A Brief History of Solar Energy – II
In our previous article, we discussed how humanity has harnessed solar energy throughout history, transforming it into a valuable resource. We explored the first steps of developing PV solar systems, the basic principles, and their integration into modern energy systems. In this second part, we will delve into recent advancements, novel applications, and the potential offered by materials science for the near future. But first, let’s examine the theoretical potential.
In 1954, at Bell Laboratories in the United States, Chapin, Fuller, and Pearson produced solar cells utilising silicon transistors, opening doors to a technology that could harness unlimited solar energy for civilization. According to MIT physics professor Washington Taylor, our planet receives 173,000 terawatts of solar energy every moment -approximately 10,000 times the total energy consumption of humanity. Thus, in theory, solar energy is an infinite and renewable resource. However, significant engineering challenges hinder our ability to use the sun to meet our entire energy demand. Some experts suggest that covering 10% of Earth’s deserts (land unsuitable for other purposes) with solar thermal systems could provide a regular energy output of 15 terawatts, sufficient to meet projected energy demands for the next 50 years. Yet, generating global energy needs from a single point and transmitting it everywhere using cables may pose challenges. Think of transmission line issues or localised climate changes. While mega-projects might appeal to our ambition, some experts advocate for numerous localised facilities to ensure sustainability.
Irrespective of the energy production approach, storing the generated energy remains a major obstacle for utilising renewable sources like solar and wind. While fossil fuels can be stored in various forms for later use, the same principle does not apply to renewable energy sources. Batteries and accumulators used for their storage are expensive, relatively short-lived, and not environmental friendly once they reach the end of their lives. One method that has recently gained popularity among scientists involves using excess energy to pump water to high points (reservoirs, dams) while energy generation continues during day hours with enough sunlight and then, subsequently releasing it to generate kinetic energy and transforming this into electricity via turbines. Producing hydrogen from solar energy (green hydrogen) and storing it in fuel cells is another promising approach. In 2017, a Swedish team discovered another method for solar energy storage, combining solar PV and solar thermal techniques to capture solar energy in chemical bonds, forming a stable liquid that can be converted into heat energy on demand. While the upper part of the prototype panel produces and stores energy in this way, the lower part heats water to produce thermal energy for use. With this approach that combines different methods, the panel is claimed to have the ability to convert 80% of the sunlight falling on it into energy.
Since the discovery of solar panels with silicon transistors, the efficiency of PV panels has steadily increased while costs have declined. The production efficiency, starting at 6% in 1954, now approaches 30% in market products. In June 2022, solar cell efficiency in laboratory experiments reached 47.6% (Fraunhofer Institute for Solar Energy Systems – ISE). Costs have plummeted from $100 per 1 Watt in 1954 to $0.5 in 2024. Panel cells continue to become smaller and thinner, technologies advance, and new materials are discovered to ease their integration into daily life. Titles such as “the world’s largest or most efficient solar system” now change hands annually.
Silicon solar panels come in two main types: monocrystalline and polycrystalline. Monocrystalline cells boast higher efficiency and are smaller in size, but they come with a higher price tag. On the other hand, polycrystalline cells are less efficient but are more cost-effective to produce. Due to their delicate nature, both types require protection within a sturdy enclosure typically made of metal and glass, which is the familiar sight of solar panels.
However, advancements in technology have led to the development of silicon-based panels known as heterojunction. Unlike traditional panels, heterojunction panels utilise multiple layers sandwiched together, allowing them to capture and “harness” various wavelengths of light more efficiently. While standard silicon panels rely on a single layer of semiconductor material, heterojunction technology incorporates a layer of crystalline silicon between thin layers of amorphous silicon, enhancing their performance.
In recent years, solar cells manufactured as thin films have also made it into the commercial market. These flexible solar cells, capable of adhering to various surfaces, exhibit lower efficiency compared to rigid panels. Nevertheless, their durability and lightweight nature make them ideal for applications in as caravans, boats, and scenarios with modest electricity demands. Additionally, they are more affordable than traditional solar panels. However, a drawback lies in their composition, often containing rare or toxic materials such as cadmium telluride (CdTe), copper indium gallium selenide (CGIS), or gallium arsenide (GaAs).
Apart from flexible panels, there has been an emergence of solar panels that can be applied onto surfaces like paint. This technology, though not entirely novel, gained attention in 2012 when a study at the University of Southern California demonstrated a method to put together solar nanocrystals in a way that enables them to conduct electrical current, thus making it possible to apply them on diverse materials. Despite its potential applicability on flexible surfaces like plastic, its cost suggests a prolonged timeline until it becomes a part of our daily lives.
Solar cells fashioned into roof tiles have been available for some time. Several manufacturers, including Elon Musk’s Tesla, have introduced these tiles into the market. They have gained popularity due to their ease of installation, minimal requirement for structural adjustment or enforcement on existing roofs, and aesthetic appeal. Although made from similar semiconductor materials as thin-film panels, their energy efficiency are often lower. While currently pricier than standard panels, they are anticipated to become more affordable over time.
Another recent significant breakthrough in renewable energy studies is silicon-perovskite tandem panel technology. Perovskite is a compound discovered in the Ural Mountains in 1839, consisting of calcium, titanium and oxygen, and named after the Russian mineral expert Lev Perovski. In 2006, Japanese scientists discovered that some types of this compound exhibit semiconducting properties. Thus, the material, known for its affordability and lower purification requirements compared to silicon, has garnered attention as a potential raw material for PV panels. However, its shorter lifespan necessitates research into enhancing durability.
Perovskite cells absorb different wavelengths of light compared to silicon cells. When utilised together in tandem configurations, these cells can harness a broader spectrum of light, enhancing energy production and decreasing cost per cell. This technology, known as perovskite-silicon tandem cells, has surpassed the 30% efficiency threshold of silicon-based ones. Notably, China’s LONGi reported 33.5% efficiency last June. Numerous organizations and companies in Europe and the US are actively exploring methods to further improve the efficiency and affordability of tandem technology.
A recent study, published this month, unveils a newly discovered organic polymer capable of yielding elastic solar cells. Conducted at the South Korean Institute of Advanced Science and Technology, the research aims to pioneer wearable energy technologies. Although the concept of flexible solar cells is not novel, Korean scientists assert that this is the most efficient elastic solar technology to date, boasting ten times the flexibility of its counterparts and achieving an impressive energy conversion efficiency of 19%.
In addition to novel materials like perovskite making their way into solar panels, there are also strategies to enhance the efficiency of existing silicon technology. One such strategy involves the implementation of bifacial, or double-sided panels. Unlike standard solar panels that only convert sunlight hitting the top surface into electricity, bifacial panels harness additional light reflected from the ground (or the roof), effectively increasing energy generation. The biggest advantage in an average of 3-10% increase in efficiency gain, making them particularly preferable for installation on white roofs or in snow-covered areas where light reflection is high.
According to the International Energy Agency (IEA), solar PV currently accounts for 4.5% of global electricity generation. While this may seem modest, it marks a significant 26% increase from 2021 to 2022, signalling a promising trajectory for the proliferation of PV energy systems in the years to come. So, where can we use the panels produced with all these new technologies and materials?
This is the question that has been keeping the minds of energy experts busy. It is possible to utilise solar panels to cover rooftops and buildings thanks to technologies that offer enhanced efficiency, flexibility, and paint-like applicability. Generating energy at the point of consumption holds promise in the long run, potentially obviating the need for extensive transmission lines. However, this alone cannot serve as the ultimate solution. Critical considerations such as the location of large-scale plants, their environmental impact, and the transformation of existing infrastructure are also at the forefront of discussions. Unfortunately, there are instances of healthy forests or agricultural land being sacrificed for the construction of solar farms. While idle land unsuitable for agricultural purposes is often favoured for large installations, these are often distant from power transmission lines or areas of energy consumption.
The concept of solar-powered highways has also garnered both acclaim and critique in recent years. Envisioned as solar cells embedded beneath tempered glass pavements, these highways purportedly mitigate the detrimental environmental effects associated with traditional asphalt pavements while concurrently generating clean energy. Additionally, proponents suggest that these solar cells could power LED-lit road markings, prevent road freezing in winter through built-in heaters, and require minimal maintenance over a 20-year lifespan. Even though these all sound sweet, several criticisms have been enunciated. Efficient operation of the panel cells require optimal sun exposure, with the correct angle, and with no shade. These are not easy to ensure on highways. Furthermore, there is need to address the cleaning issue due to frequent exposure to dust, soil, and debris. Criticisms also extend to the potential degradation of glass coverings over time, leading to diminished panel efficiency. Nonetheless, several solar highway projects have been implemented worldwide, with notable examples including one-kilometre-long installations in France in 2016 and in China in 2018, serving as live trials for this innovative technology.
Another promising approach is floatovoltaics, which involves installing solar panels on bodies of water. This concept extends beyond natural lakes, seas, and oceans to include installations on dams, wastewater treatment plants, and water canals. The primary advantages of floatovoltaics lie in its ability to utilise water surfaces without encroaching on land that could serve other purposes. Additionally, the cooling effect of water can enhance panel efficiency by 5-10%, while also reducing evaporation in freshwater reserves. However, careful consideration must be given to avoid negative impacts on aquatic life by maintaining adequate solar transmittance. Furthermore, the development of appropriate anchoring techniques remains essential, rendering floatovoltaics currently a more costly option compared to traditional panel placement practices at present.
Although covering valuable agricultural land with solar panels is generally discouraged, agrivoltaics presents a viable alternative under certain conditions. This approach involves integrating solar panels into agricultural settings, offering shade that may benefit certain crop varieties and may potentially reduce the need for irrigation. Moreover, both small and large livestock can graze beneath the panels. In the United States alone, there are 314 agrivoltaic plants with a total capacity of 2.8 Gigawatts, many of which combine livestock grazing, pollinator habitat creation, and crop production.
The concept of building a massive solar plant in the Sahara Desert capable of powering the entire planet has long captured the imagination of many. While theoretically possible and logical, caution is warranted in relying solely on such a solution. We may as well not yet have the technology to efficiently operate and maintain such a vast plant, while logistical hurdles in transporting energy globally and mitigating losses may pose additional challenges. Moreover, it is often not possible to predict the extent of damage frequent sandstorms in the desert may vause. Thus, while enticing, this idea is likely to remain a futuristic aspiration for the time being.
Speaking of large-scale facilities, the solar power generation plant in the Karapınar district of Konya province, Türkiye, stands as one of the world’s largest. With over 3 million panels, the plant produces 3 million kW/h of energy annually, meeting the needs of 2 million people.
However, when it comes to solar energy, not everything is sunny and bright. Even if everyone agrees that it is necessary to get rid of fossil fuels and switch to renewable sources that reduce carbon emissions in order to limit or reduce global warming, one must keep in mind potential dangers posed by solar energy systems. We already talked about possible mistakes in land use and that suitable lands should be preferred for this purpose.
However, when it comes to solar energy, not everything is sunny and bright. Even if everyone agrees that it is necessary to get rid of fossil fuels and switch to renewable sources that reduce carbon emissions in order to limit or reduce global warming, one must keep in mind potential dangers posed by solar energy systems. We already talked about possible mistakes in land use and that suitable lands should be preferred for this purpose.
An important issue is the minerals used in panel production, as well as the fossil fuel consumption, carbon emissions, and environmental pollution that may occur during the extraction of these minerals and the production of panels themselves. Although solar energy itself is carbon-neutral, panel production depends on fossil fuel consumption. The silicon pieces used in the production of monocrystalline panels are first turned into blocks, then cut into small pieces, and glued to the panels. This process produces the largest emissions during panel production. The process is slightly different for polycrystalline panels, and relatively less energy is consumed during the process. The carbon footprint of thin-film panels is even lower, but since they contain amorphous silicon, cadmium, copper, and similar heavy metals, they contain toxic substances that can cause serious environmental pollution.
However, these should not be an obstacle to switching to solar energy. Compared to solar panels, the footprint of thermal (coal) power plants is 18 times larger, and the footprint of other thermal (natural gas) facilities is 13 times larger. And we already know that the panels do not cause any emissions after they are produced. Therefore, there is no solid reason to not prefer solar energy over fossil fuels.
Waste issue is another concern.
Silicon solar panels contain some amount of recoverable precious metals, but the majority of the panels consist of glass and aluminium, which are not very valuable materials for recycling. Therefore, before the panels become waste, they need to be disassembled and disposed of or recycled appropriately, which is a challenging undertaking. In addition, some heavy metals in some panels are toxic pollutants, which can pose a challenge to the recycling programs of various governments. According to the International Renewable Energy Agency (IRENA), we will have 78 million tons of waste solar panels by 2050.
The lifespan of standard panels is generally given as 25-30 years. However, their energy conversion efficiency decreases by an average of 1% every year. Therefore, 10 years after installing a new system, the efficiency of your solar system decreases. Meanwhile, more efficient and cheaper panels produced with new technologies and materials appear on the market. Users make an assessment and decide that it is more logical to switch to a new panel system, and often remove their panels before reaching the end of their lives to replace them with new ones. Therefore, the calculated panel life of 25-30 years and the associated waste calculations become no longer valid. A study claims that by 2035, the total number of panels that have become waste will be 2.56 times the number of panels in use.
It is crucial to start such recycling programs early, with early measures, properly functioning waste collection and recycling systems, appropriate incentives, and similar circular economy approaches.
Let’s end our article with another solar technology that sounds like a science-fiction scenario, but has actually fascinated scientists for a long time and has begun to be tested: Space-Based Solar Power (SBSP).
The main idea is to send the energy harvested from solar panels in space to Earth wirelessly and convert it back into electricity on Earth. In fact, the idea dates back to 1923 -when Russian scientist Konstantin Tsiolkovsky proposed placing giant mirrors in orbit and focusing the sun’s rays on Earth. In 1941, famous scientist and science-fiction writer Isaac Asimov, in his short story “Reason,” imagined a facility that would capture microwave signals sent from space to Earth and convert them into electricity. Indeed, there is no obstacle at the theoretical and technological level. Solar panels placed in orbit work 10 times more efficiently since the Sun’s rays do not have to pass through the atmosphere and lose some of their energy. However, to build a system on a useful scale, it is necessary to send a series of panels at least one kilometre wide into orbit and assemble them there. Of course, the system on Earth to receive this energy must be ten times larger. This is a bit of an expensive undertaking, and thus has not yet been implemented. Recently, with the increase in private initiatives, going to space has become cheaper, and scientists have begun to reconsider this idea. A solar space facility on the planned scale will possibly produce 2 Gigawatts of energy, equivalent to a nuclear power plant. Producing the same amount of energy on Earth would require 6 million solar panels.
These dreams are now one step closer to reality with the Space Solar Power Demonstrator (SSPD-1) program.
MAPLE, which was sent into space as part of this program in January 2023, carries a panel system that is only 1.8 meters wide and has 32 different types of low-cost solar cells on it. It is also equipped with electronic circuits that convert solar energy into microwaves and two transmitters. After carrying out various tests for about a year, it was able to successfully transfer energy in space to a receiver on itself on March 3, 2024. Even though it may not seem like a big step for now, let’s not forget that every success in the realm of science begins with small steps.
It seems that investments, research, and experiments in solar energy, the pioneer of renewable energy, in space or on Earth, will continue rapidly. Technologies will become cheaper, and we will begin to encounter more panels every day. Unless we do this, we will have to bear the indisputable consequences of the climate crisis even sooner than we expect. Therefore, we must wonder, read, examine, learn our options, and start making the right choices for both ourselves and our planet.
REFERENCES
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