Civil Engineering Reference
In-Depth Information
Cooling equipment must also be provided if users of hot water are not in
operation. Exhausts will be discharged into the atmosphere if they are in excess.
Typical input-power values, in the case of natural gas input, are
250 Sm
3
/h (2,400 kW) to produce 1,000 kW of electric power and
1,100 kW of recoverable heat. The overall efficiency of the system, that
is, the ratio between output and input power, is roughly 85 %. The ratio
of output electric power to input power is roughly 40 %.
Typical values of operating parameters for several reciprocating engines fed by
natural gas are shown in Table
9.5
. As a general rule, this type of prime mover is
attractive if the process requires large quantities of low-level heat recovered from
the jacket water and lubricant oil cooling systems which may reach half of heat
rejection. An example of a reciprocating engine is reported in Sect.
9.7
.
9.6
Determining the Feasibility of Cogeneration
Cogeneration feasibility is based on economic and technical factors, which have to
be correlated to one another to complete a valid evaluation. Major factors for
consideration are as follows:
• The ratio between electricity and fuel site demand, defined as daily, monthly,
and yearly ratio. This ratio must be consistent with the ratio between electric
output power and heat recovery for the chosen cogeneration system;
• The profile of heat demand, including temperature levels of end user
requirements and typical fluctuations of the demand (hourly, daily, monthly,
yearly). Temperature levels must be consistent with the level of heat rejected
from the cogeneration system;
• The profile of electric demand and typical fluctuations as for the thermal profile.
Thermal and electric profiles must be correlated with each other;
• Purchased fuel and electricity costs, present and projected future costs;
• Working hours per year and per total life of the plant;
• Plant system sized for present site needs and for the future;
• Capital cost of the cogeneration plant and operating cost during the life of the
plant;
• Environmental issues.
Many cogeneration approaches can be followed in order to make a choice among
system types and sizes. However, in order to ensure the highest efficiency of the
system, the recovery of the rejected heat must be effective in any operating
condition of the cogeneration plant. Additional boiler plants will satisfy the end
user requirements, if these are higher than the recovery heat. Depending on the
industrial processes, this constraint can be more or less important in determining the
size of the plant.
