Physics Meets Materials Science

Solar panels operate based on the photoelectric effect, which involves the release of electrons from atoms due to photon interaction. This process is influenced by the band gap, which is the energy disparity between tightly bound electrons near the atomic nucleus (the valence band) and those that are more loosely bound (the conduction band). The predominant type of solar cell is silicon-based due to its advantageous properties: pure silicon has a band gap of 1.1 eV, making it ideal for harnessing sunlight, as demonstrated in research by William Shockley and Hans-Joachim Queisser. This band gap allows for the absorption of energy from certain photons, particularly those in the near-infrared spectrum, to free electrons. However, photons with higher energy, such as blue light, result in only partial absorption, leading to wasted energy. Consequently, the theoretical maximum efficiency for silicon-based solar cells is pegged at 30%. To enhance silicon's conductivity, it is treated with various chemical elements to form p-n junctions, which facilitates easier control of electron flow without impacting the band gap or light conversion efficiency.

This technology is not particularly recent; the first silicon-based photovoltaic cell was created in 1953 and achieved just 6% efficiency. Over the last seventy years, advancements have been made, and additional improvements are on the horizon.

In the UK, the estimated cost for solar panel installation ranges from £6000 to £8000. One factor contributing to this expense is the energy-intensive production process of silicon-based solar cells, which demands ultra-pure silicon and temperatures exceeding a thousand degrees Celsius. Since 2009, research has focused on a new material known as perovskite, which differs in its synthesis compared to silicon p-n junctions. This method employs chemistry to create the crystal structure with fewer processing steps, potentially leading to lower manufacturing costs for perovskite-based solar cells.

Currently, perovskite solar cells are still in development. The original material produced in 2009 had an efficiency of only 3.8%, but after six years of research, this increased to 22.1%. Presently, they achieve approximately 25% efficiency, comparable to silicon-based cells. However, the crystal structure of perovskites is more complex, meaning that one segment generates electrons while another creates a "hole." The perovskite crystal is positioned between layers that attract either the electron or the hole, facilitating current flow. Unfortunately, this intricate structure is also more prone to degradation than silicon. Under standard laboratory conditions (typically 25 degrees Celsius and a light intensity of 1000 Watts per square meter), the maximum operating time for a perovskite solar cell is around a thousand hours. This is significantly less than the 20 to 30 years associated with silicon-based panels, which also experience degradation over time. Most solar panels available today experience a decline in efficiency of about 0.5% to 1% per year of operation. While they continue to function beyond their 30-year design life, the loss in efficiency, combined with the deterioration of other components like weatherproof coverings and electrical circuits, usually necessitates their replacement.

Zero Waste?

In the UK, most solar panel providers claim their technology is recyclable, yet they offer little detail beyond that. Advocates often argue that, since solar panels consist of common elements found on the periodic table, they can be repurposed in numerous ways. However, other sources present a more complex reality. In the UK, suppliers are responsible for disposing of solar panels, and they are prohibited from sending them to landfills. Yet, the actual process of recycling a solar panel into a new one proves to be quite challenging. More often than not, after removing components like the aluminum frame, the silicon p-n junctions, and associated electronics, the remaining materials are ground down and redirected to other industries. I've heard that this crushed silicon and metallic elements can be integrated into concrete. While I appreciate that this material isn't ending up in landfills, it feels inappropriate to reduce such a specialized technology to mere cement filler.

One approach to enhancing the efficiency of solar cells is the use of multi-junction cells, which incorporate materials with varying band gaps, allowing different wavelengths of sunlight to be utilized more effectively for electron liberation. Another technique involves photon-multipliers, which produce multiple photons of red light upon absorbing a single photon of blue or green light. Since red light is responsible for freeing electrons from silicon, these advancements introduce additional complexity, making recycling more difficult. Although perovskite-based solar cells are not yet in production, efforts are already underway to devise effective recycling strategies for them. Perhaps in the future, these will be more easily recyclable compared to silicon-based technologies.

To Share or Be Self-Sufficient?

Some predictions indicate that the solar cells currently available should perform adequately in the UK, despite its reputation as a less sunny region. It would be logical to assume that increased sunlight correlates with more electricity generation; however, there is a trade-off, as solar panels tend to be less efficient at elevated temperatures. As the panels heat up, atomic vibrations increase, obstructing electron and hole flow. Power output is measured under standard test conditions, which set the ambient air temperature at 25 degrees Celsius (though the panels themselves may be hotter due to energy absorption) and can decrease by 0.5% for each degree increase. Given that temperatures in my area typically remain below this threshold, power output could potentially rise. However, it often remains cool due to overcast skies. Estimates for the UK’s "average" weather suggest I could generate approximately 3000 kWh of electricity annually, but what constitutes "average" is still unclear to me.

Interestingly, regions like Dubai and Qatar prove to be less effective for solar energy due to excessive heat and dust accumulation on panel surfaces, which diminishes light intensity. I find myself contemplating whether solar panels should be situated at the North and South Poles, where it’s cold but sunlight is available for half the year at each location. This would necessitate international cooperation to determine how to share this resource, and considering the recent geopolitical factors that have driven up electricity prices in the UK, this seems unlikely. Alternatively, communities could produce electricity in a manner that suits them best. The idea of having a small solar farm near my home, alongside the engineers who maintain it, is appealing. I believe I would appreciate my electricity supply more if I had a clearer understanding of its origin and upkeep. Furthermore, this arrangement would foster a sense of community as the energy could also benefit my neighbors. Currently, the most significant electricity consumption in my household likely comes from my plug-in hybrid electric car, which is usually absent during the day, making it impractical to charge it while my solar panels are generating energy. Instead, my neighbors might utilize the electricity to power their washing machines and other devices.

At the End of the Day...

At this juncture, I remain unconvinced about the viability of solar power. My electricity usage is significantly below average, so if I were to install solar panels for personal use, I would mostly be selling surplus electricity to the national grid at a minimal rate. The return on investment for installation and maintenance would take at least 27 years, and some calculators suggest I may never recoup the costs. To be honest, the maintenance appears burdensome, and given the energy-intensive manufacturing process coupled with the challenges of recycling them into equally valuable materials, they don't seem like a worthwhile investment. They may be advantageous for individuals with higher electricity expenses who could justify the initial costs, or for those less concerned about recycling high-value materials, or for those lacking reliable connections to a power grid. However, they do not appeal to me. I will wait for advancements that will make solar technology less wasteful and more accessible for everyone.

About This Story

This narrative stems from a discussion recorded as part of the podcast "Technically Speaking," which features conversations that scientists and engineers often have in the lab. These discussions blend scientific facts, imaginative speculation, and frequent film references. New episodes are released bi-weekly on platforms such as Apple, Spotify, Amazon Music, Google, Podbean, or wherever you listen to podcasts.

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