Energy is usually defined as the ability to do work. This is an anthropocentric and utilitarian perspective of energy; however, it is a useful definition for engineering where the aim of machines is to convert energy to work. As a more general description, we would say that energy is a fundamental entity whose availability and flow are required for all phenomena, natural or artificial.

An understanding of how energy is generated and measured is central to our decisions concerning the use and conservation of energy. Large-scale production of energy evolved over centuries but grew radically in the last 400 years and especially since the Industrial Revolution. A century of development and commercialization of electric power technology has ensured an easy supply, and continuous measurement.

Energy is derived in usable forms from numerous sources, such as flowing water, fossil fuels (e.g., coal and natural gas), uranium, and the sun. Electricity is a widely used form of energy. Any of these sources can be used to generate electricity. Liquid fuels such as gasoline and diesel derived from fossil fuels are a widely used source of energy. These fuels form the basis of our easy transportation. A complete understanding of the complexities of the energy systems within the natural environment requires knowledge of some basic physics and chemistry. This is discussed later in this unit in the sections under "Science Notes."

Energy Systems
An energy system may be thought of as an interrelated network of energy sources and stores of energy, connected by transmission and distribution of that energy to where it is needed. The transformation from stores of energy in food to work, and subsequent dissipation of energy is an example of such a system. The starting point of all energy in this "food chain" or "energy chain" (considering only the vegetable and cereal part of our food) is the sun.

Figure 1: Natural Energy System.

In Figure 1, each of the arrows shows transformation or transmission of energy -- that is, the energy changes form or is moved from one place to another. Plants and humans are the agents shown that store and/or transform the energy.

This natural energy system is part of a larger system that includes nutrients from the soil as input, other energy for cooking as input, etc. Figure 1 is drawn to show the parts of transformation of this initial solar energy up to its final dissipation and one storage system (fossil fuels). A complete concept map would show all the other factors. The numerous energy systems in nature include the food chain, the climate and ocean systems, and the cycles of various materials such as water, carbon, and nitrogen.

Most of the energy systems currently in use, both natural and man-made, originate in the Earth-Sun relationship. The fossil fuels we use today are stores of solar energy. Photosynthesis is an example of solar radiant energy transformed into stores of chemical energy that plants and animals (including humans) use to maintain themselves. The conversion of solar radiant energy through photosynthesis is a fundamental natural energy system. The food chain is an example of a natural, solar-based energy system that has sustained human life on Earth. Often we take for granted that energy will always be available for us to use. We fail to recognize the complexities of the energy systems that drive these environmental phenomena and sustain life on Earth. We are intricate parts of the system as end users, completing the dissipation of energy to forms that are so spread out that it is impossible to use that energy again.

Fossil fuels (coal, oil, gas) result from a transformation of plant and animal material over millions of years. The solar energy originally stored in the plant or animal is eventually converted into energy stored in carbon and hydrogen bonds of the fossil fuel. The fuels that took millions of years to make are being used at an enormously rapid rate. Figure 2 is a representation of the use of fossil fuels over time, including an estimation of how long they might last.

Figure 2: Fossil Fuel Timeline. Source: Clark, Mary E. Ariadne's Thread. St. Martin's Press, New York, 1989. Reprinted with permission of Macmillan Ltd..

Fossil fuels and fuels like uranium are "spent" once they are used to obtain energy. These are called non-renewable sources of energy. Although new plants can be planted that eventually turn to coal, the process takes millions of years and that is why coal and other fossil fuels are considered non-renewable. Solar and wind energy arrive or circulate air on the Earth everyday. These sources are called renewable.

Wood and trees used as fuel are called renewable, because they can be replanted. However, when we use them so that the rate of use far exceeds the rate of replenishment (trees take time to grow), referring to these sources as "renewable" can be a misnomer!

Energy use in each human activity has grown exponentially since the early days of human civilization. For example, technological capabilities enable us to travel more and process more food. Figure 3 shows the amount of energy (in calories) we spend for each calorie of food we get. It shows that technologies have mechanized and made large production systems of cultivation and fishing. These systems involve large expenditures of energy, as seen in Figure 3. The figure shows that for wet rice production in Asian countries, it takes between 0.02 and 0.1 calories of energy to produce 1 calorie worth of rice as food. Large-scale food production consumes enormous amounts of energy. For example, it takes over 2 calories of energy input to produce 1 calorie worth of eggs in large-scale farms, and it takes 10-15 calories of input for every calorie worth of beef produced in the U.S.. Note how the intensity of energy consumption for U.S. food production has grown almost ten-fold in the 20th century! Add to this the fact that for every calorie of energy our body gets, we have to take in over 5 calories worth of food!

Figure 3: Summary of the energy required for various types of food production. Source: Clark, Mary E. Ariadne's Thread. St. Martin's Press, New York, 1989. Reprinted with permission of Macmillan Ltd..

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