Chapter 9: Alternative Use of Energy

Solar Constant & Irradiance

• The solar constant, the rate at which solar energy arrives at Earth's atmosphere, is approximately 1361 W/m².
• The MENA region does not have the lowest solar irradiance globally; this statement is incorrect.

Solar Collector Area

• To provide 293 kWh of energy per day, assuming an average solar irradiance of 5 kWh/m²/day, a realistic solar collector area of approximately 58.6 m² would be required.

Solar Energy Classification

• Solar energy is primarily an example of a renewable energy source.

Solar Collectors

• Flat-plate collectors: These are simple and inexpensive collectors that absorb solar energy on a flat plate. They are suitable for heating water and air.
• Evacuated tube collectors: These collectors use vacuum tubes to minimize heat loss and improve efficiency. They are often used for hot water applications.
• Parabolic trough collectors: These collectors use parabolic mirrors to concentrate solar energy onto a receiver tube. They are typically used for generating electricity.
• Volumetric air collectors: These collectors capture heat from air passing through a large volume, often used for space heating.

Wind Turbine Characteristics

• Increasing the rotor blade length of a wind turbine increases the power available.
• Horizontal-axis turbines are the most common type of modern wind turbines, not vertical-axis turbines.

Wind Turbine Power Output

• The power output of a wind turbine is calculated using the formula: P = 1/2 * ρ * A * v³, where P is power, ρ is air density, A is the swept area of the rotor blades, and v is the wind speed.

Air Density

• The density of the air varies with elevation and temperature.

Wind Turbine Types

• Horizontal-axis turbines: These turbines have a horizontal axis of rotation and are generally more efficient than vertical-axis turbines.
• Vertical-axis turbines: These turbines have a vertical axis of rotation and are often used in urban environments or where space is limited.
• Offshore wind turbines: These turbines are located in the ocean and are often used to generate large amounts of electricity.

Solar Collector Orientation

• A flat-plate collector is usually oriented towards the South in the northern hemisphere to maximize solar energy absorption.

Evacuated Tube Collector Principle

• Evacuated tube collectors are based on the principle of trapping heat through convection, not conduction.

Parabolic Trough Collector Operating Temperature

• Parabolic trough collectors typically operate within a temperature range of 200°C to 400°C.

Volumetric Air Collector Advantage

• The primary advantage of volumetric air collectors over liquid collectors is that freezing is not a concern because of the use of air in the system.

Solar Technologies

• Photovoltaic (PV) cells: These cells convert sunlight directly into electricity.
• Solar thermal collectors: These collectors capture heat from the sun and use it to heat water or air.
• Concentrated solar power (CSP): These systems use mirrors to concentrate sunlight onto a receiver, generating electricity.
• Solar water heaters: These systems use solar energy to heat water for domestic use.

Solar Collector Advantages

• Linear Fresnel collectors are known for being simpler and cheaper than parabolic troughs.

Tracking Collectors

• Tracking-type collectors can utilize diffuse or scattered radiation from the sky, not necessarily direct sunlight.

Molten Salt Purpose

• The primary purpose of using molten salt (Sodium Nitrate NaNO3) in thermal systems is for thermal energy storage, allowing for electricity generation even when the sun is not shining.

Solar Cells

• Solar cells generate electricity by converting sunlight directly into electricity, not thermal energy.

Freezing Prevention

• Two main methods to prevent water from freezing in solar thermal systems are drain-down systems and anti-freeze solutions.

Solar Thermal Storage

• The most common solar thermal storage system uses water tanks.
• Rock beds are sometimes used with air-type collectors.

Solar Thermal Components

• Collector: Absorbs solar energy and transfers it to the working fluid.
• Storage: Stores the collected energy for later use.
• Heat exchanger: Transfers heat between the working fluid and the load (e.g., water or air).
• Control system: Monitors and regulates the system's operation.

Semiconductor Characteristic

• The n-type region in solar cells is characterized by excess electrons.
• The p-type region in solar cells is characterized by excess holes.

Solar Cell Function

• A p-n junction in a solar cell allows for easy flow of electrons from the n-type to p-type region, not holes.

PV Panel Power Output

• The power output from a PV panel is calculated using the formula: P = V * I, where P is power, V is voltage, and I is current.

PV System Sizing

• To determine the size of a PV system, one must analyze load properties, such as energy consumption and peak demand.