Dish/engine systems utilize concentrating solar collectors that track the sun in two axes. A reflective surface, metalized glass or plastic, reflects incident solar radiation to a small region called the focus. The size of the solar concentrato r for dish/engine systems is determined by the engine. At a nominal maximum direct normal solar insolation of 100 0 W/m2, a 25-kWe dish/Stirling system's concentrator has a diameter of approximately 10 meters.

Concentrators use a reflective surface of aluminum or silver, deposited on glass or plastic. The most durable reflective surfaces have been silver/glass mirrors, similar to decorative mirrors used in the home. Attempts to develop low-cos t reflective polymer films have had limited success. Because dish concentrators have short focal lengths, relatively thinglass mirrors (thickness of approximately 1 mm) are required to accommodate the required curvatures. In addition , glass with a low-iron content is desirable to improve reflectance. Depending on the thickness and iron content, silvered solar mirrors have solar reflectance values in the range of 90 to 94%.

The ideal concentrator shape is a paraboloid of revolution. Some solar concentrators approximate this shape wit h multiple, spherically-shaped mirrors supported with a truss structure (Figure 1). An innovation in solar concentrato r design is the use of stretched-membranes in which a thin reflective membrane is stretched across a rim or hoop. A second membrane is used to close off the space behind. A partial vacuum is drawn in this space, bringing the reflective membrane into an approximately spherical shape. Figure 2 is a schematic of a dish/Stirling system that utilizes thi s concept. The concentrator' s optical design and accuracy determine the concentration ratio. Concentration ratio, defined as the average solar flux through the receiver aperture divided by the ambient direct normal solar insolation , is typically over 2000. Intercept fractions, defined as the fraction of the reflected solar flux that passes through th e receiver aperture, are usually over 95%.

Tracking in two axes is accomplished in one of two ways, (1) azimuth-elevation tracking and (2) polar tracking. I n azimuth-elevation tracking, the dish rotates in a plane parallel to the earth (azimuth) and in another plane perpendicular to it (elevation). This gives the collector left/right and up/down rotations. Rotational rates vary throughout the day but

Stirling Engine and Alternator

Stirling Engine and Alternator

Figure 2. Schematic of a dish/engine system with stretched-membrane mirrors.

can be easily calcu lated. Most of the larger dish/engine systems use this method of tracking. In the polar trackin g method, the collector rotates about an axis parallel to the earth's axis of rotation. The collector rotates at a constan rate of 15°/hr to match the rotational speed of the earth. The other axis of rotation, the declination axis, is perpendicular to the polar axis. Movement about this axis occurs slowly and varies by +/- 23% ° over a year. Most of the smaller dish/engine systems have used this method of tracking.

Solar Stirling Engine Basics Explained

Solar Stirling Engine Basics Explained

The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.

Get My Free Ebook

Post a comment