Renewable Energy and Fuel Cells

Outcome 5: Renewable Energy Sources

Biomass Energy

  • Biomass is organic matter from terrestrial and aquatic plants and their derivatives.

  • Includes forest crops/residues, energy crops, and animal manure.

  • Renewable because plant life renews annually.

  • Form of solar energy: Solar energy \rightarrow Photosynthesis \rightarrow Biomass \rightarrow Energy generation

Biogas

  • Generated through anaerobic digestion of biomass in a biogas digester.

  • Composition:

    • Methane: 50-75%

    • Carbon dioxide: 25-50%

    • Nitrogen: 0-10%

    • Hydrogen: 0-1%

    • Trace amounts of O2 and H2S.

  • Applications: Cooking, lighting, heating, operating small IC engines

Biomass Conversion Technologies

  • Fixed Dome type

  • Floating Gas holder type

Fixed Dome Biogas Plant

  • Advantages:

    • Uses locally available materials.

    • Inexpensive.

    • Easy to construct.

  • Disadvantages:

    • Difficult to repair leaks.

    • Requires expertise for gas-tight construction.

    • Difficult to estimate gas production.

Floating Gas Holder

  • Advantages:

    • Constant gas pressure.

    • Simple operation.

    • Visible stored gas volume.

  • Disadvantages:

    • Expensive due to steel drum.

    • Steel drum may rust.

    • Requires regular maintenance.

Advantages of Biogas Technology

  1. Cheaper fuel for cooking, lighting and power generation.

  2. Good quality manure for soil fertility.

  3. Effective waste disposal, improving hygiene.

  4. Reduces burden on forests by reducing tree cutting for fuel wood.

  5. Reduces eye and lung diseases as a smokeless fuel

Biogas Digester Calculations

  • Energy from biogas digester: E = \eta Hm Fm V_b

    • \eta = burner combustion efficiency

    • H_m = heat of combustion of methane (J/m^3)

    • F_m = methane fraction in biogas

    • V_b = biogas volume (m^3)

  • Biogas volume: Vb = C m0

    • C = biogas yield per unit dry mass of whole input (m^3/Kg)

    • m_0 = digester input mass (Kg/day)

  • Fluid volume in digester: V = \frac{m0}{\rhom}

    • \rho_m = dry material density in fluid (Kg/m^3)

  • Digester volume: Vd = Vf t_r

    • V_f = digester fluid flow rate (m^3/day)

    • t_r = retention time (days)

Example Problems

  1. Given retention time = 20 days, temperature = 30°C, dry matter consumed = 10 kg/day, biogas yield = 0.24 m^3/kg, burner efficiency = 60%, methane fraction = 0.8, methane heat of combustion = 28 MJ/m^3, dry material density = 4.2 kg/m^3. Determine the power available in the digester.

  2. Design a community biogas plant for 100 families (5 adults/family). Cows: 102 (10 kg/day), Oxes: 124 (12 kg/day). Gas production: 45 liters/kg (winter), 60 liters/kg (summer). Gas required: 0.237 m^3/person/day. Determine surplus gas available.

Geothermal Energy

  • Geothermal (geo = earth, thermal = heat) energy comes from the Earth's internal heat.

  • Heat sources: radioactive decay and residual heat from earth formation

  • Core temperature can reach 7,600°F.

Earth's Structure

  • Crust

    • Oceanic: 18 km

    • Continental: 32 km

  • Mantle: Solid, includes upper mantle (Asthenosphere), lower mantle, 200°C - 1000°C, 932°C - 1652°C

  • Outer Core: Liquid, 1600°C - 4000°C

  • Inner Core: Solid, 4030°C - 5730°C

Applications

  1. Electricity Generation: Geothermal Power Plants (Dry steam, Flash steam, Binary cycle)

  2. Heating and Cooling: Geothermal Heat Pumps

Geothermal Power Plants

  1. Dry steam:

    • Uses high-pressure dry steam directly.

  2. Flash steam:

    • Uses high-pressure hot water flashed into steam. (Most common.)

  3. Binary cycle:

    • Uses lower-temperature resources with a secondary fluid (isobutene (C4H10), pentane (C5H12)).

Capacity factor

  • Capacity factor = (actual electricity generated) / (electricity at full capacity)

  • Geothermal has the highest capacity factor, providing constant 24-hour base-load power independent of weather.

  • Solar and wind are weather-dependent.

Geothermal Heat Pumps (GHP)

  • GHPs use Earth's stable temperature for heating/cooling.

  • Components: ground heat exchanger, heat pump unit, distribution system

  • Winter: extracts heat from the ground.

  • Summer: removes heat to the ground.

GHP Operation

  • GHPs exploit constant earth temperature: drilled 3 to 90 meters deep.

  • Temperature increases ~15-20°C per kilometer of depth.

Coefficient of Performance (COP)

  • COP measures heat pump efficiency: (heat transferred) / (electrical energy consumed).

  • Heating: COP = (heat output) / (electrical energy input).

  • Cooling: COP = (heat removed) / (energy consumed).

  • Higher COP = more efficient system.

Example Problems

  1. A geothermal power plant of 50 MW produces 250,000 MWh of electricity annually. Calculate the capacity factor.

  2. A geothermal plant with 12% efficiency has a thermal energy input of 50 MW. What is the electrical energy output?

  3. A geothermal heat pump extracts heat from the ground at 10°C with a COP of 4.0. How much heat can it provide a building by consuming 2 kW of electrical energy?

Ocean Energy

Types of Ocean Energy

  1. Tidal Energy: Converts tidal power to electricity; relies on gravitational forces.

  2. Wave Energy: Uses wave energy converters (WECs) to capture kinetic energy.

  3. Ocean Thermal Energy Conversion (OTEC): Uses temperature gradients to generate electricity.

Ocean Energy Classification

  • Tidal stream: captures kinetic energy of moving water currents.

  • Tidal lagoon or barrage: creates an enclosed area.

Tidal Barrage

  • Single basin: reversible turbines generate power during filling and emptying (double effect).

  • Double basin: high basin and low basin separated by a partition barrage.

Tidal Barrage Operation

  • Single-basin: Basin fills during rising tide, empties during lowering tide, driving reversible turbines.

  • Double-basin: High basin fills during high tide; low basin empties during low tide. Turbine-generators in the partition.

Advantages of Tidal Energy

  1. Renewable and Predictable.

  2. Low Greenhouse Gas Emissions.

  3. High Energy Density.

  4. Long Life span

Disadvantages of Tidal Energy

  1. High Initial Costs.

  2. Limited Locations.

  3. Environmental Impact.

  4. Maintenance Challenges

Wave Energy-WECs

  • Point Absorbers: Floating structures that move with waves.

  • Overtopping Devices: Reservoirs filled by overtopping waves.

  • Attenuators: Long structures aligned perpendicular to waves.

  • Oscillating Water Columns (OWCs): Use rising and falling water levels to generate air movement.

  • Pendulum Devices: Use swinging motion to generate mechanical energy.

Advantages of wave energy

  • Running and maintenance costs are very low

  • Wave energy machines emit no waste or pollution

  • Offshore devices have the potential to produce large amounts of energy

Disadvantages of wave energy

  • Offshore wave energy generation can interfere with shipping and wildlife

  • Waves need to be powerful to generate the maximum amount of electricity

  • Wave-energy generators can be noisy

Ocean Thermal Energy Conversion (OTEC)

  • Closed Type: Warm surface water vaporizes ammonia, which drives a turbine. Ammonia is then cooled.

  • Requires a temperature gradient > 20°C.

  • Ammonia is common due to its low boiling point.

Advantages of OTEC system

  • Power from OTEC is continuous, renewable, and pollution-free.

  • The output of OTEC shows very little daily or seasonal variation.

Disadvantages of an OTEC system

  • Capital investment is very high.

  • Conversion efficiency is very low about 3-4% due to the small temperature difference between the surface water and deep water.

  • The low efficiency of these plants coupled with high capital cost and maintenance cost makes them uneconomical for small plants.

Fuel Cell

Introduction

  • A fuel cell is an electrochemical device that converts the chemical energy of a fuel, typically hydrogen, and an oxidizing agent, commonly oxygen, directly into electricity, water, and heat

  • Uses electrochemical reactions instead of combustion to produce electricity directly

How Fuel Cells Work

  • Electrochemical reaction between hydrogen and oxygen.

  • Fuel Input (Hydrogen): Hydrogen is used as the fuel in most fuel cells. Hydrogen can be supplied to the fuel cell in various ways, often as compressed gas.

  • Oxidizing Agent (Oxygen): Ambient air/pure oxygen, serves as the oxidizing agent.

Anode (Hydrogen Electrode)

  • Hydrogen splits into protons (H⁺) and electrons (e⁻) via a catalyst.

  • H_2 \rightarrow 2H^+ + 2e^-

  • Electrons flow through an external circuit (electric current).

  • Protons pass through the electrolyte.

Cathode (Oxygen Electrode)

  • Oxygen reacts with electrons and protons to produce water and heat.

  • \frac{1}{2}O2 + 2H^+ + 2e^- \rightarrow H2O

By-products

  • Water (H₂O) and heat are the only by-products, making it environmentally friendly.

  • Hydrogen is separated by methods like Steam Methane Reforming, Electrolysis, Biomass Gasification based on availability of resources, economic considerations, and the desired purity and efficiency

  • Oxygen is from ambient air (21%) or pure oxygen when crucial (submarines and space crafts).

Types of Fuel Cells

  • Polymer Electrolyte Membrane Fuel Cell (PEMFC): Solid polymer membrane, operates at ~80°C; used in fuel cell electric vehicles.

  • Solid Oxide Fuel Cell (SOFC): Ceramic electrolyte, operates at 600-1,000°C; used in power generation.

  • Choice depends on application requirements.

Advantages

  • High Efficiency.

  • Environmental Benefits.

  • Quiet Operation.

Disadvantages

  • Hydrogen Production.

  • Hydrogen Storage.

  • Infrastructure.

Question Bank

Part A (Choose it)

  1. Renewable energy source with the highest capacity factor is (b) geothermal.

  2. The biomass conversion technology producing bioethanol is called (a) hydrolysis fermentation.

Fill up

  1. Geothermal power plant that uses lower-temperature geothermal resources to generate electricity is Binary cycle power plant.

  2. Passive systems use natural energy sources for thermal and visual comfort.

True or False

  1. Photosynthesis is the process of converting carbon dioxide and water into oxygen and carbohydrates using sunlight. (TRUE).

  2. The cut-in wind speed is the speed when the machine begins to produce power. (TRUE).

  3. Amount of gas produced in a fixed dome biogas plant can be estimated easily. (FALSE).

  4. Polymer electrolyte membrane (PEM) fuel cells operate at relatively high temperatures. (FALSE).

Part B

  1. The following data is given for a biogas digester. The retention time is 25 days, temperature is 32 degrees, dry matter consumed per day is 9500g and biogas yield is 0.34 m3/kg. The efficiency of burner is 62 %, the fraction of methane in biogas is 0.75 and heat of combustion of methane is 29 MJ/m3 . The density of dry material in the fluid is 4.2 kg/m3 . Determine the energy available in the digester. (2 Marks)

Part C

  1. Draw the block diagram of dry steam power plant which generate electricity from geothermal source. (2 Marks)

  2. List any TWO advantages and disadvantages of wave energy. (2 Marks)

  3. Explain liquid dominated low temperature type geothermal system with a neat diagram and proper labelling. (4+2+1=7 Marks)