1. Definition and Scope of Tome Deux
GSHP (Ground Source Heat Pump), also known internationally as ground-coupled or geothermal heat pump systems, corresponds in the French regulatory framework to GMI (Géothermie de Minime Importance) – “universal surface geothermal”. It relies on closed-loop vertical borehole heat exchangers (BHEs) installed in clusters at typical depths of 60–200 m, possible almost anywhere on land where drilling is feasible.
This Volume 2 focuses exclusively on the heating function: extracting heat from the ground with a relatively high Coefficient of Performance (COP) / Seasonal Performance Factor (SPF) to feed a heating network or building heating system, often for seasonal needs but also year-round where required. Simultaneously, the heat pump’s evaporator (or a dedicated free-cooling circuit) recovers cold as a useful by-product for space cooling, process cooling or district cooling networks.
The technology uses clusters (“champs de sondes”) of vertical plastic tubes connected to 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.
This Tome Deux is the direct complement to Tome Premier (Cooling Production with Heat Recovery as By-Product for Domestic Hot Water).
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, particularly in Scandinavia, Germany, Switzerland, North America and later China. Heating has historically been the dominant application in colder climates, while cooling (or free cooling) has grown in importance with rising summer loads and the desire for annual thermal balance.
In France the simplified GMI regime (Decree 2015-15 and subsequent updates, including the 2026 power threshold raise to 2 MW for closed-loop systems) allows installations generally < 200 m depth under a télé-declaration procedure outside risk zones. Internationally the same technology is marketed simply as GSHP or closed-loop ground-source systems and is governed by standards such as IGSHPA, CSA/ANSI C448, EN 17522 and national building codes.
The concept of “universal” surface geothermal rests on the near-ubiquity of suitable ground temperatures (roughly 8–18 °C at these depths in most temperate climates), 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, EN 17522, 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. 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 Heating Mode and Cold Recovery as By-Product
In heating mode the heat pump’s evaporator extracts heat from the ground loop while the condenser delivers hot water (typically 35–55 °C, occasionally up to 70 °C) to the building or district network. The ground therefore acts as a large, stable heat source.
Cold recovery occurs in two main ways:
- Simultaneous free cooling: when cooling demand exists at the same time as heating (common in mixed-use buildings, data centres, hospitals), a free-cooling heat exchanger or secondary loop can deliver chilled water directly from the borehole field without running the compressor for cooling.
- Seasonal free cooling / by-product: in summer the borehole field is used in free-cooling mode (direct heat exchange) while any residual heat-pump operation for DHW or process needs can still reject heat usefully. In heating-dominated climates the cold by-product is often used for passive or low-energy cooling of common areas, server rooms or district cooling loops.
Maximising simultaneous or sequential recovery of cold improves the annual energy balance of the ground and raises the overall system efficiency.
4. Energy Performance (Heating Focus)
Typical ranges observed in monitored systems and manufacturer data (closed-loop GSHP):
- Heating COP: 3.5 – 5.5+ (SPF heating commonly 3.8 – 5.0 for well-designed systems).
- Seasonal Performance Factor (SPF heating): 3.5 – 5.2 depending on ground conditions, field sizing, distribution temperature and controls.
- Free-cooling EER / SPF: often 15–30+ when the compressor is not used.
- Partial-load advantage: many systems show higher efficiency at 40–70 % load.
- Specific heat extraction: typically 20–50 W per metre of borehole (higher in conductive rock or with groundwater flow).
These figures are generally 30–60 % better than typical air-source heat pumps under the same winter conditions and translate into substantial electricity and peak-demand reductions. When free cooling is maximised, the overall primary-energy and carbon performance improves further and the risk of long-term ground cooling is reduced.
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 free cooling.
- Karlstad University Campus, Sweden: 204 boreholes × 240–250 m.
- Xylem Emmaboda industrial plant, Sweden: ~140 boreholes × 150 m – process heating/cooling with high SPF (≈12–13 including pumps) and progressive heat/cold recovery.
- Studenthuset, Stockholm: 20 × 200 m boreholes, five years of monitored data showing SPF ≈ 5.2 at heat-pump boundary and significant free-cooling contribution.
- Numerous US and Canadian institutional buildings (schools, universities, military bases) with multi-borehole fields; typical heating COP/SPF 3.8–5.0.
- France: tens of thousands of GMI sondes (BRGM observatory); growing number of champs de sondes for tertiary and residential ensembles under the simplified regime (now up to 2 MW closed-loop).
Telecom base stations and data centres in Sweden and elsewhere 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 heating efficiency (COP/SPF often 4–5+) and stable performance independent of outdoor air temperature extremes.
- Near-universal geographic applicability (no need for ocean access or specific aquifers).
- Simultaneous or sequential recovery of useful cold for free cooling or networks.
- 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.
- Excellent synergy with low-temperature distribution (underfloor heating, 5th-generation district heating).
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 heating-dominated systems): net heat extraction progressively cools 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 free cooling or hybrid regeneration with solar/PVT), 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, now up to 2 MW closed-loop in France): simpler permitting, lower absolute thermal impact, easier conservative oversizing, highly effective free-cooling recovery for small cooling loads. 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 GMI thresholds, rigorous 3-D modelling, long-term monitoring and adaptive control. Better suited to maximising by-product cold utilisation for process or network needs.
Decision criterion: design for near-zero net annual heat balance over a 50-year horizon, prioritise free cooling recovery, choose HDPE, and match the scale of the field to the actual heating and concurrent cooling 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 (GSHP + solar/PVT regeneration or residual heat) and intelligent controls for thermal balance.
- Improved free-cooling strategies and simultaneous heating–cooling optimisation.
- Ongoing studies on groundwater ecological impacts of large-scale thermal use.
- Material longevity and alternative plastics (still dominated by HDPE).
- 5th-generation district heating and cooling (5GDHC) networks using borehole fields as seasonal storage.
8. Prospects and Strategic Recommendations
GSHP-style borehole clusters at ≈ 200 m constitute one of the most geographically universal, high-efficiency solutions for heating with simultaneous cold 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 (or equivalent) rules with maximised free-cooling recovery. For industrial or network operators the priority is long-term thermal sustainability, multi-user load balancing and full exploitation of the cold 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 free cooling systematically, and treat the ground as a shared long-term resource rather than an infinite source.
9. Key Actors and Resources
- France: BRGM (observatory & cartography), AFPG, ADEME, certified drillers (CertiForage / new certification regime), TeleGMI portal.
- International standards & associations: IGSHPA, European Geothermal Energy Council (EGEC), national geothermal associations in Sweden, Germany, Switzerland, USA, Canada.
- Major manufacturers: heat-pump OEMs (NIBE, Viessmann, WaterFurnace, ClimateMaster, etc.) and HDPE pipe specialists (REHAU, Uponor, etc.).
10. Selected References with URLs
1. BRGM – Observatoire dynamique de la géothermie de surface. https://www.geothermies.fr/
2. AFPG – Étude de filière géothermie. https://www.afpg.asso.fr/
3. IGSHPA – International Ground Source Heat Pump Association. https://igshpa.org/
4. PPI TN-55 – Plastic Piping Materials for Ground Source Geothermal. https://plasticpipe.org/
5. ENERGY STAR / DOE Geothermal Heat Pump resources. https://www.energystar.gov/products/geothermal_heat_pumps
6. EN 17522:2023 – Design and construction of backfilled and grouted borehole heat exchangers.
7. Case studies of large Scandinavian BTES/GSHP systems (Ahus, Emmaboda, Karlstad, Studenthuset – searchable via research literature and national energy agencies).
8. IEA HPT / Annex reports on GSHP performance and hybrid systems.
9. French regulatory texts: Arrêté du 25 juin 2015 (and subsequent updates including 2024–2026 power threshold changes).
10. REHVA Journal and academic papers on long-term monitored SPF of closed-loop systems.
End of Tome Deux
This Volume 2 addresses heating production and simultaneous cold recovery. It is the direct complement to Tome Premier (Cooling Production with Heat Recovery for Domestic Hot Water).
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 Froid · Tome I
Efficiency · Geothermal section
