What is Solar Energy?

It takes a little over 8 minutes for light from the Sun to travel about 93 million miles to Earth.  This solar energy, or insolation, drives photosynthesis in plants and influences everything from our weather and climate to our health and religious traditions.  Human societies have always converted solar energy flows into resources such as food, shelter and energy, and to power a wide range of other activities.  Even petroleum originated as solar energy.

As this NREL photovoltaic resource map of the United States indicates, the solar PV resource in the U.S. is unevenly distributed, with the desert Southwest showing greater potential, and lesser potential evident in New England.  However, Germany, which lies at a more northern latitude than Vermont, actually leads the world in solar PV installations!  Solar energy works in Vermont:  there were over 1,700 PV and hot water installations in the state on May 22, 2012. Two kinds of solar energy—photovoltaics and hot water (thermal) systems—are featured in the Atlas.

  • Solar photovoltaic systems: Photovoltaic cells are made out of a semiconductor material that converts the photons in sunlight into electricity.  According to the Energy Information Administration, when photons strike a photovoltaic cell, they may be reflected, pass right through, or be absorbed.  Only the absorbed photons provide energy to generate electricity.  When enough sunlight is absorbed by the material, electrons are dislodged from the material’s atoms.  When many electrons, each carrying a negative charge, travel toward the front surface of the cell, the resulting imbalance of charge between the cell’s front and back surfaces creates a voltage potential like the negative and positive terminals of a battery.  When the two surfaces are connected through an external load, such as an appliance, electricity flows.
  • Solar hot water (thermal) systems: Humans have built south-facing structures that ‘passively’ capture solar energy for heating for millennia.  Active solar thermal systems take that same basic principle but use a collector to absorb and collect solar radiation.   Fans or pumps are then used to circulate the heated air or heat absorbing fluid.