|Fig. 1: Graph comparing Energy Returned on Investment from different sources - after Weissbach et al. |
The energy returned on investment (EROI) has been analyzed for a variety of energy sources including wind energy, photovoltaics, solar thermal, hydro, natural gas, biogas, coal and nuclear power.  A study by Weissbach et al. found that "nuclear, hydro, coal, and natural gas power systems (in this order) are significantly more effective than photovoltaic and wind power by an order of one magnitude." EROI is calculated by dividing the quantity of energy supplied with the quantity of energy used in the supply process. EROI = (Quantity of energy supplied/Quantity if energy used in supply process).  In this scenario, energy is defined as the ability to do "useful" work. Useful to do work is represented by the enthalpy of the fuel For cases of combustion, this is the heat of combustion, or the amount of heat released during combustion of a fuel. A common related concept is the energy payback period. Every energy system has initial investments of energy (e.g. facility construction). A facility produces energy for an amount of time until it reaches its expiration. Along the way, all additional energy costs incurred, for example in facility operation, facility maintenance, or self-use is accounted for and subtracted from the energy output. The energy payback period is the the "break-even" time it takes for a facility to produce an amount of energy equivalent to that used to create it.
Since the advent of energy resources, about one hundred and fifty years ago, when coal provided energy to make steel and steam, the energy obtained from these sources was obvious and it took a little energy input to get a lot of energy output. At that time, the concept EROI, the ratio of energy returned to the energy invested was simple, easy to calculate and uncomplicated to understand. When EROI is large, energy from that source is easy to obtain and cheap and the opposite is also true. When the number is one there is no return on the investment and the entire effort has been wasted.
The breakeven number was considered to be seven. The EROI for coal was large and for fossil fuels it was even larger. One joule of energy invested in oil returned 100 joules of energy, therefore oil has an EROI of 100. Initially, fossil fuels were easy to recover but as the demand increased it became more and more difficult to maintain the EROI as it required more energy investment and the return was reduced. In 2006 one joule of energy invested produced only 15 joules of energy return for oil. Financially this was still hugely profitable to the oil companies. 
New technologies and the use of unconventional sources for energy production are evaluated based on EROI. The EROI of many unconventional energy sources may be in the profitable range but lower than that of conventional sources. That is the reason why the use of the nonconventional sources becomes marginal when the price of oil drops to below 60 dollars a barrel. It is in fact the EROI and its decreasing number for oil that has provoked the exploration of other energy sources like hydroelectric, solar, and nuclear. EROI is still the method used to evaluate energy sources and decide whether it is worth investing energy in them. The above figure summarizes the EROI for different energy sources. Some sources have two values because of the need for buffering or storage EROI is critical to decision making in energy policy. The minimum EROI required for USA with its level of complexity in society and its standard of living is about 7. "An energy system must produce a surplus large enough to sustain things like food production, hospitals, and universities to train the engineers to build the plant, transport, construct, and all the elements of the civilization in which it is embedded."  Lower EROI cannot sustain a society like what currently exists in the United States with its high level of complexity and its high standard of living. The claim that renewables alone, or in the majority, can provide our energy is not consistent with their EROI. The current EROI requirement in the United States averages about 40. A portfolio of energy sources containing 50% renewables, 30% fossil fuels and 20% nuclear provides an average EROI of only about 25. The question is whether the American economy can sustain itself with an EROI drop from 40 to 25. EROI is linked to the economic policy of a country - ecomonies with access to sources of energy with higher EROI have greater potential for expansion and diversification which explains the interest in coal among developing countries like China, resulting in coal being the most sought after energy source in the world today. However, environmentalists and international organizations like the United Nations are strongly advocating for 33% nuclear, 33% renewable and 33% fossil, with an average EROI of about 36 and a reduction in carbon emissions by about 50%.
In the 21st century, debates over renewable sources are common. Regardless of the energy source, in order to attain sustainable development, it is vital to increase energy efficiencies of current processes, renewable energy is the best way to do this.  There is also a link between exergy and sustainable development. A sustainable energy system may be regarded as a cost- efficient, reliable, and environmentally friendly energy system that effectively utilizes local resources and networks. Exergy analysis has been widely used in the design, simulation and performance evaluation of energy systems. 
"Energy can be stored while power is instantaneously measured. Energy can change its form, power cannot. To oversimplify, energy if the potential and power is the output."  When it comes to thorium, a thorium atom has a fixed amount of energy stored within it. The amount of energy released when using thorium in a nuclear reactor is much higher than that of uranium. This is because thorium is such a better nuclear fuel than uranium.
We are running out of oil. We are burning oil at an increasing rate, and our supplies are declining. Paul Roberts discusses the steep cliff of oil depletion in his book, The End of Oil. He shows that extensive oil-recovery measures pushed the peak of production farther into the future, and that as a result when the peak does come, the decline will not be as gradual - when oil starts depleting we won't have a gentle downhill slope but rather a steep cliff. 
A study by Smith et al. examined the contrast between a nuclear-free, decentralized energy setup and a securtized, centralized "hard energy" dystopia.  Dystopian scenarios center around the fact that global consumption of raw materials is increasing, energy production has generally plateaued in industrial countries. Overall rising consumption of natural resources will result in additional environmental degredation. The best way to prevent these outcomes and avoid a "hard energy" dystopia trend is by increasing efficiency in the use of energy and materials, measured by energy returned on invested. 
© Caleb Kumar. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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