📖 Chapter 3 of 12 · Green Energy - Advance 2 · Class 9–10
Chapter 3: How Far Can Solar and Wind Still Go?
In this chapter
- The physics limit of a single-junction solar cell.
- Tandem cells, bifacial modules and trackers.
- Sharing land: agrivoltaics, floating solar and offshore wind.
- Meet Devika from Gujarat and Fatima from Punjab.
The Big Idea
A single-junction silicon cell can never convert more than about 33% of sunlight into electricity — the Shockley–Queisser limit. Photons with too little energy pass straight through; those with too much waste the excess as heat. To go further, engineers stack cells that absorb different colours of light: a tandem. Perovskite-on-silicon tandems have already passed 30% in laboratories.
Did You Know?
Agrivoltaics means growing crops under or between raised solar panels. The panels shade the soil and reduce water loss, while the crops keep the ground cooler, which helps the panels. Indian research farms are testing which crops grow best in this partial shade.
Story Time — Devika’s Two Harvests
Devika, age 15 · Anand, Gujarat
Devika’s family farms near Anand. When a research team proposed raising solar panels three metres above one of their plots, her grandfather objected: “Panels on farmland? We grow food here, not electricity.”
The team suggested a trial on half a hectare. The panels were mounted high and spaced apart, so tractors could pass and sunlight still reached the soil in moving stripes.
Devika kept the records. The shaded soil lost noticeably less water, so irrigation dropped. Leafy vegetables and fodder grasses did well; a sun-hungry crop in the deepest shade did worse. “It is not free,” she noted. “We trade some crop yield for a lot of electricity.”
Her grandfather’s objection softened when the first bill arrived: the farm now sold power as well as vegetables. “Two harvests from one field,” he admitted, “but only if the panels are high enough and far enough apart.”
At her school exhibition, Devika summed it up in one line: “Agrivoltaics is not about putting panels on farms. It is about designing a farm and a power plant together.”
Story Time — Fatima Stacks the Colours
Fatima, age 16 · Mohali, Punjab
Fatima won a summer internship at a materials laboratory in Mohali. On her first day, a researcher handed her two small cells: one ordinary silicon, one with a shimmering dark coating.
“Silicon is excellent at absorbing red and infrared light,” the researcher explained, “but blue photons carry more energy than silicon can use, and the extra becomes heat. That waste is a large part of why we are stuck near 33% for a single junction.”
The shimmering layer was a perovskite — a crystal that absorbs blue and green light well. Stacked on top of silicon, it takes the high-energy photons first and lets the rest pass through to the silicon below. “Two cells, two jobs. In the lab, tandems like this have crossed 30%.”
Fatima asked the obvious question: “Then why is my school roof still plain silicon?” The researcher laughed. “Because perovskites are sensitive to moisture and heat. A panel must survive 25 years on a roof in Punjab, through monsoon and 45°C summers. Making them stable is the real research problem, not making them efficient.”
Fatima wrote in her notebook: “In the laboratory, efficiency wins prizes. On a rooftop, durability wins customers.”
The Shockley–Queisser Limit and Tandem Cells
Sunlight is a mixture of colours, each photon carrying a different energy. A semiconductor has a band gap: photons below it pass through unused, and photons above it are absorbed but waste their surplus energy as heat. Balancing these two losses gives a maximum of about 33% for one junction under normal sunlight.
- Tandem: a wide-band-gap cell (perovskite) on top for blue light, silicon below for red and infrared.
- Bifacial modules also collect light reflected onto their back surface, adding roughly 5–15% on bright ground.
- Trackers turn panels to follow the Sun, raising annual output by around 15–25% at extra cost and maintenance.
- Durability matters more than records: a module must keep working for 25 years in heat, dust and humidity.
Sharing Land and Sea
India adds huge amounts of solar and wind each year, and land is becoming the limiting factor. Engineers therefore look for sites that do two jobs at once.
- Agrivoltaics: raised, widely spaced panels above crops; less irrigation, some yield trade-off.
- Floating solar: panels on reservoirs stay cooler and reduce evaporation.
- Offshore wind: stronger, steadier sea winds and very large turbines, at higher cost; India is preparing its first projects off Gujarat and Tamil Nadu.
- Rooftops and canals: already-built surfaces need no new land at all.
🔢 Numbers Corner
Band gap and photons: a photon’s energy E = hc/λ. Silicon’s band gap of about 1.1 eV matches infrared light near 1,100 nm — so silicon uses red light well, but wastes much of the energy in blue light.
Myth or Fact?
Myth: Efficiency is the only number that matters when choosing a solar panel.
Fact: Cost per unit of energy over 25 years matters more. A slightly less efficient panel that is cheaper, more durable and better suited to local heat can deliver more value.
🔬 Try it in the Green Energy Lab
Experiment with these ideas in Lab 1: Cost of a Unit (LCOE) Explorer.
Solved Examples
Work it out first, then check the answer.
1. What is the Shockley–Queisser limit, and why does it exist?
Show answer
Answer: It is the roughly 33% maximum efficiency of a single-junction cell. Photons below the band gap are not absorbed, and energy above the band gap is lost as heat.
2. How does a tandem cell get past that limit?
Show answer
Answer: It stacks two materials with different band gaps. The top cell captures high-energy photons; lower-energy photons pass through to the bottom cell.
3. Why are perovskites not yet common on rooftops?
Show answer
Answer: They degrade with moisture, heat and light over time. Long-term stability, not peak efficiency, is the current research challenge.
4. A 500 W bifacial module gains 10% from rear-side light. What is its effective output?
Show answer
Answer: 500 × 1.10 = 550 W under those conditions.
5. Give one advantage and one trade-off of agrivoltaics.
Show answer
Answer: Advantage: the same land yields crops and electricity, and shading reduces irrigation. Trade-off: some crops yield less in partial shade, and the raised structure costs more.
Check Your Understanding
A. Fill in the blanks
- The maximum efficiency of a single-junction cell is about %.
- A cell that absorbs blue light and sits above silicon is made of a .
- Modules that also use light on their rear side are called .
- Growing crops under raised solar panels is called .
- The energy gap a semiconductor needs to free an electron is its gap.
B. True or False
- Tandem cells have been measured above 30% in laboratories. ( ___ )
- Trackers reduce annual energy output. ( ___ )
- Floating solar panels run cooler than rooftop panels. ( ___ )
- Perovskite durability is already a solved problem. ( ___ )
C. Choose the correct option
- Photons with energy below the band gap are (a) absorbed fully (b) not absorbed (c) turned into X-rays
- Bifacial gain on bright ground is typically (a) 5–15% (b) 50–60% (c) 90%
- India’s first offshore wind projects are planned off (a) Gujarat and Tamil Nadu (b) Bihar and Delhi (c) Ladakh
D. Answer in detail
- Explain, with the idea of band gap, why a single material cannot use all sunlight efficiently.
- Argue for or against covering farmland with solar panels, using Devika’s data.
- Why did Fatima conclude that durability beats efficiency for real rooftops?
Brain Buster
Hold a prism or CD in sunlight and spread the spectrum. Which colours would a perovskite top cell take, and which would reach the silicon?
Check your understanding
Tap an answer to see if it is correct. This is just for practice — nothing is saved.
1. The Shockley–Queisser limit is about…
- 13%
- 33%
- 73%
Show answer
B. 33%
2. A tandem cell works by…
- Stacking materials with different band gaps
- Spinning faster
- Using bigger wires
Show answer
A. Stacking materials with different band gaps
3. Why are perovskites not yet widespread?
- They are too efficient
- They degrade with heat and moisture
- They are illegal
Show answer
B. They degrade with heat and moisture
4. Agrivoltaics means…
- Panels raised above crops
- Panels on the sea
- Panels in space
Show answer
A. Panels raised above crops
5. Bifacial modules gain extra output from…
- Light reflected onto their back
- Higher temperature
- Stronger wind
Show answer
A. Light reflected onto their back