Literature Review Chapter: Hyperboloid Solar Concentrator

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Hyperboloid Solar Concentrator

Following is a review of the literature associated with an analysis of hyperboloid solar concentrators. The structure of said review begins with a broad look at both the topics of water desalination and solar power, a more focused discussion of solar concentrators in general, and, finally, a look at the literature regarding hyperboloid solar concentrators.

Condensed History

The history of solar use goes back the "7th century B.C." (DOE). At that time ancient people used glass and crystals to concentrate the sun's rays and start fires. From the first to the fourth century, Romans used large windows to heat the baths and warm the rooms that people bathed in. From those beginnings the modern usage of solar power followed.

Moving forward to the Second World War, people did not have enough electricity for the basic functions in their houses because of restrictions imposed by the war department. Because of this people looked to solar energy generation as a way to both heat and electrify their houses. Throughout the next few decades the efficiency of the solar cells used for many applications was increasing at a slow rate, but the technologies being developed meant that smaller cells were able to generate more power (DOE).

As the past decade has progressed there have been astronomical advances in both the use, availability, and affordability of solar technologies. In the past ten years, solar concentrators have gone from large scale operations to the point where they are becoming more feasible for residential homes.

Flat Plate Solar Collectors

The flat plate solar design has become the most popular of collection designs since they are "permanently fixed in position, have simple construction, and require little maintenance" (Koo). This system generally consists of the collectors themselves and some type of conversion medium. The use of the solar power is the main concern at this stage. For residential applications the solar rays are converted to electricity for various uses around the house. The problem in the past had been the financial viability of the technology, but that is not as much of a problem now as it once was. With the advent of new material technologies and the fact that demand has increased, the cost of owning such a system has decreased, and made it easier to afford. The flat panel design made it affordable for the average consumer, but further technologies are making solar collection and concentration even more affordable and feasible for the average consumer.

Solar Concentrators

The technology associated with solar concentration is not new. However, since the cost of creating a solar concentrator is relatively high and the energy transfer has not always been the most efficient (Pitz-Paal), the technology has not always been utilized to its full capacity. Researchers have tried to determine the most effective generation methods for the technology, and have traditionally have had to work with one of three methods: a parabolic trough, a parabolic dish, or a central tower receiver (Cobble, et al.).

The concept of a central receiver for solar concentration has long been used in situations that require a large power output. Engineers realized that "to design a large-capacity (multimegawatt) plant, it is clear that a large scale with >1 MW power requirements can only be delivered by some kind of central receiver" (O'Gallagher 99). The use of these concentrators also requires large available land space to support the cells which feed the concentrator. The extremely temperatures that are generated with this type of concentrator also enhance its efficiency (Pitz-Paal). As in any ideal (such as a Carnot engine) a higher heat sources along with a low heat sink will yield the most efficient production (O'Gallagher 101). Unfortunately, there are problems with this design. The tower can only generate collect reflected solar energy, and concentrate it, if it can see the relaying field (O'Gallagher 101). This means that either the tower has to be placed on a plain that does not in any way impede its access to the reflected light, or it must be made taller to account for variations in the field (Chavez 13). The receiver inside the tower is generally a compound parabolic receiver (CPC) which was one of the first concentrator types devised and though non-complex is also relatively inefficient (Chavez 3). That is another reason for using them on large applications only. Due to lower efficiency they have to have a large enough field to generate from to make them feasible.

Both a parabolic trough and a parabolic disk use the same principle of design to accomplish the task of focusing the solar rays to a single point. "It is not necessary to use the whole part of the parabola curve to construct the concentrator. Most of the parabolic concentrator employs only a truncated portion of the parabola" (Muhammad-Suki, Ramirez-Iniguez, McMeekin, Stewart, & Clive 3). This design has advantages because of the concentration properties that it able to employ, but it also has many faults that can make it undesirable in many applications (Krewitt & Trieb). A parabolic concentrator can either be used "by rotating the two dimensional design along the x-axis to produce a parabolic dish (Muhammad-Suki, Ramirez-Iniguez, McMeekin, Stewart, & Clive 5), or by simply using the 2D trough without rotation (Ali, et al.). The disadvantages of the design come from the fact that;

"although this concentrator could provide a high concentration, it requires larger field of view to maximize the sun energy collection. To obtain maximum efficiency, it needs a good tracking system, which is quite expensive. That is why this type of concentrator is not preferred in a small residential house"

(Muhammad-Suki, Ramirez-Iniguez, McMeekin, Stewart, & Clive).

However, that is not an issue because they have been used successfully on large applications such as the Euclides-Thermie Plant at Tenerife, Canary Islands.

Other designs have also had varying amounts of success and can be used in different applications than either the central tower or parabolic designs. A Fresnel concentrator utilizes a flat upper surface and a back surface that employs canted facets" (Muhammad-Suki, Ramirez-Iniguez, McMeekin, Stewart, & Clive 6). This type has the advantage of being able to be employed in smaller applications (Townsend & Fung), it also "requires a lesser amount of material to fabricate" (Muhammad-Suki, Ramirez-Iniguez, McMeekin, Stewart, & Clive, 6). A dielectric totally internally reflecting concentrator (DTIRC) was designed to "have concentration capability close to the theoretical limit" (Muhammad-Suki, Ramirez-Iniguez, McMeekin, Stewart, & Clive 8). By increasing the efficiency of the operation, it is also possible to minimize the size to the greatest extent possible (Garcia-Botella, Fernandez-Balbuena, Vazquez, & Bernabeu). This makes it possible to use this module in more than just extreme commercial applications as long as the cost per unit can be brought to a reasonable level (Schwabach).

The final type of solar concentrator is the hyperboloid. A hyperboloid solar concentrator "produces energy by rotating the two dimensional design along its symmetrical axis. The diameters of the entrance and exit aperture are labeled as d1 and d2 respectively. If the inside wall of the hyperbolic profile is considered as a mirror, the sun rays entering the concentrator from AA' will be reflected and focused to the exit aperture BB'" (Muhammad-Suki, Ramirez-Iniguez, McMeekin, Stewart, & Clive). The major advantage of this design is that it is a very compacted relation of the original paraboloid design.

Hyperboloid Concentrators

Hyperbolic concentrators have been in use for several years and they have direct application for many different needs. However, they have not been used to their greatest efficiency because solar generation has not been able to produce as much power as a typical power plant at a similar cost (Townsend & Fung). This is a significant issue when the primary areas of the world that need clean water and the other benefits that can be provided by solar power concentration and generation are also among the poorest regions in the world (Krewitt & Trieb). Of course, the focus of researchers and engineers is not necessarily to help these regions, but to find more cost effective ways to generate power because of the present demand for sustainable technologies (Krewitt & Trieb). Due to both of these factors the hyperboloid design has become one which is discussed more and more as an option that can translate a viable energy source that can be used anywhere at a relatively low cost to the company installing the generation platform.

This technology takes the best aspects of the parabolic design and further concentrates the solar rays to make collection even more efficient (Chavez 13). According to research conducted by Garcia-Botella, et al., "By its geometric design, it is easy to prove that the 2D HPC is an ideal concentrator. All the rays incident on the entry aperture of a 2D HPC at angles minor or equal to ? will be redirected, by the parabolic section of the concentrator, to the segment F ? F0, and their second and successive reflections will cause them to emerge from the concentrator." The fact… [END OF PREVIEW]

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