Solar Flashcards
Concentrators of solar radiation
Solar concentrators use lenses, mirrors, or other reflective surfaces to focus a large area of sunlight onto a much smaller area of solar cells at the focal point. Concentrators are used to reduce the amount of solar panel material needed, thereby lowering costs and improving performance.
options for Sun tracking
- Single-Axis Tracking: Moves the concentrator side-to-side (east to west) or up-and-down (north to south). It is simpler and cheaper but less accurate.
- Dual-Axis Tracking: Moves in all directions to perfectly follow the sun’s path, capturing maximum sunlight but at a higher cost and complexity.
scattered light use
Non-imaging concentrators like CPCs and fluorescent collectors can harness diffuse or scattered sunlight, making them effective in locations with high cloud cover or atmospheric scattering.
Splitting of solar spectrum – motivation
By splitting the solar spectrum into different wavelengths, light is directed to specific devices optimized for those wavelengths, improving efficiency. This approach ensures that each section of the spectrum is utilized effectively, minimizing energy losses. It allows solar energy systems to produce more electricity from the same amount of sunlight, as different materials can better capture specific parts of the spectrum, reducing thermalization losses and enhancing overall performance.
Splitting of solar spectrum - typical implementations
Stacked/Cascade Reflection Spectral Splitters:
Uses layers of reflective materials that reflect specific wavelengths, others are transmitted. Each layer directs a different part of the light spectrum to the appropriate solar cell.
External Spectral Splitters/Concentrators:
Separate devices outside of solar cells split light into different wavelengths using mirrors or prisms. Each wavelength is then concentrated and sent to specialized solar cells or devices.
Integrated Spectral Splitters:
Built into solar panel - divide incoming light inside the system. Different wavelengths are sent to corresponding solar cell layers, reducing complexity and improving efficiency.
How do fluorescent collectors work
Fluorescent collectors: clear panels with special dyes that absorb sunlight and re-emit it at longer wavelengths. Then light is trapped inside the panel by internal reflection and guided to the edges, where solar cells convert it into electricity.
What limits fluorescent collectors efficiency
Re-absorption losses - re-emitted light is reabsorbed by the fluorescent material before reaching the solar cell
Spectral mismatch - the emission spectrum of the fluorescent material may not perfectly match the spectral response of the solar cell, leading to energy losses.
Stokes Shift and quantum efficiency - Stokes shift is the difference between the absorbed and re-emitted photon energy. Some energy is lost as heat during the emission process, thus reducing the overall quantum efficiency of the material.
Thermophotovoltaics – principles, material requirements and applications of filters.
Thermophotovoltaics – principles, material requirements and applications of filters. PV that have small band gap <1 eV. They harvest energy from IR photons. The materials must withstand high temperatures without loosing there electric or mechanical properties. They can be used for photon recycling they harvest photons that normal PV can not. In spectral splitting system. They could also harvest heat from industrial engines helping them thermoregulate and producing additional power. Filters: optimizes the spectra for TPV cells, reflecting unused light back into heat source to improve efficiency.
CdTe
CdTe structure top to bottom: front glass, n-dopedSnO2, n-CdS buffer, p-CdTe, metal back contact.
The production sequence of a CdTe solar cell involves depositing a transparent conductive oxide (TCO) onto a glass substrate, followed by an n-type cadmium sulfide (CdS) layer. A p-type CdTe absorber layer, a few um thick, is then deposited using sputtering. The back contact is applied, which is challenging due to the limited compatible metals, and doping the contact area improves electrical performance.
* Sieries junction
CIS
- Direct band gap semiconductor ~1 eV. CuInSe2.
- On glass substrate a Mo back contact is deposited; 2. Laser scribing 3. Evaporation of Cu, In, Se precursors at high temp to combine. 4. Chemical bath deposition CdS; 5. Mechanical scribing; 6. Deposition of transparent contact 7. Mechanical scribing; 8. Encapsulation.
- Series junction.
CIGS
- Direct band gap semiconductor 1-1.7 eV. Same as CuIn(Ge)Se
- On glass substrate a Mo back contact is deposited; 2. Laser scribing 3. Sputtering of Cu, In,Ge and then it is annealed at high temperature and Se gass pressure to form the semiconductor.. 4. Chemical bath deposition CdS; 5. Mechanical scribing; 6. Deposition of transparent contact 7. Mechanical scribing; 8. Encapsulation.
- Series junction
CZTS
- Direct band semiconductor made from Cu, Zn, Sn, S. It used instead of CIGS a as materials are more abundant.
- On glass substrate a Mo back contact is deposited; 2. Laser scribing 3. Sputtering of Cu, Zn, Sn, S and then it is annealed at high temperature and Se gas pressure to form the semiconductor.. 4. Chemical bath deposition CdS; 5. Mechanical scribing; 6. Deposition of transparent contact 7. Mechanical scribing; 8. Encapsulation.
- Sieries junction
motivation for P1-P3 scribing sequence
The P1-P3 scribing sequence is crucial for minimizing shading losses and effectively connecting thin-film solar cells into a module to increase its voltage. P1 involves isolating the transparent conductive oxide (TCO) layer to create series connections within the module. P2 scribes through the absorber layer but ensures no damage to the TCO, defining individual cells. Finally, P3 cuts through the cell completely, isolating each cell from one another, ensuring optimal performance and cell integrity.
Principles of electrochemical conversion
During this process, electrical energy is converted into chemical energy (or vice versa). Reduction and oxidation reactions occur at the cathode and anode, respectively. At the anode, water molecules are oxidized, producing free electrons, protons, and oxygen molecules. At the cathode, protons combine with electrons through a reduction reaction, resulting in the production of hydrogen gas (H₂).
Energy required H2O
energy required: 1.23 eV is required to split water molecule and produce H2 and O2 molecules. Minimum energy required is waters Gibs free energy change - 237 kJ/mol. Or theoretical 1,23V at standard conditions.
Operating principles of PEM electrolyser/fuel cells.
Operating principles of PEM electrolyser/fuel cells. PEM is proton exchange membrane it can transport protons from one side to another For production of H2 at one side of PEM water is injected at the contact it is oxidized and free protons travel to other side where at the cathode they reduce and H2 is produced. While In fuel cell mode, H₂ is supplied to the anode where it is oxidized, releasing protons and generating free electrons. These electrons flow through an external circuit to the cathode, where protons combine with O₂ to form water, generating electricity in the process.
Nernst equation
describes how cell potential depends on temperature, concentration and meterials
E=E0- (RT)/(nF) ln(Q)
E - electrode potential under non-standart conditions
E0 - Standart electrode potential (V), measured under standart conditions
R - universal gas constant
T - temperature
n - number of electrons transferred in the reaction
F - faraday constant
Q - reaction quotient, defined as the ratio of product and reactant activities or concentrations.
Standard hydrogen electrode SHE
Standard Hidrogen Electrode (SHE). Is a reference electrode to define electrical potential. Consists of platinum electrode in aqueous hydrogen solution of 1M, 25C and 1ATM of H2 pressure .
Relations of potentials to work functions of materials and electron affinities of semiconductors
Relations between potentials, work functions of materials, and electron affinities of semiconductors are often referenced using the Standard Hydrogen Electrode (SHE), which has a potential of 4.44 V relative to vacuum. This allows the work function of metals and the electron affinity of semiconductors to be measured accurately. By comparing these values, materials can be characterized relatively simply, facilitating the understanding and application of their electronic properties in devices.
work function - minimum energy required to move an electron from Fermi level (EF) of the material to the vacuum level (the energy at which an electron is free from the materials surface) work_function = Evac - EF
Electron affinity - electron affinity is the energy difference between the vacuum level and the conduction band minimum (CBM) El_Affinity = Evac - EC
Relation between work function and Electron affinity - in a semiconductor, the work function depends on both El_affinity and the position of the Fermi level relative to the conduction band minimum EC: work_function = El_affinity + (EF - EC)
Direct photoelectrochemical conversion to fuels – principles
Using a photoelectrochemical (PEC) cell, electrical energy is directly converted into chemical energy through oxidation and reduction reactions at the photoanode and cathode. The PEC element generates charges that facilitate the conversion of water or CO₂ into fuels or other valuable chemicals by utilizing sunlight to drive these reactions.
Requirements for bandgaps and energy level alignments
For reaction to happen the band gap must be higher then minimum energy needed to split the molecule. In H2 production Eg>1.23 eV, but not to high to harvest as much solar spectrum as possible. In practice 1.6-1.7 eV bandgaps are used. To ensure that reduction reaction happens the CB must be more negative reduction potential while VB mut be more positive then oxidation potential.
Operating principles of solar collectors
The device is made from absorber that absorbs solar radiation and turns it in to thermal energy. Then this thermal energy is carried by working fluid to applications like space or water heating.
Solar collectors - requirements for optical properties of surfaces, insulation.
such surface must have high absorptivity for maximum energy conversion, but must have small emissivity to prevent re-emission thru thermal radiation. To increase the efficiency of such device the heat exchange with environment must be minimized by using heat insulation. Anti-reflective coatings – minimize reflection losses to improve efficiency. transparent covers – glass or polymers allow light transmission and also prevent heat loss.
Construction options for various operating temperatures and their differences
Construction options for various operating temperatures and their differences. For home heating a flat plate design is sufficient. A flat black absorber converts solar energy to thermal and pipes with air or water transfer the heat. Flat plates use physical thermal isolation and glazing. For Temperatures below 300C vacuum tube absorbers are used. Glass tubes with vacuum and absorber inside have higher thermal isolation and lower losses. To transfer heat they use Glycol solution or heat pipes. This design is more common in colder climates. For temperature >300C liquid salts become the working fluid and sunlight is concentrated on to heat tubes by parabolic or Fresnel mirrors thru large area.
Concentration of solar light in solar thermal power plants and principles of their operation.
Concentration of solar light in solar thermal power plants and principles of their operation. The power plants concentrate solar energy by using light shaping thru parabolic mirrors, Fresnel lenses. This concentrated radiation heats working material (molten salt, air, oil) this heated liquid is then used to run water steam turbines and generate power.
Electrochemical energy storage systems.
Electrochemical energy storage systems. Batteries store energy in shape of chemical energy and then release it in form of electrical energy. Types: primary batteries not used for large storage as they are nor rechargeable, secondary batteries used in electronics and energy storage (Li-ion, Pb-acid…), flow batteries.
Structure and operating principles of Li ion based batteries
Structure and operating principles of Li ion based batteries – Lithium ions are stored in the anode, typically forming a compound with graphite. These ions are separated from the cathode by a electrolyte material that can transport lithium ions. During discharge, lithium ions move through the electrolyte and form a compound with the cathode material, such as cobalt oxide. Electrons flow from the anode to the cathode via an external circuit. For recharging, the positive electrode is connected to the cathode and the negative to the anode, causing lithium ions to move back to the anode.
Li-ion batteries: materials for cathodes and anodes
materials for cathodes and anodes – typical anode material is graphite as it can store large amount of Li ions and has small density providing excellent energy storage per kg. The Cathode material is more problematic needing rare earth metals (Co, V, Ni) in addition these elements are heavy reducing energy density per kg of batteries.
Flow batteries
Flow batteries. stores liquid electrolytes in external tanks. The solutions are pumped in to battery module where they are separated by membrane and redox reaction occur generating electricity. Reaction is reversable by changing polarisation.
application possibilities of Na ions
Application possibilities of Na-ion batteries: suitable for grid energy storage, electric vehicles and portable electronics due to their cost-effectiveness and sustainability, especially in regions with limited lithium resources.
