Renewable and Non-Renewable Energy Sources

Lessons

Senior 3 · Physics

Renewable and Non-Renewable Energy Sources

4 lessons

Lesson 2 of 4

Renewable Energy Sources

25 min

A light that the sun pays for

In many Rwandan villages, a small solar panel on the roof now charges a battery that lights the house at night, runs a radio and charges a phone. According to the National Institute of Statistics of Rwanda, 22% of households used solar power in 2023–24, and the share of households using electricity for lighting rose from 34% in 2016–17 to 72% (NISR, EICV7 Utilities and Amenities report). The sunlight that reaches the panel tomorrow will be as strong as today's, whatever we use. That is what makes solar energy renewable.

Definition

A renewable energy source is one that is replaced by natural processes as fast as, or faster than, we use it, so it will not run out. Examples: sunlight, flowing water, wind, geothermal heat and biomass that is regrown.

Biomass is material from living or recently living things used as fuel: firewood, charcoal, crop waste and dung, and the biogas (mostly methane) that bacteria produce from waste. Biomass is renewable only if trees and crops are regrown at least as fast as they are used.

Experiment 2: how should a solar panel face the Sun?

You need: a small solar cell (from a garden light or a science kit), an ammeter reading in mA, two wires, a protractor, and sunshine or a strong lamp.

Connect the solar cell to the ammeter.
Hold the cell so that its surface faces the light directly (angle $\theta = 0^\circ$ between the light and the normal). Record the current.
Tilt the cell so that $\theta = 30^\circ$ , 60° and 80°, and record the current each time.
Cover half of the cell with card and record the current again.

Figure 2.1: The normal is the line at 90° to the cell's surface. When $\theta = 0^\circ$ the light hits the cell face-on.

Angle $\theta$	0°	30°	60°	80°	0°, half covered
Current (mA)	60	52	30	10	31

These are illustrative results. The current is largest when the light strikes the cell face-on and falls as the cell is tilted away, because the same area of cell then catches less of the light. Covering half the cell roughly halves the output. That is why panels are pointed towards the Sun, kept clean and placed where nothing shades them.

Important

Safety. Never look directly at the Sun, even for a moment: it can damage your eyes permanently. If you use a lamp, do not touch the bulb, which gets very hot, and keep its cable away from water.

The main renewable sources

Source and how it is used	Advantages	Disadvantages and effects on the environment	In Rwanda (source)
Solar: solar (PV) cells change light into electricity; solar water heaters warm water	free, never runs out; no smoke or noise when working; works off-grid in remote villages	no output at night and less in cloudy weather, so batteries are needed; batteries and old panels must be recycled safely; large plants use land	on-grid plants at Rwamagana (8.5 MW), Nasho (3.3 MW) and Jali (0.25 MW) (REG); 22% of households use solar (NISR, 2023–24)
Hydroelectric: falling water turns turbines	cheap to run; reliable; the reservoir stores energy; dams can also control floods	output falls in the dry season; dams are costly to build, flood land and can move people from their homes; they change rivers for fish	Ntaruka, Mukungwa and Nyabarongo I and many small plants: 109.8 MW in all, plus the shared Rusumo plant (REG LCPDP 2024 Review)
Wind: wind turns the blades of a turbine	free; no fuel and no smoke	needs steady, strong winds; noisy; can harm birds; output varies	most of Rwanda is not very suitable; the Eastern Province and the Gisenyi area show the most promise (REG)
Biomass: wood and charcoal burned for cooking; biogas from dung and waste	cheap and available locally; biogas also gives fertiliser and gets rid of waste	cutting trees faster than they grow causes deforestation and soil erosion; smoke from open fires harms health	firewood is the main cooking fuel of 92.5% of rural households (NISR, 2023–24)
Geothermal: steam from hot rocks deep underground turns turbines	works day and night, in every season	only near volcanic areas or faults; exploration drilling is costly and may find nothing	not yet proven: wells drilled at Karisimbi found no usable resource, and studies continue at Gisenyi and Bugarama (REG)

Figure 2.2: A fixed-dome biogas digester. With no air inside, bacteria break down dung and other waste and give off biogas, which is mainly methane. The gas pushes out through the pipe to the stove, and the used slurry makes good fertiliser.

Rwanda's biogas programme builds on the fact that many families keep cows; REG notes that the "One cow per poor family" scheme supports it (REG, Biomass).

Worked example 1: the output of a solar panel

A solar panel has an area of $1.6\ \mathrm{m^2}$ and an efficiency of 20%. At midday, sunlight delivers $1000\ \mathrm{W\,m^{-2}}$ to it. Find (a) its electrical power; (b) the energy it gives in a day with 5 peak sun hours (the figure REG gives for Rwanda).

\text{(a)}\quad P_{\text{in}} = 1000 \times 1.6 = 1600\ \text{W}

P_{\text{out}} = 0.20 \times 1600 = 320\ \text{W}

\text{(b)}\quad E = 320\ \text{W} \times 5\ \text{h} = 1600\ \text{Wh} = 1.6\ \text{kWh}

Worked example 2: is a small solar kit enough?

A family's solar home kit has a 50 W panel and gets 5 peak sun hours a day. Each evening they use three 5 W LED bulbs for 4 hours, a 10 W radio for 3 hours and 10 Wh to charge phones. Is the kit big enough?

E_{\text{panel}} = 50 \times 5 = 250\ \text{Wh per day}

E_{\text{used}} = 3 \times 5 \times 4 + 10 \times 3 + 10 = 60 + 30 + 10 = 100\ \text{Wh per day}

Yes, with room to spare for cloudy days, as long as the battery can store at least 100 Wh.

Worked example 3: comparing a hydro plant and a solar plant

The capacity factor of a power plant is its average output divided by its installed capacity. REG's 2024 plan gives Nyabarongo I (hydro, 28 MW) a capacity factor of 68.54% and the Rwamagana solar plant (8.5 MW) 17.08%. Find the average output of each and the energy each gives in a year (8760 hours).

\text{Nyabarongo I: } 28 \times 0.6854 = 19.2\ \text{MW}

19.2 \times 8760 \approx 168\,000\ \text{MWh}

\text{Rwamagana: } 8.5 \times 0.1708 = 1.45\ \text{MW}

1.45 \times 8760 \approx 12\,700\ \text{MWh}

The solar plant produces only in daylight, so its average output is much less than its capacity.

Warning

"Renewable" does not mean "no effect on the environment". Dams flood valleys, solar panels and batteries contain materials that must be recycled, and firewood is renewable only if the trees are replanted. Charcoal made from forests that are not replanted is not a renewable fuel at all.

Tip

Where does it come from? Almost every renewable source is the Sun at work: sunlight directly (solar), the Sun lifting water as rain (hydro), the Sun heating air unevenly (wind), and plants growing in sunlight (biomass). Geothermal energy, from the Earth's hot interior, is the exception.

How do solar panels work? (TED-Ed, Richard Komp). Watch how light knocks electrons loose in the silicon of a solar cell to make a current.

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Practice

Define a renewable energy source and give four examples.
Give two advantages and two disadvantages of hydroelectric power.
A solar panel of area $2.0\ \mathrm{m^2}$ and efficiency 18% receives $800\ \mathrm{W\,m^{-2}}$ . Find its power output.
What area of 20% efficient panels is needed to give 1.0 MW in sunlight of $1000\ \mathrm{W\,m^{-2}}$ ?
Under what condition is firewood a renewable source? What happens to the land when it is not?
Explain how a biogas digester works and give two benefits of using one on a farm.
In Experiment 2, why does the current fall as the angle $\theta$ increases?
A district is choosing between a small hydro plant on a river and a solar plant. Give one reason for each choice, and say what extra equipment each one would need to supply power at night in the dry season.

Show answersHide answers

A source replaced by natural processes as fast as it is used, so it does not run out: e.g. solar, hydro, wind, geothermal, replanted biomass.
Advantages: cheap to run, reliable, stores energy, flood control. Disadvantages: low output in dry seasons, costly dams, flooded land, people moved, fish affected.
$P = 0.18 \times 800 \times 2.0 = 288\ \text{W} \approx 290\ \text{W}$
Each square metre gives $0.20 \times 1000 = 200\ \text{W}$ , so area $= \frac{1.0 \times 10^{6}}{200} = 5000\ \mathrm{m^2}$ .
Only if trees are replanted at least as fast as they are cut. Otherwise forests shrink, soil erodes and the land dries out.
Dung and water enter an airtight digester, where bacteria break them down and release biogas (mostly methane), which is piped to a stove. Benefits: clean cooking fuel, less firewood cut, less smoke, and the slurry is a fertiliser.
When tilted, the cell presents a smaller area to the light, so it catches less light energy each second and produces a smaller current.
Hydro: works day and night and can store water; but its flow drops in the dry season, so it needs a big enough reservoir. Solar: sunny all year and quick to build; but it produces nothing at night, so it needs batteries (or another source) for night-time supply.

Key points

Renewable sources are replaced naturally as fast as we use them: solar, hydro, wind, geothermal and regrown biomass.
Each has advantages and disadvantages; none is free of environmental effects.
A solar panel's output depends on its area, its efficiency, the strength of the light and the angle at which the light strikes it.
Rwanda uses hydro, solar and biomass; wind and geothermal are still being studied.

Sources and credits

7.9: World Energy Use, from College Physics (1st edition) by Paul Peter Urone and Roger Hinrichs, OpenStax, hosted by LibreTexts, licensed under CC BY 4.0. Consulted for the physics; no text was copied.
Rwandan facts: Rwanda Energy Group, Rwanda Least Cost Power Development Plan, 2024 Review (September 2025), and the REG pages Solar, Geothermal and Biomass; National Institute of Statistics of Rwanda, EICV7 Thematic Report: Utilities and Amenities 2023–24.
Photo: Cyirabo Primary School solar installation.jpg by Joachim Hauschopp, Wikimedia Commons, released under CC0.
Photo: Biogas used in a stove in a school in Rwanda by P. Feiereisen, SuSanA Secretariat, Wikimedia Commons, licensed under CC BY 2.0.
Video: How do solar panels work? - Richard Komp, TED-Ed, embedded from YouTube.

Written by Ganzaa: the lesson text, the solar-cell experiment and its illustrative results, the diagrams, the table, worked examples and practice exercises.

1A light that the sun pays for

2Experiment 2: how should a solar panel face the Sun?

3The main renewable sources

A light that the sun pays for

Experiment 2: how should a solar panel face the Sun?

The main renewable sources