Civil Engineering Reference
In-Depth Information
Afterwards, because of local regulations and tariffications, these plants have been
unevenly located and used, the choice depending on the cost of fuel and electric
energy tariffs.
9.2
Forms of Cogeneration and Trigeneration
Cogeneration plants can be grouped basically into two types referred to as topping
cycles and bottoming cycles.
The topping cycles produce power, mechanical or electric, before delivering
thermal energy to the end users. Typical examples are the backpressure or
non-condensing steam turbine, the gas turbine and combined cycles, and the
reciprocating engine where exhausts are utilized as heat for end-user needs.
The heat can also be used as input into an absorption refrigerating system
for cooling.
The bottoming cycle recovers thermal energy, which would normally be
discarded, to produce process steam and electricity. In this cycle, first thermal
energy is used for the process, and then the exhaust energy is used to produce
mechanical or electric power at the bottom of the cycle. This cycle is most
attractive where there is a large quantity of thermal energy at a temperature of
623 K (350
C; 662
F) or greater associated with exothermic reactions, as in
many chemical processes and in rotary kilns and furnaces. Recovery of the
steam to be used in a steam turbine is the commonest bottoming cycle; for
lower temperatures other cycles, like the Rankine cycle with organic fluids
(ORC—Organic Rankine Cycle), are sometimes used. In this case, the electric
efficiency is around 20 %, and the electric output power ranges from 200 kW
to 20 MW and more.
ORC plants are also used as topping cycles, usually fed by solid biomass.
The electric generator may be synchronous or asynchronous; the choice between
them depends on the working mode: if the system is independent of the utility grid,
the synchronous generator must be used; if the system is interconnected with the
utility grid, either type can conveniently be used. The generator voltage can be at
low or medium level depending on the size and on the layout of the internal
distribution network.
Table
9.1
shows the main typical parameters of cogeneration technologies based
on both fossil fuel and biomass.
Figures
9.1
and
9.2
show simplified models of cogeneration and trigeneration
plants fed by natural gas, which can also be used in the case of other fuels or
biomasses. Electrical and heating efficiency parameters from Table
9.1
are the input
together with the rated output electrical power; in trigeneration plants conversion
from heating efficiency to cooling one is made by using the absorption system COP
(practically less than 0.75).
