also be evaluated. Figure 19 illustrates these results in which combined capital cost is shown versus size. In this case, the dish diameter is the indicator of size. The minimum cost size is between 30 and 40 feet. This represents an average daytime power level of from 13 kWe to 23 kWe as shown in Figure 20. The optimum has been selected as 36 feet with an average rating of 19 kWe. This size dish has an aperture of approximately 1000 ft2 . The more expensive ("400 $/kWe") Brayton engine with higher efficiency has a slight cost advantage over the less complicated version. As with the other tracking solar collectors, the annual direct solar insola- tion which has been used is an average of 2740 BTU/ft2 day or 1 x 10^6 BTU/ft2 year (293 kWh/yr) (Ref. 10). This is felt to be representative of good locations in the Southwest such as Yuma, Arizona; China Lake, California;and Albuquerque, New Mexico. The value is based upon several consecutive years of weather from these locations. Based on an average sun load factor of 0.45 (Ref. 1), the average insolation over the 3940 hours per year that the plant is generating electricity is 0.0744 kW/ft2 (254 BTU/h ft2 ). The rated power is based on this average insolation when the plant is operating during the day. The Brayton engine has been sized at 120% of the rated capacity to be able to use most if not all of the peak energy. Some of the annual energy is lost by sizing the engine is this fashion. A relatively clear annual weather situation similar to most favorable Southwest locations is assumed to minimize the peak energy lost. Also, when this plant is operated in the sun following mode, the engine is operated over a range of operating conditions. This type of heat engine has a relatively flat heat rate versus output power characteristic. At 30% of maximum power output, the efficiency falls off only 5% of optimum. The combination of loss of some peak power and the loss of efficiency at off-load is accounted for by a correction factor of 0.955.
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