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Parabolic Trough

Parabolic trough systems use long arrays of curved mirrors to focus solar energy on a line. At the focal line a receiver tube filled with the working fluid, usually oil or molten salt, collects the heat and transports it to a centralized location to generate steam. Side view cut away below.

Parabolic trough

While the peak optical efficiency of these systems can be as high as 80%, the light to electricity efficiency is usually around 15%. The reason for the low overall efficiency is that the transfer to steam is only about 27% efficient, turbines are inefficient, and there is heat loss during transport of the working fluid. Whereas dish reflectors concentrate sunlight up to 800 fold, parabolic reflectors concentrated sunlight by a factor of 80 at best. This is a 100 fold difference in concentration that also has an impact on overall efficiency.

Like dish systems, keeping the troughs directed at the sun is critical and so they move throughout the day. Trough systems use single axis motion to track the sun and thus do not compensate for north to south variations over seasons. The reason a single axis is used in these systems is because the trough is rotating around a linear receiver tube rather than a single point as in a dish reflector. To have two axes of movement in a trough system would require that the receiver tube, which can be hundreds of meters long, also be moved.

Advantages of Trough Systems

The single largest advantage of trough systems is their maturity. These systems are the oldest and best understood CSP technologies and account for nearly 90% of the installed CSP systems in the world. There are trough systems generating power in the megawatt range throughout the United States, Spain, and Israel as well as in other sunny locations like Australia.

Another advantage of trough systems is the easy with which they can be made hybrid. Natural gas or another fossil fuel can be used to run the steam turbines when solar energy is not enough to meet demand. Most trough systems follow a hybrid design layout.

One of the major advantages that trough systems have over dish systems and even photovoltaic systems is the storage of thermal energy. Rather than having to rely on expensive batteries to store energy in its electrical form, trough systems can simply store energy as molten salt or hot oil, which can be used to produce steam even when sun is not available. Most systems are capable of generating electricity for up to 6 hours after the sun sinks below the horizon. Six hours may not seem long enough since it is not the full duration of night, but its importance is that it is plenty of time to get through peak energy usage that occurs during the early evening hours when people are cooking and preparing for bed.

Disadvantages of Trough Systems

Trough systems initially relied on a type of oil called therminol as the working fluid. Recent advances have lead to the use of molten salt as a superior replacement to therminol, but it still carries with it a particularly troublesome problem in that it can “freeze.” The concern in such systems is not that the oil or salt will become solid, but that if temperatures drop too low then the solution will become more viscous, will not flow properly, and will thus plug lines and cause damage. For molten salt, the magic number is 290 degrees Celsius.

The dependence on consistently high temperatures means that these systems, even when located in hot desert climates, have to be insulated against heat loss during cool temperatures and during overnight hours. Energy invested into keeping the solution hot when the sun is not shining decreases the overall efficiency of trough systems.

Another drawback to trough systems is the amount of land area they occupy. While perhaps a minor concern at present, increased use of such solar facilities would require vast tracts of land and in specific areas. Desert climates are ideal, which brings up the final problem.

As with all solar, transporting the energy produced in a parabolic trough plant is a barrier. The need to locate solar plants in sunny areas means that electricity must be transported great distances and current electrical grids in most countries are not up to the task. Their transmission capabilities sap a great deal of power, resulting in poor overall efficiency. For large scale solar to be viable, grids must be upgraded.

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