Xcel Energy

Introduction to Solar Energy

Directly or indirectly, virtually all energy we use is derived from solar energy: wind is created when solar energy creates differences of temperature on areas of the earth's surface, biomass energy is derived from solar energy that has been captured and stored, and fossil fuels are derived from biomass that has been trapped and transformed by geologic processes over millions of years.

In addition to the many indirect ways we use the sun's energy, there are three main ways that we use solar energy directly:

Passive Solar

Passive solar is when buildings directly take advantage of solar energy. A greenhouse is an example of passive-solar structure. While virtually all structures capture passive solar energy to some extent, sophisticated passive solar buildings are precisely designed (using window placement, building orientation and shape, thermal mass, etc) to take maximum advantage of solar energy during the course of the year.

Solar Thermal

Solar thermal systems actively collect, transport, and utilize solar energy to generate heat. The most common systems are those used to heat water, but there are also systems designed for other applications like space heating and cooking.

Solar Electric

There are two primary solar-electric technologies, photovoltaics and concentrating-collector systems:

  • Photovoltaics (PV) convert sunlight directly into electricity when sunlight is absorbed by a solar cell and the solar energy knocks electrons loose from their atoms, allowing the electrons to flow through the material to produce electricity. PV technology uses solar cells made of semiconducting materials similar to those used in computer chips. Typically, many solar cells are combined on a single panel or module, with most installations consisting of multiple panels connect together into an array. The most common type of PV modules consist of ridged solar cells within a glass and metal framework, but there are also thin-film solar cells which are flexible and very durable.
  • Concentrating-collector solar electric systems operate similarly to many typical power plants, which use a heat source to generate steam which in turn rotates a turbine activating a generator that produces electricity. Concentrating solar power systems uses the sun as a heat source. There are three main types of concentrating solar power systems: parabolic-trough, dish/engine, and power tower.


PV Solar Radiation Map (National Renewable Energy Laboratory)