The photovoltaic cell is the minimum structural unit to produce electricity through light radiation. Depending on the material from which they are made, they fall under the categories: monocrystalline silicon "mono", polycrystalline silicon "poly", thin films "ribbon" and amorphous silicon "a-Si".

Monocrystalline photovoltaic cells:

The monocrystalline elements are usually circular in shape, while often, to achieve a denser placement within a given area, bearing frame, the ends of the four antidiametric chords of the arcs are cut off.

Thin Film Photovoltaic Cells:

Thin film photovoltaic cells are created by placing at least two thin semiconductor films on a suitable substrate, which is usually either a glass surface, or a metal, or a crystal of a different semiconductor. The semiconducting films, i.e., an n-type semiconductor and a p-type semiconductor, are placed on the surface of the cell on which the solar radiation falls, creating the electrostatic potential barrier that, as mentioned above, is necessary for the operation of a photovoltaic cell.

Amorphous silicon (a-Si) photovoltaic cells:

It offers a comparative advantage over all other types of photovoltaic cells. For example, the reduction of the supplied maximum power Pmax per °C temperature increase of the amorphous silicon photovoltaic element is only -0.2%, while the corresponding factor for the rest of the photovoltaic elements (mono, poly, ribbon) ranges from -0.4% up to -0.5%.

Polycrystalline photovoltaic cells:

Polycrystalline silicon crystals are mostly made with recycled electronic purity silicon feedstock. There are two techniques (Single crystals and micro-crystals of silicon) based on which square discs of polycrystalline silicon are manufactured.

Summary Comparison

• Monocrystalline silicon cells show an efficiency of 15% - 18% but have a higher cost as well as less dense stacking per PV panel.
• Polycrystalline silicon cells have a lower efficiency of 13%-15% but a lower cost.
• Amorphous silicon (a-Si) cells have a lower efficiency drop at higher temperatures but a much lower degree of efficiency, on the order of 6%-7%.
• Thin film elements show high performance grades, but due to the possible presence of toxic elements, they are not allowed by national quality control centers of several countries in domestic installations.

Each photovoltaic cell, when it receives solar radiation, provides its output with a constant voltage equal to 0.5 V. The maximum intensity of the electric current that can be produced by each such cell depends on its surface and is estimated based, mainly, on the value of the power density of the incident radiation. A typical value of the peak power current of a photovoltaic cell range from 4 to 7 A, while the maximum power delivered is about 3 W.

Photovoltaic elements are electrically connected to each other to form modules. The photovoltaic panel is essentially a single string mounted on a common carrier substrate. Therefore, it includes many, identical photovoltaic elements connected in series to achieve an increase in the electrical power provided.

PV panels integrated in buildings can be simultaneously active elements with electricity generation, but also passive elements that contribute to the reduction of the building's energy consumption. The above is mainly carried out by PV panels which allow the passage of light, cause shading or enhance the ventilation of the interior spaces. Also, there are hybrid PV - thermal systems, which utilize the temperature developed in the PV panels to heat air or water while simultaneously improving the panel's instantaneous efficiency factor. Dual functionality is partially achieved with conventional PV panels with their proper utilization. However, there are special multipurpose PV panels that are made for a certain way of use, so that they can cope with the mechanical and practical requirements of a building while always meeting the building construction guidelines.

The integration of PV systems in buildings is characterized by additional advantages apart from the general advantages of PV systems, such as:

• Direct production of electricity at the place of demand resulting in the minimization of electrical transmission losses.
• Replacement of conventional building materials. PV panels can replace conventional building materials with the added benefit of reducing their integration costs
• Integration into existing surfaces of the building without the requirement of additional land. So, their application can also be done in densely populated areas.
• Ability to cover, if not all, a large part of the electrical consumption.
• Control and maintenance of the photovoltaic installation that can be integrated with the control and maintenance of the rest of the electrical equipment.
• In combination with bioclimatic design, mainly in the construction of a new building, it can contribute to the further reduction of the required electrical power.
• Contribution to the improvement of the aesthetics of the building in an innovative way.

In recent years there has been a significant increase in interest in solar cells based on organic materials. This is partly due to the rapid growth of the photovoltaic market, which has stimulated research into longer-term, more innovative photovoltaic technologies, and partly to the development of organic electronic materials for demonstration applications. The fast one

Advances in optoelectronic molecular materials have introduced a range of potential new photovoltaic materials, as well as an improved understanding of the capabilities of such materials and the confidence in their application and use. Organic photovoltaic devices (OPVs) are semiconductor devices that consist of one or more polymeric films.

However, they have a drawback: low energy conversion. The technology is currently struggling to deliver efficiency above 5%, compared to the 20% achieved by conventional crystalline silicon photovoltaics, or 10% by thin film. The question, therefore, is whether the promise of low cost (if indeed achieved) can sufficiently compensate for declining (albeit improving) yields to enable OPVs to become commercially viable. Despite all the research that has been done on OPVs there are still obstacles preventing their use outside the laboratory. The first is oxygen sensitivity. Many of the cells produced in laboratories today are manufactured and tested without ever being exposed to air (and the natural moisture it contains). Although encapsulation techniques exist, they should be added to the production cost. Even if the cells are not damaged by exposure to air, many of them will have a short life span because they are rapidly degraded by exposure to light.

Another issue is the size of the cells being made. Currently, most cells are manufactured for areas on the order of 1 mm2 . But to be useful, they should be manufactured for areas of the order of 10 or 100 cm2 and these cells should be able to be assembled into larger panels for areas of the order of m2.

The global installed capacity of floating PV (Floating PV - FPV) has exceeded 3 GWp in 2021, according to data from the Solar Energy Research Institute of Singapore (SERIS) at the National University of Singapore (NUS).

Apart from some installations in Europe, notably in the Netherlands, France, and the UK, floating solar power is so far mainly in Asia, with more than 85% deployed in East and Southeast Asia. In densely populated areas the proximity of water bodies to unloading centers is often an advantage. Traditional land-based solar systems face either competing uses with industrial or agricultural activities or may not be economically viable due to high land costs. This is also why Japan was an early adopter of floating PV and still has the largest number of FPV projects (~200). Floating PV is even possible in city-states such as Singapore, which commissioned a 60 MWp FPV unit in June 2021 and has requested a study for another 140 MWp. The highest installed FPV capacity to date is being developed in China (a total of 1.3 GWp), where developers have heavily exploited aquifers created when former coal mines filled with groundwater. These so-called subsidence areas are almost ideal as they are considered unstable soils (thus unsuitable for industrial or agricultural activities) and often have little bioactivity (resulting in minimal environmental impact).

Another big opportunity for floating solar energy is the combination with existing hydroelectric dams. This is even more true when solar and hydropower generation work together (instead of the pure placement of the FPV station on the reservoir). In addition to the diurnal cycle (ie solar energy generation during the day and water conservation for hydroelectric power generation at night), there is also a potential seasonal benefit in areas with dry and wet seasons. Depending on the turbines and their reaction times, it is also possible to eclipse some of the short-term fluctuations from solar energy (due to water movements) and therefore use the reservoirs as a "giant battery". Many of the announced floating solar projects are located on hydroelectric reservoirs, for example in Thailand (3.5 GWp), Korea (2.1 GWp) and Laos (1.2 GWp).