Solar Energy: Photovoltaic Cells and Collectors Explained

The Sun pours about 1361 watts onto every square metre held perpendicular to its rays at the top of the atmosphere, a value called the solar constant. After atmospheric losses, roughly 1000 W/m² reaches the ground on a clear day at noon, an enormous free flow of energy waiting to be harvested.

Two technologies harvest it. Photovoltaic (PV) cells convert sunlight directly into electricity through the photovoltaic effect, while solar collectors capture it as heat for water heating and cooking. This article explains both, since they solve different problems with the same sunshine.

Key Concepts

• Photovoltaic effect: photons striking a pn junction knock electrons loose, and the junction's built-in field sweeps them into a current.
• PV cell/module/panel: a single silicon cell gives about 0.5 V; cells are wired into modules and panels for useful voltages and powers.
• Efficiency (η): the fraction of incident light converted to electricity; commercial panels manage roughly 15 to 22%.
• Irradiance (G): solar power per unit area in W/m²; standard test conditions use G = 1000 W/m².
• Flat-plate collector: a black absorber under glass that heats water in tubes; simpler and cheaper than PV for pure heating.
• Concentrating collector: mirrors or lenses focus sunlight onto a small receiver for high-temperature heat or power.

Formulas

Panel output: P = η A G, where P is electrical power in watts, η the efficiency, A the panel area in m² and G the irradiance in W/m². Efficiency can be written η = P_out / (G A). Daily energy: E = P × (peak sun hours), giving watt-hours per day. Assumptions: the panel faces the Sun squarely, G is taken as 1000 W/m² at standard test conditions, and losses from dust, wiring and the inverter are ignored in the basic estimate.

Worked Example

A rooftop panel has area A = 2.0 m² and efficiency η = 18% (0.18). Under standard irradiance G = 1000 W/m², with 5 peak sun hours per day, find its power and daily energy.

1. Power: P = η A G = 0.18 × 2.0 × 1000.
2. Multiply: 0.18 × 2.0 = 0.36, and 0.36 × 1000 = 360 W.
3. Daily energy: E = 360 W × 5 h = 1800 Wh = 1.8 kWh per day.
4. Over a 30-day month that is 54 kWh, enough to cover a large share of a modest household's lighting and fan load.
5. Reality check: dust, heat and inverter losses trim this by 10 to 20%, so installers oversize arrays accordingly.

Common Misconceptions

• Panels want light, not heat. A PV cell's efficiency actually falls as it gets hotter; cool sunny days outperform scorching ones.
• They work in cloudy weather too. Diffuse light still generates power, just less; only night stops them entirely.
• Collectors and PV cells are different tools. Heating water with PV electricity wastes most of the energy; a thermal collector does the same job far more cheaply.

Real-World Applications

• Rooftop and solar-farm PV plants feed homes and national grids.
• Solar water heaters and solar cookers use collectors for direct heat.
• Calculators, streetlights and remote telecom towers run on small PV modules with batteries.
• Solar pumps irrigate fields where grid power is unreliable or absent.

Solar energy arrives as a free 1000 W/m² stream on clear days; photovoltaic cells turn part of it into electricity at P = η A G, while collectors turn it into heat far more cheaply where heat is all that is needed. Match the technology to the job, and sunshine becomes power.