Revised 26 September 2021



  • The International Federation of Institutes for Advanced Study (IFIAS) held the first workshop on Energy Analysis in 1974 to discuss the need for consensus on conventions and recommendations for further work

  • Energetics uses a general systems approach and circuit language diagrams to describe and analyse energy flows. 

History of Energetics

  • Energetics became a discipline in its own right when Howard Odum pointed out that "industrial man no longer eats potatoes made from solar energy; now he eats potatoes partly made of oil" in his book Environment, Power, and Society (1971).

Energy Flows

  • The study of energy flows within ecosystems is based on the First and Second Laws of Thermodynamics.

Energetic Symbols

  • Energetic symbols and diagrams used by Odum (1976) help us to visualise the laws and flows of energy within ecosystems by introducing the idea of an ecosystem as a combination of interacting parts. 

  • Different forms of energy differ in their ability to do useful work. A calorie of dispersed heat cannot do any work and sunlight must first be concentrated to be able to do useful work.

  • It takes energy to concentrate energy. Some energy must be degraded in order to concentrate what is left.

  • The degree of convertibility of energy - stored work - into applied work is often called availability.

  • Thermal energy is a special case of availability. The greater the difference between the heat source and its environment, the greater is the availability. The hot core in a nuclear reactor is energy of high availability, while that of a domestic radiator is of low availability or low-grade energy.

  • High-grade energy can be used in conjunction with a larger flow of low-grade energy to produce high-grade energy.

  • High-grade energy is wasted if it used for purposes which can be provided by using low-grade energy.

The Grading of an Energy Source

  • The grading of an energy source can be classified in terms of the energy level of the source ─ a measure of its energy inten­sity in terms of energy per unit mass ─ and its energy grade that is a measure of energy quality.

  • The energy grade of a source can be separated into either work forms or heat forms of energy.

  • Heat forms of energy ─ heat from fusion/fission, heat from combus­tion, and heat from friction ─ are graded in descend­ing order according to the tem­pera­ture of the source.


  • Exergy is available energy, the maximum fraction of an energy form which, in a reversible process, can be transformed into work.

  • Exergy can also be applied to take into account the quality of minerals used to manufacture goods.

  • A standard reference point of exergy is needed for different natural minerals in the ground, and hence the concept of ‘Thanatia’ which is a hypothetical version of our planet Earth where all mineral deposits have been exploited and their materials have been dispersed throughout the crust.

  • By adding up all the exergy expenditures of mining and refining, the rarity of minerals and their embodiment in final products can be assessed.

  • There can be no production of goods and services without exergy destruction. Unlike energy, exergy is not subject to the law of conservation.

  • An exergy destruction footprint can be established for same-purpose products which use different resources and processes during manufacturing and the full life cycle of maintenance, replacement, and recycling.


  • Emergy is the total energy used or embodied in the life cycle of a product in terms of the available energy of one kind that has to be used directly and indirectly to make a product or service

  • Different products and processes make use of different forms of energy, each of which give rise to different levels of CO2 contributions to the atmosphere. Use of Emergy alone is therefore inadequate for making true comparisons of the environmental impact of different products. The same applies for Exergy.

First & Second Law Efficiencies

  • The First Law Efficiency is the ratio of the amount of energy delivered to perform a task to the amount of energy that must be applied to achieve this task.

  • The Second Law Efficiency is the ratio of the minimum amount of avail­able energy required to carry out a task to the actual amount of available energy used.

  • The second law efficiency is a measure of how much the perform­ance of a task falls short of what is theoretically poss­ible, and can be used as a measure of the conserva­tion of free, or avail­able energy (exergy) in carrying out a task.

  • There is a trade-off between effi­ciency and power. An infinitesimally slow reversible process may be carried out with maximum efficiency, but with a penalty of a power output approaching zero. A very rapid process, on the other hand, approaches a maximum power input but at zero effi­ciency and zero power output.

  • Life forms and the activities of humankind require energy pro­cesses to be carried out at an intermediate range of rates that fall well short of the maximum second law effi­ciency.

  • The Maxi­mum Power Principle pro­poses that natu­ral systems tend to oper­ate at an effi­ciency that produces a maxi­mum power output. 

Net Energy

  • For each unit of fossil fuel energy extracted from the ground, there is an energy cost involved in doing so. Whether or not the extraction of fossil fuels from the ground results in net energy depends upon the energy cost of extraction.

  • It is of urgent priority to determine whether current or alternative renewable energy sources generate net energy by taking into account all hidden energy subsidies.

Energy Returned on Energy Invested (EROI)

  • The energy return on energy invested (EROEI or ERoEI) or energy return on investment (EROI) is the ratio of the amount of usable energy (exergy) delivered from a particular energy source to the amount of exergy used to obtain that energy resource. 

  • If the EROI of an energy source is less than or equal to 1, then the energy source is an “energy sink” and not a sustainable energy source

  • Although it takes more exergy to deliver hydrogen than the exergy of the delivered hydrogen, it might be useful in some cases to use hydrogen as a carrier of energy and likewise with the storage of energy in a battery.

  • The Energy Store on Energy Invested (ESORI) is used to analyse energy storage systems.

  • Comparisons of the EROI of different renewable energy sources and the ESORI of different storage systems are addressed in the Chapter: Issues of Sustainability.

Criticisms of Energy Analysis by Economists

  • Economists claim that price mechanisms takes energy and other resource factors into account and is a better tool than Energy Analysis for allocation and decision making. However, the pricing of fossil fuels over the past 40 years has not provided adequate signals of the need to transition from fossil fuels to renewable energy, and nor has it provided any indication of an impending peaking of all forms of fossil fuels.

  • Economist claim that Energy Analysis gives no credit for capital and no allowance is made for improvements in technology. Economics alone cannot determine physical and thermodynamic limits to technology and access to resources.  

  • Both Economic Analysis and Energy Analysis are needed to guide a transition from fossil fuels to renewable energy and infrastructure and a sustainable future beyond.