Page 341 - Sustainable On-Site CHP Systems Design, Construction, and Operations

314 Op erations

Financial pro forma models can also be created to calculate/estimate total and mar-
ginal cost, value, and amount of money saved for each piece of equipment/system
operated. Cost and values will, of course, be affected by utility time-of-use rates, sea-
sonal differences in utility costs, as well as by the cost of fuel (which can change depend-
ing upon season/purchase agreements/terms), all of which needs to be factored into
account in order to better understand equipment/system operating choices and their
resultant cost/revenue implications/consequences.
With respect to the big picture, general operating strategies are as follows:
• Maximize net revenue
• Minimize heat rate, maximize CHP plant efficiencies, minimize parasitic plant
power consumption, and minimize all losses
• Minimize carbon footprint
Maximizing net revenue (value) is probably the most common strategy employed
by CHP owners and operators, makes good financial sense, and often incorporates
other features in the list above. Under the maximizing value strategy, efforts are made
to maximize revenues (generate and sell all CHP services possible given fixed-plant
resources even if the generation/production is not the most efficient possible and
degrades overall CHP plant efficiency metrics) and to minimize costs (efficiency
improvements can be an important factor) thereby to maximize net revenue. Equip-
ment options/system operations are generally selected based on maximizing net reve-
nue. For example, at a given time, a choice might need to be made whether to operate a
steam absorption chiller or an electric-drive chiller, and a matrix, as described earlier,
can provide answers to what is the lowest-cost equipment to operate. In this case, for
example, it may be cost-effective to operate the absorption chiller during the on-peak
period but not during the off-peak period. Under this hypothetical case, a consideration
may be whether to use heat recovery produced steam in a steam turbine generator to
produce additional power, or to use the steam instead in an absorption chiller to pro-
duce chilled water for space cooling. Another option may be to produce additional
steam in the duct burner. Also, with a CHP plant, there may be times when a use for the
recovered waste heat overrides/changes cost calculations.
For example, if the heat is going to be dumped it may need to be used to meet regu-
lations and essentially becomes free. Each option should be analyzed/modeled in order
to provide CHP plant personnel and control system the information needed to maxi-
mize net revenue. In the absence of unit production cost analysis figures, it may be best
to use recovered heat in the order of highest value to lowest value which is often addi-
tional power, cooling, and heating, respectively.
Other operating strategies which are usually incorporated into the above maximiz-
ing net revenue strategy is to minimize parasitic and distribution losses, to maximize
CHP plant efficiencies, and to minimize prime mover heat rate, which, of course, are all
inextricably linked together. As with any energy project, reducing waste is step 1, mini-
mizing facility loads is step 2 (daylighting, more efficient lighting, building insulation,
more efficient windows, etc.), and minimizing CHP plant losses, where possible, should
be studied, reviewed, and implemented on an ongoing basis as step 3. A common loss
occurs in the condensate system where condensate is not returned from buildings to the
plant and/or where heat losses occur in condensate piping, losing energy. While another
common example is poorly maintained steam traps that often leak by wasting enthalpy