10.3 Low-Temperature Geothermal And Direct Use

Understanding Low-Temperature Geothermal

Low-temperature geothermal refers to geothermal resources whose temperatures are typically below about 150 °C, and often in the range of 20 to 100 °C. These resources are usually too cool to be efficient for conventional electricity generation, but they are ideal for direct use of heat. Direct use means that the heat from the ground is applied directly to a process such as space heating, bathing, or industrial use, instead of first converting it to electricity.

Such resources are widespread. In many regions, warm groundwater or shallow heat reservoirs exist at relatively modest depths. Compared to high-temperature systems, low-temperature geothermal usually involves lower drilling costs, simpler technology, and easier integration into buildings and local heating networks. This makes it an important option for reducing fossil fuel use in heating, which is a major part of total energy consumption worldwide.

Sources Of Low-Temperature Geothermal Heat

Low-temperature geothermal resources can be found in several geological settings. One common source is sedimentary basins where water is heated as it circulates through deep porous rock layers insulated by overlying sediments. These aquifers may reach temperatures suitable for district heating or industrial processes even far away from active volcanoes.

Another source is regions with moderate tectonic activity or fault systems. Water can percolate along faults to greater depths, gain heat from the surrounding rock, and then return closer to the surface as a warm or hot spring. Many historic spa towns developed around these naturally accessible warm waters.

In addition, at shallow depths almost everywhere, the ground temperature remains relatively constant through the year once you go a few meters below the surface. This near-surface heat, though not hot, is still geothermal in origin and can be very effectively used for heating or cooling when combined with simple technologies such as heat exchangers and, where needed, heat pumps. This shallow resource forms the basis for many direct-use systems around the world.

Types Of Direct Geothermal Use

Direct uses of low-temperature geothermal heat can be grouped according to where and how the heat is applied. The choice depends on the temperature of the resource, the proximity to the users, and local economic conditions.

A very visible category is bathing, wellness, and recreational uses. Hot springs and warm groundwater have been used for bathing and health resorts for centuries. These applications often require only basic water handling, such as filtration or disinfection, and a way to manage the discharged water.

A second, more energy-focused category is space heating and cooling in buildings. In colder climates, low-temperature geothermal water can be circulated through radiators, underfloor systems, or fan-coil units to heat homes, offices, and public buildings. In milder climates or for modern well-insulated buildings, even relatively modest temperatures can provide most or all of the required heating. In some cases, the same system can provide cooling by using the ground as a heat sink.

District heating systems form another important category. Here, geothermal heat feeds a network of insulated pipes that distribute hot water to many buildings in a town or neighborhood. These networks can be served entirely by geothermal heat where conditions are favorable, or geothermal can be one of several heat sources in a mixed system.

There are also many industrial and agricultural direct uses. Greenhouses often rely on geothermal heat to maintain a suitable temperature for crops, which can extend growing seasons or allow cultivation of more sensitive plants. Aquaculture facilities may use warm water to raise fish or other aquatic organisms at optimal temperatures. Certain industrial processes, such as drying of crops, food processing, or low-temperature washing and pasteurization, can also use geothermal heat directly when the temperature matches process requirements.

Direct Geothermal Heating In Buildings

Using low-temperature geothermal heat directly in buildings usually involves circulating warm water from an underground source through heat distribution systems. The key is to match the supply temperature from the geothermal resource with the needs of the building. Modern buildings with efficient envelopes and well-designed heating systems often operate with lower temperature requirements, which makes low-temperature resources especially suitable.

In a typical building-level direct-use system, geothermal water is pumped from a production well to a heat exchanger. On the other side of the heat exchanger, a closed-loop circuit circulates water through radiators, floor heating pipes, or air-handling units. This separation between the geothermal fluid and the building loop helps protect indoor equipment from mineral scaling, gas content, or any contaminants in the geothermal water.

For very low-temperature resources, designers focus on high-efficiency heat delivery. Underfloor radiant heating is a common choice, since it can provide good comfort at lower water temperatures than traditional radiators. In some cases, fan-assisted convectors or larger radiator surfaces are used to maximize heat transfer. Controls and thermostats regulate flow and maintain desired indoor temperatures, while flow rates are adjusted to maintain efficient use of the geothermal resource.

Cooling can also be supported through direct use of the ground. In some designs, relatively cool groundwater or ground-cooled loops circulate through fan-coil units or radiant systems and remove heat from the building. This reduces or replaces the need for electrically driven chillers, especially in climates where summer temperatures are moderate. The ability to serve both heating and cooling needs with one geothermal source increases the overall value of the system.

Geothermal District Heating And Cooling

District heating systems supplied by low-temperature geothermal resources can deliver large-scale benefits by serving many buildings from a central source. In such systems, one or more production wells bring warm water to the surface, which then flows through a network of insulated pipes across a neighborhood or city district. Users tap into the network at building-level heat exchangers, which transfer heat from the district loop to individual heating systems.

The design temperature of the district network is chosen to match both the geothermal resource and the needs of the connected buildings. Some systems operate at relatively high supply temperatures where the resource allows, while others intentionally operate at lower temperatures to reduce heat losses in the pipes and improve overall efficiency. Lower temperature networks are particularly well suited to new or renovated building stock with efficient heating systems.

To maintain sustainability, the district network may include injection wells that return cooled geothermal water to the same reservoir after heat extraction. This reinjection practice helps maintain reservoir pressure and temperature over time and minimizes surface disposal issues. In many cases, district heating systems add backup or peak-load boilers that use alternative fuels for very cold days, while geothermal supplies the base heating needs.

Cooling can be incorporated in district systems using the ground as a heat sink. In district cooling, relatively cool water is distributed to buildings to absorb indoor heat. Some advanced networks operate as combined heating and cooling grids, where heat is shifted between different users and the ground. Low-temperature geothermal can support such thermal networks by providing stable reference temperatures and seasonal storage options.

Agricultural And Industrial Direct Uses

Agriculture benefits significantly from low-temperature geothermal applications, particularly in regions with cool climates or where long growing seasons are desired. Greenhouses heated with geothermal water allow farmers to maintain optimal temperatures for vegetables, flowers, or other high-value crops. Heating can be provided by piping warm water through pipes under benches, along greenhouse floors, or through air heaters. The steady, often low-cost heat supply can improve yields and reduce dependence on fossil fuels.

Aquaculture is another major user. Fish, shrimp, and other aquatic species often thrive within narrow temperature ranges. Geothermal heating of ponds, tanks, or recirculating systems can keep water within this optimal band. Constant temperatures improve growth rates and survival, which can make operations more productive and resilient to weather fluctuations.

Many industrial processes require heat at moderate temperatures, which matches well with low-temperature geothermal resources. Examples include food processing such as pasteurization, washing, and drying of products, as well as drying timber or other materials. In these cases, geothermal heating can replace or supplement boilers that would otherwise burn fuel. Heat is usually transferred via heat exchangers to maintain process cleanliness and to protect equipment from geothermal fluid properties.

The key to successful agricultural and industrial direct use is matching resource characteristics and user demand. Suitable temperature, flow rate, and reliability of the geothermal source must align with the timing and intensity of process heat needs. Where this alignment exists, geothermal direct use can lower operating costs and emissions while providing predictable long-term heat prices.

Temperature Matching And Cascade Use

Because low-temperature geothermal resources have limited maximum temperature, systems must be designed carefully to use the available heat as efficiently as possible. This involves matching each use to a suitable temperature level and using the remaining heat from one process for another one that needs a lower temperature. This concept is known as cascade use.

For example, a geothermal system may first use the hottest available water to serve an industrial process that demands higher temperature. After the fluid leaves that process at a reduced temperature, it may still be warm enough to heat greenhouses. Once it cools further, it might still be adequate for space heating in well-insulated buildings or for preheating domestic hot water. In some designs, the final low-grade heat can serve aquaculture or soil warming before the fluid is finally reinjected.

The guiding principle is that each step in the cascade uses the geothermal heat at the highest practical temperature difference between the fluid and the application. This maximizes the total useful energy extracted from each unit of geothermal water. Proper sequencing of applications and careful control of flow and temperatures are essential to maintain efficient cascade systems.

By respecting this rule, designers increase the overall energy yield and economic value of a geothermal project and reduce the need for additional fuel or backup systems.

System Components And Basic Design Considerations

Low-temperature geothermal direct-use systems rely on a relatively simple set of components, although details vary by application. The starting point is one or more wells. Production wells bring water to the surface, while injection wells return used water back to the reservoir. Proper design and placement of these wells help maintain sustainable flow and temperature, prevent interference between wells, and minimize environmental impacts.

At the surface, pumps move geothermal water through the system. Submersible pumps are often installed within the production wells to lift water, while surface pumps maintain circulation in distribution networks. Pump selection balances the required flow rates and pressures against energy consumption, since pumping power is a significant part of operating costs.

Heat exchangers are central in most designs. They transfer heat from geothermal water to secondary fluids used in buildings, greenhouses, or industrial processes. Plate heat exchangers are common due to their compact size and high efficiency, but shell-and-tube designs may be used where fluids contain more particles or pose scaling risks. Separating geothermal fluid from user-side circuits protects end-use equipment from corrosion and deposition.

Control systems regulate temperatures, flows, and pressures throughout the network. Valves and sensors allow operators to respond to changing heat demands, outdoor conditions, and resource behavior. In district systems, metering at building connections supports fair billing and encourages efficient use.

Insulation of pipes and equipment is vital, especially when transporting heat over distance. Heat losses along the distribution network reduce the effective temperature and waste energy. High-quality insulation materials and careful installation preserve more of the original geothermal heat for end users.

Efficiency, Performance, And Basic Metrics

Although direct-use systems do not involve electricity generation, their performance can still be evaluated using simple energy concepts. The useful thermal power delivered to users depends on the flow rate of geothermal water and the temperature drop across the heat exchanger or application. In simplified form, the thermal power $Q$ delivered can be expressed as:

$$
Q = \dot{m} \, c_p \, \Delta T
$$

where $\dot{m}$ is the mass flow rate of the geothermal fluid, $c_p$ is its specific heat capacity, and $\Delta T$ is the temperature difference between inlet and outlet at the point of heat use.

System efficiency in direct-use applications often refers to how much of the available geothermal heat is actually transferred to useful purposes compared to how much is lost in pipes, heat exchangers, and storage. Good insulation, well-sized exchangers, and appropriate temperatures improve this efficiency.

Another important metric is the ratio of useful heat delivered to the electrical energy consumed by pumps and auxiliary equipment. Even though pumps require electricity, this ratio can be very favorable. A well-designed low-temperature geothermal direct-use system can often deliver several units of thermal energy for each unit of electricity used for pumping. This makes it a highly effective way to replace fossil fuel heating.

Advantages And Limitations Of Direct Low-Temperature Use

Low-temperature geothermal direct use has several distinctive advantages. The technology is comparatively simple, as it often involves only wells, pumps, pipes, and heat exchangers. This simplicity can translate into lower risks and easier operation, which is valuable for municipalities and small utilities. When the resource is close to users, direct use can provide very competitive heat prices over long periods, since geothermal reservoirs renew their heat naturally and have no fuel cost.

Another advantage is the significant reduction in local air pollution when geothermal replaces combustion-based heating. In urban areas, shifting from coal or oil boilers to geothermal district heating can improve air quality and reduce greenhouse gas emissions. Direct use systems are also generally quiet and have a smaller visible footprint than many other energy projects, particularly when wellheads and equipment are integrated into existing buildings.

However, there are important limitations. Low-temperature resources are site specific and cannot be transported over long distances like conventional fuels. If suitable geology does not exist near heat users, it is often uneconomic to develop long pipelines or deep wells purely for low-temperature use. The temperature and flow rate of the resource set a natural limit on how much and what kind of heat can be delivered.

Another limitation is that many existing buildings are designed with higher-temperature heating systems, which can make it harder to integrate low-temperature geothermal without significant retrofits. In such cases, combining geothermal with auxiliary boilers or renovating the heating distribution systems may be necessary. Finally, exploration and drilling still carry some financial risk, even for low-temperature projects, and careful site assessment is needed to reduce uncertainty.

Environmental And Practical Considerations

Using low-temperature geothermal directly generally has lower environmental impacts than high-temperature power projects, but it still requires careful management. Geothermal fluids may contain dissolved minerals, gases, or trace elements. If spent water is discharged at the surface without treatment, it can affect surface waters, soils, or infrastructure through scaling and corrosion. Reinjection into the reservoir is commonly used to avoid these problems and to maintain reservoir conditions.

Land use requirements for surface equipment are relatively modest. Well pads, small plants, and distribution networks can often be integrated into existing developed areas. Noise and visual impacts are usually limited to drilling periods and can be mitigated by planning and good practice.

Practical considerations for successful deployment include aligning project design with local heat demands, ensuring competent operation and maintenance capacity, and providing clear information to users. Because direct use projects are closely tied to local conditions and communities, social acceptance and transparent communication are especially important. When designed and managed well, low-temperature geothermal direct use can become a long-term, locally anchored component of sustainable energy systems.