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CPV technology today and tomorrow

Wed, 11/27/2013 - 1:25pm
Anco S. Blazev

Modern CPV equipment consists of a metal pedestal cemented into the ground onto which a large metal frame is mounted on pivoting supports thus enabling it to move on an x-y axis with the help of a gear-drive assembly. Many CPV modules, mounted on the frame, contain a dozen or more highly efficient multi-junction interconnected CPV cells, covered by a Fresnel lens. The modules with the solar cells inside, are kept always at 900 orientation to the sun, via the x-y gear-drives. Sunlight hitting the large opening of the Fresnel lens is focused (concentrated 100-1000 times) onto the 1.0 cm2 CPV cells, which generate DC electricity to be sent into the grid.

CPV isn’t catching on in mainstream due to a combination of factors including: materials inefficiency and incompatibility; manufacturing process issues; lack of reliable long term field test data; lack of experience and idea sharing forums; lack of technical understanding by the potential customers--all of which leads to lack of interest by the investment community and the utilities.

The advantages of CPV, as compared to conventional PV technologies (silicon and thin films PV modules), are many:

Table 1 shows the key advantages of the CPV technology, where: greater efficiency, longer daily power generation at near-nameplate capacity, temperature handling superiority, better land use, long term reliability, and the ability to use local materials and labor are the key and very important advantages.

The most important advantages of the CPV technology are its high efficiency and full power utilization; both several times higher than any of the conventional PV technologies and increasing by the day.

Figure 1 shows that CPV systems: a.) operate at much higher efficiency, and b.) maintain full power generation nearly 100 percent of the time from sunrise to sunset, due to precise dual-axis sun tracking.

Improved materials and manufacturing techniques will double the CPV system's power output in the near future.

The conventional PV systems are restricted to lower power output, due to their much lower overall efficiency and their marginal positioning towards the sun most of the day. PV modules are seldom installed on trackers, because the added initial and O&M expenses are unjustifiable in most cases.

Note: Solar thermal (CSP) technologies operate at efficiency close to that of CPV trackers, but the fact that they use large amount of cooling water, which is just not available in the deserts, puts them at a serious disadvantage.

Dry cooling (A/C systems) is one alternative to using cooling water, but this approach adds to the complexity and expense of the O&M operations, and uses a lot of electricity, which in turn reduces the power plant's total power output significantly.

The limitations of the CPV technology are few, but are very important to understand and consider from the onset of any solar project evaluation and analysis. CPV is most efficient and cost effective in dry, sunny climates, such as the world's deserts, where it is superior to any solar technology. Unfortunately, CPV does not function well under cloudy, foggy, and / or any diffused sunlight conditions, which limits its applications to certain geographic areas. CPV cells and Fresnel assemblies require very precise tracking, so the electro-mechanical gear-drive assemblies and x-y tracking software design are quite complex and expensive; CPV systems also contain moving parts, so periodic maintenance, tuning, and cleaning are essential for their proper and efficient operation, which increases the O&M expense significantly.

Due to the large frame design, wind gusts affect the stability of the structure, which could cause efficiency reduction and / or structural damage. Equally importantly, due to extremely high temperatures of the concentrated sunlight (100-1000 x) falling onto the CPV solar cell assemblies, they operate in near-critical heat regime, which leads to power reduction and failures due to excess and / or prolonged overheating.

The major challenges
Although CPV components and systems have been in existence and under on-and-off development for over 30 years now, there are still unresolved, or partially resolved, technical issues.  Fresnel lenses can be made out of plastic or glass, where plastics can be easily damaged by hail and sand storms, and also tend to lose transparency under prolonged, excess UV and IR radiation. Glass lenses, on the other hand, last forever, but are too heavy and expensive. There are some alternatives being developed which promise long and reliable operation even under extreme desert conditions.   

Another challenge is structural damage due to wind gusts is a problem faced by most CPV designs, because the flat top surface of the frame is usually covered with flat modules which form a large "sail." The force of wind gusts acting upon the "sail" is amplified by its large area, twisting and bending the structure. While serious structural damage can be avoided by stowing the frame face down under extreme wind conditions, even moderate wind gusts are enough to defocus the sunlight from the cells, thus reducing the overall system efficiency and even causing heat damage. Structural stress and metal fatigue are also increased due to wind action and excessive "sail" counteraction.   

One way to minimize the wind effects is to avoid the "sail" design by staggering the modules on the frame, as shown in Figure 2, thus allowing the wind to "flow" between the CPV modules instead.

Cell and module damage due to exposure to extreme heat is another reliability issue for this technology. Small multi-junction CPV cells are usually soldered onto an air-cooled heat sink, which cooling capacity is limited. Prolonged exposure to extreme heat overheats the cells, which reduces the system' efficiency and in some cases damages the CPV cell assemblies and the modules.

Using liquid flow-through heat sinks, as shown in Figure 3, is an efficient way of cooling the CPV cell assemblies, but this complicates the structure and its maintenance further, thus this approach is not in use presently.   

The future looks bright for the CPV technologies. With time, the technical challenges will be overcome, at which point CPV tracking technology is expected to become universally accepted, mass produced, and cost-effective energy source for use in large scale solar power generating plants around the world.

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