Basic Choices and Constraints on Long Term Energy Supplies clean alternative energy form It is important to understand a basic law of nature Energy once used is not regenerable So the public term renewable energy is misleading The following analysis examines the magnitudes of the world s energy supplies and the basic constraints on our ability to support in the long term society s demands using those finite supplies To put those magnitudes into a human context for policymakers and the public the longevity of our resources will be expressed on the scale of a human lifespan where 1 human lifespan is approximately 75 years Population growth and energy demand are exhausting the world s fossil energy supplies some on the timescale of a single human lifespan Increasingly sharing natural resources will require close international cooperation peace and security Paul B Weisz uman society like any system composed of dynamic H processes depends on an external energy source Historically that source was the Sun which provides heat light and photosynthesis for food to support work energy by man and animal and affects wind and water motion Since the early 19th century though the discovery of and access to a vast supply of fossil fuels within Earth has enabled the industrial revolution near exponential growth of population 1 technologies and wealth That period could well be renamed the energy revolution see figure 1 As we enter a new millennium we are growing increasingly concerned about the limits of our fossil fuels that are driving the world s economies Many journal articles committee reports and books have addressed this energy problem they contain opinions ideas and suggestions from experts within their various subdisciplines on possible ways to improve our practices or innovate technologically But a complex interdependence exists among the technological social and environmental aspects of energy use see the articles in PHYSICS TODAY April 2002 Furthermore many of the ideas researchers propose cannot significantly impact the real magnitude of the energy problem or may provide only short term relief Our basic choices are limited Nature s energy resources are confined to two categories Earth stored fossil residues and nuclear isotopes whose economic utility is limited by the finite amounts that exist on Earth and the radiation flux of solar energy whose economic utility is limited by the finite rate at which we can capture the Sun s energy and by the land areas that societies can dedicate to harness it The longevity of the fossil energy supply and the net rate of solar energy availability are both reduced by the energy consumed through their conversion to a suitable energy form and the technologies that accompany that conversion storage delivery maintenance and repair of environmental damage Solar derived consumer energy whether as electricity biomass or wind represents a Paul B Weisz is an emeritus professor of chemical and bioengineering at the University of Pennsylvania and a retired senior scientist and manager at the Central Research Laboratory of the Mobil Corp He is also currently an adjunct professor of chemical engineering at the Pennsylvania State University 2004 American Institute of Physics S 0031 9228 0407 030 9 Energy demands In viewing overall societal energy issues it is useful to express energy magnitudes in units of the quad Q where 1 Q 1015 BTU roughly equal to 2 5 1014 kcal or 1 06 1018 joule Current US energy consumption is about 100 Q year roughly a quarter of the world s total demand 2 Energy demand by humanity continues to rise An increase of about 1 5 per year is projected in the US and world demand is expected to increase by 1 2 per year for many decades mainly due to continued population growth While total demand is of course influenced by personal demand even unusually large 20 say conservation efforts would be nullified by population growth in less than 20 years Earth stored resources Petroleum In 1956 petroleum geologist M King Hubbert correctly predicted that a peak and subsequent drop in US production would occur around 1970 3 In fact foreign imports have since risen to 60 of current consumption US dependence on foreign petroleum is certain to increase In 2000 Jay Hakes of the Energy Information Administration presented a similar and extensive US Department of Energy assessment of the likely trend and peak in the world petroleum supply 4 Figure 2 shows the predicted range of years when the peak is likely to occur for demand whose growth rate may be between 0 2 Because growth rates due to population alone are anticipated to be at least 1 per year for many decades to come the pivotal event is expected to occur well within a human lifespan Moreover the analysis was based on an optimistic estimate of the world oil resource of approximately 2200 3900 billion barrels nearly twice the proven reserve 5 That would place the anticipated time to reach the peak well within a few decades Natural gas A natural gas shortage exists now in the US Yet the current growth rate of US demand is approaching 3 per year 2 6 As seen in figure 3 the proven July 2004 Physics Today 47 GROWTH IN DEMAND per year WORLD POPULATION billions total energy contents are large but highly speculative 2 7 To harvest the dilute solid carbonaceous Energy 8 contents requires drastic measures Either revolution underground combustion heating steam 7 or air to drive the carbonaceous solids toward the surface or the mining of huge vol6 umes of solids using heat solvents and 5 steam to extract the resource The extracts must be further processed to yield usable 4 hydrocarbon fuels a process that requires 3 further energy sacrifices Compared to petroleum these heavy oils present additional 2 refining and environmental problems be1 cause of the abundance of nitrogen oxygen and metal compounds found in them Also 0 the amount of CO2 released during process1200 1300 1400 1500 1600 1700 1800 1900 2000 ing and use greatly exceeds that released by YEAR the current use of petroleum fuels Nuclear energy Uranium fission Figure 1 World population growth since the 13th century plants in the US are presently supplying less than 8 of our total energy demand Were the current nuclear technology expanded to provide the electricity now supplied by coal about 23 Q the estimated US US natural gas reserve would last very few years even at uranium resources2 would be exhausted in about 35 58 constant year 2000 demand years less