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Questions and Answers
What is the primary composition of the sun?
What is the primary composition of the sun?
What will happen to the sun after its current phase of evolution?
What will happen to the sun after its current phase of evolution?
What is the temperature of the photosphere of the sun?
What is the temperature of the photosphere of the sun?
How is energy generated in the sun according to Einstein’s formula?
How is energy generated in the sun according to Einstein’s formula?
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What is the speed of the solar wind as it propagates through the solar system?
What is the speed of the solar wind as it propagates through the solar system?
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What is the primary factor affecting the amount of solar insolation received on a surface perpendicular to the sun?
What is the primary factor affecting the amount of solar insolation received on a surface perpendicular to the sun?
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Which statement about electromagnetic waves is accurate?
Which statement about electromagnetic waves is accurate?
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Which of the following pairs accurately relates wavelength and frequency of electromagnetic waves?
Which of the following pairs accurately relates wavelength and frequency of electromagnetic waves?
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What property of photons is influenced by their frequency?
What property of photons is influenced by their frequency?
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How does visible light differ from other types of electromagnetic radiation?
How does visible light differ from other types of electromagnetic radiation?
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Study Notes
Solar Energy - Lecture 3
- The sun is a plasma primarily composed of hydrogen (92%) and helium (8%).
- Plasma is where electrons are separated from nuclei due to high temperature.
- The sun is a stable main sequence star estimated to be 4.5 billion years old, and will continue burning for another 4 to 5 billion years before evolving to burn helium.
- At that point, the sun will expand beyond Earth's orbit.
- The sun's surface, the photosphere, has a temperature of about 6,000 K.
- Sunspots are cooler regions on the photosphere with diameters up to 50,000 km and temperatures of about 3,800 K.
- The sun's corona has a temperature exceeding 1 million Kelvin and extends millions of kilometers into space.
- The Sun emanates a solar wind, a low-density stream of charged particles travelling through the solar system at 450 km/s.
Sun and Earth Characteristics
- Sun Diameter: 1,392,000 km
- Sun Mass: 1.99 x 1030 kg
- Sun Surface Temperature: 5,800 K
- Earth Diameter: 12,740 km
- Earth Mass: 5.98 x 1024 kg
- Earth Surface Temperature: 300 K
Nuclei and Mass Defect
- Nuclei consist of protons (positive charge) and neutrons (no charge).
- The mass of a helium nucleus is less than the sum of the masses of four protons.
- This mass difference is converted into energy according to Einstein's equation (E=mc²).
- Energy is transferred to the Sun's surface where electromagnetic radiation and particles (solar wind) are released into space.
Solar Radiation Reaching Earth
- The sun radiates energy at a power of 3.8 x 1023 kW.
- The Earth intercepts a portion of this power, still a significant amount.
- At the top of Earth's atmosphere, the power intercepted is 1.73 x 1014 kW, or 1.35 kW/m2.
- Perpendicular to the sun. At an angle, the energy is spread over a larger area.
Solar Insolation at Earth's Surface
- On a clear day, solar insolation at the Earth's surface is about 1.0 to 1.2 kW/m2 on a surface perpendicular to the sun (9-15 hours).
- The amount depends on factors like haze and elevation.
Nature of Particles and Waves
- Particles have mass, are localized in space, and can have properties like charge. No two particles share the same space.
- Waves have no mass, are spread out over space, and can interfere with each other following the principle of superposition.
EM (Electromagnetic) Spectrum
- EM waves travel at the speed of light (c = 3 x 108 m/s).
- They are characterized by their wavelength (λ) and frequency (f), related by c = λf. Wavelength is measured in meters and frequency in Hertz (cycles/second).
EM Radiation
- EM radiation consists of oscillating electric and magnetic fields perpendicular to each other and to the direction of the wave's motion.
- The EM spectrum spans from very short wavelengths (high frequency) to very long wavelengths (low frequency).
- Visible light is a small section of the spectrum.
Photons and Energy
- At the atomic level, EM waves are discrete packets of energy called photons.
- The energy of a photon is given by E = hf or E = hc/λ.
- where h is Planck's constant (6.6 x 10-34 kg m2/s).
Visible Light
- Visible light is part of the EM spectrum.
- White light is a combination (superposition) of all visible colors.
- A rainbow demonstrates the separation of colors from white light.
Components of Radiation
- Beam radiation travels in a straight line from the sun to Earth's surface.
- Diffuse radiation is scattered by molecules/particles in the atmosphere.
Insolation
- Insolation is solar radiation at Earth's surface measured as energy/square meter per a period.
- It's dependent on latitude, climate, season, time of day, and cloud cover.
- On clear days around 1.0 kW/m2.
Greenhouse Effect
- Greenhouse gases (water vapor, carbon dioxide, methane, and other trace gases) trap heat in the atmosphere.
- A large atmosphere of carbon dioxide drastically changes the energy balance, as illustrated by Venus.
- Increased fossil fuel use has increased carbon dioxide and scientists believe this leads to global warming.
Heat Transfer
- Heat is the internal kinetic energy (random motion) of atoms/electrons in a material.
- Heat naturally flows from hotter objects to colder objects.
- Conduction, convection, and radiation are methods for heat transfer.
Conduction
- Conduction is heat transfer in solids. Heat flows from a hot side to a cold side in a solid.
- The amount of heat conducted is expressed as HCON = Uh A ∆Th. - Where: - Uh = thermal conductance - A = area - ∆Th = temperature difference - h = time (number of hours).
Resistance
- Resistance (R) is a material's property of delaying heat flow, which is the inverse of conductance. Higher R values mean higher insulation.
- The total resistance of layered materials is the sum of their individual resistances.
Active Solar Thermal Systems
- They use moving parts like pumps and fans to circulate a fluid (e.g., water) for carrying heat energy.
- Examples include systems with roof-mounted flat plate collectors.
Passive Solar Thermal Systems
- They rely on building design features to capture heat.
- They do not use mechanical parts like pumps and fans.
Solar Thermal Types Classification
- Categorized as low, medium, and high temperature systems based on the temperature they achieve.
- Low: Typically under 100°C, used for heating water and space.
- Medium: Ranging from 100°C to 250°C, used for solar ovens.
- High: Over 250°C, using reflecting mirrors to concentrate solar energy.
Solar Collectors
- Solar collectors absorb solar radiation converting it to heat that is transferred by a heating fluid (e.g., water).
- There are non-concentrating and concentrating solar collectors.
Non-concentrating Solar Collectors
- They absorb beam and diffuse radiation, making them effective even on cloudy days.
- Flat plate collectors and evacuated tube collectors are common types.
Flat Plate Solar Collectors
- They consist of an absorber plate made of a high thermal conductivity material (e.g., copper, aluminum).
- A transparent cover and tubes for the heat-transfer fluid are other key components.
- They are usually insulated and contained in a sealed housing.
Evacuated Glass Tube Collectors
- They consist of vacuum-sealed glass tubes with an absorber plate.
- They can achieve high temperatures (up to 250°C).
- Suited for commercial and industrial use or domestic use.
- Can be used effectively in locations with frequent cloudy conditions.
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