Photovoltaic (PV) cell is the technical term for solar cell, which is used to convert sunlight directly into electricity. Scientists coined the term photovoltaics to refer to this process of converting light (photons) to electricity (voltage). The process has come to be known as the PV effect. Scientists at Bell Telephone were the first to discover the PV effect in 1954 when they noticed that silicon, when exposed to sunlight, could produce an electric charge. This discovery has been pivotal in the development of various PV technologies with uses ranging from space satellites to watches.
Nowadays PV technology is being used to power homes and commercial buildings, and even in large power stations of several utility companies. For an average home, it would take about 10 to 20 solar panels to satisfy its complete power requirement. These panels are made from solar cells that are combined to form modules holding about 40 cells. To absorb maximum sunlight through the day, the panels are either assembled at a fixed angle facing south or they are fixed on a tracking device that follows the sun. When several solar panels are grouped together to form a system, it is termed as a solar array. For large industrial/commercial usage, several solar arrays are combined to form a large-scale PV system.
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Image Credits: nrel.gov
Photovoltaic Technology
PV cells are the basis of photovoltaic technology and are made up of semiconducting materials such as the often used single-crystal silicon. In case polycrystalline silicon is used, it is a thin film on the base of glass or plastic that is not expensive.
At times, thin films of amorphous non-crystalline silicon are also used (although they are not as efficient as crystalline silicon-based technologies), as amorphous silicon cells are not very expensive to manufacture due to the fact that the process is easier and the silicon required is very less. Materials such as indium, tellurium, gallium, cadmium, copper, and selenium are used in combinations in the manufacturing of PV devices.
Size of the PV cell, intensity of light source, and conversion efficiency of the cell will reflect upon the quantity of electricity produced. As PV technologies are used in applications of varying sizes, a PV system may contain the following components based on the load:
- Solar trackers – for tracking the sun and ensuring optimal solar gain of the PV array Battery-charge controllers –for controlling overcharge and over- discharge of the batteries
- Batteries - for storing electricity for later use when the sun is not shining
- Converters - for converting the voltage of the PV system to a higher or lower voltage
- Engine generators – mainly found in hybrid systems for providing backup power as well as charge for the batteries
- Inverters - for converting direct current (dc) electricity to alternating current (ac) electricity.
Possible locations for panel installation are listed below:
- Roof – PV panels can be laminated into the roof membrane or mounted above the roof
- Walls
- Skylights
- Covered parking
The following factors play a vital role in the successful installation of any PV system:
- Solar arrays and inverters must have appropriate thermal management design
- Relevant panel tilt and orientation is required so as to enhance sunlight capture and lessen soiling effects
- Proper fusing, switching, and related safety measures
- Superior quality wire and conduit must be used with no exposed wiring or wire connections
- Efficient system design to ensure functionality and serviceability
- Use of proper tools and professional installation
- Suitable attachment details and assembly operations
- Complete system inspection, testing, and commissioning
- Proper program management and project oversight
Types of PV Cells
Commonly available PV module technologies use one of the following:
- Mono-crystalline silicon
- Polycrystalline silicon
- Flexible amorphous thin film
- Copper-indium-gallium-selenide (CIGS) thin film.
Crystalline cells are made from ultra-pure silicon raw material, which is similar to those used in semiconductor chips. The silicon wafers used here are usually 150-200 µm thick. In the case of thin films, layers of semiconductor material measuring only 0.3 to 2 µm thick are placed on stainless steel or glass substrates. The thinness of the semiconductor layers brings down the costs immensely.
Another key difference between crystalline cells and thin films is their conversion efficiency. The thin film amorphous silicon PV array requires double the space than the crystalline silicon PV array as its module efficiency is halved for the same nominal capacity under standard test conditions (STC) ratings of 1,000W/m2 of sunlight, 250°C (482°F) cell temperature, and spectrum at air mass of 1.5.
Similarly in crystalline silicon PV modules, the module efficiency is less than the sum of the component cell efficiency as there are gaps between the cells and the border around the circuit. This wasted space does not generate power thereby lowering the total efficiency. The effects of temperature also play a role in differentiating the two PV modules. Basically thin film technologies tend to possess a lower negative temperature coefficient compared to crystalline technologies.
Conversion Efficiencies of Different PV Modules
The following are the conversion efficiencies of different PV modules:
- Mono-crystalline silicon - 12.5-15%
- Polycrystalline silicon - 11-14%
- Amorphous silicon (a-Si) - 5-7%
- Copper indium gallium selenide (CIGS) - 10-13%
- Cadmium telluride (CdTe) - 9-12%
Type of PV Installations
The various types of PV installations are listed below:
- Grid systems - This is the most commonly used PV system. It basically is a solar system that is connected to the utility power grid, which provides a 99% efficiency. The excess electricity produced by the solar panels can be transferred to the grid, thus lowering the electricity bill. It does not require batteries and generally is cheaper and simpler to install.
- Off-grid systems – These are also called stand-alone solar systems as they are not connected to a power grid. They require constant access to electricity, thus requiring battery storage and even diesel backup generators. Batteries have to be replaced and are expensive. However, this system is chosen as a cost-effective alternative to power remote places rather than extending power lines to those places.
- Hybrid systems – These solar systems combine the benefits of the above two into one system. They can either be a grid-tied solar system with extra battery storage, or an off-grid solar system with utility backup power. The hybrid systems require components such as battery bank, charge controller, DC disconnect, battery-based grid-tie inverter, and a power meter, to work at optimal level. They are cheaper than off-grid systems.
Applications of Photovoltaics
Photovoltaics are being increasingly used in numerous applications across the globe. Examples of some giant solar projects are the Agua Caliente Solar Project in USA with a capacity of 247 MW, China’s 200 MW Golmud Solar Park, and the Charanka Solar Park in India with a capacity of 214 MW.
The following are some of the key applications of photovoltaics:
- Building-integrated photovoltaics (BIPV) – They can be located near the building, on its roof, or integrated into the building itself.
- Cost-effective solar solutions for remote places
- Solar-powered LED lighting
- Auxiliary power for boats and cars
- As electric power for use in space
- To power calculators and novelty devices
- Solar-powered remote fixed devices - for products such as parking meters, emergency telephones, water pumps, temporary traffic signs, trash compactors, and remote guard posts and signals.
- Solar-powered lighting for roadways/highways
Sources and Further Reading