Energy balance

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Energy balance has the following meanings in several fields:

• In physics, energy balance is a systematic presentation of energy flows and transformations in a system. Theoretical basis for an energy balance is the first law of thermodynamics according to which energy cannot be created or destroyed, only modified in form. Energy sources or wave of energy are therefore inputs and outputs of the system under observation.

• In biology, total body energy balance is measured with the following equation: Energy intake = internal heat produced + external work + energy storage. The Dynamic Energy Budget theory makes explicit use of energy, mass and time balances. One Calorie (or kilogram calorie) equals the energy needed to increase the temperature of 1 kg of water by 1 °C. This is about 4.184 kJ.

• In energy economics, the energy balance of a country is an aggregate presentation of all human activities related to energy, except for natural and biological processes. National energy balances are compiled on at least an annual basis. Common methodology for compilation and presentation of energy balances allows simple addition of national energy balances to form supranational ones, such as is compiled for the European Union. United Nations compile energy balances for all member countries. International Energy Agency, a specialised agency of OECD is regularly preparing world energy balances.

• In engineering, energy balances are used to quantify the energy used or produced by a system. This can be used to build complex differential equation models to design and analyze real systems. To make an energy balance for a system is very similar to making a Mass balance but there are a few differences to remember, e.g. 1) that a specific system might be closed in a mass balance sense, but open as far as the energy balance is concerned and 2) that while it is possible to have more than one mass balance for a system there can be only one energy balance. If a balance is made for total energy, the energy balance becomes IN=OUT+ACC (where ACC stands for accumulation). Notice that there is no production (PROD) term since energy can not be produced, only converted. If instead some kinds of energy are ignored, e.g. if a heat balance is made the energy balance becomes IN+PROD=OUT+ACC (if heat is consumed the PROD term is negative, compare Mass balance.

• When comparing fuel production, energy balance is the difference between the energy produced by 1 kg of the fuel (i.e. biodiesel, petroleum, uranium ) and the energy necessary to produce it (extraction (e.g. drilling or cultivation of energetic plants), transportation, refining etc). Other factors affect fuel selection, such as portability. See also net energy gain and EROEI.

In geography, specifically climatology and hydrology, the "'energy balance'" refers to the total of all energy inputs and outputs at any location; these include solar, atmospheric transfer, and ground conducted energy.