Solar energy

The significance of solar energy

The sun's energy is critical for the occurrence of physical processes on Earth and is the basis of life, with the existence and dynamics of most ecosystems depending on it. Without continued access to this energy source, which is absorbed by green plants and algae, and used to fix carbon dioxide and water into simple sugars through photosynthesis, all ecosystems would deplete the energy stored in them, thus ceasing to function and sustain life.

The solar energy received on Earth exceeds the world's energy needs, so its use has strong potential to meet current and future demands for electrical energy. However, solar radiation does not hit the Earth in the same way due to the tilt of the Planet's axis, being higher at the equator. In Europe, radiation is greater in southern countries, such as Portugal, where the average annual number of hours of sunshine varies between 2200 and 3000 h (in contrast, for example, with Germany, where this value is 1200 to 1700 h).

Solar photovoltaic energy and how it works

Solar photovoltaic energy uses solar radiation to generate electrical energy, for which it uses technology based on the photoelectric effect through which certain materials are capable of absorbing photons (light particles) and releasing electrons. The photoelectric effect was first described in 1839 by the Frenchman Alexandre Edmond Becquerel.

Currently, the use of the photoelectric effect uses a semiconductor device (photovoltaic cell), generally made of silicon. Monocrystalline silicon cells are obtained from a single crystal of pure silicon and achieve maximum efficiency, while cells obtained from polycrystalline silicon or amorphous silicon are less efficient.

When sunlight hits the photovoltaic cells, their atoms release electrons, which, flowing through the cell, generate electricity. The photovoltaic cell is designed so that electrons move in a given direction, generating direct current (DC) electrical energy. Then, an inverter converts the DC into alternating current (AC), which can be injected into the electrical grid.

Photovoltaic systems range from simple structures installed in buildings, with a capacity of a few kW to several tens of kW, mainly aimed at self-consumption, to large centralized production plants for electricity injection into the grid, in which the structures are placed on the ground, or float on water surfaces, i.e. reservoirs.

Some of the first photovoltaic uses took place in space, on satellites, such as the Vanguard I satellite which, in 1958, used a small 1 W panel that allowed communications to remain operational for more than 6 years. Photovoltaic solar energy can still be used on a very small scale to power various devices, such as clocks, radios, flashlights, lighting, parking meters, etc.

Main components of a grid-connected solar photovoltaic plant

Photovoltaic modules/panels – consisting of groups of photovoltaic cells installed between layers of silicon, which capture solar radiation and transform light into electrical energy (photoelectric effect);
Inverters - convert the direct electrical current that the panels generate into alternating current, suitable for injection into the public service electricity grid;
Transformers – transform the low voltage alternating current coming from the inverters into medium voltage current, suitable for delivery to the local infrastructure of the public service electricity grid.

Particular aspects of a photovoltaic plant operation

In a given place, the trajectory and elevation of the sun vary during the day and throughout the year, so the solar incidence angle constantly varies. Therefore, the inclination of the solar panels must take into account their geographic location in order to maximize electricity production. In Portugal, the best orientation of the panels when fixed is the south direction, with an inclination angle of 30 to 35º. With the aim of increasing energy production, some of the photovoltaic plants installed in ground-based structures include solar trackers, which allow orienting the rows of panels, making it possible to follow the daily movement of the sun in an east/west direction through the panels.

The efficiency of photovoltaic cells increases, although insignificantly, with increases in solar radiation, but decreases with increases in ambient temperature and, even more in the temperature inside the solar modules.

Other uses of the sun's energy

In addition to photovoltaic technology, solar energy is used in solar thermal systems to provide hot water and environmental heating, and for this purpose solar thermal panels are used, very different from photovoltaics.

Thermal systems can also generate electricity in thermoelectric plants, which use techniques to concentrate the sun's energy as a heating source. The heat is then used to produce water vapor which, by driving a turbine, allows electricity to be generated through a process very similar to what occurs in thermoelectric plants powered by fossil fuels.

Advantages of photovoltaic energy

100% renewable and inexhaustible energy;
High resource availability in Portugal;
Contribution to reducing CO2 and other GHG emissions;
Contribution to Portugal achieving various international commitments and goals in the field of decarbonization and energy autonomy;
Reduction of external dependence by reducing the import of fossil fuels for burning in thermoelectric plants;
Generation of low-cost electrical energy, inducing the electrification of fossil energy consumption (e.g., electric mobility) and more competitive economic sectors (e.g., manufacturing industry) or even innovative (e.g., data centres).
Modularity, allowing the installation of different sized systems, from panels on roofs to large-centralized electricity production facilities;
Energy conversion efficiency greater than 20%;
Technology already mature, with reduced investment and operational costs;
Contribution to the generation of green jobs and to boosting the local economy, both during the construction period and during the operation and maintenance phase.

Current situation and development prospects for solar photovoltaic in Portugal

The first Portuguese photovoltaic plant was implemented in 2006, in Serpa (Hércules plant), with 11 MW of installed power. As a result of the notable growth in installed capacity, especially from 2020 onwards, solar photovoltaic already represented 15.8% (or 3.9 GW) of the total installed capacity in Portugal at the end of 2023 (24.6 GW), generating in that same year around 12% (or 5.5 GWh) of the electrical energy produced in the national generating system (45 GWh).

Evolution of photovoltaic solar power installed in Portugal (data source: DGEG)

Solar photovoltaic technology is the one with the greatest potential for progression in Portugal and in many regions of the Earth, not only due to the availability of resource (solar radiation), but also due to continued technological developments, which has allowed increasingly efficient equipment. Furthermore, compared to the two other most widely used renewable electricity production technologies, hydro and wind, it currently has the lowest penetration rate in relation to the potential identified in specialized studies.

According to the Portuguese Energy and Climate Plan for 2030 (PNEC 2030) in force, the electro-producer sector in Portugal should have a total installed photovoltaic capacity of 9.0 GW at the end of this decade, well above the 3.9 GW. previously mentioned. If the review of PNEC 2030 currently under discussion is approved, that target could more than double within the same seven-year time horizon.

Whatever target is set for the growth of solar photovoltaic in Portugal, there are obstacles to overcome, among which the following stand out i) the need to expand the electrical energy transport and distribution network, ii) the compatibility of land use with territorial planning rules, iii) the streamlining of licensing processes and iv) the market placement of electrical energy produced by new production plants in terms that compensate the risk and remunerate the capital allocated by investors in these new solar installations .

Technological areas