Cambridge Syllabus Notes

Energy & Control Systems – A‑Level Design & Technology (9705)

Designers must understand how energy is sourced, stored, transformed, transmitted and controlled in order to create efficient, safe and sustainable products. This note follows the Cambridge Topic 11 requirements and links each concept directly to design practice.

1. Primary Energy Sources

Energy for a product can be drawn from primary sources. They are classified as fossil‑fuel (finite) or renewable (essentially inexhaustible). The table summarises the most common sources used in design and technology projects.

Source	Type	Typical Energy Form(s) Produced	Key Characteristics for Designers	Common D‑Level Applications
Coal	Fossil‑fuel	Thermal → Mechanical (via steam turbine)	High CO₂, abundant, low cost, bulky storage; high particulate emissions.	Industrial heating, large‑scale power generation.
Oil / Diesel	Fossil‑fuel	Chemical → Thermal → Mechanical	Very high energy density, liquid handling, emissions of CO₂ & NOₓ.	Internal‑combustion engines, portable generators.
Natural Gas	Fossil‑fuel	Chemical → Thermal	Cleaner combustion than coal/oil, requires pressurised storage, methane slip.	Gas‑powered tools, combined‑heat‑power units.
Solar (photovoltaic)	Renewable	Electrical	Zero‑emission, intermittent, requires panels, wiring, MPPT controller.	Solar‑powered chargers, garden lighting.
Solar (thermal / radiant)	Renewable	Thermal (radiant)	Direct heat, high conversion efficiency for water heating, needs collectors.	Solar water heaters, solar ovens.
Wind	Renewable	Mechanical (rotational) → Electrical	Variable speed, requires gearbox or direct‑drive generator, site‑specific.	Small wind turbines for remote sites.
Hydro (small‑scale)	Renewable	Mechanical → Electrical	Steady flow, low environmental impact, requires suitable head and flow.	Micro‑hydro generators for off‑grid cabins.
Geothermal	Renewable	Thermal	Constant temperature, high installation cost, low surface impact.	Heat‑pump systems, ground‑source heating.

1.1 Comparative Overview (Environmental, Economic & Social Factors)

Factor	Fossil‑fuel Sources	Renewable Sources
Environmental impact	CO₂ & greenhouse gases, air pollutants, habitat disruption from extraction.	Low‑carbon, minimal emissions; impacts linked to land use (e.g., wind farms) or water use (hydro).
Economic considerations	Generally low upfront cost, price volatile with market demand.	Higher capital cost, low operating cost, often eligible for subsidies or feed‑in tariffs.
Social & cultural aspects	Established infrastructure, employment in mining/processing.	Community acceptance varies (e.g., visual impact of turbines), can provide local energy independence.

2. Energy Storage Options

Although not a primary source, storage is essential for many D‑Level projects because it decouples supply from demand.

Storage Type	Energy Form Stored	Typical Energy Density (J kg⁻¹)	Key Design Considerations
Batteries (Li‑ion, NiMH, Lead‑acid)	Chemical	0.5–1.5 × 10⁶	Voltage, capacity, self‑discharge, safety (thermal runaway), weight.
Super‑capacitors	Electrostatic	0.01–0.05 × 10⁶	Very high power, low energy, short charge cycles, limited voltage.
Flywheels	Kinetic (rotational)	0.02–0.1 × 10⁶	Low loss for short‑term storage, requires bearings, safety enclosure.
Compressed Air	Mechanical (pressure)	0.01–0.03 × 10⁶	Tank strength, heat loss during compression/expansion, moisture control.
Thermal (sensible/latent heat)	Thermal	0.001–0.01 × 10⁶	Insulation, material phase‑change properties, charging time.

3. Forms of Energy – Definitions, Equations & Design Relevance

Form	Symbol / Typical Variable	Energy Equation(s)	Power Equation(s)	Design Relevance (Why a Designer Cares)
Kinetic (translational)	\(E_k\)	\(E_k=\dfrac12 mv^{2}\)	\(P = Fv = \tau\omega\) (if rotating)	Determines forces on moving parts, sizing of flywheels, safety of rotating machinery.
Gravitational Potential	\(E_g\)	\(E_g=mgh\)	\(P = mgv\)	Used in lifts, hoists, weight‑based energy‑storage devices; influences structural design.
Elastic (Spring) Potential	\(E_s\)	\(E_s=\dfrac12 kx^{2}\)	\(P = kxv\)	Critical for spring‑loaded mechanisms, shock absorbers, tensioners.
Thermal (sensible)	\(Q\)	\(Q=mc\Delta T\)	\(P = \dot m c \Delta T\)	Guides material selection for heat‑resistant parts, cooling system design, loss calculations.
Radiant / Solar (photonic)	\(E_r\)	\(E_r = I A t\) (where \(I\) = irradiance, \(A\) = area)	\(P = I A\)	Important for solar‑thermal collectors, photovoltaic sizing, shading analysis.
Electrical	\(E\) or \(P\)	\(E = Pt = V I t\)	\(P = VI = I^{2}R = \dfrac{V^{2}}{R}\)	Determines wiring sizes, battery capacity, motor rating, and safety protection.
Chemical	\(\Delta H\)	\(\Delta H = \sum H_{\text{products}} - \sum H_{\text{reactants}}\)	Often expressed as \(\text{energy density} = \dfrac{\Delta H}{\text{mass}}\)	Impacts choice of power source (batteries, fuel cells), energy‑density calculations and safety handling.
Hydraulic / Pneumatic (fluid‑pressure)	\(E_f\)	\(E_f = pV\) (pressure × volume)	\(P = p Q_f\) (where \(Q_f\) = volumetric flow rate)	Used for high‑force actuation, requires pipe sizing, leakage control, and pressure rating.

4. Energy Conversion & Transmission

4.1 Typical Conversion Chains

Real‑world products rarely use a single conversion. The most common chains encountered in D‑Level projects are listed below. Designers should be able to identify each stage and its likely efficiency.

Chemical → Electrical – batteries, fuel cells.
Electrical → Kinetic – DC/stepper/servo motor.
Kinetic → Mechanical Work – gear train, belt, chain, cam.
Mechanical → Thermal – brakes, friction drives (useful for heat‑based sensors).
Thermal → Mechanical – steam turbine, Stirling engine, thermoelectric generator.
Fluid‑pressure (hydraulic/pneumatic) → Mechanical – cylinder, rotary actuator.

4.2 Typical Efficiencies & Loss Mechanisms

Conversion Stage	Typical Efficiency (\(\eta\))	Primary Loss Mechanisms
Chemical → Electrical (battery)	80–95 %	Internal resistance (I²R heating), self‑discharge, temperature effects.
Electrical → Kinetic (motor)	70–90 %	Joule heating, magnetic hysteresis, friction in bearings.
Kinetic → Mechanical (gears, belts, chains)	85–98 %	Gear tooth friction, belt slip, chain wear, bearing losses.
Thermal → Mechanical (steam/turbine)	30–45 %	Condensation losses, exhaust heat, friction.
Mechanical → Thermal (braking)	≈ 100 % (energy becomes heat)	Heat dissipation to surroundings; may require cooling fins.
Fluid‑pressure → Mechanical (hydraulic cylinder)	80–90 %	Seal leakage, fluid viscosity, pressure drop in lines.

4.3 Estimating Losses – Practical Guidance

Electrical (I²R) loss: \(P_{loss}=I^{2}R\). Use conductor resistance \(R = \rho \dfrac{L}{A}\) (ρ = resistivity, L = length, A = cross‑section).
Friction loss in moving parts: \(P_{fric}=F_{fric}v\) where \(F_{fric}= \mu N\) (μ = coefficient of friction, N = normal force).
Gear loss: Approximate as \(P_{loss}= \eta_{gear}\,P_{in}\) with \(\eta_{gear}=1-\dfrac{C_{gear}}{100}\) where \(C_{gear}\) is the percentage loss (typical 2–15 %).
Hydraulic pressure loss: \(\Delta p = f\frac{L}{D}\frac{\rho v^{2}}{2}\) (Darcy‑Weisbach) – useful for pipe sizing.

4.4 Sample Efficiency Calculation (Updated)

Design a portable winch that lifts a 10 kg load 0.5 m using a 7.4 V, 2 Ah Li‑ion battery, a 90 % efficient DC motor, a 95 % efficient gearbox and a 5 % loss in the winch drum (friction).

Energy stored in battery: \(E_{bat}=V I t = 7.4 \times 2 \times 3600 = 53\,280\ \text{J}\).
Motor output: \(E_{mot}=0.90 \times 53\,280 = 47\,952\ \text{J}\).
Gearbox output: \(E_{gear}=0.95 \times 47\,952 = 45\,554\ \text{J}\).
Drum (mechanical) output after friction: \(E_{out}=0.95 \times 45\,554 = 43\,276\ \text{J}\).
Gravitational potential required: \(E_{g}=mgh = 10 \times 9.81 \times 0.5 = 49.1\ \text{J}\).
Overall system efficiency: \(\displaystyle \eta_{sys}= \frac{E_{g}}{E_{bat}}\times100 \approx \frac{49.1}{53\,280}\times100 \approx 0.09\%.\)
Interpretation: most of the stored energy is lost as heat in the motor and gearbox. A designer would consider a higher‑efficiency brushless motor, a direct‑drive winch, or a different power source (e.g., super‑capacitor for short bursts).

5. Power & Sizing Equations for Designers

Translational power: \(P = Fv\) (force × velocity).
Rotational power: \(P = \tau\omega\) (torque × angular velocity).
Electrical power: \(P = VI = I^{2}R = \dfrac{V^{2}}{R}\).
Hydraulic power: \(P = p Q_f\) (pressure × volumetric flow).
Thermal power (heat transfer): \(P = \dot m c \Delta T\) (mass flow × specific heat × temperature change).

These relationships help size motors, select wire gauges, calculate required pump pressures, and estimate cooling loads.

6. Control Systems Basics

6.1 Levels of Control

Manual control – operator directly provides the input (e.g., hand‑crank, foot pedal).
Semi‑automatic control – operator initiates a cycle; the system runs autonomously until a preset condition stops it (e.g., push‑button start, limit‑switch stop).
Automatic control – continuous monitoring and adjustment without human intervention (e.g., thermostat‑controlled heater, PLC‑driven conveyor).

6.2 Core Components of a Control Loop

Component	Function in a Control Loop	Typical D‑Level Example
Sensor	Detects a physical variable and converts it to an electrical signal.	Limit switch, thermistor, photo‑electric sensor.
Controller	Processes the sensor signal, decides the required action, and sends a command.	Arduino/Arduino‑compatible microcontroller, comparator circuit, PLC.
Amplifier / Driver	Boosts the controller output to a level suitable for the actuator.	H‑bridge motor driver, MOSFET switch for a heater.
Actuator	Converts the electrical command into mechanical, thermal or fluid power.	DC motor, solenoid valve, heating element.
Feedback	Returns information about the actual output to the controller for correction.	Encoder on motor shaft, temperature reading from a thermocouple.

6.3 Open‑Loop vs Closed‑Loop Control

Open‑loop – No feedback; output is assumed to be correct (e.g., a timed motor run). Simpler but less accurate.
Closed‑loop – Feedback is continuously compared with the set‑point; controller adjusts output (e.g., thermostat). Improves accuracy and stability.

6.4 Basic PID Control (Conceptual)

Many automatic systems use a proportional‑integral‑derivative (PID) controller:

Proportional (P): Output ∝ error (difference between set‑point and measured value).
Integral (I): Corrects steady‑state error by accumulating error over time.
Derivative (D): Anticipates future error by considering the rate of change.

For D‑Level projects a simple Proportional or On‑Off control (e.g., thermostat) is usually sufficient, but understanding PID terminology helps when using Arduino libraries or PLCs.

6.5 Common Controllers & Safety Interlocks

Micro‑controllers: Arduino, Raspberry Pi (GPIO), ESP32 – easy programming, built‑in ADC/DAC.
Programmable Logic Controllers (PLC): Ladder logic, robust for industrial‑style projects.
Safety interlocks: Emergency stop (E‑stop) switches, fuse or circuit‑breaker protection, limit switches to prevent over‑travel, isolation relays for high‑voltage sections.

6.6 Example: Automatic Door Opener (Re‑visited)

Infra‑red sensor detects a person → 5 V signal to Arduino.
Arduino runs a simple on‑off routine: if sensor = high, set motor driver PWM = 80 %.
Motor driver (H‑bridge) powers a DC motor that opens the door.
Encoder on the motor shaft provides position feedback; when the door reaches the “open” count, Arduino sets PWM = 0.
After a 5‑second delay, the routine reverses the motor direction to close the door.
An E‑stop button in series with the power supply cuts all voltage instantly for safety.

7. Integrating Energy Forms in a Controlled System

When designing a product, follow these systematic steps:

Define the required output – mechanical motion, heat, light, etc.
Select the primary energy source – consider availability, cost, sustainability, and required storage.
Choose conversion devices – motor, heater, pump, turbine; note individual efficiencies.
Design the transmission path – gears, belts, hydraulic lines, wiring; calculate losses using the methods in §4.3.
Specify the control strategy – manual, semi‑automatic or automatic; decide on sensors, controller type, driver, feedback, and safety interlocks.
Quantify total system efficiency and iterate to improve performance, reduce waste, or meet regulatory limits.
Document the energy flow – a flow‑chart helps examiners see the logical sequence.

Sample Integrated Flow‑Chart (placeholder)

Chemical (battery) → Electrical → Amplifier/Driver → Motor (kinetic) → Gear train (transmission) → Mechanical work → Sensor feedback → Controller (Arduino/PLC) → (loop)

8. Key Points to Remember

Energy can be stored (gravitational, elastic, chemical, thermal, fluid‑pressure) or transferred (kinetic, electrical, radiant, thermal).
Every conversion stage incurs losses; always estimate efficiency and identify the dominant loss mechanism.
Power equations (\(P = Fv, \; P = \tau\omega, \; P = VI\)) are essential for sizing motors, wires and fluid components.
Control systems link energy flow to product function – choose the appropriate level of automation and ensure robust feedback and safety interlocks.
Design decisions must balance performance, cost, sustainability and safety. Consider environmental, economic and social impacts of the chosen primary source.
For exam answers: state the energy form, write the relevant equation, give a concise design relevance, and, where appropriate, comment on efficiency or control strategy.

Different forms of energy, including: kinetic, potential, thermal, electrical, chemical.