Comparison of the main sources of energy.

Energy and Control Systems – Comparison of the Main Sources of Energy

This note follows the Cambridge International AS & A Level Design & Technology (9705) syllabus – Topic 11 – Energy and Control Systems. It is written to support:

• AO1 – knowledge of energy forms and sources
• AO2 – communication of technical information (tables, diagrams, symbols)
• AO3 – application of knowledge to design decisions (e.g., selecting a power source)
• AO4 – evaluation of safety, cost and environmental impact

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1. Forms of Energy

Energy can be stored or transferred in several inter‑convertible forms. The syllabus requires the following seven forms to be recognised.

Form of Energy	Short Definition	Design‑relevant Example
Kinetic	Energy of motion  (½ mv²)	Rotating flywheel in a power‑tool, moving vehicle
Potential (gravitational)	Energy stored by position in a gravity field  (mgh)	Elevator counter‑weight, water stored in a dam
Elastic (stored)	Energy stored in a deformed solid  (½ kx²)	Compressed spring in a mouse‑click mechanism
Thermal	Energy associated with temperature (sensible + latent heat)	Hot water in a domestic boiler, waste‑heat recovery
Chemical	Energy stored in chemical bonds	Petrol in an internal‑combustion engine, Li‑ion battery electrolytes
Electrical	Energy carried by moving charge  (V I t)	Power supplied to a micro‑controller, mains supply
Radiant (solar)	Electromagnetic energy transmitted as photons	Solar PV panel, solar‑thermal collector

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2. Key Comparison Criteria for Energy Sources

When choosing a power source for a product, designers evaluate the following eight criteria (the syllabus wording is reproduced in italics):

• Energy density – amount of usable energy per unit mass (MJ kg⁻¹) or volume (MJ L⁻¹). Higher density usually means smaller, lighter storage.
• Availability & reliability – how continuously the source can be accessed at the required location.
• Cost – capital cost of the generation/storage system and operating cost (expressed as $/kWh).
• Environmental impact – CO₂ and other greenhouse gases, water use, land use, waste, and ecosystem effects.
• Conversion efficiency – proportion of input energy that becomes useful output (see §4).
• Control & responsiveness – how quickly the output can be varied or stopped.
• Safety & handling – hazards during storage, transport, operation and disposal.
• Typical applications – common product or system categories where the source is used.

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3. Comparison of the Main Energy Sources

Source	Energy Density (MJ kg⁻¹ or MJ L⁻¹)	Typical Conversion Efficiency (%)	Cost ($ kWh⁻¹)	CO₂ Emissions (g kWh⁻¹)	Control / Responsiveness	Safety & Handling Issues	Typical Applications
Coal (solid)	≈ 24 MJ kg⁻¹	30–40 (steam turbine)	0.05–0.10	800–1000	Steady, high‑output; limited rapid throttling	Dust, ash, CO₂, mining hazards, fire risk	Large‑scale power stations, industrial heating
Oil (diesel/gasoline)	≈ 42 MJ kg⁻¹ (≈ 35 MJ L⁻¹)	35–45 (internal‑combustion)	0.07–0.12	600–800	Very good – rapid throttle response	Spill, flammability, vapour explosion, exhaust pollutants	Transport, portable generators, small‑scale heat
Natural Gas (CH₄)	≈ 55 MJ kg⁻¹ (≈ 35 MJ m³⁻¹)	45–55 (combined‑cycle)	0.04–0.08	350–500	Very high – quick start‑up, load‑following	Leakage, explosion, asphyxiation, low‑level CO₂	Combined‑heat‑power, domestic heating, power peaking plants
Nuclear (U‑235 fission)	≈ 80 000 MJ kg⁻¹ (≈ 2 × 10⁶ MJ L⁻¹)	33–37 (steam cycle)	0.10–0.12	≈ 12 (life‑cycle)	Medium – power set by control‑rod position; load‑following possible	Radiation, radioactive waste, severe accident risk, heavy shielding	Base‑load electricity generation
Solar Photovoltaic (PV)	≈ 0.1 MJ m⁻² day⁻¹ of incident sunlight (≈ 0.04 kWh m⁻² day⁻¹)	15–22 (panel to AC)	0.12–0.20	0 (operational)	Low – output follows irradiance; can be buffered with batteries	Electrical shock, panel breakage, fire from faulty wiring	Building‑integrated power, portable chargers, off‑grid lighting
Wind (on‑shore)	≈ 0.2 MJ m⁻³ air⁻¹ (energy in moving air)	30–45 (turbine + generator)	0.08–0.15	0 (operational)	Medium – depends on wind speed; pitch control gives some rapid response	Noise, blade failure, ice throw, lightning strikes	Utility‑scale farms, offshore turbines, small‑scale rooftop turbines
Hydroelectric (dam)	≈ 0.3 MJ m⁻³ water⁻¹ (potential + kinetic)	40–60 (turbine‑generator)	0.03–0.07	0 (operational)	High – water flow can be regulated quickly	Flooding, ecosystem disruption, dam‑break risk	Large dams, run‑of‑river schemes, pumped‑storage
Biomass (solid)	≈ 15–20 MJ kg⁻¹ (dry wood)	20–30 (combustion‑steam)	0.06–0.12	200–400	Medium – limited by feedstock logistics	Particulate emissions, ash handling, fire hazard	Rural heating, combined‑heat‑power, bio‑fuel production
Geothermal (dry steam)	≈ 0.5 MJ m⁻³ steam⁻¹ (high‑temperature reservoir)	10–20 (direct heat) – up to 35 % for binary plants	0.05–0.10	0 (operational)	High – steady heat source, can be modulated with flow control	Drilling hazards, mineral scaling, possible induced seismicity	District heating, binary‑cycle power plants

Data compiled from IEA World Energy Outlook 2024, IAEA Nuclear Energy Series, and manufacturer datasheets. Typical ranges are shown; actual values vary with technology and site conditions.¹

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4. Energy Conversion & Transmission

Products rarely use the primary form of energy directly; it must be converted to the form required by the load. Three representative conversion chains are shown below.

4.1 Internal‑Combustion Engine (ICE)

1. Chemical → Thermal: Fuel combustion releases heat.
2. Thermal → Mechanical: Expanding gases drive pistons.
3. Mechanical → Electrical (optional): A generator coupled to the crankshaft produces electricity.

Typical overall efficiency for a small gasoline engine is 20–25 % because of exhaust and cooling losses.

4.2 Solar Photovoltaic (PV) System

1. Radiant → Electrical: Photons generate electron–hole pairs (photovoltaic effect).
2. Electrical → AC: An inverter converts the DC output to AC for the grid or appliances.

Losses arise from spectral mismatch (~5 %), wiring resistance (~2 %), and inverter conversion (85–95 %). Overall system efficiency is therefore 15–22 %.

4.3 Battery‑to‑Motor (Electric Tool)

1. Chemical → Electrical: Discharge of a Li‑ion cell releases stored electrochemical energy.
2. Electrical → Mechanical: A brushless DC motor converts electric power to rotary motion.
3. Mechanical → Useful Work: The motor drives a drill chuck, screwdriver bit, or propeller.

Typical efficiencies are 90 % for the battery, 85–95 % for the motor controller, and 80–90 % for the motor itself, giving an overall chain efficiency of 60–70 %.

4.4 Generic Conversion Block Diagram

4.5 Efficiency Formula

For any conversion stage:

$$\eta = \frac{P_{\text{out}}}{P_{\text{in}}}\times 100\%$$

where P_out is the useful power delivered to the load and P_in is the power supplied by the preceding source. Thermodynamic limits (e.g., Carnot efficiency) explain why heat‑based conversions cannot exceed ~60 % for typical temperature differences.

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5. Basic Control‑System Concepts

Designers regulate energy flow using feedback control. The simplest model contains four blocks (see Figure 3).

• Set‑point (input) – Desired value (e.g., temperature 80 °C).
• Sensor (feedback) – Measures the actual output (e.g., thermistor).
• Controller – Compares set‑point with feedback and generates a control signal. In many A‑Level projects the controller is a PID (Proportional‑Integral‑Derivative) unit:
  • Proportional – reacts to the magnitude of the error.
  • Integral – eliminates steady‑state error.
  • Derivative – anticipates future error, improving stability.
• Actuator – Device that modifies the energy flow (e.g., heater element, valve, motor driver).

Digital vs. Analogue Controllers

• Analogue – continuous voltage or current signals; simple, fast, but limited flexibility.
• Digital (micro‑controller) – uses ADCs, software PID, and can implement complex logic, data logging, and communication protocols.

Example: A thermostat‑controlled electric heater uses a digital PID to maintain a set temperature, turning the heating element on/off or adjusting its duty cycle via a solid‑state relay.

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6. Application to Product Design – Case Study

Scenario: Design a portable power tool (cordless drill) that must deliver 500 W for up to 30 minutes.

1. Energy demand: 500 W × 0.5 h = 250 Wh ≈ 0.9 MJ.
2. Candidate power sources:
   • Li‑ion battery (chemical → electrical)
   • Petrol‑engine generator (chemical → thermal → mechanical → electrical)
3. Comparison using the syllabus criteria:

Criterion	Li‑ion Battery	Petrol‑engine Generator
Energy density	≈ 0.9 MJ kg⁻¹ (≈ 250 Wh kg⁻¹)	≈ 42 MJ kg⁻¹ (fuel) but heavy engine & fuel tank
Weight for 250 Wh	≈ 0.3 kg (plus housing)	≈ 2 kg (engine + 0.2 kg fuel)
Conversion efficiency	≈ 90 % (battery discharge)	≈ 25 % (engine‑generator)
Cost (per unit)	$30–$50	$120–$150 (engine + fuel system)
Environmental impact	Low operational CO₂; manufacturing impact moderate	≈ 600 g CO₂ kWh⁻¹ (fuel combustion)
Control & responsiveness	Instantaneous power control via electronic speed controller	Throttle response slower; mechanical linkage required
Safety	Electrical shock, thermal runaway if damaged	Fuel spill, fire, exhaust gases, noise
Typical applications	Hand‑held power tools, drones, portable electronics	Construction sites without mains, remote‑area generators

**Conclusion (AO3)** – For a handheld drill the dominant criteria are energy density, efficiency, weight, and safety; the Li‑ion solution outperforms the petrol generator on all four, justifying its selection for the design brief.

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7. Safety, Handling & Environmental Issues (Brief Overview)

• Fire & explosion risk – high for hydrocarbons, low for electricity (but shock risk).
• Noise & vibration – significant for diesel engines, wind turbines.
• Radioactive waste – unique to nuclear, requires long‑term management.
• Resource depletion – fossil fuels vs. renewable sources.
• End‑of‑life disposal – batteries need recycling; solar panels contain hazardous metals.

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8. Emerging Technologies (Snapshot)

Technology	Potential Advantage	Design Relevance
Solid‑state batteries	Higher energy density, improved safety	Longer runtimes for portable tools, lighter weight
Hydrogen fuel cells	Zero‑emission electricity, fast refuelling	Alternative to batteries for high‑power, long‑duration devices
Advanced small‑scale wind turbines	Higher tip‑speed ratios, quieter operation	Off‑grid power for remote construction sites
Thermal‑energy storage (e.g., molten salt)	Enables 24 h renewable generation	Integration with solar‑thermal or concentrated‑solar systems

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References

1. International Energy Agency (IEA), World Energy Outlook 2024; International Atomic Energy Agency (IAEA), Nuclear Energy Series; manufacturer technical datasheets (2023‑2024). Values are typical ranges; students should cite the specific source used for any exam answer.