Photovoltaic Solar Energy

Definition and History

Photovoltaic is a method that generates electrical direct current (DC) by means of the use of a semi-conductor element when it is lit by a photon beam from the sun.

In photovoltaic applications, solar cells, which are the semiconductor elements, are interconnected and encapsulated in the so-called photovoltaic modules, so as to provide the cells with greater durability, resistance and insulation. Thus functioning over a long term period is guaranteed, usually between 20 to 25 years, with a production rate of more than 85% of the initial nominal power.

In order to build the whole system in the solar plant additional elements are needed. The invertor, the batteries, the structures where the modules are mounted on (fixed, single axis or double axis tracking) as well as the other elements required to build the plant (cables, protection, transformers, etc.) is usually called “Balance of System” (BOS).

Photovoltaic plant with Polycrystalline Silicon panels installed on non-tracked support structures

Photovoltaic plant with Polycrystalline Silicon panels installed on non-tracked support structures. Source: Institute of Solar Energy.

On a historic note, it should be mentioned that the photovoltaic effect was discovered as far back as 1839 by Becqueler, although the first silicon cell was not developed until 1954 at Bell Laboratories in the USA. In the 60’s the fundamental theory of the solar cell began to be developed with regards to all the most relevant aspects known to date and which characterize each of the solar technologies existing today, such as: materials, radiation spectrum, thermodynamics and efficiency. ars, with a production rate of more than 85% of the initial nominal power.


Types of Photovoltaics Technologies

Basically depending on the type of cell selected, we can classify photovoltaic technologies into different subgroups, whose characteristics will have a greater or lesser impact on the configuration of the BOS.

Without going into great detail the technologies commercially available at present are as follows (listed in decreasing order as per number of MW installed worldwide):

Monocrystalline and Polycrystalline Silicon

Monocrystalline Silicon before cutting

Monocrystalline Silicon before cutting

These represent almost 90% of the photovoltaic installations that exist worldwide at present. They are characterised by the ease of installation although the ratio of efficiency of the panels is not normally higher than 20%. In terms of BOS, they can be installed as either fixed structures or mounted trackers. At present, they are extensively used but the large area of cells required per square metre of installation along with the need to use many other auxiliary materials for the production of the module means that its potential for reducing costs is limited.

Thin Film

This technology is mainly based on the use of cells made from amorphous silicon, cadmium telluride (CdTe), CIGS (flexible cells) and other semiconductor materials. They are normally characterised by low efficiency rates of around 10%, which means they are not suitable for use in areas of medium and high irradiation but are appropriate for areas with low levels of irradiation. They generally need a large area for installation although the manufacturing costs are low. One of the main disadvantages is that the initial investment for production plants is the highest of all the systems described here.

Concentrated Photovoltaics (CPV)

Normally based on the use of multi-junction (MJ) cells, this is the photovoltaic technology and the one most recently commercially applied for in-land solar farms installations (it was already in use in the aerospace industry). In general, it uses optical devices to concentrate light and thus reduce the high cost of MJ cells, which means that the BOS needs to necessarily include double axis tracking. Its high yield (panels achieve efficiency of over 30% nowadays but it is believed that in the future they may reach around 40-50%) makes it ideal for the generating of electricity in large plants located in areas with a high degree of solar radiation and at the same time presents this technology as having a greater potential for reducing costs.

Installation of a photovoltaic plant with CPV modules and single post type double tracking. Source: ISFOC.

Installation of a photovoltaic plant with CPV modules and single post type double tracking.
Source: ISFOC.

From the outset, most of the efforts have been concentrated on achieving the most efficient conversion cells (difference between energy from the sun and energy generated by the device) in such as way as to ensure the modules achieve the greatest amount of energy with the lowest possible manufacturing costs. Therefore, from a more commercial point of view, modules are generally classified not only in terms of conversion efficiency but also in terms of the €/Wp ratio, where the watt peak (Wp) is the power which a module provides for a perfectly adapted load when the light proceeding from 1 kW/m² and a determined spectrum fall on it whilst the cells are maintained at 25ºC.

Once the panels have been installed, the most important parameter at plant level is the production of energy in terms of Kw/h/m², which indicates the level of electricity produced per hour and aside from the technology and configuration of the BOS will also depend on the amount of solar resources available at each location.