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1_1 Purchased Power

Fig. 23-6 Economically Dispatched Cogeneration System Operation.

Fig. 23-6 Economically Dispatched Cogeneration System Operation.

sell-back or wheeling involve additional engineering, economic, regulatory, and legal issues. Power exportation, however, is integral to many large generation projects. It is fundamental to IPPs that produce power solely for the grid. It is also an important option for cogenerators with sufficient thermal loads to support power generation in excess of their own requirements. The following items must be considered:

• Many utilities have set low values for buy-back power. Even with retail wheeling options, transmission and other costs reduce the value of exported power versus the retail value of internally used power. If power must be wheeled over several utility systems to reach a buyer, the combined wheeling charges could be cost-prohibitive.

• Engineering, operational, system protection, and monitoring requirements are greater when power is exported. The installed cost per kW of a generation system designed for exporting power is generally greater than that of a system of similar capacity designed for in-house use only.

• Interconnection and power sales contract negotiations, legal matters, and other associated activities represent non-value-added costs that may delay projects and make smaller projects uneconomical.

Despite these obstacles, power exportation continues to grow, serving the interest of individual business, the public, and many utilities. Access to wheeling markets can allow self-generators to exact a higher price for exported power than they would if the local utility were the only

Figure 23-6 depicts the operation of an electric cogener-ation system with a full-load capacity that can meet 75% of the facility's peak electric demand. The system operates at full capacity for 10 hours per day on weekdays and tracks the facility's demand for 6 hours per day. The system is shut down overnight due to the combination of low electricity cost in the off-peak period and low thermal requirements. This strategy of dynamic operation depending upon hourly economic considerations is often referred to as economic dispatch.

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