Hot water reservoir under TU Delft campus stores heat for winter

Next year, several unobtrusive wells will be drilled along the Balthasar van der Polweg on the TU Delft campus. Although these wells will only reach depths of under 200 metres, they provide access to a vast underground reservoir of hot water. According to energy storage expert Dr Martin Bloemendal (Faculty of Civil Engineering and Geosciences, CEG), the reservoir contains approximately 500,000 cubic metres of hot water, enough to fill the EEMCS tower three times over.

This high-temperature aquifer thermal energy storage system will be one of the largest, most advanced, and intensively monitored heat storage systems in Europe. Researchers, led by Professor Phil Vardon (CEG), envision it as a unique infrastructure for demonstration, education, and research. Last spring, they published a conference paper for the European Geosciences Union, the leading organisation for Earth, planetary and space science research in Europe.

Seasonal heat demand and supply

The TU Delft geothermal source pumps 350 cubic metres of hot water at temperatures of between 75°C and 80°C per hour from a depth of two kilometres. Cooled water is then reinjected into the aquifer. While geothermal systems operate best with consistent production, heating demand is seasonal.

Between April and October, when heating demand is low, surplus heat will be stored underground. This reserve will then supplement the winter heating demand, which exceeds the geothermal system’s capacity. The stored water forms a 60 metre high ‘hatbox’ in a water-bearing layer at depths of 120 to 180 metres, with a diameter of about 120 metres.

The system comprises two hot wells (80°C) and three warm wells (50°C). In winter, water from the hot wells supplies the campus and municipal heating networks through heat exchangers before flowing into the warm wells. In summer, the flow direction reverses and heat from the deep geothermal source replenishes the hot reservoir, sometimes boosting it to 90°C using a heat pump during periods of low electricity rates.

Efficiency and supplementary systems

Geothermal heat alone is expected to meet 60% of the campus’s heating demand. The HT-ATES system will increase this to 90%, with the remaining 10% covered by existing gas boilers during peak demand. “For now, we’re using the boilers because they’re already there,” says Bloemendal. “In the future, other options could replace them – or you could simply put on a jumper during those few very cold days.”

Lessons from the past

The concept of underground heat storage isn’t new. The 1973 oil crisis and later gas supply disruptions in the 1980s spurred innovation in alternative energy solutions. Early projects, like those in Beijum (Groningen) and Utrecht, faced challenges such as heat loss, insufficient water volumes, and mismatches between stored water temperatures and building heating requirements.

Learning from these experiences, TU Delft’s system addresses past shortcomings. The large storage volume minimises heat loss, and campus buildings are now well-insulated, requiring lower input temperatures.

Uncertainties and risks

Despite careful planning, some uncertainties remain, including heat recovery efficiency, groundwater spread, and potential microbial or chemical reactions. Monitoring wells have been installed to assess water quality and test the system, while sensors will track temperature and performance across the aquifer.

Initial efficiency is expected to be 40-50%, increasing to 70-80% over time as the system stabilises. Ultimately, the goal is to create a fossil-free, future-proof heating network, showcasing how conventional systems can transition to sustainable alternatives.

This cutting-edge project underscores TU Delft’s commitment to sustainable energy solutions, and makes strides toward a low-carbon future while transforming the campus into a hub for geothermal innovation.