Energy is the capacity to perform work. Power is the work carried out
during a unit of time. Equipment for the conversion of a given energy
source into final energy is employed to meet the needs of a user (e.g.,
boiler for the transformation of the chemical energy of Diesel fuel into
thermal energy).
The energy that enters the equipment is defined as consumption (C), while
the energy that leaves it directed towards the user is called the energy
requirement (R). C is always higher than R, and it can be observed that:
R=μC
where μ is the process efficiency (first law).
In a given operation (e.g., milk collection), a distinction is made between direct and indirect consumption. The former refers to the energy consumed during the operation (e.g., Diesel fuel to power the truck), while the latter concerns the energy consumed to make the necessary technical equipment available (e.g., for production of the truck, the milk containers, etc.).
The basic energy source is the sun. On the sun, matter is transformed into
energy (E), by using up its mass m, according to the equation E=mc2, where
c is the speed of light in a vacuum (≃3*108 m/s).
With the exception of the gravitational field, the renewable and non-renewable
energy sources present on the earth in various forms originate
either directly or indirectly from the sun's energy.
Energy may be superior or inferior (i.e., its qualitative level varies).
Forms of energy that are able to create mechanical energy (with which it is
possible to meet any type of requirement) are considered to be “superior”;
the term “inferior”, on the other hand, is used to denote all the lower
forms of energy.
For example, a water supply placed at a certain height with respect to a
water turbine is a superior form of energy (gravitational), since it can be
conserved over time, does not create environmental impact problems, and can
be converted into mechanical, electric or thermal energy at any time.
However, a tank of water heated to 60°C is an inferior form of energy; one
of its few uses is home heating.
In general, it is incorrect to add together forms of energy of different qualities (since each of these forms provides different results).
Energy tends to deteriorate from superior to inferior forms. For example, in an Otto engine, gasoline (superior chemical energy) explodes thereby creating mechanical and thermal energy at various temperatures (exhaust gas: 500–700°C; cooling air: 80–100°C). Once the former energy is supplied it turns into thermal energy, while the latter is diluted into the environment at lower and lower temperatures.
Edible energy (which is chemical in nature) should also be noted here. Its qualitative level is significantly different from that of other forms of energy. Consequently, from a conceptual standpoint, comparisons between these forms of energy should be avoided.
The basic forms of energy conversion are photosynthesis and combustion.
The former transforms solar energy (and substances found in nature; e.g.,
CO2 and H2O) into chemical energy stored in C- and H-based compounds. The
latter produces thermal and mechanical energy from chemical energy.
In addition, the following “transformers” convert energy: mechanical
converters (e.g., wind machines), electric generators, and biochemical and
nuclear reactions.
The efficiency μ1 (first law) of a conversion process is defined by the
equation:
μ1 = Ep/Ec
where Ep and Ec are, respectively, the energy produced and the energy consumed by the process itself (through a given technology). μ1 is always less than 1 because of the inevitable energy losses involved in transformation processes and related equipment. This type of efficiency is commonly used in practice, and it is repeatedly referred to in this report.
The efficiency μ2 (second law) is defined by the equation:
μ2=f/(E1/E2)
where E1 and E2 are energy expressed in terms of capacity to carry out
mechanical work at the exit and entrance to the conversion process,
respectively (according to the laws of thermodynamics).
When μ2 assumes low values (<20–40%), this means that a high quality source
is being used to produce low quality energy (e.g., Diesel fuel used to
produce water heated to 50°C).
When the two levels are similar, μ2>50–60% (e.g., 80°C geothermal water to
produce 50°C air).
In summary, μ2 demonstrates whether the source and user are compatible from
an energy standpoint.
The main features are:
cost.
These features are generally sufficient (otherwise, this is specified) for a definition of:
environmental impact.
Energy is measured in J (Joules) and power in W (Watts). Electric energy is generally measured in kWh. However, numerous measurement systems may be used in practice.
