Solar power Flashcards
1
Q
Advantages of using solar energy
A
- No emissions from generation
- Sunlight is a free energy source
- Sustainable
- provide electricity to remote locations
2
Q
How do you calculate the radiative emissions from the sun?
A
P = Aepssigma*T^4
3
Q
What is the average direct solar irradiation to the atmosphere?
A
1366 W/m^2
4
Q
What is the average indirect irradiation that hits the earth?
A
1000 W/m^2. The reason is that approximately 30 % of the direct radiation is scattered in the atmosphere
5
Q
How does the ozone-oxygen cycle work?
A
- Oxygen photodissociation:
O2+hv(<242 nm) –> 2 O - Ozone creation:
O+O2+A –> O3+A
- Ozone photodissociation:
O3+hv(240-310 nm) –> O2+O
If a photon with a wavelength of less than 242 nm hits the oxygen molecule, it will cause it to split into two atoms that in turn will react with other molecules forming ozone. If then a photon with a higher energy hits the ozone molecule it will cause it to split into an oxygen molecule and an oxygen atom
6
Q
How do you calculate the energy of a photon?
A
E = hc/lambda
7
Q
What is the Rayleigh scattering?
A
Dispersion of electromagnetic radiation by particles of radius less than approximately 1/10 of the wavelength of the radiation.
In other words, small particles will distribute the incoming radiation differently causing colors to form
8
Q
Why is the ozone layer important?
A
It absorbs some of the short wavelengths from the incoming solar radiation, meaning that it rejects the most dangerous UV-lights from hitting the surface of the earth
9
Q
What is the average insolation to the earth?
A
250 W/m^2
Area of the disc hit by the sun divided by earth’s surface area:
pir^2/(4pi*r^2)
10
Q
What is the capacity factor of solar?
A
25 % at best
11
Q
How does the angle affect the solar insolation?
A
As the angle between the surface and the sun increases, the insolation decreases proportionally to the angle
12
Q
What is the average insolation in sweden?
A
125 W/m^2
Reduced due to the tilt angle
13
Q
Why is the average insolation decreased at the equator?
A
More cloudy –> more solar that is irradiated
14
Q
How is the capacity factor calculated?
A
CF = actual output/installed capacity
15
Q
How does the atmospheric composition affect solar insolation?
A
Different molecules will absorb certain wavelengths and thus different compositions will let different wavelengths through
16
Q
What is the two main cost factors for solar?
A
- the efficiency of converting solar energy into electricity
- the cost of installing the necessary infrastructure
17
Q
What is the band gap?
A
Forbidden region for electrons. It is the region between two energy levels at which the electrons can exist
18
Q
Why are metals a bad material choice for solar PVs?
A
In metals, there are plenty of atoms that generates bonds forming a continuous band, thus there is no band gap
19
Q
What are the two main cauese for the motion and separation of change carriers in a solar cell?
A
- Drift of carriers: this process is driven by the electric field, which pushes electrons in one direction and holes in the opposite direction
- Diffusion of carriers: This occurs when carriers move from areas of higher concentration to areas of lower concentration, following a gradient of chemical potential
20
Q
What is a n-type material?
A
Material doped with atoms that has a HIGHER number of valence electrons as compared to the host atoms. E.g. silicon is replaced with phosphorus
21
Q
What is a p-type material?
A
Material doped with atoms that has a LOWER number of valence electrons as compared to the host atoms. E.g. silicon is replaced with boron
22
Q
What is the depletion region?
A
A region where no electrons can flow
23
Q
How does a PV cell work?
A
When a photon hits an electron in the PV-cell, it can cause the electron to jump from the valence band to the conduction band, which will generate an electron-hole pair that are separated at the junction. If there is a wire connecting the two regions, there will be a current flowing through due to the electron wanting to fill the hole.
24
Q
What is the main losses in a single junction solar cell?
A
If a photon with too much energy hits an electron it will cause it to jump further than the band gap, but then return to the band gap since it cannot exist in between two layers. This can be seen as a loss since not all solar energy is utilized.
Another loss is if a photon with too low energy hits an electron since that won’t make the electron move from the valence band to the conduction band.
Ideally, you want to excite an electron with a photon that has exactly the energy of the band gap, otherwise you haven’t utilized all energy going in.
25
What is the Shockley-Queisser limit?
It is a limit of how large percentage of the incident light energy that hits the solar cell, by taking radiative recombination into account. The Shockley-Queisser limit is around 25 -30 % at a bandgap of about 1.1 eV. This is the band gap of silicon which is why silicon is a great material for solar PVs.
26
How does temperature affect the power of a solar PV?
The efficienct of solar PVs decreases as their temperature increases. The physics behind it is increased internal carrier recombination rates, caused by increased carrier concentrations.
27
which type of solar cell offers the higher performance?
n-type since it has a greater tolerance for impurities. p-type cells suffers from light-induced degradation (LID) due to boron-oxygen defects. n-type is however more expensive since the doping process is more complex, but offers longer lifetime and performance.
28
What is a bifacial PV cell?
Its a PV cell that can absorb light from both sides, making the absorbtion greater
29
Why is silicon commonly used in solar cells for energy conversion?
Since it has a band gap that corresponds to the Stoicher-Quassie limit. It is also a semi-conductor that.
30
What is the theory behind dual and multi-junction PVs?
The thought is that different band gaps in the materials will take care of photons will different energies. Thus you can minimzed losses within mtrl
31
What is a thin-film solar cell?
It is a more flexible solar PV that can make them suitable for use in building integrated photovoltaics
32
What is the difference between direct- and indirect bandgap?
In a direct bandgap, the electron state can occur at the same momentum, meaning that no extra motion is needed as they are in indirect bandgaps
33
What are the pros of perovskite?
Simpler manufacturing process and higher tolerance for defects
34
What are the cons of perovskite?
Instability, lasts shorter than traditional silicon-PVs.
35
What are the causes of the instability of perovskite?
* Damage from moisture and oxygen
* Thermal stress
* Material instability
* Heat generated during voltage application
* Exposure to UV and visible light
* Mechanical fragility
36
What is down-conversion?
One high-energy photon is converted into two low-energy photons to better match energy in bandgap
37
What is up-conversion?
Two low-energy photons are converted into a single high-energy photon to better match energy in bandgap
38
What are the solar characteristics?
* Abundant and globally distributed: Solar energy is a vast resource available worldwide
* Seasonal variations and regional synergies: Positive synergy (USA example), high solar output during summer aligns with increased energy demand for air conditioning. Negative synergy (Norther Europe example): Low solar output in winter coincides with peak demand for heating
* Intermittency and Output variability: Varies during the day due to time and weather conditions (e.g., clouds) and no power is produced at night
* Distributed and Modular: Can be deployed close to consumers, reducing transmission losses. Large solar farms benefit from economies of scale; small systems offer localized generation
* Storage considerations: Solar output aligns with short-term energy storage solutions (e.g.. batteries, pumped hydro). Unlike wind, which can vary over 1-3 weeks, solar energy follows a predictable 24-hour cycle
* Cost structure: High initial investment but low operational expenses over time
* Land use requirements: Requires more surface area compared to wind, and nuclear power
39
Explain what a duck curve is.
A duck curve explains what happens to an energy system with a high penetration of solar power. By introducing the net-load which is the total load-solar power load, the net load will decrease during the day when the sun shines, but drastically increase during the evening when the sun stops shining and consumption increases. This will generate a sharp curve, which is called the duck curve since the net load also is high in the morning.
* The duck curve refers to a graphical representation of electricity demand from the grid on days when solar energy production is high and demand in the grid is low. This also impacts the price that follows a similar curve.
* As more solar energy is exported to the grid, usually across the middle part of the day when the sun is shining, the curves deepen. Then, as sun sets and solar energy is no longer being generated, the Duck Curve typically shows extreme changes in demand and the grid need to 'kick in' suddenly which can be difficult.
* Can result in energy system becoming unstable
40
Explain the variability in electricity demand
* Electricity demand naturally fluctuates throughout the day and across seasons
* Power systems already have decades of experience managing these variations
* Intermittent renewable sources increase the scale and unpredictability of these fluctuations
* Baseload plants can also fail unexpectedly, so backup capacity is always necessary
41
Explain how it is possible to balance net load instead of demand
* Power systems respond to net load (electricity demand minus renewable generation)
* Hydropower and thermal plants can adjust their output to balance fluctuations
* Hydropower with dams is especially valuable because it provides: energy storage, fast ramping to meet demand changes, no loss of efficiency when operating below maximum capacity
*Rule of thumb: Up to 20-25% of electricity from intermittent sources can be integrated with minimal challenges. However, a large share of renewables increases system costs-the exact impact depends on system design and the degree of adaptation
42
How can you mitigate intermittency?
* Flexible generation: mid-load plants (e.g., natural gas) are becoming more important becausre they can ramp output up or down quickly
* Geographical diversification
- Distributing renewable sources across a large geographic area smooths out local variations
- Expanding long-distance transmission helps transfer power from areas with surplus to areas with shortages
* Combining different renewable sources:
Using diverse sources (e.g., solar and wind) improves reliability because their generation patterns are uncorrelated-sunny days are oftan calm, while windy days are often cloudy
* Improved weather forecasting: Better forecasts help grid operators prepare for supply fluctuations by:
- Adjusting thermal plant outputs
- Managing demand response programs
* Energy storage solutions: Helps balance supply and demand by capturing excess energy and releasing it later. Examples of storage systems include:
- Batteries (including electric vehicle batteries)
- Pumped hydro (water reservoirs)
- Compressed air (storing air under pressure)
43
What are the differences between centralized and distributed PV systems?
* Centralized large-area PV systems are often constructed in urban areas to utilize land resources. In contrast, distributed PVs are installed on building and factory roofs to maximize space usage
* Distributed PV systems connect electricity to the local grid with minimal losses, supplementing local power. Centralized PVs transmit electricity to the grid at high voltage and then to even higher voltage levels
44
What are the three different storing technologies?
Electrochemical, mechanical and thermal
45
What are smart grids?
* Use information technology to adapt electricity use in real time
* Household appliances can be programmed to operate when electricity is abundant and cheaper
* This assumes some flexibility in when consumers need energy
46
Curtailment
* Refers to deliberately reducing renewable energy output to avoid overloading the grid
* This typically occurs when demand is low and renewable generation is high (e.g., on a windy summer Sunday)
* As renewable energy penetration increases, curtailment will likely become more common
47
What is demand side management?
* Adjusting electricity use to match supply reduces stress on the grid. Strategies include:
* Industrial DSM: Large consumers temporarily reduce power use during supply shortages
* District heating: Heat pumps and combined heat and power (CHP) systems can switch between consuming and producing electricity
* Power-to-X: Use surplus electricity to produce fuels (e.g., hydrogen)
48
How has the growth of solar cells looked recently?
Exponential
49
How has the price of solar PVs changed?
They have also decreased exponentially
50
What happens when solar cells are connected in series?
The voltage increases
51
What happens when solar cells are connected in parallell?
The current increases
52
What happens to the inertia of the system as the penetration of solar increases?
* The total inertia will decrease since solar PVs don't have any inertia.
*Turbines, generators, and motors in fossil, nuclear, and hydropower plants spin at speeds proportional to grid frequency. The rotational energy of these massive devices provides significant inertia that can counteract changes in grid frequency due to disturbances. For example, grid frequency will decrease if one power plant in a region goes offline. Other spinning generators can respond by speeding up slightly to resist the frequency shift and stabilize the grid.
Because solar energy plants don't have any moving parts (and thus inertia), the power system's inertia declines as solar penetration grows-potentially leading to rapid frequency changes. If left unchecked, such changes can cause electricity service interruptions
53
What is a solar inverter?
A solar inverter is an essential part of a grid-connected solar system. Solar panels generate DC electricity, which must be converted to AC electricity to be used in homes and businesses. The two main types are string solar string inverters and microinverters.
54
What are string inverters
The most common type of solar inverters are string solar inverters. In a standard string inverter system, the solar panels are linked together in series. The DC electricity from the strings is brought to the solar inverter via a high-voltage DC cable. The inverter then converts the DC power to AC power, which can be used in the home or fed into the electricity grid
55
What are microinverters?
In a microinverter system, each panel has a small microinverter attached to rear side of the panel. The panel still produces DC but is converted to AC on the roof, then fed to the electrical switchboard, and finally to your home appliances or the grid. Microinverters enable each panel to work independently and are well suited to complex roofs or locations with shading from trees or rooftop obstacles.
56
What are hybrid inverters?
Hybrid inverters are essentially two inverters in one and are used for both solar and battery storage
* Solar batteries, or solar power storage, allows you to store electricity generated from the sun through your solar panels, to then use in the evening or at a later date
* Given the cost and lifetime of solar batteries, this option is at present not cost efficient
57
Explain the process of solar PVs
1. An electron inside a hole is hit by light. The light's energy causes the electron to be ejected
2. The electric field created by the ions in the depletion region pushes the electron towards the n-type side
3. Now there is and additional hole in the p-side region. This hole wants to be filled.
4. An electron takes the backdoor through the wire or circuit to fill that space
58
What is the open circuit voltage?
The voltage when the current is zero
59
What is the open circuit current?
The current that flows when the voltage is zero
60
How is the SCC related to the radiation intensity?
They are proportional
61
How is the power calculated?
P=V*I
62
What is a solar module?
A solar module consists of many solar cells
63
What is a solar array?
A solar array consists of several modules
64
What is the drawback of string inverters?
Since string inverters require solar panels to be wired in series, then if one solar panel's power output is affected, the entire series of solar panels is affected in equal measure. This can pose a major issue if some part of a solar panel series will be shaded for part of the day
65
Which different types of silicon based PVs exists?
Monocrystalline
- Atoms arranged in a regular pattern
Polycrystalline
- regions of crystalline Si separated by 'grain boundaries', where bonding is irregular
Amorphous
- less regular arrangement of atoms, leading to 'dangling bonds' which can be passivated by hydrogen
66
Explain the process of producing monocrystalline PV cells
1. Silica sand is purified in an arc furnace to create 99% pure silicon
2. The 99% silicon is further refined to become almost 100% pure silicon
3. The silicon is doped with either boron or phosphorous (P-type or N-type)
4. The doped silicon is drawn into a solid crystal ingot using the Czochralski process
5. The solid round ingot is diamond wire-cut into thin square wafers
67
Explain the Czochralski process
1. Melting of polysilicon doping
2. Introduction of the seed crystal
3. Beginning of the crystal growth
4. Crystal pulling
5. Formed crystal with a residue of melted silicon
68
What is the most common type of materials in a silicon based PV
47% silver
26% aluminium
11% silicon
8% glass
8% copper
69
What are the most common degradation and faults?
Solar panels are known to be highly reliable, as they don't have any moving parts and require minimal maintenance. However, they may fail due to light-induced degradation, losing about 0.5% power per year. Most panels perform at 80% or higher of their original capacity after 20 years. The manufacturer's warranty specifies the amount of degradation.
Solar panels can experience some serious problems, such as micro-cracks and severe degradation due to various reasons. Any high stresses, such as impact, poor installation practices, or people walking on the rooftop panels, can cause tiny fractures in the cells. These problems are often hard to detect, and if left for several years, can develop into hot spots and cause catastrophic failures, such as arcing or fire.
70
How does the manufacturing facilities of solar PV look like?
Solar panels are manufactured in highly specialized facilities using highly-precise automated equipment and sensors. These plants are held to strict cleanliness standards to avoid any contamination during assembly
Advanced sensors inspect the panels and cells during manufacturing to ensure proper component placement and prevent cell damage. Manufacturers conduct tests on the final panel assembly, including EL or flash testing, to identify cell defects that could cause failure when exposed to sunlight and high temperatures over time
A typical silicon crystalline solar panel will generate enough energy to repay the embodied energy within 2 years of installation. However, as panel efficiency has increased, the payback time has reduced to less than 1.5 years in many areas with high average solar radiation
71
Name the 5 reasons why solar panels fail
1. LID - Light Induced Degradation - Normal performance loss of 0.25%-0./5 per year
2. PID - Potentional Induced Degradation - Potential long-term failure due to voltage leakage
3. General Degradation - Premature failure due to water ingress or other defects
4. LeTID - Light and elevated Temperature Induced Degradation - sudden 3%-6% loss in performance
5. Micro-cracks and hot spots - Longer-term defect and failure due to broken or damaged cells
72
Which two types of costs are related to solar PVs?
1. Module costs
2. Balance of system (BOS) costs
- on-grid: inverter and grid connection
- off-grid: battery storage
- installation work & mtrls
* Typical BOS costs \approx module costs (on-grid) today, but falling slower than module costs since the BOS costs is a mature technology
73
What is the main reason for the drastic drop of solar PVs price?
Modularity and manufacturing large batches. Simplicity of manufacturing has driven down the prices
74
What is passive solar technology?
Collects heat from the sun and stores it in material in houses. Release the stored heat during periods when the sun is absent, such as night times
Thermal mass - commonly concrete, brick, stone and tile. These materials absorb heat from the sunlight during the heating season and also absorb heat from warm interior air during the cooling season
Distribution - a method by which solar heat is transferred from where it is collected and stored to different areas of the house by conduction, convection and radiation
Passive solar cooling - Passive solar cooling systems use shading, thermal mass, and natural ventilation to reduce unwanted daytime heat and store cool night air to moderate temperatures
75
What is solar water heating?
The sun heats water in homes and businesses. Uses solar collector panels (different from PVs). The warm water is stored in a storage tank and is distributed through the building.
76
Which 3 different types of solar water heating exists?
Unglazed, flat-plate and evacuated tube.
The first two are cheap and the last one is expensive
77
What is concentrated solar power?
A technology in which you concentrate the sun's rays after which you use a heat transfer liquid or direct steam generation.
Works both with and without storage.
Typically good in desert areas due to direct radiation.
* Mirrors reflect sunlight to concentrate heat on a receiver, drives a steam turbine
* Requires direct (clear sky) insolation --> best in deserts
* But they require water for cooling! Need about 3000 l/MWh, same as nuclear. Dry cooling is possible, but reduces efficiency significantly
* Can be combined with heat storage and/or natural gas backup (solar-gas hybrid plant) --> 100% base load power
* "Solar fuels": can make hydrogen from water, or syngas (mix of CO + H2) or almost anything from fossil fuels
78
What 4 types of CSPs exists?
1. Linear fresnel reflectors
2. Parabolic through collectors
3. Solar power towers
4. Solar parabolic dishes
79
Explain the working principle behind linear fresnel reflectors
* Uses rows of mirrors to reflect sunlight to an absorber tube
* Mirrors can be flat or curved
* The absorber tube is fixed here
* Uses direct steam generation
* No heat transfer fluid is needed
* Thermodynamic efficiency 8-11%
* Requires large total area
80
Explain the working principles behind parabolic through collectors
* Parallell rows of mirrors/reflectors
* Curved in one dimension to focus the arrays on the absorber tube. Can be more than 100 m long
* Stainless steel pipes, with special coating (absorber tubes), collects the heat. Uses synthetic oil as heat transfer fluid
* The reflectors and the pipes move together to follow the sun
* The synthetic oil in the pipes transfer the heat from the sun to a heat exchanger that boils water to steam that drives a steam turbine that generates the electricity
* Most mature CSP technology
*Efficiency around 15%
* Can be connected to storage
81
Explain the working principles behind solar power towers
* Solar towers use hundreds or thousands of small reflectors (heliostats)
* Concentrate the sun's arrays at the central receiver at the top of a central tower
* Heliostats turn with the sun to maximize heat transfer
* Molten salt as both heat transfer fluid and storage
* A heat transfer fluid is heated at the central receiver that tranfer the heat to a steam turbine - generating electricity
* Direct steam generation (DSG) can be used in the receiver
* Efficiency around 17-35 %
82
Explain the working principles behind solar parabolic dishes
* Parabolic dishes concentrate the sun rays at a focal point
* The reflector follows the sun
* Most dishes have their own generator. No heat transfer fluid is needed
* Often a stirling enging is used to generate power
* Efficiency around 30 %. Highest of the CSPs
83
Discuss some environmental impacts of solar power
* Huge area --> environmental impact
* Can work against maintaining agricultural land for food production autonomy
* Many PV parks have been built on less valued land types, such as industrial sites near major highways, former landfills, and grassy areas near airports, with minimal social opposition
* Compared to wind power, solar power needs much larger areas
* PVs can be placed almost anywhere
* If located in hydropower plants reservoir it can use existing evacuation infrastructure
- Land-neutral - less space conflicts, lower plot acquisition cost
- Improved water quality - slow algae growth (could also grow bacteria under the shades --> could also degrade)
- Maintenance benefits in
dusty locations
- Higher capacity factor from a cooling effect: panels are naturally cooled hence automatically solves the issue of heating losses that occur during its operation
* Rare earth materials
* When using land for solar power generation, it is important to avoid using farmlands because they are critical for food production. Instead of farmlands, it would be better to use deserts or rooftops for solar panel installation. Photovoltaic systems require 3-10 acres of land per megawatt, while CSP systems require 4-16 acres per megawatt
* As for water usage, PV panels can be installed on lakes, but this requires careful considerations of the environmental impact. Meanwhile, CSP systems require water for cooling, which means that large amounts of water are needed for their operation. It is crucial to ensure that the water is sourced sustainably and efficiently and that the water used for cooling is treated and recycled as much as possible to reduce the environmental impact
* Solar waste is currently low because most panels installed in the past 20 year still work. However, as these systems reach their end of life, solar-related waste will increase. Fortunately, recycling is easy for many materials, including aluminium frames and mounting system
84
What are the advantages of using PV power?
* First, it has short lead times to design, install and start up a new plant
* Second, it is highly modular, so the plant economy is not strongly dependent on size
* Third, the power output matches well with peak-load demands
* Fourth, it has a static structure with no moving parts, which means there is no noise
* Fifth, it has high power capacity per unit of weight
* Sixth, it has a longer life with little maintenance because of no moving parts
* Finally, it is highly mobile and portable due to its lightweight nature
