1. Definition and Scope of Tome Premier
GMI (Géothermie de Minime Importance) designates the French regulatory category under the Mining Code for shallow geothermal installations of minor importance. In practice it corresponds to “universal surface geothermal”: closed-loop vertical borehole heat exchangers (sondes géothermiques verticales) that can be installed almost anywhere on land at depths typically between 60 m and 200 m.
This Volume 1 focuses exclusively on the cooling function: extracting cold (or more precisely rejecting heat into the ground) with a relatively high Coefficient of Performance / Energy Efficiency Ratio to feed a chilled-water network, often for seasonal needs but also year-round where required. Simultaneously, the heat pump’s condenser (or a dedicated desuperheater) recovers concentrated heat as a useful by-product, primarily for domestic hot water (DHW) and, where relevant, for heating networks.
The technology relies on clusters (“champs de sondes”) of vertical plastic tubes inserted into boreholes and connected in parallel or series to one or several reversible heat pumps. The design goal is to optimise the final service (private residential / small tertiary versus industrial or district scale) while minimising environmental impact over decades, recognising that no human energy solution is perfect.
2. Historical and Regulatory Context
Vertical borehole heat exchangers coupled to heat pumps have been used since the 1970s–1980s, with large-scale commercial deployment from the 1990s onward, especially in Scandinavia, Germany, Switzerland, North America and later China. France introduced the simplified GMI regime in 2015 (Decree 2015-15 and related orders) to accelerate surface geothermal while protecting aquifers and the subsurface. Installations under GMI (generally < 200 m depth and thermal power exchanged < 2 MW (raised from 500 kW by Décret n° 2026-537 of 25 June 2026)) require only a tele-declaration (TeleGMI portal) provided they lie outside designated risk zones.
The concept of “universal” surface geothermal rests on the fact that suitable ground temperatures (roughly 8–18 °C at these depths in most temperate climates) exist almost everywhere, in contrast to ocean-based SWAC or deep hydrothermal resources that are geographically constrained.
3. Technical Architecture
3.1 Borehole Design and Clusters
- Depth: typically 60–200 m (GMI limit ≈ 200 m investigation depth).
- Borehole diameter: commonly 110–150 mm.
- Probe configuration: single or double U-tube (or coaxial) per borehole.
- Cluster spacing: usually 6–10 m (sometimes 5–15 m) to limit thermal interference; determined by Thermal Response Test (TRT) and dynamic modelling (EED, GLHEPRO, FEFLOW, etc.).
- Grout: thermally enhanced bentonite or cement-based mixtures ensuring good thermal contact and hydraulic sealing between aquifers.
- Heat-carrier fluid: water or water–propylene glycol (preferred for lower environmental toxicity).
3.2 Materials – PVC versus Industry Standard
The request mentions clusters of PVC tubes. However, the overwhelming industry standard (IGSHPA, PPI TN-55, CSA/ANSI C448, French practice) is High-Density Polyethylene (HDPE / PE100 or PE100-RC) or, less frequently, PEX / PE-RT. These materials offer:
- Heat-fusion joints that create a monolithic, leak-proof system.
- High flexibility for insertion into deep boreholes and resistance to ground movement.
- Excellent fatigue and thermal-cycling performance over 50–100+ years.
- Proven chemical inertness with typical heat-carrier fluids.
PVC limitations for vertical BHEs: greater rigidity (harder installation), lower tolerance of temperature swings and cyclic stress, solvent-cement joints that are less reliable underground, and non-acceptance by major geothermal standards bodies. Long-term additive leaching or microplastic concerns under stress, while still debated, add further caution. For a 50-year design life and aquifer protection, HDPE is strongly preferred. If PVC is insisted upon, rigorous justification, independent certification and enhanced monitoring would be required.
3.3 Cooling Mode and Heat Recovery as By-Product
In cooling mode the heat pump’s evaporator produces chilled water for the building or district network while the condenser rejects heat into the ground loop. The ground therefore acts as a large, stable heat sink. Because ground temperature remains close to the mean annual air temperature plus the geothermal gradient, the temperature lift is far more favourable than with outdoor air, yielding higher EER/COP.
A desuperheater (small refrigerant-to-water heat exchanger) or direct use of condenser heat recovers a significant fraction of the rejected heat for domestic hot water. Depending on runtime and simultaneous loads, 10–60 % of DHW demand can be met “for free” during the cooling season. Residual heat still enters the ground, so thermal balance over the year remains a critical design parameter.
4. Energy Performance (Cooling Focus)
Typical ranges observed in monitored systems and manufacturer data (closed-loop):
- Cooling EER: 14–28+ (COP ≈ 4.1–8+). ENERGY STAR minimum for closed-loop water-to-air is 17.1 EER.
- Seasonal Performance Factor (SPF cooling): commonly 4–12 depending on ground conditions, field sizing and control.
- Partial-load advantage: many systems show higher efficiency at 50–70 % load.
- Free / passive geocooling: when fluid temperatures allow direct heat exchange without the compressor, SEER can exceed 15–30.
- Specific heat rejection: typically 20–50 W per metre of borehole (higher in conductive rock).
These figures are 20–40 % better than typical air-source chillers under the same conditions and translate into substantial electricity and peak-demand reductions. When heat recovery for DHW is maximised, the overall primary-energy and carbon performance improves further.
5. Selected Installations and Case Studies
Large closed-loop fields (clusters of 100–250 m boreholes) have operated successfully for decades, especially in Scandinavia and North America:
- Akershus University Hospital (Ahus), Norway: 228 boreholes × 200 m – one of Europe’s largest closed-loop systems serving a major hospital with both heating and cooling.
- Karlstad University Campus, Sweden: 204 boreholes × 240–250 m.
- Xylem Emmaboda industrial plant, Sweden: ~140 boreholes × 150 m – process cooling with high SPF (≈12–13 including pumps) and progressive heat recovery.
- Nydalen Business Park, Oslo: large commercial closed-loop field.
- Numerous US and Canadian institutional buildings (schools, universities, military bases) with multi-borehole fields; typical cooling EER 14–20+.
- France: tens of thousands of GMI sondes (BRGM observatory); growing number of champs de sondes for tertiary and residential ensembles under the simplified regime.
Telecom base stations in Sweden also demonstrate high-efficiency free cooling of equipment using single or small clusters of boreholes.
6. Advantages, Risks and Environmental Balance
6.1 Advantages
- High cooling efficiency (EER often 15–25+) and stable performance independent of outdoor air temperature.
- Near-universal geographic applicability (no need for ocean access or specific aquifers).
- Simultaneous recovery of useful heat for DHW or other loads.
- Long asset life of the ground loop (>50 years with HDPE).
- Low outdoor noise and visual impact; closed-loop protection of groundwater when properly sealed.
- Compatible with both private houses and large district / industrial networks.
6.2 Known and Emerging Risks
Short-term / installation risks: drilling noise and cuttings, temporary groundwater turbidity, improper sealing creating inter-aquifer pathways, antifreeze toxicity in case of leak.
Long-term thermal imbalance (critical for cooling-dominated systems): net heat injection progressively warms the borehole field, reducing ΔT and therefore COP. Documented cases of performance degradation over 10–25 years when fields were undersized or loads unbalanced. Mitigation requires 25–50-year thermal modelling, adequate spacing, annual energy balance (maximising heat recovery or hybrid operation), and monitoring of fluid temperatures.
Material durability: HDPE has an excellent track record; PVC carries higher risk of fatigue, joint failure and potential additive release under long-term thermal cycling.
Cumulative / regional effects: dense clusters or large industrial fields can create thermal plumes affecting neighbouring users or groundwater chemistry and microbiology – still an active research area.
Embodied impacts: drilling energy, plastic production, refrigerant charge of the heat pump. These are amortised over decades of high operational efficiency.
6.3 Private versus Industrial Optimisation
Private / small tertiary (often under GMI): simpler permitting, lower absolute thermal impact, easier conservative oversizing, highly effective DHW recovery via desuperheater. Optimal when loads are modest and land is limited.
Industrial / district scale: higher efficiency potential through load diversity and intentional Borehole Thermal Energy Storage (BTES), but requires full environmental authorisation above the new GMI thresholds (2 MW closed-loop), rigorous 3-D modelling, long-term monitoring and adaptive control. Better suited to maximising by-product heat utilisation for process or network needs.
Decision criterion: design for near-zero net annual heat balance over a 50-year horizon, prioritise heat recovery, choose HDPE, and match the scale of the field to the actual cooling and concurrent heating loads. Monitoring of ground-loop temperatures is essential at both scales.
7. Research Status and Innovations
- Advanced thermal modelling of large fields and long-term imbalance (EED, FEFLOW, etc.).
- Thermally enhanced grouts and larger-diameter or multi-U configurations to reduce total drilled length.
- Hybrid systems (geothermal + air-source or residual heat) and intelligent controls for thermal balance.
- Improved free-cooling strategies and simultaneous heat-recovery optimisation.
- Ongoing studies on groundwater ecological impacts of large-scale thermal use.
- Material longevity and alternative plastics (still dominated by HDPE).
8. Prospects and Strategic Recommendations
GMI-style borehole clusters at ≈ 200 m constitute one of the most geographically universal, high-efficiency solutions for cooling with simultaneous heat recovery. They are particularly attractive for inland sites, dense urban or campus environments, and mixed-use districts where SWAC is impossible.
For private users the priority is simple, robust design under GMI rules with maximised DHW recovery. For industrial or network operators the priority is long-term thermal sustainability, multi-user load balancing and full exploitation of the heat by-product. In both cases the environmental residual risks (thermal creep, material integrity, aquifer protection) are well characterised and can be kept low through standards, certified installers, modelling and monitoring.
Recommendation: adopt HDPE as the default material, size fields for 50-year thermal balance, integrate heat recovery systematically, and treat the ground as a shared long-term resource rather than an infinite sink.
9. Key Actors and Resources
- France: BRGM (observatory & cartography), AFPG, ADEME, certified drillers (CertiForage), TeleGMI portal.
- International standards & associations: IGSHPA, European Geothermal Energy Council (EGEC), national geothermal associations in Sweden, Germany, Switzerland, USA.
- Major manufacturers: heat-pump OEMs (WaterFurnace, ClimateMaster, NIBE, Viessmann, etc.) and HDPE pipe specialists (REHAU, Uponor, etc.).
10. Selected References with URLs
1. BRGM – Observatoire dynamique de la géothermie de surface (RP-71729-FR). http://infoterre.brgm.fr/rapports/RP-71729-FR.pdf
2. AFPG – Étude de filière géothermie. https://www.afpg.asso.fr/
3. Geothermies.fr – Official French portal. https://www.geothermies.fr/
4. IGSHPA – International Ground Source Heat Pump Association resources. https://igshpa.org/
5. PPI TN-55 – Plastic Piping Materials for Ground Source Geothermal. https://plasticpipe.org/
6. ENERGY STAR Geothermal Heat Pump Key Product Criteria. https://www.energystar.gov/products/geothermal_heat_pumps/key_product_criteria
7. Kensa – Ground Source Heat Pump Boreholes guidance. https://kensa.co.uk/ground-source-heat-pumps/boreholes
8. BGS – Geothermal technologies overview. https://www.bgs.ac.uk/geology-projects/geothermal-energy/geothermal-technologies/
9. Case studies of large Scandinavian BTES/GSHP systems (Ahus, Emmaboda, Karlstad – searchable via research literature and national energy agencies).
10. Arrêté du 25 juin 2015 and related GMI regulatory texts (France).
End of Tome Premier
This Volume 1 addresses cooling production and simultaneous heat recovery. A subsequent Volume 2 will treat the complementary heating-dominated operation with cold as by-product.
No human thermal solution is perfect; the residual risks identified above are real but manageable through standards, modelling, certified installation and long-term monitoring.
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GMI Chaud · Tome II
Efficiency · Geothermal section
