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Figure 3 shows an example breakdown of cycling costs for a large super-critical gas-fired unit. The numbers in this figure represent costs per on-off cycle and load-follow cycle.
Although the cost per load-follow cycle is low, since there can be hundreds of them per year for a unit, their annual total cost contribution can add up to be very significant.
Evaluation results
For the periods simulated, overall generation cost reductions (total production cost, on a monthly basis) was substantial and ranged from 5-14 percent (Figure 4). This range was the same whether we included 50 percent of cycling related cost or 100 percent of these costs. The energy benefit from DR was highest during periods when system loads were high and there was low wind availability (summer months).
Note that the simulations were the optimal dispatch of the fossil generation by including cycling related costs. Hence, the model purchases power from wholesale markets to offset the cost of cycling power plants. The amount of DR available in the system was unable to reduce the total cost associated with low load operation, which was exacerbated due to the inclusion of variable generation in the system. The model allowed units to operate at low load levels despite the availability of DR.
Finally, total monthly cycling-related wear and tear costs on the system reduced by about 2-30 percent, due to the availability of DR (Figure 5). This was true for both sets of simulations, including half and all cycling costs in the simulations. Benefits of DR to reduce cycling-related costs were greater under volatile MW demand and when there were outage events resulting in unavailability of large fossil units.
The authors emphasize that this analysis was limited to one system and DR programs are still being researched and developed. We expect that sub-hourly production cost simulations, including DR, would provide better insights into the benefit of DR to reduce cycling costs of non-renewable conventional generation. Also, further research is required to include variations of different market rules and transmission topologies. Fast DR implementations also require better forecast of loads, which is complex given the variability of renewable resources. However, with expected increases in fuel prices, environmental regulations, increased renewable integration (and variability) and load growth, the economic benefits of DR in terms of avoided cycling costs at power plants are of substantial value.
Conclusion
Many utilities have been forced to cycle aging fossil units that were originally designed for base-load operation. This has been primarily due to the recent addition of new, more efficient base load units and “must take” base loaded Independent Power Producer (IPP) power. Moreover, the large scale introduction of renewable resources has only increased cycling of these power plants.
DR can provide significant benefits if utilized to reduce cycling and startups for generation fleets with a mix of coal and gas/oil fueled plants that are primarily steam-boiler type plants, and with a significant portion of older plants. Also, DR increases system reliability in a variable generation scenario by reducing power plant cycling, both on/off and load following.
The significant benefit of DR to reduce cycling, in addition to other energy benefits should be included in any estimate of total benefits of DR when evaluating the merits of an overall value proposition for a specific DR project.
Acknowledgements
The authors would like to thank Sila Kiliccote and Girish Ghatikar at LBNL for their support, as well as Phil Besuner, Gene Danneman, Dwight Agan and Steve R. Paterson for their valuable guidance during the drafting of this paper.
Editor’s note: This paper, PWR2011-55200, was printed with permission from ASME and was edited from its original format. To purchase this paper in its original format or find more information, visit the ASME Digital Store at www.asme.org.
