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Research Papers: Gas Turbines: Electric Power

# An Expanded Cost of Electricity Model for Highly Flexible Power Plants

[+] Author and Article Information
S. Can Gülen

Principal Engineer

Indrajit Mazumder

GE Energy,

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the Journal of Engineering for Gas Turbines and Power. Manuscript received June 25, 2012; final manuscript received July 13, 2012; published online November 21, 2012. Editor: Dilip R. Ballal.

J. Eng. Gas Turbines Power 135(1), 011801 (Nov 21, 2012) (11 pages) Paper No: GTP-12-1217; doi: 10.1115/1.4007379 History: Received June 25, 2012; Revised July 13, 2012

## Abstract

Cost of electricity (COE) is the most widely used metric to quantify the cost-performance trade-off involved in comparative analysis of competing electric power generation technologies. Unfortunately, the currently accepted formulation of COE is only applicable to comparisons of power plant options with the same annual electric generation (kilowatt-hours) and the same technology as defined by reliability, availability, and operability. Such a formulation does not introduce a big error into the COE analysis when the objective is simply to compare two or more base-loaded power plants of the same technology (e.g., natural gas fired gas turbine simple or combined cycle, coal fired conventional boiler steam turbine, etc.) and the same (or nearly the same) capacity. However, comparing even the same technology class power plants, especially highly flexible advanced gas turbine combined cycle units with cyclic duties, comprising a high number of daily starts and stops in addition to emissions-compliant low-load operation to accommodate the intermittent and uncertain load regimes of renewable power generation (mainly wind and solar) requires a significant overhaul of the basic COE formula. This paper develops an expanded COE formulation by incorporating crucial power plant operability and maintainability characteristics such as reliability, unrecoverable degradation, and maintenance factors as well as emissions into the mix. The core impact of duty cycle on the plant performance is handled via effective output and efficiency utilizing basic performance correction curves. The impact of plant start and load ramps on the effective performance parameters is included. Differences in reliability and total annual energy generation are handled via energy and capacity replacement terms. The resulting expanded formula, while rigorous in development and content, is still simple enough for most feasibility study type of applications. Sample calculations clearly reveal that inclusion (or omission) of one or more of these factors in the COE evaluation, however, can dramatically swing the answer from one extreme to the other in some cases.

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## Figures

Fig. 1

Example cyclic duty profile (one day—Monday after the weekly shutdown) for a gas turbine combined cycle power plant. (Adopted from Ref. [7].) This particular profile is sometimes described as the plant “two-cycled” daily. Possible variations are myriad (e.g., the plant being brought down to a minimum load overnight instead of being shut down).

Fig. 2

A sample annual load profile table. Entries in each cell are the operating hours at the corresponding ambient temperature with the corresponding power output. If a GTCC product is unable to satisfy a particular load requirement (e.g., for example a high load point on a very hot day), it is left out when calculating its total MW h production (marked by o). The balance is made up while evaluating the COE via capacity/energy replacement terms.

Fig. 3

Annual ambient temperature variation for a typical site in southern Europe

Fig. 4

Typical advanced heavy duty gas turbine CC power plant ambient and part load correction factors

Fig. 5

Conventional combined cycle start (per Fig. 2 of Ref. [10]). The parameter α quantifies the true MW h generated (the area below the start curve) as a fraction of the simple estimate (the triangular area below the straight line between t = 0 and t = ΔH).

Fig. 6

Typically expected GTCC nonrecoverable performance loss due to parts aging, deformation, and similar degradation, which cannot be rectified via scheduled maintenance and repairs

Fig. 7

Impact of better part load heat rate, fast start, and load ramps on plant efficiency (normalized basis)

Fig. 8

Breakdown of COE difference between base and advanced GTCC plants (per Eq. (11), $135.5/MW h and$112.1/MW h, respectively, in Table 6)

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