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Redefining PV Capacity

Effective metrics give solar its due credit.

Fortnightly Magazine - February 2009

Photovoltaic (PV) power generation is an intermittent, non-dispatchable resource generally considered as energy-only with no capacity credit. However, there is ample evidence that solar energy reliably is available at peak demand time when loads are driven by day-time commercial air conditioning, and can contribute effectively to increasing the capacity available on a regional grid.

Several studies have shown that the availability of solar power plants often is high during times of peak electrical demand when peaks occur in summer and are driven by day-time commercial air conditioning. These peaks are intensified during heat waves, which are fueled by solar gain. Thus, the resource creating the demand also can be used to meet that demand and provide local grid decongestion. This relieves load pockets and transmission bottlenecks, providing the equivalent of firm peaking capacity, i.e., effective capacity.

Effective capacity can be verified tangibly in the case of dispersed PV. For instance, the demand-response program run by the New York Independent System Operator (NYISO) is designed to provide the equivalent of peaking capacity via load curtailment and user-sited power generation. When dispersed PV generation is available, the program can be reduced substantially while maintaining the same degree of peak reduction effectiveness. 1 When the power grid is under stress due to high summer demand, high power transfers, and reaches the point of rolling blackouts, the PV resource generally is close to ideal. A case in point is the Aug. 14, 2003, Northeast blackout, which could have been prevented with less than 500 MW of PV dispersed over the entire northeastern United States, keeping all unattended failures from feeding into one another to the point of regional outage cascade. 2

Figure 1

This non-traditional effective capacity credit can be quantified by: 1) identifying and comparing different metrics that have been proposed and sometimes used by the utility and renewable energy industries; 2) comparing these metrics through experimental case studies; and 3) reviewing the results of a consensus building effort involving the utility, solar and research industries.

Effective Capacity Metrics

Simple metrics can be estimated directly from the knowledge of load demand and power generation history (see Figure 2) . These fall into four broad categories.

Metrics can be based on the concept of loss-of-load probability. Utilities used effective load-carrying capability to quantify the capacity of their power generation units before the strengthening of continental/regional interconnectivity. The methodology still was applied at Pacific Gas and Electric Co. 3 in the 1980s. As defined by Garver, 4 the effective load-carrying capability of a power plant represents its ability to increase the total generation capacity available on a local grid (

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