6. Hydropower, Marine and Geothermal Energy

Introduction and Context

Renewable energy systems differ not only in their resource base but also in their ability to provide controllable power, energy storage, and useful heat. Hydropower, marine energy, and geothermal energy are especially significant because they convert naturally occurring flows of water, seawater, or subsurface heat into electricity and, in some cases, direct thermal services. Unlike variable solar and wind generation, conventional hydropower and geothermal plants can often provide dispatchable output. Marine technologies, although less mature, offer predictable tidal resources and substantial long-term potential.

These technologies are not interchangeable. Hydropower depends on hydraulic head and flow, geothermal systems depend on subsurface temperature, permeability, and fluid chemistry, while marine systems must withstand corrosion, biofouling, waves, and difficult offshore access. Sound engineering therefore requires simultaneous consideration of resource assessment, conversion efficiency, civil works, equipment selection, environmental effects, grid requirements, economics, and risk.

This chapter develops an integrated framework for assessing these systems. It considers conventional, small, and pumped-storage hydropower; turbine selection and cavitation; tidal, wave, and ocean-thermal conversion; and geothermal reservoirs, power cycles, heat pumps, drilling, and environmental management.

Learning Objectives

After studying this chapter, the reader should be able to:

1. Explain the energy-conversion principles of hydropower, marine, and geothermal systems.

2. Estimate hydraulic power and distinguish the principal hydropower configurations.

3. Select turbines using head, discharge, speed, efficiency, and cavitation criteria.

4. Explain the operating principle and system value of pumped-storage hydropower.

5. Compare tidal-range, tidal-stream, wave, and ocean-thermal technologies.

6. Characterize geothermal resources and select appropriate power or heating cycles.

7. Assess drilling, reservoir, environmental, regulatory, and financial constraints.

8. Apply techno-economic and life-cycle reasoning to project decisions.

Hydropower: From Water Resource to Grid Service

The theoretical hydraulic power available from a water stream is

\[

P_h=\rho g Q H

\]

where \(\rho\) is water density, \(g\) is gravitational acceleration, \(Q\) is discharge, and \(H\) is the effective hydraulic head. Actual electrical output is

\[

P_e=\rho g Q H\eta_t\eta_g\eta_a

\]

where \(\eta_t\), \(\eta_g\), and \(\eta_a\) represent turbine, generator, and auxiliary efficiencies. The net head is lower than the gross elevation difference because of intake, penstock, bend, valve, and draft-tube losses. A feasibility assessment must therefore use a duration curve of available flows rather than a single design discharge.

A hydropower project generally includes a catchment and reservoir or diversion structure, intake, trash rack, gates, waterways, penstock or tunnel, turbine, generator, transformer, switchgear, tailrace, and control system. Civil works commonly dominate project cost and construction risk. Geological conditions, sediment transport, floods, seismicity, and access can be more decisive than turbine efficiency.

Conventional configurations

A run-of-river plant uses the natural flow with limited storage. It can have relatively low inundation but its output varies with seasonal discharge. A storage plant uses a reservoir to shift water across time and can provide energy, reserve, frequency regulation, irrigation support, and flood-management benefits. A diversion plant transfers water through a canal or tunnel to create head at a powerhouse, often with a smaller reservoir.

Installed capacity should not be confused with annual energy. Capacity is the maximum rated power, whereas annual generation depends on the flow regime and capacity factor. A plant designed for a high peak discharge may have excellent peak output but low annual utilization. Conversely, a plant with modest rated capacity may generate more consistently. Design discharge is therefore selected by balancing energy yield, equipment cost, spill, ecological flow requirements, and the value of firm capacity.

Small hydropower

Small hydropower is commonly categorized by national regulation rather than by a universal international threshold. It may include pico, micro, mini, and small systems. These plants are useful for rural electrification, isolated industrial loads, water-supply infrastructure, and hybrid mini-grids. Their principal advantage is often not scale efficiency but the ability to use existing weirs, irrigation canals, or water-supply pipelines.

Low civil-work cost, simple controls, maintainability, and local manufacturing can be more important than the highest peak efficiency. Cross-flow, Turgo, Pelton, Francis, and propeller-type machines may all be appropriate depending on head and flow. Environmental flow, fish passage, sediment exclusion, and community access must be incorporated from the outset rather than treated as post-construction additions.

Pumped-storage hydropower

A pumped-storage plant contains upper and lower reservoirs connected by reversible pump-turbines. During periods of low electricity demand or abundant variable renewable generation, electricity drives pumping. During high demand, water is released through the turbine to generate electricity. The round-trip efficiency is less than unity because of hydraulic, mechanical, electrical, and pumping losses, but the plant supplies valuable storage and grid services.

Pumped storage is not a primary energy source; it is an energy-shifting and flexibility technology. Its value includes peak shaving, frequency control, reserve provision, black start, and reduction of renewable curtailment. Site selection depends on elevation difference, reservoir volume, geology, transmission access, water availability, environmental acceptability, and construction cost. Closed-loop systems, which use two artificial reservoirs with limited natural-water interaction, can reduce some river impacts but still involve substantial excavation and land disturbance.

Turbine Selection, Performance, and Cavitation

Turbine selection begins with the available head and discharge. Pelton turbines are impulse machines suited to high head and relatively low flow. Water jets strike buckets at atmospheric pressure, making them comparatively tolerant of changes in tailwater level. Francis turbines are reaction machines used across a broad intermediate-head range and are common in large storage and run-of-river plants. Kaplan and propeller turbines are suitable for low head and high flow; Kaplan runner blades and guide vanes can be adjusted to maintain efficiency over changing discharge. Cross-flow turbines are mechanically simple and useful for small projects with variable flow.

No boundary is absolute. The final choice depends on specific speed, unit size, sediment content, part-load behavior, transient operation, manufacturability, and life-cycle cost. For a given site, several turbines may be technically feasible, but their annual energy, maintenance burden, and cavitation risk may differ substantially.

A reaction turbine operates with pressure changes through the runner and draft tube. If local absolute pressure falls below the water-vapor pressure, vapor bubbles form and subsequently collapse in higher-pressure regions. This phenomenon, cavitation, causes noise, vibration, pitting, efficiency loss, and potentially severe runner damage. It is promoted by excessive turbine setting above tailwater, high temperature, low atmospheric pressure, poor hydraulic design, and off-design operation.

The Thoma cavitation coefficient is commonly used to relate available pressure margin to turbine head. In practice, designers use model tests, manufacturer limits, site elevation, tailwater variation, transient analysis, and a safety margin to determine the permissible setting. Mitigation includes lowering the turbine, improving runner and draft-tube design, controlling operating ranges, maintaining smooth surfaces, and using erosion-resistant materials or coatings. Cavitation should not be treated as merely a maintenance problem; it is a design, operation, and hydraulic-transient issue.

Water hammer is another important transient. Rapid gate closure changes momentum and produces pressure waves in the penstock. Excessive pressure can damage pipes and supports, while excessive negative pressure can cause column separation. Surge tanks, controlled closure laws, air valves, pressure relief devices, and appropriate penstock design are used to manage these effects.

Marine Energy: Predictable Resource, Difficult Environment

Marine energy converts the motion, elevation difference, or thermal gradient of seawater into useful energy. Its technical promise is considerable, but marine deployment is demanding because equipment must operate in corrosive, biologically active, storm-prone environments with expensive maintenance access.

Tidal-range systems

Tidal-range plants use the difference in water level between a basin and the sea. A barrage or lagoon creates a hydraulic head, and turbines generate during ebb generation, flood generation, or both. Reversible bulb turbines may pump as well as generate, allowing limited control of generation timing. The resource is highly predictable, but large structures can alter sediment transport, estuarine habitats, navigation, and water quality. Environmental assessment must examine the whole estuary rather than only the powerhouse.

Tidal-stream systems

Tidal-stream turbines extract kinetic energy from moving tidal currents. The idealized power relation is

\[

P=\frac{1}{2}\rho A v^3 C_p

\]

where \(A\) is swept area, \(v\) is current velocity, and \(C_p\) is the power coefficient. The cubic dependence on velocity makes site selection critical. Devices may use horizontal-axis rotors, vertical-axis rotors, ducted turbines, or oscillating hydrofoils. Arrays require analysis of wake interaction, seabed anchoring, navigation, acoustic emissions, cable routing, and fatigue under reversing flow.

Wave energy

Wave devices convert the oscillatory motion of waves into electricity. Terminology includes point absorbers, attenuators, oscillating water columns, overtopping devices, and oscillating wave-surge converters. The power available in waves depends on significant wave height, wave period, water depth, and spectral distribution. A device optimized for one sea state may perform poorly in another. Survivability during extreme storms often governs design more strongly than average energy capture. Control systems that tune damping or phase can improve capture, but they add sensing, actuation, and failure modes.

Ocean thermal energy conversion

Ocean thermal energy conversion, or OTEC, uses the temperature difference between warm surface water and cold deep water. In a closed cycle, a working fluid such as ammonia vaporizes at warm-water temperature and condenses using cold deep water. An open cycle flashes seawater under vacuum to produce low-pressure steam, which can also yield desalinated water. Hybrid systems combine both concepts.

The thermodynamic efficiency is low because the temperature difference is small. Consequently, large seawater flow rates, heat exchangers, intake pipes, and pumping power are required. Net power, not gross turbine power, is the relevant metric. OTEC is most attractive where warm surface water and deep cold water are geographically close and where electricity, cooling, or desalinated water have high value.

Geothermal Energy: Reservoir, Well, and Cycle

Geothermal energy derives from the Earth’s internal heat. A geothermal project requires a heat source, a reservoir or engineered fracture system, permeability, fluid, and wells capable of sustaining production and reinjection. Resource quality is characterized by temperature, pressure, flow rate, chemistry, depth, and long-term decline behavior.

Hydrothermal reservoirs contain naturally heated water or steam. Dry-steam plants use steam directly in the turbine. Flash-steam plants reduce the pressure of high-temperature water so that part of it vaporizes; single-flash or 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 cycles and Kalina cycles are examples of binary approaches. Binary systems can exploit moderate-temperature resources and keep mineral-laden brine largely separate from the turbine loop.

The first law describes energy transfer, but the second law is especially important because geothermal heat is supplied at finite temperature. Exergy analysis identifies where useful work potential is destroyed through throttling, heat transfer across large temperature differences, condensation, and reinjection. A plant with a high thermal efficiency is not necessarily optimal if it causes excessive reservoir pressure decline or pumping demand.

Geothermal heat pumps

Ground-source heat pumps use the relatively stable temperature of shallow ground as a heat source in winter and a heat sink in summer. A vapor-compression cycle transfers heat through buried horizontal loops, vertical boreholes, or groundwater systems. The coefficient of performance for heating is

\[

COP_h=\frac{Q_h}{W}

\]

and for cooling is \(COP_c=Q_c/W\). Seasonal performance depends on ground temperature, loop design, compressor efficiency, building loads, auxiliary pumps, and thermal balance. A ground-source heat pump is not the same as a deep geothermal power plant: it uses near-surface thermal stability rather than a high-temperature reservoir.

Enhanced geothermal systems

Enhanced geothermal systems seek to create or improve permeability in hot rock by injecting fluid through wells. Hydraulic stimulation can increase heat-transfer area, but it introduces uncertainty in fracture connectivity, fluid losses, induced seismicity, reservoir longevity, and well integrity. Careful monitoring, traffic-light protocols for seismic events, and staged stimulation are essential.

Drilling, Reservoir Management, and Project Economics

Drilling is frequently the largest single source of geothermal project uncertainty. Costs increase sharply with depth, hard or fractured formations, high temperature, lost circulation, corrosive fluids, pressure control requirements, and difficult well trajectories. Casing, cement, logging tools, elastomers, pumps, and wellhead equipment must tolerate the thermal and chemical environment.

Exploration commonly progresses from geological and geochemical surveys to geophysics, temperature-gradient wells, slim holes, and production-scale wells. Exploration risk is not eliminated by a promising surface manifestation. A commercial decision requires evidence of sustainable flow, adequate temperature, acceptable chemistry, reservoir connectivity, and reinjection capacity.

Reservoir management aims to maintain pressure and heat extraction over the project life. Reinjection prevents surface disposal of brine and can support pressure maintenance, but poorly placed injection wells may cause thermal breakthrough. Tracers, pressure interference tests, microseismic monitoring, chemical analysis, and numerical reservoir models help estimate subsurface behavior. Models should be treated as decision tools with uncertainty ranges, not as exact descriptions of an inaccessible reservoir.

Economic appraisal should include exploration success probability, drilling schedule, transmission, water treatment, make-up wells, decommissioning, and the value of capacity and ancillary services. Levelized cost alone may misrepresent a dispatchable plant’s value. A portfolio or system model can assign additional value to firm renewable output, avoided fuel use, and flexibility.

Environmental, Regulatory, and Professional Responsibilities

Hydropower can fragment rivers, obstruct fish migration, change sediment transport, inundate land, alter downstream temperature and flow, and affect communities dependent on water and land. Environmental flows, sediment passage, fish ladders or lifts, resettlement safeguards, transparent consultation, and cumulative basin planning are central design responsibilities.

Marine projects may affect benthic habitats, marine mammals, fisheries, navigation, electromagnetic exposure from cables, and coastal processes. Monitoring should compare observed effects with predicted thresholds and provide adaptive management. Early engagement with fishers, coastal communities, port authorities, and conservation agencies can reduce conflict and improve design quality.

Geothermal projects may produce hydrogen sulfide, silica-rich brine, dissolved metals, noise, land subsidence, and induced seismicity. Reinjection, gas abatement, closed-loop handling, well integrity management, and continuous monitoring reduce risk. Water rights and competing uses must be considered, particularly in arid regions.

Professional practice requires compliance with dam safety rules, electrical codes, pressure-vessel requirements, marine spatial planning, environmental permitting, occupational safety standards, and emergency procedures. Safety cases should address dam failure, flood routing, penstock rupture, turbine overspeed, confined spaces, high voltage, high temperature, hydrogen sulfide, diving, lifting, and offshore weather. Energy access and local livelihood impacts also raise questions of procedural fairness and benefit sharing.

Digital Engineering and Emerging Directions

Modern projects increasingly use geographic information systems, remote sensing, digital elevation models, hydrological forecasting, supervisory control and data acquisition, condition monitoring, and digital twins. Machine-learning models can support short-term inflow forecasting, anomaly detection, predictive maintenance, and wave-resource characterization. Their outputs require physical validation because rare floods, extreme storms, and reservoir nonstationarity may lie outside training data.

Variable-speed pumped storage, hybrid hydro-solar plants, floating solar on reservoirs, low-head fish-friendly turbines, and offshore tidal arrays are active areas of development. Closed-loop geothermal concepts, advanced drilling, superhot-rock research, and improved binary cycles may expand geothermal potential. The principal engineering challenge remains integration: equipment must be designed with storage, transmission, environmental constraints, and system-level flexibility in mind rather than optimized in isolation.

Applied Case: Screening a Multipurpose Renewable-Energy Portfolio

Background

Consider an illustrative coastal region with a seasonal river, a narrow tidal channel, and a high-temperature geothermal prospect near an industrial load center. The regional grid experiences evening peaks and increasing solar curtailment at midday.

Situation and problem

The planning authority must decide whether to develop one large hydropower reservoir, a pumped-storage scheme, a tidal-stream array, or a geothermal plant. Capital is limited, environmental approval is uncertain, and the authority requires both annual energy and reliable evening capacity.

Analysis

Hydrological analysis shows that a storage reservoir could provide substantial firm capacity, but it would inundate a populated valley and modify downstream sediment transport. A run-of-river alternative has lower social impact but little ability to shift generation. A pumped-storage site has favorable elevation and geology, but transmission reinforcement and upper-reservoir construction are expensive.

The tidal channel has predictable currents, yet its mean velocity is insufficient for attractive output except during a limited part of each tidal cycle. Array wake losses and difficult maintenance access reduce its initial economic attractiveness. The geothermal prospect has high temperature, but exploration drilling does not guarantee a productive reservoir. A binary or flash plant would require reinjection and careful management of non-condensable gases and mineral scaling.

Strategy and solution

Rather than choosing solely by nameplate capacity, the authority evaluates four dimensions: firm capacity, annual energy, environmental burden, and development risk. It selects a smaller run-of-river plant using an existing diversion structure, develops pumped storage in a staged manner, and funds geothermal exploration with a decision gate after the first appraisal well. The tidal project is retained as a monitored demonstration rather than treated as the principal capacity resource.

Outcome

The portfolio produces less theoretical energy than the largest reservoir option, but it offers better flexibility, lower social disruption, and a staged exposure to geothermal and marine technologies. Operational data from the demonstration projects improve later investment decisions. The result illustrates that technology selection is a system-design problem, not a competition based only on conversion efficiency.

Learning points

• Resource predictability does not eliminate construction and maintenance risk.
• Firm capacity and annual energy must be evaluated separately.
• Environmental constraints can change the technically preferred option.
• Staging and decision gates are valuable when subsurface or offshore uncertainty is high.
• Hybrid portfolios may create greater system value than a single oversized project.

Common Errors and Best Practices

A frequent hydropower error is using average river flow as design flow without a duration curve. Another is selecting a turbine from head alone while ignoring sediment, part-load efficiency, tailwater variation, and cavitation. In pumped storage, planners may overlook round-trip losses, water balance, transmission constraints, or the operating value of ancillary services.

Marine assessments often use laboratory performance without adequately accounting for biofouling, storm survival, cable failure, wake interaction, and vessel access. OTEC studies can report gross output while underestimating pumping power. Geothermal assessments may overinterpret surface manifestations, omit exploration failure probability, or neglect scaling, corrosion, reinjection, and make-up wells.

Best practice is to combine measured or defensible resource data with uncertainty analysis, life-cycle costing, environmental baseline studies, constructability review, and stakeholder engagement. Design margins should reflect credible extremes, not only average conditions. Independent review, transparent assumptions, staged investment, and post-installation performance verification substantially improve professional decisions.

Chapter Summary

Hydropower converts hydraulic head and discharge into electricity and can provide energy, storage, reserve, and grid flexibility. Run-of-river, storage, small hydro, and pumped-storage schemes have different resource, environmental, and operational characteristics. Pelton, Francis, Kaplan, and cross-flow turbines are selected primarily through head, flow, specific speed, transient behavior, sediment, and cavitation considerations. Cavitation and water hammer require deliberate hydraulic and mechanical design.

Marine energy includes tidal-range, tidal-stream, wave, and ocean-thermal systems. Tidal resources are predictable, but offshore corrosion, storms, environmental effects, wake interaction, and maintenance costs remain significant. OTEC is thermodynamically constrained by its small temperature difference and must be evaluated on net power.

Geothermal projects depend on temperature, permeability, fluid chemistry, wells, and sustainable reservoir management. Dry-steam, flash, binary, enhanced-geothermal, and heat-pump systems serve different temperature and application ranges. Drilling and exploration dominate uncertainty in many projects. Across all technologies, responsible planning integrates resource assessment, exergy and performance analysis, economics, digital monitoring, environmental protection, regulation, and community interests.

Key Terms

Hydraulic head: Elevation and pressure energy available per unit weight of water.

Design discharge: Flow rate selected for sizing a hydropower unit.

Run-of-river plant: Hydropower plant with limited storage that largely follows natural river flow.

Pumped-storage hydropower: Reversible hydroelectric system that stores electricity by pumping water uphill.

Specific speed: Dimensionless or standardized turbine parameter relating speed, flow, and head.

Reaction turbine: Turbine in which pressure changes occur through the runner.

Cavitation: Formation and collapse of vapor bubbles caused by local pressure falling below vapor pressure.

Water hammer: Pressure transient caused by rapid change in fluid velocity in a conduit.

Tidal range: Difference in water level between high and low tide.

Tidal-stream turbine: Machine extracting kinetic energy from tidal currents.

Wave energy converter: Device that converts wave motion into useful power.

Ocean thermal energy conversion: Process using the temperature difference between warm surface and cold deep seawater.

Hydrothermal reservoir: Naturally permeable subsurface formation containing geothermal fluid.

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

Binary cycle: Geothermal cycle transferring heat to a separate low-boiling-point working fluid.

Enhanced geothermal system: Engineered geothermal reservoir created or improved by stimulation.

Reinjection: Return of geothermal fluid to the subsurface reservoir.

Coefficient of performance: Useful heating or cooling delivered by a heat pump per unit electrical work.

Exergy: Maximum theoretical useful work obtainable as a system reaches environmental equilibrium.

Capacity factor: Actual energy generated divided by energy possible at continuous rated output.

Review Questions

Short Answer Questions

1. State the equation for theoretical hydraulic power and explain its variables.

2. Distinguish a run-of-river plant from a storage hydropower plant.

3. Why are Pelton turbines generally associated with high-head applications?

4. What causes cavitation in a reaction turbine?

5. Define reinjection in a geothermal project.

Long Answer Questions

1. Explain the technical and environmental considerations involved in selecting a hydropower turbine.

2. Describe the operating principle, services, and limitations of pumped-storage hydropower.

3. Compare tidal-range, tidal-stream, and wave-energy systems.

4. Explain dry-steam, flash-steam, and binary geothermal power cycles.

5. Discuss drilling and reservoir-management risks in geothermal development.

Analytical / Application Questions

1. A proposed plant has a gross head of 80 m, discharge of 25 m³/s, and combined turbine-generator-auxiliary efficiency of 0.82. Estimate its electrical output and identify additional information required for annual-energy estimation.

2. A low-head site has highly variable discharge and substantial sediment. Compare Kaplan, Francis, and cross-flow options, considering efficiency, maintenance, and operating flexibility.

3. A coastal region must choose between a tidal-stream array and pumped storage to complement midday solar generation. Develop a decision framework using predictability, storage value, environmental effects, infrastructure, and life-cycle risk.

22. Multiple Choice Questions with Answers

Question 1

Which expression gives the theoretical hydraulic power available at a hydropower site?

A. P = ρgQH

B. P = ρQ/Hg

C. P = 1/2 ρAv²

D. P = mcpΔT

Correct Answer: A. P = ρgQH

Question 2

A hydropower project requiring high head and relatively low discharge would most appropriately consider which turbine?

A. Kaplan

B. Pelton

C. Propeller

D. Bulb turbine

Correct Answer: B. Pelton

Question 3

The principal purpose of pumped-storage hydropower is to

A. create primary energy from water

B. increase the chemical energy of water

C. shift electricity across time and provide grid flexibility

D. eliminate all transmission losses

Correct Answer: C. shift electricity across time and provide grid flexibility

Question 4

Cavitation damage occurs when

A. water velocity becomes exactly zero

B. local absolute pressure falls below water-vapor pressure

C. generator frequency exceeds grid frequency

D. sediment concentration becomes negligible

Correct Answer: B. local absolute pressure falls below water-vapor pressure

Question 5

The power available from a tidal-stream turbine is especially sensitive to current velocity because power varies approximately with

A. v

B. v²

C. v³

D. 1/v

Correct Answer: C. v³

Question 6

A major limitation of ocean thermal energy conversion is that it

A. requires a very large temperature difference

B. has a small temperature difference and substantial pumping requirements

C. can operate only in freshwater

D. cannot produce electricity from a heat engine

Correct Answer: B. has a small temperature difference and substantial pumping requirements

Question 7

Which geothermal cycle is particularly suitable for moderate-temperature resources and keeps geothermal brine separate from the turbine loop?

A. Binary cycle

B. Dry-steam cycle

C. Open chimney cycle

D. Diesel cycle

Correct Answer: A. Binary cycle

Question 8

In geothermal reservoir management, reinjection is used primarily to

A. dispose of or return geothermal fluid and support reservoir management

B. increase surface evaporation

C. remove the need for production wells

D. convert all brine directly into steam

Correct Answer: A. dispose of or return geothermal fluid and support reservoir management

Question 9

Which factor most directly reduces the net output of an OTEC plant relative to its gross turbine output?

A. Pumping power for large seawater flows

B. Absence of seawater

C. Very high combustion losses

D. High-temperature flame instability

Correct Answer: A. Pumping power for large seawater flows

Question 10

A project decision gate after an exploratory geothermal well is useful because it

A. guarantees a commercial reservoir before drilling

B. allows investment to continue only if temperature, flow, chemistry, and reinjection evidence are adequate

C. eliminates the need for environmental assessment

D. makes reservoir models unnecessary

Correct Answer: B. allows investment to continue only if temperature, flow, chemistry, and reinjection evidence are adequate