Decentralized Poly-generation of Energy: Basic Concepts - Design for Micro-Combined Cooling, Heating and Power Systems - page 36

decreased, but only transformed (converted) from one form of energy into another.
The most common energy conversion processes are presented in Fig. 2 .
The few possibilities of direct conversion of energy from one form to another are
shown on Table 2 and Figure 2
According to energy conservation law, a system

s total quantity of energy
remains constant and is called the primary energy of that system. The elements that
the primary energy of a system is stored in are called sources of energy.
Quantitatively, the primary energy of a system represents the sum of the
quantities of energy contained in all its sources of energy.

'

Today, the main sources of energy are:

￿

fossil fuels (coal, petroleum, and natural gases);

￿

bio fuels ( (firewood, wood residue, agricultural residue etc.);

￿

renewable as hydraulic, geothermal, solar, wind energy;

￿

nuclear energy.

￿

In the case of fuels (whether fossil or bio), primary energy is obtained through
burning and is evaluated by multiplying the quantity of burned fuel by its calori c

power. Numerically, the calori c power of a fuel is the energy resulting from

burning a unitary quantity of fuel (for example, a kg—if

—

if the fuel is solid or liquid,

or an m 3

—

if it is gaseous).
The great majority of fuels contain water which, in the burning process, is
released under the form of vapors. That is why, in the burning process, part of the
energy resulting from the chemical reactions is used to evaporate the water. Con-
sequently, the primary energy which is converted into heat during the burning of the
fuel cannot be measured directly; it can only be evaluated, and the evaluation is
done relative to the reference state of the water in the fuel.

Fusion

Steam Turbine

Electrolysis Lighting

Photosynthesis

Electric
Motor

Joule
Effect

Electricity Heat

Energy

Light
Energy

Nuclear
Energy

Chemical
Energy

Mechanical
Energy

Thermo-
couple

Generator

Cells, batteries

Combustion

Compression

Chemical Reactions

Fig. 2 Conversion processes