Solar energy is radiant light and heat that are emitted from the sun, it is considered to be the cleanest and most abundant renewable energy source available.
Even though only a small part of this light ever reaches the earth, the amount radiated by the sun in an hour and a half is sufficient to sustain the whole world’s energy consumption for an entire year (Source: energy.gov)
Consequently, finding and developing technologies to harness and capture solar energy is a practical solution for replacing fossil energy, reducing GHG emissions and facing climate change. (Source: seia.org)
Solar energy technologies
There are three primary technologies by which solar energy is harnessed:
Photovoltaic PV
Photovoltaic (PV) gets its name from the process of converting light (photons) to electricity (voltage), which is called the photovoltaic effect, this phenomenon was first exploited in 1954 by scientists at Bell Laboratories in the US.
Today, electricity from PV is one of the fastest-growing renewable energy technologies, it has become cost competitive in many regions and its systems are being deployed at large scales to help power the electric grid. (Source: nrel.gov)
Photovoltaic (PV) devices can be used to power anything from small electronics such as calculators and road signs up to homes and large commercial businesses. (Source: seia.org)
How it works
Solar cells are appliances that convert sunlight directly into electricity, they are made of layers of semiconductor materials similar to those used in computer chips. The photovoltaic effect through which these PV cells work can be explained as follows:
- Photons strike the PV cell, they can then reflect off the cell, pass through the cell, or be absorbed by the semiconductor material, only in this last case the photons provide energy to generate electricity.
- Once the semiconductor material absorbs enough sunlight, electrons are dislodged from the material’s atoms and flow freely, creating a current.
- The current is captured and transferred through wires. (Source: news.energysage.com)
In order for the electric current to be created, the electrons must flow in the same direction. This is accomplished through a combination between two different layers of silicon that work much like a battery.
A first silicon layer that is doped with atoms of phosphorus, which has one more electron than silicon and is referred to as the negative terminal (n-layer) and a second layer that is doped with atoms of boron, which has one less electron and is referred to as the positive terminal (p-layer).
An electric field is be created at the junction between the two layers, as it is illustrated in the following figure. (Source: planete-energies.com)
Special treatment of the material surface during manufacturing makes the front surface of the cell more receptive to the dislodged, electrons so that the electrons naturally migrate to the surface of the cell. (Source: eia.gov)
The PV cell is the basic building block of a PV system, the efficiency at which it converts sunlight to electricity varies by the type of semiconductor material and PV cell technology. Individual cells can vary in size from about 0.5 inches to about 4 inches across. However, one cell produces 1 or 2 Watts, which is only enough electricity for small uses, such as for powering calculators or wristwatches, they are thus, combined in a packaged, weather-tight PV module or panel of about 40 cells or more. PV module electricity generating capacity increases with the number of cells in the module or in the surface area of the module, PV modules can be connected in groups to form a PV array (A PV array can be composed of two or hundreds of PV modules). (Source: eia.gov)
The below figure illustrates the composition of a solar panel.
The following figure illustrates the functioning principle of a photovoltaic solar system.
Types of photovoltaic cell technologies
There are three main types of PV cell technologies that dominate the world market: Monocrystalline silicon, Polycrystalline silicon and thin film silicon, as described in the table below. (Source: greenmatch)
Higher efficiency PV technologies, including gallium arsenide and multi-junction cells, are less common due to their high cost, but are ideal for use in concentrated photovoltaic systems and space applications.
Concentrating solar power (CSP)
Concentrated solar power (CSP) is an approach for generating electricity through mirrors to concentrate the sun’s rays and produce heat for electricity generation via a conventional thermodynamic cycle.
CSP systems are a flexible source of renewable energy since they have the ability to store energy.
CSP can be used to deliver heat to a variety of industrial applications, like water desalination, enhanced oil recovery, food processing, chemical production, and mineral processing. (Source: energy.gov)
How it works
As shown in the following figure, the mirrors reflect, concentrate and focus natural sunlight onto a specific point. The sunlight is then converted into heat that is afterwards used to produce steam. The steam enables a turbine to rotate; thus, generating electrical power.
When using CSP systems it is possible to store the electricity produced, this process can consequently be continuously repeated and can be used on days where there is no sunlight, or before sunrise and after sunset. (Source: brunel.net)
Types of CSP technologies
There are four main types of CSP technologies: Parabolic trough systems, Power tower systems, Linear fresnel systems and Parabolic dish systems, as described below. (Source: brunel.net, e3solar.com)
The above stated technologies are illustrated in the below figure.
Solar heating and cooling systems (SHC)
Solar heating & cooling (SHC) technologies harness thermal energy from the sun in order to provide hot water, space heating, cooling, and pool heating for residential, commercial, and industrial applications. (Source: seia.org)
About 30% of the total energy consumption of IEA member countries is spent on energy for heating, domestic hot water, cooling and lighting. Most of these applications can be covered using solar thermal technologies. The objective of the IEA solar heating and cooling programs is to cover 50 % of low temperature needs in regard to heating and cooling with solar by 2030. (Source: nachhaltigwirtschaften.at)
Types of SHC technologies
There are several solar heating and cooling technologies. We can distinguish the following:
Water heating technology
Solar water heating systems are composed of three main elements: the solar collector, insulated piping, and a hot water storage tank, as illustrated in the figure below. The solar collector gathers heat from solar radiation and transfers it to water, this heated water flows out of the collector to a hot water tank, and is then used. (Source: seia.com)
The main function of solar collectors is to absorb solar energy and convert it into heat.
There are various types of solar collectors from which we can cite: Flat-plate collectors, Evacuated-tube collectors and unglazed solar collector.
Solar air technology
Solar air heating is a solar thermal technology that is used for commercial and industrial building heating, in this system solar energy is captured and used to heat air. This system is illustrated in the following figure. (Source: seia.gov)
Solar energy data
The below maps give the share of solar electricity production in the whole world and in Europe in 2020.
A report by independent climate think-tank Ember said that:
- Solar power supply in the European Union during June and July rose to a record in 2021, accounting for 10% of total electricity produced in the region.
- The 27 countries in the bloc generated nearly 39 terawatt hours (TWh) of power from solar panels during June and July, up 10.9 TWh from 2018. (Source: reuters.com)
According to the IEA solar PV 2021 report:
- Solar PV generation increased a record 156 TWh (23%) in 2020 to reach 821 TWh. It demonstrated the second-largest absolute generation growth of all renewable technologies in 2020, slightly behind wind and ahead of hydropower.
- Solar PV development will continue to break records, with annual additions reaching 162 GW by 2022 – almost 50% higher than the pre-pandemic level of 2019. (Source: International Energy Agency)
The following figure illustrates annual solar PV capacity additions in certain countries from 2019 to 2022.
The following figure mpas countries by their solar power capicity on an overall and per capita basis.
The following figure gives a comparison between PV, CSP and coal GHG emissions.
