Photovoltaic Systems

Photovoltaic SystemsSolar cells, also called photovoltaic (PV) cells by scientists, convert sunlight directly into electricity. PV gets its name from the process of converting light (photons) to electricity (voltage), which is called the PV effect. To boost the power output of PV cells, they are connected together to form larger units called modules. Modules, in turn, can be connected to form even larger units called arrays, which can be interconnected to produce more power, and so on. In this way, PV systems can be built to meet almost any electric power need, small or large.


Solar panels used to power homes and businesses are typically made from solar cells combined into modules that hold about 40 cells. A typical home will use about 10 to 20 solar panels to power the home. Many solar panels combined together to create one system is called a solar array. For large electric utility or industrial applications, hundreds of solar arrays are interconnected to form a large utility-scale PV system.

Depending on the functional and operational requirements of the system, the specific components required may include major components such as a DC-AC power inverter, battery bank, system and battery controller, auxiliary energy sources and sometimes the specified electrical load (appliances). In addition, an assortment of balance of system (BOS) hardware, including wiring, overcurrent, surge protection and disconnect devices, and other power processing equipment.

Batteries are often used in PV systems for the purpose of storing energy produced by the PV array during the day, and to supply it to electrical loads as needed (during the night and periods of cloudy weather). Other reasons batteries are used in PV systems are to operate the PV array near its maximum power point, to power electrical loads at stable voltages, and to supply surge currents to electrical loads and inverters. In most cases, a battery charge controller is used in these systems to protect the battery from overcharge and overdischarge.

When considering a solar PV system, local sunlight is an important factor in the output and economics. Electric output is essentially proportional to the amount of local sunlight, and cost is inversely proportional to output. It's relatively easy to estimate the amount of annual output from a solar photovoltaic (PV) system, its comparative price with other solar systems, and its economics in terms of cents per kilowatt-hour (c/kWh). See NREL's PVWatts calculator, which helps to determine the energy production and cost savings of grid-connected photovoltaic energy systems throughout the world. Version 1 allows you to select a location from a map or text list of pre-determined locations. Version 2 allows you to select any location in the United States.


Photovoltaic Solar Resource
National Solar Photovoltaics (PV) Resource Potential of the United States
for additional solar maps see: National Renewable Energy Laboratory (NREL)

Advantages of Photovoltaic Solar Power

Photovoltaic solar power is one of the most promising renewable energy sources in the world. Compared to nonrenewable sources such as coal, gas, oil, and nuclear, the advantages are clear:

  • Generates free energy from the sun.
  • Has no moving parts to break down thus requiring minimal maintenance.
  • Non-polluting energy reduces emissions: Has no direct impact on the environment.
  • Photovoltaic (PV) cells are modular, you can start with a small system and expand as your needs increase.
  • Systems have a long life & durability. Cells last 25-30 years.
  • Grid-Tie systems allow you to sell excess electricity back to the utility.
  • Can be installed and operated anywhere including areas of difficult access and remote locations.
  • Helps get us off dependence on foreign oil.
  • PV cells make no noise and give off no exhaust.
  • Allow the use of electricity in remote areas where it would be expensive or impossible to run power lines.
  • Have electrical power during blackouts.
  • Rebates and incentives available.
Reliability

Reliability is an important factor when considering the costs of a PV system. Since the building blocks of solar arrays are PV cells, which are considered "solid-state" devices with no moving parts and, therefore, are highly reliable and long-lived; reliability of the entire PV system is usually considered when evaluating photovoltaic 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.

Today's photovoltaic modules are extremely safe and reliable products, with minimal failure rates and projected service lifetimes of 20 to 30 years. Most major manufacturers offer warranties of 20 or more years for maintaining a high percentage of initial rated power output. When selecting PV modules, look for the product listing (UL), qualification testing and warranty information in the module manufacturer's specifications.

Additional Resources
Websites
  • DOE's EERE Photovoltaics Energy Basics website gives a brief description of how the photovoltaic materials convert sunlight into electrical energy.
  • Database of State Incentives for Renewables & Efficiency (DSIRE) provides a comprehensive list of federal, state, and local incentives that promote renewable energy and energy efficiency.
  • Go Solar California has information about the PVUSA Test Conditions energy production of most modules.
  • North American Board of Certified Energy Practitioners (NABCEP) provides an industry certification of experienced photovoltaic installers. NABCEP was designed to raise industry standards and promote consumer confidence in photovoltaic and solar thermal system installations.
  • The National Association of Home Builders NAHB Research Center's ToolBase.org is the housing industry's best resource for technical information, providing builders with a reliable source of information on new products or processes, details on each technology, and contact information for manufacturers. The site details solar benefits, costs, and ease of implementation compared with other technologies.
  • National Center for Photovoltaics (NCPV) focuses on innovations in photovoltaic technology that drive industry growth in photovoltaic manufacturing nationwide. Formed by the U.S. Department of Energy (DOE) and based at NREL, the NCPV focuses on research and development and increasing U.S. competitiveness.
  • NREL's PVWatts calculator determines the energy production and cost savings of grid-connected photovoltaic energy systems throughout the world. Version 1 allows you to select a location from a map or text list of pre-determined locations. Version 2 allows you to select any location in the United States.
  • Procuring Solar Energy: A Guide for Federal Facility Decision Makers provides an overview of federal facility managers and their procurement terms on the process of installing solar electric and solar thermal systems.
  • Sandia National Laboratories teams with the U.S. Department of Energy, industry, and academia to improve the performance and reliability of photovoltaic technologies and grid integration.
Societies
Publications