Geothermal energy systems are designed to harness the natural heat stored beneath the Earth’s surface and deliver it in a usable form for heating, cooling and in some cases energy generation. The exact configuration of a geothermal system depends on local geology, subsurface temperature, reservoir permeability and the nature of the end-user heat demand. Star Energy approaches geothermal development through a technically led process that evaluates these factors in detail to determine the most appropriate system design for each project.
While multiple geothermal technologies exist globally, Star Energy’s core expertise lies in the development of deep geothermal doublet systems. These systems are particularly suited to the geological conditions found across parts of the United Kingdom and provide a reliable, large-scale source of low-carbon heat for district heating networks, industrial users and major public sector facilities.
Deep Geothermal Doublet Systems
The primary geothermal system developed by Star Energy is the deep geothermal doublet. This configuration involves the drilling of two deep wells into a geothermal reservoir, typically at depths of up to 5 kilometres depending on the geological setting.
The first well, known as the production well, is used to abstract naturally heated formation water from a permeable reservoir. This water, commonly referred to as brine due to its high salinity, has been heated by surrounding rock formations over geological timescales. Reservoir temperatures in suitable UK formations can exceed 80°C and in some cases approach or exceed 100°C.
Once the hot brine is brought to the surface, its thermal energy is transferred to a secondary heating circuit through plate heat exchangers. This process allows the geothermal fluid to remain within a closed operational loop while delivering heat to a district heating network or industrial process. After heat extraction, the cooled brine is reinjected into the reservoir through a second well, known as the reinjection well.
This reinjection process is fundamental to the sustainability of geothermal systems. By returning fluid to the reservoir, pressure conditions are maintained and long-term heat extraction can continue without significant depletion of the resource. The production and injection wells together form the geothermal doublet system.
The design and placement of these wells require detailed subsurface modelling. Geological interpretation, seismic data analysis, hydrogeological modelling and reservoir simulation are used to identify suitable formations with sufficient permeability and reservoir thickness. Directional drilling techniques are frequently employed to ensure accurate well placement and maximise reservoir contact.
Deep geothermal doublet systems are capable of delivering large and continuous heat outputs. A single doublet installation can typically provide between 5 MW and 20 MW of thermal capacity, depending on reservoir characteristics and flow rates. This scale of output makes deep geothermal systems particularly suitable for district heating networks, hospitals, universities, industrial facilities and other large heat consumers.
Closed-Loop Geothermal Systems
In some geological environments, natural permeability within the target formation may be limited. Where sufficient fluid flow cannot be achieved, closed-loop geothermal systems may be considered as an alternative.
Closed-loop systems operate by circulating a working fluid through sealed pipework installed in deep boreholes. Heat is transferred from the surrounding rock to the circulating fluid through thermal conduction rather than fluid extraction. Because formation water is not produced, closed-loop systems avoid challenges associated with brine handling, reinjection and reservoir pressure management.
Although closed-loop systems typically deliver lower thermal outputs than open-loop doublets, they offer flexibility in areas where permeability is insufficient for conventional geothermal production. These systems may be used to support smaller district heating schemes or individual large facilities.
Geothermal Heat with Heat Pump Integration
In some applications, geothermal heat may be combined with large-scale heat pumps to optimise system performance and achieve required delivery temperatures. This is particularly relevant where geothermal reservoir temperatures are lower than the operating temperatures required by existing heating networks.
Heat pumps operate by raising the temperature of a heat source through the application of electrical energy. When coupled with geothermal systems, the relatively stable subsurface temperature provides an efficient and reliable heat source. Because geothermal systems maintain consistent temperatures throughout the year, heat pump performance is significantly more stable than air-source alternatives.
This hybrid approach allows geothermal resources with temperatures of 50–70°C to be effectively integrated into heating networks designed for higher supply temperatures.
District Heating Integration
A defining feature of Star Energy’s geothermal systems is their integration with district heating infrastructure. District heating networks distribute hot water through insulated pipe systems to multiple buildings across a defined urban area.
Geothermal doublets are particularly well suited to this application because they provide continuous baseload heat. Unlike intermittent renewable technologies, geothermal energy is available 24 hours a day and is not dependent on weather conditions. This reliability allows geothermal plants to form the backbone of heat networks, with supplementary systems providing peak demand support where required.
Heat extracted from geothermal brine is transferred through plate heat exchangers to the district heating circuit. Thermal buffers and control systems are often incorporated to balance supply and demand fluctuations and maintain network stability.
Subsurface Engineering and Drilling
The development of deep geothermal systems relies heavily on subsurface engineering capabilities developed within the oil and gas sector. Star Energy’s experience in well design, directional drilling, reservoir analysis and subsurface modelling provides a strong technical foundation for geothermal deployment.
Drilling geothermal wells to depths of several kilometres requires advanced drilling technologies and careful materials selection to account for high salinity fluids, elevated temperatures and potential corrosion risks. Electric submersible pumps are typically installed in production wells to lift geothermal brine to the surface.
Well spacing and orientation must be carefully designed to avoid thermal breakthrough, where reinjected cooled water returns prematurely to the production well. Reservoir simulation models are used to optimise flow rates and ensure sustainable heat extraction over multi-decade project lifetimes.
System Selection and Project Design
Star Energy evaluates each geothermal project on a site-specific basis. Key factors considered during system selection include geological conditions, expected reservoir temperatures, permeability, available subsurface data, and the scale and stability of local heat demand.
Where suitable reservoirs exist, deep geothermal doublet systems represent the preferred solution due to their ability to deliver large-scale, stable thermal output. In other cases, closed-loop systems or hybrid geothermal-heat pump configurations may provide the most technically viable approach.
This flexible engineering approach ensures that geothermal systems are designed to match both the available resource and the requirements of the end user.
A Scalable Solution for Low Carbon Heat
As the UK seeks to decarbonise its heating sector, geothermal systems offer a scalable and domestically sourced solution. Deep geothermal doublets in particular provide long-term baseload heat capable of supporting district heating networks and major energy users.
Through careful geological assessment, engineering design and integration with heat distribution infrastructure, Star Energy is developing geothermal systems that harness the UK’s subsurface heat resource in a sustainable and technically robust manner.