Renewable Energy and Photovoltaics Notes

Renewable Energy Overview

• Definition: Energy generated from natural resources like sunlight, wind, rain, tides, and geothermal heat that are naturally replenished.

• Global Consumption (2006): Approximately 18% of global final energy consumption was from renewable sources.

• Most Promising Source: Solar energy due to its abundance.

• Current Contribution: Solar energy accounts for less than 1% of total global energy demand.

Renewable Energy Consumption in the US (2004)

• Total US Energy Consumption: 100.278 Quadrillion Btu

  • Petroleum: 40%

  • Natural Gas: 23%

  • Coal: 23%

  • Nuclear Energy: 6%

  • Renewable Energy: 8% (6.117 Quadrillion Btu)

• Renewable Energy Breakdown:

  • Biomass: 47%

  • Hydroelectric: 45%

  • Geothermal: 6%

  • Wind: 2%

  • Solar: 1%

Renewable Energy Technologies

• Photovoltaic (PV) solar systems

• Solar thermal collectors

• Wind power

• Hydroelectric power

• Biomass

• Integration into electricity networks in developed and developing countries.

• Solar water heating systems.

Solar Photovoltaic (PV) Systems

• Examples:

  • 4 kW solar PV system at BWP employee parking area.

  • Robert Beher's 2 kW solar PV system on his roof.

  • 7.3 kW residential ground mount solar PV.

  • 45 kW solar system at Big Dog Productions.

  • 7.3 kW solar PV and solar pool heater.

Solar Cells and Photovoltaic Effect

• Solar Cell Definition: A device that converts light directly into electricity through the photovoltaic effect.

• Photovoltaic Effect: The basic physical process through which a PV cell converts sunlight into electricity.

• Photons: Sunlight is composed of photons, which contain different amounts of energy corresponding to different wavelengths of the solar spectrum.

• Photon Interaction with PV Cell:

  • Photons can be reflected, absorbed, or pass through the cell.

  • Absorbed photons generate electricity.

  • Energy of a photon is transferred to an electron in an atom of the semiconductor material.

  • The electron escapes its normal position and becomes part of the current in an electrical circuit.

Solar Cell Assemblies

• Components:

  • Single Photovoltaic Cells: individual units.

  • Photovoltaic Module: connected in series.

  • Photovoltaic System: several modules assembled together.

• Material: Silicon in crystalline form is most commonly used to make photovoltaic cells. momcrystlien

• Process:

  • Photovoltaic panels absorb photons and initiate an electric current.

  • Photons striking the solar panel surface cause electrons to be knocked out of their atomic orbits.

  • These free electrons are pulled into a directional current by the electric field generated by the solar cells.

Albert Einstein and the Photoelectric Effect

• Einstein's Contribution (1905): Described light as composed of discrete quanta (photons).

• Photon Energy: A photon above a threshold frequency has the required energy to eject a single electron.

• Nobel Prize: Einstein received the Nobel Prize in Physics in 1921 for this discovery.

Generations of Solar Cells

• Classification: Solar cells are classified into three generations based on their order of importance.

• First Generation:

  • Composed of large-area, high-quality, single-junction devices.

  • High energy and labor inputs.

  • Approaching theoretical limiting efficiency of 31%.

  • Energy payback period of 5–7 years.

  • Dominated commercial production in 2007 (89.6%).

• Second Generation:

  • Developed to address energy requirements and production costs.

  • Examples: Amorphous silicon and silicon microamorphous.

  • Applied in a thin film, reducing material mass and costs.

  • Promise of higher conversion efficiencies and cheaper production costs.

• Third Generation:

  • Aim to enhance poor electrical performance of second-generation cells while maintaining low production costs.

  • Targeting conversion efficiencies of 30-60%.

  • Approaches:

    • Multi-junction photovoltaic cells.

    • Concentration of the incident spectrum.

    • Use of thermal generation by UV light to enhance voltage or carrier collection.

    • Use of the infrared spectrum for night-time operation.

Multi-Junction Solar Cells

• Limitation of Single-Junction Cells: Can only use photons with energy equal to or greater than the band gap of the cell material.

• Multi-Junction Approach: Use two or more different cells with multiple band gaps to generate voltage.

• Efficiency: Higher total conversion efficiency because they can convert more of the energy spectrum of light to electricity.

• Structure: Stack of individual single-junction cells in descending order of band gap ($E_g$).

• Process: The top cell captures high-energy photons, and the rest pass to lower-band-gap cells.

Photo Generation of Charge Carriers

• Photon Interactions with Silicon:

  1. Photon passes straight through (lower energy photons).

  2. Photon reflects off the surface.

  3. Photon is absorbed (if energy is higher than the silicon band gap value), generating an electron-hole pair and sometimes heat.

High-Efficiency Solar Cells

• Definition: Solar cells that generate more electricity per incident solar power unit (watt/watt).

• Focus: Cost-efficient technologies in terms of cost per generated power.

• Strategies to Reduce Cost:

  • Increasing efficiency.

  • Decreasing the cost of solar cells per generated unit of power.

• Challenge: Increasing photovoltaic efficiency is of great interest from both academic and economic perspectives.

Solar Modules and Arrays

• Construction: Solar cells are often electrically connected and encapsulated as a module (panel).

• Protection: PV modules often have a sheet of glass on the front side to protect the semiconductor wafers from the elements.

• Interconnection: Modules are interconnected in series, parallel, or both to create an array with the desired peak DC voltage and current.

• Power Output: Measured in watts or kilowatts.

Practical Use of Solar Energy

• Grid Connection: Electricity is often fed into the electricity grid using inverters.

• Energy Storage: Batteries are used to store energy that is not needed immediately.

• Applications:

  • Solar cell phone chargers.

  • Solar bike lights.

  • Solar camping lights.

Charge Carrier Separation

• Modes:

  • Drift: Driven by an electrostatic field across the device.

  • Diffusion: From zones of high carrier concentration to zones of low carrier concentration.

• p-n Junction Solar Cells: The dominant mode of charge carrier separation is by drift.

Equivalent Circuit of a Solar Cell

• Purpose: To understand the electronic behavior of a solar cell using well-known electrical components.

• Components: Equivalent circuit includes elements that model the behavior of the solar cell under different conditions.

Solar Cell Efficiency Factors

• Energy Conversion Efficiency (η): The percentage of power converted from absorbed light to electrical energy and collected when a solar cell is connected to a circuit.

• Calculation: $\eta = \frac{P<em>m}{E \times A</em>c}$

  • Where:

    • $P_m$ is the maximum power point.

    • $E$ is the input light irradiance in W/m\text{^2}.

    • $A_c$ is the surface area of the solar cell in m\text{^2}.

Maximum Power Point

• Definition: The point that maximizes $V \times I$ (voltage times current) on an irradiated cell.

• Determination: Determined by increasing the resistive load on the cell continuously from zero to a very high value.

Lifespan and Research

• Lifespan: Commercially available solar cells can produce electricity for at least twenty years without a significant decrease in efficiency.

• Current Research: Active research in universities and institutions worldwide, divided into three areas (unspecified in the transcript).

Advantages and Disadvantages of Solar Energy

• Advantages:

  • Free energy source.

  • No fuel required.

  • Produces no waste or pollution.

  • Useful in remote locations without easy access to electricity.

  • Suitable for low-power applications.

• Disadvantages:

  • Doesn't work at night.

  • Expensive to build solar power stations.

  • Can be unreliable in less sunny climates.

  • Requires a large area of solar panels for high-power applications.

Solar Cell Applications

• Satellites: Powering satellites for TV, telephones, navigation, weather forecasting, and internet.

Real-World Examples

• GPS (Global Positioning System): Relies on Einstein's theory of relativity for accurate calculations.

• Hubble Space Telescope: Provides clear view of the universe.

Concentrating Solar Photovoltaic Technology

• France Project: A 36-acre photovoltaic park generates 18.2 MW with an annual supply of 26 million kWh, powering 8,000 families.