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Hastening Genco Obsolescence?

DER: This final installment of Oak Ridge National Laboratory's series on distributed energy resources investigates efficiency, the environment, and generation displacement.

Fortnightly Magazine - May 2005
Figure 5 - Net Emissions From DER Operating During Weekdays Only for Different Scenarios

based on a low-NO X Solar Mars 90 combustion turbine and are shown in Table 2, along with pertinent parameters of the new CC plants and existing non-electrical boilers. 2,3 Both the DER and the CC facility are modeled as low-NO X emitters, while the thermal boiler modeled has emissions based on the average value for gas-fired steam turbine boilers in the region. The Solar Mars 90 is a 9.5-MW turbine with dry low-NO X combustion and selective catalytic reduction. With an electrical efficiency of 29 percent and a heat exchanger efficiency of 62 percent, the total efficiency of the DER is 73 percent.

Typically, NO X emissions are reported in terms of lbs./MMBtu of thermal energy in. In Table 2, we also calculate the emissions in terms of lbs./mmBtu of useful energy out. For the new CC, the value is the amount of energy in divided by electrical efficiency; for the boiler, the value is the amount of energy in divided by thermal efficiency. However, DER technology in CHP mode creates both electrical and thermal output, and its relative emissions are the input amount divided by its combined efficiency of 73% [29% + 62% x (1-29%)]. Similarly, since all three technologies use natural gas, they have the same CO 2 emissions based on input energy. However, based on useful energy out, the DER with CHP is the least polluting. The CO 2 emissions of the Mars 90 also can be stated on an electrical output basis for comparison with the Regulatory Assistance Project (RAP) rule. 4 With a 29 percent electrical generation efficiency, 117 lbs/MMBtu into the system corresponds to about 1,400 lbs/MWh out of the system. The RAP model emissions rule for DER is 1,650 lbs/MWh in 2012.

When the added emissions from the DER (with CHP) are summed with the reductions from the CC and thermal system, the result is a net lowering of emissions in all cases. Figure 5 shows the net NO X emissions for the peaking DER scenarios. In the scenario in which 2,000 MW of new CC is canceled, the central station emissions actually increase; therefore, if the DER provided only electricity generation, then emissions would be higher. However, the large savings from thermal system displacements results in a large overall emissions reduction. In all of the other scenarios, and in all of the scenarios for baseload DER, the reduction in electric system NOX emissions alone is more than the increase from DER. Compound that with the savings from thermal systems, and DER is clearly a cleaner option.

Table 3 shows the net primary energy (fuel) and emissions changes from all of the scenarios studied. A positive value means that the overall amount of fuel used or emissions released is greater when the DER is included in the system than when it is not. A negative value means the system that includes DER uses less fuel or releases fewer emissions. As an example from the first line, if DER was used to generate 1,000 MWh, then the total net fuel use (if the DER provided