A Detailed Analysis of Power Demand Compensation by Using Photovoltaic Power Generation

Available Information On Photovoltaic Power

There is an enormous supply of articles on the subject of photovoltaic power. Most articles are narrow in scope, perhaps announcing a recent breakthrough or discussing a particular project or application. The internet provides a great deal of information as well, with web sites sponsored by government agencies, industry groups, and manufacturers. We did have some difficulty finding an overview of the subject. Most books on photovoltaics are at least five years old and cover the technical aspect of photovoltaics without providing an assessment of the practicality of using photovoltaics for power generation.

Why Photovoltaic Power Requires Study

The high cost of generating electrical power using photovoltaic cells compared to conventional coal-, gas-, and nuclear-powered generators has kept PV power generation from being in widespread use. Less than 1% of electricity is generated by photovoltaics. However, there are a few applications in which PV power is economical. These applications include satellites, developing countries that lack a power distribution infrastructure, and remote or rugged areas where running distribution lines are not practical. As the cost of photovoltaic systems drops, more applications become economically feasible. The non-polluting aspect of PV power can make it an attractive choice even when conventional generating systems are more economical. The manufacture of photovoltaic systems has increased steadily for the last 25 years. It is inevitable that engineers will be called upon to develop photovoltaic technology or will be involved in projects using this technology. Many existing reports on photovoltaics cover only one facet of the technology and sometimes writers inflate their reports on behalf of the company involved. There is a need for an up-to-date, objective understanding of photovoltaic power generation. With this goal in mind we have created this report.

Photovoltaic Technology

Scientists have known of the photovoltaic effect for more than 150 years. Photovoltaic power generation was not considered practical until the arrival of the space program. Early satellites needed a source of electrical power and any solution was expensive. The development of solar cells for this purpose led to their eventual use in other applications.

Power Output and Efficiency Ratings

The figures given for power output and efficiency of photovoltaic cells, modules, and systems can be misleading. It is important to understand what these figures mean and how they relate to the power available from installed photovoltaic generating systems.

Power Ratings

Photovoltaic power generation systems are rated in peak kilowatts (kWp). This is the amount of electrical power that a new, clean system is expected to deliver when the sun is directly overhead on a clear day. We can safely assume that the actual output will never quite reach this value. System output will be compromised by the angle of the sun, atmospheric conditions, dust on the collectors, and deterioration of the components. When comparing photovoltaic systems to conventional power generation systems, one should bear in mind that the PV systems are only productive during the daytime. Therefore, a 100 kW photovoltaic system can produce only a fraction of the daily output of a conventional 100 kW generator.

Efficiency Ratings

The efficiency of a photovoltaic system is the percentage of sunlight energy converted to electrical energy. The efficiency figures most often reported are laboratory results using small cells. A small cell has a lower internal resistance and will yield a higher efficiency than the larger cells used in practical applications. Additionally, photovoltaic modules are made up of numerous cells connected in series to deliver a usable voltage. Due to the internal resistance of each cell, the total resistance increases and the efficiency drops to about 70% of the single-cell value. Efficiency is higher at lower temperatures. Temperatures used in laboratory measurements may be lower than those in a practical installation.

Converting Sunlight to Electricity

A typical photovoltaic cell consists of semiconductor material (usually silicon) having a pn junction as shown in Figure 1.

Figure 1.Implementation of  solar cells

Sunlight striking the cell raises the energy level of electrons and frees them from their atomic shells. The electric field at the pn junction drives the electrons into the n region while positive charges are driven to the p region. A metal grid on the surface of the cell collects the electrons while a metal back-plate collects the positive charges .

Light Generates

Electron and Hole

p-Type

n-Type

Thin Film Technology

Thin-film solar cells are manufactured by applying thin layers of semiconductor materials to a solid backing material. The composition of a typical thin-film cell is shown in Figure 2. Sunlight entering the intrinsic layer generates free electrons. The p-type and n-type layers create an electric field across the intrinsic layer. The electric field drives the free electrons into the ntype layer while positive charges collect in the p-type layer. The total thickness of the p-type, intrinsic, and n-type layers is about one micron. Although less efficient than single- and polycrystal silicon, thin-film solar cells offer greater promise for large-scale power generation because of ease of mass-production and lower materials cost. Thin-film is also suitable for building-integrated systems because the semiconductor films may be applied to building materials such as glass, roofing, and siding .

Fig.2.

Using thin films instead of silicon wafers greatly reduces the amount of semiconductor material required for each cell and therefore lowers the cost of reducing photovoltaic cells. Gallium arsenide (GaAs), copper indium diselenide (CuInSe2), cadmium telluride (CdTe) and titanium dioxide (TiO2) are materials that have been used for thin film PV cells. Titanium dioxide thin films have been recently developed and are interesting because the material is transparent and can be used for windows.

Tin Oxide Tin oxide is a conductive material that is transparent when in a thin layer. Tin oxide is used in place of a metallic grid for the top layer of thin film photovoltaic sheets .

Amorphous Silicon (a-Si) Amorphous (uncrystallized) silicon is the most popular thin-film technology. It is prone to degradation and produces cell efficiencies of 5-7%. Double- and triple-junction designs raise efficiency to 8-10%. The extra layers capture different wavelengths of light. The top cell captures blue light, the middle cell captures green light, and the bottom cell captures red light. Variations include amorphous silicon carbide (a-SiC), amorphous silicongermanium (a-SiGe), microcrystalline silicon (mc-Si), and amorphous silicon-nitride (a-SiN)

.

Cadmium Telluride (CdTe) and Cadmium Sulphide (CdS) Photovoltaic cells using these materials are under development by BP Solar and Solar Cells Inc .

Poly-crystalline Silicon Poly-crystalline silicon offers an efficiency improvement over amorphous silicon while still using only a small amount of material.

Concentrating Collectors

By using a lens or mirror to concentrate the sun’s rays on a small area, it is possible to reduce the amount of photovoltaic material required. A second advantage is that greater cell efficiency can be achieved at higher light concentrations. To accommodate the higher currents in the photocells, a larger metallic grid is used. For example, in a system with a 22X concentration ratio, the grid covers about 20% of the surface of the solar cell. To prevent this from blocking 20% of the sunlight, a prism is used to redirect sunlight onto the photovoltaic material, as shown in Figure 3. A second problem is the higher temperatures of a concentrating system. The cells may be cooled with a heat sink or the heat can be used to heat water .

Fig.3.

Only direct sunlight, not scattered by clouds or haze, can be concentrated. Therefore, the concentrating collectors are less effective in locations that are frequently cloudy or hazy, such as coastal areas .

How much power is available from the sun?

Sunlight reaches the Earth’s outer atmosphere at strength of 1367 watts per square meter, defined as AM0, or “air mass zero.” Atmospheric losses reduce the sun’s power to about 1000 W/m2 when the sun is directly overhead on a cloudless day . Figure 4 shows the average daily sunlight falling on a square meter surface which has been tilted toward the southern horizon at an angle equal to the latitude of the location. Note that diffused as well as direct sunlight is considered, making this map applicable to flat plate collectors.

Fig.4.Average daily sunlight in kWh/m2

Conversion Efficiency

The most efficient PV modules usually employ single-crystal silicon cells, with efficiencies up to 15%. Poly-crystalline cells are less expensive to manufacture but yield module efficiencies of about 11%. Thin-film cells are less expensive still, but give efficiencies to about 8% and suffer greater losses from deterioration.

Production Considerations

In the past, low-grade silicon was bought from semiconductor manufacturers for use in building solar cells. With improvements in the manufacturing process, silicon manufacturers are able to consistently produce the more profitable semiconductor-grade silicon. As a result, it is becoming difficult to buy low-grade silicon. There has been much discussion about building a production facility dedicated to the production of silicon for solar cells.

Photovoltaic Applications

Photovoltaic power generation has been most useful in remote applications with small power requirements where the cost of running distribution lines was prohibitive. As PV power becomes more affordable, the use of photovoltaics for grid-connected applications is increasing. However, the high cost of PV modules and the large area they require continue to be obstacles to using PV power to supplement existing electrical utilities. An interesting approach to both of these problems is the integration of photovoltaics into building materials.

Building-Integrated Systems

Building-integrated photovoltaic (BIPV) systems offer advantages in cost and appearance by incorporating photovoltaic properties into building materials such as roofing, siding, and glass. When BIPV materials are substituted for conventional materials in new construction, the savings involved in the purchase and installation of the conventional materials are applied to the cost of the photovoltaic system. BIPV installations are architecturally more attractive than roof mounted PV structures.

For example, United Solar Corporation produces photovoltaic shingles that replace normal asphalt shingles. Each PV shingle replaces a seven-foot long row of asphalt shingles, and any roofer can install them. Normally, only one-third of a roof needs to be covered with PV panels to produce sufficient power for the average home. Glass manufactured with photovoltaic properties is available for use in skylights and windows. The architect can select from several colors of transparent photovoltaic glass. The tint color and depth is controlled by the type and amount of semiconductor material used in the construction of the photovoltaic glass.

Off-Grid Applications

The majority of photovoltaic power generation applications are remote, off-grid applications. These include communication satellites, terrestrial communication sites, remote homes and villages, and water pumps. These are sometimes hybrid systems that include an engine-driven generator to charge batteries when solar power is insufficient.

Grid-Connected Applications

In grid-connected application, the DC power from solar cells runs through an inverter and feeds back into the distribution system. Grid-connected systems have demonstrated an advantage in natural disasters by providing emergency power capabilities when utility power was interrupted. Although PV power is generally more expensive than utility-provided power, the use of grid connected systems is increasing.

The Economics Of Photovoltaic Power Generation

Photovoltaic efficiency and manufacturing costs have not reached the point that photovoltaic power generation can compete with conventional coal-, gas-, and nuclear-powered facilities. The cost of photovoltaic power (when storage is not required) is two to four times that of conventionally produced power. It is difficult to define this relationship precisely due to wide variations in the cost of producing and distributing conventional electrical power and other variables. Due to the wide range of these variables, some applications of photovoltaic power are economically superior to conventional systems.

Conclusion

However, large variations in cost of conventional electrical power, and other factors, such as cost of distribution, create situations in which the use of PV power is economically sound. PV power is used in remote applications such as communications, homes and villages in developing countries, water pumping, camping, and boating. Grid connected applications such as electric utility generating facilities and residential rooftop installations make up a smaller but more rapidly expanding segment of PV use. Furthermore, as technological advances narrow the cost gap, more applications are becoming economically feasible at an accelerating rate.