Geothermal Energy in Construction

Geothermal energy in construction exploits the fact that ground temperature stabilises below 10-15 m depth, remaining virtually constant year-round. On the Iberian Peninsula, ground temperature at 15 m depth ranges from 12 °C (northern plateau) to 18 °C (Mediterranean and southern coast), with annual variations of less than ±1 °C. This stability allows the ground source heat pump (GSHP) to operate with moderate temperature differences between the cold source (ground) and the hot source (building), maximising efficiency.

The normal geothermal gradient is 25-30 °C per kilometre of depth. Building applications work within the shallow geothermal range (0-400 m depth), where ground temperature is driven by solar radiation and surface climate rather than deep terrestrial heat flow. Ground thermal conductivity varies by geology: 1.0-1.5 W/m·K in clays, 2.0-3.5 W/m·K in granite and gneiss, and 1.5-2.5 W/m·K in limestone, according to the IGME Geothermal Atlas (Spanish Geological and Mining Institute).

Horizontal collectors consist of PE 100 polyethylene pipe circuits buried at 1.2-1.8 m depth. They require a ground area 1.5-2.5 times the heated floor area and extract 15-40 W/m² depending on soil type and climate (EN 15450). Installation cost is €20-40/m² of ground area, but they are limited to plots with sufficient open space.

Vertical borehole heat exchangers (BHE) are the dominant solution in urban construction. They consist of 80-200 m deep boreholes with single or double U-tubes through which a water-propylene glycol mixture circulates. Thermal extraction capacity per linear metre of borehole depends on the ground: 30-40 W/m in sediments, 50-60 W/m in crystalline rock, per VDI 4640 (Part 2). For a 150 m² dwelling in Madrid with heating demand of 50 kWh/m²·year, typically 2 boreholes of 100 m are required. Drilling costs range from €40 to €80 per linear metre depending on geology.

The ground source heat pump extracts heat from the geothermal fluid (at 8-15 °C) and raises it to the interior distribution temperature. With underfloor heating (35-40 °C supply temperature), the SCOP reaches 4.5-5.5, while with conventional radiators (55-65 °C) it drops to 3.0-3.5. In passive cooling (free cooling) mode, the geothermal fluid at 12-15 °C circulates directly through the underfloor system without activating the compressor, with electricity consumption limited to circulation pumps (equivalent COP of 15-30).

Commission Regulation (EU) 813/2013 (Ecodesign) classifies ground source heat pumps with SCOP of 4.5 as A++ and 5.0 as A+++. Heat pump service life is 20-25 years, while geothermal boreholes last over 50 years (GeoTrainet, 2014). The Thermal Response Test (TRT), per ASHRAE 1118-RP, measures in situ ground thermal conductivity and borehole thermal resistance—essential data for precise sizing of the borehole field.

The cost of a complete geothermal installation for a 150 m² single-family home in Spain (2 boreholes of 100 m + heat pump + underfloor heating) ranges from €18,000 to €30,000, compared to €3,000-6,000 for a gas boiler with radiators. However, annual operating costs are 50-70% lower: approximately €400-700/year (geothermal with underfloor heating) versus €1,200-1,800/year (gas boiler), at 2024 energy prices.

The simple payback period is 10-15 years without subsidies and 6-10 years with grants from Spain's PREE Programme (Incentives for Building Energy Rehabilitation) and Next Generation EU funds, which cover 40-70% of the investment depending on intervention type and income level. Over 25 years, cumulative savings from a geothermal installation versus natural gas exceed €15,000-25,000, accounting for gas price escalation and the elimination of direct CO₂ emissions (a gas boiler household emits 2.5-4.0 tCO₂/year, while geothermal with renewable electricity emits zero).

District-scale geothermal multiplies efficiency by balancing thermal loads between buildings with different demand profiles. The Ecoenergies Barcelona project supplies heating and cooling to 70 ha of the 22@ district using groundwater wells and centralised heat pumps, achieving a seasonal COP of 5.2 and a 60% emission reduction versus conventional systems. The Drake Landing Solar Community (Okotoks, Canada) combines 800 m² of solar thermal collectors with 144 geothermal boreholes at 35 m depth for seasonal storage, covering 97% of heating demand for 52 homes.

In Spain, the IGME has published the Geothermal Atlas of Spain with ground thermal conductivity maps at 1:200,000 and 1:1,000,000 scales, facilitating feasibility studies. The autonomous communities of Castilla y León, Madrid, and the Basque Country have the highest density of building geothermal installations. The Spanish Geothermal Technology Platform (GEOPLAT) estimates a market potential of 50,000 new geothermal building installations by 2030, compared to approximately 15,000 existing in 2023.