Renewable Energy Technologies Quiz

Renewable Energy Technologies

• PV Conversion Theory is covered.
• Bibliography: Documents are available in Moodle and on pveducation.org.

Basics

• Peripheral electrons are grouped into conduction electrons and valence electrons.
• Conduction electrons carry electricity in the conduction band and are free to move in an electric field.
• Valence electrons have limited mobility and form chemical bonds with other atoms.
• The band gap of a semiconductor is the minimum energy required to excite an electron.
• The gap between energy bands dictates if a material is a metal, insulator, or semiconductor.

Energy Gap

• Conducting materials have overlapping conduction and valence bands.
• Semiconductors have a small energy gap between the bands (approximately 1-2 eV).
• Insulating materials have a large energy gap between the bands.

Other Topics

• Conductive materials have a very small bandgap that overlaps or are already in the conduction band.
• Other elements have a wide bandgap preventing conduction electrons at room temperature, which makes them insulators.
• The bandgap for semiconductors is in the range of 1 to 1.5 eV.
• In semiconductors, electrons in the conduction band and holes in the valence band are responsible for electrical conduction.
• The energy required to break bonds corresponds to the width of the bandgap.
• Silicon energy gap value: 1.12 eV
• Gallium arsenide energy gap value: 1.42 eV
• Intrinsic carriers concentration (n₁) is equal to the concentration of both electrons and holes in pure intrinsic semiconductors.
• The concentration of intrinsic carriers in silicon is 1.5x10^10 cm-3.
• The concentration of intrinsic carriers in gallium arsenide is 1.8x10^6 cm-3.
• Activation energy E₂ is approximately equal to Eg / 2, where k = 1.38 x 10^-23 J/K and T is the absolute temperature.
• Calculation to determine the concentration of silicon atoms in a crystalline solid, using the given data.
• Doping: Adding impurities to a crystal to increase the number of electrons or holes in the semiconductor to improve electrical conductivity.
• Phosphorus is a donor impurity, adding extra electrons.
• Boron is an acceptor impurity, creating holes.
• The ionization energy of phosphorus in silicon is smaller than the energy gap, which allows electrons to move to the conduction band.
• The ionization energy of an acceptor in silicon is also smaller.
• In semiconductors, both electrons in the conduction band and holes in the valence band take part in electrical conductivity.
• For pure semiconductors, the concentration of negative charges (conduction electrons) equals that of positive charges (conduction holes), which is represented as n₁.
• Factors impacting intrinsic carrier concentration: bandgap and temperature.

Doping

• Adding impurities changes the number of electrons in the conduction band or holes in the valence band.
• Extrinsic semiconductors are formed after introducing impurities.
• Impurity atoms are either donors (extra electrons) or acceptors (missing electrons/holes).

Energy Level

• Probability that an energy level is occupied by an electron, which depends on Fermi energy and temperature.
• In intrinsic materials the number of energy states is approximately the same for both bands.
• By doping with n-type atoms, the electron concentration increases leading to an increase in Fermi energy level.
• Conversely, doping with p-type atoms leads to a decrease in Fermi energy level.

Junction

• P-N junction formation: one side of semiconductor material is doped with p-type atoms (e.g boron) and the other side with n-type atoms (e.g. phosphorus).
• Two layers of material that are originally neutral, form an electric field through the contact forming the P-N junction.
• Charge concentration gradient leads to electron diffusion in the p-type and holes in the n-type region.
• This diffusion generates a potential barrier between regions, with the p-type becoming negatively charged and the n-type positively charged.
• The internal electric field distorts the energy bands where the Fermi level is constant across the junction.

I-V Characteristics

• I-V characteristics show the relationship between current and voltage across the junction.
• With no voltage, there is no current.
• With direct polarization, an exponential increase in current (diode effect) occurs.
• With reverse polarization, current is very low (ideally zero), acting as a diode.
• The curve depends on incident light.

Photon Energy

• Only photons with energy equal to or greater than the bandgap of semiconductor material can excite electrons into the conduction band.
• When a photon has enough energy, it's absorbed and creates an electron-hole pair.
• Amount of photons reaching the surface each second depends on the wavelength.
• The wavelength of solar radiation that can be fully converted is determined from the energy gap.

Preliminary Evaluation of Electric Power

• Calculation of the electric current generated in ideal conditions under solar radiation.

Solar Cell

• Photovoltaic cell converting light into electricity by exploiting the electric field separating charges generated during light absorption.
• Excess energy is converted to heat which is a major loss mechanism in the PV cell.
• Semiconductor materials that use different processes to convert solar energy into electricity are the main subjects of the topic.
• PV cell consists of p-type and n-type semiconductor materials joined together forming a junction.

Efficiency of a Solar Cell

• Comparison of the power output to the amount of power input considering standard conditions.

Conversion Efficiency of a Cell

• Relationship between the maximum power (Pstc) from the cell under standard test conditions.

Main Inefficiencies in Solar Cells

• Impurities or defects may lead to the annihilation of electron-hole pairs.
• Surface reflections and shading affect top contacts.
• Series resistances through emitter and base of the solar cell, contact resistance, top and rear metal contacts increase resistances.
• Impact on fill factor (reduces) and short circuit current.

Factors Affecting Performance

• Efficiency, peak power, irradiance, cell temperature, and air mass.

NOCT - Nominal Operating Cell Temperature

• Standardized conditions to calculate cell temperature under operational conditions.

Single Junction Cell

• The material for solar cells affects their efficiency as high Eg implies less photons in the solar spectrum with E > Eg.

Multi-Junction Cell

• Efficiency roughly 32% today with high costs and used in aerospace and similar applications
• Stacked materials to absorb different frequencies of sunlight are the most efficient.

Different Approaches to PV

• Conventional approach: uses silicon.
• Thin film approach: uses materials like cadmium telluride (CdTe), copper indium gallium selenide (CIGS), and amorphous silicon in a thin film form.
• Concentrator approach: uses lenses to focus sunlight onto a smaller area.

Solar Cell Components

• Structures for multi-layers and different materials used.
• Descriptions and diagrams for cells, panels, modules, and photovoltaic generator.

Photovoltaic Modules

• Modules have various parts: cells panels, modules, and photovoltaic generator etc.
• Silicon based components used.

Silicon Ingots

• Silicon refinement into single crystal polysilicon versus multi-crystalline polysilicon.
• Ingot growth and wafer processing.

Costs and Energy Sharing in Silicon PV

• Division of costs among different system elements (e.g., system, installation, BOS, modules etc.)

Costs of PV

• Historical price trend for crystalline silicon PV cells. This involves tracking the cost per watt over time.
• Implementation of Swanson's law: demonstrates a relationship between cumulative installations and cost per watt.

PV Module Production by Region

• Global productions across different regions.

Different Approaches to PV

• Approaches to developing PV materials with lower cost: conventional, thin film, and concentrator approaches.

BIPV (Building Integrated Photovoltaics)

• Technologies for integrating PV into buildings.
• Categories according to their level of integration: least, more, and fully integrated.

Potential Applications

• Low and high-rise buildings considering available space, facades, and technical restrictions.
• Important advantage: heat reduction, reducing temperatures in urban areas by 20 to 30 degrees.

AgriVoltaics

• Combination of agriculture and solar.
• Descriptions of advantages and constraints for agrivoltaics.
• Crop yield increase and lower temperatures.
• Potential for increased harvest sizes.
• High investment and shade tolerance needed.

Floating PV

• PV technology that is built over water systems.
• Global trends/growth and examples of applications.

PV Software

• Freeware software for PV energy performance estimation.

Test your knowledge on the fundamentals of photovoltaic conversion theory and energy gap concepts in renewable energy technologies. Explore the differences between conductors, semiconductors, and insulators, and understand the significance of electron mobility in materials.