AP® Environmental Science review sheet from Aim for Five (aimforfive.com/enviro/units/6/6-8)
Unit 6 · Topic 6.8
6.8 Solar Energy
Solar energy can be turned directly into electricity with photovoltaic cells, concentrated to make steam, or captured as heat for buildings and water. It produces no pollution while running, but output depends on the sun, panels require mined materials, and large solar farms take up land.
Key terms
- photovoltaic (PV) cell
- active solar
- passive solar
- concentrated solar power
Photovoltaic (PV) cells
A photovoltaic cell is a thin layer of semiconductor material, usually silicon. When sunlight strikes it, the light's energy knocks electrons loose, and the cell's structure pushes them in one direction, creating an electric current. No fuel burns and nothing moves. Cells are wired into panels, and panels into arrays on rooftops or in large solar farms.
PV panels make direct current (DC), so an inverter converts it to the alternating current (AC) used in homes and the grid. Typical panels turn about 15 to 22 percent of the sunlight that hits them into electricity.
Active and passive solar heating
Active solar heating uses pumps or fans to move a heated fluid. A common example is a rooftop solar water heater: a dark collector absorbs sunlight, water or antifreeze flowing through it heats up, and a pump moves it to a storage tank.
Passive solar design uses the building itself to collect, store and spread heat, with no pumps or fans. In the Northern Hemisphere that means large south-facing windows to let in winter sun, thick walls or floors of stone, brick or concrete (thermal mass) that soak up heat by day and release it at night, and roof overhangs that shade windows from the high summer sun but let in the low winter sun.
Concentrated solar power
Concentrated solar power (CSP) uses mirrors to focus sunlight onto a small area. In a power tower, thousands of mirrors aim at a receiver on top of a tower; in other designs, curved troughs focus light on a pipe. The concentrated heat warms a fluid that makes steam to spin a turbine. Some CSP plants store heat in molten salt so they can keep generating after sunset. CSP needs strong, direct sunlight, so it is built in deserts.
Advantages and disadvantages
| Advantages | Disadvantages |
|---|---|
| No air pollution or CO₂ while generating | Output stops at night and drops on cloudy days |
| Fuel (sunlight) is free and renewable | Storing energy for later needs batteries or other storage |
| Rooftop panels use space already built on | Large solar farms cover land and can disturb desert habitats and species |
| Low operating cost; panels last decades | Panels need mined metals and energy to manufacture, and old panels must be disposed of or recycled |
| Can power remote places with no grid | Concentrated beams at CSP towers can burn birds that fly through them |
Worked examples
Try each one yourself first, then open the solution.
- Example 1Calculator allowed
Payback period for rooftop solar
A family installs solar panels that cost 18,000 dollars. A tax credit covers 30% of the cost. The panels save the family 1,500 dollars per year on electricity. How many years until the savings pay back the family's cost?
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- Step 1: Find the tax credit: 30% of 18,000 dollars = 0.30 × 18,000 dollars = 5,400 dollars.
- Step 2: Family's actual cost: 18,000 dollars − 5,400 dollars = 12,600 dollars.
- Step 3: Payback period = cost ÷ annual savings = 12,600 dollars ÷ 1,500 dollars per year = 8.4 years.
Answer: About 8.4 years.
- Example 2
Trap: passive versus active solar
Classify each as active or passive solar: (a) a house with large south-facing windows and a thick concrete floor; (b) a rooftop collector with a pump that circulates water to a tank; (c) a roof overhang sized to block summer sun.
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- Step 1: Ask: does the system use a pump or fan to move heat? If yes, it is active. If the building's design alone does the work, it is passive.
- Step 2: (a) Windows and thermal mass, no moving parts: passive.
- Step 3: (b) A pump moves the heated water: active.
- Step 4: (c) An overhang is a fixed design feature: passive. Photovoltaic panels that make electricity are usually treated as their own category, though some textbooks group them with active solar because they rely on electrical equipment.
Answer: (a) Passive. (b) Active. (c) Passive.
Common mistakes
- Mixing up PV with solar heating. PV turns light directly into electricity; active and passive solar heating capture the sun's heat for buildings and water.
- Saying solar has no environmental impact. Manufacturing, mining, land use and disposal all have costs, even though operation is clean.
- Forgetting the hemisphere: passive solar windows face south in the Northern Hemisphere but north in the Southern Hemisphere.
On the exam
- Payback-period and cost-savings calculations are common. Subtract rebates or credits first, then divide by the annual savings.
- When asked for a disadvantage of a large solar farm, habitat disruption in deserts and intermittency are specific, accepted answers.
Connected topics
Videos
Check yourself
4 questions on 6.8 Solar Energy. Pick an answer to see if you got it, and why.
A homeowner adds large south-facing windows and a thick stone floor that soaks up sunlight by day and releases heat at night. Compared with installing rooftop photovoltaic panels, this approach
Which is an environmental drawback of building a very large solar farm in a desert?
A home's rooftop photovoltaic system is rated at 5 kW. On average, it produces as much as it would in 4.5 hours of full sun each day. About how much electricity does it make in a year?
A concentrated solar power plant uses thousands of mirrors aimed at a central tower. How does it differ from a photovoltaic solar farm?
0 of 4 answered