Introductory Chapter: Solar Photovoltaic Energy

1. Introduction

The concept of energy transition is defined as a transformation of fossil-based energy resources to non-carbonated during the upcoming years [1]. Hence, supplying energy through renewable resources that can be naturally replenished on a human timescale is being of great importance. This form of energy is named renewable energy and is mostly sustainable and environmentally friendly. Renewable energy can be easily converted into different types of energy (e.g., electricity, heat) via recent technologies.

Accordingly, in 2015, the international community set the Sustainable Development Goals (SDGs) as a part of the UN 2030 Agenda for Sustainable Development [2]. The goals include pledges to eliminate poverty, starvation, etc. Of all the goals set by the international community, some goals such as to supply clean energy, to protect the climate, and so on were energy-related. It is mentioned that the seventh goal (known as SDG7) attempts to provide services for clean, affordable, and modern energy all over the world and increase the portion of renewable energy among the other types by 2030. Also, since all countries in the world are prone to suffer from climate change, SDG13 tries to increase the immunity by either enhancing the resilience of different countries or educating people and raising awareness.

2. Solar radiation

The main source of energy to move the atmosphere is the sun. This energy is radiated in the form of electromagnetic waves with a wavelength between 0.2 and 4 μm (see Figure 1). The smallest measurable amount of an electromagnetic field is called a photon. The modernized definition of photon is derived from research (which were based on those carried out by the German physicist Max Planck) done by Albert Einstein from 1900s to 1920s. In 1926, the term “photon” was popularized by Gilbert Lewis in his letter written to the Nature magazine.

Figure 1.

The Earth’s radiation budget [3].

The power that is received from the sun in the form of solar electromagnetic radiation per unit area over a given time period is named solar irradiance and is measured in W/m² in SI units. Irradiance can be measured in space or at the Earth’s surface after it has partially been absorbed by the atmosphere as well as scattered. On the Earth’s surface, the amount of irradiance is a function of the tilt of the measuring surface, the height of the sun above the horizon, and also the atmospheric conditions. Figure 2 depicts the irradiance of the sun at the Earth’s surface in both a direct normal irradiation (DNI) and a global horizontal irradiation (GHI).

Figure 2.

(a) The world’s map of the direct normal solar irradiation (DNI) at the Earth’s surface and (b) the world’s map of the global horizontal irradiation (GHI) at the Earth’s surface according to Global Solar Atlas 2.0 [4].

3. Solar energy

The light and heat that are radiated from the sun are often named solar energy and are one of the most significant sources of renewable energy. Solar energy can be harnessed through some technologies that are categorized into two main classes namely active solar technologies such as photovoltaic systems and passive solar technologies that include a wide variety of techniques such as orienting a building to the sun.

3.1 Solar photovoltaics

The history of photovoltaics (PV) dates back to 1800s when Alexandre Edmond Becquerel observed PV effect. This was followed by testing the first solar cell with the efficiency of less than 1% in 1883. It was then in the first two decades of the twentieth century when Albert Einstein published his paper on photoelectric effect that resulted in his first and only Noble Prize in 1921. A decade later, in 1931, the first pure semiconductor was developed. At first, in 1950s, solar cells were utilized for space applications. In 1957, solar cells with around 8% of efficiency were developed, a record that was soon broken by Hoffman Electronics to a high of 10 and 14% in 1959 and 1960, respectively. Soon after, the first amorphous silicon PV cell was developed and the global PV capacity rose to 500 kW. This amount grew even further and reached a high of 21.3 MW in 1983. In about 20 years, in 2002, 175-kW high-concentrating PV plant was installed in Arizona, United States. Four years later, the world witnessed a new record of 40% efficiency for PV technology. With the increase in the global PV capacity to 100 GW in 2012, the manufacturing costs reduced significantly to $1.25 per watt. In 2016, the first solar-powered plane flew around the world [5]. Figure 3 depicts the PV power potential in the world.

Figure 3.

The world’s map of photovoltaic power potential [4].

Solar photovoltaic generation has broken the record of 156 GWh (23%) in 2020 to reach 821 GWh, which proved the second largest growth of all renewable technologies in 2020, slightly behind wind and ahead of hydropower. In China, the United States, and Vietnam, an unprecedented surge (a record of 134 GW) in PV capacity additions took place. Solar PV is undeniably becoming the lowest cost option for electricity generation all around the world and is expected to attract a vast amount of investment in the coming years [6].

Furthermore, solar energy is predicted to play a key role in the future global energy system owing to the scale of the solar resource. The installed solar photovoltaic (PV) throughout the world exceeded 1 TW at the beginning of 2022. This brought the world into the era of TW-scale PV [7]. This will definitely be fortified by the rapid expansion of PV industry as well as everyday cost decreases. The world envisions a future with nearly 10 TW of PV by 2030 and 30–70 TW by 2050, which can provide a majority of global energy [8].

3.2 Solar cells

The term “solar cell” was previously mentioned in the history of photovoltaics. In fact, solar cell is attributed to any device that directly converts the energy of light into electrical energy through the photovoltaic effect. The vast majority of solar cells are fabricated from silicon with rising efficiency and decreasing cost as the materials range from amorphous (non-crystalline) to polycrystalline and monocrystalline (single crystal) silicon forms. Solar cells, in comparison with batteries or fuel cells, do not utilize chemical reactions or require fuel to produce electricity, and, compared with electric generators, they do not have any moving parts [9].

As previously mentioned, solar cells are usually categorized into four main classes including the following:

(1) Monocrystalline solar cells that are also known as single crystalline cells and are very easy to identify due to their dark black color. They are made from a very pure form of silicon that has made them become the most efficient material for the process of sunlight conversion into electricity, (2) polycrystalline cells (multi-silicon cells) that were the first solar cells to be developed in the industry in the beginning of 1980s, (3) amorphous solar cells that, as the word “amorphous” meaning “shapeless” suggests, are not structured or crystallized on a molecular level and were commonly used for small-scale applications, and (4) thin film solar cells that are manufactured by placing several thin layers of photovoltaic on top of each other to create the module [10].

Another important concept in this area is named “the spectral response.” A solar cell’s spectral response to light of a single wavelength is its response at that specific wavelength multiplied by the intensity of the light. If the actual irradiance and device spectral response profiles are symmetrical around the center wavelength, then the currents generated from light on each side of the center are equal, and their sum is equivalent to the current that the device would generate if illuminated by a single-wavelength source of the same intensity (see Figure 4) [12].

Figure 4.

The solar spectral irradiance at air mass 0 (AM0) and global air mass 1.5 (AM1.5G) and the cutoff wavelength of semiconductor materials for common solar cell applications [11].

On the other hand, efficiency is the most commonly used parameter to compare the performance of one solar cell to another. It is defined as the ratio of the output energy from a solar cell to the input energy from the sun. Moreover, the efficiency also depends on the spectrum and intensity of the sunlight and the temperature of the solar cell. Therefore, conditions under which efficiency is measured must be carefully controlled in order to properly compare the performance of one device to another. Terrestrial solar cells are measured under AM1.5 conditions and at an ambient temperature of 25°C. Whereas for space uses, solar cells are measured under AM0 conditions. A comprehensive report on the past, and recent and projected solar cell efficiency results is provided in Figure 5 [13].

Figure 5.

Solar cell efficiencies throughout the history [13].

3.3 Solar photovoltaic systems

Solar cells are arranged into large groupings, which are called solar arrays. These arrays, composed of thousands of solar cells, can be considered as central electric power stations which convert sunlight into electrical energy to be distributed to industrial, commercial, and residential consumption. On the other hand, in a smaller scale, the configuration is commonly referred to as solar modules, which are mostly installed by homeowners on their rooftops to replace their conventional electric supply. Solar modules are also used to provide electric power in many remote areas where conventional electric power sources are either unavailable or prohibitively expensive to install. Solar cells also provide power for most space installs, from communications and weather satellites to space stations owing to the fact that they do not have any moving parts; therefore, there is no need for maintenance or any fuels that would require replenishment. Solar cells (as will be discussed further ahead) have also been used in consumer products, such as electronic toys, calculators, and radios [9]. However, in a large-scale version, in solar PV plants (see Figure 6), thermal energy from the sun is utilized and further transformed into electrical energy using photovoltaic modules installed in an optimal configuration. The thermal energy is abundant, easy to access, and cheap. Another type of solar power plant (which does not seem to be as common as the previous type) is the concentrated solar power plant, which contains plenty of mirrors or lenses that are carefully placed in an organized way to concentrate on collected heat to one specific position, which is further utilized to supply power for a steam turbine that generates electricity [15].

Figure 6.

An PV power plant located in Hungary [14].

3.4 Current global status of photovoltaics

According to Feldman et al. [16], from 2010 to 2020, the addition to global PV capacity grew from 17 to 139 GW_DC in a way that the global PV installations reached 760 GW_DC at the end of 2020. In 2020, approximately 100 MW of concentrated solar power was added in China. At the end of that year, 57% of cumulative PV installations were in Asia, 22% were in Europe, and 15% were in the Americas. The United States is now the country with the second largest cumulative installed PV capacity.

Also, China, the United States, Japan, Germany, and India were the leading five markets in cumulative PV installations at the end of 2020. However, Vietnam, with more than 11 GW of installations in 2020, took India away from the top five for annual deployment.

According to IEA estimation, in 2020, PV was the main source of 3.7% of global electricity generation. Although the United States was a leading PV market, it was below the average and other leading markets concerning PV generation as a percentage of total country electricity generation, with 3.4%.

From Q1 2020 to Q1 2021, installations in China, the United States, and Germany increased from 35 to 45%, and specifically those in India rose 89% although analysts argued that India’s large increase was due to developers finishing delayed 2020 projects. Despite the growth in installations, it was not necessarily indicative of 2021 as a whole. A significant portion of deployment often comes toward the end of the year. Significant supply constraints, increased costs, and resurgent waves of the pandemic (particularly in India) might suppress installations, see Figure 7.

Figure 7.

Current and projected installations throughout the world [17].

Analysts also predict continued growth in annual global PV installations, with a median estimate of 209 GW_DC in 2022 and 231 GW_DC in 2023. China, Europe, the United States, and India are anticipated to involve in about two-thirds of global PV installations over this period. Analysts note that these projections come despite many projects in 2022 risking delay or cancelation because of increasing material and shipping costs, see Figure 8 [11]. Table 1 lists the top 14 PV power plants around the world.

Figure 8.

Installation growth from 2020 to 2021 [18].

Table 1.

The world largest solar power plants [19].

4. Trends and applications of photovoltaics

Photovoltaic technology has many applications to improve human life. To date, many applications of this technology have been utilized in industry as well as by ordinary users (see Figure 9). However, the advances in this technology do not certainly end with its current applications, and therefore, it sees many bright horizons ahead. Numerous examples of new applications of photovoltaic technology are as follows.

Figure 9.

Various examples of novel applications of photovoltaic technology are as the follows: (a) BAPV [20], (b) BIPV [21], (c) LSC PV [22], (d) VIPV [23], (e) solar street lights [24], (f) PV charging stations [25], (g) PV bike path [26], (h) PV windrail [27], (i) solar-powered pavement [28], (j) PV tree [29], (k) floating PV systems [30], and (l) agri-voltaic system [31].

4.1 Building-attached photovoltaic (BAPV)

BAPV is the classic arrangement of photovoltaic systems and solar cells mounted on the roofs or building surfaces. Although it is probable to be aesthetically problematic, this is to avoid any sort of shading as much as possible. Moreover, in this application, the installer is required to utilize several pieces of equipment such as those needed for the mounting system.

4.2 Building-integrated photovoltaic (BIPV)

In comparison with BAPV, BIPV is an innovative design in which solar cell and in general the photovoltaic system are integrated with the construction itself, either the façade (e.g., walls, windows) or the roofs as skylights, shingles, etc. [32]. Therefore, it is targeted to be practiced as a phase of aesthetics with any utilitarian views.

4.3 Luminescent solar concentrator photovoltaic (LSC PV)

LSC PV devices consist of transparent optical light guides typically made of a polymer or glass containing luminophores with one or more photovoltaic (PV) solar cells mounted on one or more edges and sometimes rear of the light guide [33, 34]. The sunlight is then intercepted by small photovoltaic cells and sequently converted into electricity. The main advantage of LSC is that it is capable of producing electricity even in low-light conditions and can be incorporated into architectural structures particularly as transparent elements. LSC’s outstanding versatility is undeniable since it can be integrated either with houses and buildings as a colored window, a leaf roof, a smart window, etc., or with urban facilities as a noise barrier, a parking shed, etc.

Recently, Aghaei et al. have developed attractive mosaic LSC PV devices made by miniaturizing cubical light guides and mounting bifacial solar cells to the edges of neighboring light guides, as well as optionally attaching monofacial PV to the bottom sides. These mosaic LSC PV devices could be applied to make solar energy ubiquitous to the urban setting where it requires making visually appealing devices that can function in the challenging lighting conditions found in cities. Thus, by developing such colorful, visually appealing mosaic LSC PV devices, one can accelerate the general acceptance of solar energy in the built environment, even with the modest efficiency devices [35].

4.4 Vehicle-integrated photovoltaic (VIPV)

Although photovoltaic technology is mostly utilized in grid-connected applications, a new application could be the integration of photovoltaic into battery electric vehicles, creating a VIPV. Photovoltaic can help recharging the vehicle battery, without being connected to a charging station. VIPV can make transport more sustainable and seems to be cost-efficient. Accompanied with a systematic appropriate installation, it can also appear in solar cars, buses, spacecrafts, boats, UAVs (unmanned aerial vehicles), trains, hybrid airships, AUVs (autonomous undersea vehicles), bikes, etc.

4.5 Solar street lights

Another impactful application of solar cells is to be installed especially on top of street or roadway lights as a power supply. The installment is usually accompanied with an oversized battery not only to power the system at nights or in low-light conditions but also to enable an autonomous performance in aforementioned conditions up to even five days [36]. Furthermore, the autonomy helps the system to remain needless to be constantly connected to the grid so that the whole system will be able to work properly for a specific period of time even off-grid.

4.6 Photovoltaic charging stations

Slightly different from VIPVs, charging stations are usually roof-mounted and much simpler in terms of power electronic devices. These stations, therefore, are lighter in weight and smaller in the area they occupy. As a result, the stations are often cheaper and easier to be maintained for a long time.

4.7 Photovoltaic bike path

A tremendous amount of sunlight scatters regularly all over the ground, which can be harvested with the aid of specifically designed machines, vehicles, and equipment. An example of these vehicles is a bike ridden along a solar-cell-covered path and is known as a PV bike path. The bike is designed in a way that the wheels are in a high level of friction with the path [37]. This might be the point of entry into the maximization of land utilization along with power generation.

4.8 Photovoltaic-integrated zero/low-energy buildings

Zero-energy buildings (ZEBs) generally refer to a category of buildings with very high energy performance, characterized by a very low or approximately zero annual energy requirement. The required amount of energy is entirely or significantly covered by renewable energy, including energy from renewable sources produced on-site or nearby [38]. This renewable energy can be comprised of PV systems being used to supply electric energy demand, etc.

4.9 Photovoltaic-powered air conditioners

Air conditioning has always been an important issue either in industrial, commercial, or residual consumption. The use of PV systems in this area can produce a noticeable reduction in energy costs and bring about economic benefits. Also, using a PV-powered air conditioner has proved to conserve nearly 67 and 77% of the grid energy in summer during the day and at night, respectively [39].

4.10 Photovoltaic windrail

Owing to the great flexibility of PV technology, PV windrails are a novel combination of both wind and solar energy. The device is mostly installed on the rooftop of a building to obtain the maximum speed of the blowing wind. The wind then heads toward a channel to generate electricity using turbine generators.

4.11 Photovoltaic trees

PV trees are one of the most intriguing applications, which can be achieved from PV systems. The problem of land shortage or even the urban aesthetics can cause the system to be lifted and mounted on top of steel stems, which physically resembles a tree with PV modules as the leaves on top. The system can bring numerous advantages from simply charging small handy gadgets to supplying the power needed for street lights and electric vehicles. The future of PV trees seems to be extremely promising.

4.12 Photovoltaic pavements

Pavements have covered 30–40% of the urban surface [40] and are considered as enormous potential for PV installments and energy generation through solar PV modules. Surprisingly, studies have shown that even the temperature on the surface of walkable PV pavements or cycling tracks is proved to be lower than that on conventional pavements.

4.13 Landscape-integrated photovoltaic

A novel idea in PV systems installation is to integrate the natural landscape with solar PV modules. The idea is developed in a way that bifacial PV modules can be utilized vertically to avoid using flat spacious surfaces, rooftops, etc., and therefore taking advantage of more installation space to increase the amount of power generated by the system.

4.14 Product-integrated photovoltaic (PIPV)

Integrated with several products, solar photovoltaic energy can be exploited to yield grid-independent, battery saving, and wasteless devices. Many calculators and watches have been utilizing solar energy for ages to supply power. Moreover, the area of usage can be (and even is in some cases) expanded to lamps, chargers, scales, etc., even though the design is definitely under development and progression for further utilizations [41].

4.15 Floating photovoltaic systems

In areas suffering from land limitation, the concept of floating photovoltaic systems and utilizing the area on the surface of water can be highly advantageous. The system is even capable of being integrated with hydropower electricity transmission system to end up a higher efficiency [30].

4.16 Submerged photovoltaic

Another novel idea that can be achieved through the flexibility of PV technology is to use the spacious area underwater to locate the PV arrays and to take advantage of the natural cooling system. One appropriate space is a swimming pool where the modules can be installed both on the edges and on the pool floor.

4.17 Agri-voltaic system

The term “Agri-voltaic system” refers to a combined production of photovoltaic power and agricultural products on the same area of land. Solar modules and crops share light and radiation so that modules that are located above part of the crops generate shade and create a kind of microclimate over the mentioned area. Therefore, the result will be more fresh products, less water requirement for the plants, and lower losses due to evaporation [42].

5. Summary

To wrap up, the world has long been transitioning from fossil fuels to renewables. Not only does this help preserve the environment, but it also brings about other benefits under the sustainable development goals (SDGs) by the international community, including the fight against poverty and hunger. Moreover, photovoltaic technology has been facilitating people’s lives for many years. Over 183 GW of photovoltaic systems was installed worldwide, which is nearly 40 GW more than 2020. Many countries, including China, the United States, and India, have paid special attention to photovoltaic technology to meet their electrical and thermal needs, both in industry and in the field of home consumption. As of 2020, cumulative installed solar power capacity in China that leads the whole world in this field had reached almost 253 GW. Also, as mentioned, with the increase in progress in the photovoltaic industry and also the daily increasing reduction of prices in this field, the solar resources will reach the level of Terawatt-scale in the coming years. Moreover, areas of photovoltaic use are transitioning from conventional to more advanced areas such as PV pavements, BIPV, agri-voltaic systems. Undoubtedly, due to the increasing advances in photovoltaic technology, its use in the future will be much wider and more common than today.