Page 168 - Boiler plant and distribution system optimization manual

Over 100 Ways to Improve Efficiency 153

Some Combined Cycle facilities (Figure 10. INSTALL BACKPRESSURE TURBINES
10.48) are over 80% efficient. This plus the high
cost of electricity puts a Combined Cycle facility Cost
at a good economic advantage. High
Peak shaving is another consideration, a
large part of many electric utility bills is for “de- Potential Savings
mand” charges which is a charge based on the High
peak demand for electricity and not actual energy
use. Often the peak demand for electricity occurs Description
at the same time as the demand for other types of Steam systems extract the most heat from
energy such as heat and cooling, so if the onsite steam sources when pressure is reduced to match
generation of electricity can reduce electrical de- needed temperatures of pressures.(Table 10.4) In
mand and provide other forms of energy at the a larger facility with sufficient steam flows, tur-
same time, large savings are possible. bo-generators can be used instead of PRV valves
The key to effective cogeneration is to have to do the same function while generating electric-
both electrical and heat demands occur at the ity at high efficiency. A electrical utility plant has
same time. an efficiency of about 32%; by using turbo-gener-
ators this way an efficiency of 80% is possible.
Advantages/Disadvantages This is a design option requiring a high pres-
This option may take an extensive analysis sure boiler up front. Backpressure turbines are in-
but should not be overlooked as a way to improve stalled from the high pressure header to the me-
the bottom line of your operation. High cost, load dium pressure and low pressure headers. When
matching, utility intertie, red tape and pollution steam pressure is reduced this way more heat
control are a few of the challenges that must be becomes available and the steam is used more
taken into account. It is a very good way to re- efficiently. This is the same reason why pressure
duce steam and electrical power costs and may be reducing valves are used. The reason for the high
well worth the trouble. efficiency is that the turbo-generators do not con-

Figure 10.48—Combined Cycle electrical generation.