Solar Concentrator PV Systems

In Concentrating Photovoltaics (CPV), a large area of sunlight is focused onto the solar cell with the help of an optical device. By concentrating sunlight onto a small area, this technology has three competitive advantages: Requires less photovoltaic material to capture the same sunlight as non-concentrating pv. It makes the use of high-efficiency but expensive multi-junction cells economically viable due to smaller space requirements.

The optical system comprises standard materials, manufactured in proven processes. Thus, it is less dependant on the immature silicon supply chain. Moreover, optics are less expensive than cells.

Concentrating light, however, requires direct sunlight rather than diffuse light, limiting this technology to clear, sunny locations. It also means that, in most instances, tracking is required.

Despite having been researched since the 1970s, it has only now entered the solar electricity sector as a viable alternative. Being a young technology, there is no single dominant design.

Here are some examples of concentrator technologies and examples for both line and point concentrators. Although there might be differences in execution or materials used, most designs will follow one of those concepts.

Fresnel Lens

A Fresnel lens, named after the French physicist, comprises several sections with different angles, thus reducing weight and thickness in comparison to a standard lens. With a Fresnel lens, it is possible to achieve short focal lenght and large aperture while keeping the lens leight, fresnel lenses can be constructed in a shape of a circle to provide a point focus with concentration ratios of around 500, or in cylindrical shape to provide line focus with lower concentration ratios.

With the high concentration ratio in a Fresnel point lens, it is possible to use a multi-junction photovoltaic cell with maximum efficiency. In a line concentrator, it is more common to use high efficiency silicon.

Because the Fresnel lenses are small and need a vertical beam, So arranged next to each other (Array of lenses) to build a large solar panel (CPV panel), the CPV panel can move by tracking systems and follow the sun where the falling sunlight is vertical

Parabolic Mirrors

Here, all incoming parallel light is reflected by the collector (the first mirror) through a focal point onto a second mirror. This second mirror, which is much smaller, is also a parabolic mirror with the same focal point. It reflects the light beams to the middle of the first parabolic mirror where it hits the solar cell.

The advantage of this configuration is that it does not require any optical lenses. However, losses will occur in both mirrors. So focus has achieved a concentration ratio of 500 in point concentrator shape.

The first and second mirrors are made of plastic with reflector coating and are also arranged in array to build a large solar panel for the same reasons that were mentioned with the Fresnel lens.

There is another type of parabolic mirrors that are large in size, made of glass and achieve a concentration of 1000 times, this type does not require small mirrors because it reflects direct sunlight on the CPV Dense Array Module.

A CPV Dense Array module consists of many multi-junction solar cell assemblies (triple junction solar cell) packaged onto one 10″x10″ module. The triple junction solar cell converts the concentrated solar power light into electricity.

The advantage of using a CPV dense array module compared to Fresnel CPV systems is that it is compact, low cost, and low maintenance.

Reflectors

Low concentration photovoltaic modules use mirrors to concentrate sunlight onto a solar cell. Often, these mirrors are manufactured with silicone-covered metal. This technique lowers the reflection losses by effectively providing a second internal mirror. The angle of the mirrors depends on the inclination angle and latitude as well as the module design, but is typically fixed. The concentration ratios achieved range from 1.5 – 2.5.

Low concentration cells are usually made from monocrystalline silicon. No cooling is required. The largest low-concentration photovoltaic plant in the world is Sevilla PV with modules from three companies: Artesa, Isofoton and Solartec.

Luminescent Concentrators

In a luminescent solar concentrator (LSC), light is refracted in a luminescent film, and then being channelled towards the photovoltaic material. This is a very promising technology, as it does not require optical lenses or mirrors.

Moreover, it also works with diffuse light and hence does not need tracking. The concentration factor is around 3. There are many ongoing developments on the LSCPV system.

For instance, Covalent are using an organic material for the film, whilst Prism Solar use holographic film. Furthermore, this concentrator does not need any cooling, as the film could be constructed such that wavelengths that can not be converted by the solar cell would just pass thru. Hence, unwanted wavelengths would be removed.

Cooling

Monocrystalline silicon cells is that they lose their efficiency with the temperature, for every temperature increase, the efficiency of cells falls by around 0.5 percent above 25°C, so they need an efficient cooling system to improve their efficiency particularly with concentrating PV systems, there are two types of cooling:-

Passive Cooling: Here, the cell is placed on a cladded cermaic substrate with high thermal conductivity. The ceramic also provides electrical isolation.

Active Cooling: Typically, liquid metal is used as a cooling fluid, capable of cooling from 1,700°C to 100°C.

Hybrid Solar Panels

We showed how the efficiency of electricity generating of cells and PV solar panels is greatly reduced by the temperature of the panels, the hotter the panel, the lower the efficiency and the lower the power generated – not ideal when PV solar panels are dark, non-reflective, and pointing at the sun all day.

The solution was a hybrid system combining PV solar panel with a solar water heating panel so that the PV solar panel could generate electricity at a higher efficiency where the efficiency of the silicon cell may up to 22% instead of 18% thanks to the cooling effect of the water which would in turn be heated in the panel and put to use pools, showers, or commercial use.

This PVT (Photovoltaic Thermal) technology enables more power per square metre than standalone solar PV or solar thermal can achieve.

The hybrid solar panel consists of five basic parts,

the first part is PV module and
the second part a copper absorber it is a slice of copper working heat absorbent and placed directly below the PV module,
the third part is the copper pipe web below the copper absorber which passes cold water inside it and reacts with the copper absorber,
the fourth part the insulator which is a layer of insulating material to prevent thermal leakage and Aluminum frame for the cohesion of the previous parts.

The hybrid solar panel can be Static or in the sun tracking system.

How is cooled panel and water heating?

See the following illustration:-

Hybrid Panel (PVT), typically mounted on your home’s roof, absorb the sun’s thermal energy.
A pump circulates a heat transfer fluid from the hybrid Panel (PVT), through a heat exchanger in your solar storage tank.
The heat exchanger transfers the sun’s warmth to your water supply.
Hot water flows to your existing water heater, which provides backup heating when the sun isn’t shining.

Comparison table between the three types of solar panel.

CPVT System

Here is a mix between the two previous system, solar focus system and solar hybrid system where mirrors are used to create an average concentration of 4 suns to 12 suns on a hybrid solar panel.

With concentration degree 12 sun cooling system is not more effective, the silicon cell efficiency decreases from 18 % to 7% but 7% with 12 suns produce 84 % electrical energy than any other cell.