Alternative Energy Sources Overview

Alternative Use of Energy

• Alternative energy sources are non-depleting or renewable energy sources.
• Examples of alternative energy sources include solar and wind.
• Conventional energy sources like natural gas, coal, and oil were once considered alternative energy sources.
• Funding for research and development (R&D) and tax incentives for alternative energy decreased significantly in the 1980s and early 1990s.

Solar Thermal Systems

• Subtopics covered include Introduction, Solar Irradiance, Solar Thermal Collectors, and Thermal Storage Systems

Solar Energy Introduction

• Surface temperature of the sun is 5778 K (5505°C).
• Core temperature of the sun is 15.6 × 10⁶ K.
• Power output of the sun is 3.85 × 10²⁶ Watts.
• Energy output of the sun is 3.37 × 10²⁷ KW·hr/yr.
• Annual energy intercepted by Earth is 1.5 × 10¹⁸ KW·hr/year (0.000000045%).
• World energy demand is 1.4 × 10¹⁴ KW·hr/year (0.0093%).
• Total ratio of world energy demand to intercepted solar energy is 4.2 × 10^-12%.

Solar Irradiance (Distribution)

• The Middle East and North Africa (MENA) region receives the highest solar irradiance globally.
• Distribution maps show differences in solar energy received geographically.

Solar Irradiance (Orientation)

• Maximizing solar energy collection involves orienting collectors according to the sun's position for optimal radiation.
• This ensures the collector remains perpendicular to the sunlight.

Solar Irradiance (Numerical Facts)

• Solar energy arrives at the Earth's atmosphere at a rate of approximately 1353 W/m² (428 Btu/hr ft²).
• This is known as the solar constant.
• Some of the incoming solar radiation is reflected back into space, absorbed by the atmosphere, or scattered.
• Only about two-thirds of the initial solar energy reaches the Earth's surface.
• A calculation of typical noontime solar radiation can be about 946 W/m² (300 Btu/hr • ft²) on a cloudless day at 40° North latitude.
• A solar collector that tracks the sun will theoretically maximize its collection efficiency.
• In practice, energy collection efficiencies are lower (for example, approximately 70%).
• A more realistic estimate of solar panel area needed to absorb a certain amount of energy to provide 293 kWh (1e6 Btu) of energy per day is about 53 m² (567 ft²).

Solar Thermal Collectors

• These collectors categorized into non-concentrating and concentrating types.
• Non-concentrating types include flat-plate, evacuated-tube, and volumetric air collectors.
• Concentrating types include parabolic trough, compound parabolic concentrator (CPC), solar power tower, and solar dish Stirling engine.

Flat Plate Collectors

• Commonly used for relatively lower temperature applications like heating water.
• Typically oriented South (in the Northern Hemisphere)
• Consist of glazing, absorber plates, flow tubes, insulation, and casing.

Evacuated Tubes (Vacuum Tubes)

• Similar in concept to flat-plate collectors but employ a vacuum to reduce heat loss via convection.

Volumetric Air Collector

• Advantages over liquid collectors include no freezing concerns, less damage from leaks, and reduced corrosion issues.

Parabolic Trough Collectors

• High-temperature collectors using parabolic troughs to focus sunlight.
• Temperature range generally from 250-450°C.
• Concentrating collectors are not as effective with diffuse or scattered radiation from the sky and must track direct sunlight

Compound Parabolic Concentrator (CPC)

• Uses smaller, flat mirrors to concentrate solar radiation, similar to parabolic troughs, but they are cheaper and simpler to manufacture

Solar Power Tower

• Uses many mirrors to focus solar radiation on a central receiver at the top of a tall tower.
• Molten salt (sodium nitrate) is often used as a heat transfer fluid to store thermal energy

Solar Dish Concentrator

• Uses a parabolic dish to concentrate sunlight on a receiver to directly generate AC power.

Thermal Storage Systems

• Solar energy generation is not aligned with demand, thus storage systems needed.
• Common solar thermal storage method involves water tanks.
• Air-based systems can use a rock bed for storing energy as heat.

Solar Cells (photovoltaics)

• Convert sunlight directly into electricity.
• Semiconductor materials utilized to create p-n junctions.
• Types of solar cells exist, some examples given include n-type or p-type.
• Solar cells output depends on the solar irradiance and temperature.

Wind Energy

• Wind is a form of solar energy generated by uneven heating, irregularities of the Earth's surface, and the Earth's rotation.
• Harvested wind energy used from modern wind turbines converts motion energy into usable electricity.

Wind Calculations

• Wind speed and the amount of energy in the wind vary with air density and the blade's swept area.
• Air density varies with temperature and elevation.
• Warm air is less dense than cold air, and air is less dense at higher elevations.
• Swept area increases with the blade's length.
• Wind energy is related to the cube of wind speed.

Wind Turbine Types

• Modern wind turbines are generally categorized as horizontal-axis or vertical-axis types.
• Horizontal wind turbines are similar to airplane propellers.
• Vertical wind turbines are sometimes compared to egg beaters, like the Darrieus wind turbine.

Wind Turbine Types-Horizontal Axis

• Advantages:
  • Blades positioned offset from center of gravity, leading to greater stability.
  • Ability to adjust blade pitch to minimize damage during storms.
  • Tall towers allow access to stronger winds due to wind shear
  • Self starting ability.
• Disadvantages:
  • Difficulty operating in areas with low wind
  • High transportation and installation costs due to large components.
  • Require skilled maintenance.

Wind Turbine Types-Vertical Axis

• Advantages:
  • Easy to maintain
  • Low construction and transportation costs
  • No directional requirements
• Disadvantages:
  • Less efficient
  • Low starting torque that may require energy to start turning
  • Operates in low and turbulent winds

Turbine Components

• Horizontal turbines consist of blades, a drive train (including gearboxes and generators), a tower, and various supporting equipment.