Civil Engineering Reference
In-Depth Information
Rob Adams' estimation technique was chosen not only for its logic and
simplicity, but because it splits the difference between two other maintenance
estimates that we received from other sources. Figure 2 compares the three
estimates on a monthly cost-per-station-size scale. It is not surprising that the
three estimates are so different because the contractors rely heavily on station-
specific circumstances that were not available for these general estimates.
Given the choice between three qualified industry experts, we selected the
middle estimate.
Next, the "8% to 5%" was distilled into an equation so it could be inserted
into the model. As shown in Figure 2, a polynomial equation fit the line very
well for the range from 0 to 300,000 DGE throughput. After that, it was set to
rise 0.06% per DGE. The polynomial equation used in the model is:
Electricity is the primary operation cost considered in the base-case
scenario. "Commercial" electricity clients in most states pay both an energy
charge and a capacity charge (often called demand charge) for electricity. The
capacity charge reflects how much electricity the utility needs to be prepared
to produce for you and therefore depends on how quickly you draw electricity
from the grid, which is especially important for CNG stations because they can
have a very large ampere draw. The model assumes the energy charge to be
$0.10/kWh, which is between the mid-peak and on-peak prices in California in
January 2009. The assumed capacity charge is $12/kW/month for the same
reasons. The combined electricity charges result in different monthly
electricity prices for the three fleets based on throughput, as seen in Figure 3.
The two trendlines were converted to equations and inserted into the model. It
should be noted that the transit station's electric costs start higher than the
refuse and school's cost because its smaller refueling window requires larger
compressors, which leads to higher electric capacity requirements. This
demand charge represents a fixed cost portion of the electric bill. This
difference is minimized as throughput increases because the variable cost
portion of the electrical bill (the energy charge) becomes more pronounced,
which decreases the previous advantage that these stations had over the transit
station.
Labor for hostlers (people who refuel, clean, and maintain fleets) is not
considered an additional cost in the base case because diesel vehicles need
them also. Furthermore, hostlers are not an additional cost because it is
generally more economical to use a hostler than to have drivers or other staff
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