ENERGY RESOURCES AND THEIR USES

1. Introduction


Source: Solar and Heliospheric Observatory

Most livings thing depend on energy from the sun, a medium-sized star 1.5x108 km away from the earth. Green plants capture the energy of visible light, and in an energy-dependent process called PHOTOSYNTHESIS, carbon dioxide and water are converted to carbohydrates (sugars, starches and cellulose), and oxygen. These carbohydrates are stores of chemical energy that are used for the plants' own use. HERBIVORES are plant-eating animals that utilise the carbohydrates for their own energy needs. CARNIVORES are meat-eating animals that feed on herbivores, and thereby get their energy from the sun third-hand as it were.

Human beings are no different - as a species, Homo sapiens is an OMNIVORE, feeding both on vegetable and animal foods. However, where humans differ from animals is that they require energy over and above that which is required for maintaining life and growth. At the lowest level, humans need energy for warmth and for cooking. This can be supplied by the burning of plant material, such as wood.

Our modern world requires huge quantities of energy, for the generation of electricity, transportation, for industrial processes, and communication. This energy has to be obtained somehow, and since the start of the Industrial Revolution (around 1750), we have relied principally on the so-called FOSSIL FUELS, notably coal, petroleum, and natural gas. These resources originated millions of years ago when the climate of the earth was much warmer, and large areas of the earth's land mass was covered with plants. As these plants died, the vegetable matter was converted over long periods of time into these fossil fuels, which we exploit today.

All sorts of ways exist to supplement or even replace the energy needs presently met by fossil fuels, which slowly but surely are running out. Supplying the world with alternatives to fossil fuels is a major scientific and technological challenge in the 21st century.

Useful on-line information on energy resources may be found at

  1. http://en.wikipedia.org/wiki/Renewable_energy
  2. http://home.clara.net/darvill/altenerg/index.htm

2. Renewable and non-renewable energy resources

Energy resources may be classified as RENEWABLE and NON-RENEWABLE. Non-renewable resources are resources that will ultimately run out, while renewable resources should last at long as the sun keeps providing light energy to the planet, a period of time that will last several billion years.

Renewable Non-renewable
Solar energy
Wind
Water power
Tidal
Wave
Geothermal
Biological (biogas, biofuel, biomass)
Fossil fuels (coal, oil, natural gas)
Nuclear fuels (uranium, plutonium)

Note that nuclear energy, tidal energy and geothermal energy do not originate from the sun.

What about hydrogen as a source of energy?

(Click here for a discussion)

3. Properties of energy resources

Ideally, an energy resource should:

It turns out that no resource meets all the above ideal requirements. If a resource is plentiful, it is not necessarily cheap - wind energy, for example is plentiful, but it requires a large capital investment in order to harness it on a large scale. Nuclear energy does not produce greenhouse gases, but generates long-lived radioactive substances which have to be disposed of safely.

Most renewable resources require large land or sea areas. Further, several of these resources are dependent on the weather, and are thus not continuously reliable.

3.1 Solar energy:

The sun provides an unlimited amount of energy in the form of electromagnetic radiation, reaching the earth chiefly as light and heat. Harnessing this energy is neither simple nor without problems. Present day applications are:

Solar heating: Solar heaters are not infrequently found on the roofs of houses, where they are used to heat water for domestic purposes.



Photovoltaic cells: These are devices that convert sunlight directly into electricity. They are useful for generating small amounts of electricity for specialised applications (for example in telecommunications and satellites). Fairly modest installations, such as the one shown on the right, can generate enough electricity for lighting purposes. The electricity produced may be stored in batteries. The current produced by photovoltaic cells is direct current (DC), whereas most appliances use alternating current (AC).



Solar furnace at Odeillo, France (Source: H Zell, Wikipedia Commons)

Solar furnaces: These are huge assemblies of mirrors that focus the sun's rays on a particular spot. Very high temperatures may be achieved, and this could be used to generate steam to drive steam turbines, and thus generate electricity.



Rance River tidal power station, France (Source: Wikipedia - Public domain)

3.2 Tidal power

Much thought has been given to the use of tides in order to provide energy. The basic idea is to store water in a large dam with a rising tide, and letting it run out through turbines as the tide goes out. In order for this to be feasible, there must be a large difference between the sea levels at low and high tides, and the geography of the site must be favourable. No such conditions exist in South Africa. A tidal power station exists in the Rance river estuary in France, with an average output of 96 MW.



3.3 Wave power:

Waves carry a lot of energy, which could be captured to drive turbines and thus generate electricity. Pilot plants have been constructed, but the whole technology is still very much in its infancy.

3.4 Fossil fuels:

Coal is plentiful in South Africa, with large deposits near Witbank, Ermelo and Secunda. In 2003, South Africa produced about 230 Mt (megatons) of coal, about 30% of which was exported, making South Africa the world's second largest coal exporter after Australia. No significant oil deposits have been discovered in South Africa, but offshore drilling on the Agulhas Bank has led to the exploitation of natural gas reserves off Mossel Bay.

Coal is cheap, and was the fossil fuel that powered the Industrial Revolution in Western Europe in the 18th Century. Petroleum used to be cheap, but its price and availability are subject to political factors. The Western industrialised nations are heavily dependent on supplies from the Middle East, which is a politically unstable region.

The biggest drawback of fossil fuels is the fact that their combustion produce greenhouse gases contributing to global warming, and acids that produce acid rain. Roughly speaking, every kilogram of oil burned produces 3.1 kilogram of carbon dioxide. A jetliner flying from Cape Town to London will produce more than 230 tonnes of CO2 during the trip!

3.5 Nuclear fuels

The primary nuclear fuel is uranium-235, which constitutes about 0.7% of the mass of naturally-occuring uranium, most of which comprises uranium-238, which is useless for energy purposes. Naturally-occurring uranium has to be enriched to about 3% U-235 before it can be used for energy production.

Uranium is a non-renewable resource (South Africa produces uranium as a by-product of some gold mining operations). It is "clean", in the sense that nuclear power stations do not emit greenhouse gases, but the waste products are highly radioactive and

South Africa has a nuclear power station at Koeberg, on the West Coast, which is fairly modest compared with some in other countries, several of which, notably France, are heavily dependent on nuclear energy for the generation of electricity. Nuclear power generators are also used to power large aircraft carriers and certain classes of submarines.

The Nesjavellir Geothermal Power Plant, Iceland (Author: Gretar varsson, in public domain)

3.6 Geothermal energy

This involves tapping into high pressure steam generated by volcanic activity, or by running water deep underground, where hot rocks convert the water into high pressure steam. The steam is used to drive turbines that generate electricity, or simply for heating purposes. Iceland, Japan, New Zealand and Italy have sufficient sources of geothermal energy to make electricity generating installations commercially viable.



3.7 Water power

By water power, we mean here the utilisation of the kinetic energy of falling or fast running water in order to drive machinery. HYDROELECTRIC power stations are often built near large dams, where high water pressures are available. The best example of this in Southern Africa is the Cahora Bassa scheme in Mozambique, which supplies electricity to the ESKOM power grid.

3.8 Wind energy

The old: Mostert's Mill, near Cape Town, built in the late 18th century, used wind energy to produce flour.
The new: Wind turbines such as these in use in the United kingdom, supplement electricity generated by fossil fuels and nuclear energy.

Wind energy is a clean renewable energy source, and some countries, notably Portugal, have developed large "wind farms" that contribute significantly to electricity production. However, the installation of wind turbines is expensive, and many ecologists consider these modern windmills to be unsightly. South Africa has a few experimental turbines.

3.9 Biologically derived fuels

BIOMASS is any material from biological sources that can be used as a source of energy, normally by burning. Examples of this are:

An obvious drawback is the emission of carbon dioxide, a greenhouse gas. On the other hand, it can be argued that the biomass was originally generated from atmospheric carbon dioxide in the first place, so no long-term increase occurs. Proponents of biomass usage play down the fact that energy is necessarily used up in production and distribution of biomass fuels.

In some parts of the world, crops are planted not for human consumption, but for production of biomass and biofuel. Consequently, many informed persons fear that this might result in food shortages and higher prices, thus having a negative impact on Third World countries. Those in favour of such practices seldom mention the necessary use of fertilizers in producing the crops, and the energy cost of producing and transporting these. There seems to be a disenchantment about this "solution" to the global energy problem.

Additional questions












The sun, our nearest star

Some statistics about the sun
Mean distance of earth from the sun 1.496x1011 m

Putting these statistics into perspective:

  • The sun's light takes 8.31 minutes to reach the earth.

  • The sun's siameter is 109 times that of the earth.

  • The sun's mass is 322 956 times that of the earth.

  • The sun's volume is 1 300 000 times that of the earth.

Mass 1.891x1030
Mean diameter 1.392x109 m
Volume 1.4122x1027 m
Mean density 1.409x103kg·m-3
Surface temperature 5778 K
Temperature of corona about 5x106 K
Temperature of core about 16x106 K
Composition
   Hydrogen
   Helium
   Oxygen
   Others
 
   73.48%
   24.85%
   0.77%
   0.63%
Data from http://en.wikipedia.org/wiki/Sun

The figure on the left shows the comparative sizes of the earth and the sun.


The figure on the right gives a rough idea of the relative size of the sun and the mean sun-earth distance.












Hydrogen as an energy resource

At first glance, hydrogen would seem to be an ideal fuel. There are unlimited supplies of hydrogen in water, from which hydrogen may be obtained by electrolysis. Alternately, hydrogen can be produced by various reactions, carried out on a large scale in industry:

All these processes require a high energy input, so it is not correct to think of hydrogen as an "energy resource". It is better to consider hydrogen as an "energy carrier", produced by an input of energy, and used to provide energy somewhere else, bearing in mind always that more energy is used up in the overall process than is produced in the end.

The attractiveness of hydrogen as a fuel lies in the fact that it is a "clean" fuel - combustion only produces water. However, there are several drawbacks to its widespread use:

Nevertheless, much research has been carried out on the use of hydrogen as a motor car fuel. In Iceland, where hydroelectric power is cheap, hydrogen is produced by electrolysis on a large scale. There one can find service stations that supply the gas to motorists (who need specially converted engines).

Hydrogen is used in fuel cells, and some classes of submarines are equipped with such cells, allowing them to remain submerged for an appreciable time.












The solar heater

The simplest type of solar heater is of the so-called "thermosiphon" design. It consists basically of a collector, which absorbs light in a length of copper or (less desirably) plastic pipe, and re-emits energy as heat into water carried in the pipe. The surface of the pipe is black (copper pipe painted black is best), as black objects are most efficient at absorbing light. The pipe is coiled, and placed in a glass-fronted box to minimise heat loss by convection to the atmosphere.

Hot water rises out of the collector, since it is less dense than cold water, and is conveyed to the top of the domestic hot-water cylinder, thus relieving the need to heat the water electrically, since the thermostat will control at what temperature the electric heater will be activated. Cooler water from the bottom of the cylinder is fed back by gravity to the bottom of the collector. (The above diagram is simplified. For more information on water solar heaters, see http://www.greenbuilder.com/sourcebook/HeatCool.html)












Photovoltaic cells

When light of a suitable wavelength strikes certain materials, electrons are knocked out of their orbitals, and can flow in a circuit. Thus, light energy is directly converted into electricity, a phenomenon discovered in 1839 by Edmond Becquerel, and explained by Albert Einstein in 1905. Photovoltaic cells are made of semiconducting materials, and achieve efficiencies around 30% (meaning that 30% of the light energy falling onto the cells is converted to electricity). The greater the light intensity, the greater the current produced.

A single photovoltaic cell does not produce much electricity. For this reason, they are linked to form modules, which in turn are linked to form arrays. The greater the array, the more light can be collected, and hence the more the current that can be produced.

The image below is that of a small commercially available array used to top up a motor car battery. It is placed on the dashboard of the car, and plugged into the cigarette lighter socket. A small "trickle" current is produced which charges the battery while the car is parked - in the sun, of course!

(For more information on photovoltaic cells, see http://science.nasa.gov/headlines/y2002/solarcells.htm)