NOTES –OPTION C: ENERGY

Part 1: Energy Sources (C.1) – pp. 766-773

C.1 – Societies are completely dependent on energy resources. The quantity of energy is conserved in any conversion but the quality is degraded.

Conservation of energy- states that energy cannot be created or destroyed. It can only be converted from one form into another. The problem is that the quality of energy is being degraded – if we lose energy to the environment it is no longer available to do useful work.

Sources of energy

  • Either hot bodies, like the ______, or objects that store high-quality ______.
  • Need to be cheap, plentiful and readily accessible.
  • Needs to be able to release energy at a reasonable and controllable rate.
  • Useful energy needs to produce minimal ______.

Fuel:a substance that can release energy by changing its chemical or nuclear structure.

Non-renewable sources of energy:

  • Used at a faster rate than it is replenished.
  • Examples:

Renewable sources of energy:

  • Replenished at a faster rate than it is used.
  • Examples:

Calculating energy density and specific energies:

Energy density = Specific energy =

Table 1.1: specific energies of some common fuels

Fuel / Specific energy / MJ kg-1
fusion fuel
238U
hydrogen (g)
gasoline (petrol)
natural gas
Coal
Wood

Example: Calculate the specific energy and energy density of hexane from its density, which is 0.6548 g cm-3 under standard conditions (see section 13 of the IB data booklet).

Energy conversions are never 100% efficient. Energy is always dissipated during energy transformations, as explained by the second law of thermodynamics.

A primary source of energy is often not used directly, but instead converted into a secondary source such as electricity. Energy is inevitable lost during the conversion (in the form of heat transferred to the environment).

The efficiency of an energy transfer:

efficiency =

Efficiency of different power stations depend on design, but typical efficiency values for fossil fuel power stations are below:

Table 1.2:Typical efficiency values for fossil fuels

Fuel / Typical efficiency values
Gas
Oil
coal

Example: a coal-burning power station generates electrical power at a rate of 550 x 106 J s-1. The power station has an overall efficiency of 36% for the conversion of heat into electricity.

a)Calculate the total quantity of electrical energy generated in one year of operation

b)Calculate the total quantity of heat energy used in the generation of this amount of electricity.

c)Calculate the mass of coal that will be burned in one year of operation, assuming that coal has the enthalpy of combustion of graphite.