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Photovoltaics: A Dispatchable Peak-Shaving Option
PV technology combined with storage offers a cost-effective alternative to capacity additions.By John Byrne,
Ralph Nigro, and
Steven E. Letendre
Until recently, both regulators and electric utilities have considered photovoltaic (PV) technology (i.e., solar cells) an unattractive
energy-supply option because of its relatively high cost. Now, however, a number of utilities have shown interest in using PV for peak-shaving. Analyses of the Mid-Atlantic region (which has an average insolation rate of only 1,550 Kwh/m2/yr) indicate that PV deployed in a peak-shaving role is cost-effective if modest targets for improved efficiency and cost reductions in PV modules are met, or if nontraditional environmental and distribution benefits are included.
A PV system's peak load-reduction capacity is ordinarily equal to the power it generates at any moment. However, integrating PV technology with storage makes it possible to displace a load greater than an array's output at peak demand periods. This application is currently being tested at Delmarva Power & Light Co. (DP&L), and will be tested at four additional sites in 1995 as part of the PV:BONUS program under a contract with the U.S. Department of Energy.
The dispatchable PV peak-shaving system under investigation at DP&L incorporates energy storage and focuses on commercial building applications to take advantage of the demand-sensitive rate structures used to price electricity services to commercial customers. The presence of demand (kilowatts) as well as energy (kilowatt-hours) charges means that commercial customers can realize the advantages of peak-shaving through bill savings. Often, demand charges constitute a greater portion of a commercial customer's bill than energy charges. Because PV in a peak-shaving role is similar to (and will have to compete with) conventional demand-side management (DSM) technologies, we will use the term PV-DSM to represent the application.
Demand savings are maximized by designing PV-DSM to offer dispatchable load-reduction capacity to utilities. Dispatchability of a PV-DSM system can be achieved either by integrating the solar component with a direct load-control device or by incorporating some form of energy storage. Possible forms of storage include batteries and cool storage (for a system designed to manage air-conditioning loads). The advantage of storage is that it avoids the need to interrupt service.
The cost-effectiveness of a PV-DSM system depends upon a number of variables, including the capacity reduction credited to the system, the level of demand charges, and the amount of solar energy available over the course of a year. A regression analysis based on 72 different cases demonstrated that credited capacity is the most important variable in determining the economic value of PV-DSM. The credited capacity of dispatchable PV-DSM is sensitive to the number of hours per day the system will be expected to be available for dispatch. The larger the number of hours, the smaller the power output the system will be able to maintain (em and thus, its credited capacity.
Shaving Commercial Building Demand
The PV-DSM system is designed to shave commercial building demand during periods of utility system peak load. The system stores the energy produced by the PV array during periods of relatively low demand (early to mid-morning). By