1,547 m down, about 65 °C: Basel votes 89–1 to grow Riehen's heat network
Basel's Grosser Rat lends CHF 7.6 million to grow Riehen's 1994 geothermal heat network by 5.5 km and 620 connections. What it means for your heating design.
Since April 1994, a pair of wells under Riehen has done what most Swiss municipalities are still planning. They pull water up from 1,547 metres at about 65 °C, take the heat out and send it back underground at about 25 °C. The hydrothermal doublet sits in the Upper Rhine Graben and is run by Wärmeverbund Riehen AG (WVR). The tiefegeothermie.de project sheet lists 5.25 MW of installed thermal capacity and a flow of 20 litres per second. In 2018 the plant delivered 23.3 GWh of heat, which WVR equates to about 2.6 million litres of crude oil.
Do the division first. 23.3 GWh over 5.25 MW comes to roughly 4,400 full-load hours, which agrees with the sheet’s 4,500 to 5,500 operating hours a year. This is a utility with 32 years of data behind it, not a demonstrator.
This heat is not the sun’s. The top few metres of ground follow the solar year, and that shallow layer is what a building-level ground-source heat pump draws on. At 1,500 metres you draw on the Earth’s own heat flow. Going from about 10 °C at the surface to about 65 °C implies roughly 35 °C per kilometre.
The vote, and what it builds
On 16 September 2026, Basel-Stadt’s Grosser Rat voted 89 to 1 to lend WVR CHF 7.6 million. As Baublatt reported the next day, the loan is interest-free and conditionally repayable, with a 15-year drawdown. Canton and municipality are meant to share the cost equally. Riehen’s Einwohnerrat votes on its own CHF 7.6 million in October, and that result is still open. The responsible officials are Regierungsrat Kaspar Sutter and Gemeinderat Daniel Hettich. WVR, founded in 2009, is owned 50/50 by the municipality and IWB Industrielle Werke Basel.
The money pays for distribution, not drilling: 5.5 km of network, up to 620 new building connections and more plant capacity, saving about 6,875 tonnes of CO₂ a year. According to the joint Regierungsrat and Gemeinderat release of 5 March 2026, the expansion should replace the remaining 47 % of non-renewable heat. Today, gas CHP units back up the geothermal base load. Basel-Stadt’s target is net zero by 2037. WVR is also developing geo2riehen, a second plant at Grendelmatten. It is meant to serve about 4,000 more residents and lift the renewable share from about 55 % to over 80 %. In 2023 CEO Matthias Meier said: „Die positiven Resultate zeigen, dass der Untergrund sehr gute Voraussetzungen für eine zweite Geothermieanlage bietet.“ (The positive results show that the subsurface offers very good conditions for a second geothermal plant.)
←TODAY: A Swiss deep-heat doublet running since 1994 just won a cantonal vote, 89 to 1, to grow by 5.5 km.
→3012: Zurich 3012 maps its subsurface as plainly as its street grid, and every building lists a return temperature on its datasheet.
Fulcrum: How cold the water leaves a radiator in Riehen decides how much heat a well 1,547 m below can deliver.
The survey under the border
The groundwork for this was laid in the field. From 3 to 17 February 2022, about 9,400 geophones went out across Riehen, Basel and neighbouring municipalities, including Grenzach-Wyhlen in Germany. The Swiss Federal Office of Energy (BFE) committed CHF 1.2 million, and the University of Geneva took part in the research. No release names the crews who placed those sensors in two winter weeks, or the operators who have run the plant since 1994. Without them, none of today’s numbers would exist. Meier put the social side plainly in 2021: „Für eine erfolgreiche Projektumsetzung braucht es den Rückhalt der Politik wie auch der Bevölkerung.“ (A successful project needs the backing of both politics and the public.)
The same approach is moving west. ThinkGeoEnergy reported on 25 September 2026 on the Interreg project SUFRAGE (CHF 2.4 million, running to the end of 2028), co-led by AURA-EE and Géothermie-Suisse with Hydro-Géo Environnement and GEOTEST. It will map the subsurface on the French side of Geneva in 2D and 3D and build a monitoring platform for geothermal installations.
On the drawing board
A supply of about 65 °C suits radiators in older buildings and floor heating in new ones. A network connection also frees up the roof and cellar space that a heat pump or tank would otherwise take. The trade-off is that you swap your own machine for a utility contract, and the network’s temperatures become fixed design constraints. From the late 2070s, the lesson is to read the error bar before you specify. Riehen’s error bar is three decades of production.
Hack: Work out how much heat the Riehen doublet gives up depending on how cold its water goes back down. Heat equals flow times specific heat times temperature drop, so the return temperature sets the power just as much as the well does. Paste this into a GhPython component in Rhino or any Python shell:
cp = 4.18e3 # J/(kg·K), water; 1 L ≈ 1 kg
flow = 20 # L/s, Riehen production well
for t_ret in (25, 35, 45):
print(t_ret, "°C return:", round(flow * cp * (65 - t_ret) / 1e6, 2), "MW")With a 25 °C return you get about 3.3 MW. At 45 °C the same well gives about 1.7 MW. This is wellhead arithmetic, not the 5.25 MW plant rating, but it shows why a radiator matters to a well 1.5 km below it.
Atelier: Basel-region offices will see more renovation briefs inside the expanded network, where the heating concept starts from a connection instead of a boiler. If your team drafts energy concepts with AI, the AI will fill in default temperatures unless someone supplies the real ones. Monday move: add one line to your project-start checklist, “District heating area? Request the operator’s supply and return temperatures”, and fill it in before anyone sizes an emitter.
Before you size a single radiator, ask the operator for the supply and return temperatures.
Sources & Further Reading
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