Geothermal and Ambient Heat Systems

Introduction and Context

Geothermal and ambient heat systems use naturally available thermal energy rather than relying solely on combustion or electrical resistance. Geothermal energy is heat stored in the Earth, while ambient heat is low-temperature energy present in air, surface water, groundwater, soil, and waste streams. Because ambient heat is often below the useful temperature required by a building or industrial process, a heat pump is used to raise its temperature.

These technologies are important in decarbonizing heating, cooling, hot-water production, and selected industrial processes. Their environmental performance depends not only on the energy source but also on electricity-carbon intensity, refrigerant choice, drilling impacts, system sizing, operating temperatures, and the long-term balance between heat extraction and heat rejection.

Learning Objectives

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

1. Classify geothermal resources and explain the nature of geothermal reservoirs.
2. Describe geothermal exploration, drilling, wells, and production constraints.
3. Distinguish direct-use geothermal systems from geothermal-electric power cycles.
4. Explain the operating principles of ground-, air-, and water-source heat pumps.
5. Calculate coefficient of performance and interpret seasonal performance.
6. Select refrigerants and size major heat-pump components using engineering criteria.
7. Evaluate practical, environmental, economic, and regulatory issues in system deployment.

1. Geothermal Resources and Reservoirs

1.1 Origin of geothermal heat

Geothermal heat originates mainly from primordial heat retained from Earth’s formation and radioactive decay of elements within the crust and mantle. Temperature generally increases with depth. The geothermal gradient varies by location, but a commonly used continental average is approximately 25–30 °C/km. This value is not universal: volcanic regions may have much larger gradients, whereas stable crystalline regions may have smaller ones.

A geothermal resource is useful when heat can be economically accessed and transferred to a customer or power cycle. Resource quality depends on temperature, fluid flow rate, permeability, chemistry, depth, reservoir volume, and sustainability of extraction.

1.2 Resource classification

Temperature boundaries are approximate rather than universal design rules. A reservoir is commonly described as hydrothermal when naturally occurring hot water or steam can circulate through permeable rock. A hot-dry-rock or EGS project creates or improves permeability so that an injected fluid can recover heat from hot rock.

1.3 Reservoir engineering

A geothermal reservoir contains heat, fluid, and a transport pathway. Production wells withdraw hot fluid, while injection wells return cooled fluid to maintain pressure and reduce surface disposal. Reservoir performance is governed by Darcy flow through porous media, although fractured-rock reservoirs may require more complex models.

For a porous medium, Darcy’s law may be written as:

Here, Q is volumetric flow rate (m³/s), k is permeability (m²), A is flow area (m²), μ is dynamic viscosity (Pa·s), ∆P is pressure difference (Pa), and L is flow length (m). The equation assumes single-phase, laminar flow through a representative porous medium. The negative sign indicates flow from high to low pressure. In fractured reservoirs, permeability may be directional and the simple homogeneous form may be inadequate.

The recoverable thermal power is approximately:

where Q̇_th is useful thermal rate (W), ṁ is mass flow rate (kg/s), c_p is specific heat capacity (J/(kg·K)), and T_in and T_out are inlet and outlet temperatures (°C or K difference). For water, c_p is approximately 4.18 kJ/(kg·K) over many building-system temperature ranges. The equation neglects phase change, heat losses, and variation of properties.

A reservoir must be managed so that pressure, temperature, and chemistry remain acceptable over the project life. Excessive extraction can cause pressure decline and thermal depletion; poorly designed injection can cause premature thermal breakthrough, in which cooled injected fluid reaches production wells before sufficient heat recovery has occurred.

2. Exploration, Drilling, and Well Systems

2.1 Exploration sequence

A geothermal project normally progresses through reconnaissance, geological and geochemical surveys, geophysical investigation, exploratory drilling, well testing, reservoir modelling, and development drilling. Surface indicators such as hot springs, fumaroles, altered minerals, and elevated soil-gas concentrations can identify promising regions, but they do not prove commercial productivity.

Geophysical methods include resistivity surveys, magnetotellurics, gravity, seismic methods, and temperature-gradient drilling. Each method provides indirect evidence; reliable decisions require integration of multiple datasets.

2.2 Drilling and completion

Geothermal wells may be several kilometres deep and must withstand high temperature, pressure, corrosion, and scaling. Major activities include drilling, casing installation, cementing, directional control, well logging, perforation or open-hole completion, and pressure or flow testing.

Well design must consider:

• rock stability and lost circulation;
• casing strength at temperature and pressure;
• cement integrity and thermal expansion;
• corrosion from dissolved gases and salts;
• silica, carbonate, and other mineral scaling;
• wellhead pressure and flow-control requirements;
• worker safety during drilling and production.

Production wells bring hot fluid to the surface. Injection wells return brine or condensate to the reservoir. A separator may divide steam and liquid, while pumps may be required where reservoir pressure cannot sustain flow.

2.3 Environmental and regulatory controls

Potential impacts include induced seismicity, land disturbance, noise, groundwater interaction, air emissions from naturally occurring hydrogen sulfide or carbon dioxide, and disposal of saline brine. Reinjection, closed-loop designs, monitoring networks, chemical control, and emergency procedures reduce risks. Regulation commonly addresses water protection, well integrity, seismic monitoring, emissions, land access, and abandonment obligations.

3. Direct-Use Geothermal Systems

Direct-use systems transfer geothermal heat to a load without first converting it to electricity. Applications include district heating, greenhouse heating, aquaculture, bathing, crop drying, snow melting, industrial washing, and some process heating.

A direct-use system may contain a production well, pump, heat exchanger, distribution network, reinjection well, and auxiliary boiler. A heat exchanger is often preferred when geothermal fluid chemistry could foul or corrode the building-side circuit. Cascaded use is particularly effective: high-temperature fluid can first serve an industrial process, then a district-heating network, and finally a greenhouse or bathing load before reinjection.

The annual useful energy is:

For a constant load operating for time t, this becomes E_useful = Q̇_useful t. Units must be consistent; for example, kW multiplied by hours gives kWh. Pumping electricity and auxiliary energy must be subtracted when assessing net benefit.

4. Geothermal Power Cycles

4.1 Dry-steam plants

Dry-steam plants use naturally produced steam directly in a turbine. The steam expands through the turbine, producing shaft work, and is then condensed. Such resources are uncommon and require suitable steam quality and chemistry.

4.2 Flash-steam plants

In a flash plant, high-pressure geothermal liquid is depressurized. Some liquid flashes into steam because the saturation temperature falls with pressure. The steam drives a turbine, while separated brine may be flashed again in a double-flash plant or reinjected.

Flash systems require careful control of moisture, dissolved gases, scaling, and turbine corrosion. Their performance depends strongly on wellhead enthalpy, separator pressure, condenser pressure, and mass flow.

4.3 Binary-cycle plants

Binary plants use geothermal fluid to heat a secondary working fluid with a lower boiling point. The secondary fluid circulates in a closed Rankine cycle consisting of a pump, preheater, vaporizer, turbine, and condenser. Organic Rankine cycles are common for moderate-temperature resources.

The geothermal fluid and working fluid remain separate, reducing direct turbine exposure to brine. However, the plant has additional heat exchangers, pumps, controls, and working-fluid safety requirements. Net electric output is:

The net electrical efficiency is:

where Q̇_geo,in is the geothermal heat supplied to the plant. This efficiency is generally modest because geothermal source temperatures are much lower than combustion temperatures, but geothermal plants can operate for many hours per year.

5. Heat-Pump Fundamentals

A heat pump transfers heat from a lower-temperature source to a higher-temperature sink by consuming work. In heating mode, the useful output is condenser heat. In cooling mode, the useful output is evaporator heat removed from the conditioned space.

For a steady heat pump:

where Q̇_H is heat delivered at the high-temperature side, Q̇_L is heat absorbed at the low-temperature side, and Ẇ_in is compressor and associated input power. The coefficient of performance in heating mode is:

For cooling mode:

COP is dimensionless, although capacity and power must be expressed in the same units. It is not an efficiency in the strict thermodynamic sense and can exceed 1 because the system moves heat rather than converting work directly into heat.

For an ideal reversible heat pump operating between absolute temperatures T_H and T_L:

Temperatures must be in kelvin. Real systems have lower COP because of finite temperature differences, compressor inefficiency, pressure drops, fan and pump power, frosting, cycling, and heat loss. Lower delivery temperature and higher source temperature generally improve heating COP.

Worked example 1: heating COP

A heat pump delivers 12 kW of heat while consuming 3 kW of electrical power. Its heating COP is:

The remaining relation gives source heat: Q̇_L = 12 − 3 = 9 kW. Thus, 9 kW is extracted from the source and 3 kW is supplied as electrical work.

6. Ground-, Air-, and Water-Source Heat Pumps

6.1 Ground-source heat pumps

A ground-source heat pump (GSHP) exchanges heat with soil or rock through horizontal ground loops, vertical boreholes, or groundwater wells. Closed-loop systems circulate water or a water-antifreeze mixture. Open-loop systems pump groundwater through a heat exchanger and then discharge or reinject it subject to water regulations.

Ground temperature at sufficient depth changes less than outdoor air temperature. Consequently, a GSHP can have a relatively stable source temperature and high seasonal performance. Its disadvantages include drilling or excavation cost, land requirements, loop design complexity, and possible long-term ground-temperature imbalance.

6.2 Air-source heat pumps

An air-source heat pump (ASHP) exchanges heat with outdoor air. It is easier and usually less expensive to install than a GSHP, but its source temperature varies widely. Heating capacity and COP fall during cold weather. Frost can form on the outdoor coil when its surface is below the air dew point and near or below 0 °C, requiring periodic defrosting. Defrost consumes energy and can temporarily reverse or interrupt heating operation.

Cold-climate ASHPs use variable-speed compressors, enhanced vapor injection, larger heat exchangers, improved controls, and carefully selected refrigerants to maintain useful capacity at low temperature.

6.3 Water-source heat pumps

Water-source systems use groundwater, lakes, rivers, cooling-tower loops, wastewater, or hybrid thermal networks. Water often provides a more stable temperature and better heat transfer than air. Design must address fouling, corrosion, biological growth, water rights, discharge temperature, freezing risk, and environmental protection.

Comparison

7. Seasonal Performance and Energy Analysis

A rated COP is measured at a specified operating condition and does not represent an entire year. Seasonal performance accounts for changing source temperature, load, cycling, auxiliary energy, and defrost.

A useful seasonal performance factor is:

where Q_H,i is useful heating delivered during interval i and W_in,i is total electrical input during that interval. The input should include compressor, fans, pumps, controls, crankcase heaters, and defrost energy when evaluating whole-system performance.

If annual useful heating is 24,000 kWh and total electricity use is 6,000 kWh, then:

The annual heat extracted from the environment is 18,000 kWh, since delivered heat equals environmental heat plus electricity.

For a combined heating and cooling assessment, heating and cooling should be reported separately or converted using a clearly stated weighting method. A single annual number can hide poor performance in one mode.

8. Refrigerants and Heat-Pump Components

A vapor-compression heat pump includes a compressor, condenser, expansion device, evaporator, refrigerant, heat-transfer fluids, controls, and safety devices. In heating mode, the indoor heat exchanger acts as the condenser and the outdoor or ground loop heat exchanger acts as the evaporator. A reversing valve permits cooling operation in many systems.

Refrigerant selection must consider:

• global warming potential (GWP);
• ozone depletion potential (ODP);
• flammability classification;
• toxicity and occupational exposure;
• operating pressure and discharge temperature;
• volumetric heating capacity;
• compatibility with oils, seals, and materials;
• regulatory availability and end-of-life recovery.

No refrigerant is universally optimal. Hydrofluorocarbon phase-down policies have encouraged lower-GWP alternatives such as hydrocarbons, carbon dioxide, ammonia, and newer hydrofluoroolefin-based blends. Hydrocarbons can be highly efficient but are flammable; ammonia has excellent thermodynamic properties but requires toxicity controls; carbon dioxide operates at high pressure and is especially relevant to hot-water systems. Selection must follow applicable safety standards and codes rather than GWP alone.

Refrigerant leakage can substantially increase lifecycle climate impact. Good practice includes factory-sealed circuits where feasible, leak detection, trained servicing, recovery and reclamation, accurate charging, and documentation.

9. System Sizing and Design Criteria

9.1 Heating-load sizing

The design heating load is estimated from transmission, ventilation, infiltration, solar effects, internal gains, and hot-water demand. A simplified transmission-loss estimate is:

where U_j is thermal transmittance of building element j (W/(m²·K)), A_j is its area (m²), and ∆T is indoor-outdoor design temperature difference (K). Ventilation and infiltration load may be approximated by:

The complete load calculation must comply with the relevant building standard and include local climate data. Oversizing is undesirable because it increases cost, cycling, standby losses, and sometimes minimum modulation problems. Undersizing may require backup heat and produce unacceptable indoor temperatures.

9.2 Source-side sizing

For a closed ground loop, approximate heat extraction is:

Loop length depends on ground conductivity, undisturbed temperature, borehole resistance, spacing, fluid temperature limits, annual heating and cooling balance, and allowable temperature drift. Manufacturer data and validated ground-loop software are required for final design; a single rule-of-thumb length is not transferable between sites.

9.3 Distribution temperature

Heat pumps perform best with low-temperature emitters such as oversized radiators, fan coils, or underfloor heating. Reducing the required supply temperature reduces compressor pressure ratio and improves COP. Domestic hot water may require a higher temperature, so a separate high-temperature stage, controlled pasteurization cycle, or auxiliary heater may be necessary.

9.4 Controls and commissioning

Important controls include variable-speed modulation, weather compensation, supply-temperature reset, flow verification, freeze protection, defrost control, thermal storage, and demand management. Commissioning should verify flow rates, temperature differences, refrigerant charge, sensor calibration, electrical power, noise, safety interlocks, and actual capacity at representative conditions.

10. Worked Design Example

Consider an illustrative small building with a design heating load of 18 kW. A selected heat pump has a design heating COP of 4.0 at the required source and delivery temperatures.

Electrical input is:

Ground-side extraction is:

If a preliminary closed-loop calculation indicates an average design extraction rate of 45 W per metre of active borehole, the required active borehole length is:

This might be provided as three 100 m boreholes, subject to spacing, drilling constraints, thermal-response testing, and annual simulation. The result is not a final design because 45 W/m is site-specific and must be verified. If the system operates 1,800 equivalent full-load heating hours, the approximate delivered heating energy is 32,400 kWh, but actual annual energy depends on part-load operation and weather.

11. Illustrative Case: Electrifying a Rural Community Heating System

Background

This fictional illustrative case concerns a rural community currently using individual oil boilers. The buildings have moderate insulation, a cold winter climate, and access to a suitable shallow aquifer. A community energy cooperative is considering a water-source heat-pump network.

Problem

The cooperative must reduce fuel use without imposing excessive electricity demand or compromising groundwater quality. Peak heating demand is 500 kW, while average seasonal demand is much lower. The aquifer temperature is relatively stable, but its sustainable yield and reinjection characteristics are uncertain.

Analysis

Hydrogeological testing is performed before final design. The proposed network uses production and reinjection wells, plate heat exchangers, variable-speed heat pumps, low-temperature distribution, and a thermal-storage tank. A backup boiler is retained for extreme peaks and maintenance. The design team compares annual electricity use using simulated hourly source temperatures and building loads rather than applying a rated COP to all hours.

Suppose annual useful heat is 900 MWh and modeled total heat-pump electricity is 225 MWh. The seasonal performance factor is:

The study also includes pumping energy, well drawdown, reinjection temperature, electricity-carbon intensity, refrigerant leakage, and lifecycle cost.

Approach and solution

The cooperative adopts staged construction: one well pair and a limited distribution zone are commissioned first. Continuous monitoring measures groundwater temperature, pressure, flow, reinjection chemistry, indoor comfort, and electricity consumption. Expansion is permitted only if the aquifer shows acceptable pressure and thermal recovery.

Outcome and learning points

The staged approach reduces resource risk and produces site-specific operating evidence. The retained backup system improves resilience, while low-temperature distribution improves heat-pump performance. The case demonstrates that successful projects integrate thermodynamics, hydrogeology, controls, finance, regulation, and community governance. A high COP alone does not guarantee sustainability; source recovery, peak capacity, water protection, and lifecycle emissions must also be demonstrated.

12. Challenges, Limitations, and Contemporary Developments

Key challenges include high upfront cost, uncertain drilling success, ground-temperature imbalance, variable air-source performance, grid constraints, refrigerant regulation, water permits, and limited skilled installers. Economic comparisons should include capital cost, maintenance, electricity and fuel prices, replacement risk, incentives, and discount rate.

Hybrid systems combine heat pumps with boilers, solar thermal systems, thermal storage, or district-energy networks. Digital controls can forecast weather and electricity prices, coordinate thermal storage, detect performance degradation, and participate in demand response. Artificial intelligence may assist fault detection and predictive control, but it cannot replace sensor validation, physical commissioning, or engineering accountability. Models trained on biased or incomplete data can misdiagnose faults, while cybersecure control architecture is essential for connected systems.

Best practices are to reduce building loads first, use low-temperature distribution, select equipment from verified performance maps, size for realistic design conditions, include total auxiliary power, protect groundwater and geothermal reservoirs, choose refrigerants through a whole-life risk assessment, provide monitoring access, and commission the installed system against measured performance.

Chapter Summary

Geothermal resources range from shallow ground heat to high-temperature hydrothermal reservoirs and engineered hot-rock systems. Resource quality depends on temperature, flow, permeability, chemistry, depth, and sustainable reservoir management. Exploration combines geological, geochemical, geophysical, and drilling evidence. Direct-use systems can provide efficient heat for buildings, agriculture, bathing, and industry, while dry-steam, flash, and binary cycles generate electricity from different resource conditions.

Heat pumps move heat from a low-temperature source to a useful sink. Heating COP is the ratio of delivered heat to input work, but seasonal performance is more meaningful because source temperature, load, defrost, cycling, and auxiliary power vary with time. Ground-source systems offer stable temperatures but require costly loops or wells; air-source systems are easier to install but are sensitive to cold weather and frost; water-source systems can perform well but require careful water-quality and regulatory management.

Effective design requires accurate load calculations, appropriate source-side sizing, low delivery temperatures, suitable refrigerants, safety controls, commissioning, and long-term monitoring. Sustainability requires more than a high COP: it includes reservoir or aquifer balance, lifecycle emissions, refrigerant containment, environmental protection, resilience, and responsible governance.

Key Terms

Geothermal energy: Thermal energy stored within the Earth.

Geothermal reservoir: A subsurface volume containing heat, fluid, and a pathway for fluid movement.

Hydrothermal resource: A naturally permeable geothermal system containing hot water or steam.

Enhanced geothermal system: An engineered system that improves fluid circulation through hot rock.

Geothermal gradient: Increase in temperature with depth.

Production well: Well that extracts geothermal fluid.

Injection well: Well that returns fluid to a geothermal reservoir.

Direct use: Application of geothermal heat without conversion to electricity.

Flash-steam cycle: Power cycle in which pressurized hot liquid is depressurized to produce steam.

Binary cycle: Power cycle in which geothermal heat vaporizes a separate working fluid.

Heat pump: Device that uses work to transfer heat from a lower to a higher temperature.

COP: Ratio of useful heat transfer rate to power input under specified conditions.

Seasonal performance factor: Seasonal useful heat delivered divided by seasonal electrical input.

Ground-source heat pump: Heat pump exchanging heat with soil or rock.

Air-source heat pump: Heat pump exchanging heat with outdoor air.

Water-source heat pump: Heat pump exchanging heat with groundwater, surface water, or a water loop.

Refrigerant: Working fluid circulating through a vapor-compression heat pump.

Thermal breakthrough: Premature arrival of cooled injection fluid at a production well.

Defrost cycle: Operation that removes frost from an air-source heat exchanger.

Thermal response test: Field test used to estimate ground thermal properties for loop design.

Review Questions

Short Answer Questions

1. What physical characteristics determine the usefulness of a geothermal reservoir?
2. Why is reinjection used in many geothermal projects?
3. Define heating-mode COP and distinguish it from efficiency.
4. State one major advantage and one limitation of ground-source heat pumps.
5. Why does frost form on an air-source heat-pump coil?

Long Answer Questions

1. Explain the exploration, drilling, completion, and testing sequence for a geothermal project.
2. Compare dry-steam, flash-steam, and binary geothermal power cycles.
3. Explain how source temperature and delivery temperature affect real heat-pump COP.
4. Discuss refrigerant selection using environmental, safety, thermodynamic, and regulatory criteria.
5. Describe a complete engineering approach to sizing and commissioning a heat-pump system.

Analytical / Application Questions

1. A heat pump delivers 20 kW of heat at a COP of 4.0. Calculate electrical input and source-side heat extraction.
2. A building requires 360 MWh of useful annual heat and consumes 90 MWh of total heat-pump electricity. Calculate the seasonal performance factor and interpret the result.
3. A proposed aquifer heat-pump project has high modeled COP but uncertain reinjection performance. Identify the technical measurements and governance conditions that should be satisfied before expansion.

Multiple Choice Questions with Answers

Question 1

A heat pump delivers 12 kW of heat while consuming 3 kW of electrical power. What is its heating COP?

A. 0.25

B. 3.0

C. 4.0

D. 9.0

Correct Answer: C. 4.0

Question 2

Which geothermal power cycle uses a separate secondary working fluid in a closed loop?

A. Dry-steam cycle

B. Binary cycle

C. Single-flash cycle

D. Direct steam cycle

Correct Answer: B. Binary cycle

Question 3

For a heat pump delivering 18 kW at a heating COP of 4.0, what is the approximate source-side heat extraction?

A. 4.5 kW

B. 13.5 kW

C. 18 kW

D. 22.5 kW

Correct Answer: B. 13.5 kW

Question 4

Which condition generally improves the heating COP of a heat pump?

A. Higher delivery temperature

B. Lower source temperature

C. Higher source temperature

D. Greater refrigerant leakage

Correct Answer: C. Higher source temperature

Question 5

What is the principal purpose of reinjection in a geothermal reservoir?

A. To increase combustion temperature

B. To return fluid and help manage pressure and disposal

C. To eliminate the need for production wells

D. To convert all brine into steam

Correct Answer: B. To return fluid and help manage pressure and disposal

Question 6

Which heat-pump type is most directly affected by outdoor frost and defrost operation?

A. Air-source

B. Ground-source

C. Closed-loop water-source

D. Deep geothermal direct-use

Correct Answer: A. Air-source

Question 7

A system delivers 900 MWh of annual heat and uses 225 MWh of electricity. What is its seasonal performance factor?

A. 0.25

B. 2.0

C. 4.0

D. 1,125

Correct Answer: C. 4.0

Question 8

In Darcy’s law, permeability primarily represents the ability of what medium to transmit fluid?

A. A turbine blade

B. A porous or fractured geological formation

C. A refrigerant compressor

D. An electrical conductor

Correct Answer: B. A porous or fractured geological formation

Question 9

Which factor is especially important when selecting a refrigerant?

A. Only its color

B. Only its purchase price

C. GWP, flammability, pressure, toxicity, and compatibility

D. Its ability to remain solid during operation

Correct Answer: C. GWP, flammability, pressure, toxicity, and compatibility

Question 10

Why can a heat pump have a COP greater than one?

A. It creates energy from nothing

B. It transfers environmental heat in addition to using input work

C. It violates the first law of thermodynamics

D. It converts all electricity into mechanical work with no losses

Correct Answer: B. It transfers environmental heat in addition to using input work