Overview of Solar Energy

Overview of Solar Energy

• Solar insolation is a primary energy source, used for space heating, water heating, and thermoelectric power generation.
• Solar cells convert solar energy directly into electricity.
• The chemistry of semiconductors is key to solar cell function.
• Solar PV (photovoltaic) systems, involving solar panels and solar arrays, and the economics of solar systems are discussed.

Lecture Outline

• 99.9% of Earth's surface energy comes from the sun.
• Most primary energy sources rely on the sun directly or indirectly.
• Solar radiation can be used directly for work.
• Four major solar energy forms: passive solar, solar heating, concentrated solar, and solar photovoltaics (PV).
• Solar PV systems consist of solar panels, which contain solar cells, made of semiconductors.
• Semiconductors have varying conductivity, ranging from conductors to insulators, dependent on how tightly electrons are bound. The difference is based on how tightly the outer electrons are bound in the outermost electron shell around the atomic nuclei.
• Electrons in a conductor can easily move about.
• Electrons in a valence band of an atom will break out into a conductive band with a certain amount of energy (band gap).
• Insulators have a wide band gap, making it difficult for electrons to jump.
• Conductors have a small or nonexistent band gap.
• Semi-conductors have a moderate or small band gap.

Semiconductors (Detailed)

• Semiconductors exist on a spectrum from conductors to insulators.
• Conductors conduct electricity, while insulators do not.
• The difference in conductivity lies in how tightly electrons are held in the outermost electron shell of the atom (valence shell).

Semiconductor Doping

• Semi-conductors are created by doping silicon with other elements to alter its electron properties.
• N-type semiconductors have extra electrons.
• P-type semiconductors have fewer electrons.

P-N Junction

• A P-N junction forms when N-type and P-type semiconductors are joined.
• Excess electrons in the N-type material are attracted to excess electron holes in the P-type material.

Solar Cell Operation

• Electrons move from the N-side to the P-side when sunlight is present.
• The depletion zone creates an electric field opposing electron movement.
• The width of the band gap is the same for both sides of the semiconductor, but the absolute energy level at the bottom of the band gap is greater on the P-side than the N-side.
• Electrons tend to concentrate on the N-side, and holes concentrate on the P-side.

Solar Cell Construction

• Solar panel construction uses multiple solar cells in series.
• The total panel voltage is the sum of the cell voltages.

Types of PV Cells

• Monocrystalline PV cells: Made from single silicon crystals, most expensive, highest efficiency (17-22%).
• Polycrystalline PV cells: Made from multiple interlocking silicon crystals, faster to create, lower cost, lower efficiency (15-17%).
• Film PV cells: Made from rapidly quenched doped silica, no crystals, flexible, lowest efficiency (10-13%).

Solar PV System

• Solar panels are arranged in series to increase voltage, and in parallel to increase current.
• Inverters convert DC current from panels to AC current.

Solar Resource and Output

• Solar insolation is predictable, but it's intermittent due to clouds/night.
• Peak solar insolation often occurs near peak electricity demand.

Solar Panel Orientations

• Fixed panels: Fixed orientation, equates to the least solar energy.
• Panels with seasons: Fixed orientation but tilt varies with the seasons.
• Panels with tracking: Tracks the sun, generally the most solar energy.

Solar Economics

• Main cost components include PV modules, inverters, balance-of-system hardware (wiring, mounting, etc.), and installation/permitting costs.
• Incentives are key to reducing solar costs for residential customers.