Table of Contents
The word energy gets readily bandied about. In this website, it’s meaning is reserved to be a fundamental unit of measurement. With this, energy is directly related to mass as in;
|Eq. 1||Energy = Mass * (speed of light)2|
|– or –||E = m c2|
There is a choice of units for energy though each is interchangeable. This website uses a Joule. This is in honour of James Prescott Joule.
A calories is another unit of energy. With its association with food, particularly for dieters, it is perhaps better known. In a science laboratory, a calorie is the energy needed to raise the temperature of one gram of water by one degree Celsius (at standard temperature and pressure). A warning to the dieters. Their calorie which they see on food boxes is actually a thousand ‘lab’ calories. For lab work, the following relation holds;
|Eq. 2||4.184 Joules = 1 calorie|
This is the relationship used in this website.
Commonly, we relate energy to action. Often the action is considered to be work or heat. The following list relates energy
|Yearly solar emissions||1034|
|Energy to launch space shuttle||1013|
|One litre of gasoline||107|
|Human’s daily energy needs||107|
At one time we considered matter to be indestructible. That is, matter is neither created nor destroyed. Since then, we’ve learned about nuclear fission and fusion. Now we know that matter can be destroyed via fission as within a manmade nuclear reactor and created via fusion as within a star. But we consider matter/energy to be a constant as shown in Equation 1 above. This is significant with regard to entropy.
Energy is finite and regulated. Our Sun fuses hydrogen atoms into the bigger helium atoms and thus sends forth the energy. We see it as light, feel it as heat and watch it encourage plants to grow. This is a good thing as we eat the grown plants or other animals which themselves eat plants. Our biological energy needs are completely and solely satisfied by what we eat.
Our civilization also needs energy. As we started down the path of advanced civilizations, we simply burned wood for cooking food. Later we harnessed wind power using sails and directed water flow onto wheels to grind wheat into flour. Now, we’re splitting atoms to use the huge quantity of released energy to send electricity through our power grids. But the most ubiquitous source of energy is the fossil fuels. Petroleum, natural gas and coal are the real energy provides for
our civilization today.
These fossil fuels power electrical generating stations, push airplanes through the skies, and, propel cars along the freeways. Yet, these energy sources aren’t limitless. They all started as plants about some hundreds of millions of years ago. These plants captured the Sun’s energy, just like plants of today, however, they weren’t able to release it. Thus the energy accumulated and through a variety of processes they were transformed. Given the
hundreds of millions of years for this process, we can readily consider fossil fuels as non-renewable. Hence, we will eventually consume all that is available today.
The following numbers give an indication as to the degree of dependence upon fossil fuel in our civilization today.
The Sun is our energy source. Measurements in space have shown that a fairly steady energy flux arrives at the top of our atmosphere. Similar measurements quantify the amount of energy that reaches the land surface. We can consider this energy to be renewable. This energy limit is all that is available for all living things on Earth.
Table 2: Estimated availability
|From sun to top of atmosphere||1387 Watts per square metre|
|To land surface||1000 Watts per square metre, high noon|
|To land surface, average||240 Watts per square metre, average over Earth|
|To land surface, in Joules||1.127 x 1024 J/yr|
Effectively all the useful energy available to people comes from the Sun. This energy comes in a variety of shapes and forms. Because of it, the wind blows. Because of it, water evaporates from the oceans and falls onto the highlands so as to allow the water to flow back down to the oceans. Direct rays power photosynthesis in plants.
|Solar power incident on earth, rate||1 x 1017||Watts|
|Solar power incident on earth, yearly total||3.2 x 1024||J/yr|
|Solar power incident on land||2.2 x 1024||J/yr|
|Maximum solar power taken by net primary producers on land||2.2 x 1023||J/yr|
|Maximum solar power taken by herbivores on land||1.1 x 1022||J/yr|
|Maximum solar power taken by carnivores||5.5 x 1020||J/yr|
|Global human energy consumption||4 x 1020||J/yr|
The above table quantifies the theoretical maximum energy available. It assumes a 5% efficiency rate in transferring energy from plants to nimals and from animals into other animals. From this, in theory, humans are now capturing for themselves almost all the available energy or carnivores. True, the energy source is now mainly fossil fuels but this serves to highlight that without fossil fuels, we’d have difficulties meeting our energy needs.
|Human Energy Consumption (2002)|
|Global human energy consumption||4 x 1020||J/yr|
|Primary energy consumption||9405.0||mtoe|
|Primary energy consumption||3.95 x 1020||J/yr|
|Primary energy consumption – oil||1.48 x 1020||J/yr|
|Primary energy consumption – natural gas||0.96 x 1020||J/yr|
|Primary energy consumption – coal||1.0 x 1020||J/yr|
|Primary energy consumption – nuclear||0.26 x 1020||J/yr|
|Primary energy consumption – hydro||0.25 x 1020||J/yr|
The above table shows where people get their energy. It is interesting to note that the above value for the primary energy consumption doesn’t even include biomass. That is, the amount of energy we derive from wood and other vegetation is so small that it isn’t worth including.
The units of mtoe are million tons of oil equivalent. It’s a huge amount of energy and is regularly used in annual summaries for national consumption. One mtoe has about 4 x 1016 joules of energy.
|Minimum Energy Intake|
|Average human male||2500 kCalories/day|
|Average human male||10.5 megajoules/day|
|Earth’s human population (2005)||6 413 956 859|
|Base living requirements||2.452 x 1019J/yr|
But is all this energy really used by our civilization? Surely we must be using energy just to keep our bodies functioning? This is true, our bodies do need energy. But, we are using much more as the table above shows. With over 6 billion people alive today, and assuming each needs the average adult male energy requirements, our bodies use a little over 2 x 1019joules of energy each year. This is a small fraction of the amount of the primary energy consumption of our civilization which we saw in the preceding table.
With the above, we see that now our question gets a bit more complicated. A lot of people live on the Earth’s surface. They all need to eat to live. Many want the comforts of an advanced civilization which requires lots of energy. The Earth is finite. Eventually we will reach a maximum number of people and a maximum energy consumption. At least this is true if we rely solely upon renewable energy sources. However, if we rely upon non-renewable energy sources and the supply of energy runs out, what then?
|Earth Parameters (area in km2)|
|Area of Earth’s land surface||148 847 000|
|Snow land – 20%||29 769 400|
|Dry land – 20%||29 769 400|
|Mountains – 20%||29 769 400|
|Farmable land – 30%||44 654 100|
|Land with no topsoil – 10%||14 884 700|
|Area of Earth’s water surface – 70%||361 254 000|
The Earth is magnificent and has the necessary features to support life. Yet, even with its fantastic facilities, it isn’t perfect. Only a small fraction of the Earth’s surface is suitable for farming. Farms are where we get our energy needs. Or, at least our biological energy needs. The question remains as to whether the farms can support today’s current population and the level of civilization to which they are acustomed. The reference text looks at this question and considers a future
without the energy we derive from fossil fuels.
Entropy is a simple concept with far reaching importance. Entropy tells us that energy, though indestructible, becomes less able. This is our experience on Earth and, except for black holes of outer space, seems to hold for everywhere in the universe. Less capable means that energy, once used, can’t do as much work or create as much heat.
Let’s picture this idea. Imagine a lake filled with water. At one end of the lake, the water empties through a river and descends over a waterfall. At the base of the waterfall is a mill. The mill uses the water to spin stones that grind the grain to make flour. After the water spins the stones, it flows to another lower lake. In this closed example, water is neither created nor destroyed. The water that starts at the top lake has the ability to do work, which it does when it descends through the waterfall and turns the wheel. The water at the bottom lake is the same to all appearances as the water in the top lake but it can’t do work. It’s ability has gone. That’s the picture.
Energy is like the water in this imaginary scenario. It exists. It starts with the ability to do work or perform an action which it naturally will do. Once it does so, it remains but its ability is gone. The entropy relates to where energy exists in this imaginary scenario. Energy that is akin to water in the top lake has the greatest amount of energy. Energy that is akin to water in the bottom lake has the least amount of energy. And, like water being unable to flow uphill, entropy of a system will not naturally improve.
What does entropy mean for us on Earth? Well, consider our solar system as a closed system. It has a given entropy. This entropy will naturally increase (capability decrease). Though there may be localized areas of the solar system where entropy decreases, on the whole it increases. The same is true for any system on Earth. Small locations may see drops in entropy especially with man-made works such as electrical generating station. But expand the boundaries of these small locations for the system to include the complete thermal cycle and the natural tendency to increase entropy remains.
The implication of entropy for humanity is that energy capability is naturally continuing to decrease. As we burn wood in furnaces, capture photons with photovoltaic cells or split atoms in reactors, we make a brief local increase in energy ability which we use. But overall we increase entropy. In effect, our action increases the size of the topmost river in our imaginary scenario. Our actions allow more water to descend over the falls and driver more mills. But this means the water more quickly loses its capability. And we know it never recovers this.
This one-way nature of energy should be a warning to humanity. There is lots of energy about us but its entropy is continually increasing. Would a wise person be sure to make the best utility of the available energy? Do we make the best choices? Take a look at the consumption patterns to see where we are using energy today.
One question may still perplex you. Where does energy’s ability go to? Simply, it dissipates as heat. Cold areas get warmer. Cold areas don’t get colder. As heat energy radiates out from stars or planets, the energy transfers to the atoms spread throughout the universe. If this were the only force, eventually all material objects of the universe will all have the same temperature. But that temperature would still be very darn cold so don’t expect any paradise.