A solar concentrator is a concentration system of solar energy that is used to convert solar energy into thermal energy. Its operation is based on the exploitation of the reflection of the solar rays obtained through reflective surfaces (substantially of mirrors), in order to concentrate on a receiver of contained size. Similar to a magnifying glass focusing its light on a point, the concentrators reflect sunlight by means of an arrangement of mirrors aligned towards a lens capable of capturing that energy for its use.
In a solar concentration system, in comparison with the classic flat solar collectors, the following advantages are obtained:
- Energy: expressible in terms of solar-thermal conversion efficiency (since the losses by convection and radiation are directly proportional to the area of the receiver, which by definition is smaller than the area of the reflector). According to the laws of thermodynamics.
- Economic: construction costs move mainly on reflecting surfaces and relative pointing devices.
Types of solar concentrators
There are two types of solar concentrators that allow to maximize the performance of the solar installation:
- Cylindrical parabolic solar concentrators. These concentrators have a unique curvature; for this reason they are called "2D" concentrators because they are "two-dimensional". This system allows to obtain average temperatures in the focal environment of 200-300 ºC.
- The paraboloid solar concentrators of revolution. These concentrators have a double curvature. For this reason they are called "3D" or three-dimensional concentrators. These systems allow temperatures even higher than 3000 ºC.
One of the difficulties of these systems based on solar concentrators, is to ensure at all times of the day, that the solar radiation falls parallel to the axis of the paraboloid.
For this a system of tracking the apparent movement of the sun (tracking) guided by a control system is needed. This system is basically formed by a collimator or narrow tube to which the radiation enters, which has a photovoltaic cell at the bottom. When the concentrator is oriented towards the sun, the radiation reaches the bottom of the collimator tube, and the photovoltaic cell generates electricity. When this does not happen, a rotation of the concentrator is generated until the cell voltage is restored.
In small solar concentrators, such as a solar cooker, this monitoring of the apparent movement of the sun can be guaranteed with a tracking system applied directly to the concentrator (or even its position can be adjusted by hand every x time).
But in large solar concentrators the technology is complicated. In these cases deformations of the parabolic surface appear that affect the operation.
In these cases, one or more intermediate plane mirrors, called heliostats, are used.
A concentrator system then has two reflections: The first, of sunlight on the reflective surface of the heliostat, and then this reflected beam is reflected again in the concentrator passing through the focal environment. These two reflectivities cause a loss of intensity, since in each step there is a percentage of the solar radiation that is not reflected (absorbed or refracted).
However, the benefits of moving flat heliostats outweigh this loss of reflectivity.
Applications of solar concentrators
The applications of solar concentrators are multiple. The most common use is the generation of electricity, but there are also solar plants that take advantage of solar radiation for use in the metallurgical industry or can even be used to create solar cookers.
The generation of electricity is the most widely used application of solar concentrators. For this purpose, 2D or 3D solar concentrators can be used interchangeably, but the technologies are different.
In the case of 2D solar concentrators, a confined fluid circulates through pipes that coincide with the focal environment, heating up to temperatures of around 200ºC and becoming vapor.
On the other hand, in 3D solar concentrators, electricity is generated, placing in the focal area a thermal machine capable of converting heat into electricity, for example, a hydrogen-confined cycle Stirling engine.
With small 3D concentrators, solar cookers can be realized. The concentrator can have between 80 cm and 2 meters. In the focal area a "hornalla" is placed where the container with the contents to be prepared is supported.
Solar concentrators allow the use of solar energy in multiple industrial applications, such as, for example, in the metallurgical industry.
The high temperatures achieved in the focal environment in large 3D concentrators allow them to be used for metallurgical applications such as the obtaining of alloys for the industry.
History of solar concentrators
Already in ancient times, it is said that Archimedes of Syracuse succeeded in repelling the Romans, who tried to besiege the Sicilian city of the sea, by using rudimentary solar concentrators obtained from polished bronze shields.
Especially in the late nineteenth and early twentieth century many machines were built with solar concentration systems. This positive approach towards an extensive use of solar energy, however, was stopped due to the advent of oil and the First World War, and then resumed in the early 70s after the oil crisis of 1973.
To date, we went from a test phase to a pre-commercialization of CSP plants, mainly for the production of electricity. A new class of concentrating plants called CPV (concentrated photovoltaic in English) is also emerging, able to take full advantage and make the use of high efficiency photovoltaic cells, such as multi-junction cells, advantageous.
Last review: February 21, 2018