Hydropower, Marine, Geothermal and Bioenergy Technologies

Introduction

Renewable energy systems differ not only in their energy source but also in resource variability, conversion efficiency, environmental effects, geographical requirements and ability to respond to demand. Hydropower converts the potential and kinetic energy of water; marine technologies use tides, waves and ocean currents; geothermal systems extract heat from the Earth; and bioenergy converts chemical energy stored in biological matter. Together, these technologies can provide electricity, heat, transport fuels and grid-support services.

A central engineering distinction is between energy and power. Energy is the quantity of work available, measured in joules or kilowatt-hours, whereas power is the rate at which energy is delivered, measured in watts. Hydropower and geothermal plants can often be dispatchable, while tidal and wave resources are more predictable but generally less controllable. Biomass can be dispatchable if fuel is stored, but its sustainability depends on feedstock production, land use and emissions management.

Learning Objectives

By the end of this chapter, students should be able to:

1. Explain the operating principles and principal components of hydropower, marine, geothermal and bioenergy systems.
2. Distinguish small, large and pumped-storage hydropower.
3. Calculate basic hydraulic, tidal, wave, geothermal and biomass energy potential.
4. Compare technologies according to resource, environmental, dispatchability and electrical-integration characteristics.
5. Evaluate practical design constraints, sustainability issues and regulatory considerations.
6. Apply the concepts to an integrated renewable-energy planning problem.

1. Hydropower Technology

1.1 Physical principle

Hydropower uses the gravitational potential energy of water. For a mass m raised through a vertical height h, the ideal potential energy is E = mgh, where g is gravitational acceleration, approximately 9.81 m/s². In a flowing river, the mass flow rate is ṁ = ρQ, where ρ is water density, approximately 1000 kg/m³ for fresh water, and Q is volumetric flow rate in m³/s.

The ideal hydraulic power is therefore:

where P_h is hydraulic power in watts, H is the effective hydraulic head in metres, and Q is flow rate. The electrical output is:

where η_t, η_g and η_o are turbine, generator and other system efficiencies. The effective head is normally the gross elevation difference less hydraulic losses in the intake, penstock and other waterways. The equation assumes a steady flow and a defined net head; it is not a substitute for a full water-balance or transient analysis.

Worked Example 1: Run-of-river plant

A plant receives 12 m³/s at a net head of 18 m. Turbine efficiency is 90%, generator efficiency is 96% and other losses reduce output by 3%. Taking ρ = 1000 kg/m³:

If the plant operates at this output for 4000 equivalent full-load hours per year, its annual energy is:

The result is an idealized estimate. Seasonal river flow, outages, environmental releases and part-load efficiency must be included in a bankable assessment.

1.2 Main hydropower configurations

Run-of-river plants use the natural river discharge with limited storage. They usually have lower inundation impacts, but their output follows seasonal flow and is less dispatchable.

Storage or reservoir plants retain water behind a dam. Storage permits production during high-demand periods and can provide reserve, frequency regulation and black-start capability. Their impacts may include land inundation, altered sediment transport, fish migration barriers, methane emissions from some reservoirs and relocation of communities.

Diversion plants route part of a river through a canal, tunnel or penstock to create head without a large dam. Ecological flow requirements are essential.

Small hydropower is commonly classified by national regulation rather than by one universal threshold. A practical engineering distinction is that small plants generally have lower civil-works scale and serve local or distributed loads. Micro- and pico-hydropower can supply isolated communities, farms or remote facilities. Their advantages include modularity and relatively short construction periods; constraints include limited seasonal output, debris, sediment and maintenance access.

Large hydropower benefits from economies of scale and can provide bulk energy, storage and ancillary services. However, construction involves major geological, financial, social and environmental risks. Installed capacity alone does not determine performance: annual energy depends on the flow-duration curve, head, turbine availability and operating policy.

1.3 Components and conversion chain

A conventional plant includes a watershed and reservoir or intake, trash rack, gates, spillway, tunnel or canal, penstock, turbine, draft tube, generator, transformer, switchgear, transmission connection and control system. Water energy is converted in sequence:

water potential energy → hydraulic energy → turbine mechanical energy → generator electrical energy → transformed and transmitted electrical energy.

Impulse turbines, such as Pelton turbines, are suitable for high head and relatively low flow. Reaction turbines, such as Francis and Kaplan turbines, operate with pressure change through the runner. Francis turbines suit medium head; Kaplan and propeller turbines suit low head and high flow. Turbine selection must consider head range, flow variation, cavitation risk, sediment and efficiency across the operating envelope.

1.4 Pumped-storage hydropower

Pumped storage is an energy-storage system rather than a primary energy source. During low-demand or low-price periods, electricity drives pumps that move water to an upper reservoir. During demand peaks, water flows back through turbines.

The stored energy is approximately:

where V is stored water volume in m³ and η_d is the discharge-path efficiency. Round-trip efficiency includes pumping, generation, hydraulic and electrical losses:

Typical practical values are often about 70–85%, depending on plant design and operating conditions. Pumped storage provides shifting, reserve, ramping, inertia and black-start services. Its limitations include suitable topography, water availability, permitting, construction cost and ecological effects. Closed-loop systems reduce dependence on an existing river but still require two reservoirs and substantial civil works.

2. Marine Energy

Marine energy is attractive because oceans contain large energy flows and some resources, especially tides, are highly predictable. Commercial deployment remains more limited than land-based wind and solar because marine equipment faces corrosion, biofouling, storms, difficult maintenance and costly subsea electrical infrastructure.

2.1 Tidal-range systems

Tidal-range schemes exploit the water-level difference created by the tide. A barrage or lagoon forms a basin, and turbines generate during filling, emptying or both. The theoretical energy available from a basin of area A and tidal range R is approximately:

This estimate assumes a simplified constant-area basin and neglects turbine, sluice, hydraulic and electrical losses. Real output is lower, and tidal-range operation changes sediment transport, habitats and estuarine water exchange.

2.2 Tidal-stream systems

Tidal-stream turbines extract kinetic energy from moving seawater. Their ideal power relationship is:

where A is rotor swept area, v is current velocity, and C_p is the power coefficient. The coefficient is less than the theoretical actuator-disk limit and is further reduced by drivetrain and generator efficiency. Because power varies with the cube of velocity, site assessment must characterize the full current-speed distribution, turbulence and flow direction.

Tidal-stream arrays require foundations or moorings, subsea cables, power-conditioning equipment and corrosion-resistant materials. They can be highly predictable, but maximum generation occurs at particular phases of the tidal cycle and may not coincide with demand peaks.

2.3 Wave energy

Wave-energy converters extract energy from heave, surge, pitch, overtopping or oscillating water columns. Ocean-wave power depends on wave height, period, water density and the sea state. For an idealized regular deep-water wave, the energy flux per unit crest length is approximately:

where H_s is significant wave height in metres and T_e is an appropriate energy period in seconds. The formula is a simplified linear-wave estimate and assumes deep water; shallow-water effects, directional spreading, spectral shape and device response must be included for design.

Wave devices generally include a floating or fixed absorber, mechanical or hydraulic power take-off, generator, mooring or foundation, subsea cable and control system. Survivability during extreme storms is often more important than peak efficiency. Wave output is variable but can complement wind and solar in some coastal systems.

2.4 Marine resource and integration assessment

Marine projects require metocean measurements, environmental baseline studies, navigation analysis, fisheries consultation, seabed surveys and subsea-cable planning. Electrical integration must account for long cable lengths, reactive power, harmonics, protection coordination and limited accessibility for repairs. Predictability improves scheduling but does not eliminate variability or the need for reserves.

3. Geothermal Energy

3.1 Resource types

Geothermal energy is heat stored within the Earth. Resources include hydrothermal reservoirs, hot dry rock or enhanced geothermal systems, geopressured resources and shallow ground-source heat. Electricity generation usually requires sufficiently high temperature and flow, whereas direct heating can use lower-temperature resources.

The basic heat-transfer relationship is:

where Q̇_h is useful thermal power in watts, ṁ is fluid mass flow rate in kg/s, c_p is specific heat capacity in J/(kg·K), and temperatures are in °C or K differences. For a geothermal plant, the net electrical output is approximately:

Pumping, injection and cooling loads can be significant.

3.2 Conversion technologies

Dry-steam plants use steam directly from the reservoir to drive a turbine.

Flash-steam plants reduce the pressure of hot water so that part of it vaporizes. Single- and double-flash arrangements may be used.

Binary-cycle plants transfer heat from geothermal brine to a secondary working fluid with a lower boiling point. Organic Rankine Cycle and Kalina-type systems can use moderate-temperature resources while keeping geothermal fluid largely in a closed loop.

A geothermal system normally includes production wells, separators or heat exchangers, turbine and generator, condenser, cooling system, reinjection wells, pipelines, valves and monitoring equipment. Reinjection helps sustain reservoir pressure and reduces surface disposal, although scaling, corrosion and induced seismicity must be managed.

3.3 Resource assessment and environmental issues

Exploration combines geological mapping, geochemistry, geophysics, temperature-gradient drilling and reservoir modelling. The key uncertainties are reservoir temperature, permeability, sustainable flow, well productivity and decline rate. Unlike solar and wind, geothermal output can be steady and dispatchable, but resource depletion or well failure may reduce long-term output.

Potential impacts include land disturbance, noise, hydrogen sulfide, brine disposal, subsidence and induced seismicity. Closed-loop designs and reinjection can reduce emissions and contamination. Regulatory approval should address water rights, drilling safety, seismic monitoring, worker protection and community consent.

4. Bioenergy Technologies

4.1 Feedstocks and conversion pathways

Bioenergy uses organic material such as forestry residues, agricultural residues, energy crops, animal manure, municipal organic waste and biogas. Feedstock properties include moisture content, ash content, heating value, particle size, contaminants and seasonal availability.

Major pathways are:

• Combustion: biomass is burned to produce hot gases, steam and electricity or useful heat.
• Anaerobic digestion: microorganisms convert wet organic matter into biogas, primarily methane and carbon dioxide.
• Gasification: limited oxygen converts solid biomass into producer gas containing carbon monoxide, hydrogen, methane and carbon dioxide.
• Pyrolysis: heating without oxygen produces bio-oil, syngas and biochar.
• Fermentation and transesterification: biological or chemical processes produce ethanol and biodiesel.

A biomass power plant contains fuel reception and storage, preparation equipment, a boiler or gasifier, steam turbine or engine, generator, emissions-control equipment, ash handling and grid-interconnection equipment. A biogas facility adds digesters, gas cleaning, storage, engines or upgrading equipment and digestate management.

4.2 Energy potential and efficiency

The energy in a biomass quantity is estimated by:

where m is mass of usable biomass and LHV is lower heating value, normally in MJ/kg. Electrical output is:

For cogeneration, total useful-energy efficiency is:

where E_h is useful delivered heat. Moisture strongly affects available energy. If a wet feedstock contains a high water fraction, part of the fuel energy is consumed in evaporating water, and the effective heating value is lower than the dry-basis value.

Worked Example 2: Biomass electricity

A facility receives 10,000 tonnes of dry residue annually. The lower heating value is 16 MJ/kg and net electrical efficiency is 25%.

Using 1 kWh = 3.6 MJ:

Thus:

This estimate assumes all feedstock is collected, transported and usable. A realistic project subtracts parasitic electricity, downtime, storage losses and contamination-related losses.

4.3 Sustainability and emissions

Bioenergy is not automatically carbon-neutral. Lifecycle emissions arise from cultivation, fertilizer, harvesting, transport, drying, processing, methane leakage and land-use change. Residue removal can reduce soil carbon and nutrients. Energy crops may compete with food production or biodiversity. Sustainable practice prioritizes wastes and residues, maintains soil quality, protects ecosystems and verifies supply-chain traceability.

Combustion requires controls for particulate matter, nitrogen oxides, sulfur compounds, carbon monoxide and acid gases. Anaerobic digestion can reduce uncontrolled methane emissions from manure, but leaks from digesters and gas-upgrading systems must be monitored. Digestate may be a fertilizer, although pathogens, nutrients and contaminants require regulation.

5. Comparative Engineering Assessment

The capacity factor is a useful but limited metric:

where E_actual is actual energy over time t and P_r is rated power. Capacity factor does not indicate whether generation occurs when needed, nor does it capture environmental or fuel-supply risks.

Electrical integration includes generator type, voltage transformation, protection, forecasting, ramp rates, inertia, fault contribution, reactive-power capability and grid-code compliance. Hydropower and synchronous geothermal or biomass generators can provide inertia and voltage support. Power-electronic marine devices may require converters and control systems to meet grid requirements. Variable resources benefit from forecasting, storage, demand response, transmission and geographic diversity.

6. Integrated Applied Case: Illustrative Coastal-Regional Plan

Background

Consider an illustrative coastal region with a small river, a moderate geothermal prospect, agricultural residues and a strong tidal channel. Peak demand is 42 MW, annual demand is 210 GWh and the existing grid depends heavily on imported diesel.

Problem

The regional authority seeks lower fuel use while retaining reliability. It must compare a 6 MW run-of-river plant, a 10 MW geothermal plant, a 5 MW biomass CHP plant and a 4 MW tidal-stream array. The resources have different output profiles and environmental risks.

Analysis and approach

The run-of-river project is estimated at 6 MW with a 35% capacity factor, yielding:

The geothermal plant is expected to operate at 90% capacity factor:

The biomass CHP plant operates at 75% capacity factor:

The tidal array operates at a 32% capacity factor:

Total annual generation is approximately 141.3 GWh, or 67% of annual demand. The remaining energy must come from the existing grid, additional resources or efficiency measures. The annual values do not prove adequacy: hourly simulation is needed because geothermal and biomass can provide firm output, river generation varies seasonally, and tidal generation follows a predictable but non-dispatchable cycle.

The preferred first phase is geothermal exploration and rehabilitation of the river site, combined with sustainable biomass contracts and a grid study for tidal connection. A small battery or demand-response program may address short-duration mismatch, while a future pumped-storage or transmission option could provide longer-duration balancing.

Outcome and learning points

The case demonstrates that a technology portfolio should not be selected by capacity factor alone. Resource diversity, seasonal correlation, fuel logistics, environmental permitting, transmission capacity, reserve requirements and lifecycle cost determine system value. Dispatchable technologies can reduce the reserve burden, while predictable marine resources improve scheduling but cannot necessarily follow demand.

7. Challenges, Governance and Contemporary Developments

Key challenges include climate-driven hydrological change, extreme floods and droughts, marine storm survivability, geothermal drilling risk, biomass supply competition and aging infrastructure. Climate assessment should use multiple hydrological and temperature scenarios rather than a single historical average.

Environmental governance should include cumulative-impact assessment, ecological-flow rules, fish passage where feasible, sediment management, marine spatial planning, Indigenous and local-community participation, land and water rights, worker safety and transparent monitoring. Projects should report lifecycle greenhouse-gas emissions and disclose assumptions about land use, methane leakage and end-of-life disposal.

Digital technologies increasingly support condition monitoring, digital twins, hydrological forecasting, reservoir optimization, geothermal reservoir modelling and predictive maintenance. Artificial intelligence can identify abnormal vibration or forecast inflow, but its outputs require quality data, physical validation, cybersecurity controls and human accountability. AI should support—not replace—engineering judgment and regulatory compliance.

Best practice is to begin with resource measurement, establish a defensible baseline, model annual and hourly performance, quantify uncertainty, engage affected communities early, design for maintainability and evaluate the full lifecycle rather than only construction cost.

Chapter Summary

Hydropower converts water head and flow into electricity and ranges from small run-of-river installations to large reservoirs and pumped-storage facilities. Its output can be highly valuable for dispatch, reserve and grid stability, but dams and diversions may cause substantial ecological and social impacts. Marine technologies include tidal-range, tidal-stream and wave systems. Tidal resources are predictable, but marine equipment faces corrosion, storms, difficult access and expensive subsea connections.

Geothermal systems provide firm heat and electricity where productive reservoirs exist. Dry-steam, flash-steam and binary-cycle plants differ according to resource temperature and fluid condition. Reinjection, reservoir management and seismic monitoring are important. Bioenergy is dispatchable when fuel is stored and can provide combined heat and power, but its sustainability depends on feedstock origin, land use, transport, emissions and methane control.

The fundamental calculations are based on hydraulic power P = ρgQH, tidal or wave kinetic power, geothermal heat flow Q̇ = ṁc_p∆T and biomass energy E = mLHV. Effective renewable planning combines these calculations with resource variability, lifecycle assessment, grid integration, environmental governance and social acceptability.

Key Terms

Hydraulic head: Effective vertical energy difference available to a hydropower turbine.

Run-of-river: Hydropower system with limited storage whose output follows river flow.

Reservoir hydropower: Hydropower using stored water to control generation timing.

Pumped storage: Electricity-storage system that pumps water uphill and later generates electricity.

Tidal range: Difference in water level caused by the tide.

Tidal stream: Kinetic energy in moving tidal currents.

Wave-energy converter: Device that extracts mechanical or electrical energy from ocean waves.

Geothermal reservoir: Subsurface formation containing usable heat and, commonly, fluid flow.

Flash-steam plant: Geothermal plant that vaporizes hot pressurized water by reducing pressure.

Binary-cycle plant: Geothermal plant using a secondary working fluid in a heat exchanger.

Reinjection: Return of geothermal fluid to the subsurface reservoir.

Biomass: Biological material used as an energy feedstock.

Anaerobic digestion: Microbial conversion of organic matter without oxygen.

Biogas: Gas, mainly methane and carbon dioxide, produced by anaerobic digestion.

Gasification: Thermochemical conversion of biomass with limited oxygen.

Lower heating value: Fuel energy estimate excluding the latent heat recovered by condensing water vapor.

Capacity factor: Actual energy divided by rated power multiplied by the operating period.

Dispatchability: Ability of a plant to control output when required by the system operator.

Ancillary services: Grid-support functions such as frequency regulation, voltage control and reserves.

Lifecycle assessment: Evaluation of environmental impacts across a technology’s entire life.

Review Questions

Short Answer Questions

1. Define net hydraulic head and explain why it is lower than gross head.
2. What is the main difference between tidal-range and tidal-stream systems?
3. Name three principal geothermal electricity-generation technologies.
4. Why does biomass moisture content reduce useful energy output?
5. Define capacity factor and state one limitation of using it alone.

Long Answer Questions

1. Explain the components and energy-conversion process of a conventional reservoir hydropower plant.
2. Compare small hydropower, large hydropower and pumped storage in terms of resource, cost, environmental impact and grid services.
3. Describe the operating principles, advantages and limitations of wave-energy converters.
4. Explain how geothermal resources are explored, developed and managed during operation.
5. Compare combustion, anaerobic digestion, gasification and pyrolysis as bioenergy pathways.

Analytical / Application Questions

1. A hydropower site has Q = 8 m³/s, net head H = 25 m and total efficiency of 82%. Calculate electrical output using ρ = 1000 kg/m³ and g = 9.81 m/s². State the assumptions.
2. A tidal-stream turbine has swept area 300 m², current speed 2.5 m/s and combined power coefficient and drivetrain efficiency of 0.38. Estimate output using the kinetic-energy equation and explain why actual annual energy will be lower than a constant-speed estimate.
3. Design a preliminary renewable portfolio for a remote coastal community. Justify the roles of hydropower, geothermal, marine energy, biomass, storage and demand response using resource, environmental and electrical-integration criteria.

Multiple Choice Questions with Answers

Question 1

For a hydropower plant, which expression gives ideal hydraulic power?

A. P = ρgQH

B. P = ρQ/(gH)

C. P = gH/(ρQ)

D. P = ρg/(QH)

Correct Answer: A. P = ρgQH

Question 2

A key function of pumped-storage hydropower is to:

A. Create primary energy from groundwater

B. Shift electrical energy between periods of low and high demand

C. Eliminate hydraulic losses in rivers

D. Convert biomass directly into electricity

Correct Answer: B. Shift electrical energy between periods of low and high demand

Question 3

Tidal-stream power varies most directly with the cube of:

A. Water density

B. Rotor diameter

C. Current velocity

D. Tidal range

Correct Answer: C. Current velocity

Question 4

Which geothermal plant uses a secondary working fluid heated by geothermal brine?

A. Dry-steam plant

B. Binary-cycle plant

C. Tidal barrage

D. Pelton hydropower plant

Correct Answer: B. Binary-cycle plant

Question 5

If 5 MW of biomass generation operates at a 75% capacity factor for one year, its approximate annual energy is:

A. 3.75 GWh

B. 32.9 GWh

C. 43.8 GWh

D. 65.7 GWh

Correct Answer: B. 32.9 GWh

Question 6

Which biomass process occurs without oxygen and produces methane-rich gas?

A. Anaerobic digestion

B. Combustion

C. Flash evaporation

D. Mechanical screening

Correct Answer: A. Anaerobic digestion

Question 7

The main purpose of geothermal reinjection is to:

A. Increase tidal range

B. Return fluid to the reservoir and reduce surface disposal

C. Dry biomass before combustion

D. Store electricity in batteries

Correct Answer: B. Return fluid to the reservoir and reduce surface disposal

Question 8

Which technology is generally most directly associated with seasonal river-flow variability?

A. Run-of-river hydropower

B. Binary geothermal power

C. Stored biomass combustion

D. Pumped storage in a closed loop

Correct Answer: A. Run-of-river hydropower

Question 9

In the capacity-factor equation, the denominator is:

A. Actual annual energy

B. Rated power multiplied by the operating period

C. Fuel mass multiplied by heating value

D. Net head multiplied by flow

Correct Answer: B. Rated power multiplied by the operating period

Question 10

Which issue is particularly important for marine-energy equipment?

A. Corrosion and difficult offshore maintenance

B. Absence of any mechanical loads

C. Guaranteed generation at all times

D. Zero requirement for subsea cables

Correct Answer: A. Corrosion and difficult offshore maintenance