have been built and operated successfully.
Advantages of PV include demonstrated low operation and maintenance costs, no moving parts, silent and simple operation, almost unlimited lifetime if properly cared for, no recurring fuel costs, modularity, and minimal environmental effects. The disadvantages are its cost, need for large collector areas due to the diluteness of insolation, and the diurnal and seasonal variability of the output.
PV systems can be flat-plate or concentrating type. While flat-plate systems utilize the global (direct and diffuse) radiation, concentrator systems harness only the direct or beam radiation. As such, concentrating systems must track (one axis or two axis) the sun. Flat-plate systems may or may not be mounted on trackers.
By 1990, efficiencies of flat-plate crystalline and thin-film cells had reached 23 and 15%, respectively. Efficiencies as high as 34% were recorded for concentrator cells. Single-crystal and amorphous PV module efficiencies of 12 and 5% were achieved by the early ’90s. For an average module efficiency of 10% and an insolation of 1 kW/m2 on a clear afternoon, 10 m 2 of collector area is required for each kW of output.
The output of a PV system is dc and inversion is required for supplying ac loads or for utility-interactive operation. While the required fuel input to a conventional power plant depends on its output, the input to a PV system is determined by external factors such as cloud cover, time of day, season of the year, geographic location, orientation, and geometry of the collector.
Therefore, PV systems are operated, as far as possible, at or near their maximum outputs. Also, PV plants have inertialess generation and are subject to rapid changes in their outputs due to moving clouds. The current-voltage (IV) characteristic of an illuminated solar cell is shown in Figure 60.1. It is given as
where
Io and Is are the dark and source currents, respectively, k is the Boltzmann constant (1.38×10–23 J/K), T is the temperature in K, and e is the electronic charge. Under ideal conditions (identical cells), for a PV module with a series-parallel arrangement of cells, the IV characteristic will be similar, except that the current scale should be multiplied by the number of parallel branches and the voltage scale by the number of cells in series in the module.
The source current varies linearly with insolation. The dark current increases as the cell
operating temperature increases. Also, the larger the energy gap of the material, the smaller the dark current. The ratio of source current to dark current should be made as large as possible for improved operation.
Single-crystal silicon is still the dominant technology for fabricating PV devices. Polycrystalline, semicrystalline, and amorphous silicon technologies are developing rapidly to challenge this. Highly innovative technologies such as spheral cells are being introduced to reduce costs. Concentrator systems typically employ gallium arsenide or multiple junction cells. Many other materials and thin-film technologies are under investigation as potential candidates.
PV applications range from milliwatts (consumer electronics) to megawatts (central station plants). They are suitable for portable, remote, stand-alone, and utility-interactive applications. PV systems should be considered as energy sources and their design should maximize the conversion of insolation into useable electrical form.
Power requirements of practical loads are met using an energy storage and reconversion system or utility interconnection. Concentrating systems have been designed and operated to provide both electrical and low grade thermal outputs with combined peak utilization efficiencies approaching 60%.
The vigorous growth of PV technology is manifested by a doubling of world PV module shipments in six years — from 42 MW in 1989 to 84 MW in 1995. Tens of thousands of small (<1 kW) systems are in operation around the world. Thousands of kilowatt-size systems (1 to 10s of kW) also have been installed and are in operation.
Many intermediate-scale systems (10 to 100s of kW) and large-scale systems (1 MW or larger) are being installed by utility- and government-sponsored programs as proof-of-concept experiments and to glean valuable operational data.
By 1988, nearly 11 MW of PV was interconnected to the utility system in the United States alone. Most were in the 1- to 5-kW range. The two major exceptions are the 1-MW Hesperia-Lugo project installed in 1982 and the 6.5-MW Carrisa Plains project installed in 1984, both in California.
In Germany, a 340-kW system began operation in 1988 as part of a large program. Switzerland had a plan to install 1 MW of PV in 333 roof-mounted units of 3 kW each. By 1990, the installed capacity of PV in Italy exceeded 3 MW.
Many nations have recognized the vast potential of PV and have established their own PV programs within the past decade. A view of the 300 kW flat-plate grid-connected PV system installed and operated by the city of Austin electric utility department in Austin, Texas.
From a capital cost of $7000/kW in 1988 with an associated levelized energy cost of 32¢/kWh, even with a business-as-usual scenario, a twofold reduction to $3500/kW by 2000 and an additional 3-to-1 reduction to $1175/kW by 2030 are being projected. The corresponding energy costs are 15 and 5¢/kWh, respectively. These estimates put the cost of energy from PV in par with the cost of energy from conventional plants in the early part of the twenty-first century.
Most people typically don't understand when I try to explain basically the graph you've constructed (which is quite nice) I get a very... Lost look on people's faces if I try too hard. But very nice guide for rookies, wil shoot it to a few of my colleagues.
ReplyDelete-Sharone Tal