Concentrator Photovoltaic Systems

Solar panels contain solar cells that convert sunlight into electricity which is conducted away by metal conductors for conversion, distribution and use in appliances. Conventional solar cells are made by heating raw quartz sand to produce pure silicon wafers which are then doped with semiconductors that creates a junction along which electricity can flow. Concentrator/concentration photovoltaics (CPV) is a photovoltaic technology that generates electricity from sunlight with the aid of solar cells but in a slightly different way.

Source: Solarlits.com

How Do CPV Systems Work?

They Incorporate Lenses, Mirrors and Few Solar Cells

Unlike conventional photovoltaic systems, CPV systems use optical devices like lenses or curved mirrors to collect sunlight from a larger area and concentrate it onto small, high-efficiency, and often multi-junction solar cells.


They Often Incorporate High-Efficiency Multi-Junction Solar Cells

The high-efficiency requirement of solar cells can be only achieved with multi-junction cells. They come with the highest field efficiencies of any solar cell so far, with field efficiencies reaching up to 30% to 36%, and laboratory efficiencies up to 46%. At the present, field efficiencies of conventional solar cells is still at 23%.

A Multi-Junction Silicon-Based Solar Cell, Source: Fraunhofer ISE
A Multi-Junction Silicon-Based Solar Cell, Source: Fraunhofer ISE

MJ solar cells were originally designed for application in space science. It wasn't till the early 2000s, that their application expanded into solar solutions.


Multi-junction (MJ) solar cells are made by stacking solar cells on top of one another and tuning each independent cell to a specific wavelength of light to maximize the efficiency conversion of incoming light into electricity. To boost of the solar cells, the spectrum of white light is widened. At the present, most multi-junction concentrator solar cells compose of 3 solar cells but researchers are expanding to 4 and 5 junction solar cells to boost efficiencies.


They Can Only Generate Electricity From Direct Sunlight

Unlike conventional PV systems, CPV systems can only use direct sunlight. They are incapable of utilizing diffused sunlight from clouds and atmosphere. They are therefore best suited for areas with high amount of direct sunlight and have to be fitted with trackers to maintain their performance. This represents a disadvantage in comparison with conventional solar cells.

Source: Pennsylvania State University, Credit: Mark Fedkin
They Are Often Ground-Mounted

They are often ground-mounted and typically require solar trackers, that is, devices that allows them to follow the sun as it moves across the sky thus enabling the system to produce more energy. This adds to their complexity and costs.


They Often Require Cooling Systems

Also, because of the amount of heat they generate, they often require cooling systems to avoid the thermal destruction of their materials. This also adds to their complexity and costs.

A Concentrator Solar Panel In Between Two Conventional Solar Panels, Image Credit: Brücke-Osteuropa - via Wikimedia Commons

Types of CPV Systems

CPV systems are classified in terms of their solar concentration ratio.


Low-concentration CPV (LCPV)

LCPV are systems with a solar concentration ration of 2–10. In LCPV systems, conventional or modified silicon solar cells are typically used. The cells do not require active cooling systems and no tracking systems are needed.


Medium-concentration CPV

They have concentration ratio of 10 to 100 and require one-axis solar tracking system. They also require cooling, either passive or active but are mostly passive. They employ silicon, CdTe, and CIGS (copper indium gallium selenide) solar cells.


High-concentration CPV (HCPV)

HCPV systems have a concentration ratio of 100-400 and above. They require a two-axis solar tracking system. They require active cooling in the form of high-capacity heat sinks. They incorporate multi-junction solar cells.

An Array of 25-kW CPV Systems in Hawthorn, Australia, Image Source: R. McConnell

Attractions

CPV systems have these competitive advantages over the conventional or non-concentrating solar cell technology:


1. They require less photovoltaic material to capture the same sunlight as conventional non-concentrating solar cell technologies.


2. By making use of few high-efficiency cells, space requirements are reduced.


3. They use materials that are not dependent on the still growing silicon supply chain.


4. Again, by making use of few photovoltaic materials, CPV systems have the potential to mitigate two major challenges of conventional solar, namely, the loss of land space and the accumulation of toxic solar waste and unrecyclable materials.

Disadvantages

The most significant disadvantage of CPV systems is that unlike conventional non-concentrating 'flat-plate' PV systems, they can only use direct sunlight. They are incapable of utilizing diffused sunlight from clouds and the atmosphere. They are therefore best suited for areas with high amount of direct sunlight. Otherwise, they have to be fitted with trackers to maintain their performance. The complex tracking systems, extra parts like mirrors, lenses and the cooling systems, adds significantly to the cost of investing in them. In comparison, CPV systems cost from 2.5 to 4 times higher than conventional silicon solar cells. This makes them financially unattractive and explains why are still not yet common.


Also, CPV systems are often immensely large, and more suitable for utility scale installations than for residential installations. Some manufacturers are however, developing smaller CPV products for the residential market.


As CPV systems get cheaper, more efficient and smaller with the optimization of the MJ solar cell technology, CPV systems may yet play a big role in the future of renewable energy.


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