For today's lecture we shall examine the historical energy usage patterns for the United States, including the mix of different energy sources we have used over time, and consider how these patterns link to population and economics. In addition, we will use what we see to project future usage and production patterns, and consider strategies that might help prolong current energy sources.
We start by looking at the US energy consumption over the past 50 years.
- Figure 1)
- (source: Energy by G. Aubrecht)
One can see that the portions devoted to industrial, residential & commercial (heating) and transportation sectors have grown steadily during this time.
But wait. Is this an exponentially-growing curve? What is Exponential Growth?
Segments of the total use curve depicted in Figure 1 appear similar to the example exponential curve discussed on the Exponential Growth page. In 1970, however, in response to the first of two "energy crises" (1973 & 1979), the energy consumption curve falls away from that expected for continuous exponential growth. During these two crises the world's supply of oil was restricted by OPEC-led restriction of the supply of oil.
It is easier to observe the behavior of exponentially-growing functions by plotting them on a semi-log graph. Figure 4, below, shows the annual energy consumption (now in in quadrillion (1015) Btus) during the past 140 years. Notice that for each major division of the vertical axis the amount increases by a factor of ten. This means that each big "tick" represents ten times the amount of the previous tick.
- Figure 4
- (Source: Energy & Problems of a Tech. Society- Kraushaar & Ristenen)
Let's examine the curves depicting energy consumption by source on the graph above more carefully. In 1850 there were, essentially, two sources of energy in the US-- wood and coal. During the next 40 years wood use gradually declines while the use of coal skyrockets. Coal replaced wood as a source of heat energy for steam engines/heat engines during that time because it carries more chemical potential energy per mass.
Around the turn of the century oil, natural gas, and hydroelectricity enter the mix. Oil was valued for its ease in handling and how cleanly it burns relative to coal. During the next 40 years, roughly from 1890 to 1930, oil consumption grows rapidly at the expense of coal usage. This time frame also sees the gradual replacement of horse-drawn transportation by early automobiles ("horseless carriages"). The growth in the use of hydroelectricity and, to some extent, natural gas, corresponds to the electrification of America during the first half of the twentieth century.
By 1940 oil and natural gas have surpassed coal in usage, while hydroelectricity consumption has begun to level off. Between 1940 and 1970 the highest long-term growth in energy consumption is sustained by increasing reliance on oil and natural gas-- to provide transportation, fuel increases in agricultural yields, and to generate electricity.
By 1970 US production of oil has peaked and world supply is strongly impacted by the formation of OPEC. Oil and natural gas consumption fall off and coal is envisioned as a replacement, at least for the electricity-generating sector. However, a substantial increase in the consumption of coal and hydroelectricity are insufficient to maintain the growth in total consumption.
Figure 4.5 shows how the energy source "mix" has varied historically in the U.S.
- Figure 4.5
The answer is pretty simple. Assuming that, on the average, everyone in the US has a similar lifestyle, energy consumption will grow as population grows. As depicted in Figure 5, below, the US population grew steadily between 1790 and about 1870 (at about 2.9% per year), then started to decrease in growth, reaching an average value of about 1.8% per year.
- Figure 5
- (source: U.S. Census Bureau via Energy- G. Aubrecht)
- Figure 6
As energy consumption per capita increased, so did the nation's GNP. Essentially, we convert energy into food, housing and products; and the more energy we put into this process, the better we live (some may debate whether we actually live "better" than people did during the 19th century).
Recent information can be found at the US Energy Information Agency. The following figure shows energy sources and uses for 1996.
- Figure 7
- (source: US Energy Information Agency: 1998 Annual Energy Review)
The US Energy Information Agency also generates predictions concerning future energy usage. You can see by the figure below, that they anticipate that the world energy source mix will include more natural gas and renewable energy sources in the coming years.
- Figure 8
Also note that this is a regular "linear" plot of energy usage versus year. The vertical scale is NOT logarithmic. The projected curves for oil and coal consumption appear to gently increase in slope, at best.