Decentralized Poly-generation of Energy: Basic Concepts - Design for Micro-Combined Cooling, Heating and Power Systems - page 34

Nuclear energy is the energy obtained following the fission reaction of the

atom fission of a

nucleus means its splitting into several fragments. The difference between the mass
of the initial nucleus and the sum of the fragment masses is found in the kinetic
energy that these fragments acquire. In the nuclear reactor, this energy is trans-
formed into thermal energy. In the process of nuclear

' s nucleus, for instance uranium-235 or plutonium-239. The

fission, only 0.1 % of the

atom is total energy is emitted, the rest of 99.9 % remaining stored in the mass of

the newly created fragments. But this quantity is millions of times larger than that
obtained following the oxidation (burning) reaction of a fossil fuel (coal, for
instance), where the chemical energy (that is the energy of the atomic and molecular
links) is transformed into thermal energy.

'

1.2 Energy Units

The consumer is interested in being delivered energy to satisfy his needs, such as
the heating and lighting of the residence. For him, essential are the quantity of
delivered energy, the form in which it is delivered, and the amount due. The basic
forms of delivered energy are heat and power. Afterward, the energy delivered is
converted, transformed, or transmitted to other bodies, the result being called useful
energy. For example, the heat is transmitted to the radiator in the room, for its
heating, and the electricity is converted into light energy, for lighting the house
area. These energies are derived from primary energy supplied fuel. Figure 1
exempli es the three notions regarding energy: primary energy (obtained from

natural gas, in a thermal station), delivered energy (by the electric energy distributor),
and useful energy (necessary for the consumer).
The energy units have been de

ned either in keeping with the useful energy, or
in keeping with the primary energy of the energy source.
(a) When the energy units are de ned in keeping with the useful energy, the

following cases occur:

If the useful energy is electrical, its unit is the kilowatt-hour (kWh)

￿

If the useful energy is mechanical, its unit is the Joule (J).

￿

Delivered
energy

Useful
energy

Primary
energy

Fig. 1 Primary, delivered, and useful energy