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1.0 System Description
Dish/engine systems convert the thermal energy in solar radiation to mechanical energy and then to electrical energy
in much the same way that conventional power plants convert thermal energy from combustion of a fossil fuel to
electricity. As indicated in Figure 1, dish/engine systems use a mirror array to reflect and concentrate incoming direct
normal insolation to a receiver, in order to achieve the temperatures required to efficiently convert heat to work. This
requires that the dish track the sun in two axes. The concentrated solar radiation is absorbed by the receiver and
transferred to an engine.
Dish/engine systems are characterized by high efficiency, modularity, autonomous operation, and an inherent hybrid
capability (the ability to operate on either solar energy or a fossil fuel, or both). Of all solar technologies, dish/engine
systems have demonstrated the highest solar-to-electric conversion efficiency (29.4%)[1], and therefore have the
potential to become one of the least expensive sources of renewable energy. The modularity of dish/engine systems
allows them to be deployed individually for remote applications, or grouped together for small-grid (village power) or
end-of-line utility applications. Dish/engine systems can also be hybridized with a fossil fuel to provide dispatchable
power. This technology is in the engineering development stage and technical challenges remain concerning the solar
components and the commercial availability of a solarizable engine. The following describes the components of
dish/engine systems, history, and current activities.
SOLAR DISH ENGINE
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Figure 2. Schematic of a dish/engine system with
stretched-membrane mirrors.
Concentrators
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 concentrator
for dish/engine systems is determined by the engine. At a nominal maximum direct normal solar insolation of 1000
W/m2, a 25-kW dish/Stirling system’s concentrator has a diameter of approximately 10 meters.
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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-cost
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 with
multiple, spherically-shaped mirrors supported with a truss structure (Figure 1). An innovation in solar concentrator
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 this
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 the
receiver aperture, are usually over 95%.