Renewable Sources of Energy:

RENEWABLE ENERGY – BASIC CONCEPT

• Renewable energy: Energy obtained from naturally replenished sources.
• Main sources: Solar, Wind, Biomass, Hydro, Tidal, Geothermal, Ocean and Hydrogen/Fuel Cells.
• Major advantages: low fuel cost, sustainable resource and lower operational emissions.
• Major limitation: many renewable sources are intermittent and require storage/grid integration.

SOLAR RADIATION

• Solar radiation: Electromagnetic energy emitted by the Sun.
• Solar energy reaching Earth consists mainly of direct and diffuse radiation.
• Beam/Direct radiation: Reaches the surface directly from the Sun.
• Diffuse radiation: Scattered by atmosphere/clouds and reaches the surface from different directions.
• Global radiation: Direct + diffuse radiation on a surface.

SOLAR THERMAL ENERGY

• Solar thermal systems convert solar radiation into heat.
• Heat can be used for water heating, space heating, drying, cooling and power generation.
• Main collectors:
  • Flat Plate Collector (FPC)
  • Focusing/Concentrating Collectors

FLAT PLATE COLLECTOR (FPC)

Principle: Solar radiation is absorbed by a dark absorber plate and transferred to a working fluid.

• Usually uses a black/selective-coated absorber surface.
• Common absorber materials: copper, aluminium.
• Cover material: generally glass.
• Working fluids: water, air or heat-transfer fluids.
• FPC generally does not require solar tracking.
• Suitable for relatively low-to-moderate temperature applications.

FOCUSING / CONCENTRATING SOLAR COLLECTORS

• Use mirrors/lenses to concentrate solar radiation onto a smaller receiver.
• Can achieve higher temperatures than flat plate collectors.
• Usually require tracking of the Sun.
• Best suited to areas having high direct normal solar radiation.

FLAT PLATE VS FOCUSING COLLECTOR

SOLAR COLLECTOR PERFORMANCE

• Collector efficiency:

\[ \eta_c = \frac{\text{Useful heat gained}}{\text{Solar energy incident}} \]

\[ \eta_c = \frac{Q_u}{A_c I} \]

• \( Q_u \) = Useful heat collected.
• \( A_c \) = Collector area.
• \( I \) = Incident solar irradiance.
• Performance decreases when heat losses increase.
• Higher absorber absorptivity and lower thermal emissivity improve performance.
• Insulation reduces conductive heat losses.

SOLAR THERMAL ENERGY STORAGE

Purpose: Store collected solar heat for use when solar radiation is unavailable.

• Sensible heat: \( Q = mc\Delta T \)
• Latent heat: \( Q = mL \)
• Molten salts are important in high-temperature solar thermal storage.

APPLICATIONS OF SOLAR THERMAL ENERGY

SOLAR THERMAL COOLING

• Solar heat can drive a thermally activated refrigeration system.
• Common technology: absorption refrigeration.
• Solar energy provides heat instead of directly driving a conventional compressor.
• Useful for cooling when solar availability is high.

SOLAR PHOTOVOLTAIC (PV) CONVERSION

Photovoltaic effect: Direct conversion of solar radiation into electrical energy using semiconductor materials.

• Common PV material: Silicon.
• Solar cell is the basic PV unit.
• Multiple cells form a module/panel.
• Multiple modules can form an array.
• PV output is DC electricity.
• An inverter converts DC to AC for AC loads/grid connection.

PV PERFORMANCE

• Electrical efficiency: \( \eta = \text{Electrical output} / \text{Solar input} \).
• PV cell output depends on solar irradiance, cell temperature, shading and electrical load.
• Higher cell temperature generally reduces the electrical efficiency of conventional silicon PV cells.
• Important operating points: Open-circuit voltage (\( V_{oc} \)), short-circuit current (\( I_{sc} \)), maximum-power point.
• Fill Factor: \( \text{FF} = \frac{P_{max}}{V_{oc} \times I_{sc}} \)

WIND ENERGY

Wind energy: Kinetic energy of moving air converted into mechanical/electrical energy.

• Wind turbine converts wind energy into shaft power.
• Generator converts shaft power into electrical energy.
• Wind turbines are mainly classified as:
  • Horizontal Axis Wind Turbine (HAWT)
  • Vertical Axis Wind Turbine (VAWT)

WIND TURBINE POWER

Available wind power:

\[ P = \frac{1}{2} \rho A V^3 \]

• \( \rho \) = Air density.
• \( A \) = Swept area.
• \( V \) = Wind velocity.
• Power varies with the cube of wind speed.

Betz Limit: Maximum theoretical fraction of wind power extractable by an ideal wind turbine ≈ 59.3%.

• Actual turbine power is lower than the theoretical Betz limit.
• Major components: blades, rotor, hub, gearbox/direct drive, generator, nacelle and tower.

HAWT VS VAWT

BIOMASS ENERGY

Biomass: Organic material of biological origin used as an energy resource.

• Sources: wood, agricultural residues, animal waste, municipal organic waste, energy crops.
• Biomass can be converted into heat, electricity, biogas, biofuels and producer/synthesis gas.

BIOGAS

• Produced by anaerobic digestion of organic matter.
• Main combustible component: Methane (CH₄).
• Other major component: CO₂.
• Hydrogen sulphide and moisture may also be present.
• Uses: cooking, heating, electricity generation and upgraded biomethane.

TIDAL ENERGY

Tidal energy: Energy obtained from periodic rise/fall and movement of seawater caused mainly by gravitational effects of the Moon and Sun.

• Tidal energy is highly predictable compared with solar and wind.
• Main limitation: suitable sites are geographically limited.
• Barrage systems may use ebb, flood or two-way generation.

FUEL CELLS

Fuel Cell: Electrochemical device that directly converts chemical energy of a fuel into electrical energy.

• Works continuously as long as fuel and oxidant are supplied.
• Fuel cells are not conventional heat engines.
• Hydrogen fuel cells produce electricity, heat and water when hydrogen reacts electrochemically with oxygen.

Basic Hydrogen Fuel Cell:

Anode: \( \text{H}_2 \rightarrow 2\text{H}^+ + 2e^- \)

Cathode: \( \frac{1}{2}\text{O}_2 + 2\text{H}^+ + 2e^- \rightarrow \text{H}_2\text{O} \)

Overall: \( \text{H}_2 + \frac{1}{2}\text{O}_2 \rightarrow \text{H}_2\text{O} + \text{Electrical energy} + \text{Heat} \)

FUEL CELL COMPONENTS

TYPES OF FUEL CELLS

FUEL CELL VS BATTERY

IMPORTANT COMPARISON – RENEWABLE SOURCES

HIGH-YIELD EXAM FACTS

• Solar thermal → Solar radiation to heat.
• Solar PV → Solar radiation directly to electricity.
• Flat plate collector → Non-concentrating collector.
• Focusing collector → Concentrates direct solar radiation.
• Flat plate collector → Generally no tracking.
• Concentrating collector → Usually requires tracking.
• FPC → Suitable for low/moderate temperature applications.
• CSP → Concentrated Solar Power.
• Sensible storage → Temperature change.
• Latent storage → Phase change.
• Thermochemical storage → Chemical reaction.
• PV output → DC.
• Inverter → DC to AC.
• Wind power \( \propto V^3 \).
• Betz limit ≈ 59.3%.
• Biogas → Mainly methane + CO₂.
• Anaerobic digestion → Biogas.
• Gasification → Producer gas/syngas.
• Pyrolysis → Bio-oil + char + gas.
• Tidal energy → Highly predictable renewable source.
• Fuel cell → Direct electrochemical conversion.
• Hydrogen fuel cell overall product → Water + electricity + heat.

⚡ ONE-MINUTE REVISION