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Questions and Answers
What is one of the main disadvantages of horizontal-axis wind turbines?
What is one of the main disadvantages of horizontal-axis wind turbines?
Vertical-axis wind turbines are the most common type of wind turbine in use today.
Vertical-axis wind turbines are the most common type of wind turbine in use today.
False
What is the primary function of the blade or rotor in a wind turbine?
What is the primary function of the blade or rotor in a wind turbine?
To convert the energy in the wind to rotational shaft energy
The Darrieus wind turbine is a type of vertical-axis turbine that resembles a giant, __________.
The Darrieus wind turbine is a type of vertical-axis turbine that resembles a giant, __________.
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Match the following wind turbine types with their characteristics:
Match the following wind turbine types with their characteristics:
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Which of the following is considered an alternative energy source?
Which of the following is considered an alternative energy source?
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The MENA region has the lowest solar irradiance compared to other regions of the world.
The MENA region has the lowest solar irradiance compared to other regions of the world.
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What is the solar constant value, representing the amount of solar energy arriving at Earth’s atmosphere?
What is the solar constant value, representing the amount of solar energy arriving at Earth’s atmosphere?
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At 40° north latitude, the noontime radiation rate on a flat surface normal to the sun’s rays is about _____ W/m2 on a clear day.
At 40° north latitude, the noontime radiation rate on a flat surface normal to the sun’s rays is about _____ W/m2 on a clear day.
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Which of the following describes the main components of solar thermal systems?
Which of the following describes the main components of solar thermal systems?
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Match the components of solar energy systems with their functions:
Match the components of solar energy systems with their functions:
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Part of the solar energy that reaches the Earth is reflected back to space.
Part of the solar energy that reaches the Earth is reflected back to space.
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What is the realistic area of a solar collector needed to provide 293 kWh of energy per day, given the efficiency rates mentioned?
What is the realistic area of a solar collector needed to provide 293 kWh of energy per day, given the efficiency rates mentioned?
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What is the formula used to calculate the power output of a wind turbine?
What is the formula used to calculate the power output of a wind turbine?
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Warm air is denser than cold air.
Warm air is denser than cold air.
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What factors affect the amount of energy in the wind?
What factors affect the amount of energy in the wind?
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Horizontal-axis turbines typically have ____ blades.
Horizontal-axis turbines typically have ____ blades.
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Match the following wind turbine types with their descriptions:
Match the following wind turbine types with their descriptions:
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What is the swept area (A) of a turbine given a rotor blade length (r) of 5 m?
What is the swept area (A) of a turbine given a rotor blade length (r) of 5 m?
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The larger the swept area of the turbine, the less power it can capture from the wind.
The larger the swept area of the turbine, the less power it can capture from the wind.
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At what elevation is air density typically lower?
At what elevation is air density typically lower?
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Wind turbines convert ____ energy into electricity.
Wind turbines convert ____ energy into electricity.
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What is the result of increasing rotor blade length on a wind turbine?
What is the result of increasing rotor blade length on a wind turbine?
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What type of collector is best suited for high-temperature applications above 120°C?
What type of collector is best suited for high-temperature applications above 120°C?
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Evacuated tubes utilize vacuum technology to prevent heat loss through convection.
Evacuated tubes utilize vacuum technology to prevent heat loss through convection.
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What is the primary advantage of air collectors compared to liquid collectors?
What is the primary advantage of air collectors compared to liquid collectors?
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The __________ uses two-axis tracking mirrors to focus sunlight on a central tower.
The __________ uses two-axis tracking mirrors to focus sunlight on a central tower.
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Match the solar collector types with their primary characteristics:
Match the solar collector types with their primary characteristics:
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Which collector is known for being simpler and cheaper due to its construction?
Which collector is known for being simpler and cheaper due to its construction?
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Flat plate collectors are typically oriented towards the North in the northern hemisphere.
Flat plate collectors are typically oriented towards the North in the northern hemisphere.
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What is the role of the heated fluid in a flat plate collector?
What is the role of the heated fluid in a flat plate collector?
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The __________ is a concentrating collector that cannot utilize diffuse radiation from the sky.
The __________ is a concentrating collector that cannot utilize diffuse radiation from the sky.
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Which of the following types of collectors utilizes an absorber plate?
Which of the following types of collectors utilizes an absorber plate?
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What is used as a coolant in solar thermal systems?
What is used as a coolant in solar thermal systems?
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A rock-bed type of storage is typically used with water-type collectors.
A rock-bed type of storage is typically used with water-type collectors.
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What is one benefit of thermal storage systems in solar energy applications?
What is one benefit of thermal storage systems in solar energy applications?
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Photovoltaic solar cells convert sunlight directly into __________.
Photovoltaic solar cells convert sunlight directly into __________.
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Which component separates collector fluid from storage fluid in solar thermal systems?
Which component separates collector fluid from storage fluid in solar thermal systems?
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Electrons flow easily through p-type semiconductor material.
Electrons flow easily through p-type semiconductor material.
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What happens to excess electron-hole pairs in a p-type semiconductor when illuminated?
What happens to excess electron-hole pairs in a p-type semiconductor when illuminated?
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At 60 °C, the power output from a PV panel must be calculated using the formula 𝑃𝑚𝑝 = 𝑃𝑆𝑇𝐶(1 − 𝛽 (𝑇 − 25) × 𝑛). The variable 𝑛 refers to __________.
At 60 °C, the power output from a PV panel must be calculated using the formula 𝑃𝑚𝑝 = 𝑃𝑆𝑇𝐶(1 − 𝛽 (𝑇 − 25) × 𝑛). The variable 𝑛 refers to __________.
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What is the typical STC value for incident solar radiation used in calculations?
What is the typical STC value for incident solar radiation used in calculations?
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Study Notes
Chapter 9: Alternative Use of Energy
- Alternative energy sources are non-depleting or renewable energy sources. Examples include solar and wind energy. Historically, natural gas, coal, and oil have served as alternative energy sources at some point.
- Government funding for research and development (R&D) and tax incentives for alternative energy sources sharply decreased in the 1980s and early 1990s.
9.1 Solar Thermal Systems
- This section introduces solar thermal systems.
- Topics covered include:
- Introduction
- Solar Irradiance
- Solar Thermal Collectors
- Thermal Storage Systems
Solar Energy Introduction
- Surface temperature of the Sun is 5778 K (5505°C).
- Core temperature of the Sun is 15.6 x 106 K.
- Power output of the Sun is 3.85 x 1026 Watts.
- Annual energy intercepted by Earth is 1.5 x 1018 kWh/year.
- World energy demand is 1.4 x 1014 kWh/year.
Solar Irradiance (Distribution)
- The MENA region has the highest global average solar irradiance.
- Solar irradiance distribution is presented on a world map with different intensity levels.
Solar Irradiance (Orientation)
- Optimal solar energy collection requires understanding solar radiation angles for maximum energy capture.
- Diagrams illustrate how the angle of the sun's rays changes throughout the day and year.
Solar Irradiance (Numerical Facts)
- The solar constant, the rate at which solar energy arrives at the Earth's atmosphere, is approximately 1353 W/m² (428 Btu/hr ft²).
- Only about two-thirds of the sun's energy reaches the Earth's surface. The rest is reflected, absorbed, or scattered by the atmosphere.
- Noontime solar radiation on a flat surface normal to the sun's rays at 40° north latitude is approximately 946 W/m² (300 Btu/hr •ft²). This number varies with time and latitude.
- Solar collectors are often designed to track the sun to maximize their exposure to direct sunlight.
Solar Thermal Collectors
- Non-concentrating collectors include:
- Flat plate collectors
- Evacuated tube collectors
- Volumetric air collectors
- Concentrating collectors include:
- Parabolic troughs
- Compound parabolic concentrators (CPCs)
- Fresnel collectors
- Solar power towers
- Solar dish Stirling engines
Flat Plate Collectors
- Flat plate collectors are commonly used for lower temperature applications like water heating.
- Collector components include glazing, absorber plate, insulation, flow tubes, fluid inlet and outlet.
- Collectors are typically oriented south for optimal sunlight in the Northern Hemisphere.
- Heated fluid is used for water heating, space heating, refrigeration systems, moisture removal, etc.
Evacuated Tubes (Vacuum Tubes)
- Evacuated tube collectors are similar to flat plate collectors but have a vacuum layer to reduce heat loss.
- Heat transfer is improved by reducing convection loss from the vacuum.
Volumetric Air Collectors
- Volumetric air collectors use air as the heat transfer fluid, which is less susceptible to freezing and corrosion.
- Air collectors are good for applications that do not require high temperatures.
Parabolic Trough Collectors
- Parabolic trough collectors concentrate sunlight using parabolic troughs to heat a fluid.
- Collectors generally operate using a fluid like oil to increase the efficiency and reduce the size needed for the collector.
Compound Parabolic Concentrator (CPC)
- CPCs concentrate sunlight using multiple smaller flat mirrors.
- These collectors tend to be more cost-effective than parabolic troughs.
Solar Power Tower
- Solar power towers use a large array of heliostats (mirrors) to concentrate sunlight onto a receiver atop a tower.
- Molten salts are frequently used to store the thermal energy.
Solar Dish Concentrators
- Solar dish concentrators use a curved reflector to concentrate sunlight onto a receiver.
- The concentrator often includes a generator that turns the heat from concentrated sunlight directly into electricity.
Thermal Storage Systems
- Thermal storage systems are used to store solar energy when solar energy availability does not match the demand.
- A rock-bed storage is sometimes used with air collectors.
- Water tanks are a common choice to store thermal energy.
- Antifreeze or similar methods are used in climates where freezing could affect the system.
Solar Cells (Photovoltaics)
- Solar cells use semiconductor materials to directly convert sunlight into electricity.
- They employ p-n junction diodes for their operation.
- Factors affecting PV panel output include incident sunlight intensity, temperature, and conversion efficiency.
Wind Energy Introduction
- Winds are a form of solar energy caused by uneven atmospheric heating, Earth's surface irregularities, and Earth's rotation.
- Wind turbines capture wind energy to generate electricity.
Wind Energy Calculations
- Wind energy calculations depend on air density, swept area of the turbine, and wind speed. The speed of the wind cubed (v3), plays a significant role in determining the power harnessed (power increase is proportional to increase in wind speed cubed).
- Density of air varies with elevation and temperature. Warm air is less dense than cold air.
- Swept area of the turbine (the area the blades cover and are in motion) is important, larger swept areas capture more wind energy. Larger circular areas capture more wind energy than smaller circular areas. Larger propeller-like blades increase energy capture.
- The Betz criterion defines the theoretical maximum efficiency for wind turbines at 59%.
Wind Turbine Types
- Horizontal-axis turbines are most common and resemble airplane propellers. Larger horizontal-axis turbines can be quite tall.
- Vertical-axis turbines resemble egg beaters. They have advantages in certain circumstances, but may not be as energy-efficient or cost-effective compared to the common horizontal-axis designs.
Wind Turbine Components
- Horizontal-axis turbines typically consist of blades, a rotor, a drive train including a gearbox and a generator, a tower that supports the rotor and drive train, and other equipment.
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Description
Explore alternative energy sources such as solar and wind, alongside historical context and government influence on their development. This chapter delves into solar thermal systems, solar irradiance, and the power output of the Sun. Learn how these technologies impact energy demand and sustainability.
