Photovoltaic (PV)

Photovoltaics (PV) is the field of technology and research related to the application of solar cells for energy production by converting Sun energy (sunlight, including Sun ultra violet radiation) directly into electricity by the photovoltaic effect. The latter refers to the process of converting light (photons) to electricity (voltage). Solar cells are photovoltaic devices that use semi-conducting materials to convert sunlight directly into electricity. When sunlight is absorbed by these materials, it causes electrons to flow through the material generating electric currents. Solar cells produce direct current (DC) electricity. There are two broad categories of solar cells; thin film and crystalline.

The key components of a photovoltaic power system are the photovoltaic cells (also called solar cells) interconnected and encapsulated to form a photovoltaic module (the commercial product), the mounting structure for the module or array (several modules mounted and interconnected together to produce a desired voltage and current (power capacity)), the inverter (essential for grid-connected systems and required for many off-grid systems), the storage battery and the charge controller (for off-grid systems only). Solar cells are typically combined into modules that hold up to 40 cells to generate substantial voltages (typically 12 V or 24V) and currents that can be used to power various devices. The power output of a module is measured under standardized test conditions in Watt Peak (Wp).

Performance of PV modules depends on the amount of solar irradiation received which varies by location and season.  For this reason, systems normally need to be carefully designed.  A typical commercial solar cell has an efficiency of 15%. The first solar cells, built in the 1950s, had efficiencies of less than 4%.

Applications and Efficiency

PV technology can be employed in a variety of applications: Typical applications of PV technology include remote telecommunications, cathodic protection of pipelines, PV home systems, vaccine refrigeration, water pumping, grid connected  or building integrated systems, miniature electronic devices and toys:

Off-grid domestic PV systems like solar home systems:

• Provide electricity to households and villages that are not connected to the utility electricity network (also referred to as the grid)
• Provide electricity for lighting, phone charging, refrigeration and other low power loads
• Are often the most appropriate technology to meet the energy demands of off-grid communities

Off-grid non-domestic PV installations:

• Are used in locations where small amounts of electricity have a high value
• Were the first commercial application for terrestrial PV systems
• Provide power for a wide range of applications, such as telecommunication, water pumping, vaccine refrigeration and navigational aids
• Make PV commercially cost competitive with other small generating sources

Grid-connected distributed PV systems:

• Provide power to grid-connected customers or directly to the electricity network (specifically where that part of the electricity network is configured to supply power to a number of customers rather than to provide a bulk transport function)
• May be on or integrated into the customer's premises, often on the demand side of the electricity meter, on public and commercial buildings, or elsewhere in the built environment

Grid-connected centralized PV systems:

• Perform the functions of centralized power stations
• Supply power that is not associated with a particular electricity customer
• Primarily supply bulk power, rather existing on the electricity network to perform specific functions.

To design a system for PV application the following information is required: daily energy requirement, voltage and current draw of appliances, average insolation (kWh/m2 day), the yearly variation in insolation levels for the specific area and the equipment type, availability and costs to enable appropriate selection.

Capability and Limitations

• The number and type of appliances that can be used with SHS is limited.  Lights, TVs, sound systems and low-wattage DC appliances are appropriate.  
• Costs of well designed, installed and maintained systems are relatively high on a cash basis although the life cycle cost of PV is often less than comparison technologies.
• Sales, installation and support infrastructure for systems is largely underdeveloped leading to higher delivery and maintenance cost.
• Experiences show that security is essential for most PV installations.  In remote unguarded locations, there is risk of the modules and other system components being stolen or vandalized.

Links to further reading

http://en.wikipedia.org/wiki/Photovoltaics
http://www1.eere.energy.gov/solar/photovoltaics.html
http://www.greenenergy.org.uk/pvuk2/technology/index.html