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Dish Reflectors

Dish reflectors systems use parabolic shaped disks, much like satellite television disks, that concentrate sunlight (up to 800 fold) on a single point of focus. At that point of focus is a heat engine, which is often a Stirling engine but can be a simple steam engine. Dish reflectors are generally capable of producing 5 to 25 kW of power and operate at temperatures around 750 degrees Celsius (over 1300 degrees Fahrenheit).

Keeping the reflectors pointed at the sun is critical to ensuring performance in dish systems. Because the engine is mounted at the focus of the dish, it is quite easy to move these systems in multiple dimensions. In most cases, two axes movement is the standard. The dish moves to accommodate the path of the sun across the sky from morning to night and also moves to accommodate the level of the sun above the horizon during different seasons. As an added benefit, the ability to perform complex movements allows these dishes to protect themselves by inverting during evening hours. Besides protecting the dish, this helps prevent dust, dirt, and water from settling on the mirrors and keeps maintenance to a minimum.

The reflectors on a dish are not the integral part of design. What makes these systems work is the lightweight Stirling engine that has an entire page devoted to its operation. As an overview, the Stirling engine uses the cyclic expansion and contraction of a gas in a fixed space to drive a crankshaft. The engine is well known for being quite and highly efficient. At 40% efficiency, the Stirling engine is roughly twice as efficient as a standard internal combustion engine. Because Stirling engines can be built in almost any size, they are ideal for use in scalable applications like concentrated solar power.

Before leaving the concept of the Stirling engine, it is important to point out that while many solar applications produce direct current (D/C) electricity (especially photovoltaic systems), Stirling engines produce A/C current, which is what is used in all homes and most other applications. Because there is no need for conversion from D/C to A/C, Stirling engine based systems are simple and efficient.

Advantages of Dish Reflectors

The primary advantage of a dish reflector is efficiency. Form light to electricity (that is to say from beginning of the cycle to the very end), dish reflectors are generally around 25% efficient, a staggering number for any solar application and the best of any of the CSP systems. This efficiency allows dish systems to be relatively compact when compared to other CSP technologies. Most predictions are that dish systems will be able to provide a consistent 25% efficiency and a peak of 30% in the near future. While not applicable to all applications, this efficiency would make them highly competitive for stationary, terrestrial power generation.

Smaller dishes can produce about 3 kW each, meaning 4 to 5 dishes would supply the power needs of a single home. Larger dishes can produce up to 25 kW, which is almost double what the average home requires in a day. Because they are scalable and modular, dishes can be installed over time with little complication, making it easier for consumers to slowly implement a solar solution.

One of the biggest advantages of these systems is that they can be installed almost anywhere. Many solar applications require a flat surface for installation, but the dual axis tracking of dish systems means they can be installed on hills, roofs, or anywhere that the sun shines.

The last major advantage to point out with dish systems is their hybrid capabilities. All a Stirling engine needs to produce electricity is heat. Since heat can be supplied in a number of ways, it is possible to create hybrid dishes that can use standard fuels when sunlight is not available. This eliminates the need for complex backup systems and makes dish units “all-in-one” solutions for many installations, particularly remote areas.

Disadvantages

Despite their many positives, dish systems do have drawbacks. The first and foremost is the complexity of the tracking system. The dual axis design requires more mechanical parts than most other systems and is thus more prone to failure. Careful maintenance is necessary to keep these dishes in top working order. If maintenance is performed correctly, however, a 25 year service life is guaranteed by most manufacturers.

Because electricity is generated directly at the dish by the Stirling engine (by moving magnets through a coil), energy can only be stored as electricity, which means batteries are required as in a photovoltaic system. The downsides to batteries are their cost and relatively short life span. There is no option for thermal energy storage with dish systems.

Dish technology is relatively new compared to other CSP solutions. As such, some are concerned about the technology development risk. Whether or not this is a legitimate concern rests on how “proven” other CSP and solar technologies are and the potential of major dish companies, like Infinia, to continue to improve the efficiencies and operation of dish systems.
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