Introduction

In physics, energy is the property that must be transferred to an object in order to perform work on, or to heat, the object. It can be converted in form, but not created or destroyed.

To analyse an energy system, we propose the following rational approach :

• start with the analysis of the needs, not with the source
• elaborate a cost function from the need to the source
• minimize it

Energy needs

World-wide average annual energy consumption per capita is 17500 kilowatt hours. This is equivalent to a continuous consumption of 2000 watts. The figure for western Europe is close to 6000 watts per capita. By contrast, in some Asian and African countries, consumption is a mere fraction of the global average. The vision of a 2000-watt society allows us to make comparisons between industrialised and developing nations, and thus paves the way for everyone to enjoy a high standard of living.

55% of Energy is for industries, 25 % for transport and 20% for buildings. The energy demand for buildings accounts for 25% of total energy consumption in the world and 40% in Europe. Space heating represents the main part of this energy: 53% in the world and about 80% in Europe.

Energy forms

Common forms of energy include the kinetic energy of a moving object, the potential energy stored by an object's position in a force field (gravitational, electric or magnetic), the elastic energy stored by stretching solid objects, the chemical energy released when a fuel burns, the radiant energy carried by light, and the thermal energy due to an object's temperature.

Energy in many of its forms may be used in natural processes, or to provide some service to society such as heating, refrigeration, lightening or performing mechanical work to operate machines. For example, in order to heat your home, your furnace can burn fuel, whose chemical potential energy is thus converted into thermal energy, which is then transferred to your home's air in order to raise its temperature. In another example, an internal combustion engine burns gasoline to cause pressure that pushes the pistons, thus performing work in order to accelerate your vehicle, ultimately converting the fuel's chemical energy to your vehicle's additional kinetic energy corresponding to its increase in speed.

Energy quality

The concept of energy quality is intuitive if one considers examples where the form of energy remains constant. For instance if we consider only the inertial form of energy, then the energy quality of a moving body is higher when it moves with a greater velocity. If we consider only the heat form of energy, then a higher temperature has higher quality. And if we consider only the light form of energy then light with higher frequency has greater quality. All these differences in energy quality are therefore easily measured with the appropriate scientific instrument.

The situation becomes more complex when the form of energy does not remain constant. In this context Ohta formulated the question of energy quality in terms of the conversion of energy of one form into another, that is the transformation of energy. Here, energy quality is defined by the relative ease with which the energy transforms, from form to form.

The different forms of energy does not have the same quality. The quality is related to the :

• Ability to be transformed into an other form of energy : the limitation can either come from some physical or technological constraints. The transformation is associated to an increase of entropy.

  Electricity have a special place in this ranking. Electricity is a high grade energy : easy to transform in lot of different needs, easy to transport, lot of experience in all machines. It is even possible to transport information with electricity. On the contrary, heat, especially when its temperature is low, is a low grade energy : difficult to transform into something else.

• Ability to be concentrated (either in term of volume or mass depending on the application) : the concentration of energy has a great impact on the cost needed to tranform it.

  Energy density is the amount of energy stored in a given system or region of space per unit volume. Colloquially it may also be used for energy per unit mass, though the accurate term for this is specific energy. Often only the useful or extractable energy is measured, which is to say that inaccessible energy (such as rest mass energy) is ignored.

  Energy per unit volume has the same physical units as pressure, and in many circumstances is a synonym: for example, the energy density of a magnetic field may be expressed as (and behaves as) a physical pressure, and the energy required to compress a compressed gas a little more may be determined by multiplying the difference between the gas pressure and the external pressure by the change in volume. In short, pressure is a measure of the enthalpy per unit volume of a system. A pressure gradient has the potential to perform work on the surroundings by converting enthalpy to work until equilibrium is reached.

Energy source

Energy transformations in the universe over time are (generally) characterized by various kinds of energy which has been available since the Big Bang, later being "released" (that is, transformed to more active types of energy such as kinetic or radiant energy), when a triggering mechanism is available to do it.

• Release of energy from gravitational potential
• Release of energy from radioactive potential
• Release of energy from hydrogen fusion potential

Energy flow

Since energy cannot be created or destroyed, all is about energy transformation, ie changing energy from one of its forms into another. In addition to being convertible, energy is transferable to a different location or object.

To fullfill our energy needs, difference ressources are used and transformed through an existing energy system to the form of needed energy.