What Is Solar?

Solar, in an engineering and scientific context, refers to the capture and conversion of energy from the sun for practical use. The sun radiates roughly 173,000 terawatts of power continuously onto the earth's surface, a quantity that dwarfs all human energy consumption. Solar technologies extract this energy through two principal pathways: the direct conversion of sunlight into electricity using photovoltaic (PV) cells, and the collection of solar heat for thermal processes or for driving turbines in concentrating solar-thermal power plants.

The field draws from semiconductor physics, optics, thermodynamics, materials science, and power systems engineering. Advances in each of these disciplines have steadily increased conversion efficiency and lowered the cost of solar installations over the past four decades, making solar the fastest-growing electricity source worldwide.

Photovoltaic Systems

Photovoltaic technology converts sunlight directly into direct-current electricity through the photovoltaic effect, a phenomenon in which photons absorbed by a semiconductor material free electrons to produce a current. Silicon is the dominant semiconductor, with monocrystalline silicon cells achieving commercial efficiencies of 20 to 24 percent and polycrystalline cells reaching 16 to 18 percent. Thin-film technologies based on cadmium telluride or copper indium gallium selenide offer lower efficiency but reduced manufacturing cost and better performance at high temperatures.

Individual PV cells are wired into modules, modules are combined into arrays, and arrays feed inverters that convert direct current to grid-compatible alternating current. The U.S. Department of Energy's solar photovoltaic technology overview describes the full system chain from individual cells through utility-scale power plants, including the California Solar Star facility, which operates at 579 megawatts. Rooftop and distributed installations operate at the kilowatt scale and connect to the low-voltage distribution grid.

Concentrating Solar-Thermal Power

Concentrating solar-thermal power (CSP) uses mirrors or lenses to focus a large area of sunlight onto a receiver, generating high-temperature heat rather than electricity directly. Parabolic trough systems focus sunlight onto a fluid-filled tube running along the focal line of a curved mirror; power tower systems use a field of tracking flat mirrors called heliostats to concentrate sunlight onto a central receiver atop a tower. The heat is used to generate steam, which drives a conventional turbine-generator set. The U.S. Department of Energy's concentrating solar-thermal power resource explains how CSP plants are frequently paired with thermal energy storage systems, allowing them to deliver power after sunset and providing a dispatchability advantage that PV systems require batteries to replicate.

Solar Resource Assessment

The engineering potential of any solar installation depends on the local solar resource, quantified as irradiance in watts per square meter or as cumulative daily insolation in kilowatt-hours per square meter per day. Direct normal irradiance (DNI) governs CSP performance, while global horizontal irradiance (GHI) governs flat-panel PV. The International Energy Agency's solar PV market analysis tracks global deployment trends and notes that solar PV accounted for 5.4 percent of total global electricity generation in 2023, with investment in the sector that year exceeding all other power generation technologies combined.

Applications

Solar has applications in a wide range of disciplines, including:

• Utility-scale electricity generation for grid supply
• Residential and commercial rooftop power systems
• Space and satellite power supply systems
• Agricultural water pumping and rural electrification
• Desalination and industrial process heat
• Solar charging infrastructure for electric vehicles