Alternative Energy: Chapter 9

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.