Wind Turbines

Energy and the Environment

Energy is needed for two functions:

1. To provide heating, cooking and processing of fluids
2. To provide electricity to drive machines, or power lights.

The following sections will discuss the various forms of energy, and how energy can be converted from one form to another which convenient for heating, cooling etc.

Forms of energy

We associate energy with devices whose inputs are fuel based such as electrical current, coal, oil or natural gas; resulting in outputs such as movement, heat or light.

Unit of energy is the Joule (J). The rate of producing energy is POWER which has the unit of Joule per second or the Watt (W).

There are FIVE forms of Energy:

1.  Mechanical Energy
2.  Electrical Energy
3. Chemical Energy
4. Nuclear Energy
5. Thermal Energy

These energy forms are discussed in the following sections.

Mechanical energy

This type of energy is associated with the ability to perform physical work.

There are two forms in which this energy is found; namely potential energy and kinetic energy.

Potential energy

As the name implies is contained in a body due to its height above its surroundings, examples such as the gravitational energy of the water behind a dam, and the energy stored in batteries.

Potential Energy = mass x acceleration due to gravity (9.81) x height above datum

E_p = m x g x h

The energy produced by one kilogram of water falling from a height of 100m above ground is a potential energy, which can be calculated as follows:

Potential Energy = mass x acceleration due to gravity x height above datum

E_p = 1 x 9.81 x 100 = 981 J/kg

Kinetic energy

Kinetic energy is related to the movement of the body in question. Examples of KE such as the flywheel effect and the energy of water flowing in a stream.

Kinetic Energy = ½ mass × velocity squared

E_k = ½ × m × v²

The water stream in a river flowing at a velocity of 2 m/s has a kinetic energy of:

Kinetic Energy = ½ mass × velocity squared = ½ × 1 × (2)² = 2 J/kg

Electrical energy

This type of energy as the name implies is associated with the electrons of materials. Electrical energy exists in two forms:

1. Electrostatic electricity
   This type of electrical energy is produced by the accumulation of charge on the plates of a capacitor. Charles Coulomb first described electric field strengths in the 1780's. He found that for point charges, the electrical force varies directly with the product of the charges. The greater the charges, the stronger the field. And the field varies inversely with the square of the distance between the charges. This means that the greater the distance, the weaker the force becomes. The formula for electrostatic force, F, is given as:
   F = k (q₁ × q₂) / d²
   Where q₁ and q₂ are the charges, d is the distance between the charges. And k is the proportionality constant which depend on the material separating the charges.
2. Electromagnetic energy
   This type is produced with a combination of magnetic and electric forces. It exists as a continuous spectrum of radiation. The most useful type of electromagnetic energy comes in the form of solar radiation transmitted by the sun that forms the basis of all terrestrial life.

Chemical energy

This type of is associated with the release of thermal energy due to a chemical reaction of certain substances with oxygen. Burning wood, coal or gas is the main source of energy we commonly use in heating and cooking.

Calculation of chemical energy

The energy liberated from the combustion of a given mass of fuel, with a known calorific value in a combustion chamber of known efficiency is given by:

Chemical Energy = Mass of fuel × calorific value × efficiency of combustion

Nuclear energy

This energy is stored in the nucleus of matter, and is released as a result of interactions within the atomic nucleus.

There are three nuclear reactions:

1. Radioactive decay:
   In which one unstable nucleus (radioisotope) decays into a more stable configuration resulting in the release of matter and energy.
2. Fission:
   A heavy nucleus absorbs a neutron splitting it into two or more nuclei accompanied by a release of energy. Uranium U235 has the ability to produce 70x10⁹ J/kg
   Einstein proposed the following equation to calculate the energy produced from nuclear fissioning (i.e. conversion of matter (m) into energy, E are related to the speed of light C) :
   E = m C²
   This reaction forms the bases for current nuclear power generation plants.
3. Fusion:
   Two light nuclei combine to produce a more stable configuration accompanied by the release of energy. Heavy water (Deuterium) fusion reaction may produce energy at the rate of 0.35x10¹² J/kg.
   This reaction is yet to be realized to produce electricity on commercial basis.

Thermal energy 

Thermal energy is associated with intermolecular vibration resulting in heat and a temperature rise above that of the surroundings. Thermal energy is calculated for two different regimes:

When the substance in a pure phase, say if it is in a liquid, gas or solid, then

Thermal Energy = mass x specific heat capacity x temperature difference

During a change of phase, such as evaporation or condensation, it can be calculated by:

Thermal Energy = mass x latent heat

However, if there is a change of phase, say during the condensation of water vapour into liquid, there is an additional amount of heat released while the temperature remains constant during the change of phase. For 1 kg of water to be heated at ambient pressure from 20 to 120 oC, the requirement is

Thermal energy = heating water (20-100)° C + evaporation at 100° C + super-heating vapour (100-120)° C

Thermal energy = 1 × 4.219 × (100-20) + 1x2256.7 + 1 × 2.01 × (120-100)

                      = 337.52 + 2256.7 + 40.2

                      = 2634.42 kJ

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