SOLAR ENERGY SYSTEMS
1. Context
India’s imports of solar photovoltaic (PV) cells from China jumped 141 per cent, seemingly driven by the increase in domestic solar PV module manufacturing capacity.
2. Electricity generation
- Electricity can primarily be generated through two key methods. The first is electromagnetic induction, a principle discovered by Michael Faraday in 1821, which became commercially applicable around 1890. Even today, this method remains central to most of the world's electricity generation.
- The second approach involves photovoltaic (PV) technology, which relies on semiconductor materials like elemental silicon to convert sunlight directly into electricity. The photovoltaic effect was first observed by Alexander Becquerel in 1839.
- However, it wasn't until 1954 that a functional and efficient solar cell was developed at Bell Laboratories by Chapin, Fuller, and Pearson using doped silicon.
- This achievement was made possible thanks to two pivotal scientific contributions: Albert Einstein’s Nobel Prize-winning explanation of the photoelectric effect, and Jan Czochralski’s method for producing single-crystal silicon, which remains the foundation for most PV cells today.
- Unlike PV systems that feed regulated, taxable electricity into national power grids, technologies such as solar water heaters, solar air heaters, and solar-based cooling systems usually operate independently.
- For example, solar cooling uses an absorption refrigeration process that can cool interiors to as low as 19°C even when the ambient temperature hits 40°C. These standalone technologies are similar to PV panels used in off-grid regions, typically for purposes like charging batteries and powering basic lighting systems.
- Globally, solar insolation—the measure of solar energy received—varies significantly by region. Though solar energy is plentiful, it is scattered over wide areas, making it less concentrated.
- To harness it effectively, various focusing technologies such as parabolic troughs, Fresnel lenses, and other solar concentrators are employed for tasks ranging from cooking and water desalination to thermal heating and electricity production
3. How does sunlight generate power?
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- Photovoltaic (PV) cells are typically composed of semiconducting materials like elemental silicon. Unlike metals such as copper, which are Ohmic conductors (their electrical resistance increases with temperature), silicon behaves as a non-Ohmic material. At room temperature, silicon is a poor conductor, but its conductivity improves as it gets warmer.
- From a quantum mechanical perspective, electrical conduction occurs when electrons occupy a higher energy level known as the conduction band, where they can move freely, similar to how water flows in the ocean. Electrons that remain in the valence band, a lower-energy state, are immobile and do not contribute to electrical current.
- To move an electron from the valence band to the conduction band, energy must be supplied. This energy can come from thermal excitation (increased atomic motion at higher temperatures) or from other energy sources such as light.
- Light, depending on the experiment, behaves either as a wave or as individual energy packets called photons. When photons strike electrons in the valence band, they can transfer energy to the electrons, allowing them to rise to the conduction band—if the photon’s energy matches or exceeds the required energy gap.
- This energy gap, known as the band gap and measured in electron volts (eV), must be matched precisely by the photon’s energy for the transition to occur. If a photon has more energy than necessary, the excess is converted to heat, which not only leads to energy loss but can also cause electrons to escape.
- Besides the energy requirement, there's also a symmetry condition for these transitions, though it plays a lesser role in this context. Due to these constraints, silicon-based PV cells cannot utilize the entire solar spectrum efficiently—around 50.4% of the sunlight is unusable. About 20.2% of photons lack sufficient energy to initiate the transition, while 30.2% carry excess energy that’s lost as heat.
- Other semiconducting materials like gallium arsenide, cadmium telluride, and copper indium selenide can absorb different parts of the solar spectrum more effectively. However, their widespread use is restricted by challenges such as limited availability, environmental hazards, and handling complexities
4. Silicon Solar Cells
- In silicon-based photovoltaic (PV) cells, trace amounts of phosphorus and boron are intentionally introduced to create regions that either have an excess of electrons or a shortage of them (known as "holes").
- This results in the formation of a p-n junction, where the difference in electrical charge establishes an electric field. When sunlight hits the surface, this setup acts like a battery, generating an electric potential that drives current.
- When an external circuit is connected, electrons travel from the negatively charged side through the load toward the positively charged side, completing the circuit. This process can continue as long as the cell is exposed to light.
- However, even within the 49.6% of the solar spectrum that is usable, several energy losses still occur. For example, PV cells often heat up to 30–40°C above the surrounding temperature, and this radiative heat loss accounts for around 7% of energy loss.
- An additional 10% loss is caused by differences in the mobility of positive and negative charges, a phenomenon known as the saturation effect, which reduces the generated voltage over time.
- These factors contribute to a theoretical efficiency ceiling of 33.7% for single-junction silicon PV cells, a value known as the Shockley-Queisser limit. Moreover, practical inefficiencies such as non-uniform sunlight exposure and manufacturing inconsistencies between cells (which cause differences in open-circuit voltage) also reduce performance.
- When real-world losses like converting DC to AC and managing peak power output are considered, the actual efficiency of silicon-based crystalline PV cells averages about 25% in laboratory settings, while the best commercial models achieve roughly 20% efficiency. By comparison, natural photosynthesis captures only 3–6% of the sunlight it receives
5. What can solar energy and cannot do?
- Because natural silicon reflects a significant amount of light, photovoltaic (PV) cells are coated with a transparent anti-reflective layer, typically made from tin oxide or silicon nitride, which also gives them their distinctive blue hue.
- Unlike biological photosystems, which assemble proteins using minimal energy at ambient temperatures, PV technology demands high energy inputs during manufacturing.
- The production of PV cells starts with the Czochralski process, which purifies elemental silicon to about 99% purity by melting and slowly forming it into single-crystal ingots. When these ingots are sliced into thin wafers, roughly 20% of the material is lost as silicon dust.
- Due to the high costs associated with single-crystal silicon, alternative techniques have been developed — for instance, ribbon technology avoids sawing losses, while amorphous silicon cells are more economical. Their natural structural imperfections can be corrected by adding hydrogen, improving performance.
- To capture a wider range of the solar spectrum, multijunction amorphous silicon cells have been engineered. These can theoretically reach efficiencies as high as 42%, though real-world performance typically peaks around 24%. PV technologies are broadly grouped into three generations:
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First-generation uses thick crystalline wafers (~200 µm),
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Second-generation relies on thin-film wafers (1–10 µm), and
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Third-generation includes advanced designs like multijunction tandem cells and quantum dots, which can generate more charge carriers per photon, potentially surpassing the Shockley–Queisser efficiency limit.
- The cost of PV-generated electricity has declined sharply—from $4–5 per watt in 2010 to about $2.8 per watt in 2023 (and as low as $1.27 per watt for utility-scale systems), nearly achieving the U.S. SunShot Initiative’s goal of $1 per watt. Breaking down system costs:
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Modules account for 38%,
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Inverters and other electronics for 8%,
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Wiring and mounting contribute 22%, and
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The remaining 33% is spent on balance-of-system (BoS) costs, including labour, permits, administrative expenses, and profit margins.
- Since single-crystal silicon cells are close to their theoretical peak, future cost reductions are most likely in the BoS components. In terms of durability, PV systems lose efficiency at a rate of about 0.5% per year, with most modules lasting 20 to 25 years.
- Interestingly, although tropical and desert climates receive higher solar irradiance, PV modules perform more efficiently in cool, clear conditions due to reduced thermal losses. This makes it challenging for low- and middle-income countries — many of which lie in tropical zones — to fully leverage PV systems, especially given infrastructural limitations and climatic constraints.
- Additionally, air pollution can block 2–11% of solar radiation, and dust accumulation (soiling) can cause an extra 3–4% loss annually.
- Cleaning solar panels is both risky and water-intensive, since the panels become electrically active under sunlight. In densely populated urban areas, PV systems may also intensify the urban heat island effect by trapping heat.
- While other solar technologies can offer some complementary benefits, the extent to which PV systems alone can support a fully carbon-neutral energy future remains a topic of active scientific debate
6. Government supported schemes
Jawaharlal Nehru National Solar Mission (JNNSM) – 2010
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Launched under the National Action Plan on Climate Change (NAPCC).
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Aimed to establish India as a global leader in solar energy.
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Target revised to 100 GW of solar capacity by 2022 (out of 175 GW total renewable energy target).
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Focus on both grid-connected and off-grid solar power systems.
PM-KUSUM (Pradhan Mantri Kisan Urja Suraksha evam Utthaan Mahabhiyan) – 2019
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Promotes the use of solar energy in the agriculture sector.
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Three components:
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Component A: Solar power plants (up to 2 MW) on barren land.
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Component B: Standalone solar-powered agricultural pumps.
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Component C: Solarisation of existing grid-connected pumps
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For Prelims: General issues on Environmental ecology, Bio-diversity & climate change For Mains: GS-III: Conservation, environmental pollution and degradation, environmental impact assessment. |
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Previous Year Questions
1.Consider the following statements: (2016)
Which of the statements given above is/are correct? (a) 1 only Answer (a)
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Source: Indianexpress
